In vitro pharmacodynamics of polymyxin B and tigecycline alone and in combination against carbapenem-resistant Acinetobacter baumannii.

Carbapenem-resistant Acinetobacter baumannii is increasing in prevalence. Polymyxin B and tigecycline are among the most active antibiotics used against this pathogen in vitro. Past in vitro studies, however, neglected the importance of simulating exposures observed in humans to determine their antibacterial effects. In this study, four carbapenem-resistant A. baumannii isolates were evaluated using an in vitro pharmacodynamic model. Free-drug exposures using 1 mg/kg of body weight of polymyxin B every 12 h (q12h), 100 and 200 mg tigecycline q12h, and the combination of these regimens were simulated. The microbiological responses to these treatments were measured by the change in log10 CFU/ml over 24 h and the area under the bacterial killing and regrowth curve (AUBC). Resistance was assessed by a population analysis profile (PAP) conducted after 24 h of treatment. Polymyxin B achieved a reduction on the order of -2.05 ± 0.68 log10 CFU/ml against these A. baumannii isolates, while all isolates grew to control levels with tigecycline monotherapy. Combination therapy with polymyxin B plus 200 mg tigecycline q12h achieved a greater reduction in bacterial density than did therapy with polymyxin B alone (-3.31 ± 0.71 versus -2.05 ± 0.68 log10 CFU/ml, P < 0.001) but not significantly different than combination therapy with 100 mg tigecycline q12h (-2.45 ± 1.00 log10 CFU/ml, P = 0.370). Likewise, combination therapy with polymyxin B plus 200 mg tigecycline q12h significantly reduced the AUBC compared to that with polymyxin B alone (62.8 ± 8.9 versus 79.4 ± 10.5 log10 CFU/ml, P < 0.05). No changes in the PAP from baseline were observed for either antibiotic alone. In this study, combination therapy with simulated exposures of polymyxin B and tigecycline at an aggressive dose of 200 mg q12h produced synergistic or additive effects on humans against these multidrug-resistant A. baumannii strains.