Reclaiming the efficacy of β-lactam-β-lactamase inhibitor combinations: Avibactam restores the susceptibility of CMY-2-producing Escherichia coli to ceftazidime

Abstract

CMY-2 is a plasmid-encoded Ambler class C cephalosporinase that is widely disseminated in Enterobacteriaceae and is responsible for expanded-spectrum cephalosporin resistance. As a result of resistance to both ceftazidime and p-lactamase inhibitors in strains carrying blaCMY, novel β-lactam-β-lactamase inhibitor combinations are sought to combat this significant threat to p-lactam therapy. Avibactam is a bridged diazabicyclo [3.2.1]octanone non-β-lactam β-lactamase inhibitor in clinical development that reversibly inactivates serine β-lactamases. To define the spectrum of activity of ceftazidime-avibactam, we tested the susceptibilities of Escherichia coli clinical isolates that carry blaCMY-2 or bla cmy-69 and investigated the inactivation kinetics of CMY-2. Our analysis showed that CMY-2-containing clinical isolates of E. coli were highly susceptible to ceftazidime-avibactam (MIC90, ≥0.5 mg/liter); in comparison, ceftazidime had a MIC90 of > 128 mg/liter. More importantly, avibactam was an extremely potent inhibitor of CMY-2 β-lactamase, as demonstrated by a second-order onset of acylation rate constant (k2/K) of (4.9 ± 0.5) X 104 M-1 s-1 and the off-rate constant (koff) of (3.7 ± 0.4) x10-4 s-1. Analysis of the reaction of avibactam with CMY-2 using mass spectrometry to capture reaction intermediates revealed that the CMY-2-avibactam acylenzyme complex was stable for as long as 24 h. Molecular modeling studies raise the hypothesis that a series of successive hydrogen-bonding interactions occur as avibactam proceeds through the reaction coordinate with CMY-2 (e.g., T316, G317, S318, T319, S343, N346, and R349). Our findings support the microbiological and biochemical efficacy of ceftazidime-avibactam against E. coli containing plasmid-borne CMY-2 and CMY-69. Copyright © 2014, American Society for Microbiology. All Rights Reserved.