2020-08-12| Trials & Approvals

Seres Therapeutics’ Microbiome Therapy for Recurrent C. difficile Infections, Shines in Phase 3

by Tulip Chakraborty
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The Cambridge, MA-based company’s oral microbiome therapy, SER-109 exceeds the FDA’s expectations in treating recurrent Clostridium difficile Infections in Phase 3, ECOSPOR III Study trial.

By T. Chakroborty, Ph.D.

In the last few decades, the US and other countries have seen a rise in frequent, difficult to treat cases of Clostridium difficile Infections (CDI). Every year in the U.S alone, approximately half a million individuals are diagnosed with C. difficile with an estimated 20,000 deaths annually. CDI is known to be one of the most common causes of diarrhea and impacts the quality of life in individuals. Current approaches in treating CDI include vancomycin, fecal based microbiota treatment which provides modest results while overlooking patients with recurrent disease. Seres Therapeutics aims to improve these statistics by introducing their game-changer oral microbiome therapy (SER-109) for recurrent CDI.

Over the course of the ECOSPOR III Study, the efficacy results obtained exceeded the benchmark set in discussion with the US Food and Drug Administration (FDA). This would enable Seres Therapeutics to apply for a Biologics License Application (BLA) based on a single study by itself. Additionally, the FDA has granted SER-109 Breakthrough Therapy and Orphan Drug designations for the treatment of CDI.

“We are extremely pleased with these highly clinically meaningful SER-109 Phase 3 study results, greatly exceeding the statistical threshold provided by the FDA,” said Eric D. Shaff, President, and CEO of Seres. “ Based on our prior discussions with the FDA, we believe this trial should provide the efficacy basis for submitting an application for product approval. We look forward to meeting with the FDA as soon as possible to discuss the regulatory path forward with the goal of bringing SER-109 to patients as a first-in-class microbiome therapeutic. Our results represent the first-ever positive pivotal clinical study results for a targeted microbiome drug candidate. We believe these Phase 3 data provide strong validation for our underlying microbiome therapeutics platform, which has been the scientific basis for the Company, as well as persuasive clinical evidence supporting our other active pipeline programs.”

Mark Wilcox, M.D., Professor of Medical Microbiology, University of Leeds said “Recurrent C. difficile infection is a serious disease that devastates patients’ quality of life, and in many severe cases may result in a patient’s death. Today’s treatment options have important shortcomings related to efficacy, safety, and route of administration, and novel approaches that target the root causes of the disease are urgently needed. The SER-109 Phase 3 results are highly impressive and represent an exceptional advance in the fight against this disease. I believe that SER-109 has the potential to fundamentally transform the treatment of recurrent C. difficile infection”.



The ECOSPOR III study is a randomized, multicenter, placebo-controlled study that had approximately 182 patients with recurrent CDI. The study randomized patients to receive SER-109 or placebo orally for three consecutive days. The requirements for the study included patients having a positive C. difficile toxin test at study entry to confirm recurrence and accuracy of the primary endpoint which was indicated by the number of patients with recurrent CDI at upto 8 weeks. The secondary endpoint included patients evaluated at 24 weeks post-treatment.

At an 8-week timepoint, a significantly lower recurrence rate of roughly 11.1% was found in patients treated with SER-109 versus 41.3% in placebo patients. On an absolute basis, a 30.2% lower rate of recurrence was observed in patients who were given SER-109 as compared to the placebo group. The clinical response rate for this study was 88.9% with SER-109 and 58.7% with placebo. Generally, SER-109 was well tolerated amongst the patients with adverse events that occurred in the eight-week period including flatulence, abdominal distention, and abdominal pain. These symptoms were no different from those in the placebo group.



SER-109 is an investigational, oral, microbiome therapeutic that is intended to reduce recurrent CDI and helps to restore the diversity of the gastrointestinal microbiome population. SER-109 consists of a group of purified spores from the Firmicute species, obtained from the excreta of healthy individuals after inactivating pathogens which makes it fundamentally different from fecal microbiota transplantation.

Lisa von Moltke, M.D., FCP, Chief Medical Officer, said, “We would like to thank all those who participated in this landmark study. Based on these highly positive SER-109 ECOSPOR III results, we believe that this novel microbiome therapeutic candidate could potentially provide a much-needed effective oral treatment option for the approximately 170,000 patients in the U.S. that suffer from recurrent CDI annually. Seres applied a data-driven and scientifically rigorous approach to developing SER-109. The proprietary scientific learnings we have obtained continue to drive our overall R&D efforts and the advancement of our other ongoing microbiome therapeutic programs.”

Editor: Rajaneesh K. Gopinath, Ph.D.

Related Article: The Influential Role of Gut Microbiome in Cancer Therapy



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