In Vitro Activity of the Ultra-Broad-Spectrum Beta-Lactamase Inhibitor QPX7728 in Combination with Meropenem against Clinical Isolates of Carbapenem-Resistant Acinetobacter baumannii

QPX7728 is a recently discovered ultra-broad-spectrum beta-lactamase inhibitor (BLI) with potent inhibition of key serine and metallo-beta-lactamases. QPX7728 enhances the potency of many beta-lactams, including carbapenems, in beta-lactamase-producing Gram-negative bacteria, including Acinetobacter spp. The potency of meropenem alone and in combination with QPX7728 (1 to 16 mu g/ml) was tested against 275 clinical isolates of Acinetobacter baumannii (carbapenem-resistant A. baumannii [CRAB]) collected worldwide that were highly resistant to carbapenems (MIC50 and MIC90 for meropenem, 64 and >64 mu g/ml). Addition of QPX7728 resulted in a marked concentration-dependent increase in meropenem potency, with the MIC90 of meropenem alone decreasing from >64 mu g/ml to 8 and 4 mu g/ml when tested with fixed concentrations of QPX7728 at 4 and 8 mu g/ml, respectively. In order to identify the mechanisms that modulate the meropenem-QPX7728 MIC, the whole-genome sequences were determined for 135 isolates with a wide distribution of meropenem-QPX7728 MICs. This panel of strains included 116 strains producing OXA carbapenemases (71 OXA-23, 16 OXA-72, 16 OXA-24, 9 OXA-58, and 4 OXA-239), 5 strains producing NDM-1, one KPC-producing strain, and 13 strains that did not carry any known carbapenemases but were resistant to meropenem (MIC >= 4 mu g/ml). Our analysis indicated that mutated PBP3 (with mutations localized in the vicinity of the substrate/inhibitor binding site) is the main factor that contributes to the reduction of meropenem-QPX7728 potency. Still, >90% of isolates that carried PBP3 mutations remained susceptible to <= 8 mu g/ml of meropenem when tested with a fixed 4 to 8 mu g/ml of QPX7728. In the absence of PBP3 mutations, the MICs of meropenem tested in combination with 4 to 8 mu g/ml of QPX7728 did not exceed 8 mu g/ml. In the presence of both PBP3 and efflux mutations, 84.6% of isolates were susceptible to <8 mu g/ml of meropenem with 4 or 8 mu g/ml of QPX7728. The combination of QPX7728 with meropenem against CRAB isolates with multiple resistance mechanisms has an attractive microbiological profile.