Publication of ECOSPOR lll Phase 3 Study Data Evaluating SER-109 in Recurrent C. Difficile Infection

Seres Therapeutics, Inc. (Nasdaq: MCRB), a leading microbiome therapeutics company, today announced the publication in the New England Journal of Medicine (NEJM) of data from its Phase 3 ECOSPOR III study evaluating SER-109, an investigational oral microbiome therapeutic for the treatment of recurrent C. difficile infection (rCDI). The publication highlights data that found SER-109 to be superior to placebo in reducing CDI recurrence, with 88% of SER-109 patients achieving a sustained clinical response compared to 60% on placebo. SER-109 was also found to be well tolerated, with a side effect profile comparable to placebo and no serious drug-related adverse events observed. Seres expects to finalize a BLA submission for SER-109 with the U.S. Food and Drug Administration (FDA) in mid-2022.

“The publication of these data in the New England Journal of Medicineunderscores the potential of SER-109 and its ability to provide safe, effective treatment to prevent recurrent C. difficile infection – of which there are 170,000 annual cases in the U.S.,”

said Lisa von Moltke, M.D., Chief Medical Officer at Seres.

“These robust findings reinforce our belief that microbiome therapeutics have the potential to transform the way we treat serious diseases and help maximize opportunities in infection prevention based upon the proven mechanism of SER-109.”

The SER-109 ECOSPOR III Phase 3 study ( identifier: NCT03183128) was a multicenter, randomized, placebo-controlled study. Previously reported topline data demonstrated that the study achieved its primary endpoint where SER-109 was superior to placebo in reducing CDI recurrence at eight weeks, reflecting a sustained clinical response rate of approximately 88% at eight weeks post-treatment. SER-109 resulted in a 27% absolute reduction of recurrence of CDI compared to placebo at eight weeks post-treatment, which is a relative risk reduction of 68%. In May 2021, Seres presented 24-week clinical data from the study that demonstrated significantly reduced recurrence rates compared to placebo.

“Recurrent C. difficile infection can have a serious and debilitating impact on patients’ lives, and there are currently very few treatment options available. As a clinician, I am excited by the data presented in this publication and the potential for an effective, safe and orally administered microbiome therapeutic that could alter the devastating impacts of this disease,”

said Paul Feuerstadt, MD, FACG, AGAF, Yale University School of Medicine and lead author.

“SER-109 is an entirely new treatment modality that shows promise to change the current standard of care.”

The published manuscript, titled “SER-109, an oral investigational microbiome therapeutic for patients with recurrent Clostridioides difficile infection,” is available online and will appear in the January issue of the New England Journal of Medicine (N Engl J Med 2022;386(3):220-229).

About SER-109

SER-109 is an oral microbiome therapeutic candidate consisting of a consortium of highly purified Firmicutes spores, which normally live in a healthy microbiome. SER-109 is designed to prevent further recurrences of CDI by modulating the disrupted microbiome to a state that resists C. difficile colonization and growth. The SER-109 manufacturing purification process is designed to remove unwanted microbes, thereby reducing the risk of pathogen transmission beyond donor screening alone. The U.S. FDA has granted SER-109 Breakthrough Therapy designation and Orphan Drug designation for the treatment of rCDI.