Effects of voriconazole on tacrolimus metabolism in a kidney transplant recipient.

Infection occurs frequently in the organ transplant recipients during the post-transplant period because of immunosuppression. Therefore, prophylactic antimicrobial agents are often used. The azole antifungals, widely prescribed prophylactically, are known to have many drug-drug interactions. This report presents a case of drug-drug interaction between voriconazole and tacrolimus in a kidney transplant recipient. Voriconazole treatment led to a dramatic increase in tacrolimus concentration that required its discontinuation in spite of the manufacturer's guidelines that recommend a reduction of tacrolimus dosage by one-third. The present drug-drug interaction can be attributed to a strong inhibitory effect on cytochrome P450-3A4 activity by voriconazole. When voriconazole and tacrolimus are coadministered, close monitoring of tacrolimus blood levels is recommended as the rule-of-thumb reduction of tacrolimus dose by one-third may not be satisfactory.