ABSTRACT: Resistance to polymyxin antibiotics is increasing. Without new antibiotic classes, combination therapy is often required. We systematically investigated bacterial killing with polymyxin-based combinations against multidrug-resistant (including polymyxin-resistant), carbapenemase-producing Klebsiella pneumoniae Monotherapies and double- and triple-combination therapies were compared to identify the most efficacious treatment using static time-kill studies (24 h, six isolates), an in vitro pharmacokinetic/pharmacodynamic model (IVM; 48 h, two isolates), and the mouse thigh infection model (24 h, six isolates). In static time-kill studies, all monotherapies (polymyxin B, rifampin, amikacin, meropenem, or minocycline) were ineffective. Initial bacterial killing was enhanced with various polymyxin B-containing double combinations; however, substantial regrowth occurred in most cases by 24 h. Most polymyxin B-containing triple combinations provided greater and more sustained killing than double combinations. Standard dosage regimens of polymyxin B (2.5?mg/kg of body weight/day), rifampin (600?mg every 12 h), and amikacin (7.5?mg/kg every 12 h) were simulated in the IVM. Against isolate ATH 16, no viable bacteria were detected across 5 to 25 h with triple therapy, with regrowth to ?2-log10 CFU/ml occurring at 48 h. Against isolate BD 32, rapid initial killing of ?3.5-log10 CFU/ml at 5 h was followed by a slow decline to ?2-log10 CFU/ml at 48 h. In infected mice, polymyxin B monotherapy (60?mg/kg/day) generally was ineffective. With triple therapy (polymyxin B at 60?mg/kg/day, rifampin at 120?mg/kg/day, and amikacin at 300?mg/kg/day), at 24 h there was an ?1.7-log10 CFU/thigh reduction compared to the starting inoculum for all six isolates. Our results demonstrate that the polymyxin B-rifampin-amikacin combination significantly enhanced in vitro and in vivo bacterial killing, providing important information for the optimization of polymyxin-based combinations in patients.
