LAUSR

ABSTRACT Klebsiella pneumoniae carbapenemases (KPCs) are widespread β-lactamases that are a major cause of clinical non-susceptibility of Gram-negative bacteria to carbapenems and other β-lactam antibiotics. Ceftazidime combined with the β-lactamase inhibitor avibactam (CAZ–AVI) has been effective in treating infections by KPC-producing bacteria, but emerging KPC variants confer resistance to the combination. Taniborbactam and ledaborbactam are bicyclic boronate β-lactamase inhibitors currently under development with cefepime and ceftibuten, respectively, to treat carbapenem-resistant bacterial infections. Here, we assessed the effects of clinically important KPC-2 and KPC-3 variants (V240G, D179Y, and D179Y/T243M) on the antibacterial activity of cefepime–taniborbactam (FEP–TAN) and ceftibuten–ledaborbactam (CTB–LED) and examined catalytic activity and inhibition of these variants. Unlike CAZ–AVI, FEP–TAN and CTB–LED were highly active against Escherichia coli strains expressing these KPC variants. Experiments with purified enzymes showed that FEP and CTB were poorly hydrolyzed by the KPC variants and had weak affinity for variants containing D179Y. In addition, the D179Y substitution in KPC-2 reduced inhibition by TAN and LED, but inactivation efficiencies (k2/K) for these inhibitors were significantly higher than those for AVI. K2/K was less affected for D179Y-containing KPC-3 variants, and robust inhibition was observed by TAN, LED, and AVI. Together, the findings illustrate a biochemical basis for FEP–TAN and CTB–LED efficacy in KPC variant-mediated CAZ–AVI resistance backgrounds, whereby the boronate inhibitors have sufficient inhibitory activity, while FEP and CTB are poor substrates and bind to the variant enzymes with reduced affinity.