In vitro pharmacodynamics of vancomycin and cefazolin alone and in combination against methicillin-resistant Staphylococcus aureus

Abstract

Previous studies employing time-kill methods have observed synergistic effects against methicillin-resistant Staphylococcus aureus (MRSA) when a β-lactam is combined with vancomycin. However, these time-kill studies have neglected the importance of human-simulated exposures. We evaluated the effect of human simulated exposures of vancomycin at 1 g every 8 h (q8h) in combination with cefazolin at 1 g q8h against various MRSA isolates. Four clinical isolates (two MRSA isolates [vancomycin MICs, 0.5 and 2.0 μg/ml], a heterogeneous vancomycin-intermediate S. aureus [hVISA] isolate [MIC, 2.0 μg/ml], and a vancomycin-intermediate S. aureus [VISA] isolate [MIC, 8.0 μg/ml]) were evaluated in an in vitro pharmacodynamic model with a starting inoculum of 10(6) or 10(8) CFU/ml. Bacterial density was measured over 48 to 72 h. Time-kill curves were constructed, and the area under the bacterial killing and regrowth curve (AUBC) was calculated. During 10(6) CFU/ml studies, combination therapy achieved greater log(10) CFU/ml changes than vancomycin alone at 12 h (-4.31 ± 0.58 versus -2.80 ± 0.59, P < 0.001), but not at 48 h. Combination therapy significantly reduced the AUBC from 0 to 48 h (122 ± 14) compared with vancomycin alone (148 ± 22, P = 0.017). Similar results were observed during 10(8) CFU/ml studies, where combination therapy achieved greater log(10) CFU/ml changes at 12 h than vancomycin alone (-4.00 ± 0.20 versus -1.10 ± 0.04, P < 0.001) and significantly reduced the AUBC (275 ± 30 versus 429 ± 37, P < 0.001) after 72 h of incubation. In this study, the combination of vancomycin and cefazolin at human-simulated exposures improved the rate of kill against these MRSA isolates and resulted in greater overall antibacterial effect, but no differences in bacterial density were observed by the end of the experiments.

Fig 1

Mean bacterial densities over 48 h for MRSA Hosp2-23 (A), MRSA STA 336 (B), hVISA STA 449 (C), and VISA STA 454 (D). Closed circles, growth control; closed triangles (up), vancomycin monotherapy for MRSA Hosp2-23; closed triangles (down), vancomycin monotherapy for MRSA STA 336; closed squares, vancomycin monotherapy for hVISA STA 449; closed diamonds, vancomycin monotherapy for VISA STA 454; open triangles (up), combination therapy for MRSA Hosp2-23; open triangles (down), combination therapy for MRSA STA 336; open squares, combination therapy for hVISA STA 449; open diamonds, combination therapy for VISA STA 454. Data are plotted as the means of the two models for the treatments and the mean of all corresponding growth control isolates. The lower limit of detection (dashed lines) was 1.7 log₁₀ CFU/ml.

Fig 2

Mean bacterial densities over 72 h for MRSA STA 336 and hVISA STA 449. Closed circles, growth control; closed triangles (down), monotherapy for MRSA STA 336; closed squares, vancomycin monotherapy for hVISA STA 449; open triangles (down), monotherapy for MRSA STA 336; open squares, combination therapy for hVISA STA 449. Data are plotted as the means of the two models for the treatments and the mean of all corresponding growth control isolates. The lower limit of detection (dashed line) was set to 1.7 log₁₀ CFU/ml.