The combination of aztreonam and avibactam has been proposed for the treatment of infections caused by metallo-β-lactamase-producing Gram-negative organisms, given the stability of aztreonam against metallo-β-lactamases plus the broad coverage of avibactam against AmpC β-lactamases and ESBLs. This study aimed to evaluate the efficacy of the combination against four clinical isolates with defined but diverse β-lactamase profiles.Methods
The MICs of aztreonam were determined without and with avibactam (1, 2, 4, 8 and 16 mg/L). Using the MIC values, the static time–kill kinetic studies were designed to encompass aztreonam concentrations of 0.25, 0.5, 1, 2 and 4 times the MIC at the respective avibactam concentrations from 0 to 8 mg/L. Aztreonam and avibactam concentrations were determined by LC-MS/MS during the course of the time–kill kinetic studies to evaluate whether avibactam protects aztreonam from degradation.Results
Three of the four isolates had aztreonam MICs ≥128 mg/L in monotherapy. Dramatically increasing susceptibility associated with a decrease in aztreonam MIC was observed with increasing avibactam concentration. Against all isolates, the combinations resulted in greater killing with a much lower dose requirement for aztreonam. The resulting changes in base-10 logarithm of cfu/mL at both the 10 h and 24 h references (versus 0 h) were synergistic. In contrast, a significantly higher concentration of aztreonam in the monotherapy was required to produce the same kill as that in the combination therapy, due to rapid aztreonam degradation in two isolates.Conclusions
The aztreonam/avibactam combination protects aztreonam from hydrolysis and provides synergy in antimicrobial activity against multiple β-lactamase-expressing strains with a wide MIC range.