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.