SER-155

July 8, 2026: Seres Therapeutics, Inc. (NASDAQ: MCRB) reported encouraging top-line results from an investigator-sponsored, open-label trial (NCT06801067) at Memorial Sloan Kettering Cancer Center evaluating SER-155 for immune checkpoint inhibitor–related enterocolitis (irEC) in 15 immunosuppressive-naïve patients with Grade 2–3 disease.

Efficacy (primary endpoint, Day 15): 12/15 (80%) achieved an immunosuppressive-free clinical response (≥1-grade improvement in diarrhea without corticosteroids/biologic immunomodulators); 8/12 responders (67%) had ≥2-grade improvement. Immunosuppressive-free complete clinical remission (Grade 0 diarrhea) was 5/15 (33%) at Day 15.

Durability (Day 43): 5/15 (33%) maintained immunosuppressive-free clinical response and 2/15 (13%) maintained complete remission. All Day 15 responders had the same or better diarrhea grade at Day 43; 7 received GI-targeted, non-systemic immunosuppressives after Day 15.

Safety/pharmacology: SER-155 was generally well tolerated through Day 43 with no drug-related serious AEs and no bloodstream infections reported. Strain engraftment was robust; fecal calprotectin and fecal albumin decreased post-treatment with statistically significant reductions by Day 43.

Upcoming: Management webcast July 8, 2026 (8:30 a.m. ET); investigators intend to present results at a future medical conference. Seres stated it is engaging potential partners and seeking funding to advance a Phase 2 study in allo-HCT and broader pipeline development.