Seres Therapeutics will present expanded biomarker data from its SER-155 Phase 1b study at the 2025 American Society of Clinical Oncology (ASCO) Annual Meeting, revealing new insights into the investigational live biotherapeutic's ability to promote immune recovery in patients undergoing allogeneic hematopoietic stem cell transplantation (allo-HSCT).
The new data builds on previously reported findings that demonstrated SER-155's 77% relative risk reduction in bacterial bloodstream infections. Post hoc analyses from the Phase 1b study show significantly higher levels of the homeostatic cytokine IL-7 both after the second course of SER-155 and at HSCT Day 100, compared to placebo. Additionally, a higher frequency of CD4+ T cells was observed in peripheral blood at these same timepoints in the SER-155 treatment arm.
Immune Reconstitution Mechanism
The biomarker findings suggest SER-155 may play a crucial role in promoting T-cell populations essential for immune reconstitution after allo-HSCT. The observed increases in IL-7 levels and CD4+ T cells are particularly significant, as these markers indicate the restoration of a functional immune system following transplantation.
"Patients undergoing allo-HSCT are highly susceptible to life-threatening bloodstream infections, making effective immune reconstitution and restoration of a diverse, functional T-cell population after allo HSCT important for positive long-term outcomes," said Marcel van den Brink, M.D., Ph.D., President of City of Hope Cancer Center and National Medical Center. "The observed increases in IL-7 levels and CD4+ T-cells associated with SER-155 administration are particularly encouraging and consistent with a beneficial immunoregulatory effect of SER-155."
Clinical Development Progress
SER-155 is an investigational, oral, live biotherapeutic designed to decolonize gastrointestinal pathogens, improve epithelial barrier integrity, and induce immune homeostasis. The therapy aims to prevent bacterial bloodstream infections, including those harboring antimicrobial resistance, in patients undergoing allo-HSCT.
The Phase 1b placebo-controlled study demonstrated not only the significant reduction in bacterial bloodstream infections but also decreased systemic antibiotic exposure and lower incidence of febrile neutropenia. These results have earned SER-155 both Breakthrough Therapy designation for reducing bloodstream infections in adults undergoing allo-HSCT and Fast Track designation for reducing infection risk and graft-versus-host disease.
Regulatory and Partnership Strategy
Seres Therapeutics plans to submit a Phase 2 trial protocol for SER-155 in allo-HSCT to the FDA in the coming weeks. The company is actively seeking partners to support continued clinical development of the program, which was initially supported by the Combating Antibiotic-Resistant Bacteria Biopharmaceutical Accelerator (CARB-X).
The company envisions expanding SER-155's application beyond allo-HSCT to other medically vulnerable patient populations, including autologous-HSCT patients, cancer patients with neutropenia, CAR-T recipients, individuals with chronic liver disease, solid organ transplant recipients, and patients in intensive care and long-term acute care facilities.
ASCO Presentation Details
The new biomarker data will be presented in a poster session titled "Exploratory analyses of immune reconstitution biomarkers from a Ph1b study of an investigational, oral, live biotherapeutic product, SER-155, in adult allo-HCT" on Sunday, June 1, 2025, from 9:00 AM to 12:00 PM CT. Emily Walsh, Ph.D., Director of Research Technologies at Seres Therapeutics, will present the findings.
