Evaluation of vancomycin and daptomycin against methicillin-resistant Staphylococcus aureus and heterogeneously vancomycin-intermediate S. aureus in an in vitro pharmacokinetic/pharmacodynamic model with simulated endocardial vegetations

Glycopeptides have historically been the drugs of choice for the treatment of infections caused by methicillin-resistant Staphylococcus aureus (MRSA). However, the continued selective pressure has led to the emergence of non-susceptible strains including heterogeneously vancomycin-intermediate S. aureus (hVISA). Infections with hVISA have been associated with poor outcomes including vancomycin treatment failures. The objective of this study was to evaluate vancomycin and daptomycin against vancomycin-susceptible MRSA and hVISA in a pharmacokinetic/pharmacodynamic (PK/PD) model with simulated endocardial vegetations. Six clinical isolates obtained from patients at the Detroit Medical Center were used: MRSA 494, MRSA 67, hVISA R1720, hVISS R2295, hVISA R3640 and hVISA R1629. All heteroresistant strains were confirmed by a population analysis profile ratio, with Mu3 as a control strain. Vancomycin regimens of 1 g every 12 h and 2 g every 12 h and daptomycin regimens of 6, 10 and 12 mg/kg daily were utilized in a PK/PD model over 72 h. Against MRSA isolates, vancomycin displayed minimal activity and minimal-to-no activity against hVISA. In general, the use of high dose vancomycin over standard dose vancomycin did not improve activity except against one of six isolates (MRSA 494). Daptomycin was bactericidal against both MRSA and hVISA isolates, although the rate of kill was slower against hVISA. Overall, daptomycin achieved rapid and effective kill against both MRSA and hVISA while vancomycin displayed slow and minimal kill against MRSA and minimal-to-no activity against hVISA, regardless of high dose exposure.