Can Ceftazidime-Avibactam and Aztreonam Overcome β-Lactam Resistance Conferred by Metallo-β-Lactamases in Enterobacteriaceae?

Abstract

Based upon knowledge of the hydrolytic profile of major β-lactamases found in Gram-negative bacteria, we tested the efficacy of the combination of ceftazidime-avibactam (CAZ-AVI) with aztreonam (ATM) against carbapenem-resistant enteric bacteria possessing metallo-β-lactamases (MBLs). Disk diffusion and agar-based antimicrobial susceptibility testing were initially performed to determine the in vitro efficacy of a unique combination of CAZ-AVI and ATM against 21 representative Enterobacteriaceae isolates with a complex molecular background that included bla_IMP, bla_NDM, bla_OXA-48, bla_CTX-M, bla_AmpC, and combinations thereof. Time-kill assays were conducted, and the in vivo efficacy of this combination was assessed in a murine neutropenic thigh infection model. By disk diffusion assay, all 21 isolates were resistant to CAZ-AVI alone, and 19/21 were resistant to ATM. The in vitro activity of CAZ-AVI in combination with ATM against diverse Enterobacteriaceae possessing MBLs was demonstrated in 17/21 isolates, where the zone of inhibition was ≥21 mm. All isolates demonstrated a reduction in CAZ-AVI agar dilution MICs with the addition of ATM. At 2 h, time-kill assays demonstrated a ≥4-log₁₀-CFU decrease for all groups that had CAZ-AVI with ATM (8 μg/ml) added, compared to the group treated with CAZ-AVI alone. In the murine neutropenic thigh infection model, an almost 4-log₁₀-CFU reduction was noted at 24 h for CAZ-AVI (32 mg/kg every 8 h [q8h]) plus ATM (32 mg/kg q8h) versus CAZ-AVI (32 mg/kg q8h) alone. The data presented herein require us to carefully consider this new therapeutic combination to treat infections caused by MBL-producing Enterobacteriaceae.

Keywords: avibactam; aztreonam; ceftazidime; disk diffusion; metallo-β-lactamases.

FIG 1

ATM placed directly on the CAZ-AVI disk to evaluate synergy. E. cloacae isolate 6.31 was used in this assay.

FIG 2

Time-kill curve for K. pneumoniae isolate 1.41. ATM concentrations were held constant at 8 μg/ml for all combinations, with two exceptions: (i) the growth control (no antibiotics added) and (ii) CAZ-AVI with no ATM at 8× the MIC. Various ceftazidime-avibactam (CAZ-AVI) concentrations were added corresponding to 1× (1 μg/ml CAZ plus 0.25 μg/ml AVI), 2× (2 μg/ml CAZ plus 0.5 μg/ml AVI), 4× (4 μg/ml CAZ plus 1 μg/ml AVI), and 8× (8 μg/ml CAZ plus 2 μg/ml AVI) the MIC of the combination CAZ-AVI plus ATM obtained by agar dilution (1 μg/ml). Three replicates were conducted for each of the conditions reported in the time-kill assay.

FIG 3

Individual and mean counts of log₁₀ CFU per thigh for various antibiotic–β-lactamase inhibitor combinations in the neutropenic thigh infection model for K. pneumoniae 1.41-infected mice, with 5 log₁₀ CFU/mouse and 5 mice per treatment group. LOQ, limit of quantitation (2.35 log₁₀ CFU/thigh).