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Background There is a pressing need for development of oral antibiotics with activity against SBL-EB, particularly carbapenemase-producers, for use in the community or as step-down therapy for complicated urinary tract infection. VNRX-7145 is a novel boronic acid-based SBL inhibitor with no intrinsic activity that was designed as an orally bioavailable prodrug. The active moiety (VNRX-5236) is known to restore in vitro susceptibility to (CTB), an oral cephalosporin, among CTB-resistant SBL-EB. Methods CTB-resistant SBL-EB (N = 21) with CTB MICs ≥32 µg/mL and CTB/VNRX-5236 MIC range 0.12–2 µg/mL (VNRX-5236 fixed at 4 µg/mL) were evaluated. Carbapenemases were produced by 9 strains (4 OXA, 5 KPC). Bacterial suspensions (~107 CFU/mL) were used to inoculate the thighs of neutropenic mice. A human-simulated regimen of ceftibuten (CTB HSR) equivalent to a 400 mg q12h dosage was developed in infected mice. In dose ranging studies, groups of 3 animals each received the CTB HSR as monotherapy or combined with escalating VNRX-5236 exposures (CTB:VNRX-5236 dose ratios ranging from 10:1 to 1:4). Efficacy was assessed as the change in log10 CFU/thigh at 24 hours from 0 hour burden. With previous in vivo dose fractionation studies indicating the free area under the VNRX-5236 concentration–time curve to MIC ratio (fAUC0-24/MIC) as the PK/PD driver of efficacy, the Hill equation was used to estimate the magnitude required to achieve a static endpoint. Results Compared with 0 hour controls (mean log10 CFU/thigh, 5.7 ± 0.3), the bacterial burden for all isolates increased in saline-dosed controls and CTB HSR groups by 3.1 ± 0.8 and 2.5 ± 0.8 log10 CFU/thigh, respectively. The addition of VNRX-5236 resulted in bacterial stasis in 20/21 strains; the mean reduction in bacterial burden with the 1:1 CTB:VNRX-5236 dose ratio was −0.2 ± 0.7 log10 CFU/thigh. A composite assessment of exposure-responses indicated a fAUC0-24/MIC of 9.0 (R2 = 0.70) was associated with stasis. Conclusion Against CTB-resistant SBL-EB, inclusive of OXA-48- and KPC-producing strains, VNRX-5236 potentiated the in vivo activity of the CTB human-simulated exposure. The identified fAUC0-24/MIC target associated with bacterial stasis should be considered when selecting VNRX-7145 doses for clinical studies. Disclosures All authors: No reported disclosures.

open forum infectious diseases

682. In Vivo Pharmacodynamics of VNRX-7145 in the Neutropenic Murine Thigh Infection Model When Administered in Combination with Humanized Exposures of Twice Daily Ceftibuten (CTB) Against Serine β-Lactamase-Producing Enterobacteriaceae (SBL-EB)