Seres Therapeutics, Inc. announced that enrollment is complete in the placebo-controlled Cohort 2 of its Phase 1b trial of SER-155 in patients who received Allogeneic Hematopoietic Stem Cell Transplantation (Allo HSCT). SER-155 is an orally administered, consortium of bacteria, cultivated from cell banks and designed to reduce the incidence and severity of enteric-derived infections and resulting bloodstream infections, including those that may harbor antibiotic resistance. Infections are a leading cause of mortality and morbidity in this immunocompromised patient population.

SER-155 is also designed to induce immune tolerance responses to reduce the incidence of GvHD. The SER-155 Phase 1b study (NCT04995653) is being conducted across 13 clinical centers in the US, including Memorial Sloan Kettering. Study Cohort 1, which included 13 participants, was designed to assess safety and drug pharmacology, including the engraftment of drug bacteria in the gastrointestinal tract.

Cohort 1 clinical data, announced in May 2023, showed favorable tolerability, successful drug bacteria engraftment, and a substantial reduction in pathogen domination in the gastrointestinal microbiome. Study Cohort 2, which includes 45 participants, incorporates a randomized, double-blinded placebo-controlled 1:1 design to further evaluate safety and engraftment, as well as clinical outcomes. SER-155 is a consortium of bacterial species selected using Seres?

reverse translation discovery and development MbTx platform technologies. The design incorporates microbiome biomarker data from human clinical data and nonclinical human cell-based assays, and in vivo disease models. The SER-155 composition is designed to prevent and decrease the colonization and abundance of bacterial pathogens that can harbor antibiotic resistance and to enhance epithelial barrier integrity in the GI tract to both reduce the likelihood of pathogen translocation and decrease the incidence of bloodstream infections and GvHD.

SER-155 has received FDA Fast Track Designation.