Seres Therapeutics Advances SER-155 Bloodstream Infection Prevention; With FDA Feedback And Streamlines Operations To Extend Cash Runway

Seres Therapeutics, Inc., a leader in live biotherapeutics, announced that it has received additional feedback from the U.S. Food and Drug Administration (FDA) on the Phase 2 study design for its lead candidate, SER-155. The therapy is being developed to prevent bloodstream infections (BSIs) in adults undergoing allogeneic hematopoietic stem cell transplant (allo-HSCT). This feedback will help the company finalize its protocol as it works to advance the study, which has Breakthrough Therapy designation.

“We are progressing start-up activities for our Breakthrough Therapy designated SER-155 live biotherapeutic program and are pleased with the further constructive feedback from the FDA, including on key parameters such as study size, primary endpoint, and interim analysis plan, that we expect will allow us to finalize the Phase 2 study protocol. While we advance SER-155 Phase 2 study start-up activities, we continue to engage in active discussions with multiple parties seeking capital to initiate the study, and to support our broader portfolio of product candidates with applications for inflammatory diseases. The cost reduction actions, and the resultant operating runway extension, are intended to provide the Company with additional opportunity to advance these strategic priorities. We recognize the outstanding contributions our departing team members have made and thank them for their efforts in bringing meaningful live biotherapeutic products to patients,” said Thomas DesRosier and Marella Thorell, co-CEOs of Seres.

The planned Phase 2 trial of SER-155 will be a placebo-controlled, well-powered study with a primary endpoint of preventing BSIs through 30 days post-transplant. Approximately 248 patients are expected to be enrolled, with an interim analysis planned when half of participants reach the primary endpoint. This interim readout, projected within 12 months of trial initiation, will assess efficacy and futility, potentially accelerating FDA discussions for Phase 3 design. Positive outcomes could enable a single Phase 3 study to support registration and expand development into additional high-risk patient populations.

Alongside trial preparations, Seres is enacting cost-saving measures, including a 25% workforce reduction, effective since August 2025. This restructuring is expected to extend the company’s cash runway into the second quarter of 2026 while retaining key capabilities for advancing SER-155. Severance costs related to the reduction are estimated at $1.0–$1.4 million, to be paid in Q4 2025. The company continues to engage with potential partners to secure resources for SER-155’s development and other pipeline programs.