Polymyxin triple combinations against polymyxin-resistant, multidrug-resistant KPC-producing Klebsiella pneumoniae.

: 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 Mono, double and triple combination therapies were compared to identify the most efficacious treatment using static time-kill studies (24h, six isolates), an in vitro pharmacokinetic/pharmacodynamic model (IVM; 48h, two isolates), and the mouse thigh infection model (24h, six isolates). In static time-kill studies, all monotherapies (polymyxin B/rifampicin/amikacin/meropenem/minocycline) were ineffective. Initial bacterial killing was enhanced with various polymyxin B-containing double combinations, however substantial regrowth occurred in most cases by 24h. 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/d), rifampicin (600 mg/12-hourly) and amikacin (7.5 mg/kg/12-hourly) were simulated in the IVM. Against isolate ATH 16, no viable bacteria were detected across 5-25h with triple therapy, with regrowth to ∼2-log10 CFU/mL occurring at 48h. Against isolate BD 32, rapid initial killing of ∼3.5-log10 CFU/mL at 5h was followed by a slow decline to ∼2-log10 CFU/mL at 48h. In infected mice, polymyxin B monotherapy (60 mg/kg/d) was generally ineffective. With triple therapy (polymyxin B 60 mg/kg/d, rifampicin 120 mg/kg/d, and amikacin 300 mg/kg/d), at 24h 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/rifampicin/amikacin combination significantly enhanced in vitro and in vivo bacterial killing, providing important information for the optimization of polymyxin-based combinations in patients.