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SERES THERAPEUTICS, INC. filed this Form 10-K on 03/16/2017
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Phase 1b/2 clinical study design. The Phase 1b/2 clinical study was a two-part trial designed to evaluate the safety and efficacy of SER-109 in 30 patients with recurrent CDI, defined as three or more occurrences of CDI in the previous 12 months. Part 1 of the study evaluated a single dose of SER-109 administered orally in 30 capsules over two days, with the dose derived from approximately 75 grams of stool. Part 2 of the study evaluated a single dose of SER-109 administered orally in a range of one to 7 capsules over one day. The target dose in Part 2 was 1x108 spores per dose, which was approximately 17-fold lower than the mean dose in Part 1.




Mean Dose

(spore units)





Median (Range)


Number of CDI

Recurrences in

Prior 12 months

Median (Range)





5 / 10


71 years (22 – 88)


3 (2 – 6)





5 / 10


58 years (39 – 83)


3 (2 – 5)


Phase 1b/2 clinical study results.  The primary efficacy measure was the absence of CDI (defined in this study as more than three unformed bowel movements in a 24-hour period with laboratory confirmation of a positive C. difficile stool test) during the eight weeks after initiating therapy. Twenty-six of 30 patients, or 87% of patients, in the Phase 1b/2 clinical study achieved the primary efficacy endpoint, consisting of 13 patients in each of Part 1 and Part 2 of the study. Among the 26 patients was one patient who experienced an initial recurrence on Day 26 and was re-treated, per protocol, with a dose from the same donor. Following re-treatment, this patient also achieved the primary efficacy endpoint. Of the patients who did not meet the primary efficacy endpoint, one had a recurrence of CDI on Day 5 and did not receive a second treatment with SER-109 and the three other patients were determined by their attending investigator to be recovering from their diarrheal episode by the time they submitted their stool sample for CDI testing. The three patients were determined to be clinically CDI free at eight weeks. As a result, the clinical cure rate for the study, which we defined as the absence of CDI requiring antibiotic treatment during the eight-week period after SER-109 dosing, was 97%, or 29 of 30 patients.

The primary safety measures were an evaluation of adverse events, laboratory values, vital signs and physical examination findings prior to and after dosing with SER-109 over a 24-week time period. In Part 1 of the study, 80% of the patients experienced at least one adverse event, all of which were treatment emergent adverse events, or TEAEs. A TEAE was defined as an adverse event that started or worsened at or during the time of or after the date of the first dose of SER-109 through the final follow-up visit. Five, or 33%, of the patients were judged by the investigator to have a TEAE attributable to SER-109 and all were mild or moderate. In Part 2, 100% of the patients experienced at least one adverse event, all of which were TEAEs. Nine, or 60%, of the patients were judged by the investigator to have a TEAE attributable to SER-109 and all were mild or moderate. The most common adverse events were gastrointestinal disorders and diarrhea. The majority of TEAEs were mild in severity and consistent with post-antibiotic recovery from CDI. One patient in Part 2 had a severe adverse event of chest pain, which was not considered related to SER-109. Two patients each in Part 1 and Part 2 had more than one serious adverse event, none of which was considered related to SER-109. There were no deaths in Part 1 or Part 2.

Phase 2 clinical study design. The Phase 2 clinical study was a randomized, double-blinded, placebo-controlled, parallel-group two arm trial that enrolled a total of 89 patients with a history of multiply-recurrent CDI, defined as 3 or more CDI episodes within 9 months. The Phase 2 clinical study enrolled 89 subjects with multiply recurrent CDI, in a randomized, double-blind, placebo-controlled 24-week study conducted to evaluate the safety and efficacy of SER-109. Subjects were randomized at a 2:1 ratio with 59 subjects receiving SER-109 and 30 subjects receiving placebo. SER-109 was administered orally as a single dose of 1x108 bacterial spores, following the completion of antibiotic treatment for CDI. The study was conducted at 36 centers across the United States. The primary endpoint was the absence of recurrence of C. difficile positive diarrhea requiring antibiotic treatment up to 8 weeks following treatment with SER-109 or placebo.

Phase 2 clinical study results. The predefined study primary efficacy endpoint was the relative risk of CDI recurrence up to 8 weeks after treatment with SER-109 compared to treatment with placebo.  CDI recurrence was defined as diarrhea for 2 or more consecutive days, a positive CDI test, and the requirement for antibiotic treatment. Based on 8-week data, CDI recurrence occurred in 44% of subjects (26 of 59) who received SER-109, compared to 53% of subjects (16 of 30) who received placebo. The relative risk of CDI recurrence for the placebo population compared to the SER-109 population was not statistically significant. As part of the prespecified design, subjects were stratified into two groups: less than 65 years old and 65 years old and older . In subjects less than 65 years old, CDI recurrence occurred in 43% of subjects who received SER-109 (12 of 28) and in 27% of subjects who received placebo (4 of 15). In subjects 65 years and older, CDI recurrence occurred in 45% of subjects who received SER-109 (14 of 31), and in 80% of those who received placebo (12 of 15).

The most commonly reported adverse events in both the SER-109 and placebo arms were in the gastrointestinal category, and were diarrhea (25% vs 14%), abdominal pain (22% vs 14%), flatulence (12% vs 3%), and nausea (10% vs 10%), for SER-109 and Placebo, respectively. No drug-related serious adverse events were observed. The SER-109 analyses were recently shared with the FDA. Based on feedback received from the FDA, we plan to initiate a new Phase 2 SER-109 clinical study in approximately 320 patients with multiply recurrent C. difficile infection. Study participants will be randomized 1:1 between SER-109 and placebo and will receive a dose that is approximately 10-fold higher than in the first Phase 2, administered over


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