Seres Therapeutics SER-109, Investigational Microbiome Drug to Reduce rCDI, What Was Learned From a Phase 2 Clinical Trial



Recurrent C. difficile infection (rCDI) is associated with loss of microbial diversity and microbe-derived secondary bile acids, which inhibit C. difficile germination and growth. SER-109, an investigational microbiome drug of donor-derived, purified spores, reduced recurrence in a dose-ranging, open-label Phase (Ph)1 study in subjects with multiply rCDI.

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In a Ph2 double-blind trial, subjects with clinical resolution on standard-of-care antibiotics were stratified by age (< or ≥65 years) and randomized 2:1 to single-dose SER-109 or placebo. Subjects were diagnosed at study entry by PCR or toxin testing. Safety, C. difficile-positive diarrhea through week 8, SER-109 engraftment and bile acid changes were assessed.


89 subjects were enrolled; 67% were female; 80.9% diagnosed by PCR. rCDI rates were lower in the SER-109 arm than placebo (44.1% versus 53.3%, respectively) but did not meet statistical significance. In a pre-planned analysis, rates were reduced among subjects ≥65 years (45.2% versus 80%, respectively; RR:1.77, 95% CI:1.11-2.81) while the <65 group showed no benefit. Early engraftment of SER-109 was associated with non-recurrence (p <0.05) and increased secondary bile acid concentrations (p<0.0001). Whole metagenomic sequencing from this study and our prior Ph1 study revealed previously unappreciated dose-dependent engraftment kinetics and confirmed an association between early engraftment and nonrecurrence. Engraftment kinetics suggest that Ph2 dosing was suboptimal. Adverse events were generally mild-to-moderate in severity.


Early SER-109 engraftment was associated with reduced CDI recurrence and favorable safety was observed. A higher dose of SER-109 and requirements for toxin testing were implemented in the current Ph3 trial.