Seres Therapeutics, Inc. (Nasdaq: MCRB) recently announced encouraging translational biomarker results from its SER-155 Phase 1b placebo-controlled study in patients undergoing allogeneic stem cell transplantation (allo-HSCT). These findings support the previously reported 77% relative risk reduction in bloodstream infections (BSIs) and further validate the potential of SER-155 as a novel therapeutic approach for preventing BSIs and modulating inflammatory pathways.



SER-155 is an investigational, oral, live biotherapeutic designed to decolonize GI pathogens, improve epithelial barrier integrity, and induce immune tolerance to prevent bacterial bloodstream and antimicrobial resistant (AMR) infections, as well as other pathogen-associated negative clinical outcomes, in patients undergoing allo-HSCT for the treatment of hematological malignancies. The new biomarker results demonstrate that SER-155 promotes epithelial barrier integrity and decreases systemic inflammatory biomarkers compared to placebo, supporting its mechanism of action and potential for reducing BSIs.



The statistically significant decrease in fecal albumin, an established biomarker of epithelial barrier integrity, indicates that SER-155 helps strengthen the gut barrier, reducing the likelihood of bacteria translocating from the gastrointestinal tract into the bloodstream. Additionally, the lower concentrations of plasma biomarkers of systemic inflammation and changes in immune homeostasis observed following SER-155 dosing reinforce the positive impact in allo-HSCT patients. These data also support the potential role for Seres' live biotherapeutic platform to provide clinical benefit to patients with inflammatory and immune diseases, such as IBD, including ulcerative colitis and Crohn's disease.

Wendy Garrett, M.D., Ph.D., Professor of Immunology and Infectious Diseases at Harvard T.H. Chan School of Public Health, commented on the SER-155 translational biomarker data, stating, "The SER-155 translational biomarker data, combined with the promising clinical results demonstrating an impressive reduction in bloodstream infection rates, provide strong biological evidence supporting Seres' live biotherapeutic candidates as a novel potential therapeutic approach to improve epithelial barrier integrity and to positively modulate multiple inflammatory pathways. These data support potential opportunities for live biotherapeutics, beyond infection, in inflammatory and immune diseases."

Seres' SER-155 Phase 1b clinical study results have been accepted for an oral presentation in the Best Abstracts in Infectious Diseases track and the related drug pharmacology results have been accepted as a poster presentation at the February 2025 TANDEM Meeting. Market research recently completed by the Company with US healthcare professionals and payers confirmed the high unmet need to prevent BSIs in allo-HSCT patients and a desire for better prophylactic options, suggesting a strong value proposition for SER-155.

In December 2024, the US Food and Drug Administration (FDA) granted Breakthrough Therapy designation to SER-155 for reduction of BSIs in adults undergoing allo-HSCT. The Company submitted a Briefing Book in support of its planned interaction with FDA on a potential next registrational study of SER-155 in allo-HSCT and expects feedback from the agency this quarter.

Seres Therapeutics' SER-155 Phase 1b study biomarker data represents a significant scientific breakthrough in the treatment of allo-HSCT patients. The 77% relative risk reduction in bloodstream infections, coupled with statistically significant improvements in epithelial barrier integrity markers, provides compelling evidence of the treatment's efficacy. The FDA's Breakthrough Therapy designation further validates the potential clinical impact of SER-155. The Company's cash position of $31 million, combined with expected $75 million in Nestlé payments in 2025, extends runway into Q1 2026, providing adequate resources for advancing the SER-155 program through critical development stages.

The market research indicating strong payer receptivity and outpatient pharmacy benefit coverage potential significantly de-risks the commercial pathway for SER-155. The dual application potential in both infection prevention and inflammatory diseases expands the total addressable market substantially. The translational biomarker results demonstrate a dual mechanism of action - strengthening epithelial barrier integrity while modulating inflammatory pathways. This biological validation strengthens the clinical significance of the previously reported infection reduction data. The acceptance for presentation at the TANDEM Meeting in the Best Abstracts category underscores the scientific community's recognition of these findings.

The potential expansion into inflammatory bowel disease (IBD) applications, including ulcerative colitis and Crohn's disease, represents a strategic pivot that could significantly broaden the therapeutic utility of Seres' live biotherapeutics platform. The mechanistic insights from the SER-155 study provide a strong scientific rationale for this expansion. The healthcare professional and payer research reveals important market dynamics supporting SER-155's commercial potential. The identified high unmet need in BSI prevention for allo-HSCT patients, combined with the lack of effective prophylactic options, creates a compelling market opportunity. The anticipated outpatient pharmacy benefit coverage pathway could facilitate broader market access and adoption.

Seres Therapeutics' strategic pursuit of partnerships for both SER-155 and inflammatory disease applications demonstrates a pragmatic approach to maximizing commercial potential while managing development costs. The Breakthrough Therapy designation enhances partnering leverage and could accelerate the path to market for SER-155. As the Company continues to advance its live biotherapeutic platform, investors should closely monitor its progress and consider the potential of SER-155 and other pipeline candidates to address unmet medical needs and generate value for shareholders.