Time-Kill Evaluation of Antibiotic Combinations Containing Ceftazidime-Avibactam against Extensively Drug-Resistant Pseudomonas aeruginosa and Their Potential Role against Ceftazidime-Avibactam-Resistant Isolates

Abstract

Ceftazidime-avibactam (CZA) has emerged as a promising solution to the lack of new antibiotics against Pseudomonas aeruginosa infections. Data from in vitro assays of CZA combinations, however, are scarce. The objective of our study was to perform a time-kill analysis of the effectiveness of CZA alone and in combination with other antibiotics against a collection of extensively drug-resistant (XDR) P. aeruginosa isolates. Twenty-one previously characterized representative XDR P. aeruginosa isolates were selected. Antibiotic susceptibility was tested by broth microdilution, and results were interpreted using CLSI criteria. The time-kill experiments were performed in duplicate for each isolate. Antibiotics were tested at clinically achievable free-drug concentrations. Different treatment options, including CZA alone and combined with amikacin, aztreonam, meropenem, and colistin, were evaluated to identify the most effective combinations. Seven isolates were resistant to CZA (MIC=/16/4 mg/liter), including four metallo-B-lactamase (MBL)-carrying isolates and two class A carbapenemases. Five of them were resistant or intermediate to aztreonam (MIC=/16 mg/liter). Three isolates were resistant to amikacin (MIC=/64 mg/liter) and one to colistin (MIC=/4 mg/liter). CZA monotherapy had a bactericidal effect in 100% (14/14) of the CZA-susceptible isolates. Combination therapies achieved a greater overall reduction in bacterial load than monotherapy for the CZA-resistant isolates. CZA plus colistin was additive or synergistic in 100% (7/7) of the CZA-resistant isolates, while CZA plus amikacin and CZA plus aztreonam were additive or synergistic in 85%. CZA combined with colistin, amikacin, or aztreonam was more effective than monotherapy against XDR P. aeruginosa isolates. A CZA combination could be useful for treating XDR P. aeruginosa infections, including those caused by CZA-resistant isolates.