Title: Successful treatment of methicillin-resistant Staphylococcus aureus mitral valve endocarditis with sequential linezolid and telavancin monotherapy following daptomycin failure
Abstract: Sir, We are grateful for the informative comment by Camou and would like to take the opportunity to address some of the points. With regard to our first-line treatment choice, several factors led us to initially use vancomycin as empirical therapy. First, the patient presented as an intravenous drug user without abnormalities on transthoracic two-dimensional echocardiogram, prompting coverage for methicillin-resistant Staphylococcus aureus (MRSA) bacteraemia. Though the patient had renal dysfunction, the dosage was adjusted with the help of our clinical pharmacy department to provide adequate AUC/MIC, while attempting to minimize exacerbating the patient’s kidney dysfunction. Secondly, vancomycin comes ready to administer in pre-made bags (Baxter, Deerfield, IL, USA). Daptomycin, on the other hand, requires reconstitution using aseptic techniques; and, in our experience, requires at least 15 min of undisturbed reconstitution time, which may not be practical in an emergency department. Lastly, our institutional price for 1 g of vancomycin intravenously every 8 h (our most common dose) is $23.09/day, versus $256.29 for 500 mg of daptomycin intravenously every 24 h. For our patient, who might have benefited from a 10 mg/kg dose, the cost of daptomycin would have been $512.58/day. For comparison, our price for 750 mg of telavancin is $186.71. In response to comments about the lack of therapeutic drug monitoring of vancomycin: given vancomycin’s time-dependent bactericidal activity, interpretation of trough levels before reaching steady state can be misleading. Typically, a trough is drawn during a drug’s steady state to ensure proper therapeutic response (and that administration of the drug is equal to its clearance). In a patient with acute kidney injury, such as ours, the patient-specific half-life of the drug increases, and thus the time to steady state does as well. Our patient only received two doses of vancomycin; his initial concentration [Cp01⁄4dose (mg)/V] of 17.5 mg/L was well below the toxic level. Camou is correct to point out that daptomycin is a reasonable alternative to vancomycin in MRSA infections. The literature notes, however, that no agent has proven superior to vancomycin and that even the use of high-dose daptomycin (8–10 mg/kg) requires further study to determine whether improved outcomes occur. Should daptomycin replace vancomycin for empirical treatment of MRSA infections? We feel that optimizing the use of vancomycin (utilizing clinical pharmacy can be helpful in this regard) is still the most cost-effective solution. If in the face of factors that predict vancomycin failure, an alternative agent should be sought. Indeed, once blood cultures returned a vancomycin MIC of 2 mg/ L (by broth microdilution), we opted for daptomycin.