Pharmacodynamics of Ceftaroline against Staphylococcus aureus Studied in anIn VitroPharmacokinetic Model of Infection

Abstract

ABSTRACTAnin vitrosingle-compartment dilutional pharmacokinetic model was used to study the pharmacodynamics of ceftaroline againstStaphylococcus aureus(both methicillin-susceptibleS. aureus[MSSA] and methicillin-resistantS. aureus[MRSA]). Mean serum free concentrations of ceftaroline (the active metabolite of the prodrug ceftaroline fosamil) dosed in humans at 600 mg every 12 h (q12h) were simulated, and activities against 12S. aureusstrains (3 MSSA strains and 9 MRSA strains, 3 of which had a vancomycin-intermediate phenotype) were determined. Ceftaroline produced 2.5- to 4.0-log10-unit reductions in viable counts by 24 h with all strains and a 0.5- to 4.0-log-unit drop in counts at 96 h. The antibacterial effect could not be related to the strain MIC across the ceftaroline MIC range from 0.12 to 2.0 μg/ml. In dose-ranging studies, the cumulative percentage of a 24-h period that the free drug concentration exceeded the MIC under steady-state pharmacokinetic conditions (fTMIC) of 24.5% ± 8.9% was associated with a 24-h bacteriostatic effect, one of 27.8% ± 9.5% was associated with a −1-log-unit drop, and one of 32.1% ± 8.1% was associated with a −2-log-unit drop. The MSSA and MRSA strains had similarfTMICvalues.fTMICvalues increased with increasing duration of exposure up to 96 h. Changes in ceftaroline population analysis profiles were related tofTMIC.fTMICs of <50% were associated with growth on 4× MIC recovery plates at 96 h of drug exposure. These data support the use of ceftaroline fosamil at doses of 600 mg q12h to treatS. aureusstrains with MICs of ≤2 μg/ml. AnfTMICof 25 to 30% would make a suitable pharmacodynamic index target, butfTMICvalues of ≥50% are needed to suppress the emergence of resistance and require clinical evaluation.