FDA Grants Priority Review for Novel C. Difficile Therapeutic


If approved, SER-109 would be the first oral microbiome therapeutic available. The Prescription Drug User Fee Act action date is April 26, 2023.

The FDA has accepted for review Seres Therapeutics’ biologics license application (BLA) for SER-109 for the prevention of recurrent C. difficile infection (rCDI), a hard-to-treat infection. The application has been granted priority review designation with a Prescription Drug User Fee Act (PDUFA) action date of April 26, 2023.

SER-109 is microbiome therapeutic composed of purified Firmicutes spores for the treatment of recurrent C. difficile infection. Firmicutes spores normally live in a healthy microbiome. SER-109 is designed to modulate 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 to the risk of pathogen transmission. The FDA has granted SER-109 breakthrough therapy designation and orphan drug designation for the prevention of recurrent C. difficile infection.

With nearly 170,000 cases of recurrent C. diff each year in the United States, it is one of the top three most urgent bacterial threats in the country, according to the Centers for Disease Control and Prevention, and is a leading cause of hospital-acquired infection. The CDC estimates that there are about half a million infections a year, and one in six patients will have a second infection within two to eight weeks. In those over the age of 65, one in 11 patients with C. difficile will die within one month.

Eric Shaff

Eric Shaff

“We are working as quickly as possible to bring this first-ever oral microbiome therapeutic to patients most in need, if approved,” Eric Shaff, president and chief executive officer at Seres, said in a press release.

The application is supported by two phase 3 trials. Results of the ECOSPOR III trial, which were published in the New England Journal of Medicine in January, showed that 88% of people in the SER-109 group were free from C. difficile recurrence at eight weeks post-treatment, compared with 60% in the placebo group. At 24 weeks post-treatment, 79% of the SER-109 group were still free from C. difficile recurrence, compared with 53% in the placebo group. SER-109 was observed to be well tolerated with no drug-related serious adverse events. The trial enrolled 182 adults.

ECOSPOR IV, an open-label extension study of ECOSPOR III, evaluated 263 adults with rCDI at the commercial dose to fulfill FDA requirements for the SER-109 safety database. The study duration for both cohorts was about 27. SER-109 was well-tolerated and there was a 91% sustained clinical response at eight weeks in the overall population. At 24 weeks post-treatment, 86% of those treated with SER-109 experienced sustained clinical response.

In July 2021, Seres entered into an agreement with Nestlé Health Science to jointly commercialize SER-109 in the United States and potentially Canada. Nestlé Health Science’s pharmaceutical business, Aimmune Therapeutics, will assume the role of lead in commercialization.

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