Vancomycin: We Can't Get There from Here

Background. We sought to characterize the pharmacodynamic profile of the more intensive vancomycin dosing regimens currently used in response to the recent vancomycin guidelines. Methods. A series of Monte Carlo simulations was performed for vancomycin regimens ranging from .5 g intravenous (IV) Q12H to 2 g IV Q12H. The probability of achieving an AUC/MIC ratio >= 400 for each dosing regimen was calculated for minimum inhibitory concentrations (MICs) from .5 to 2 mg/L. The risk of nephrotoxicity for each regimen was derived from a previously published vancomycin trough-nephrotoxicity logistic regression function. Restricted analyses were performed that only included subjects with troughs between 15 and 20 mg/L. Results. At a MIC of 2 mg/L, even the most aggressive dosing regimen considered (2 g every 12 h) only yielded a probability of target attainment (PTA) of 57% while generating a nephrotoxicity probability upward of 35%. At a MIC of 1 mg/L, >= 3 g per day provided PTA in excess of 80% but were associated with unacceptable risks of nephrotoxicity. In the restricted analyses of subjects with troughs between 15 and 20 mg/L, all regimens produced a PTA of 100% at MICs <= 1 mg/L. The PTA was variable among the regimens at a MIC of 2 mg/L and was highly dependent on the total daily dose administered. Conclusions. This study indicates that vancomycin may not be useful for treating serious methicillin-resistant Staphylococcus aureus (MRSA) infections with MIC values > 1 mg/L where PTA is questionable. Since an AUC/MIC ratio >= 400 is target associated with efficacy, one should consider incorporating computation of AUC when monitoring vancomycin.