Seres Therapeutics, Inc. reported initial clinical data about SER-155. SER-155 is an oral, cultivated bacterial consortia investigational therapeutic designed to prevent enteric-derived infections and resulting blood stream infections, as well as induce immune tolerance responses to reduce the incidence of graft-versus-host disease (GvHD) in patients undergoing allogeneic hematopoietic stem cell transplantation (allo-HSCT). Gastrointestinal (GI) microbiome data from the first 100 days post HSCT in Cohort 1 of the SER-155 Phase 1b open-label study showed the successful engraftment of SER-155 bacterial strains and a substantial reduction in the cumulative incidence of pathogen domination, a biomarker associated with the risk of serious enteric infections and bloodstream infections, as well as GvHD.

The tolerability profile observed was favorable, with no serious adverse events attributed to SER-155 administration. Enrollment in the placebo-controlled Cohort 2 portion of the study is ongoing and topline results are anticipated in mid-2024. SER-155 Phase 1b Study Design and Summary of Cohort 1 Results: SER-155 is an investigational, oral, 16 strain, cultivated microbiome therapeutic designed to prevent colonization and reduce the abundance of ESKAPE pathogens (e.g., from families such as Enterococcaceae, Enterobacteriaceae, Streptococcaceae, Staphylococcaceae) in the GI tract to reduce the risk of enteric driven bloodstream infections and other downstream consequences, such as GvHD, in patients receiving allo-HSCT.

SER-155 has the potential to impact antimicrobial resistance (AMR), including infections caused by carbapenem-resistant Enterobacteriaceae (CRE) and vancomycin-resistant Enterococci (VRE). The development of SER-155 is informed by preclinical data that showed that allo-HSCT patients across multiple study sites had disrupted microbiomes and were susceptible to pathogen overgrowth in the GI tract, as well as in vivo studies that demonstrated SER-155's ability to significantly decolonize VRE and CRE, and further modulate epithelial barrier integrity and T cell biology or relevance to GvHD. Previously published exploratory results from the SER-109 ECOSPOR III Phase 3 study showed decolonization of gut pathogens, including bacteria that carry antibiotic resistance genes, providing clinical proof-of-concept for the SER-155 program.

The SER-155 Phase 1b study includes two cohorts, with Cohort 1 designed to assess safety and drug pharmacology, including the engraftment of drug bacteria in the gastrointestinal tract. Cohort 1 included 13 subjects who received any dosing of the SER-155 regimen, with 11 of these subjects subsequently receiving an allo-HSCT. Nine subjects had evaluable samples for microbiome data analyses.

The average age in Cohort 1 was 60, and most subjects had acute myeloid leukemia, myelodysplastic syndrome or myeloproliferative neoplasia as their primary disease and received reduced-intensity conditioning pre-transplant. Most subjects received peripheral blood stem cells from a matched unrelated donor. Neutrophil engraftment was observed in all subjects.

The majority of subjects received a tacrolimus-based regimen for GvHD prophylaxis. Published data have demonstrated an association between the incidence of GI domination with ESKAPE pathogens in allo-HSCT patients and the risk of serious infections, GvHD, and mortality. Reducing the incidence of domination of ESKAPE pathogens in the GI microbiome through administration of a microbiome therapeutic has the potential to meaningfully reduce enteric infections, enteric driven blood stream infections and GvHD in this medically compromised patient population and more broadly.

Enrollment of Cohort 2 is ongoing, incorporating a randomized, double-blinded placebo-controlled design to further evaluate safety, drug strain engraftment, and incidence of gastrointestinal ESKAPE pathogen domination, as well as the incidence of enteric infections, enteric driven blood stream infections and GvHD. Cohort 2 will enroll approximately 60 subjects administered either SER-155 or placebo at a 1:1 ratio. The Company anticipates obtaining top-line placebo-controlled day-100 data from study Cohort 2 in mid-2024.

Seres believes that the medical benefit and commercial potential for SER-155 is substantial. Nearly 30,000 allo-HSCT procedures are performed in the U.S. and Europe per year. Complications are frequent and costly, with additional costs for patients with complications averaging approximately $180,000/year.

Infections and GvHD are estimated to result in nearly half of mortality associated with allo-HSCT.4 With the exception of an exclusive license from MSK, Seres fully owns worldwide rights for commercialization of SER-155. In addition to SER-155, the Company is evaluating other microbiome therapeutic preclinical programs for additional medically compromised patient populations who are at risk of life-threatening infections.