Nacubactam Enhances Meropenem Activity against Carbapenem-Resistant Klebsiella pneumoniae Producing KPC

Carbapenem-resistant Enterobacteriaceae (CRE) are resistant to most antibiotics, making CRE infections extremely difficult to treat with available agents. Klebsiella pneurnoniae carbapenemases (KPC-2 and KPC-3) are predominant carbapenemases in CRE in the United States. Nacubactam is a bridged diazabicyclooctane (DBO) beta-lactamase inhibitor that inactivates class A and C beta-lactamases and exhibits intrinsic antibiotic and beta-lactam "enhancer" activity against Enterobacteriaceae. In this study, we examined a collection of meropenem-resistant K pneumoniae isolates carrying bla(KPC-2) or bla(KPC-3); meropenem-nacubactam restored susceptibility. Upon testing isogenic Escherichia coli strains producing KPC-2 variants with single residue substitutions at important Ambler class A positions (K73, S130, R164, E166, N170, D179, K234, E276, etc.), the K234R variant increased the meropenemnacubactam MIC compared to that for the strain producing KPC-2, without increasing the meropenem MIC. Correspondingly, nacubactam inhibited KPC-2 (apparent K-i [K-iapp] = 31 +/- 3 mu M) more efficiently than the K234R variant (K-i app = 270 +/-27 mu M) and displayed a faster acylation rate (k(2)/K, which was 5,815 +/- 582 M-1 s(-1) for KPC-2 versus 247 +/- 25 M-1 s(-1) for the K234R variant. Unlike avibactam, timed mass spectrometry revealed an intact sulfate on nacubactam and a novel peak (+337 Da) with the K234R variant. Molecular modeling of the K234R variant showed significant catalytic residue (i.e., 570, K73, and 5130) rearrangements that likely interfere with nacubactam binding and acylation. Nacubactam's aminoethoxy tail formed unproductive interactions with the K234R variant's active site. Molecular modeling and docking observations were consistent with the results of biochemical analyses. Overall, the meropenem-nacubactam combination is effective against carbapenem-resistant K. pneumoniae. Moreover, our data suggest that p-lactamase inhibition by nacubactam proceeds through an alternative mechanism compared to that for avibactam.