Increased activity of linezolid in combination with rifampicin in a murine pneumonia model due to MRSA.

Objectives: The chloramphenicol/florfenicol resistance gene cfr, which mediates cross-resistance to linezolid and other classes of antimicrobial agents, represents a global therapeutic challenge due to its dissemination among MDR nosocomial pathogens, including MRSA. This study aimed to compare the efficacy of the linezolid/rifampicin combination in a murine pneumonia model caused by cfr-positive and cfr-negative clinical MRSA strains. Methods: Synergistic activity between linezolid and rifampicin was evaluated by chequerboard and time-kill assays. Pharmacokinetic profiles in plasma and epithelial lining fluid (ELF) as well as the therapeutic efficacy of linezolid alone and in combination with rifampicin were investigated in a murine pneumonia model. The Emax Hill equation was used to model the dose-response relationship. Results: Increased susceptibility of the study MRSA strains to linezolid was observed with the rifampicin combination (MIC decreased 2- to 16-fold versus linezolid alone). The combination had synergistic activity (fractional inhibitory concentration index ≤0.5) against all cfr-positive MRSA isolates. Linezolid demonstrated excellent pulmonary penetration with an ELF/fplasma AUC ratio of 2.68 ± 0.17. The addition of rifampicin significantly improved the efficacy of linezolid in the pneumonia model due to cfr-positive and cfr-negative MRSA strains. The fAUC/MIC targets of linezolid associated with stasis, 1 log10 kill and 2 log10 kill were 15.9, 38.8 and 175 in plasma, and 43.5, 108 and 415 in ELF, respectively. Importantly, the linezolid fAUC/MIC targets in both plasma and ELF were 2.4-6.7 times lower in combined linezolid/rifampicin therapy versus linezolid monotherapy (P < 0.005). Conclusions: Combination of linezolid with rifampicin significantly improved the efficacy of linezolid in the murine pneumonia model caused by MRSA strains in the presence and absence of the cfr gene.