2063. Elucidating the Gram-Positive Selective Spectrum Activity of Ibezapolstat; Secondary Analysis from the phase 2a trial

BACKGROUND: Ibezapolstat (IBZ) is a non-absorbable antimicrobial currently in phase 2 clinical trials for the treatment of Clostridioides difficile infection (CDI). In vitro and human studies have shown potent activity of IBZ against C. difficile but selective activity against other beneficial Gram-positive gut microbiota shown to reduce the risk of recurrent CDI. As the target DNA Pol IIIC enzyme is present in most Gram-positive species, the reasons for this selectivity are unclear. The purpose of this study was to assess the selectivity of IBZ against Gram-positive gut commensal microbiota. METHODS: Using stool samples and microbiome data from the phase 2a CDI study, concentration changes in certain Gram-positive commensals were analyzed by qPCR over time in patients with CDI given IBZ. Gram-positive isolates were cultured from stool, speciated, and MIC determined. AlphaFold2-enabled drug docking was used to assess in silico differences in drug-binding residue sites predictive of IBZ binding across a large range of Gram-positive bacteria isolated from the stool. RESULTS: From the phase 2a study, relative abundance of Firmicutes and concentrations of certain species (Clostridium leptum and Clostridium coccoides groups) increased while on IBZ therapy. Thirty-six strains were isolated from Families Clostridiaceae (n=25), Enterococcaceae (n=3), Lachnospiraceae (n=3), and Peptostreptococcaceae (n=5). MIC50/90 values averaged 1.5/3.0 ug/mL (geometric mean) for strains isolated at baseline and early therapy (days 1-3) and 12/126 ug/mL for samples taken later on therapy and three-day follow-up. Two strains isolated at day 30 follow-up were < 2 ug/mL and another was >128 ug/mL. All C. difficile strains were IBZ susceptible (MIC<2 ug/mL). Using a protein sequence alignment from more than 500 Firmicute PolC sequences, potential contact points of interest were identified that helped explain the selectivity of Ibezapolstat. CONCLUSION: Increased Firmicute abundance and concentration may be explained by selection of non-susceptible Firmicute species during therapy. Distinct IBZ drug-binding residues in the Pol IIIC enzyme may explain this selectivity. Structural biology experiments will confirm these results. DISCLOSURES: Eugenie Basseres, PhD, Accurx: Grant/Research Support Kevin W. Garey, PharmD, MS, Acurx: Grant/Research Support|Ferring: Advisor/Consultant|Paratek: Grant/Research Support