Seres' biotherapeutic resolves checkpoint inhibitor colitis without systemic immunosuppression

Cambridge, Massachusetts-based [Seres Therapeutics](https://app.allsci.com/news/ASC-NR-0000003873073-1.0-1783509008) (Nasdaq: MCRB) reported early clinical data suggesting that [SER-155](https://app.allsci.com/drugs/ASC-DR-0000000034512-1.0-1772718717), its oral live biotherapeutic, may resolve diarrhea associated with immune checkpoint inhibitor-related enterocolitis without requiring systemic immunosuppression — a finding that, if confirmed in larger studies, could address one of the more clinically consequential adverse reactions limiting cancer immunotherapy use. The data, from a 15-patient Phase I investigator-sponsored trial conducted at Memorial Sloan Kettering Cancer Center, showed that 80% of participants achieved an immunosuppressive-free clinical response at Day 15, the primary endpoint. In the single-arm [Phase I study](https://app.allsci.com/clinical-trial/ASC-CT-0000000048590-1.0-1745763661), 12 of 15 patients (80%) achieved the primary endpoint of at least a one-grade improvement in diarrhea without systemic immunosuppression by Day 15, while one-third achieved complete remission. Biomarkers of gastrointestinal inflammation also improved, and no serious treatment-related adverse events were reported. The trial was single-arm and open-label, enrolling 15 patients at a single center, which substantially limits interpretation. The absence of a control arm means the observed response rates cannot be attributed to SER-155 with certainty, and the small sample size precludes conclusions about durability or patient subgroup effects. The study was sponsored by [Memorial Sloan Kettering](https://app.allsci.com/organization/ASC-OH-0000000001534-1.0-1764853177) rather than Seres, and is designed as an exploratory study to inform further development rather than as a registration-enabling trial. Efficacy data beyond Day 43 were not reported. SER-155 is a defined, cultivated multi-strain bacterial consortium administered orally; it is designed to engraft in the gastrointestinal tract and restore microbiome homeostasis, targeting mucosal epithelial barrier integrity and immune regulation at the gut-immune interface. Current standard of care for moderate-to-severe irEC requires halting checkpoint inhibitor therapy and initiating systemic corticosteroids, with infliximab (Remicade) or vedolizumab (Entyvio) used in steroid-refractory cases. Both agents carry systemic immunosuppressive effects that may blunt anti-tumor immunity — a concern that underpins the rationale for a microbiome-based approach. The irEC indication represents a strategic expansion for Seres, whose primary development program for SER-155 targets bloodstream infection prevention in patients undergoing allogeneic hematopoietic cell transplantation, where the drug holds Breakthrough Therapy and Fast Track designations and is described as Phase II-ready pending funding. The company said it is engaging potential partners, including companies with checkpoint inhibitor franchises, to advance SER-155 in irEC, and is separately seeking financing to progress the transplant program. Seres divested its approved microbiome product, fecal microbiota spores (Vowst), to Nestlé Health Science and is now a clinical-stage company dependent on external capital for both programs. *** This article was generated with AI assistance and reviewed and edited by the AllSci editorial team Explore more at AllSci News: [https://allsci.com/news/](https://allsci.com/news/) --- Spot something wrong? [Report an issue with this article](https://newsgen-prod.reframedata.com/feedback/checkpoint-inhibitor-colitis-treatment-seres-1)