CAMBRIDGE, Mass., July 8, 2026
Seres Therapeutics announced encouraging topline results from an investigator-sponsored clinical trial (IST) evaluating SER-155 in patients with immune checkpoint inhibitor-related enterocolitis (irEC), a common and potentially severe complication of cancer immunotherapy. Conducted by Memorial Sloan Kettering Cancer Center (MSK), the open-label study enrolled 15 patients with moderate-to-severe (Grade 2–3) irEC who had not yet received immunosuppressive treatment. The trial met its primary endpoint, with 80% of participants (12 of 15) achieving an immunosuppressive-free clinical response by Day 15, defined as at least a one-grade improvement in diarrhea without requiring corticosteroids or biologic immunosuppressive therapy. Notably, 67% of responders experienced at least a two-grade improvement, while 33% of patients achieved complete resolution of diarrhea symptoms without immunosuppressive treatment. These findings suggest that SER-155 could provide a novel therapeutic option that allows patients to continue life-saving immune checkpoint inhibitor (ICI) therapy while avoiding the complications associated with systemic immunosuppression.
Safety Profile and Biomarker Data Support Therapeutic Potential
SER-155 demonstrated a favorable safety profile throughout the study, with no drug-related serious adverse events, no bloodstream infections, and only four moderate, non-serious adverse events reported in two participants, all of which resolved successfully. By Day 43, one-third of participants maintained an immunosuppressive-free clinical response, while two patients remained in complete clinical remission without systemic immunosuppressive therapy. Importantly, all participants who responded by Day 15 maintained the same or improved diarrhea grade at the end of follow-up, even if some later required localized gastrointestinal-targeted immunosuppressive treatment. Pharmacological analyses also confirmed successful engraftment of SER-155 bacterial strains, consistent with previous clinical studies, while key biomarkers of gastrointestinal inflammation and mucosal epithelial barrier integrity, including fecal calprotectin and fecal albumin, showed statistically significant reductions by Day 43. These biological improvements provide evidence that SER-155 functions as designed by restoring the intestinal barrier and reducing inflammation through targeted microbiome modulation.
Microbiome Therapy May Expand Beyond Cancer Treatment Side Effects
According to Seres Therapeutics, the encouraging clinical and pharmacological findings support continued development of SER-155 not only for irEC but also for a broader range of inflammatory and immune-mediated diseases. Company leaders emphasized that the therapy’s mechanism of repairing the mucosal epithelial barrier and restoring immune balance addresses disease pathways not targeted by existing treatments. Executive Chairman and Interim Chief Executive Officer Richard Kender stated that the results demonstrate the potential for SER-155 to reduce reliance on systemic immunosuppressive drugs while allowing patients to remain on critical immune checkpoint inhibitor therapies. President and Chief Scientific Officer Dr. Matthew Henn added that the findings strengthen confidence in the company’s live biotherapeutics platform, supporting future development for additional conditions such as ulcerative colitis and Crohn’s disease. Investigators at Memorial Sloan Kettering Cancer Center also highlighted the therapy’s potential to improve outcomes for patients who develop severe gastrointestinal complications during cancer immunotherapy.
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Positive Data Strengthen Development of Seres’ Live Biotherapeutics Platform
The positive IST results further reinforce Seres Therapeutics’ strategy of developing live biotherapeutic medicines that target the interaction between the gut microbiome, intestinal barrier, and immune system. The company is currently engaging potential strategic collaborators and financing partners to support both the continued development of SER-155 in irEC and the planned Phase 2 study in patients undergoing allogeneic hematopoietic cell transplantation (allo-HCT), where the therapy has already received Breakthrough Therapy and Fast Track designations. Beyond SER-155, Seres is also advancing SER-603 for inflammatory bowel disease, expanding its pipeline of microbiome-based therapeutics. With strong early efficacy, a favorable safety profile, and evidence of biological activity, SER-155 has the potential to become a first-in-class microbiome therapy that enables cancer patients to continue immunotherapy while addressing one of oncology’s most challenging treatment-related complications.
Source: Seres Therapeutics, press release
