Seres Therapeutics, Inc. (NASDAQ:MCRB) (Seres or the Company), a leading live biotherapeutics company, today announced that the US Food and Drug Administration (FDA) has granted Breakthrough Therapy designation to SER-155, the Company's lead investigational program, for the reduction of bloodstream infections (BSIs) in adults undergoing allogeneic hematopoietic stem cell transplant (allo-HSCT) for the treatment of hematological malignancies. In September 2024, Seres reported topline clinical data from Cohort 2 of its SER-155 Phase 1b placebo-controlled study in patients undergoing allo-HSCT, in which SER-155 was associated with a significant reduction in BSIs (77% relative risk reduction), a significant reduction in systemic antibiotic exposure, and lower incidence of febrile neutropenia, in each case as compared to placebo, through day 100 post-HSCT. Additionally, SER-155 was generally well tolerated, with no observed treatment-related serious adverse events.

FDA Breakthrough Therapy designation ensures communication and guidance from FDA to expedite the development of medicines which are intended to treat serious or life-threatening diseases, and in which preliminary clinical evidence indicates that the drug may demonstrate substantial improvement on a clinically significant endpoint over available therapies. The receipt of Breakthrough Therapy designation for SER-155, the Company's second program to receive such designation, grants Seres access to FDA senior management and a commitment from FDA to work closely with the Company, facilitating an efficient drug development process. In December 2023, SER-155 received FDA Fast Track designation for reducing the risk of infection and GvHD in this patient population.