Background:In patients undergoing allogeneic hematopoietic stem cell transplantation (HCT), loss of gastrointestinal microbial diversity is associated with risk of bloodstream infections (BSI), acute graft-versus-host disease (aGvHD), and death. SER-155 is a cultivated investigational microbiome therapeutic rationally designed to improve clinical outcomes in patients undergoing HCT by restoring colonization resistance to pathobionts, promoting epithelial barrier integrity, and reducing colonic inflammation. SER-155-001 is a Phase 1b study to evaluate the efficacy, safety, and pharmacokinetics (PK) of SER-155 in adults undergoing HCT. Methods:This study will enroll approximately 70 subjects ≥ 18 years in an open-label Cohort 1 (n = 10) followed by a double-blind, placebo-controlled Cohort 2 (n = 60) randomized 1:1 to SER-155 or placebo and stratified by conditioning regimen intensity. Exclusion criteria include history of severe colitis of any etiology or active inflammatory bowel disease or total colectomy, transplant using umbilical cord blood or ex vivo T-cell therapy, receipt of fecal microbiota transplant or any live microbial therapeutic within 3 months prior to screening, and evidence of relapse or progression of hematologic malignancy (minimal residual disease is allowed). Following screening, subjects in both cohorts will receive 2 treatment courses (before and after HCT), each comprised of microbiome conditioning with oral vancomycin or placebo followed by SER-155 or placebo, and a conditional 3rd treatment course if the subject receives antibiotics (Table). Safety outcomes will be followed through 52 weeks post HCT. The primary endpoint is to evaluate the incidence and severity of adverse events, serious adverse events, and adverse events of special interest. Secondary endpoints include rates of BSI, gastrointestinal infections, aGvHD, febrile neutropenia and overall survival in placebo vs SER-155 arms. Microbiome related endpoints include engraftment of SER-155 bacterial strains in the gastrointestinal tract (PK endpoint) and fecal microbiome diversity, composition and metabolites. Clinical trial information: NCT04995653.

*A third course will be administered without microbiome conditioning following completion of antibiotics for subjects who receive 3 days of systemic antibiotics following HCT.