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Background CDI is a 2-hit process requiring C. difficile spores and antibiotic-mediated dysbiosis, a low diversity state of the gut microbiome. Recurrent CDI (rCDI) is common and may be related to inadequate antibiotic concentrations (e.g., metronidazole; MET) or persistent dysbiosis (e.g., vancomycin; VAN). SER-262 is an oral investigational microbiome drug rationally designed to reduce rCDI by restoring colonization resistance. Methods SERES-262-001 was a Phase 1b randomized placebo (PBO)-controlled single and multidose study. Subjects with primary CDI (n = 96) were enrolled in 8 cohorts (SER-262: PBO, 5:1). Subjects were dosed after MET (n = 57) or VAN (n = 39) per investigator discretion. Engraftment of SER-262 strains was evaluated using strain-specific molecular probes in fecal samples; microbial diversity was measured via whole metagenomic shotgun sequencing. Endpoints included safety and rCDI rates up to 8 weeks posttreatment and strain engraftment at 1, 4, 8, 12, and 24 weeks. Results SER-262 safety was comparable to PBO. Although overall rCDI rates were similar in SER-262 (n = 80) and PBO (n = 16) subjects (18.8% vs. 12.5%, respectively), in a post-hoc analysis we observed reduced rates of rCDI in the VAN+SER-262 arm compared with MET+SER-262 (6.3 vs. 27.1%, respectively, P = 0.02, Figure 1). Overall, 8 of 12 SER-262 strains showed significant engraftment relative to PBO. However, greater SER-262 strain engraftment was observed in VAN-treated subjects compared with MET-treated subjects (P &lt; 0.001, Figure 2). To better understand the impact of dysbiosis on engraftment, we evaluated baseline microbial diversity by prior antibiotic received and observed that the diversity of Bacteroidetes and Firmicute species was lower in VAN-treated subjects compared with MET-treated subjects (P &lt; 0.001, Figure 3). Conclusion In this first phase 1b study of a fermented microbiome drug in subjects with primary CDI, SER-262 was safe and well-tolerated. The higher efficacy rates of SER-262 in reducing rCDI among VAN-treated subjects may be due to low baseline microbial diversity, which creates an ecologic niche for greater engraftment of dose species. Treatment of C. difficile with VAN, followed by restoration of colonization resistance with SER-262, is a promising 2-pronged therapeutic paradigm to reduce rCDI. Disclosures All authors: No reported disclosures.

open forum infectious diseases

1503. Engraftment of Investigational Microbiome Drug, SER-262, in Subjects Receiving Vancomycin Is Associated with Reduced Rates of Recurrence after Primary Clostridium Difficile Infection (CDI)