SER-155

November 3, 2026: Seres Therapeutics (NASDAQ: MCRB) announced publication in Nature Medicine of its Phase 1b SER-155 study in adults undergoing allogeneic hematopoietic cell transplantation (allo-HCT). In the randomized, double-blind, placebo-controlled Cohort 2, SER-155 was generally well tolerated and reduced bloodstream infections to 10% versus 42.9% with placebo, a 77% relative risk reduction (p=0.0423). Systemic antibiotic exposure was lower with SER-155 (mean 9.2 vs. 21.1 days; p=0.0494), and febrile neutropenia was numerically lower (65% vs. 78.6%). No treatment-related serious adverse events were observed, and SER-155 strains engrafted in the GI tract.

Translational analyses showed significantly lower fecal albumin and lower plasma biomarkers of systemic inflammation versus placebo, supporting SER-155’s proposed barrier-protective and anti-inflammatory mechanism. SER-155 has FDA Breakthrough Therapy and Fast Track designations for reducing BSIs in adults undergoing allo-HCT; the company said it has aligned with FDA on a proposed Phase 2 protocol and that the program is Phase 2-ready pending funding.

The release also cited July 2026 investigator-sponsored irEC data: in 15 patients with Grade 2-3 immune checkpoint inhibitor-related enterocolitis who had not received immunosuppressive therapy, 12/15 (80%) achieved the day-15 primary endpoint of immunosuppressive-free clinical response, with no SER-155-related serious adverse events. Seres plans to submit a prevention study design for irEC to FDA.