Seres’ SER-155 previously showed a 77% relative reduction in bloodstream infections versus placebo in a Phase 1b allo-HSCT study, alongside lower systemic antibiotic exposure and fewer febrile neutropenia events. The program holds Breakthrough Therapy and Fast Track designations.
The company now reports additional FDA feedback, enabling finalization of a Phase 2 protocol in adults undergoing allogeneic stem cell transplant. The placebo-controlled study will enroll approximately 248 participants and use prevention of bloodstream infections through 30 days post-transplant as the primary endpoint. A pre-specified interim analysis for efficacy and futility is planned once roughly half the cohort has reached the primary endpoint, with an interim read targeted within 12 months of study initiation. Seres expects that a positive Phase 2 could support advancement into a single Phase 3 trial to underpin registration. To fund initiation, the company is seeking capital and other resources while cutting operating costs, including a workforce reduction of about 25%, extending the runway into the second quarter of 2026.
Strategically, Seres is compressing timelines to an actionable signal while conserving cash, a pragmatic move in a capital-constrained microbiome sector. Anchoring the primary endpoint at 30 days post-transplant tightens follow-up, accelerates the interim read, and focuses on an outcome with clear clinical and regulatory relevance. The design also sets up a binary value inflection to catalyze financing or a strategic transaction. After divesting VOWST, Seres is repositioning as a leaner, cultivated-consortia platform company; shifting to products manufactured from clonal cell banks may simplify supply, batch release, and CMC reproducibility relative to donor-sourced approaches, but it raises the bar on demonstrating consistent potency and colonization performance at scale.
For transplant centers and sites, the operational footprint is defined and time-limited: a 30-day primary endpoint with objective BSI adjudication, layered onto standard allo-HSCT care. Sites will need alignment on antibiotic stewardship and infection surveillance to minimize confounding, plus readiness for stool sampling and centralized microbiome analytics. CROs with established HSCT networks and capabilities in complex inpatient workflows and central lab coordination will be advantaged. Regulators and payers are likely to focus on the magnitude and durability of BSI reduction, interactions with prophylactic antimicrobials, and any downstream effects on graft-versus-host disease and hospitalization metrics. Vendors supporting GMP cultivation, QC assays for live consortia, and microbiome bioinformatics may see near-term demand if the study moves forward as planned.
The near-term watchlist is executional and financial. A financing or partnership to open sites is the gating event; absent that, start-up momentum stalls despite FDA-aligned design. Once initiated, enrollment velocity in a specialized HSCT setting, adherence amid heavy antibiotic use, and lot-to-lot consistency of a cultivated product become the key risks to the interim. FDA’s receptivity to a single definitive Phase 3 will likely depend on the robustness of the Phase 2 effect size, safety in the context of immunosuppression, and signals beyond 30 days. Positive interim data would strengthen the case to expand into adjacent high-risk populations such as autologous HSCT and CAR-T recipients. Conversely, a muted signal would leave limited options given the narrowed resourcing. In a field recalibrating around pragmatic, clinically anchored endpoints, this protocol will test whether a standardized microbiome consortium can deliver a registrational path in infection prevention.
Jon Napitupulu is Director of Media Relations at The Clinical Trial Vanguard. Jon, a computer data scientist, focuses on the latest clinical trial industry news and trends.
