Peer-reviewed publication reports a 77% relative risk reduction in bloodstream infections and significantly lower systemic antibiotic exposure with SER-155 compared with placebo

Translational data from the study support the SER-155 mechanisms of strengthening the mucosal epithelial barrier and reducing systemic inflammation

CAMBRIDGE, Mass., Nov. 03, 2026 (GLOBE NEWSWIRE) -- Seres Therapeutics, Inc. (Nasdaq: MCRB), a leading live biotherapeutics company, today announced that results from its Phase 1b clinical study of SER-155 in adults undergoing allogeneic hematopoietic cell transplantation (allo-HCT) have been published in the November 3, 2026 issue of Nature Medicine. SER-155 is an investigational, oral, cultivated multi-strain live biotherapeutic product (LBP) designed to reduce the risk of bloodstream infections (BSIs) in medically vulnerable patients. The publication reports that SER-155 was generally well tolerated and was associated with a 77% relative risk reduction in BSIs compared with placebo. SER-155 has received Breakthrough Therapy designation from the U.S. Food and Drug Administration (FDA) for the reduction of BSIs in adults undergoing allo-HCT, as well as Fast Track designation, and the Company has aligned with the FDA on its proposed Phase 2 study protocol. SER-155 is Phase 2-ready in allo-HCT, pending receipt of funding.

“Publication in Nature Medicine is an important milestone for the SER-155 program and recognition of the innovative science being advanced by Seres,” said Matthew Henn, Ph.D., President and Chief Scientific Officer of Seres Therapeutics. “Many bloodstream infections in transplant patients originate in the gut, when pathogens expand and cross a damaged intestinal barrier. SER-155 was designed to address both sides of that problem by preventing the colonization and expansion of pathogens, many that can harbor antibiotic resistance, in the gastrointestinal tract that results from extensive antibiotic use in these patients and also protecting and improving mucosal epithelial barrier integrity that is disrupted by conditioning cancer therapy to limit bacterial translocation from the GI into the bloodstream and potential mortality due to infection. In this study, SER-155 reduced bloodstream infections while improving biomarkers of barrier integrity and systemic inflammation, providing clinical validation of these mechanisms. We believe our live biotherapeutic approach could be highly differentiated in multiple diseases where loss of gut mucosal epithelial barrier integrity is thought to play a central role. In addition, SER-155 has the potential to address the continued emergence of antimicrobial resistant infections, particularly those in immunocompromised patients, and we thank CARB-X for their prior support of this program, our collaborators and the patients who participated in this clinical study.”

The Phase 1b study (NCT04995653) evaluated SER-155 in adults undergoing allo-HCT for hematologic malignancies. Cohort 1 was an open-label cohort that assessed safety and drug pharmacology, including engraftment of SER-155 strains in the gastrointestinal (GI) tract. Cohort 2 was a randomized, double-blind, placebo-controlled cohort that evaluated safety, drug pharmacology and clinical outcomes, including BSIs, systemic antibiotic exposure and febrile neutropenia, through day 100 after transplant.

In Cohort 2, BSIs occurred in 10% of patients receiving SER-155 compared with 42.9% of patients receiving placebo, a 77% relative risk reduction (p=0.0423). Patients receiving SER-155 also had significantly lower systemic antibiotic exposure, with a mean of 9.2 days compared with 21.1 days for placebo (p=0.0494), and a numerically lower incidence of febrile neutropenia (65% compared with 78.6%). SER-155 strains engrafted in the GI tract, and SER-155 was generally well tolerated, with no treatment-related serious adverse events observed.

Translational biomarker analyses from the study further support SER-155 mechanisms of action. Compared with placebo, SER-155 led to a statistically significant decrease in fecal albumin, an established biomarker of epithelial barrier integrity, and lower plasma levels of biomarkers of systemic inflammation. SER-155 also improved biomarkers of immune homeostasis. Improved mucosal epithelial barrier integrity may reduce bacterial translocation from the GI tract into the bloodstream, consistent with the reduction in BSIs observed in the study. These barrier-protective and anti-inflammatory effects also support the potential of Seres’ LBPs in inflammatory and immune diseases, including immune checkpoint inhibitor related enterocolitis (irEC) and inflammatory bowel disease (IBD).

“These results, now peer-reviewed and published in Nature Medicine, reinforce our belief in the potential of SER-155 to prevent bloodstream infections in immunocompromised patients like allo-HCT patients,” said Richard Kender, Executive Chairman and Interim Chief Executive Officer of Seres Therapeutics. “We believe the ability of SER-155 to strengthen the gut barrier and reduce inflammation gives it potential well beyond transplant, including in immune checkpoint inhibitor-related enterocolitis (irEC), a serious complication of cancer immunotherapy where an immunosuppressive-free treatment could allow patients to stay on their cancer therapy.”

Seres is also developing SER-155 for irEC, a frequent and often severe adverse reaction that can force patients to interrupt or discontinue immune checkpoint inhibitor (ICI) cancer therapy and initiate immunosuppressive therapy. In July 2026, Seres reported results from an open-label investigator-sponsored trial at Memorial Sloan Kettering Cancer Center in 15 patients with Grade 2-3 irEC who had not received immunosuppressive therapy. Twelve of 15 participants (80%) achieved an immunosuppressive-free clinical response at day 15, the study’s primary endpoint, and SER-155 was generally well tolerated, with no serious adverse events assessed as related to SER-155. In the irEC study, fecal calprotectin and fecal albumin, markers of gut inflammation and mucosal barrier integrity, decreased significantly from baseline by day 43, consistent with the barrier-protective activity observed in allo-HCT.

Publication Details
Journal: Nature Medicine
Title: A live biotherapeutic product for adult allogeneic hematopoietic cell transplantation recipients: a partially randomized, placebo-controlled phase 1b trial
Publication Date & Access: November 3, 2026
Authors: Doris M. Ponce*, Zachariah DeFilipp*, Christopher B. Ford, Bina Tejura, Mary-Jane Lombardo, David I. Lichter, Meghan Chafee, Satyajit Kosuri, Nandita Khera, Hemant S. Murthy, Karamjeet S. Sandhu, David N. Fredricks, Tessa Andermann, Rajneesh Nath, Kelly Brady, Brooke R. Hasson, Emily Walsh, Nathan Hicks, Jonathan U. Peled, Lisa von Moltke, Matthew R. Henn, Marcel R.M. van den Brink**, Dennis M. Walling** and the SER-155-001 Study Team***. *These authors contributed equally. **These authors jointly supervised this work. ***A list of consortium authors and their affiliations appears at the end of the paper.

About Seres Therapeutics
Seres Therapeutics, Inc. (Nasdaq: MCRB) is a clinical-stage biotechnology company developing novel live biotherapeutics products in oncology that address causes of mortality and discontinuation of life-saving cancer treatment, and in inflammatory and immune (I&I) diseases, by modulating host function to protect and improve mucosal epithelial barrier integrity, induce immune homeostasis and tolerance, and prevent the colonization and overgrowth of pathogens in the gastrointestinal (GI) tract. The Company previously led the development and FDA approval of VOWST™, the first orally administered microbiome therapeutic, which was subsequently divested to Nestlé Health Science. SER-155, an investigational cultivated multi-strain biotherapeutic, which has received Breakthrough Therapy and Fast Track designations, is being advanced for the prevention of bloodstream infections in patients undergoing allogeneic hematopoietic stem cell transplant (allo-HCT), and is Phase 2 ready, pending receipt of funding. SER-155 is also being developed to prevent immune checkpoint inhibitor-related enterocolitis (irEC), with the aim of enabling patients to continue their ICI cancer therapy and avoid the need for immunosuppressive steroid treatment. Following promising results from an investigator-sponsored trial, Seres plans to submit the design of a study for the prevention of irEC in patients initiating ICI therapy to the FDA. The Company is advancing IND-enabling studies for SER-603, which is in development for inflammatory bowel disease. Mechanistically, Seres' biotherapeutics target the mucosal epithelial barrier-immune interface and are optimized to modulate host function to increase mucosal epithelium integrity, induce immune homeostasis, and prevent the colonization and overgrowth of harmful bacteria in the GI tract. For more information, please visit www.serestherapeutics.com. 

About CARB-X
CARB-X (Combating Antibiotic-Resistant Bacteria Biopharmaceutical Accelerator) is the world’s leading non-profit partnership accelerating the research and development of innovative products to prevent, diagnose and treat bacterial infections and address the global threat of antimicrobial resistance. CARB-X pairs non-dilutive funding with comprehensive technical and scientific support, resulting in supported projects being five times more likely to advance into clinical development than comparable projects that did not receive CARB-X support. Research reported in this press release is partially supported by CARB-X. CARB-X is funded in part with federal funds from the U.S. Department of Health and Human Services (HHS); Administration for Strategic Preparedness and Response; Biomedical Advanced Research and Development Authority (BARDA) under agreement number 75A50122C00028 and by awards from Wellcome (WT224842), the UK Department of Health and Social Care’s Global Antimicrobial Resistance Innovation Fund (GAMRIF), the Gates Foundation, Germany’s Federal Ministry of Research, Technology and Space (BMFTR), the Novo Nordisk Foundation, Italy’s Ministry of Economy and Finance (MEF), Japan’s Ministry of Health, the European Commission’s DG Health Emergency Preparedness and Response Authority (DG HERA), and KfW Development Bank. The U.S. National Institute of Allergy and Infectious Diseases (NIAID), part of the National Institutes of Health (NIH) in HHS, provides support in the form of in-kind services through access to a suite of preclinical services for product development. The content of this press release is solely the responsibility of the authors and does not necessarily represent the official views of CARB-X or its funders

Forward-Looking Statements
This press release contains forward-looking statements within the meaning of the Private Securities Litigation Reform Act of 1995. All statements contained in this press release that do not relate to matters of historical fact should be considered forward-looking statements, including, without limitation, statements about: SER-155 and its intended uses and benefits in allo-HCT and irEC; our clinical development plans for SER-155; the timing and results of clinical studies and data readouts; current or future product candidates and their potential impacts and outcomes; our ability to obtain funding or enter into partnerships; the anticipated timing of any of the foregoing; and other statements that are not historical fact.

These forward-looking statements are based on management's current expectations. These statements are neither promises nor guarantees, but involve known and unknown risks, uncertainties and other important factors that may cause our actual results, performance or achievements to be materially different from any future results, performance or achievements expressed or implied by the forward-looking statements, including, but not limited to, the following: (1) our need for additional funding; (2) our ability to continue as a going concern; (3) we have incurred significant losses, are not currently profitable and may never become profitable; (4) our cost reduction actions may not achieve their intended benefits, including an extended cash runway; (5) our limited operating history; (6) we may not be able to realize the anticipated benefits of the VOWST sale, and may face new challenges as a smaller, less diversified company; (7) we have in the past and may in the future receive notice of the failure to satisfy a continued listing rule from The Nasdaq Stock Market LLC; (8) our novel approach to therapeutic intervention; (9) our reliance on third parties to conduct our clinical trials and manufacture our product candidates; (10) our ability to achieve market acceptance necessary for commercial success; (11) the competition we will face; (12) our ability to protect our intellectual property; (13) impact of our recent management transitions and appointments and our ability to retain key personnel; and (14) disruptions at the FDA or other government agencies. These and other important factors discussed under the caption "Risk Factors" in our Quarterly Report on Form 10-Q for the quarterly period ended June 30, 2026, filed with the Securities and Exchange Commission (SEC), as well as our other reports filed with the SEC, could cause actual results to differ materially from those indicated by the forward-looking statements made in this press release. Any such forward-looking statements represent management's estimates as of the date of this press release. While we may elect to update such forward-looking statements at some point in the future, except as required by law, we disclaim any obligation to do so, even if subsequent events cause our views to change. These forward-looking statements should not be relied upon as representing our views as of any date subsequent to the date of this press release.

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