The recent Medical Letter article on vancomycin dosing and monitoring1 briefly mentioned use of an alternative antibiotic for treatment of methicillin-resistant Staphylococcus aureus (MRSA) with reduced susceptibility to vancomycin. Some readers have asked for more information on this subject.
REDUCED SUSCEPTIBILITY TO VANCOMYCIN — Microbiology laboratories generally report isolates of MRSA as “susceptible” to vancomycin when the minimum inhibitory concentration (MIC) is ≤2 mg/L. Recent guidelines suggest considering an alternative antibiotic when the MIC is ≥2 mg/L.2
ALTERNATIVES TO VANCOMYCIN — For complicated skin or soft tissue infections caused by MRSA with reduced susceptibility to vancomycin, either daptomycin (Cubicin) or linezolid (Zyvox) would be a reasonable choice. Linezolid, which is available for both oral and IV use, is bacteriostatic and may not be effective for treatment of endocarditis; it is not recommended for use in catheter-associated bacteremias. Linezolid can cause bone marrow suppression, particularly if it is used for more than 10 days.
When bacteremia is present, especially when it is due to right-sided endocarditis, daptomycin, which is given IV, would be a better choice, unless the patient has concomitant pneumonia (daptomycin is inactivated by surfactant). Daptomycin is rapidly bactericidal, but some S. aureus strains with decreased susceptibility to vancomycin have decreased susceptibility to daptomycin as well.
Other drugs that may be effective against some strains of MRSA include quinupristin-dalfopristin (Synercid), which causes thrombophlebitis and has substantial potential for adverse drug interactions, and tigecycline (Tygacil), which achieves only low serum concentrations and is bacteriostatic. Another alternative is trimethoprim-sulfamethoxazole (Bactrim, Septra, and others), but experience with its use in serious MRSA infections is limited.
1. Vancomycin dosing and monitoring. Med Lett Drugs Ther 2009; 51:25.
2. M Rybak et al. Therapeutic monitoring of vancomycin in adult patients: a consensus review of the American Society of Health-System Pharmacists, the Infectious Diseases Society of America, and the Society of Infectious Diseases Pharmacists. Am J Health-Syst Pharm 2009; 66:82.