Seres Therapeutics Inc. reported that it has completed in-depth analyses of the previously reported SER-109 Phase 2, eight-week clinical study data in patients with multiply recurrent Clostridium difficile infection. The company also reported the full, 24-week SER-109 Phase 2 study results and open label extension study data. Analysis was conducted to evaluate both the role of C. difficile diagnostic testing in defining the correct SER-109 Phase 2 study entry population, and in the proper diagnosis of C. difficile recurrences during the study. In the Phase 2 study, 81% of study subjects (72 of 89 subjects) were enrolled based on polymerase chain reaction (PCR) based testing for C. difficile, as well as clinical evaluation. An important and increasingly well-appreciated limitation of PCR testing is that while a positive result indicates that C. difficile cytotoxin genes are present, a positive PCR test does not necessarily indicate that the organism is viable and producing disease causing cytotoxins, nor that C. difficile is the source of clinical symptoms. The company performed an in-depth analysis to examine SER-109 biological activity in the Phase 2 trial, as measured by microbiome changes in patients and downstream biological effects in the gastrointestinal tract. Results demonstrated a statistically significant increase in the richness of commensal spore-forming bacterial species in patients treated with SER-109, as compared to those receiving placebo. These data demonstrate that SER-109 successfully engrafted and was biologically active in the Phase 2 study. In addition, among those patients with an increased prevalence of specific SER-109 associated bacterial species, a decreased rate of high confidence recurrences (i.e., recurrences confirmed by C. difficile cytotoxin assay) was demonstrated. The company also conducted a thorough and detailed investigation of the potential impacts of manufacturing and formulation changes implemented in the Phase 2 study. No issues regarding product quality or formulation were identified which would have impacted the Phase 2 study results. The full, 24-week Phase 2 study results continue to demonstrate that SER-109 was generally well tolerated. The most common adverse events associated with SER-109 included diarrhea, abdominal pain and flatulence. The Phase 2 study population represented older individuals, many in poor health, and a high rate of serious adverse events (SAEs) was reported in both study arms. A numerically higher rate of SAEs was observed in the SER-109 arm (15.0% versus 10.3% for placebo), however there was no detectable pattern in the SAEs observed, and none of these were considered to be SER-109 drug-related by the study investigators.