The Food and Drug Administration (FDA) has accepted for Priority Review the Biologics License Application (BLA) for the oral microbiome therapeutic SER-109 for the treatment of recurrent Clostridioides difficile infection (CDI).

SER-109 contains purified bacterial spores of multiple Firmicute species obtained from the stool of healthy human donors. The BLA is supported by data from the double-blind, randomized, placebo-controlled phase 3 ECOSPOR III study ( Identifier: NCT03183128), which evaluated the efficacy and safety of SER-109 in 182 adults who had 3 or more episodes of CDI within 12 months, inclusive of the current episode. Patients were randomly assigned to receive 4 capsules of SER-109 or placebo orally daily for 3 days after standard-of-care antibiotic treatment.

Results showed that treatment with SER-109 met the primary endpoint demonstrating superiority to placebo in reducing CDI recurrence. At 8 weeks post treatment, 88% of patients in the SER-109 arm did not have CDI recurrence compared with 60% of those in the placebo arm. At 24 weeks post treatment, 79% of patients remained free from CDI recurrence compared with 53% of those in the placebo arm. 

The application also includes efficacy and safety data from ECOSPOR IV ( Identifier: NCT03183141), an open-label extension study, which evaluated SER-109 in 263 adults with recurrent CDI. Results showed that 91% and 86% of patients treated with SER-109 had sustained clinical response at 8 and 24 weeks, respectively.

A Prescription Drug User Fee Act target date of April 26, 2023 has been set for the application.


Seres Therapeutics announces FDA acceptance of Biologics License Application for investigational microbiome therapeutic SER-109 for recurrent C. difficile infection for Priority Review. News release. Seres Therapeutics, Inc. Accessed October 26, 2022.