[1]	[2] Includes $13,311 and $6,292 for the years ended December 31, 2025 and 2024 related to reimbursement received from SPN (related party) for transition services provided by the Company

Property and Equipment, Net - Additional Information (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2025	Dec. 31, 2024	Dec. 31, 2023
Property, Plant and Equipment [Abstract]
Depreciation and amortization expense	$ 4,133	$ 5,468	$ 6,243
Disposal of fully depreciated assets cost basis	734	679
Net book value of disposed of certain assets	$ 18	0
Impairment loss of leasehold improvements		$ 1,536

- Definition

The aggregate expense recognized in the current period that allocates the cost of tangible assets, intangible assets, or depleting assets to periods that benefit from use of the assets.

+ References + Details

Cybersecurity Risk Management, Strategy, and Governance	12 Months Ended
Cybersecurity Risk Management, Strategy, and Governance	Dec. 31, 2025
Cybersecurity Risk Management, Strategy, and Governance [Line Items]
Cybersecurity Risk Management Processes for Assessing, Identifying, and Managing Threats [Text Block]	Item 1C. Cybersecurity Cybersecurity Risk Management and Strategy We have developed and implemented a cybersecurity risk management program intended to protect the confidentiality, integrity, and availability of our critical systems and information. We design and assess our program based on the National Institute of Standards and Technology Cybersecurity Framework, or NIST CSF. This does not imply that we meet any particular technical standards, specifications, or requirements, only that we use the NIST CSF as a guide to help us identify, assess, and manage cybersecurity risks relevant to our business. Our cybersecurity risk management program is integrated into our overall risk management program, and shares common methodologies, reporting channels and governance processes that apply across the risk management program to other legal, compliance, strategic, operational, and financial risk areas. Key elements of our cybersecurity risk management program include but are not limited to the following: • risk assessments designed to help identify material risks from cybersecurity threats to our critical systems and information; • a security team principally responsible for managing (1) our cybersecurity risk assessment processes, (2) our security controls, and (3) our response to cybersecurity incidents; • the use of external service providers, where appropriate, to assess, test or otherwise assist with aspects of our security processes; • cybersecurity awareness training of our employees, including incident response personnel, and senior management; • a security incident response plan that includes procedures for responding to cybersecurity incidents; • a third-party risk management process for key service providers based on our assessment of their criticality to our operations and respective risk profile, suppliers, and vendors who have access to our critical systems and information; and • cybersecurity insurance to cover us for costs and expenses we may incur due to a cybersecurity incident. We have not identified risks from known cybersecurity threats, including as a result of any prior cybersecurity incidents, that have materially affected, including our operations, business strategy, results of operations, or financial condition. We face risks from cybersecurity threats that, if realized are reasonably likely to materially affect us, including our operations, business strategy, results of operations, or financial condition. For more information, “Risk Factors— Risks Related to Our Operations—Our business and operations may suffer in the event of information technology system failures, cyberattacks or deficiencies in our cybersecurity.” Cybersecurity Governance Our Board considers cybersecurity risk as part of its risk oversight function and has delegated to the Audit Committee oversight of management’s implementation of our cybersecurity risk management program. The Audit Committee receives quarterly reports from our Vice President, Bioinformatics & Information Technology, or VP of IT, on our cybersecurity risks. In addition, our VP of IT updates the Audit Committee, where it deems appropriate, regarding any cybersecurity incidents it considers to be significant or potentially significant. The Audit Committee regularly reports to the full Board regarding its activities, including those related to cybersecurity. Our management team, including our VP of IT, is responsible for assessing and managing our material risks from cybersecurity threats. The team has primary responsibility for our overall cybersecurity risk management program and supervises both our internal cybersecurity personnel and our retained external cybersecurity consultants. Our VP of IT has nearly three decades of IT and computational experience in life sciences organizations. His cybersecurity work includes the development and implementation of cybersecurity policies, platforms, and robust end-user training curriculums. Our VP of IT and IT Group work together to monitor and report cybersecurity trends and threats to management. Additionally, we work with an external IT partner and external cybersecurity counsel to assess, identify, and manage risks from cybersecurity threats. The IT Group undertakes table-top business disruption, disaster recovery and related response strategies, and plans on a periodic basis, and aims to review, and if appropriate update, applicable policies and procedures annually. Our management team and IT Group takes steps to stay informed about and monitor efforts to prevent, detect, mitigate, and remediate cybersecurity risks and incidents through various means, which may include briefings from internal security personnel; threat intelligence and other information obtained from public or private sources, including external consultants engaged by us; and alerts and reports produced by security tools deployed in our IT environment.
Cybersecurity Risk Management Processes Integrated [Flag]	true
Cybersecurity Risk Management Processes Integrated [Text Block]	Our cybersecurity risk management program is integrated into our overall risk management program, and shares common methodologies, reporting channels and governance processes that apply across the risk management program to other legal, compliance, strategic, operational, and financial risk areas.
Cybersecurity Risk Management Third Party Engaged [Flag]	true
Cybersecurity Risk Third Party Oversight and Identification Processes [Flag]	true
Cybersecurity Risk Materially Affected or Reasonably Likely to Materially Affect Registrant [Flag]	false
Cybersecurity Risk Board of Directors Oversight [Text Block]	Cybersecurity Governance Our Board considers cybersecurity risk as part of its risk oversight function and has delegated to the Audit Committee oversight of management’s implementation of our cybersecurity risk management program. The Audit Committee receives quarterly reports from our Vice President, Bioinformatics & Information Technology, or VP of IT, on our cybersecurity risks. In addition, our VP of IT updates the Audit Committee, where it deems appropriate, regarding any cybersecurity incidents it considers to be significant or potentially significant. The Audit Committee regularly reports to the full Board regarding its activities, including those related to cybersecurity. Our management team, including our VP of IT, is responsible for assessing and managing our material risks from cybersecurity threats. The team has primary responsibility for our overall cybersecurity risk management program and supervises both our internal cybersecurity personnel and our retained external cybersecurity consultants. Our VP of IT has nearly three decades of IT and computational experience in life sciences organizations. His cybersecurity work includes the development and implementation of cybersecurity policies, platforms, and robust end-user training curriculums. Our VP of IT and IT Group work together to monitor and report cybersecurity trends and threats to management. Additionally, we work with an external IT partner and external cybersecurity counsel to assess, identify, and manage risks from cybersecurity threats. The IT Group undertakes table-top business disruption, disaster recovery and related response strategies, and plans on a periodic basis, and aims to review, and if appropriate update, applicable policies and procedures annually. Our management team and IT Group takes steps to stay informed about and monitor efforts to prevent, detect, mitigate, and remediate cybersecurity risks and incidents through various means, which may include briefings from internal security personnel; threat intelligence and other information obtained from public or private sources, including external consultants engaged by us; and alerts and reports produced by security tools deployed in our IT environment.
Cybersecurity Risk Board Committee or Subcommittee Responsible for Oversight [Text Block]	Our Board considers cybersecurity risk as part of its risk oversight function and has delegated to the Audit Committee oversight of management’s implementation of our cybersecurity risk management program.
Cybersecurity Risk Process for Informing Board Committee or Subcommittee Responsible for Oversight [Text Block]	The Audit Committee regularly reports to the full Board regarding its activities, including those related to cybersecurity
Cybersecurity Risk Role of Management [Text Block]	Our management team, including our VP of IT, is responsible for assessing and managing our material risks from cybersecurity threats. The team has primary responsibility for our overall cybersecurity risk management program and supervises both our internal cybersecurity personnel and our retained external cybersecurity consultants. Our VP of IT has nearly three decades of IT and computational experience in life sciences organizations. His cybersecurity work includes the development and implementation of cybersecurity policies, platforms, and robust end-user training curriculums. Our VP of IT and IT Group work together to monitor and report cybersecurity trends and threats to management. Additionally, we work with an external IT partner and external cybersecurity counsel to assess, identify, and manage risks from cybersecurity threats. The IT Group undertakes table-top business disruption, disaster recovery and related response strategies, and plans on a periodic basis, and aims to review, and if appropriate update, applicable policies and procedures annually. Our management team and IT Group takes steps to stay informed about and monitor efforts to prevent, detect, mitigate, and remediate cybersecurity risks and incidents through various means, which may include briefings from internal security personnel; threat intelligence and other information obtained from public or private sources, including external consultants engaged by us; and alerts and reports produced by security tools deployed in our IT environment.
Cybersecurity Risk Management Positions or Committees Responsible [Flag]	true
Cybersecurity Risk Management Positions or Committees Responsible [Text Block]	Our management team, including our VP of IT, is responsible for assessing and managing our material risks from cybersecurity threats. The team has primary responsibility for our overall cybersecurity risk management program and supervises both our internal cybersecurity personnel and our retained external cybersecurity consultants.
Cybersecurity Risk Management Expertise of Management Responsible [Text Block]	Our VP of IT and IT Group work together to monitor and report cybersecurity trends and threats to management.
Cybersecurity Risk Process for Informing Management or Committees Responsible [Text Block]	Our VP of IT has nearly three decades of IT and computational experience in life sciences organizations.
Cybersecurity Risk Management Positions or Committees Responsible Report to Board [Flag]	true

Consolidated Statements of Cash Flows - USD ($)
$ in Thousands

12 Months Ended

Dec. 31, 2025

Dec. 31, 2024

Dec. 31, 2023

Cash flows from operating activities:

Net Income (Loss)

$ 5,696

$ 136

$ (113,724)

Adjustments to reconcile net loss to net cash used in operating activities:

Stock-based compensation expense

10,840

20,971

34,101

Depreciation and amortization expense

4,133

5,468

6,243

Non-cash operating lease cost

8,420

9,230

8,871

Net (accretion) amortization of (discounts) premiums on investments

(236)

Gain on sale of VOWST Business, net of transaction costs

(146,707)

Amortization of debt issuance costs

1,413

1,139

Loss associated with extinguishment of debt

23,351

1,625

(Gain)/loss on disposal of fixed assets

(192)

317

Impairment of long-lived assets

3,267

Change in fair value of warrant liabilities

(546)

(1,554)

Collaboration (profit) loss sharing - related party (1)

^[1]

5,158

Changes in operating assets and liabilities:

Accounts receivable due from SPN - related party

1,708

(2,068)

Prepaid expenses and other current and non-current assets

2,720

987

(29,124)

Accounts receivable

(157)

Collaboration receivable - related party

8,674

(8,674)

Inventories

(33,795)

(29,647)

Deferred income - related party

(4,124)

7,730

Deferred revenue - related party

(1,325)

Accounts payable

(2,397)

(2,940)

(11,578)

Accrued liabilities due to SPN - related party

(14,472)

(15,708)

Operating lease liabilities

(8,674)

(6,339)

(2,197)

Accrued expenses and other current and long-term liabilities

(6,508)

(10,196)

15,838

Net cash provided by (used in) operating activities

1,117

(148,609)

(117,354)

Cash flows from investing activities:

Purchases of property and equipment

(252)

(380)

(7,975)

Proceeds from sales of property and equipment

210

Purchases of investments

(4,426)

Sales and maturities of investments

22,983

Sales of restricted investments

1,401

Proceeds from sale of VOWST Business

141,272

Net cash (used in) provided by investing activities

(42)

142,293

10,582

Cash flows from financing activities:

Proceeds from at the market equity offering, net of issuance costs

13,167

23,530

18,159

Proceeds from exercise of stock options

501

877

Proceeds from Securities Purchase Agreement - related party

13,516

Issuance of common stock under ESPP

230

487

2,151

Proceeds from issuance of debt, net of issuance costs

103,378

Repayment of notes payable

(127,905)

(52,860)

Net cash provided by (used in) financing activities

13,898

(90,372)

71,705

Net increase (decrease) in cash, cash equivalents and restricted cash

14,973

(96,688)

(35,067)

Effect of exchange rate changes on cash, cash equivalents and restricted cash

(1)

Cash, cash equivalents and restricted cash at beginning of year

39,461

136,150

171,215

Cash, cash equivalents and restricted cash at end of year

54,434

39,461

136,150

Supplemental disclosure of cash flow information:

Cash paid for interest

10,858

12,547

Supplemental disclosure of non-cash investing and financing activities:

Property and equipment purchases included in accounts payable and accrued expenses

Lease liability arising from obtaining right-of-use assets

3,046

Prepaid rent reclassified to right-of-use assets

4,634

Recognition of warrant liabilities

2,100

Warrants issued related to prior credit facility with Oaktree and recorded as debt discount

$ 0

$ 2,785

[1]	Includes non-cash collaboration profits and losses related to pre-launch activities; subsequent to the approval of VOWST in April 2023, collaboration (profit) loss sharing - related party is included within changes in operating assets and liabilities.

Stock and Stock-Based Awards - Summary of Stock Based Compensation Expense (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2025	Dec. 31, 2024	Dec. 31, 2023
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total stock-based compensation expense	$ 10,840	$ 20,971	$ 34,101
Research and Development Expenses [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total stock-based compensation expense	5,080	10,996	19,341
General and Administrative Expenses [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total stock-based compensation expense	$ 5,760	$ 9,975	$ 14,760

Income Taxes - Schedule of Net Deferred Tax Assets (Detail) - USD ($) $ in Thousands	Dec. 31, 2025	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Deferred tax assets:
Net operating loss carryforwards	$ 167,203	$ 156,157
Research and development tax credit carryforwards	54,149	54,648
Section 174 capitalized research and development expenses	39,759	53,600
Stock-based compensation expense	28,594	28,819
Lease liability	22,689	23,917
Contingent consideration	13,437	0
Accrued expenses	1,788	7,032
Section 163(j) limitation	1,843	5,599
Depreciation and amortization	1,223	953
Other	98	132
Total deferred tax assets	330,783	330,857
Deferred tax liabilities:
Depreciation and amortization	0	0
Right of use assets	(19,822)	(21,115)
Total deferred tax liabilities	(19,822)	(21,115)
Valuation allowance	(310,961)	(309,742)	$ (312,809)	$ (277,370)
Net deferred tax assets	$ 0	$ 0

Segment Reporting - Schedule of reportable segment net income (loss) (Detail) - USD ($)
$ in Thousands

12 Months Ended

Dec. 31, 2025

Dec. 31, 2024

Dec. 31, 2023

Segment Reporting Information [Line Items]

Grant revenue

$ 789

$ 0

Operating Expenses

94,760

121,315

195,097

Gain on sale of VOWST Business

80,685

5,684

Net income (loss) from continuing operations

5,696

(125,771)

(190,130)

Net income from discontinued operations, net of tax

125,907

76,406

Net income (loss)

5,696

136

(113,724)

Operating Segments [Member]

Segment Reporting Information [Line Items]

Grant revenue

789

R&D personnel-related (including stock-based compensation)

21,553

28,689

65,251

G&A personnel-related (including stock-based compensation)

14,413

22,679

36,069

Professional fees

8,991

9,805

18,784

Facility-related and other

15,752

20,699

22,647

Gain on sale of VOWST Business

(80,685)

(5,684)

Other segment (income) expense

^[1]

(12,191)

13,773

(3,722)

Net income (loss) from continuing operations

5,696

(125,771)

(190,130)

Net income from discontinued operations, net of tax

^[2]

125,907

76,406

Net income (loss)

5,696

136

(113,724)

Live Biotherapeutics Platform [Member] | Operating Segments [Member]

Segment Reporting Information [Line Items]

Operating Expenses

23,573

29,006

43,342

SER-155 [Member] | Operating Segments [Member]

Segment Reporting Information [Line Items]

Operating Expenses

$ 3,687

$ 6,804

$ 7,759

[1]	Other segment (income) expense includes manufacturing services expenses, research and development expenses on early stage programs, interest income and other (income) expense, net.
[2]	See Note 3, Discontinued Operations and TSA, for further details.

Accrued Expenses and Other Current Liabilities - Summary of Accrued Expenses and Other Current Liabilities (Detail) - USD ($) $ in Thousands	Dec. 31, 2025	Dec. 31, 2024
Payables and Accruals [Abstract]
Clinical and development costs	$ 246	$ 422
Manufacturing and quality costs	145	478
Payroll and employee-related costs	2,685	7,656
Facility and other	896	2,163
Total accrued expenses and other current liabilities	$ 3,972	$ 10,719

Discontinued Operations and TSA

12 Months Ended

Dec. 31, 2025

Discontinued Operations and Disposal Groups [Abstract]

Discontinued Operations and TSA

3. Discontinued Operations and TSA

On September 30, 2024, the Company completed the sale of its VOWST Business to SPN. The Company has determined the sale of the VOWST Business represents a strategic shift that will have a major effect on its business and therefore met the criteria for classification as discontinued operations on September 30, 2024. Accordingly, the VOWST Business is reported as discontinued operations in accordance with ASC 205-20, Discontinued Operations. The related assets and liabilities of the VOWST Business are classified as assets and liabilities of discontinued operations in the consolidated balance sheets and the results of operations from the VOWST Business as discontinued operations in the consolidated statements of operations. Applicable amounts in prior years have been

recast to conform to this discontinued operations presentation. The Company recognized a gain on the sale of the VOWST Business upon closing.

As of December 31, 2025 and 2024, there were no assets or liabilities of discontinued operations.

The following table presents the gain on the sale of the VOWST Business as of December 31, 2024, pursuant to the Purchase Agreement:


	December 31,
	2024
Consideration received
Upfront payment (1)	$	79,788
Prepaid milestone		60,000
Deferred revenue from termination of 2016 License Agreement		95,364
Settlement of net collaboration payable at close		27,465
Premium on equity financing		1,484
Deferred income from termination of 2021 License Agreement		3,606
Accrued liabilities due to SPN - related party		(33,458	)
Total fair value transferred for business	$	234,249

Net assets transferred
Inventory	$	63,442
Prepaid expenses and other current assets		2,219
Property and equipment, net		3,966
Operating lease assets		17,929
Other non-current assets		39,328
Accrued expenses and other current liabilities		(31,547	)
Operating lease liabilities		(14,413	)
Net assets transferred	$	80,924

Transaction costs	$	6,618

Gain on sale, pre-tax	$	146,707
Income tax		—
Gain on sale, net of tax	$	146,707

[1] The upfront payment consists of $100,000, less $17,857 owed by the Company to an affiliate of SPN under the prior license agreement between the Company and the SPN affiliate, less approximately $2,355 in satisfaction of costs due under the Bacthera Manufacturing Agreement.

For the year ended December 31, 2024, the gain from sale of the VOWST Business, net of tax of $146,707 was included in the net income (loss) from discontinued operations, net of tax line item of the Company’s consolidated statements of operations and comprehensive income (loss). While the Company has net income from discontinued operations for the year ended December 31, 2024, the Company realized a tax loss for the full year ended December 31, 2024, for which it is more likely than not that the Company will not realize a benefit. The Company has recorded a full valuation allowance against its net deferred tax assets as of December 31, 2025, 2024 and 2023.

The following table presents the financial results of the discontinued operations:


	Year Ended December 31,
	2025		2024			2023
Revenue:
Collaboration revenue - related party	$	—	$	—		$	126,325
Total revenue		—		—			126,325
Operating expenses:
Research and development expenses		—		5,809			28,263
General and administrative expenses		—		4,066			10,244
Collaboration (profit) loss sharing - related party		—		(1,496	)		704
Total operating expenses		—		8,379			39,211
(Loss) income from discontinued operations		—		(8,379	)		87,114
Other income (expense):
Gain on sale of VOWST business		—		146,707			—
Interest expense		—		(12,192	)		(10,708	)
Other expense		—		(229	)		—
Income (loss) from discontinued operations, pre-tax	$	—	$	125,907		$	76,406
Income tax		—		—			—
Net income (loss) from discontinued operations, net of tax	$	—	$	125,907		$	76,406

In accordance with ASC 205-20, only expenses specifically identifiable and related to a business to be disposed may be presented in discontinued operations. As such, the research and development and general and administrative expenses in discontinued operations include corporate costs incurred directly to solely support the VOWST Business.

The Company also entered into a Transition Services Agreement (“TSA”) with NESA, an affiliate of SPN, in connection with the Transaction, through which the Company provided certain manufacturing services until December 31, 2025, and other transition services, for the duration specified in the schedule to the TSA for each service. For the years ended December 31, 2025 and 2024, the Company recognized $13,311 and $6,292 of TSA reimbursement income in other income in the Company’s consolidated statements of operations and comprehensive income. For the years ended December 31, 2025 and 2024, the Company incurred $6,544 and $3,532 of expenses related to manufacturing services and $3,592 and $3,136 of TSA labor and passthrough expenses to support the transition services, including finance and accounting, information technology, human resources, operations, and other services.

For the years ended December 31, 2025 and 2024, $14,897 and $3,724 was billed to NESA related to transition services performed by the Company, and the Company received $91,605 and $1,656 from NESA during the periods including the first installment payment of $50,000 received in January 2025 and the second installment payment of $25,000 received in July 2025 which were conditioned on the Company’s material compliance with obligations under the TSA. The $75,000 of installment payments were recognized in Gain on sale of VOWST Business within continuing operations in the Company’s consolidated statements of operations and comprehensive income for the year ended December 31, 2025 as the gain was realizable. As of December 31, 2025 and 2024, the Company had $360 and $2,068 in accounts receivable due from SPN - related party in the Company’s consolidated balance sheets.

The Company has estimated costs associated with certain accrued liabilities due to SPN - related party as a loss contingency in accordance with ASC 450, Contingencies. These contingent liabilities are presented as Accrued Liabilities due to SPN - related party from continuing operations on the consolidated balance sheets as of December 31, 2025 and December 31, 2024 and consist of the following (in thousands):


	December 31,		December 31,
	2025		2024
Profit Sharing Payments	$	1,701	$	11,230
Royalties associated with the MSK Agreement		1,309		2,786
VOWST post-marketing safety surveillance study		268		771
80.1% of lease cost of Waltham facility		—		1,501
Employment-related costs for conveying employees		—		1,462
Total accrued liabilities due to SPN - related party	$	3,278	$	17,750

The contingent liabilities accrued on the Company's consolidated balance sheets are remeasured at December 31, 2025 based on i) cash payments made by the Company to reduce the accrued liabilities due to SPN - related party and ii) revised estimates of the total remaining liabilities due to SPN - related party. For the years ended December 31, 2025 and 2024, the Company recognized a gain

on sale of VOWST Business of $5,685 and $5,684, respectively, as a result of the change in the accrued liabilities due to SPN - related party.

The Company has excluded from its consolidated balance sheets the effects of certain milestone payments received by the Company after the Product has achieved net sales-based milestones. These contingent receivables will be recognized as a gain contingency, in accordance with ASC 450, Contingencies, in continuing operations in the period when the contingencies are resolved.

The cash flows related to discontinued operations have not been segregated and are included in the consolidated statements of cash flows. There were no cash flows related to discontinued operations for the year ended December 31, 2025. For the years ended December 31, 2024 and 2023, capital expenditures related to the VOWST Business were $112 and $2,272, respectively. Depreciation expense related to the VOWST Business for the same periods was $989 and $2,019, respectively. Non-cash operating lease costs related to the VOWST Business for the years ended December 31, 2024 and 2023 were $1,447 and $2,123, respectively, while the share based compensation expense for the same periods were $1,884 and $2,773, respectively. The collaboration loss sharing (related party) related to the VOWST Business was $0 and $5,158 for the years ended December 31, 2024 and 2023, respectively. Excluding the gain of $146,707 recognized on the sale of the VOWST Business presented in the consolidated statements of cash flows for the year ended December 31, 2024, there were no other material operating or investing non-cash items related to the VOWST Business for any period presented.

Segment Reporting

12 Months Ended

Dec. 31, 2025

Segment Reporting [Abstract]

Segment Reporting

14. Segment Reporting

The Company's interim Chief Executive Officer, who is the Chief Operating Decision Maker ("CODM"), manages and allocates resources to the operations of the Company on a total company basis by assessing the overall level of resources available and how to best deploy these resources across functions and research and development projects that are in line with the Company's long-term company-wide strategic goals. The CODM uses the Company’s consolidated net income (loss) to monitor actual results versus the budget in assessing segment performance and the allocation of resources. The measure of segment assets is reported on the consolidated balance sheets as total assets.

The Company's reportable segment net income (loss) for the years ended December 31, 2025, 2024 and 2023, consisted of the following (in thousands):


	Year Ended December 31,
	2025			2024			2023
Grant revenue	$	789		$	—		$	—
Significant segment expenses:
Live biotherapeutics platform	$	23,573		$	29,006		$	43,342
SER-155		3,687			6,804			7,759
R&D personnel-related (including stock-based compensation)		21,553			28,689			65,251
G&A personnel-related (including stock-based compensation)		14,413			22,679			36,069
Professional fees		8,991			9,805			18,784
Facility-related and other		15,752			20,699			22,647
Gain on sale of VOWST Business		(80,685	)		(5,684	)		-
Other segment (income) expense (1)		(12,191	)		13,773			(3,722	)
Net income (loss) from continuing operations		5,696			(125,771	)		(190,130	)
Net income from discontinued operations, net of tax (2)		—			125,907			76,406
Net income (loss)	$	5,696		$	136		$	(113,724	)

[1] Other segment (income) expense includes manufacturing services expenses, research and development expenses on early stage programs, interest income and other (income) expense, net.

[2] See Note 3, Discontinued Operations and TSA, for further details.

- Definition

The entire disclosure for reporting segments including data and tables. Reportable segments include those that meet any of the following quantitative thresholds a) it's reported revenue, including sales to external customers and intersegment sales or transfers is 10 percent or more of the combined revenue, internal and external, of all operating segments b) the absolute amount of its reported profit or loss is 10 percent or more of the greater, in absolute amount of 1) the combined reported profit of all operating segments that did not report a loss or 2) the combined reported loss of all operating segments that did report a loss c) its assets are 10 percent or more of the combined assets of all operating segments.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 280
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 21
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482810/280-10-50-21

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 280
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 21
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482810/280-10-50-21

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Topic 280
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 32
-Subparagraph (e)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482810/280-10-50-32

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Topic 280
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 31
-Publisher FASB
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+ Details

Property and Equipment, Net (Tables)

12 Months Ended

Dec. 31, 2025

Property, Plant and Equipment [Abstract]

Schedule of Property and Equipment, Net

Property and equipment, net consisted of the following (in thousands):


	December 31,
	2025			2024
Laboratory equipment	$	23,852		$	24,468
Computer equipment		4,058			3,924
Furniture and office equipment		4,523			4,523
Leasehold improvements		30,954			30,954
Construction in progress		861			861
		64,248			64,730
Less: Accumulated depreciation and amortization		(56,613	)		(53,196	)
	$	7,635		$	11,534

Discontinued Operations and TSA - Gain on Sale of the VOWST Business (Parenthetical) (Details) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2024	Dec. 31, 2025
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Upfront Payment	$ 15,000	$ 15,000
SPN [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Upfront Payment		15,000
Upfront Payment		17,857
Upfront payment satisfaction fees		2,355
VOWST Business [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Upfront Payment		$ 100,000

Income Taxes (Tables)

12 Months Ended

Dec. 31, 2025

Income Tax Disclosure [Abstract]

Schedule of Reconciliation of U.S Federal Statutory Income Tax Rate to Company's Effective Income Tax Rate

A reconciliation of the U.S. federal statutory income tax rate to the Company’s effective income tax rate after the adoption of ASU 2023-09 is as follows:


	Year Ended December 31, 2025
	Amount			%
Federal statutory income tax rate	$	1,202			21.0	%
State and local income taxes, net of federal income tax effect(1)		—			—	%
Tax credits
Research and development tax credits		334			5.8	%
Changes in valuation allowance		(3,445	)		(60.2	)%
Nontaxable or nondeductible items
Meals and entertainment		14			0.3	%
Stock-based compensation		3,094			54.1	%
Adjustment to gain on sale of VOWST business		(1,133	)		(19.8	)%
Changes in unrecognized tax benefits		(67	)		(1.2	)%
Other adjustments		1			0.0	%
Effective income tax rate	$	—			—	%

[1] State and local taxes in Massachusetts comprise the majority of this category.

A reconciliation of the U.S. federal statutory income tax rate to the Company’s effective income tax rate for years prior to the adoption of ASU 2023-09 is as follows:


	Year Ended December 31,
		2024			2023
Federal statutory income tax rate			(21.0	)%		(21.0	)%
Research and development tax credits			(1.8	)		(2.5	)
State taxes, net of federal benefit			(3.6	)		(7.2	)
Stock-based compensation			1.0			0.8
Uncertain tax position reserves			0.4			0.5
Other			(0.4	)		(0.1	)
Change in deferred tax asset valuation allowance			25.4			29.5
Effective income tax rate			—	%		—	%

Schedule of Net Deferred Tax Assets

Net deferred tax assets as of December 31, 2025 and 2024 consisted of the following:


	December 31,
	2025			2024
Deferred tax assets:
Net operating loss carryforwards	$	167,203		$	156,157
Research and development tax credit carryforwards		54,149			54,648
Section 174 capitalized research and development expenses		39,759			53,600
Stock-based compensation expense		28,594			28,819
Lease liability		22,689			23,917
Contingent consideration		13,437			—
Accrued expenses		1,788			7,032
Section 163(j) limitation		1,843			5,599
Depreciation and amortization		1,223			953
Other		98			132
Total deferred tax assets	$	330,783		$	330,857
Deferred tax liabilities:
Depreciation and amortization		—			—
Right of use assets		(19,822	)		(21,115	)
Total deferred tax liabilities		(19,822	)		(21,115	)
Valuation allowance	$	(310,961	)	$	(309,742	)
Net deferred tax assets	$	—		$	—

Summary of Changes In Valuation Allowance for Deferred Tax Assets

Changes in the valuation allowance for deferred tax assets during the years ended December 31, 2025, 2024 and 2023 related primarily to the increases or decreases in NOLs, research and development tax credit carryforwards, capitalized research and development expenses pursuant to IRC Section 174, and non-deductible stock-based compensation expenses. A rollforward of the valuation allowance is as follows:


	Year Ended December 31,
	2025			2024			2023
Valuation allowance at beginning of year	$	(309,742	)	$	(312,809	)	$	(277,370	)
Decreases recorded as benefit to income tax provision					3,067			—
Increases recorded to income tax provision		(1,219	)		—			(35,439	)
Valuation allowance as of end of year	$	(310,961	)	$	(309,742	)	$	(312,809	)

Schedule of Change In Unrecognized Tax Benefits Roll Forward

The changes in the Company's unrecognized tax benefits for the years ended December 31, 2025, 2024, and 2023 were as follows:


	Year Ended December 31,
	2025			2024		2023
Balance at beginning of year	$	14,015		$	13,529	$	12,528
Increase in unrecognized tax benefits as a result of tax positions taken during the year		129			486		1,001
(Decrease) in unrecognized tax benefits as a result of tax positions taken during the year		(289	)		—		—
Reduction to unrecognized tax benefits		—			—		—
Balance at end of year	$	13,855		$	14,015	$	13,529

Summary of Significant Accounting Policies - Schedule of Property and Equipment Estimated Useful Life (Details)	Dec. 31, 2025
Laboratory Equipment [Member]
Property, Plant and Equipment [Line Items]
Property and Equipment Useful Lives (in years)	5 years
Computer Equipment, Furniture and Office Equipment
Property, Plant and Equipment [Line Items]
Property and Equipment Useful Lives (in years)	3 years
Leasehold Improvements [Member]
Property, Plant and Equipment [Line Items]
Property, Plant, and Equipment, Useful Life, Term, Description [Extensible Enumeration]	us-gaap:UsefulLifeTermOfLeaseMember

Nature of the Business and Basis of Presentation - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands				9 Months Ended	12 Months Ended
	Jul. 01, 2025	Jan. 15, 2025	Aug. 05, 2024	Sep. 30, 2024	Dec. 31, 2025	Dec. 31, 2024	Dec. 31, 2023
Nature Of Business And Basis Of Presentation [Line Items]
Entity incorporated month and year					2010-10
Entity incorporation, state or country code					DE
Payments to acquire business				$ 15,000	$ 15,000
Stock issued during period, shares, new Issues				714,285	714,285
Business combination percentage of all rent and other cost due					80.10%
Payment offset amount of employment obligation	$ 1,421
Share Price				$ 21	$ 21
Milestone Prepayment			$ 60,000		$ 60,000
Payment in cash for first sales milestone			150,000
Future milestone payment			125,000
Closing period milestone payment one			400,000
Closing period milestone payment two			150,000
Closing period milestone payment three			750,000
Accumulated deficit					(972,403)	$ (978,099)
Cash and cash equivalents					45,766	30,793
Net loss from continuing operations					(93,971)	(121,315)	$ (195,097)
Operating cash outflows					73,883
Installment payments					$ 75,000
Transition Services Agreement [Member]
Nature Of Business And Basis Of Presentation [Line Items]
Payments to acquire other productive assets	$ 25,000	$ 50,000
Nestle Health Science [Member]
Nature Of Business And Basis Of Presentation [Line Items]
Payments to acquire business			100,000
Satisfaction fee paid						$ 9,016
Adjusted account settlement amount			$ 17,857
Fixed interest rate			10.00%
Derivative fixed interest rate thereafter			5.00%

Segment Reporting (Tables)

12 Months Ended

Dec. 31, 2025

Segment Reporting [Abstract]

Schedule of reportable segment net income (loss)

The Company's reportable segment net income (loss) for the years ended December 31, 2025, 2024 and 2023, consisted of the following (in thousands):


	Year Ended December 31,
	2025			2024			2023
Grant revenue	$	789		$	—		$	—
Significant segment expenses:
Live biotherapeutics platform	$	23,573		$	29,006		$	43,342
SER-155		3,687			6,804			7,759
R&D personnel-related (including stock-based compensation)		21,553			28,689			65,251
G&A personnel-related (including stock-based compensation)		14,413			22,679			36,069
Professional fees		8,991			9,805			18,784
Facility-related and other		15,752			20,699			22,647
Gain on sale of VOWST Business		(80,685	)		(5,684	)		-
Other segment (income) expense (1)		(12,191	)		13,773			(3,722	)
Net income (loss) from continuing operations		5,696			(125,771	)		(190,130	)
Net income from discontinued operations, net of tax (2)		—			125,907			76,406
Net income (loss)	$	5,696		$	136		$	(113,724	)

[1] Other segment (income) expense includes manufacturing services expenses, research and development expenses on early stage programs, interest income and other (income) expense, net.

[2] See Note 3, Discontinued Operations and TSA, for further details.

Subsequent Events - Additional Information (Detail) - Subsequent Event [Member]	Feb. 27, 2026
Subsequent Event [Line Items]
Subsequent event, date	Feb. 27, 2026
Subsequent event, description	On February 27, 2026, the Board of Directors (the “Board”) of the Company appointed Richard N. Kender, a current member of the Board, to serve as Executive Chair of the Board and as Interim Chief Executive Officer of the Company, Matthew Henn, Ph.D., the Company’s Chief Scientific Officer, to the additional role of President, and Kelly Brady, M.S., the Company’s Senior Vice President, Clinical Development, to the role of Executive Vice President, Chief Operating Officer, each effective March 2, 2026 (the “Effective Date”).As a result of the appointment of Mr. Kender as Interim Chief Executive Officer and Dr. Henn as President, Thomas J. DesRosier and Marella Thorell ceased serving as Co-Chief Executive Officers and Co-Presidents of the Company as of the Effective Date. Mr. DesRosier will continue to serve as the Company’s Executive Vice President, Chief Legal Officer, and Ms. Thorell will continue to serve as the Company’s Executive Vice President, Chief Financial Officer.

Pay vs Performance Disclosure - USD ($) $ in Thousands	12 Months Ended
Pay vs Performance Disclosure - USD ($) $ in Thousands	Dec. 31, 2025	Dec. 31, 2024	Dec. 31, 2023
Pay vs Performance Disclosure
Net Income (Loss)	$ 5,696	$ 136	$ (113,724)

- Definition

The portion of profit or loss for the period, net of income taxes, which is attributable to the parent.

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+ Details

Leases (Tables)

12 Months Ended

Dec. 31, 2025

Leases [Abstract]

Summary of Operating Lease Assets and Liabilities

The following table summarizes the presentation in the Company’s consolidated balance sheets of its operating leases (in thousands):


	December 31,
	2025		2024
Assets:
Operating lease assets	$	72,483	$	80,903

Liabilities:
Operating lease liabilities	$	10,390	$	8,674
Operating lease liabilities, net of current portion		72,576		82,966
Total operating lease liabilities	$	82,966	$	91,640

Summary of Lease Costs

The following table summarizes the effect of lease costs in the Company’s consolidated statements of operations and comprehensive income (loss) (in thousands):


	Year Ended December 31,
	2025			2024			2023
Operating lease costs	$	19,175		$	19,514		$	22,324
Short-term lease costs		—			—			1,477
Variable lease costs		6,467			6,589			7,229
Sublease income		(3,272	)		(2,708	)		—
Total lease costs	$	22,370		$	23,395		$	31,030

Schedule of Future Payments of Operating Lease Liabilities

As of December 31, 2025, future payments of operating lease liabilities are as follows (in thousands):


	As of December 31, 2025
2026	$	19,983
2027		20,582
2028		20,863
2029		20,132
2030		11,545
2031 and thereafter		25,317
Total future payments of operating lease liabilities	$	118,422
Less: imputed interest		(35,456	)
Present value of operating lease liabilities	$	82,966

Discontinued Operations and TSA - Gain on the Sale of the VOWST Business (Details) - USD ($)
$ in Thousands

12 Months Ended

Dec. 31, 2025

Dec. 31, 2024

Dec. 31, 2023

Net assets transferred

Gain on sale, pre-tax

$ 0

$ 146,707

$ 0

VOWST Business [Member]

Consideration received

Upfront payment

^[1]

79,788

Prepaid milestone

60,000

Deferred revenue from termination of 2016 License Agreement

95,364

Settlement of net collaboration payable at close

27,465

Premium on equity financing

1,484

Deferred income from termination of 2021 License Agreement

3,606

Accrued Liabilities due to SPN - related party

(33,458)

Total fair value transferred for business

234,249

Net assets transferred

Inventory

63,442

Prepaid expenses and other current assets

2,219

Property and equipment, net

3,966

Operating lease assets

17,929

Other non-current assets

39,328

Accrued expenses and other current liabilities

(31,547)

Operating lease liabilities

(14,413)

Net assets transferred

80,924

Transaction costs

6,618

Gain on sale, pre-tax

146,707

Income tax

$ 0

Gain on sale, net of tax

$ 146,707

[1]

The upfront payment consists of $100,000, less $17,857 owed by the Company to an affiliate of SPN under the prior license agreement between the Company and the SPN affiliate, less approximately $2,355 in satisfaction of costs due under the Bacthera Manufacturing Agreement.

The following table presents the financial results of the discontinued operations:


	Year Ended December 31,
	2025		2024			2023
Revenue:
Collaboration revenue - related party	$	—	$	—		$	126,325
Total revenue		—		—			126,325
Operating expenses:
Research and development expenses		—		5,809			28,263
General and administrative expenses		—		4,066			10,244
Collaboration (profit) loss sharing - related party		—		(1,496	)		704
Total operating expenses		—		8,379			39,211
(Loss) income from discontinued operations		—		(8,379	)		87,114
Other income (expense):
Gain on sale of VOWST business		—		146,707			—
Interest expense		—		(12,192	)		(10,708	)
Other expense		—		(229	)		—
Income (loss) from discontinued operations, pre-tax	$	—	$	125,907		$	76,406
Income tax		—		—			—
Net income (loss) from discontinued operations, net of tax	$	—	$	125,907		$	76,406

Property and Equipment, Net - Schedule of Property and Equipment, Net (Detail) - USD ($) $ in Thousands	Dec. 31, 2025	Dec. 31, 2024
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 64,248	$ 64,730
Less: Accumulated depreciation and amortization	(56,613)	(53,196)
Property and equipment, net	7,635	11,534
Laboratory Equipment [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	23,852	24,468
Computer Equipment [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	4,058	3,924
Furniture and Office Equipment [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	4,523	4,523
Leasehold Improvements [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	30,954	30,954
Construction in Progress [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 861	$ 861

Discontinued Operations and TSA (Tables)

12 Months Ended

Dec. 31, 2025

Discontinued Operations and Disposal Groups [Abstract]

Gain on the Sale of the VOWST Business

The following table presents the gain on the sale of the VOWST Business as of December 31, 2024, pursuant to the Purchase Agreement:


	December 31,
	2024
Consideration received
Upfront payment (1)	$	79,788
Prepaid milestone		60,000
Deferred revenue from termination of 2016 License Agreement		95,364
Settlement of net collaboration payable at close		27,465
Premium on equity financing		1,484
Deferred income from termination of 2021 License Agreement		3,606
Accrued liabilities due to SPN - related party		(33,458	)
Total fair value transferred for business	$	234,249

Net assets transferred
Inventory	$	63,442
Prepaid expenses and other current assets		2,219
Property and equipment, net		3,966
Operating lease assets		17,929
Other non-current assets		39,328
Accrued expenses and other current liabilities		(31,547	)
Operating lease liabilities		(14,413	)
Net assets transferred	$	80,924

Transaction costs	$	6,618

Gain on sale, pre-tax	$	146,707
Income tax		—
Gain on sale, net of tax	$	146,707

The following table presents the financial results of the discontinued operations:


	Year Ended December 31,
	2025		2024			2023
Revenue:
Collaboration revenue - related party	$	—	$	—		$	126,325
Total revenue		—		—			126,325
Operating expenses:
Research and development expenses		—		5,809			28,263
General and administrative expenses		—		4,066			10,244
Collaboration (profit) loss sharing - related party		—		(1,496	)		704
Total operating expenses		—		8,379			39,211
(Loss) income from discontinued operations		—		(8,379	)		87,114
Other income (expense):
Gain on sale of VOWST business		—		146,707			—
Interest expense		—		(12,192	)		(10,708	)
Other expense		—		(229	)		—
Income (loss) from discontinued operations, pre-tax	$	—	$	125,907		$	76,406
Income tax		—		—			—
Net income (loss) from discontinued operations, net of tax	$	—	$	125,907		$	76,406

Summary of Accrued Liabilities of Continuing Operations

These contingent liabilities are presented as Accrued Liabilities due to SPN - related party from continuing operations on the consolidated balance sheets as of December 31, 2025 and December 31, 2024 and consist of the following (in thousands):


	December 31,		December 31,
	2025		2024
Profit Sharing Payments	$	1,701	$	11,230
Royalties associated with the MSK Agreement		1,309		2,786
VOWST post-marketing safety surveillance study		268		771
80.1% of lease cost of Waltham facility		—		1,501
Employment-related costs for conveying employees		—		1,462
Total accrued liabilities due to SPN - related party	$	3,278	$	17,750

- Definition

Tabular disclosure of information related to a disposal group. Includes, but is not limited to, a discontinued operation, disposal classified as held-for-sale or disposed of by means other than sale or disposal of an individually significant component.

+ References

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+ Details

Stock and Stock-Based Awards - Summary of Estimated Fair Value of Employee Stock Options (Details)	12 Months Ended
	Dec. 31, 2025	Dec. 31, 2024	Dec. 31, 2023
Employees Stock Option [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Risk-free interest rate	4.26%	2.70%	3.64%
Expected term (in years)	6 years	6 years	6 years
Expected volatility	121.00%	76.20%	107.20%
Expected dividend yield	0.00%	0.00%	0.00%
Employee Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Risk-free interest rate	4.34%	5.16%	5.01%
Expected term (in years)	6 months	6 months	6 months
Expected volatility	115.80%	118.30%	79.10%
Expected dividend yield	0.00%	0.00%	0.00%

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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-Subparagraph (c)(1)(iv)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480429/718-10-50-2

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Reference 19: http://www.xbrl.org/2003/role/disclosureRef
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Reference 22: http://www.xbrl.org/2003/role/disclosureRef
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Reference 23: http://www.xbrl.org/2003/role/disclosureRef
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Reference 24: http://www.xbrl.org/2003/role/disclosureRef
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Reference 25: http://www.xbrl.org/2003/role/disclosureRef
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Reference 26: http://www.xbrl.org/2003/role/disclosureRef
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Reference 27: http://www.xbrl.org/2003/role/disclosureRef
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Reference 28: http://www.xbrl.org/2003/role/disclosureRef
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Reference 29: http://www.xbrl.org/2003/role/disclosureRef
-Topic 718
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Reference 30: http://www.xbrl.org/2003/role/disclosureRef
-Topic 718
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-URI https://asc.fasb.org/1943274/2147480429/718-10-50-2

+ Details

Restructuring - Additional Information (Detail) $ in Thousands			12 Months Ended
	Sep. 23, 2025	Nov. 02, 2023 Employee	Dec. 31, 2025 USD ($)	Dec. 31, 2024 USD ($)	Dec. 31, 2023 USD ($)
Restructuring Cost And Reserve [Line Items]
Restructuring and related cost, number of positions eliminated, period percent	25.00%
Workforce Reduction Rate		41.00%
Number of positions eliminated \| Employee		160
Restructuring Charges			$ 1,027		$ 5,606
Restructuring Incurred Cost Statement of Income or Comprehensive Income Extensible Enumeration Not Disclosed Flag			true
Research and development expenses			$ 49,060	$ 64,600	117,597
General and administrative expenses			39,156	53,183	77,500
Accrued Expenses and Other Current Liabilities [Member]
Restructuring Cost And Reserve [Line Items]
Outstanding restructuring liabilities			$ 0	$ 20
Restructuring Charges [Member]
Restructuring Cost And Reserve [Line Items]
Research and development expenses					3,481
General and administrative expenses					2,125
Severance and other employee costs
Restructuring Cost And Reserve [Line Items]
Restructuring Charges					5,345
Acceleration of unvested equity awards
Restructuring Cost And Reserve [Line Items]
Restructuring Charges					$ 261

401(k) Savings Plan

12 Months Ended

Dec. 31, 2025

Retirement Benefits [Abstract]

401(k) Savings Plan

13. 401(k) Savings Plan

The Company has a defined contribution savings plan under Section 401(k) of the IRC. This plan covers substantially all employees who meet minimum age and service requirements and allows participants to defer a portion of their annual compensation on a pre-tax basis. Effective January 1, 2016, the Company elected to match 50% of the first 6% of an employee’s deferral. Company contributions are expensed in the year for which they are declared. During the years ended December 31, 2025, 2024, and 2023 the Company recorded expense of $592, $1,068, and $2,003, respectively, for 401(k) match contributions.

- Definition

The entire disclosure for retirement benefits.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
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-Name Accounting Standards Codification
-Section 50
-Paragraph 1
-Subparagraph (o)
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-URI https://asc.fasb.org/1943274/2147480506/715-20-50-1

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
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Reference 4: http://www.xbrl.org/2003/role/disclosureRef
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-Name Accounting Standards Codification
-Section 50
-Paragraph 3
-Subparagraph (c)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480266/715-60-50-3

Reference 5: http://www.xbrl.org/2003/role/disclosureRef
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-Name Accounting Standards Codification
-Section S99
-Paragraph 2
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480126/715-20-S99-2

Reference 6: http://www.xbrl.org/2003/role/disclosureRef
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-Name Accounting Standards Codification
-Section 50
-Paragraph 1
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480794/715-70-50-1

Reference 7: http://www.xbrl.org/2003/role/exampleRef
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-Name Accounting Standards Codification
-Section 55
-Paragraph 17
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480482/715-20-55-17

Reference 8: http://www.xbrl.org/2003/role/disclosureRef
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-Paragraph 1
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Reference 9: http://www.xbrl.org/2003/role/disclosureRef
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-URI https://asc.fasb.org/1943274/2147480506/715-20-50-1

Reference 10: http://www.xbrl.org/2003/role/disclosureRef
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-URI https://asc.fasb.org/1943274/2147480506/715-20-50-1

Reference 11: http://www.xbrl.org/2003/role/disclosureRef
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-URI https://asc.fasb.org/715/tableOfContent

Reference 12: http://www.xbrl.org/2003/role/disclosureRef
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-URI https://asc.fasb.org/1943274/2147480506/715-20-50-1

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-URI https://asc.fasb.org/1943274/2147480506/715-20-50-1

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-Paragraph 1
-Subparagraph (r)(2)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480506/715-20-50-1

+ Details

Summary of Significant Accounting Policies - Cash, Cash Equivalents and Restricted Cash (Details) - USD ($) $ in Thousands	Dec. 31, 2025	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Accounting Policies [Abstract]
Cash and cash equivalents	$ 45,766	$ 30,793
Restricted cash, non-current	8,668	8,668
Total cash, cash equivalents and restricted cash	$ 54,434	$ 39,461	$ 136,150	$ 171,215

Stock and Stock-Based Awards - Stock Based Compensation - Additional Information (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2025 USD ($)
Equity [Abstract]
Unrecognized stock-based compensation cost	$ 9,986
Weighted average period over which unrecognized compensation cost is expected to be recognized	3 years 2 months 12 days

Leases - Summary of Operating Lease Assets and Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2025	Dec. 31, 2024
Assets:
Operating lease assets	$ 72,483	$ 80,903
Liabilities:
Operating lease liabilities	10,390	8,674
Operating lease liabilities, net of current portion	72,576	82,966
Total operating lease liabilities	$ 82,966	$ 91,640

Leases

12 Months Ended

Dec. 31, 2025

Leases [Abstract]

Leases

6. Leases

The Company leases real estate, primarily laboratory, office and manufacturing space. The Company’s leases have remaining terms ranging from approximately three to seven years. Certain leases include one or more options to renew, exercisable at the Company’s sole discretion, with renewal terms that can extend the lease from approximately five years to ten years. The Company evaluated the renewal options in its leases to determine if it was reasonably certain that the renewal option would be exercised, given the Company’s current business structure, uncertainty of future growth, and the associated impact to real estate, the Company concluded that it is not reasonably certain that any renewal options would be exercised. Therefore, the operating lease assets and operating lease liabilities only contemplate the initial lease terms. All the Company’s leases qualify as operating leases.

In January 2024, the Company entered into a sublease agreement with an unrelated third party to sublease a portion of its office and laboratory space in Cambridge, Massachusetts. The term of the sublease agreement commenced in March 2024 and ends on January 13, 2030. The Company will receive lease payments over the sublease term totaling $10,400. The sublessee is obligated to pay all real estate taxes and costs related to the subleased premises, including cost of operations, maintenance, repair, replacement and property management. Sublease income is recorded as Other (expense) income, net in the Company’s consolidated statements of operations and comprehensive income (loss).

During the year ended December 31, 2024, the Company identified an indicator of impairment of its donor collection facility in Cambridge, Massachusetts, as the facility is no longer being used by the Company and is being marketed for sublease. The Company determined that this represents a significant adverse change in the extent in which the long-lived asset was being used. The Company determined that the location contains multiple asset groups for the purpose of the long-lived asset impairment assessment. The Company concluded that the carrying value of each asset group was not recoverable as it exceeded the future net undiscounted cash flows that are expected to be generated from the assets within the asset group. In the first quarter of 2024, the Company recognized an impairment loss of $3,267, consisting of $1,731 on the operating lease right-of-use asset and $1,536 on the leasehold improvements. $2,727 of the total impairment loss is included in research and development expenses and the remaining $540 is included in general and administrative expenses in the accompanying consolidated statements of operations and comprehensive income (loss).

The following table summarizes the presentation in the Company’s consolidated balance sheets of its operating leases (in thousands):


	December 31,
	2025		2024
Assets:
Operating lease assets	$	72,483	$	80,903

Liabilities:
Operating lease liabilities	$	10,390	$	8,674
Operating lease liabilities, net of current portion		72,576		82,966
Total operating lease liabilities	$	82,966	$	91,640

The following table summarizes the effect of lease costs in the Company’s consolidated statements of operations and comprehensive income (loss) (in thousands):


	Year Ended December 31,
	2025			2024			2023
Operating lease costs	$	19,175		$	19,514		$	22,324
Short-term lease costs		—			—			1,477
Variable lease costs		6,467			6,589			7,229
Sublease income		(3,272	)		(2,708	)		—
Total lease costs	$	22,370		$	23,395		$	31,030

During the years ended December 31, 2025, 2024, and 2023, the Company made cash payments for operating leases of $19,442, $17,330 and $15,656, respectively.

As of December 31, 2025, future payments of operating lease liabilities are as follows (in thousands):


	As of December 31, 2025
2026	$	19,983
2027		20,582
2028		20,863
2029		20,132
2030		11,545
2031 and thereafter		25,317
Total future payments of operating lease liabilities	$	118,422
Less: imputed interest		(35,456	)
Present value of operating lease liabilities	$	82,966

As of December 31, 2025, the weighted average remaining lease term was 6.07 years and the weighted average incremental borrowing rate used to determine the operating lease liability was 13%. As of December 31, 2024, the weighted average remaining lease term was 6.98 years and the weighted average incremental borrowing rate used to determine the operating lease liability was 13%.

Discontinued Operations and TSA - Additional Information (Details) - USD ($)

1 Months Ended

12 Months Ended

Jul. 31, 2025

Jan. 31, 2025

Dec. 31, 2025

Dec. 31, 2024

Dec. 31, 2023

Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]

Assets of discontinued operations

$ 0

Liabilities of discontinued operations

Other (expense) income, net

^[1]

16,755,000

(14,107,000)

$ 134,000

Manufacturing services

6,544,000

3,532,000

Gain on sale, pre-tax

146,707,000

Gain on sale of VOWST Business

80,685,000

5,684,000

Installment payment

$ 25,000,000

$ 50,000,000

Accounts receivable due from SPN - related party

360,000

2,068,000

Share based compensation expenses

10,840,000

20,971,000

34,101,000

Transition Services Agreement [Member]

Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]

Other (expense) income, net

13,311,000

6,292,000

Transition Services Agreement [Member] | Passthrough Expenses [Member]

Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]

Other (expense) income, net

3,592,000

3,136,000

Transition Services Agreement [Member] | NESA [Member]

Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]

Billed Contracts Receivable

14,897,000

3,724,000

Related party partial payments

91,605,000

1,656,000

VOWST Business [Member]

Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]

Gain on sale, pre-tax

146,707,000

Gain on sale of VOWST Business

75,000,000

Capital expenditures

112,000

2,272,000

Depreciation

989,000

2,019,000

Non-cash operating lease costs

1,447,000

2,123,000

Share based compensation expenses

1,884,000

2,773,000

Collaboration loss sharing

$ 5,158,000

VOWST Business [Member] | Transition Services Agreement [Member]

Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]

Gain on sale, pre-tax

$ 5,685,000

$ 5,684,000

[1]	[2] Includes $13,311 and $6,292 for the years ended December 31, 2025 and 2024 related to reimbursement received from SPN (related party) for transition services provided by the Company

Stock and Stock-Based Awards - Summary of Restricted Stock Unit Activity (Details) - RSU and PSU [Member]	12 Months Ended
	Dec. 31, 2025 $ / shares shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of Shares, Unvested restricted stock units, Beginning balance \| shares	81,459
Number of Shares, Granted \| shares	79,846
Number of Shares, Forfeited \| shares	(12,300)
Number of Shares, Vested \| shares	(124,855)
Number of Shares, Unvested restricted stock units, Ending balance \| shares	24,150
Weighted Average Grant Date Fair Value, Unvested restricted stock units, Beginning balance \| $ / shares	$ 55.36
Weighted Average Grant Date Fair Value, Granted \| $ / shares	18.29
Weighted Average Grant Date Fair Value, Forfeited \| $ / shares	62.76
Weighted Average Grant Date Fair Value, Vested \| $ / shares	30.54
Weighted Average Grant Date Fair Value, Unvested restricted stock units, Ending balance \| $ / shares	$ 57.31

Stock and Stock-Based Awards - Additional Information (Details) $ / shares in Units, $ in Thousands			9 Months Ended	12 Months Ended
	Apr. 10, 2025	May 21, 2021 USD ($)	Sep. 30, 2024 USD ($) $ / shares shares	Dec. 31, 2025 USD ($) $ / shares shares	Dec. 31, 2024 USD ($) $ / shares shares	Feb. 22, 2024 shares	Feb. 21, 2024 shares	Jul. 01, 2015 $ / shares shares
Subsidiary, Sale of Stock [Line Items]
Preferred stock, shares authorized \| shares				10,000,000	10,000,000
Preferred stock, par value \| $ / shares				$ 0.001	$ 0.001
Stockholders equity note stock split, conversion ratio	20
Common stock, shares issued \| shares			714,285	714,285
Share Price \| $ / shares			$ 21	$ 21
Payments to acquire business			$ 15,000	$ 15,000
Common stock, shares authorized \| shares				360,000,000	360,000,000	360,000,000	240,000,000
Aggregate fair value of common stock issued			13,516		$ 13,516
VOWST Business [Member]
Subsidiary, Sale of Stock [Line Items]
Payments to acquire business				$ 100,000
Proceeds from issuance of common stock			$ 1,484
Initial Public Offering ("IPO") [Member]
Subsidiary, Sale of Stock [Line Items]
Preferred stock, shares authorized \| shares								10,000,000
Preferred stock, par value \| $ / shares								$ 0.001
At The Market Equity Offering Program [Member] \| 2021 Sales Agreement [Member] \| Cowen And Company, LLC [Member]
Subsidiary, Sale of Stock [Line Items]
Gross proceeds from sale of common stock		$ 150,000,000
Common stock, shares issued \| shares				739,545	1,030,846
Common stock, average share price \| $ / shares				$ 18.48	$ 23.67
Proceeds from issuance of common stock				$ 13,167	$ 23,530
Percentage of commission on sale of common stock				3	3

Stock and Stock-Based Awards - Summary of Stock Option Activity (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2025	Dec. 31, 2024
Equity [Abstract]
Number of Shares, Beginning Balance	886,123
Number of Shares, Granted	546,805
Number of Shares, Exercised	(23,900)
Number of Shares, Forfeited	(284,820)
Number of Shares, Ending Balance	1,124,208	886,123
Number of Shares, Vested and expected to vest	1,124,208
Number of Shares, Options exercisable	502,733
Weighted Average Exercise Price, Beginning Balance	$ 114.47
Weighted Average Exercise Price, Granted	15.14
Weighted Average Exercise Price, Exercised	20.96
Weighted Average Exercise Price, Forfeited	114.77
Weighted Average Exercise Price, Ending Balance	68.07	$ 114.47
Weighted Average Exercise Price, Vested or expected to vest	68.07
Weighted Average Exercise Price, Options exercisable	$ 129.06
Weighted Average Remaining Contractual Term, Outstanding	7 years 2 months 1 day	6 years 7 months 2 days
Weighted Average Remaining Contractual Term	7 years 2 months 1 day
Weighted Average Remaining Contractual Term, Options exercisable	5 years 1 month 24 days
Aggregate Intrinsic Value, Outstanding	$ 228	$ 87
Aggregate Intrinsic Value, Vested or expected to vest	228
Aggregate Intrinsic Value, Options exercisable	$ 31

Leases - Schedule of Future Payments of Operating Lease Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2025	Dec. 31, 2024
Leases [Abstract]
2026	$ 19,983
2027	20,582
2028	20,863
2029	20,132
2030	11,545
2031 and thereafter	25,317
Total future payments of operating lease liabilities	118,422
Less: imputed interest	(35,456)
Present value of operating lease liabilities	$ 82,966	$ 91,640

Accrued Expenses and Other Current Liabilities

12 Months Ended

Dec. 31, 2025

Payables and Accruals [Abstract]

Accrued Expenses and Other Current Liabilities

5. Accrued Expenses and Other Current Liabilities

Accrued expenses and other current liabilities consisted of the following:


	December 31,
	2025		2024
Clinical and development costs	$	246	$	422
Manufacturing and quality costs		145		478
Payroll and employee-related costs		2,685		7,656
Facility and other		896		2,163
	$	3,972	$	10,719

Document and Entity Information - USD ($)	12 Months Ended
Document and Entity Information - USD ($)	Dec. 31, 2025	Mar. 06, 2026	Jun. 30, 2025
Document Information [Line Items]
Document Type	10-K
Amendment Flag	false
Document Period End Date	Dec. 31, 2025
Document Fiscal Year Focus	2025
Document Fiscal Period Focus	FY
Trading Symbol	MCRB
Entity Registrant Name	Seres Therapeutics, Inc.
Entity Central Index Key	0001609809
Entity Current Reporting Status	Yes
Current Fiscal Year End Date	--12-31
Entity Filer Category	Accelerated Filer
Entity Small Business	true
Entity Emerging Growth Company	false
ICFR Auditor Attestation Flag	false
Document Financial Statement Error Correction [Flag]	false
Entity Shell Company	false
Entity File Number	001-37465
Entity Tax Identification Number	27-4326290
Entity Address, Address Line One	101 Cambridgepark Drive
Entity Address, City or Town	Cambridge
Entity Address, State or Province	MA
Entity Address, Postal Zip Code	02140
City Area Code	617
Local Phone Number	945-9626
Entity Public Float			$ 71,941,772
Entity Common Stock Shares Outstanding		9,586,298
Entity Incorporation, State or Country Code	DE
Document Annual Report	true
Document Transition Report	false
Entity Well Known Seasoned Issuer	No
Entity Voluntary Filers	No
Entity Interactive Data Current	Yes
Title of 12(b) Security	Common stock, par value $0.001 per share
Security Exchange Name	NASDAQ
Documents Incorporated by Reference	Portions of the registrant’s definitive Proxy Statement relating to its 2026 Annual Meeting of Stockholders to be filed with the SEC within 120 days after the end of the fiscal year ended December 31, 2025 are incorporated herein by reference in Part III.
Auditor Firm ID	238
Auditor Name	PricewaterhouseCoopers LLP
Auditor Location	Boston, Massachusetts
Auditor Opinion [Text Block]	Opinions on the Financial Statements We have audited the accompanying consolidated balance sheets of Seres Therapeutics, Inc. and its subsidiaries (the “Company”) as of December 31, 2025 and 2024, and the related consolidated statements of operations and comprehensive income (loss), of stockholders' equity (deficit) and of cash flows for each of the three years in the period ended December 31, 2025, including the related notes (collectively referred to as the “consolidated financial statements”). In our opinion, the consolidated financial statements present fairly, in all material respects, the financial position of the Company as of December 31, 2025 and 2024, and the results of its operations and its cash flows for each of the three years in the period ended December 31, 2025 in conformity with accounting principles generally accepted in the United States of America.

Leases - Additional Information (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2024	Dec. 31, 2025	Dec. 31, 2024	Dec. 31, 2023
Operating Leased Assets [Line Items]
Existence of option to extend		true
Option to extend, description		Certain leases include one or more options to renew, exercisable at the Company’s sole discretion, with renewal terms that can extend the lease from approximately five years to ten years.
Impairment loss of leasehold improvements			$ 1,536
Lease payment over sublease term		$ 10,400
Leases Payments		$ 19,442	$ 17,330	$ 15,656
Weighted average remaining lease term		6 years 25 days	6 years 11 months 23 days
Weighted average incremental borrowing rate		13.00%	13.00%
Cambridge Massachusetts [Member]
Operating Leased Assets [Line Items]
Impairment loss of leasehold improvements	$ 1,536
Impairment loss	3,267
Impairment loss	1,731
General and Administrative Expenses [Member] \| Cambridge Massachusetts [Member]
Operating Leased Assets [Line Items]
Impairment loss	540
Research and Development Expenses [Member] \| Cambridge Massachusetts [Member]
Operating Leased Assets [Line Items]
Impairment loss	$ 2,727
Maximum [Member]
Operating Leased Assets [Line Items]
Operating lease, remaining term		7 years
Lease renewal term (in years)		10 years
Minimum [Member]
Operating Leased Assets [Line Items]
Operating lease, remaining term		3 years
Lease renewal term (in years)		5 years

Net Income (Loss) per Share

12 Months Ended

Dec. 31, 2025

Earnings Per Share [Abstract]

Net Income (Loss) per Share

8. Net Income (Loss) per Share

Basic and diluted net income (loss) per share attributable to common stockholders was calculated as follows (in thousands, except share and per share data):


	Year Ended December 31,
	2025		2024			2023
Basic Earnings Per Share:
Numerator:
Net income (loss) from continuing operations attributable to common stockholders	$	5,696	$	(125,771	)	$	(190,130	)
Net income from discontinuing operations attributable to common stockholders	$	—	$	125,907		$	76,406
Net income (loss) attributable to common stockholders	$	5,696	$	136		$	(113,724	)

Denominator:
Weighted average common shares outstanding - basic		8,858,975		7,769,910			6,400,339
Net income (loss) from continuing operations per share attributable to common stockholders - basic	$	0.64	$	(16.20	)	$	(29.71	)
Net income from discontinued operations per share attributable to common stockholders - basic	$	—	$	16.20		$	11.94
Net income (loss) per share attributable to common stockholders - basic	$	0.64	$	0.00		$	(17.77	)

Diluted Earnings Per Share:
Numerator:
Net income (loss) from continuing operations attributable to common stockholders	$	5,696	$	(125,771	)	$	(190,130	)
Net income from discontinuing operations attributable to common stockholders	$	—	$	125,907		$	76,406
Net income (loss) attributable to common stockholders	$	5,696	$	136		$	(113,724	)

Denominator:
Weighted average common shares outstanding - basic		8,858,975		7,769,910			6,400,339

Dilutive impact from:
Stock options to purchase common stock		—		—			—
Unvested restricted stock units		10,749		—			—
Shares issuable under employee stock purchase plan		18		—			—
Weighted average common shares outstanding - diluted		8,869,742		7,769,910			6,400,339
Net income (loss) from continuing operations per share attributable to common stockholders - diluted	$	0.64	$	(16.20	)	$	(29.71	)
Net income from discontinued operations per share attributable to common stockholders - diluted	$	—	$	16.20		$	11.94
Net income (loss) per share attributable to common stockholders - diluted	$	0.64	$	—		$	(17.77	)

Anti-dilutive potential common stock equivalents excluded from the calculation of net income (loss) per share:
Stock options to purchase common stock		1,124,208		886,123			742,335
Unvested restricted stock units		24,150		81,459			168,730
Shares issuable under employee stock purchase plan		2,047		7,160			14,889
Warrants to purchase common stock		32,379		32,379			58,871

The Company's potential dilutive securities include stock options, unvested restricted common stock and shares issuable under the 2015 Employee Stock Purchase Plan. The effect of dilutive securities was calculated using the treasury stock method. The anti-dilutive potential common stock equivalents for the year ended December 31, 2025 were excluded from the computation of diluted net income per share attributable to common stockholders because those stock options to purchase common stock and restricted stock units had an anti-dilutive impact due to the assumed proceeds per share using the treasury stock method being greater than the average fair value of the Company’s common shares for those periods. For that same period, the warrants to purchase common stock were excluded because the exercise price of the Tranche A Warrants is greater than the average fair value of the Company’s common shares. The anti-dilutive potential common stock equivalents for the years ended December 31, 2024 and 2023 were excluded from the computation of diluted net loss per share attributable to common stockholders because those stock options to purchase common stock, restricted stock units, and shares issuable under employee stock purchase plan had an anti-dilutive impact as the Company reported a net loss from continuing operations attributable to common stockholders for those periods

The Company utilizes the control number concept in the computation of diluted earnings per share to determine whether potential common stock equivalents are dilutive. The control number used is income (loss) from continuing operations. The control number concept requires that the same number of potentially dilutive securities applied in computing diluted earnings per share from continuing operations be applied to all other categories of income or loss, regardless of their anti-dilutive effect on such categories. Since the Company had a net loss from continuing operations for all periods in which discontinued operations is presented, no dilutive effect has been recognized in the calculation of income from discontinued operations per share. Therefore, the weighted average number of common shares outstanding used to calculate both basic and diluted net loss per share attributable to common stockholders is the same for the years ended December 31, 2024 and 2023.

Leases - Summary of Lease Costs (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2025	Dec. 31, 2024	Dec. 31, 2023
Leases [Abstract]
Operating lease costs	$ 19,175	$ 19,514	$ 22,324
Short-term lease costs	0	0	1,477
Variable lease costs	6,467	6,589	7,229
Sublease income	(3,272)	(2,708)	0
Total lease costs	$ 22,370	$ 23,395	$ 31,030

Related Party Transactions

12 Months Ended

Dec. 31, 2025

Related Party Transactions

12. Related Party Transactions

As described in Note 1, Nature of the Business and Basis of Presentation and Note 3, Discontinued Operations, in September 2024, the Company sold the VOWST Business, including inventory and equipment, certain patents and patent applications, know-how, trade secrets, trademarks, domain names, marketing authorizations and related rights, documents, materials, business records and data and contracts that are used or held for use primarily in the development, commercialization and manufacturing of VOWST, to SPN, and SPN assumed certain liabilities from the Company. As consideration for the Transaction, the Company received an upfront cash payment of $139,788, which consists of $100,000, less $17,857 owed by the Company to an affiliate of SPN under the prior license agreement between the Company and the SPN affiliate, less approximately $2,355 in satisfaction of fees due under the Bacthera Manufacturing Agreement; plus a prepayment of the $60,000 milestone payment tied to the achievement of worldwide annual net sales of VOWST of $150,000; plus an equity investment of $15,000 based on the Securities Purchase Agreement pursuant to which SPN purchased 714,285 shares of common stock at a purchase price of $21.00 per share.

For the years ended December 31, 2025 and 2024, the Accrued Liabilities due to SPN - related party on the Company's consolidated balance sheets were $3,278 and $17,750, respectively, which represents amounts due to SPN pursuant to the Purchase Agreement, which are further described in Note 3, Discontinued Operations and TSA. During the years ended December 31, 2025 and 2024, the Company paid $8,787 and $9,608, respectively, related to the outstanding liability. The Company also recognized a gain on sale of VOWST Business of $5,685 and $5,684, for the years ended December 31, 2025 and 2024, respectively, as a result of the change in the accrued liabilities due to SPN - related party, primarily as a result of SPN's actual Profit Sharing Payments as compared to the estimate as of the Closing.

As described in Note 3, Discontinued Operations and TSA, the Company entered into the TSA with NESA, an affiliate of SPN, in connection with the Transaction, through which the Company provided certain manufacturing services until December 31, 2025, and other transition services, for the duration specified in the schedule to the TSA for each service. For the years ended December 31, 2025 and 2024, the Company recognized $13,311 and $6,292 of TSA reimbursement income in other (expense) income, net in the Company’s consolidated statements of operations and comprehensive income (loss). For the years ended December 31, 2025 and 2024, the Company incurred $6,544 and $3,532 of expenses related to manufacturing services and $3,592 and $3,136 of TSA labor and passthrough expenses to support the transition services, including finance and accounting, information technology, human resources, operations, and other services.

conditioned on the Company's material compliance with obligations under the TSA. The $75,000 of installment payments were recognized in Gain on sale of VOWST Business within continuing operations in the Company's consolidated statements of operation and comprehensive income for the year ended December 31, 2025 as the gain was realizable. As of December 31, 2025, the Company had $360 in accounts receivable due from SPN - related party in the Company’s consolidated balance sheets.

For the year ended December 31, 2025, Nestlé purchase certain manufacturing and laboratory equipment from the Company. The Company received $169 in proceeds from the sale, which is recognized in Other income (expense), net in the Company's consolidated statements of operations and comprehensive income (loss).

- Definition

The entire disclosure for related party transactions. Examples of related party transactions include transactions between (a) a parent company and its subsidiary; (b) subsidiaries of a common parent; (c) and entity and its principal owners; and (d) affiliates.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 850
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 1
-Subparagraph (d)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147483326/850-10-50-1

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 946
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480990/946-20-50-2

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Topic 946
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 5
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480990/946-20-50-5

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Topic 946
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 6
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480990/946-20-50-6

Reference 5: http://www.xbrl.org/2003/role/disclosureRef
-Topic 946
-SubTopic 235
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-Subparagraph (c)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147477968/946-235-50-2

Reference 6: http://www.xbrl.org/2003/role/disclosureRef
-Topic 946
-SubTopic 235
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-Subparagraph (e)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147477968/946-235-50-2

Reference 7: http://www.xbrl.org/2003/role/disclosureRef
-Topic 946
-SubTopic 220
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.6-07(2)(g)(3))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479134/946-220-S99-1

Reference 8: http://www.xbrl.org/2003/role/disclosureRef
-Topic 946
-SubTopic 220
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.6-07(2)(c))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479134/946-220-S99-1

Reference 9: http://www.xbrl.org/2003/role/disclosureRef
-Topic 946
-SubTopic 220
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.6-07(2)(e))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479134/946-220-S99-1

Reference 10: http://www.xbrl.org/2003/role/disclosureRef
-Topic 850
-Name Accounting Standards Codification
-Publisher FASB
-URI https://asc.fasb.org/850/tableOfContent

Reference 11: http://www.xbrl.org/2003/role/disclosureRef
-Topic 850
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 6
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147483326/850-10-50-6

Reference 12: http://www.xbrl.org/2003/role/disclosureRef
-Topic 850
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 1
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147483326/850-10-50-1

Reference 13: http://www.xbrl.org/2003/role/disclosureRef
-Topic 850
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 1
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147483326/850-10-50-1

+ Details

Discontinued Operations and TSA - Summary of Financial results of the discontinued operations (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2025	Dec. 31, 2024	Dec. 31, 2023
Revenues [Abstract]
Total revenue	$ 789	$ 0	$ 0
Operating Expenses [Abstract]
Research and development expenses	49,060	64,600	117,597
General and administrative expenses	39,156	53,183	77,500
Total operating expenses	94,760	121,315	195,097
Other Nonoperating Income (Expense) [Abstract]
Gain on sale of VOWST business	0	146,707	0
Interest expense	0	0	(2,468)
Net income (loss) from discontinued operations, net of tax	0	125,907	76,406
VOWST Business [Member]
Revenues [Abstract]
Collaboration revenue - related party	0	0	126,325
Total revenue	0		126,325
Operating Expenses [Abstract]
Research and development expenses	0	5,809	28,263
General and administrative expenses	0	4,066	10,244
Collaboration (profit) loss sharing - related party	0	(1,496)	704
Total operating expenses	0	8,379	39,211
(Loss) income from discontinued operations	0	(8,379)	87,114
Other Nonoperating Income (Expense) [Abstract]
Gain on sale of VOWST business	0	146,707	0
Interest expense	0	(12,192)	(10,708)
Other expense	0	(229)	0
Income (loss) from discontinued operations, pre-tax	0	125,907	76,406
Income tax	0	0	0
Net income (loss) from discontinued operations, net of tax	$ 0	$ 125,907	$ 76,406

Discontinued Operations and TSA - Summary of Accrued Liabilities from Continuing Operations (Details) - USD ($) $ in Thousands	Dec. 31, 2025	Dec. 31, 2024
Discontinued Operations and Disposal Groups [Abstract]
Profit Sharing Payments	$ 1,701	$ 11,230
Royalties associated with the MSK Agreement	1,309	2,786
VOWST post-marketing safety surveillance study	268	771
80.1% of lease cost of Waltham facility	0	1,501
Employment-related costs for conveying employees	0	1,462
Total accrued liabilities due to SPN	$ 3,278	$ 17,750

Restructuring

12 Months Ended

Dec. 31, 2025

Restructuring and Related Activities [Abstract]

Restructuring

. Restructuring

On September 23, 2025, the Company announced that it implemented cost reduction actions, including decreasing its workforce by approximately 25%. During the year ended December 31, 2025, the Company incurred approximately $1,027 in restructuring costs, primarily related to severance costs. These costs were paid out in the fourth quarter of 2025.

On November 2, 2023, the Company announced a restructuring plan to prioritize the commercialization of VOWST and the completion of the SER-155 Phase 1b study, while significantly reducing costs and supporting longer-term business sustainability. The restructuring plan included (i) a reduction of the Company’s workforce by approximately 41% across the organization, resulting in the elimination of approximately 160 positions; (ii) significantly scaling back all non-partnered research and development activities other than the completion of the SER-155 Phase 1b study; and (iii) reducing general and administrative expenses, including consolidating office space.

During the year ended December 31, 2023, the Company recognized a restructuring charge of $5,606, which was incurred entirely in the fourth quarter of 2023, and which represents all restructuring charges expected to be incurred. $3,481 of the total restructuring charges was included in research and development expenses and the remaining $2,125 in general and administrative expenses in the accompanying consolidated statements of operations and comprehensive income (loss) for the year ended December 31, 2023. Restructuring charges included approximately $5,345 of employee related termination costs in the form of salary continuation and cash severance payments, and $261 related to the acceleration of vesting of certain previously granted RSUs and PSUs.

The unpaid restructuring charge included in accrued expenses and other current liabilities in the Company’s consolidated balance sheets was $0 and $20 for the years ended December 31, 2025 and 2024, respectively.

Accrued Expenses and Other Current Liabilities (Tables)

12 Months Ended

Dec. 31, 2025

Payables and Accruals [Abstract]

Summary of Accrued Expenses and Other Current Liabilities

Accrued expenses and other current liabilities consisted of the following:


	December 31,
	2025		2024
Clinical and development costs	$	246	$	422
Manufacturing and quality costs		145		478
Payroll and employee-related costs		2,685		7,656
Facility and other		896		2,163
	$	3,972	$	10,719

Nature of the Business and Basis of Presentation

12 Months Ended

Dec. 31, 2025

Accounting Policies [Abstract]

Nature of the Business and Basis of Presentation

Seres Therapeutics, Inc. (the “Company”) was incorporated under the laws of the State of Delaware in October 2010 under the name Newco LS21, Inc. In October 2011, the Company changed its name to Seres Health, Inc., and in May 2015, the Company changed its name to Seres Therapeutics, Inc. The Company is a clinical-stage company focused on improving patient outcomes in medically vulnerable populations through discovery and development of novel live biotherapeutics. The Company led the successful development and approval of VOWST (previously referred to as SER-109), the first FDA-approved orally administered microbiome therapeutic and a Breakthrough Therapy designated drug, which was sold to Nestlé Health Science (as defined below) in September 2024. The Company's live biotherapeutic product (“LBP”) candidates, including SER-155, SER-603, SER-147, and other potential candidates in earlier development, are consortia of bacteria designed to optimize specific, targeted pharmacological properties, and are formulated for oral delivery. The Company is designing LBP candidates to target the prevention and treatment of a broad swath of infections, and to treat inflammatory and immune (“I&I”) diseases by modulating host function to increase epithelium integrity and to induce immune homeostasis and tolerance, as well as to prevent the colonization and overgrowth of pathogens in the gastrointestinal (“GI”) tract.

SER-155, the Company's most advanced LBP candidate, is an investigational, oral, live biotherapeutic designed to decolonize GI pathogens, improve GI epithelial barrier integrity, and induce immune homeostasis to prevent bacterial bloodstream infections (“BSIs”), including those that can harbor antimicrobial resistance (“AMR”), as well as other pathogen-associated negative clinical outcomes in patients undergoing allogeneic hematopoietic stem cell transplantation (“allo-HSCT”). In the Company's placebo-controlled Phase 1b study of SER-155 in allo-HSCT, SER-155 was associated with a 77% relative risk reduction in bacterial BSIs and a significant reduction in systemic antibiotic exposure as well as a lower incidence of febrile neutropenia, as compared to placebo through day 100 post HSCT. SER-155 was generally well tolerated, with no observed treatment-related serious adverse events. Following advancement of key startup activities for the SER-155 Phase 2 study in allo-HSCT, including the submission of a final protocol to the FDA in January 2026, study site evaluation and qualification with its CRO, and manufacturing of drug substance, the Company has paused additional investment in that program while continuing to seek funding for the Phase 2 study.

The clinical data from the SER-155 Phase 1b study in allo-HSCT, along with the Company's extensive preclinical and translational clinical data compiled over the past decade support and inform the advancement of the Company's earlier stage programs targeting I&I diseases. The Company is evaluating SER-155 in immune checkpoint-related enterocolitis, or irEC, and the Company is developing SER-603, broadly in inflammatory bowel disease, or IBD, including ulcerative colitis, or UC, and Crohn's disease. The Company is currently exploring potential collaborations related to those I&I disease programs.

The Company has built and deploys a reverse translational platform and knowledge base for the discovery and development of live biotherapeutics, and maintains extensive proprietary know-how that may be used to support future research and development efforts. This platform incorporates high-resolution analysis of human clinical data to identify microbiome biomarkers associated with disease and non-disease states; preclinical screening using human cell-based assays and in vitro/ex vivo and in vivo disease models customized for live biotherapeutics; and a strain library and associated microbiological capabilities that spans broad biological and functional breadth. This platform and knowledge base are integrated through a proprietary knowledge graph and agentic artificial intelligence, enabling rapid identification of specific microbes, microbial genes, and microbial metabolites/peptides associated with disease and the design of therapeutic consortia of bacteria for specific pharmacological properties to restructure the gut microbiome and modulate functional pathways associated with disease. In addition, the Company owns a valuable intellectual property estate related to the development and manufacture of live biotherapeutics.

At a special meeting of stockholders held on September 26, 2024, the Company’s stockholders approved, and on September 30, 2024 (the "Closing Date"), the Company completed, the sale (the “Transaction”) of its VOWST microbiome therapeutic business (the “VOWST Business”), including inventory and equipment, certain patents and patent applications, know-how, trade secrets, trademarks, domain names, marketing authorizations and related rights, documents, materials, business records and data and contracts that are used or held for use primarily in the development, commercialization and manufacturing of the microbiome product sold under the brand name VOWST as provided for in accordance with the terms of the Purchase Agreement (the “Product”), to Société des Produits Nestlé S.A. (“SPN”), a wholly-owned subsidiary of Nestlé S.A., and its designated affiliates (collectively, “Nestlé Health Science”) pursuant to the Asset Purchase Agreement, dated as of August 5, 2024 (the “Purchase Agreement”), by and among the Company and SPN, and a wholly-owned subsidiary of Nestlé S.A. As consideration for the Transaction, SPN paid or agreed to pay, as applicable, the following Transaction Consideration:

(i)

a cash payment, which was paid upon completion of the Transaction (“Closing”), of $100,000, less approximately $17,857 owed by the Company to an affiliate of SPN as of March 31, 2024 under the prior license agreement between the Company and the SPN affiliate, less approximately CHF 2,000 in satisfaction of fees due under the Bacthera Manufacturing Agreement (defined below);

(ii)

cash installment payments of $50,000, which was received on January 15, 2025 and $25,000, which was received on July 1, 2025 (offset by $1,421 paid by the Company to Nestlé on July 1, 2025 related to certain employment obligations assumed by SPN, as described below), conditioned on the Company’s material compliance with obligations under the Transition

Services Agreement (the “TSA”) (as described below) entered into at Closing between the Company and Nestlé Enterprises S.A., an affiliate of SPN, or NESA;

(iii)

prepayment of the $60,000 milestone payment tied to the achievement of worldwide annual net sales of the Product of $150,000 (the “First Sales Milestone”), which was paid in cash at Closing (the “Prepaid Milestone”), which Prepaid Milestone will accrue interest at a fixed rate of 10% per annum until the First Sales Milestone is achieved and 5% per annum thereafter until the earlier of (x) the date on which the Prepaid Milestone, plus accrued interest thereon, has been repaid in full by set-off and (y) the last day of the Milestone Period (as defined below); and

(iv)

future milestone payments of (x) $125,000 tied to the achievement of worldwide annual net sales of the Product of $400,000 and (y) $150,000 tied to the achievement of worldwide annual net sales of the Product of $750,000, during the period from Closing until December 31 of the calendar year in which the tenth anniversary of Closing occurs (the “Milestone Period”) (together, the “Future Milestone Payments” and, together with the Prepaid Milestone, the “Milestone Payments”).

As they are earned, the Milestone Payments will be satisfied as follows: (i) first, by set-off against all accrued interest on the Prepaid Milestone until the amount of such accrued interest has been paid in full, (ii) second, by set-off against the outstanding balance of the Prepaid Milestone until the Prepaid Milestone has been repaid in full and (iii) thereafter, in cash. If any amount of the Prepaid Milestone (and any accrued interest thereon) remains outstanding as of following the last day of the Milestone Period (defined below), the balance thereof (together with any interest accrued thereon) will be forgiven and the right of set-off of SPN with respect thereto will be deemed forfeited. The installment payment received on July 1, 2025 was offset by $1,421 paid by the Company to Nestlé on the same date related to certain employment obligations assumed by SPN with respect to the period ended as of the Closing Date.

The Company and SPN shared 50/50 in the net profit or net loss (the “Profit Sharing Payments”) achieved during the period from the date of Closing until December 31, 2025 (the “Profit Sharing Period”), with the net profit or net loss calculated as (i) the net sales of VOWST in the United States and Canada, plus (ii) other income received in connection with the grant of a license or sublicense with respect to VOWST in the United States and Canada as described in the Purchase Agreement, minus (iii) allowable expenses directly attributable or reasonably allocable to certain development activities, commercialization activities, medical affairs activities, manufacturing activities or other relevant activities, as described in the Purchase Agreement. During the Profit Sharing Period, the Company reimbursed SPN for (i) certain payments under the exclusive license agreement between the Company and Memorial Sloan Kettering Cancer Center, (ii) certain costs incurred in connection with an ongoing post-marketing safety study of VOWST and (iii) 80.1% of all rent and other costs due to the landlord under the lease for the Company’s Waltham facility.

At Closing, in exchange for a payment to be made by SPN to Bacthera AG, the Long Term Manufacturing Agreement, dated November 8, 2021 between the Company and Bacthera AG (the “Bacthera Manufacturing Agreement”) was terminated and each of Bacthera and Seres released one another from any and all losses, liabilities or other obligations arising thereunder with respect to the period ending at the Closing Date, including without limitation any milestone payments required to be paid to Bacthera thereunder.

On the Closing Date, the Company and SPN entered into a securities purchase agreement (the “Securities Purchase Agreement”) pursuant to which SPN purchased 714,285 shares (the “Shares”) of common stock at Closing, at a purchase price per share of $21.00, for an aggregate purchase price of $15,000. Under the terms of the Securities Purchase Agreement, SPN agreed not to sell or transfer the Shares for a period of six months after Closing, subject to certain customary exceptions. The Company agreed to register the resale of the Shares by SPN within 90 days of Closing. On October 1, 2024, the Company filed a registration statement to register the Shares, which became effective on October 11, 2024. In addition, under the terms of the Securities Purchase Agreement, for as long as SPN, together with its affiliates, beneficially owns at least 10% of the Company’s outstanding shares of common stock, the Company has agreed to take such action within its control to include one individual designated by SPN in the slate of nominees recommended by the Company’s board of directors (or the applicable committee of the board of directors) to the Company’s stockholders for election to the board of directors at the applicable stockholder meeting. SPN designated Hans-Juergen Woerle, M.D., Ph.D. and on February 4, 2025, the Company’s board of directors appointed Dr. Woerle to the board as a Class III director, with a term expiring at the Company’s 2027 annual meeting of stockholders. The Securities Purchase Agreement contains customary representations and warranties and closing conditions.

On the Closing Date, the Company entered into a TSA with NESA, which provides for services to be performed by the Company in order to facilitate a transition of the business associated with the VOWST Business to NESA and its affiliates. The scope of the transition services includes the provision of certain manufacturing services and certain administrative functions related to the VOWST Business and operations, including the maintenance of certain manufacturing services and the related facility in which such services are currently conducted. The Company provided the manufacturing services until December 31, 2025 and other services for the duration specified in the schedule to the TSA for each service. NESA paid the Company for certain fixed costs, including a monthly fixed fee and a variable per batch fee for preserved raw material suspension ("PRMS") manufacturing, and reimbursed the Company for certain costs of the transition services performed by the Company under the TSA, including labor. The know-how and other intellectual property

generated in connection with the performance of the TSA will be owned by NESA with the Company having a non-exclusive license to such know-how and other intellectual property under the Cross-License Agreement. During the term of the TSA, the Company transferred the specifications for materials and documentation necessary to enable PRMS manufacturing services to a third party service provider designated by NESA. In the event of a material failure by the Company to deliver PRMS under the TSA, NESA had step-in rights to negotiate to enter into a direct lease with the landlord of the manufacturing facility with respect to the portion of such facility used in connection with the VOWST Business or to cause such services to be performed, with any reasonable out-of-pocket costs and expenses incurred in connection therewith reimbursed by the Company.

At the Closing Date, the Company entered into a cross-license agreement with SPN under which the Company granted to SPN a perpetual, worldwide, non-exclusive, fully paid-up license under certain Company patents that have been issued or will issue in the future and current know-how controlled by the Company that is not transferred to SPN pursuant to the Purchase Agreement. In the field of the treatment of Clostridioides difficile infections ("CDI") and recurrent CDI and associated complications (collectively, the “CDI Field”) the license to SPN under such Company patents and know-how will be exclusive to SPN for five years after Closing and co-exclusive between SPN and the Company following that five year period. The license from the Company to SPN is to issued Company patents that currently or in the future cover the Product or improvements thereof, and know-how that is used or reasonably useful in connection with the exploitation of the VOWST Business. The Company also granted SPN an exclusive, perpetual, worldwide, fully paid-up license under issued Company patents that currently or in the future cover the Product and improvements thereof and know-how that is used or reasonably useful in connection with the exploitation of the Product to exploit SER-262 in the CDI Field. SPN granted to the Company a perpetual, worldwide, non-exclusive license under the patents and know-how that are transferred to SPN pursuant to the Purchase Agreement or developed under the TSA, for the Company’s products for use outside of the CDI Field, and after five years from Closing for Company products containing designed, cultivated, bacterial consortia not manufactured using human stool (excluding SER-262) in the CDI Field. From and after Closing, certain license agreements between the Company, SPN, and/or their respective affiliates terminated and are of no further force or effect, except as contemplated by the Purchase Agreement. For example, on September 30, 2024, in connection with the Transaction, the 2016 License Agreement (the “2016 License Agreement”) with Nestec, Ltd., as succeeded by SPN (together with NHSc Rx License GmbH, their affiliates, and their subsidiaries “Nestlé”), and the 2021 License Agreement (the “2021 License Agreement”) with NHSc Pharma Partners, succeeded by NHSc Rx License GmbH (together with Société des Produits Nestlé S.A, their affiliates, and their subsidiaries “Nestlé”) were terminated upon mutual agreement of the parties, with provisions related to record retention, confidentiality obligations, indemnification obligations, intellectual property ownership, and any outstanding payment obligations surviving the termination of each of the 2016 License Agreement and 2021 License Agreement, respectively.

On the Closing Date, the parties entered into assignment and assumption of lease agreements (the “Assignment and Assumption Agreements”). Under the Assignment and Assumption Agreements, the Company assigned to SPN the Company’s rights in, to and under certain real property leases, and SPN assumed the liabilities related thereto.

On the Closing Date, the parties entered into an employee support agreement (the “Employee Support Agreement”). Under the Employee Support Agreement, among other things and subject to the terms and conditions therein, certain employees of the Company related to the VOWST Business who accepted employment with SPN or one of its designated affiliates provided the services they provided to the Company prior to the Transaction to SPN, as well as other services as SPN may reasonably request, from Closing until the day prior to the beginning of SPN’s or its designated affiliate’s next pay period following the Closing. SPN reimbursed the Company’s out of pocket costs in connection with such employees’ services, including certain compensation and benefits paid or provided to such employees pursuant to the terms of the Employee Support Agreement. All such employees were transferred to SPN as of December 31, 2024.

In connection with the Transaction, the Company fully retired its senior secured debt facility with Oaktree Capital Management. The Company intends to use the remaining proceeds to support the further advancement of SER-155 and the Company’s other wholly-owned cultivated live biotherapeutic candidates for medically vulnerable patient populations with potential to address large commercial opportunities.

The Company incurred certain significant costs relating to the Transaction, such as legal, accounting, financial advisory, printing and other professional services fees, as well as other customary payments. For the year ended December 31, 2024, these costs amounted to approximately $9,016, which is included within the net income (loss) from discontinued operations, net of tax line item on the Company's consolidated statements of operations.

Going Concern

The Company is subject to risks common to companies in the biotechnology industry including, but not limited to, new technological innovations, protection of proprietary technology, dependence on key personnel, compliance with government regulations and the need to obtain additional financing. The Company operates in an environment of rapid change in technology and substantial

competition from pharmaceutical and biotechnology companies. In addition, the Company is dependent upon the services of its employees and consultants.

The accompanying consolidated financial statements have been prepared on a basis which assumes that the Company will continue as a going concern and which contemplates the realization of assets and satisfaction of liabilities and commitments in the normal course of business. As of December 31, 2025, the Company had an accumulated deficit of $972,403 and cash and cash equivalents of $45,766.

The Company’s product candidates are in development, and will require significant additional research and development efforts, including extensive preclinical and clinical testing and regulatory approval, prior to potential commercialization. There can be no assurance that the Company’s research and development will be successfully completed, that adequate protection for the Company’s intellectual property will be obtained, or maintained, that any product candidate developed will obtain necessary government regulatory approval, or that any approved product will be commercially viable. Even if the Company’s product development efforts are successful, it is uncertain when, if ever, the Company will generate significant revenue from product sales.

Primarily as a result of the costs associated with continuing the research and development efforts for SER-155 and the Company’s live biotherapeutics platform, the Company incurred a loss from operations of $93,971 and had operating cash outflows of $73,883, excluding the $75,000 installment payments received from Nestlé, for the year ended December 31, 2025. The Company expects that its operating losses and negative cash flows will continue for the foreseeable future. Based on the Company’s currently available cash resources, current and forecasted level of operations, and forecasted cash flows for the 12-month period subsequent to the date of issuance of these consolidated financial statements, the Company will require additional funding to support its ongoing operations and meet its obligations as they come due. These conditions raise substantial doubt about the Company’s ability to continue as a going concern.

The Company’s ability to continue as a going concern is dependent upon its ability to obtain the necessary financing to meet its obligations and repay its liabilities arising from normal business operations when they come due, and to generate profitable operations in the future. Management plans to provide for the Company’s capital requirements through financing or other strategic transactions, including potential business development transactions, and selling shares under the Company's at the market equity offering. There can be no assurance that the Company will be able to raise additional capital to fund operations with terms acceptable to the Company, or at all. Because certain elements of management’s plans to mitigate the conditions that raised substantial doubt about the Company’s ability to continue as a going concern are outside of the Company’s control, including the ability to raise capital through an equity or other financing, those elements cannot be considered probable according to Accounting Standards Codification (“ASC”) 205-40, Going Concern ("ASC 205-40"), and therefore cannot be considered in the evaluation of mitigating factors. As a result, management has concluded that substantial doubt exists about the Company’s ability to continue as a going concern for 12 months from the date these consolidated financial statements are issued. The accompanying consolidated financial statements do not include any adjustments that may result from the outcome of this uncertainty.

Reclassifications

Certain amounts in prior periods have been reclassified to reflect the impact of the discontinued operations treatment of the VOWST Business.

On April 21, 2025, the Company effected a 1-for-20 reverse stock split of the Company’s common stock. All share and per share amounts in the financial statements and notes thereto have been retroactively adjusted for all periods presented to give effect to this reverse stock split. The reverse stock split had no impact on the number of authorized shares or the par value of preferred and common stock. Therefore, the Company reclassified an amount equal to the reduction in the number of shares of common stock at par value to additional paid-in capital. No fractional shares were issued in connection with the reverse split, and stockholders who would otherwise be entitled to receive a fractional share instead received a cash payment equal to the fraction of a share of common stock in lieu of such fractional share. Proportionate adjustments were made to the number of shares authorized under the Company’s equity incentive plans, the number of shares subject to any award or purchase right under the Company’s equity incentive plans, and the exercise price or purchase price with respect to any stock option award or purchase right under the Company’s equity incentive plans. See Note 7, Stock and Stock-Based Awards, for further information.

The accompanying financial statements have been prepared in conformity with accounting principles generally accepted in the United States of America (“GAAP”) and include the accounts of the Company and its wholly owned subsidiaries after elimination of all intercompany accounts and transactions.

Net Income (Loss) per Share - Basic and Diluted Net Income (Loss) Per Share Attributable to Common Stockholders (Detail) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2025	Dec. 31, 2024	Dec. 31, 2023
Numerator:
Net income (loss) from continuing operations attributable to common stockholders	$ 5,696	$ (125,771)	$ (190,130)
Net income from discontinuing operations attributable to common stockholders	0	125,907	76,406
Net income (loss) attributable to common stockholders	$ 5,696	$ 136	$ (113,724)
Denominator:
Weighted average common shares outstanding - basic	8,858,975	7,769,910	6,400,339
Net income (loss) from continuing operations per share attributable to common stockholders - basic	$ 0.64	$ (16.2)	$ (29.71)
Net income from discontinued operations per share attributable to common stockholders - basic	0	16.2	11.94
Net income (loss) per share attributable to common stockholders - basic	$ 0.64	$ 0	$ (17.77)
Numerator:
Net income (loss) from continuing operations attributable to common stockholders	$ 5,696	$ (125,771)	$ (190,130)
Net income from discontinuing operations attributable to common stockholders	0	125,907	76,406
Net income (loss) attributable to common stockholders	$ 5,696	$ 136	$ (113,724)
Denominator:
Weighted average common shares outstanding - basic	8,858,975	7,769,910	6,400,339
Dilutive impact from:
Stock options to purchase common stock	0	0	0
Unvested restricted stock units	10,749	0	0
Shares issuable under employee stock purchase plan	18	0	0
Weighted average common shares outstanding, diluted	8,869,742	7,769,910	6,400,339
Net income from discontinued operations per share attributable to common stockholders - diluted	$ 0.64	$ (16.2)	$ (29.71)
Net income (loss) from continuing operations per share attributable to common stockholders - diluted	0	16.2	11.94
Net income (loss) per share attributable to common stockholders - diluted	$ 0.64	$ 0	$ (17.77)
Employee Stock Option
Anti-dilutive potential common stock equivalents excluded from the calculation of net income (loss) per share:
Anti-dilutive potential common stock equivalents excluded from the calculation of net loss per share:	1,124,208	886,123	742,335
Unvested Restricted Stock Units [Member]
Anti-dilutive potential common stock equivalents excluded from the calculation of net income (loss) per share:
Anti-dilutive potential common stock equivalents excluded from the calculation of net loss per share:	24,150	81,459	168,730
Shares Issuable under employee stock purchase plan [Member]
Anti-dilutive potential common stock equivalents excluded from the calculation of net income (loss) per share:
Anti-dilutive potential common stock equivalents excluded from the calculation of net loss per share:	2,047	7,160	14,889
Shares Issuable under employee stock purchase plan [Member] \| Warrants [Member]
Anti-dilutive potential common stock equivalents excluded from the calculation of net income (loss) per share:
Anti-dilutive potential common stock equivalents excluded from the calculation of net loss per share:	32,379	32,379	58,871

Notes Payable - Additional Information (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2025	Dec. 31, 2024	Dec. 31, 2023
Debt Instrument [Line Items]
Loss on extinguishment	$ 0	$ (23,351)	$ (1,625)
Adjustments To Additional Paid In Capital, Warrant Issued			2,785
Repayments of Notes Payable	0	127,905	52,860
Interest expense	$ 0	$ 0	$ 2,468

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
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Reference 2: http://www.xbrl.org/2003/role/disclosureRef
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Reference 3: http://www.xbrl.org/2009/role/commonPracticeRef
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+ Details

Commitments and Contingencies - Additional Information (Detail) - USD ($)	Dec. 31, 2025	Dec. 31, 2024
Indemnification Agreement [Member]
Other Commitments [Line Items]
Obligations accrued	$ 0	$ 0
Legal Contingencies [Member]
Other Commitments [Line Items]
Obligations accrued	$ 0	$ 0

Stock and Stock-Based Awards - 2015 and 2025 Incentive Award Plan - Additional Information (Details)			12 Months Ended
	Apr. 10, 2025 shares	Jun. 16, 2015	Dec. 31, 2025 shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stockholders equity note stock split, conversion ratio	20
2015 Incentive Award Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Percentage of common stock shares outstanding		4.00%
Stock incentive plan description			The number of shares initially reserved for issuance under the 2015 Plan was the sum of (i) 110,000 shares of common stock and (ii) the number of shares subject to awards outstanding under the 2012 Plan that expire, terminate or are otherwise surrendered, canceled, forfeited or repurchased by the Company on or after the effective date of the 2015 Plan. In addition, the number of shares of common stock that may be issued under the 2015 Plan was subject to increase on the first day of each calendar year, beginning in 2016 and ending in 2025, equal to the lesser of (i) 4% of the number of shares of the Company’s common stock outstanding on the last day of the preceding applicable calendar year and (ii) an amount determined by the Company’s board of directors.
Number of shares authorized for issuance under Incentive Award Plan	1,750,493
2025 Incentive Award Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares authorized for issuance under Incentive Award Plan	2,230,243
Number of additional shares authorized for issuance under Incentive Award Plan	479,750
Shares available for future grant			740,410
2025 Incentive Award Plan [Member] \| Minimum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock option granted vesting period			4 years
2025 Incentive Award Plan [Member] \| Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock option granted vesting period			4 years
Stock option expiration period			10 years

Stock and Stock-Based Awards - 2015 Employee Stock Purchase Plan - Additional Information (Details) - 2015 Employee Stock Purchase Plan [Member] - shares		12 Months Ended
	Jun. 16, 2015	Dec. 31, 2025
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Aggregate shares of common stock reserved for issuance	18,250	106,064
Shares issued under ESPP	20,000	17,839
Percentage of common stock shares outstanding	1.00%
Percentage of eligible earning contributed toward semi-annual purchase of common stock		15.00%
Offering period under the plan		6 months
Percentage of purchase price with respect to each offering period		85.00%

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

+ Details

Consolidated Balance Sheets - USD ($)
$ in Thousands

Dec. 31, 2025

Dec. 31, 2024

Current assets:

Cash and cash equivalents

$ 45,766

$ 30,793

Accounts receivable due from SPN - related party

360

2,068

Accounts receivable

157

Prepaid expenses and other current assets

^[1]

3,093

5,813

Total current assets

49,376

38,674

Property and equipment, net

7,635

11,534

Operating lease assets

72,483

80,903

Restricted cash

8,668

Other non-current assets

Total assets

138,193

139,810

Current liabilities:

Accounts payable

1,682

4,079

Accrued expenses and other current liabilities

3,972

10,719

Accrued liabilities due to SPN - related party

3,278

17,750

Operating lease liabilities

10,390

8,674

Total current liabilities

19,322

41,222

Operating lease liabilities, net of current portion

72,576

82,966

Other long-term liabilities

2,077

1,838

Total liabilities

93,975

126,026

Commitments and contingencies (Note 10)

Stockholders' equity:

Preferred stock, $0.001 par value; 10,000,000 shares authorized at December 31, 2025 and 2024; no shares issued and outstanding at December 31, 2025 and 2024

Common stock, $0.001 par value; 360,000,000 shares authorized at December 31, 2025 and 2024, respectively; 9,556,446 and 8,650,227 shares issued and outstanding at December 31, 2025 and 2024, respectively

Additional paid-in capital

1,016,611

991,874

Accumulated deficit

(972,403)

(978,099)

Total stockholders' equity

44,218

13,784

Total liabilities and stockholders' equity

$ 138,193

$ 139,810

[1]	Includes $0 as of December 31, 2025 and $2,683 as of December 31, 2024 of unbilled receivable from SPN (related party) related to certain costs of the transition services performed by the Company. See Note 3, Discontinued Operations and TSA, for further details.

Stock and Stock-Based Awards - Stock Options - Additional Information (Details) - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended	3 Months Ended	12 Months Ended
	Apr. 30, 2023	Mar. 31, 2024	Dec. 31, 2025	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2021
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted average grant-date fair value of stock options			$ 13.3	$ 18.58	$ 90.37
Intrinsic Value, Stock options exercised			$ 98	$ 0	$ 438
Number of stock options granted			546,805
Weighted average exercise price of stock options granted			$ 15.14
Stock-based compensation expense			$ 10,840	20,971	34,101
Performance Shares [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted average grant-date fair value of stock options						$ 110.6
Number of stock options granted						28,100
Percentage of performance-based stock options	50.00%
Stock-based compensation expense			0	8	$ 2,051
Percentage of Performance-Based Stock Options Vesting	50.00%
Employee Stock Option Market Condition [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of stock options granted		127,500
Stock-based compensation expense			$ 641	$ 775
Employees Stock Option [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted average exercise price of stock options granted		$ 22
Expiration period of stock options granted		10 years

Income Taxes - Schedule of Reconciliation of U.S Federal Statutory Income Tax Rate to Company's Effective Income Tax Rate (Detail) - USD ($)

12 Months Ended

Dec. 31, 2025

Dec. 31, 2024

Dec. 31, 2023

Income Tax Disclosure [Abstract]

Federal statutory income tax rate

$ 1,202,000

Federal statutory income tax rate percent

(21.00%)

Research and development tax credits

$ 334,000

Research and development tax credits percent

5.80%

(1.80%)

(2.50%)

State and local income taxes, net of federal income tax effect

^[1]

$ 0

State and local income taxes, net of federal income tax effect percent

0.00%

^[1]

(3.60%)

(7.20%)

Nontaxable or nondeductible items

Meals and entertainment

$ 14,000

Meals and entertainment percent

0.30%

Stock-based compensation

$ 3,094,000

Stock-based compensation percent

54.10%

1.00%

0.80%

Adjustment to gain on sale of VOWST business

$ (1,133,000)

Adjustment to gain on sale of VOWST business percent

(19.80%)

Uncertain tax position reserves percent

0.40%

0.50%

Changes in unrecognized tax benefits

$ (67,000)

Changes in unrecognized tax benefits percent

(1.20%)

Other adjustments

$ 1,000

Other adjustments percent

0.00%

(0.40%)

(0.10%)

Change in deferred tax asset valuation allowance

$ (3,445,000)

Change in deferred tax asset valuation allowance percent

(60.20%)

25.40%

29.50%

Effective income tax rate

$ 0

Effective income tax rate percent

0.00%

[1]	State and local taxes in Massachusetts comprise the majority of this category.

Consolidated Statements of Operations and Comprehensive Income (Loss) - USD ($)
$ in Thousands

12 Months Ended

Dec. 31, 2025

Dec. 31, 2024

Dec. 31, 2023

Revenue:

Grant revenue

$ 789

$ 0

Total revenue

$ 789

$ 0

Revenue from Contract with Customer, Product and Service [Extensible Enumeration]

us-gaap:GrantMember

Operating expenses:

Research and development expenses

$ 49,060

$ 64,600

$ 117,597

General and administrative expenses

39,156

53,183

77,500

Manufacturing services

6,544

3,532

Total operating expenses

94,760

121,315

195,097

Loss from operations

(93,971)

(121,315)

(195,097)

Other income (expense):

Gain on sale of VOWST Business

80,685

5,684

Interest income

2,227

3,967

7,301

Interest expense

(2,468)

Other income (expense), net

^[1]

16,755

(14,107)

134

Total other income (expense), net

99,667

(4,456)

4,967

Net income (loss) from continuing operations

5,696

(125,771)

(190,130)

Net income from discontinued operations, net of tax

125,907

76,406

Net income (loss)

$ 5,696

$ 136

$ (113,724)

Net income (loss) from continuing operations per share attributable to common stockholders - basic

$ 0.64

$ (16.2)

$ (29.71)

Net income from discontinued operations per share attributable to common stockholders - basic

16.2

11.94

Net income (loss) per share attributable to common stockholders - basic

0.64

(17.77)

Net income (loss) from continuing operations per share attributable to common stockholders - diluted

0.64

(16.2)

(29.71)

Net income from discontinued operations per share attributable to common stockholders - diluted

16.2

11.94

Net income (loss) per share attributable to common stockholders - diluted

$ 0.64

$ 0

$ (17.77)

Weighted average common shares outstanding - basic

8,858,975

7,769,910

6,400,339

Weighted average common shares outstanding - diluted

8,869,742

7,769,910

6,400,339

Other comprehensive income:

Unrealized gain on investments, net of tax of $0

$ 0

$ 10

Currency translation adjustment

Total other comprehensive income

Comprehensive income (loss)

$ 5,696

$ 136

$ (113,712)

[1]	[2] Includes $13,311 and $6,292 for the years ended December 31, 2025 and 2024 related to reimbursement received from SPN (related party) for transition services provided by the Company

Property and Equipment, Net

12 Months Ended

Dec. 31, 2025

Property, Plant and Equipment [Abstract]

Property and Equipment, Net

. Property and Equipment, Net

Property and equipment, net consisted of the following (in thousands):


	December 31,
	2025			2024
Laboratory equipment	$	23,852		$	24,468
Computer equipment		4,058			3,924
Furniture and office equipment		4,523			4,523
Leasehold improvements		30,954			30,954
Construction in progress		861			861
		64,248			64,730
Less: Accumulated depreciation and amortization		(56,613	)		(53,196	)
	$	7,635		$	11,534

Depreciation and amortization expense was $4,133, $5,468 and $6,243 for the years ended December 31, 2025, 2024 and 2023, respectively, which includes amounts related to both continuing and discontinued operations. During the years ended December 31, 2025 and 2024, the Company disposed of certain assets with a cost basis of $734 and $679, respectively, and a net book value of $18 and $0, respectively. In addition, during the year ended December 31, 2024, the Company recorded an impairment loss of $1,536 related to leasehold improvements at one of the Company's locations for which impairment indicators were determined to exist as of December 31, 2024. See Note 6, Leases, for further details.

401(k) Savings Plan - Additional Information (Detail) - USD ($) $ in Thousands		12 Months Ended
	Jan. 01, 2016	Dec. 31, 2025	Dec. 31, 2024	Dec. 31, 2023
Retirement Benefits [Abstract]
Employer match of employee contributions	50.00%
Employee deferral subject to employer match	6.00%
Employer contribution expenses		$ 592	$ 1,068	$ 2,003

Summary of Significant Accounting Policies

12 Months Ended

Dec. 31, 2025

Accounting Policies [Abstract]

Summary of Significant Accounting Policies

Discontinued Operations

The Company accounted for the sale of its VOWST Business in accordance with ASC 205 Discontinued Operations and ASU No. 2014-08, Reporting of Discontinued Operations and Disclosures of Disposals of Components of an Entity. The Company followed the held-for-sale criteria as defined in ASC 205. ASC 205 requires that a component of an entity that has been disposed of or is classified as held for sale and has operations and cash flows that can be clearly distinguished from the rest of the entity be reported as assets held for sale and discontinued operations. In the period a component of an entity has been disposed of, the results of operations for the periods presented are reclassified into separate line items in the consolidated statements of operations. In the period a discontinued operation is classified for sale, the assets and liabilities of the discontinued operation are also reclassified into separate line items on the related consolidated balance sheets for the periods presented.

Due to the sale of the VOWST Business during the third quarter of 2024 (see Note 3, Discontinued Operations and TSA), in accordance with ASC 205, Discontinued Operations, the Company has classified the results of the VOWST Business as discontinued operations in its consolidated statements of operations and cash flows for all periods presented. All amounts included in the notes to the consolidated financial statements relate to continuing operations unless otherwise noted.

Use of Estimates

The preparation of financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities, the disclosure of contingent assets and liabilities at the date of the consolidated financial statements and the reported amounts of revenue and expenses during the reporting periods. In these consolidated financial statements, the Company uses estimates and assumptions related to the accrual of research and development expenses. Estimates are periodically reviewed in light of changes in circumstances, facts and experience. Actual results could differ from the Company’s estimates.

Restricted Cash

The Company held restricted cash of $8,668 as of December 31, 2025 and 2024, which represents cash held for the benefit of the landlord for certain of the Company’s leases. The Company has classified the restricted cash as long-term on its consolidated balance sheets as the underlying leases are greater than 1 year.

Cash, cash equivalents and restricted cash were comprised of the following (in thousands):


	December 31,
	2025		2024
Cash and cash equivalents	$	45,766	$	30,793
Restricted cash, non-current		8,668		8,668
Total cash, cash equivalents and restricted cash	$	54,434	$	39,461

Concentration of Credit Risk

Financial instruments that potentially expose the Company to concentrations of credit risk consist primarily of cash balances at financial institutions in amounts that exceed federally insured limits. The Company does not believe that it is subject to unusual credit risk beyond the normal credit risk associated with commercial banking relationships.

Property and Equipment

Property and equipment are stated at cost less accumulated depreciation. Depreciation expense is recognized using the straight-line method over the useful life of the asset, which are as follows:


		Estimated Useful Life (In Years)
Laboratory equipment		5
Computer equipment, furniture and office equipment		3
Leasehold improvements		Lesser of useful life or lease term

Expenditures for repairs and maintenance of assets are charged to expense as incurred. Upon retirement or sale, the cost and related accumulated depreciation of assets disposed of are removed from the accounts and any resulting gain or loss is included in loss from operations.

Impairment of Long-Lived Assets

Long-lived assets consist of property and equipment and right-of-use assets associated with our lease agreements. All of the Company's long-lived assets are to be held and used and have definitive lives and accordingly are tested for recoverability whenever events or changes in business circumstances indicate that the carrying amount of the assets may not be fully recoverable. Factors that the Company considers in deciding when to perform an impairment review include significant underperformance of the business in relation to expectations, significant negative industry or economic trends and significant changes or planned changes in the use of the assets. If an impairment review is performed to evaluate a long-lived asset for recoverability, the Company compares forecasts of undiscounted cash flows expected to result from the use and eventual disposition of the long-lived asset or asset group to its carrying value. An impairment loss would be recognized when estimated undiscounted future cash flows expected to result from the use of an asset or asset group are less than its carrying amount. The impairment loss would be based on the excess of the carrying value of the impaired asset over its fair value, determined based on discounted cash flows. For the years ended December 31, 2025, 2024 and 2023, the Company recorded an impairment loss on long-lived assets of $0, $3,267 and $0, respectively.

Research and Development Costs

Research and development costs are expensed as incurred. Research and development expenses include salaries, stock-based compensation and benefits of employees, third-party license fees and other operational costs related to the Company’s research and development activities, including allocated facility-related expenses and external costs of outside vendors engaged to conduct both preclinical studies and clinical trials, and beginning in the fourth quarter of 2024 certain costs of fulfilling the Company's obligations under the TSA which are reimbursable by Nestlé.

Research Contract Costs and Accruals

The Company has entered into various research and development contracts with research institutions and other companies. These agreements are generally cancelable, and related payments are recorded as research and development expenses as incurred. The Company records accruals for estimated ongoing research costs based on reporting provided by third parties, typically contract research organizations. When evaluating the adequacy of the accrued liabilities, the Company analyzes progress of the studies, including the phase or completion of events, invoices received and contracted costs. Significant judgments and estimates are made in determining the accrued and prepaid balances at the end of any reporting period. Actual results could differ from the Company’s estimates. The Company’s historical accrual estimates have not been materially different from the actual costs.

Patent Costs

All patent-related costs incurred in connection with filing and prosecuting patent applications are expensed as incurred due to the uncertainty about the recovery of the expenditure. Amounts incurred are classified as general and administrative expenses.

Accounting for Stock-Based Compensation

The Company measures all stock options and other stock-based awards granted to employees, non-employees, and directors based on the fair value on the date of the grant and recognizes compensation expense of those awards over the requisite service period, which is generally the vesting period of the respective award. Generally, the Company issues stock options, restricted stock units and restricted stock awards with only service-based vesting conditions and records the expense for these awards using the straight-line method. For stock options or restricted stock units issued with performance-based vesting conditions, the stock compensation expense related to these awards is recognized based on the grant date fair value when achievement of the performance condition is deemed probable.

The Company classifies stock-based compensation expense in its consolidated statements of operations and comprehensive income (loss) in the same manner in which the award recipient’s payroll costs are classified or in which the award recipients’ service payments are classified.

The Company accounts for forfeitures of stock-based awards as they occur rather than applying an estimated forfeiture rate to stock-based compensation expense.

The fair value of each stock option grant is estimated on the date of grant using the Black- Scholes option-pricing model. The Company estimates its expected common stock volatility based on its historical common stock volatility for the same time period. The Company uses the simplified method prescribed by Securities and Exchange Commission Staff Accounting Bulletin No. 107, Share-Based Payment, to calculate the expected term of options granted to employees, non-employees and directors. The risk-free interest rate is determined by reference to the U.S. Treasury yield curve in effect at the time of grant of the award for time periods approximately equal to the expected term of the award. Expected dividend yield is based on the fact that the Company has never paid cash dividends and does not expect to pay any cash dividends in the foreseeable future.

Grant Revenue

The Company generates revenue from government contracts that reimburse the Company for certain allowable costs for funded projects. For contracts with government agencies, when the Company has concluded that it is the principal in conducting the research and development expenses, and where the funding arrangement is considered central to the Company’s ongoing operations, the Company classifies the recognized funding received as revenue.

The Company has concluded to recognize funding received as revenue, rather than as a reduction of research and development expenses, because the Company is the principal in conducting the research and development activities and these contracts are central to its ongoing operations. Revenue is recognized as the qualifying expenses related to the contracts are incurred. Revenue recognized upon incurring qualifying expenses in advance of receipt of funding is recorded in the Company’s consolidated balance sheet as accounts receivable. The related costs incurred by the Company are included in research and development expense in the Company’s consolidated statements of operations and comprehensive loss.

Income Taxes

The Company accounts for income taxes using the asset and liability method, which requires the recognition of deferred tax assets and liabilities for the expected future tax consequences of events that have been recognized in the consolidated financial statements or in the Company’s tax returns. Deferred taxes are determined based on the difference between the financial statement and tax basis of assets and liabilities using enacted tax rates in effect in the years in which the differences are expected to reverse. Changes in deferred tax assets and liabilities are recorded in the provision for income taxes. The Company assesses the likelihood that its deferred tax assets will be recovered from future taxable income and, to the extent it believes, based upon the weight of available evidence, that it is more likely than not that all or a portion of the deferred tax assets will not be realized, a valuation allowance is established through a charge to income tax expense.

The Company applies ASC 740-10, Accounting for Uncertain Tax Positions. The Company accounts for uncertainty in income taxes recognized in the financial statements by applying a two-step process to determine the amount of tax benefit to be recognized. First, the tax position must be evaluated to determine the likelihood that it will be sustained upon external examination by the taxing authorities. If the tax position is deemed more-likely-than-not to be sustained, the tax position is then assessed to determine the amount of benefit to recognize in the financial statements. The amount of the benefit that may be recognized is the largest amount that has a greater than 50% likelihood of being realized upon ultimate settlement. The provision for income taxes includes the effects of any resulting tax reserves, or unrecognized tax benefits, that are considered appropriate as well as the related net interest and penalties.

Segment Reporting

The Company manages its operations as a single operating segment for the purposes of assessing performance and making operating decisions, and thus reports as a single reportable segment. The Company’s singular focus is on developing live biotherapeutics for medically vulnerable patient populations to prevent bacterial bloodstream and antimicrobial resistant (AMR) infections as well as to treat GI-related immune diseases. Revenue to date has been generated solely through the Company's agreements with its collaborators, all of which has been earned in the United States. All tangible assets are held in the United States.

Comprehensive Income (Loss)

Comprehensive income (loss) includes net income (loss) as well as other changes in stockholders’ equity (deficit) that result from transactions and economic events other than those with stockholders. For the year ended December 31, 2023, other comprehensive income consisted of changes in unrealized gains from available-for-sale investments and a currency translation adjustment.

Net Income (Loss) per Share

Basic net income (loss) per share is computed using the weighted average number of common shares outstanding during the period. Diluted net income (loss) per share is computed using the sum of the weighted average number of common shares outstanding during the period and, if dilutive, the weighted average number of potential shares of common stock, including the assumed exercise of stock options and unvested restricted stock. The Company applies the two-class method to calculate its basic and diluted net loss per share attributable to common stockholders. The two-class method is an earnings allocation formula that treats a participating security as having rights to earnings that otherwise would have been available to common stockholders. However, the two-class method does not impact the net loss per share of common stock for periods in which the Company is in a net loss position.

The Company’s restricted stock awards entitle the holder of such awards to dividends declared or paid by the board of directors, regardless of whether such awards are unvested, as if such shares were outstanding common shares at the time of the dividend. However, the unvested restricted stock awards are not entitled to share in the residual net assets (deficit) of the Company. Accordingly, in periods in which the Company reports a net loss attributable to common stockholders, diluted net loss per share attributable to common stockholders is the same as basic net loss per share attributable to common stockholders, since dilutive common shares are not assumed to have been issued if their effect is anti-dilutive.

Leases

In accordance with ASC 842, Leases, the Company determines if an arrangement is or contains a lease at inception. A contract is or contains a lease if the contract conveys the right to control the use of an identified asset for a period of time in exchange for consideration. The Company classifies leases at the lease commencement date as operating or finance leases and records a right-of-use asset and a lease liability on the consolidated balance sheets for all leases with an initial lease term of greater than 12 months. Leases with an initial term of 12 months or less are not recorded on the balance sheets, but payments are recognized as expense on a straight-line basis over the lease term. The Company has elected not to record a right-of-use asset or lease liability for leases with terms of 12 months or less.

A lease qualifies as a finance lease if any of the following criteria are met at the inception of the lease: (i) there is a transfer of ownership of the leased asset to the Company by the end of the lease term, (ii) the Company holds an option to purchase the leased asset that it is reasonably certain to exercise, (iii) the lease term is for a major part of the remaining economic life of the leased asset, (iv) the present value of the sum of lease payments equals or exceeds substantially all of the fair value of the leased asset, or (v) the nature of the leased asset is specialized to the point that it is expected to provide the lessor no alternative use at the end of the lease term.

The Company enters into contracts that contain both lease and non-lease components. Non-lease components may include maintenance, utilities, and other operating costs. The Company combines the lease and non-lease components of fixed costs in its lease arrangements as a single lease component. Variable costs, such as utilities or maintenance costs, are not included in the measurement of right-of-use assets and lease liabilities, but rather are expensed when the event determining the amount of variable consideration to be paid occurs.

Finance and operating lease assets and liabilities are recognized at the lease commencement date based on the present value of the lease payments over the lease term using the discount rate implicit in the lease. If the rate implicit is not readily determinable, the Company utilizes an estimate of its incremental borrowing rate based upon the available information at the lease commencement date. Operating lease assets are further adjusted for prepaid or accrued lease payments. Operating lease payments are expensed using the straight-line method as an operating expense over the lease term. The Company’s lease terms may include options to extend or terminate the lease when it is reasonably certain that the Company will exercise that option. Finance lease assets are amortized to depreciation expense using the straight-line method over the shorter of the useful life of the related asset or the lease term. Finance lease payments are bifurcated into (i) a portion that is recorded as imputed interest expense and (ii) a portion that reduces the finance liability associated with the lease.

Right-of-use assets and lease liabilities are reassessed and remeasured when amendments to the terms of the lease agreement require reassessment and remeasurement of the lease payments and other inputs to the calculation of right-of-use assets and lease liabilities. The Company accounts for remeasurements and modifications to lease liabilities using the present value of remaining lease payments and estimated incremental borrowing rate at the date of remeasurement. The adjustment to the lease liability is recognized as

a gain or loss in operating expenses, or as an adjustment to the right-of-use asset, as appropriate, based on the terms and conditions within the lease that are amended.

Recently Adopted Accounting Pronouncements

In December 2023, the FASB issued ASU 2023-09, Income Taxes (Topic 740): Improvements to Income Tax Disclosures which requires public entities to disclose specific categories in the effective tax rate reconciliation, as well as expanded disclosures on income taxes paid by jurisdictions. ASU 2023-09 is effective for fiscal years beginning after December 15, 2024, with early adoption permitted. The Company adopted this standard and applied the disclosure requirements on a prospective basis effective for the annual reporting period ended December 31, 2025. There was no impact on the Company’s consolidated financial statements and additional required disclosures have been included in Note 11, Income Taxes.

Recently Issued Accounting Pronouncements

In November 2024, the FASB issued ASU 2024-03, Disaggregation of Income Statement Expenses (Topic 220), which requires disclosure in the notes to financial statements about specific types of expenses included in the expense captions presented on the face of the statement of operations. The requirements of the ASU are effective for annual periods beginning after December 15, 2026, and for interim periods beginning after December 15, 2027, with early adoption permitted. The requirements will be applied prospectively with the option for retrospective application. The Company is currently evaluating the impact related to the adoption of ASU 2024-03 on its financial statement disclosures.

In December 2025, the FASB issued ASU 2025-10, Government Grants (Topic 832): Accounting for Government Grants Received by Business Entities, which standardizes the accounting for government grants and distinguishes between a grant related to an asset and a grant related to income. The requirements of the ASU are effective for annual periods beginning after December 15, 2028, and for interim periods within those annual periods, with early adoption permitted. The requirements can be applied using a modified prospective or a modified retrospective approach. The Company does not expect the impact of ASU 2025-10 to be material to its financial statement disclosures as it is consistent with the Company's current accounting policy for government grants.

Income Taxes

12 Months Ended

Dec. 31, 2025

Income Tax Disclosure [Abstract]

Income Taxes

11. Income Taxes

During the years ended December 31, 2025, 2024 and 2023, the Company recorded no income tax benefits for the net operating losses incurred in each year or interim period, due to its uncertainty of realizing a benefit from those items.

A reconciliation of the U.S. federal statutory income tax rate to the Company’s effective income tax rate after the adoption of ASU 2023-09 is as follows:


	Year Ended December 31, 2025
	Amount			%
Federal statutory income tax rate	$	1,202			21.0	%
State and local income taxes, net of federal income tax effect(1)		—			—	%
Tax credits
Research and development tax credits		334			5.8	%
Changes in valuation allowance		(3,445	)		(60.2	)%
Nontaxable or nondeductible items
Meals and entertainment		14			0.3	%
Stock-based compensation		3,094			54.1	%
Adjustment to gain on sale of VOWST business		(1,133	)		(19.8	)%
Changes in unrecognized tax benefits		(67	)		(1.2	)%
Other adjustments		1			0.0	%
Effective income tax rate	$	—			—	%

[1] State and local taxes in Massachusetts comprise the majority of this category.

A reconciliation of the U.S. federal statutory income tax rate to the Company’s effective income tax rate for years prior to the adoption of ASU 2023-09 is as follows:


	Year Ended December 31,
		2024			2023
Federal statutory income tax rate			(21.0	)%		(21.0	)%
Research and development tax credits			(1.8	)		(2.5	)
State taxes, net of federal benefit			(3.6	)		(7.2	)
Stock-based compensation			1.0			0.8
Uncertain tax position reserves			0.4			0.5
Other			(0.4	)		(0.1	)
Change in deferred tax asset valuation allowance			25.4			29.5
Effective income tax rate			—	%		—	%

Net deferred tax assets as of December 31, 2025 and 2024 consisted of the following:


	December 31,
	2025			2024
Deferred tax assets:
Net operating loss carryforwards	$	167,203		$	156,157
Research and development tax credit carryforwards		54,149			54,648
Section 174 capitalized research and development expenses		39,759			53,600
Stock-based compensation expense		28,594			28,819
Lease liability		22,689			23,917
Contingent consideration		13,437			—
Accrued expenses		1,788			7,032
Section 163(j) limitation		1,843			5,599
Depreciation and amortization		1,223			953
Other		98			132
Total deferred tax assets	$	330,783		$	330,857
Deferred tax liabilities:
Depreciation and amortization		—			—
Right of use assets		(19,822	)		(21,115	)
Total deferred tax liabilities		(19,822	)		(21,115	)
Valuation allowance	$	(310,961	)	$	(309,742	)
Net deferred tax assets	$	—		$	—

The Tax Cuts and Jobs Act (“TCJA”), which was enacted in 2017, required taxpayers to capitalize and amortize research and experimental expenditures under IRC Section 174 for tax years beginning after December 31, 2021. This rule became effective for the Company during the year ended December 31, 2022 and resulted in the capitalization of research and development costs for the years ended December 31, 2024, 2023, and 2022. The TCJA required the Company to amortize these costs for tax purposes over five years if the research and development was performed in the U.S. and over 15 years if the research and development was performed outside the U.S. On July 4, 2025, new U.S. tax legislation was signed into law (known as the “One Big Beautiful Bill Act”, or “OBBA”) which generally allows taxpayers to (i) immediately deduct research and experimental expenditures attributable to U.S.-based research paid or incurred in taxable years beginning after December 31, 2024 and (ii) elect to accelerate, over a period of one or two years, any unamortized research and experimental expenditures attributable to U.S.-based research incurred in taxable years beginning after December 31, 2021 and before January 1, 2025. The Company deducted 100% of its research and experimental expenditures incurred in 2025 during the year ended December 31, 2025. Further, the Company has elected not to accelerate the unamortized research and experimental expenditures from prior years and instead recognize them over the remaining amortization period of five years from when they were initially capitalized.

For the year ended December 31, 2025, the Company did not have any material cash payments or refunds for income taxes.

As of December 31, 2025, the Company had net operating loss carryforwards (“NOLs”) for federal and state income tax purposes of $616,412 and $597,758, respectively. Federal NOLs of $98,756, generated before 2018, will begin expiring in varying amounts in 2035 unless utilized. The remaining federal NOLs of $517,656, generated after 2017, will be carried forward indefinitely and could be used to offset up to 80% of taxable income in future tax years. The Company's state NOLs will expire at various times starting in 2035. As of December 31, 2025, the Company also had available gross research and development tax credit carryforwards for federal and state income tax purposes of $55,341 and $14,767, respectively, which begin to expire in 2031 and 2028, respectively. The federal research and development tax credits include an orphan drug credit carryforward of $25,876.

Utilization of the NOLs and research and development tax credit carryforwards may be subject to a substantial annual limitation under Sections 382 and 383 of the IRC due to ownership changes that have occurred previously or that could occur in the future. These ownership changes may limit the amount of carryforwards that can be utilized annually to offset future taxable income. In general, an ownership change, as defined by Section 382, results from transactions increasing the ownership of certain shareholders or public groups in the stock of a corporation by more than 50% over a three-year period. Since its formation, the Company has raised capital through the issuance of capital stock on several occasions. These financings, combined with the purchasing shareholders' subsequent disposition of those shares, may have resulted in an ownership change or could result in an ownership change in the future upon subsequent disposition. The Company conducted an analysis to determine if historical changes in ownership through December 31, 2024 would limit or otherwise restrict its ability to utilize these NOLs and research and development credit carryforwards. As a result of this analysis, the Company does not believe there are any significant limitations on its ability to utilize these carryforwards. However, future changes in ownership after December 31, 2024 could affect the limitation in future years. Any limitation may result in expiration of a portion of the NOLs or research and development credit carryforwards before utilization.

The Company has evaluated the positive and negative evidence bearing upon its ability to realize the deferred tax assets. Management has considered the Company’s history of cumulative net losses incurred since inception and its lack of commercialization of any products or generation of any revenue from product sales since inception and has concluded that it is more likely than not that the Company will not realize the benefits of the deferred tax assets. Accordingly, a full valuation allowance has been established against the deferred tax assets as of December 31, 2025 and 2024. Management reevaluates the positive and negative evidence at each reporting period.


	Year Ended December 31,
	2025			2024			2023
Valuation allowance at beginning of year	$	(309,742	)	$	(312,809	)	$	(277,370	)
Decreases recorded as benefit to income tax provision					3,067			—
Increases recorded to income tax provision		(1,219	)		—			(35,439	)
Valuation allowance as of end of year	$	(310,961	)	$	(309,742	)	$	(312,809	)

During the year ended December 31, 2023, the Internal Revenue Service ("IRS") concluded their examination of the Company for the period ended December 31, 2018 related to the Company's 2018 research and development tax credits ("R&D Credit(s)"). The Company has adjusted its 2018 R&D Credits and its overall federal and state R&D Credit carryforward balance from the Company's inception to December 31, 2025 to account for the conclusions drawn by the IRS. Also, the Company has reviewed each of its overall filing positions since inception and has not identified any additional positions that do not meet the more likely than not threshold. The Company does not anticipate a material change to its uncertain tax position reserves in the next 12 months. The changes in the Company's unrecognized tax benefits for the years ended December 31, 2025, 2024, and 2023 were as follows:


	Year Ended December 31,
	2025			2024		2023
Balance at beginning of year	$	14,015		$	13,529	$	12,528
Increase in unrecognized tax benefits as a result of tax positions taken during the year		129			486		1,001
(Decrease) in unrecognized tax benefits as a result of tax positions taken during the year		(289	)		—		—
Reduction to unrecognized tax benefits		—			—		—
Balance at end of year	$	13,855		$	14,015	$	13,529

The Company has not yet conducted a study of its research and development credit carry forwards. This study may result in further adjustment to the Company’s R&D Credits; however, a full valuation allowance has been provided against the Company’s R&D Credits, and if an adjustment is required, this adjustment would be offset by an adjustment to the valuation allowance. Thus, there would be no impact to the consolidated balance sheets or statements of operations if an adjustment were required. The Company had no other unrecognized tax benefits accrued for the years ended December 31, 2025 and 2024, or related interest and penalties as of such dates. The Company will recognize any interest and penalties related to uncertain tax positions in income tax expense.

The Company files tax returns as prescribed by the tax laws of the jurisdictions in which it operates. In the normal course of business, the Company is subject to examination by federal and state jurisdictions, where applicable. The Company's tax years are still open under statute from 2011 to present. All years may be examined to the extent the tax credit or net operating loss carryforwards are used in future periods.

- Definition

The entire disclosure for income tax.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 942
-SubTopic 740
-Name Accounting Standards Codification
-Section 50
-Paragraph 1
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147477617/942-740-50-1

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 235
-SubTopic 10
-Name Accounting Standards Codification
-Section S50
-Paragraph 1
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480738/235-10-S50-1

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Topic 944
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-Name Accounting Standards Codification
-Section 50
-Paragraph 1
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147478822/944-740-50-1

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
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-URI https://asc.fasb.org/1943274/2147482685/740-10-50-12

Reference 5: http://www.xbrl.org/2003/role/exampleRef
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-Name Accounting Standards Codification
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-Paragraph 231
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482663/740-10-55-231

Reference 6: http://www.xbrl.org/2003/role/disclosureRef
-Topic 740
-SubTopic 10
-Name Accounting Standards Codification
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-Paragraph 12C
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482685/740-10-50-12C

Reference 7: http://www.xbrl.org/2003/role/disclosureRef
-Topic 740
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-Name Accounting Standards Codification
-Section 50
-Paragraph 12B
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482685/740-10-50-12B

Reference 8: http://www.xbrl.org/2003/role/disclosureRef
-Topic 740
-SubTopic 270
-Name Accounting Standards Codification
-Section 50
-Paragraph 1
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147477891/740-270-50-1

Reference 9: http://www.xbrl.org/2003/role/disclosureRef
-Topic 740
-SubTopic 10
-Name Accounting Standards Codification
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-Paragraph 1
-Subparagraph (SAB Topic 6.I.5.Q1)
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-URI https://asc.fasb.org/1943274/2147479360/740-10-S99-1

Reference 10: http://www.xbrl.org/2003/role/disclosureRef
-Topic 946
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-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480990/946-20-50-13

Reference 11: http://www.xbrl.org/2003/role/disclosureRef
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Reference 12: http://www.xbrl.org/2003/role/disclosureRef
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-URI https://asc.fasb.org/740/tableOfContent

Reference 13: http://www.xbrl.org/2003/role/disclosureRef
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-URI https://asc.fasb.org/1943274/2147482685/740-10-50-14

Reference 14: http://www.xbrl.org/2003/role/disclosureRef
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-URI https://asc.fasb.org/1943274/2147482685/740-10-50-21

Reference 15: http://www.xbrl.org/2003/role/disclosureRef
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Reference 16: http://www.xbrl.org/2003/role/disclosureRef
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Reference 17: http://www.xbrl.org/2003/role/disclosureRef
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-Section 50
-Paragraph 2
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482603/740-30-50-2

+ Details

Stock and Stock-Based Awards - Retention Awards - Additional Information (Details) - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended	12 Months Ended
	Sep. 30, 2025	Dec. 31, 2025	Dec. 31, 2024	Dec. 31, 2023
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Stock based compensation expense for stock options		$ 10,840	$ 20,971	$ 34,101
Service Based Restricted Stock Units (RSUs) [Member]
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Number of shares granted	68,596	79,846	75,066	154,904
Weighted average grant-date fair value of stock options	$ 18.93
Stock based compensation expense for stock options		$ 1,299
Retention Awards [Member]
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Stock based compensation expense for stock options		$ 163	$ 655	$ 92

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

+ Details

Summary of Significant Accounting Policies - Additional Information (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2025	Dec. 31, 2024	Dec. 31, 2023
Accounting Policies [Line Items]
Restricted cash	$ 8,668	$ 8,668
Impairment of long-lived assets	$ 0	$ 3,267	$ 0
Minimum percentage of likelihood of tax benefit being realized upon settlement	50.00%

Net Income (Loss) per Share (Tables)

12 Months Ended

Dec. 31, 2025

Earnings Per Share [Abstract]

Basic and Diluted Net Income (Loss) Per Share Attributable to Common Stockholders

Basic and diluted net income (loss) per share attributable to common stockholders was calculated as follows (in thousands, except share and per share data):


	Year Ended December 31,
	2025		2024			2023
Basic Earnings Per Share:
Numerator:
Net income (loss) from continuing operations attributable to common stockholders	$	5,696	$	(125,771	)	$	(190,130	)
Net income from discontinuing operations attributable to common stockholders	$	—	$	125,907		$	76,406
Net income (loss) attributable to common stockholders	$	5,696	$	136		$	(113,724	)

Denominator:
Weighted average common shares outstanding - basic		8,858,975		7,769,910			6,400,339
Net income (loss) from continuing operations per share attributable to common stockholders - basic	$	0.64	$	(16.20	)	$	(29.71	)
Net income from discontinued operations per share attributable to common stockholders - basic	$	—	$	16.20		$	11.94
Net income (loss) per share attributable to common stockholders - basic	$	0.64	$	0.00		$	(17.77	)

Diluted Earnings Per Share:
Numerator:
Net income (loss) from continuing operations attributable to common stockholders	$	5,696	$	(125,771	)	$	(190,130	)
Net income from discontinuing operations attributable to common stockholders	$	—	$	125,907		$	76,406
Net income (loss) attributable to common stockholders	$	5,696	$	136		$	(113,724	)

Denominator:
Weighted average common shares outstanding - basic		8,858,975		7,769,910			6,400,339

Dilutive impact from:
Stock options to purchase common stock		—		—			—
Unvested restricted stock units		10,749		—			—
Shares issuable under employee stock purchase plan		18		—			—
Weighted average common shares outstanding - diluted		8,869,742		7,769,910			6,400,339
Net income (loss) from continuing operations per share attributable to common stockholders - diluted	$	0.64	$	(16.20	)	$	(29.71	)
Net income from discontinued operations per share attributable to common stockholders - diluted	$	—	$	16.20		$	11.94
Net income (loss) per share attributable to common stockholders - diluted	$	0.64	$	—		$	(17.77	)

Anti-dilutive potential common stock equivalents excluded from the calculation of net income (loss) per share:
Stock options to purchase common stock		1,124,208		886,123			742,335
Unvested restricted stock units		24,150		81,459			168,730
Shares issuable under employee stock purchase plan		2,047		7,160			14,889
Warrants to purchase common stock		32,379		32,379			58,871

Segment Reporting - Additional Information (Detail)	12 Months Ended
Segment Reporting - Additional Information (Detail)	Dec. 31, 2025
Segment Reporting [Abstract]
Segment Reporting, CODM, Individual Title and Position or Group Name [Extensible Enumeration]	srt:ChiefExecutiveOfficerMember
Segment Reporting, CODM, Profit (Loss) Measure, How Used, Description	The CODM uses the Company’s consolidated net income (loss) to monitor actual results versus the budget in assessing segment performance and the allocation of resources. The measure of segment assets is reported on the consolidated balance sheets as total assets.

Stock and Stock-Based Awards - Restricted Stock Units - Additional Information (Details) - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended	12 Months Ended
	Sep. 30, 2025	Dec. 31, 2025	Dec. 31, 2024	Dec. 31, 2023
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense		$ 10,840	$ 20,971	$ 34,101
Aggregate Intrinsic Value, Outstanding		228	87
Share based compensation expenses		$ 10,840	$ 20,971	$ 34,101
Service Based Restricted Stock Units (RSUs) [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares granted	68,596	79,846	75,066	154,904
Stock option granted vesting period		4 years
Weighted average grant-date fair value of stock options	$ 18.93
Stock-based compensation expense		$ 1,299
Aggregate Intrinsic Value, Outstanding		$ 1,903	$ 1,899	$ 4,729
Service Based Restricted Stock Units (RSUs) [Member] \| Share-Based Payment Arrangement, Tranche One [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock option granted vesting period		1 year
Vesting rights percentage		25.00%
Service Based Restricted Stock Units (RSUs) [Member] \| Share-Based Payment Arrangement, Tranche Two [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting rights percentage		75.00%
Performance Shares [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense		$ 0	8	$ 2,051
Performance Based Restricted Stock Units (PSUs) [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares granted				66,135
Weighted average grant-date fair value of stock options				$ 110
Stock-based compensation expense		$ 0	$ 792	$ 4,293

Insider Trading Arrangements	3 Months Ended
Insider Trading Arrangements	Dec. 31, 2025 shares
Trading Arrangements, by Individual
Material Terms of Trading Arrangement	On November 25, 2025, Teresa L. Young, Ph.D., our Chief Commercial and Strategy Officer, adopted a Rule 10b5-1 plan (the “Young 10b5-1 Plan”) intended to satisfy the affirmative defense of Rule 10b5-1(c) of the Exchange Act. The plan provides for the periodic sale of (1) up to 31,680 shares of common stock and (2) the net number of shares of common stock underlying 1,152 unvested restricted stock units after giving effect to the number of shares to be sold outside of the Young 10b5-1 Plan to satisfy tax withholding obligations (which net amount is not currently determinable) between March 17, 2026 and March 12, 2027, unless otherwise terminated according to its terms. On November 26, 2025, Kelly Brady, M.S., our Executive Vice President, Chief Operating Officer, adopted a Rule 10b5-1 plan (the “Brady 10b5-1 Plan”) intended to satisfy the affirmative defense of Rule 10b5-1(c) of the Exchange Act. The plan provides for the periodic sale of (1) up to 7,611 shares of common stock and (2) the net number of shares of common stock underlying 797 unvested restricted stock units after giving effect to the number of shares to be sold outside of the Brady 10b5-1 Plan to satisfy tax withholding obligations (which net amount is not currently determinable) between March 17, 2026 and November 17, 2026, unless otherwise terminated according to its terms.
Teresa L. Young [Member]
Trading Arrangements, by Individual
Name	Teresa L. Young
Title	Chief Commercial and Strategy Officer
Rule 10b5-1 Arrangement Adopted	true
Adoption Date	November 25, 2025
Arrangement Duration	360 days
Aggregate Available	31,680
Kelly Brady [Member]
Trading Arrangements, by Individual
Name	Kelly Brady
Title	Executive Vice President, Chief Operating Officer
Rule 10b5-1 Arrangement Adopted	true
Adoption Date	November 26, 2025
Arrangement Duration	245 days
Aggregate Available	7,611

Condensed Consolidated Statement of Stockholders' Equity (Deficit) (Parenthetical) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2025	Dec. 31, 2024	Dec. 31, 2023
Market Equity Offering [Member]
Issuance costs	$ 498	$ 867	$ 772

Consolidated Statements of Stockholders' Equity (Deficit) - USD ($) $ in Thousands	Total	Common Stock [Member]	Additional Paid-in Capital [Member]	Accumulated Other Comprehensive Income [Member]	Accumulated Deficit [Member]
Beginning balance at Dec. 31, 2022	$ 10,783	$ 6	$ 875,300	$ (12)	$ (864,511)
Beginning balance, shares at Dec. 31, 2022		6,261,400
Issuance of common stock upon exercise of stock options, shares		13,028
Issuance of common stock upon exercise of stock options, value	877		877
Issuance of common stock upon vesting of RSUs and PSUs, net of tax withholdings, shares		61,958
Issuance of common stock under ESPP, shares		30,133
Issuance of common stock under ESPP, value	2,151		2,151
Issuance of common stock from at the market equity offering, shares		385,554
Issuance of common stock from at the market equity offering, value	18,159	$ 1	18,158
Issuance of warrants	2,785		2,785
Stock-based compensation expense	34,101		34,101
Other comprehensive income	12			12
Net income (loss)	(113,724)				(113,724)
Ending balance at Dec. 31, 2023	(44,856)	$ 7	933,372		(978,235)
Ending balance, shares at Dec. 31, 2023		6,752,073
Issuance of common stock upon vesting of RSUs and PSUs, net of tax withholdings, shares		123,602
Issuance of common stock under ESPP, shares		29,421
Issuance of common stock under ESPP, value	487		487
Issuance of common stock from at the market equity offering, shares		1,030,846
Issuance of common stock from at the market equity offering, value	23,530	$ 1	23,529
Issuance of common stock from Securities Purchase Agreement - related party, shares		714,285
Issuance of common stock from Securities Purchase Agreement - related party, value	13,516	$ 1	13,515
Stock-based compensation expense	20,971		20,971
Net income (loss)	136				136
Ending balance at Dec. 31, 2024	$ 13,784	$ 9	991,874		(978,099)
Ending balance, shares at Dec. 31, 2024	8,650,227	8,650,227
Issuance of common stock upon exercise of stock options, shares	23,900	23,900
Issuance of common stock upon exercise of stock options, value	$ 501		501
Issuance of common stock upon vesting of RSUs and PSUs, net of tax withholdings, shares		124,855
Issuance of common stock under ESPP, shares		17,839
Issuance of common stock under ESPP, value	230		230
Issuance of common stock from at the market equity offering, shares		739,545
Issuance of common stock from at the market equity offering, value	13,167	$ 1	13,166
Stock-based compensation expense	10,840		10,840
Adjustment for fractional shares due to reverse stock split		100
Net income (loss)	5,696				5,696
Ending balance at Dec. 31, 2025	$ 44,218	$ 10	$ 1,016,611	$ 0	$ (972,403)
Ending balance, shares at Dec. 31, 2025	9,556,446	9,556,466

Income Taxes - Schedule of Change In Unrecognized Tax Benefits Roll Forward (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2025	Dec. 31, 2024	Dec. 31, 2023
Income Tax Disclosure [Abstract]
Balance at beginning of year	$ 14,015	$ 13,529	$ 12,528
Increase in unrecognized tax benefits as a result of tax positions taken during the year	129	486	1,001
(Decrease) in unrecognized tax benefits as a result of tax positions taken during the year	(289)	0	0
Reduction to unrecognized tax benefits	0	0	0
Balance at end of year	$ 13,855	$ 14,015	$ 13,529

Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Thousands	Dec. 31, 2025	Dec. 31, 2024
Preferred stock, par value	$ 0.001	$ 0.001
Preferred stock, shares authorized	10,000,000	10,000,000
Preferred stock, shares issued	0	0
Preferred stock, shares outstanding	0	0
Common stock, par value	$ 0.001	$ 0.001
Common stock, shares authorized	360,000,000	360,000,000
Common stock, shares issued	9,556,446	8,650,227
Common stock, shares outstanding	9,556,446	8,650,227
SPN [Member]
Unbilled receivable	$ 0	$ 2,683

Summary of Significant Accounting Policies (Tables)

12 Months Ended

Dec. 31, 2025

Accounting Policies [Abstract]

Cash, Cash Equivalents and Restricted Cash

Cash, cash equivalents and restricted cash were comprised of the following (in thousands):


	December 31,
	2025		2024
Cash and cash equivalents	$	45,766	$	30,793
Restricted cash, non-current		8,668		8,668
Total cash, cash equivalents and restricted cash	$	54,434	$	39,461

Schedule of Property and Equipment Estimated Useful Life

Depreciation expense is recognized using the straight-line method over the useful life of the asset, which are as follows:


		Estimated Useful Life (In Years)
Laboratory equipment		5
Computer equipment, furniture and office equipment		3
Leasehold improvements		Lesser of useful life or lease term

Subsequent Events

12 Months Ended

Dec. 31, 2025

Subsequent Events [Abstract]

Subsequent Events

15. Subsequent Events

Executive Officer Transition

On February 27, 2026, the Board of Directors (the “Board”) of the Company appointed Richard N. Kender, a current member of the Board, to serve as Executive Chair of the Board and as Interim Chief Executive Officer of the Company, Matthew Henn, Ph.D., the Company’s Chief Scientific Officer, to the additional role of President, and Kelly Brady, M.S., the Company’s Senior Vice President, Clinical Development, to the role of Executive Vice President, Chief Operating Officer, each effective March 2, 2026 (the “Effective Date”).

As a result of the appointment of Mr. Kender as Interim Chief Executive Officer and Dr. Henn as President, Thomas J. DesRosier and Marella Thorell ceased serving as Co-Chief Executive Officers and Co-Presidents of the Company as of the Effective Date. Mr. DesRosier will continue to serve as the Company’s Executive Vice President, Chief Legal Officer, and Ms. Thorell will continue to serve as the Company’s Executive Vice President, Chief Financial Officer.

Stock and Stock-Based Awards

12 Months Ended

Dec. 31, 2025

Equity [Abstract]

Stock and Stock-Based Awards

7. Stock and Stock-Based Awards

On July 1, 2015, in connection with the closing of the initial public offering of the Company’s common stock (“IPO”), the Company effected its Restated Certificate of Incorporation, which authorizes the Company to issue 10,000,000 shares of preferred stock, $0.001 par value per share.

On April 10, 2025, at the Company's 2025 annual meeting of stockholders (the “2025 Annual Meeting”), stockholders approved an amendment to the Company’s Restated Certificate of Incorporation in order to effect a reverse stock split of all outstanding shares of the Company's common stock. On April 21, 2025, the Company effected a 1-for-20 reverse stock split of its common stock. The reverse stock split had no impact on the number of authorized shares or the par value of preferred and common stock. As of the effective time of the reverse stock split, the number of shares of common stock issuable upon exercise, vesting or settlement of outstanding awards, the exercise price of all outstanding options and any stock price vesting goals with respect to any outstanding awards under the Company’s equity plans (including the 2025 Plan, as defined below) was proportionately adjusted (and rounded down to the nearest whole share in the case of shares and up to the nearest whole cent in the case of exercise prices, as applicable) based on the 1-for-20 ratio. In addition, the number of shares available for future issuance and any share‑based award limits under the Company’s equity plans were proportionately reduced based on the 1-for-20 ratio. Trading of the Company's common stock on The Nasdaq Global Select Market commenced on a split-adjusted basis on April 22, 2025.

On September 30, 2024, the Company entered into the Securities Purchase Agreement with SPN, pursuant to which SPN purchased 714,285 shares at the Closing at a purchase price per share of $21.00, for an aggregate purchase price of $15,000. Under the terms of the Securities Purchase Agreement, SPN has agreed not to sell or transfer the shares for a period of six months after Closing, subject to certain customary exceptions. The Company agreed to register the resale of the Shares by SPN within 90 days of Closing. On October 1, 2024, the Company filed a registration statement to register the Shares, which became effective on October 11, 2024. In addition, under the terms of the Securities Purchase Agreement, for as long as SPN, together with its affiliates, beneficially owns at least 10% of the Company's outstanding shares of common stock, the Company has agreed to take such action within its control to include one individual designated by SPN in the slate of nominees recommended by the Company's board of directors (or the applicable committee of the board) to the Company's stockholders for election to the board at the applicable stockholder meeting. The Securities Purchase Agreement contains customary representations and warranties and closing conditions. The aggregate fair value of $13,516 for the common stock issued to SPN was recorded in equity, with the remaining $1,484 cash received from SPN under the Securities Purchase Agreement allocated to the consideration transferred for the VOWST Business.

On February 22, 2024, the Company’s board of directors adopted a resolution to amend the Restated Certificate of Incorporation, subject to stockholder approval, by increasing the number of authorized shares of the Company’s Common Stock from 240,000,000 shares to 360,000,000 shares (the “Share Increase Amendment”). At the Company’s annual meeting of stockholders held on April 4, 2024, the Company’s stockholders approved the Share Increase Amendment. On April 5, 2024, the Company amended its Restated Certificate of Incorporation to reflect the Share Increase Amendment.

In May 2021, the Company entered into a Sales Agreement (the “Sales Agreement”) with Cowen and Company, LLC ("Cowen") to sell shares of the Company’s common stock, with aggregate gross sales proceeds of up to $150,000, from time to time, through an "at-the-market" equity offering program (“ATM”) under which Cowen acts as sales agent. During the year ended

December 31, 2025, the Company sold 739,545 shares of common stock under the 2021 Sales Agreement, at an average price of approximately $18.48 per share, raising aggregate net proceeds of approximately $13,167 after deducting an aggregate commission of approximately 3% and other issuance costs. During the year ended December 31, 2024, the Company sold 1,030,846 shares of common stock under the 2021 Sales Agreement, at an average price of approximately $23.67 per share, raising aggregate net proceeds of approximately $23,530 after deducting an aggregate commission of approximately 3% and other issuance costs.

2015 and 2025 Incentive Award Plans

On June 16, 2015, the Company’s stockholders approved the 2015 Incentive Award Plan (the “2015 Plan”), which became effective on June 25, 2015. The 2015 Plan was subsequently amended on December 14, 2022, and provides for the grant of incentive stock options, nonqualified stock options, stock appreciation rights, restricted stock awards, restricted stock units and other stock-based awards. The number of shares initially reserved for issuance under the 2015 Plan was the sum of (i) 110,000 shares of common stock and (ii) the number of shares subject to awards outstanding under the 2012 Plan that expire, terminate or are otherwise surrendered, canceled, forfeited or repurchased by the Company on or after the effective date of the 2015 Plan. In addition, the number of shares of common stock that may be issued under the 2015 Plan was subject to increase on the first day of each calendar year, beginning in 2016 and ending in 2025, equal to the lesser of (i) 4% of the number of shares of the Company’s common stock outstanding on the last day of the preceding applicable calendar year and (ii) an amount determined by the Company’s board of directors.

On April 10, 2025, the Company's stockholders approved the 2025 Incentive Award Plan (the “2025 Plan”) as an amendment and restatement of the Seres Therapeutics, Inc. 2015 Incentive Award Plan. The amendment, among other things, authorized the issuance of 2,230,243 shares of the Company’s common stock for awards under the 2025 Plan, which includes 1,750,493 shares previously authorized for issuance under the 2015 Plan plus an increase of 479,750 shares (in each case, which amounts reflect the 1-for-20 reverse stock split), and extended the term of the 2025 Plan to March 3, 2035, the tenth anniversary of the approval of the 2025 Plan by the Company’s board of directors.

Stock awards granted under the 2025 Plan generally vest over four years and expire after ten years, although options have been granted with vesting terms less than four years. As of December 31, 2025, there were 740,410 shares available for future grant under the 2025 Plan.

2015 Employee Stock Purchase Plan

On June 16, 2015, the Company’s stockholders approved the 2015 Employee Stock Purchase Plan (the “ESPP”), which became effective on June 25, 2015. A total of 18,250 shares of common stock were reserved for issuance under the ESPP. In addition, the number of shares of common stock that may be issued under the ESPP automatically increase on the first day of each calendar year, beginning in 2016 and ending in 2025, by an amount equal to the lesser of (i) 20,000 shares, (ii) 1% of the number of shares of the Company’s common stock outstanding on the last day of the applicable preceding calendar year and (iii) an amount determined by the Company’s board of directors. Offering periods under the ESPP will commence when determined by the plan administrator. During the year ended and as of December 31, 2025, there were 17,839 shares issued and 106,064 shares were reserved and available for issuance under the ESPP, respectively.

The ESPP provides that eligible employees may contribute up to 15% of their eligible earnings toward the semi-annual purchase of the Company's common stock. Purchase rights issued under the ESPP are intended to be qualified under Section 423 of the Internal Revenue Code of 1986, as amended ("IRC"). The employee's purchase price is derived from a formula based on the closing price of the common stock on the first day of the offering period versus the closing price on the date of purchase (or, if not a trading day, on the immediately preceding trading day). The offering period under the ESPP has a duration of six months, and the purchase price with respect to each offering period beginning on or after such date is, until otherwise amended, equal to 85% of the lesser of (i) the fair market value of the Company's common stock at the commencement of the applicable six-month offering period or (ii) the fair market value of the Company's common stock on the purchase date.

2022 Employment Inducement Award Plan

On December 14, 2022, the Company’s board of directors approved the 2022 Employment Inducement Award Plan (the "2022 Plan"), which became effective on such date without stockholder approval pursuant to Rule 5635(c)(4) of The Nasdaq Stock Market LLC listing rules (“Rule 5635(c)(4)”). The 2022 Plan provides for the grant of nonqualified stock options, stock appreciation rights, restricted stock awards, restricted stock units and other stock- or cash-based awards. In accordance with Rule 5635(c)(4), awards under the 2022 Plan may only be made to a newly hired employee who has not previously been a member of our board of directors, or an employee who is being rehired following a bona fide period of non-employment by us as a material inducement to the employee’s entering into employment with us. A total of 125,000 shares of common stock were reserved for issuance under the 2022 Plan. Any shares subject to awards previously granted under the 2022 Plan that expire, terminate or are otherwise surrendered, canceled, or forfeited in any case, in a manner that results in the Company acquiring the shares covered by the award at a price not greater than the price (as adjusted to reflect any equity restructuring) paid by the Participant for such shares or not issuing any shares covered by the award, the unused shares covered by the award will again be available for award grants under the 2022 Plan.

As of December 31, 2025, there were 86,644 shares available for future grant under the 2022 Plan.

Stock Options

The following table summarizes the Company’s stock option activity for the year ended December 31, 2025:


	Number of Shares			Weighted Average Exercise Price		Weighted Average Remaining Contractual Term		Aggregate Intrinsic Value
						(in years)
Outstanding as of December 31, 2024		886,123		$	114.47		6.59	$	87
Granted		546,805		$	15.14
Exercised		(23,900	)	$	20.96
Forfeited		(284,820	)	$	114.77
Outstanding as of December 31, 2025		1,124,208		$	68.07		7.17	$	228
Vested and expected to vest as of December 31, 2025		1,124,208		$	68.07		7.17	$	228
Options exercisable as of December 31, 2025		502,733		$	129.06		5.15	$	31

In February 2024, the Company approved a repricing of certain stock option awards to reduce the original exercise price to the Company’s current fair market value as of February 12, 2024 (the “Repricing”). The Repricing became effective 18 months from the approval date, or August 2025. The remaining terms and conditions of each option affected by the Repricing remained the same, including the expiration date, vesting commencement date, vesting schedule and any vesting acceleration. As the Repricing was retrospective to February 2024, the weighted average exercise price for stock options outstanding as of December 31, 2024 in the table above has been adjusted to reflect the Repricing. The Repricing was accounted for as a modification, and an immaterial amount of incremental fair value is being recognized from the date the Repricing was approved over the remaining vesting term of the options.

The weighted average grant-date fair value of stock options granted during the years ended December 31, 2025, 2024, and 2023 was $13.30, $18.58, and $90.37 per share, respectively. The total intrinsic value of stock options exercised during the years ended December 31, 2025, 2024, and 2023 was $98, $0, and $438, respectively.

The aggregate intrinsic value of stock options is calculated as the difference between the exercise price of the stock options and the fair value of the Company’s common stock for those stock options that had exercise prices lower than the fair value of the Company’s common stock.

During the year ended December 31, 2021, the Company granted performance-based stock options to employees for the purchase of an aggregate of 28,100 shares of common stock with a grant date fair value of $110.60 per share. These stock options are exercisable only upon achievement of specified performance targets. In April 2023, the performance target associated with 50% of the performance-based stock options was achieved. Accordingly, the Company recorded $0, $8 and $2,051 of compensation expense during the years ended December 31, 2025, 2024 and 2023, respectively, with respect to these performance-based stock options, which represents a cumulative catch-up from the grant date through the achievement of the performance targets, and vesting of the remaining 50% of the options beginning in April 2023. The remaining compensation expense associated with these performance-based stock options was recognized as of April 2024, for all such options for which ongoing performance targets were achieved and service requirements were met.

During the three months ended March 31, 2024, the Company granted stock options to certain executives for the purchase of an aggregate of 127,500 shares of common stock. These awards will vest only to the extent that the 30-day trailing simple average public market closing price of the Company's common stock reaches certain price thresholds. These awards have an exercise price of $22.00 and vest and become exercisable when the market conditions are satisfied or, if later, on the first anniversary of the grant date. These awards expire 10 years from the date of grant. The fair value of these market-based stock options was estimated using a Monte Carlo valuation method. During the years ended December 31, 2025 and 2024, the Company recognized $641 and $775 of compensation expense related to these awards.

Restricted Stock Units

The Company has granted restricted stock units with service-based vesting conditions ("RSUs") and restricted stock units with performance-based vesting conditions ("PSUs"). RSUs and PSUs represent the right to receive shares of common stock upon meeting specified vesting requirements. Restricted stock units may not be sold or transferred by the holder and vest according to the vesting conditions of each award. The table below summarizes the Company’s RSU and PSU activity for the year ended December 31, 2025:


	Number of Shares			Weighted Average Grant Date Fair Value
Unvested restricted stock units as of December 31, 2024		81,459		$	55.36
Granted		79,846		$	18.29
Forfeited		(12,300	)	$	62.76
Vested		(124,855	)	$	30.54
Unvested restricted stock units as of December 31, 2025		24,150		$	57.31

During the years ended December 31, 2025, 2024, and 2023, the Company granted 79,846, 75,066 and 154,904 RSUs, respectively. RSUs generally vest over four years, with 25% vesting after one year, and the remaining 75% vesting quarterly over the next 3 years, subject to continued service to the Company through the applicable vesting date. PSUs vest according to the performance requirements of the awards, generally when the Company has determined that the specified performance targets have been achieved.

During the year ended December 31, 2023, the Company granted PSUs to employees covering an aggregate of 66,135 shares of common stock with a grant date fair value of $110.00. These PSUs begin to vest ratably only upon achievement of specified performance targets, which were achieved in April 2023. Accordingly, the Company recorded $0, $792 and $4,293 in compensation expense during the years ended December 31, 2025, 2024, and 2023 respectively, with respect to these PSUs.

The aggregate intrinsic value of RSUs, including PSUs for which the performance conditions have been met, that vested during the years ended December 31, 2025, 2024 and 2023 was $1,903, $1,899, and $4,729, respectively.

Retention Awards

In September 2025, the Company issued retention awards to employees of the Company in the form of RSUs covering 68,596 shares of common stock with a grant date fair value of $18.93 per share. The retention RSUs fully vested on November 15, 2025 subject to the employee's continued employment with the Company through such date. The compensation expense associated with these awards was recognized ratably over the vesting period. For the year ended December 31, 2025, the Company recognized $1,299 in compensation expense with respect to the retention RSUs.

In November 2023, as part of the corporate restructuring described in Note 9, Restructuring, the Company issued retention awards to employees of the Company in the form of RSUs, which vested as to the first tranche on August 15, 2024 and as to the second tranche on May 15, 2025, subject to remaining actively employed with the Company through such date. The compensation expense associated with these awards was recognized ratably over the vesting period. For the years ended December 31, 2025, 2024, and 2023, the Company recognized $163, $655 and $92, respectively, in compensation expense with respect to the retention awards.

Stock-based Compensation Valuation

The assumptions that the Company used to determine the fair value of the stock options granted to employees and directors were as follows, presented on a weighted average basis:


	Year Ended December 31,
	2025			2024			2023
Risk-free interest rate		4.26	%		2.70	%		3.64	%
Expected term (in years)		6.0			6.0			6.0
Expected volatility		121.0	%		76.2	%		107.2	%
Expected dividend yield		0	%		0	%		0	%

The Company estimates the fair value of rights to acquire common stock under the ESPP using a Black-Scholes valuation model on the date of grant and the straight-line attribution approach to recognize the expense. The assumptions that the Company used to determine the fair value of rights to acquire common stock under the ESPP were as follows, presented on a weighted average basis:


	Year Ended December 31,
	2025			2024			2023
Risk-free interest rate		4.34	%		5.16	%		5.01	%
Expected term (in years)		0.5			0.5			0.5
Expected volatility		115.8	%		118.3	%		79.1	%
Expected dividend yield		0	%		0	%		0	%

Stock-based Compensation

The Company recorded stock-based compensation expense related to stock options and restricted stock units in the following expense categories of its consolidated statements of operations and comprehensive income (loss):


	Year Ended December 31,
	2025		2024		2023
Research and development expenses	$	5,080	$	10,996	$	19,341
General and administrative expenses		5,760		9,975		14,760
	$	10,840	$	20,971	$	34,101

As of December 31, 2025, the Company had an aggregate of $9,986 of unrecognized stock-based compensation cost, which is expected to be recognized over a weighted average period of 3.2 years.

Summary of Significant Accounting Policies (Policies)

12 Months Ended

Dec. 31, 2025

Accounting Policies [Abstract]

Discontinued Operations

Use of Estimates

Restricted Cash

Cash, cash equivalents and restricted cash were comprised of the following (in thousands):


	December 31,
	2025		2024
Cash and cash equivalents	$	45,766	$	30,793
Restricted cash, non-current		8,668		8,668
Total cash, cash equivalents and restricted cash	$	54,434	$	39,461

Concentration of Credit Risk

Property and Equipment

Property and equipment are stated at cost less accumulated depreciation. Depreciation expense is recognized using the straight-line method over the useful life of the asset, which are as follows:


		Estimated Useful Life (In Years)
Laboratory equipment		5
Computer equipment, furniture and office equipment		3
Leasehold improvements		Lesser of useful life or lease term

Impairment of Long-Lived Assets

Research and Development Costs

Research Contract Costs and Accruals

Patent Costs

Accounting for Stock-Based Compensation

The Company accounts for forfeitures of stock-based awards as they occur rather than applying an estimated forfeiture rate to stock-based compensation expense.

Grant Revenue

Income Taxes

Segment Reporting

Comprehensive Income (Loss)

Net Loss per Share

Net Income (Loss) per Share

Leases

a gain or loss in operating expenses, or as an adjustment to the right-of-use asset, as appropriate, based on the terms and conditions within the lease that are amended.

Recently Adopted Accounting Pronouncements

Recently Issued Accounting Pronouncements

Consolidated Statements of Operations and Comprehensive Income (Loss) (Parenthetical) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2025	Dec. 31, 2024	Dec. 31, 2023
Unrealized (loss) gain on investments, net of tax	$ 0	$ 0	$ 0
SPN [Member]
Reimbursement received	$ 13,311	$ 6,292

Segment Reporting - Schedule of reportable segment net income (loss) (Parenthetical) (Detail)	12 Months Ended
	Dec. 31, 2025
Segment Reporting [Abstract]
Other segment, Description	Other segment (income) expense includes manufacturing services expenses, research and development expenses on early stage programs, interest income and other (income) expense, net.

Stock and Stock-Based Awards (Tables)

12 Months Ended

Dec. 31, 2025

Subsidiary, Sale of Stock [Line Items]

Summary of Stock Option Activity

The following table summarizes the Company’s stock option activity for the year ended December 31, 2025:


	Number of Shares			Weighted Average Exercise Price		Weighted Average Remaining Contractual Term		Aggregate Intrinsic Value
						(in years)
Outstanding as of December 31, 2024		886,123		$	114.47		6.59	$	87
Granted		546,805		$	15.14
Exercised		(23,900	)	$	20.96
Forfeited		(284,820	)	$	114.77
Outstanding as of December 31, 2025		1,124,208		$	68.07		7.17	$	228
Vested and expected to vest as of December 31, 2025		1,124,208		$	68.07		7.17	$	228
Options exercisable as of December 31, 2025		502,733		$	129.06		5.15	$	31

Summary of Restricted and Performance Stock Unit Activity

The table below summarizes the Company’s RSU and PSU activity for the year ended December 31, 2025:


	Number of Shares			Weighted Average Grant Date Fair Value
Unvested restricted stock units as of December 31, 2024		81,459		$	55.36
Granted		79,846		$	18.29
Forfeited		(12,300	)	$	62.76
Vested		(124,855	)	$	30.54
Unvested restricted stock units as of December 31, 2025		24,150		$	57.31

Summary of Stock Based Compensation Expense


	Year Ended December 31,
	2025		2024		2023
Research and development expenses	$	5,080	$	10,996	$	19,341
General and administrative expenses		5,760		9,975		14,760
	$	10,840	$	20,971	$	34,101

Employees Stock Option [Member]

Subsidiary, Sale of Stock [Line Items]

Summary of Estimated Fair Value of Employee Stock Options

The assumptions that the Company used to determine the fair value of the stock options granted to employees and directors were as follows, presented on a weighted average basis:


	Year Ended December 31,
	2025			2024			2023
Risk-free interest rate		4.26	%		2.70	%		3.64	%
Expected term (in years)		6.0			6.0			6.0
Expected volatility		121.0	%		76.2	%		107.2	%
Expected dividend yield		0	%		0	%		0	%

Employee Stock [Member]

Subsidiary, Sale of Stock [Line Items]

Summary of Estimated Fair Value of Employee Stock Options

The assumptions that the Company used to determine the fair value of rights to acquire common stock under the ESPP were as follows, presented on a weighted average basis:


	Year Ended December 31,
	2025			2024			2023
Risk-free interest rate		4.34	%		5.16	%		5.01	%
Expected term (in years)		0.5			0.5			0.5
Expected volatility		115.8	%		118.3	%		79.1	%
Expected dividend yield		0	%		0	%		0	%

Income Taxes - Summary of Changes In Valuation Allowance for Deferred Tax Assets (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2025	Dec. 31, 2024	Dec. 31, 2023
Valuation Allowance [Abstract]
Valuation allowance at beginning of year	$ (309,742)	$ (312,809)	$ (277,370)
Decreases recorded as benefit to income tax provision		3,067
Increases recorded to income tax provision	(1,219)		(35,439)
Valuation allowance as of end of year	$ (310,961)	$ (309,742)	$ (312,809)

Stock and Stock-Based Awards - 2022 Employment Inducement Award Plan - Additional Information (Details) - 2022 Employment Inducement Award Plan [Member] - shares	12 Months Ended
	Dec. 31, 2025	Dec. 14, 2022
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Stock incentive plan description	125,000 shares of common stock were reserved for issuance under the 2022 Plan. Any shares subject to awards previously granted under the 2022 Plan that expire, terminate or are otherwise surrendered, canceled, or forfeited in any case, in a manner that results in the Company acquiring the shares covered by the award at a price not greater than the price (as adjusted to reflect any equity restructuring) paid by the Participant for such shares or not issuing any shares covered by the award, the unused shares covered by the award will again be available for award grants under the 2022 Plan.
Number of shares initially reserved for issuance		125,000
Shares available for future grant	86,644

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

+ Details

Income Taxes - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2025	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Income Tax Disclosure [Line Items]
Income tax expense (benefit)	$ 0	$ 0	$ 0
Unamortized research and experimental expenditures acceleration period	one or two years
Percentage of research and experimental expense	100.00%
Cash payments or refunds for income taxes	$ 0
Unrecognized tax benefits	$ 13,855,000	$ 14,015,000	$ 13,529,000	$ 12,528,000
Massachusetts [Member]
Income Tax Disclosure [Line Items]
Net operating loss carry forwards, expiration year, start	2035
Future Tax Years [Member] \| Maximum [Member]
Income Tax Disclosure [Line Items]
Taxable income on NOL	80.00%
Federal [Member]
Income Tax Disclosure [Line Items]
Net operating loss carryforwards	$ 616,412,000
Research and development performance period	5 years
Federal [Member] \| Before 2018 [Member]
Income Tax Disclosure [Line Items]
Net operating loss carryforwards	$ 98,756,000
Net operating loss carry forwards, expiration year, start	2035
Federal [Member] \| After 2017 [Member]
Income Tax Disclosure [Line Items]
Net operating loss carryforwards	$ 517,656,000
Federal [Member] \| Research and Development Tax Credit [Member]
Income Tax Disclosure [Line Items]
Tax credit carryforwards	$ 55,341,000
Tax credit carryforwards, expiration year, start	2031
Federal [Member] \| Orphan Drug Credit Carryforwards [Member]
Income Tax Disclosure [Line Items]
Tax credit carryforwards	$ 25,876,000
Foreign [Member]
Income Tax Disclosure [Line Items]
Research and development performance period	15 years
State [Member]
Income Tax Disclosure [Line Items]
Net operating loss carryforwards	$ 597,758,000
State [Member] \| Research and Development Tax Credit [Member]
Income Tax Disclosure [Line Items]
Tax credit carryforwards	$ 14,767,000
Tax credit carryforwards, expiration year, start	2028

Commitments and Contingencies

12 Months Ended

Dec. 31, 2025

Commitments and Contingencies Disclosure [Abstract]

Commitments and Contingencies

10. Commitments and Contingencies

Leases

Refer to Note 6, Leases, for discussion of the commitments associated with the Company’s lease portfolio.

Indemnification Agreements

In the ordinary course of business, the Company may provide indemnification of varying scope and terms to vendors, lessors, business partners and other parties with respect to certain matters including, but not limited to, losses arising out of breach of such agreements or from intellectual property infringement claims made by third-parties. In addition, the Company has entered into indemnification agreements with members of its board of directors and its officers that will require the Company, among other things, to indemnify them against certain liabilities that may arise by reason of their status or service as directors or officers. The maximum

potential amount of future payments the Company could be required to make under these indemnification agreements is, in many cases, unlimited. To date, the Company has not incurred any material costs as a result of such indemnifications. The Company does not believe that the outcome of any claims under indemnification arrangements will have a material effect on its financial position, results of operations or cash flows, and it has not accrued any liabilities related to such obligations in its consolidated financial statements as of December 31, 2025 or 2024.

Legal Contingencies

The Company accrues a liability for legal contingencies when it believes that it is both probable that a liability has been incurred and that the Company can reasonably estimate the amount of the loss. The Company reviews these accruals and adjusts them to reflect ongoing negotiations, settlements, rulings, advice of legal counsel and other relevant information. To the extent new information is obtained and the views on the probable outcomes of claims, suits, assessments, investigations or legal proceedings change, changes in the Company’s accrued liabilities would be recorded in the period in which such determination is made.

In addition, in accordance with the relevant authoritative guidance, for any matters in which the likelihood of material loss is at least reasonably possible, the Company will provide disclosure of the possible loss or range of loss. If a reasonable estimate cannot be made, however, the Company will provide disclosure to that effect. The Company expenses legal costs as they are incurred.

The Company did not accrue any liabilities related to legal contingencies in its consolidated financial statements as of December 31, 2025 or 2024.