1299. In Vitro-In Vivo Discordance with β-lactams against Metallo-β-lactamase-Producing Enterobacterales: Implications for Susceptibility Testing

BACKGROUND: Using murine models of thigh and lung infection, we previously reported the potent in vivo activity of carbapenem human-simulated regimens against metallo-β-lactamase-producing Enterobacterales despite the observed resistance in vitro (JAC 2020 Apr 1;75(4):997-1005, AAC 2014;58(3):1671-7). In the current study, we examined the in vivo activity of cefepime human-simulated regimen against metallo-β-lactamase-producing Enterobacterales in a murine thigh infection model. METHODS: A population of clinical (n=21) and isogenic engineered (n=5) metallo-β-lactamase-producing Enterobacterales isolates expressing VIM, IMP or NDM but not co-expressing ESBLs or serine carbapenemases were utilized. KPC-producing strains (n=3) were included as positive controls. MICs of cefepime, piperacillin-tazobactam and meropenem were determined using broth microdilution in conventional cation-adjusted Muller Hinton and EDTA-supplemented broth at EDTA concentration of 300 mg/L (zinc-limited). The in vivo efficacy of a cefepime human-simulated regimen (2 g q8h as 2 h infusion) was determined in the neutropenic murine thigh infection model against the test isolates. Efficacy was measured as the change in log(10)cfu/thigh at 24 h compared with 0 h controls. RESULTS: Metallo-β-lactamase-producing Enterobacterales were found to be cefepime, piperacillin-tazobactam and meropenem non-susceptible in conventional broth. Supplementation with EDTA resulted in multi-fold reduction in the MICs and restoration of susceptibility. In accordance with the MICs generated in the zinc-limited broth, the administration of cefepime human-simulated regimen was associated with substantial bacterial reductions among mice infected with the clinical as well as the isogenic engineered metallo-β-lactamase-producing isolates. As anticipated with serine-based resistance, absence of MIC reduction in zinc-limited broth and lack of in vivo activity against KPC-producers were observed. CONCLUSION: For metallo-β-lactamase-producing Enterobacterales, in vitro susceptibility testing to β-lactams with conventional media such as cation-adjusted Muller Hinton broth, a zinc-rich testing medium, is flawed since it does not recapitulate the host environment in which zinc concentrations are low. DISCLOSURES: David P. Nicolau, PharmD, Cepheid (Other Financial or Material Support, Consultant, speaker bureau member or has received research support.)Merck & Co., Inc. (Consultant, Grant/Research Support, Speaker’s Bureau)Wockhardt (Grant/Research Support)