An Open-Label and Randomized, Double-Blind, Placebo-Controlled, Multicenter Study to Evaluate Safety, Tolerability, Pharmacokinetics, and Efficacy of SER-155 in Adults Undergoing Hematopoietic Stem Cell Transplantation to Reduce the Risk of Infection and Graft vs. Host Disease

This is a Phase 1b randomized, double-blind, placebo-controlled, multiple dose, multicenter study designed to evaluate the safety, efficacy, and microbiome alterations associated with SER-155 dosing, after microbiome conditioning with oral vancomycin, in adult subjects aged ≥18 years who are undergoing Hematopoietic Stem Cell Transplantation (HSCT).

Inclusion Criteria:

• Male and female subjects ≥ 18 years of age undergoing HSCT.
• Planning to undergo allogeneic hematopoietic stem cell transplantation from a human leukocyte antigen matched related donor, haploidentical related donor, HLA-matched unrelated donor, or HLA 1-antigen mismatched unrelated or related donor, with either bone marrow or peripheral blood stem cells as a graft source, and with any conditioning regimen

Exclusion Criteria:

• Severe colitis of any etiology or active/currently-treated inflammatory bowel disease (IBD) or total colectomy.
• Evidence of relapse or progression of hematologic malignancy (minimal residual disease is allowed).
• Transplant using umbilical cord blood or ex vivo T-cell-depleted HSCT
• Receipt of chimeric antigen receptor T-cell (CAR-T) therapy.
• Received a fecal microbiota transplant (FMT) or any live microbial therapeutic within 3 months prior to Screening.
• Known allergy or intolerance to oral vancomycin.
• Concomitant participation or participation within 14 days or 5 half-lives of another investigational unapproved treatment, whichever is longer.