A Phase 1b Study to Evaluate the Efficacy, Safety, and Pharmacokinetics of an Investigational Microbiome Therapeutic, SER-155, in Adults Undergoing Hematopoietic Stem Cell Transplantation

Background: In patients undergoing allogeneic hematopoietic 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 an investigational cultivated microbiome therapeutic rationally designed to improve clinical outcomes in HCT by restoring colonization resistance to potential pathogens, 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 that is currently enrolling at 4 sites.

Study Design and 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 or active inflammatory bowel disease or total colectomy, umbilical cord blood or ex vivo T-cell depletion, exposure to 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 1; Figure 1). 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.

In April 2022, the SER-155 Data and Safety Monitoring Committee met as part of a planned review of available safety data from Cohort 1 and supported continuation of Cohort 1 enrollment.

Summary: In this Phase 1b study, the feasibility, safety, and clinical efficacy of SER-155 is being assessed in the setting of allogeneic HCT. This trial will add to the expanding clinical experience of microbiome-directed interventions in allogeneic HCT recipients.

Ponce:Seres Therapeutics: Research Funding. Peled:Seres Therapeutics: Consultancy, Patents & Royalties, Research Funding; Maat Pharma: Consultancy; DaVolterra: Consultancy; CSL Behring: Consultancy; Post Biotic Research: Consultancy, Current equity holder in publicly-traded company. Tejura:Seres Therapeutics: Current Employment, Current equity holder in private company, Current holder of stock options in a privately-held company. Ford:Seres Therapeutics: Current Employment. van den Brink:Vor Biopharma: Honoraria; Pluto Therapeutics: Current holder of stock options in a privately-held company, Honoraria; Notch Therapeutics: Current holder of stock options in a privately-held company, Honoraria; Seres Therapeutics: Current holder of stock options in a privately-held company, Honoraria, Other: IP Licensing , Research Funding; Rheos Medicines: Honoraria; Frazier Healthcare Partners: Honoraria; Nektar Therapeutics: Honoraria; Ceramedix: Honoraria; Lygenesis: Honoraria; GlaskoSmithKline: Honoraria; Da Volterra: Honoraria; Thymofox: Honoraria; Garuda: Honoraria; Novartis (Spouse): Honoraria; Synthekine (Spouse): Honoraria; Beigene (Spouse): Honoraria; Kite (Spouse): Honoraria; Juno Therapeutics: Other: IP Licensing ; DKMS: Other: fiduciary role on the Foundation Board ; Wolters Kluwer: Patents & Royalties. Kosuri:Seres Therapeutics: Research Funding. Khera:Incyte: Consultancy; Optum: Honoraria. Defilipp:Kadmon: Consultancy; MorphoSys: Consultancy; Incyte: Research Funding; Regimmune: Research Funding; Taiho Oncology: Research Funding; Incyte: Consultancy; Omeros: Consultancy; Syndax Pharmaceuticals: Consultancy. von Moltke:Seres Therapeutics: Current Employment.