A mechanism-based pharmacokinetic/pharmacodynamic analysis of polymyxin B-based combination therapy against carbapenem-resistant Klebsiella pneumoniae isolates with diverse phenotypic and genotypic resistance mechanisms.

Increased resistance to β-lactams/β-lactamase inhibitors by mutations in β-lactamase genes, porins, and efflux pumps complicates the management of carbapenem-resistant Klebsiella pneumoniae (CRKP). Polymyxin B (PMB)-based combination therapy is the best alternative treatment for middle and low-income countries that cannot access the latest medicines. It is crucial to know both phenotypic and genotypic characteristics of a pathogen to understand the killing effect of each drug and its combinations. Hence, our objective was to incorporate mechanistic insights gained from resistance mechanisms of each isolate to develop a mechanism-based pharmacokinetic/pharmacodynamic model. Six clinical CRKP isolates with diverse genotypic resistance expressing blaKPC, blaNDM, porin, and mgrB mutations were used for static concentration time kill (SCTK) assays to evaluate the rate and extent of killing by monotherapy, double and triple combinations using PMB (0.5-64 mg/L), meropenem (10-120 mg/L), and fosfomycin (75-500 mg/L). Isolate BRKP28 expressed non-functional MgrB (a regulatory protein) and high-level phenotypic resistance (PMB MIC: >128 mg/L). In line with the observed resistance, the model estimated that BRKP28 had a reduced maximum killing rate constant for PMB (3.61 h⁻¹) relative to other isolates. The mechanistic synergy of PMB, due to outer membrane disruption, was incorporated into three isolates with porin mutations. PMB demonstrated 83%-88% mechanistic synergy with meropenem and 81%-98% with fosfomycin. The model further estimated that a very low concentration of PMB (0.49-0.64 mg/L) was sufficient to achieve 50% of the maximum synergy. Simulations using population pharmacokinetic models showed that combination therapy of PMB (1 mg/kg q12h) and fosfomycin (8 g q8h) achieved >73% reduction in area under the bacterial load-versus-time curve across four isolates. The triple combination therapy achieved a 67.7% reduction in non-carbapenamase producing isolate. These findings demonstrates that a low PMB dosing regimen (1 mg/kg q12h) can produce synergistic effects in combination therapy and may be effective in managing infections caused by CRKP, including PMB resistant isolates.