Live Biotherapeutic SER-155 Cut Bloodstream Infection Risk by 77% in Transplant Patients: Phase 1b Results Published in Nature Medicine
Seres Therapeutics, Inc. (Nasdaq: MCRB), a live biotherapeutics company based in Cambridge, Massachusetts, announced on November 3 that results from its Phase 1b clinical study of SER-155 in adults undergoing allogeneic hematopoietic cell transplantation (allo-HCT) have been published in the November 3, 2026 issue of Nature Medicine. SER-155 is an investigational, oral, cultivated multi-strain live biotherapeutic product designed to reduce the risk of bloodstream infections (BSIs) in medically vulnerable patients.
The Phase 1b study (NCT04995653) had two cohorts: an open-label cohort assessing safety and drug pharmacology, and a randomized, double-blind, placebo-controlled cohort evaluating safety, drug pharmacology and clinical outcomes, including BSIs, systemic antibiotic exposure and febrile neutropenia, through day 100 after transplant.
In Cohort 2, BSIs occurred in 10% of patients receiving SER-155 compared with 42.9% of patients receiving placebo, a 77% relative risk reduction (p=0.0423). Patients receiving SER-155 also had significantly lower systemic antibiotic exposure, a mean of 9.2 days compared with 21.1 days for placebo (p=0.0494), and a numerically lower incidence of febrile neutropenia, 65% compared with 78.6%. SER-155 strains engrafted in the gastrointestinal tract, and the product was generally well tolerated, with no treatment-related serious adverse events observed.
Translational biomarker analyses support the drug's mechanisms, the company said: many bloodstream infections in transplant patients originate in the gut, when pathogens expand and cross a damaged intestinal barrier. Compared with placebo, SER-155 led to a statistically significant decrease in fecal albumin, an established biomarker of epithelial barrier integrity, and lower plasma levels of biomarkers of systemic inflammation.
"Publication in Nature Medicine is an important milestone for the SER-155 program and recognition of the innovative science being advanced by Seres," said Matthew Henn, Ph.D., president and chief scientific officer of Seres Therapeutics. Executive chairman and interim chief executive officer Richard Kender said the peer-reviewed results reinforce the company's belief in SER-155's potential to prevent bloodstream infections in immunocompromised patients such as allo-HCT recipients.
SER-155 has received Breakthrough Therapy designation from the U.S. Food and Drug Administration (FDA) for the reduction of BSIs in adults undergoing allo-HCT, as well as Fast Track designation, and the company has aligned with the FDA on its proposed Phase 2 study protocol; the program is Phase 2-ready in allo-HCT, pending receipt of funding. Seres is also developing SER-155 for immune checkpoint inhibitor-related enterocolitis, where a July 2026 investigator-sponsored trial at Memorial Sloan Kettering Cancer Center reported that 12 of 15 patients achieved an immunosuppressive-free clinical response at day 15.
(Source: Seres Therapeutics)
Source: Seres Therapeutics
Published by GSM · USA (original Chinese edition)
Photo: 图：实验室人员检查血液样本（资料图）。National Eye Institute / Rhoda Baer Photography，CC BY 2.0，经 Wikimedia Commons。 / Photo: A laboratory worker examines a blood sample (file photo). National Eye Institute / Rhoda Baer Photography, CC BY 2.0, via Wikimedia Commons.
