Background: Acute graft-versus-host disease (aGvHD) is a life-threatening complication following allo-HCT. Microbiome dysfunction and associated GI barrier disruption may contribute to aGvHD via activation of inflammatory immune responses. Circulating biomarkers in the peri-transplant period are correlated with risk of severe aGvHD, including suppression of tumorigenicity 2 (ST2), regenerating islet-derived 3-alpha (Reg3a) and inflammatory cytokines (IL6, IFNϒγ& TNFα). SER-155 is an investigational cultivated oral microbiome therapeutic designed to restructure the GI microbiome, improve GI barrier integrity and reduce GI inflammation and is being assessed in a 2-part Phase 1b study in adults undergoing allo-HCT. We present preliminary safety, GvHD, PK, and exploratory biomarker data from the completed open-label Cohort 1 through Day 100 post-HCT. Methods: Adult recipients of allo-HCT were eligible. HCT conditioning and aGvHD prophylaxis were per investigator discretion. Patients received 2 courses of SER-155 (pre-HCT and post-neutrophil engraftment, each comprised of 4 days of microbiome conditioning with oral vancomycin followed by 10 days oral SER-155. The primary endpoint was safety. Strain engraftment (PK) and biomarkers were assessed in stool and plasma samples. Results: Fifteen adults enrolled; 13 received at least 1 course of study drug (median age 67; 54% male), and 11 underwent allo-HCT. Following each course, the majority of the 16 SER-155 strains were detected in stool. Treatment-emergent adverse events (AEs) were reported in all patients with GI AEs the most common. No serious AEs (SAEs) were deemed related to SER-155. Most SAEs and AEs of special interest (ie, BSIs, invasive or GI infections) occurred between HCT and the 2nd SER-155 course. There were no deaths before Day 100. No BSIs were attributable to SER-155 strains. The rate of grade 2-4 aGvHD based on MAGIC criteria through Day 100 was 45.5%; no severe grade 3-4 aGvHD was observed. Median levels of plasma biomarkers REG3α, ST2 and inflammatory cytokines were not elevated relative to a reference range (Table). Conclusions: In this small open-label cohort, SER-155 was generally well-tolerated through Day 100 without drug-related SAEs and no severe aGvHD. Plasma biomarkers were not elevated at HCT Day 0 or neutrophil recovery. These preliminary observations align with the design and preclinical evaluation of SER-155. Clinical trial information: NCT04995653.

#pg/mL; median (range); *N=8.