Tacrolimus is a potent calcineurin inhibitor currently used for prophylaxis and treatment of allograft rejection. As tacrolimus metabolism occurs for the most part via the cytochrome P450-3A4 (CYP3A4) enzyme system, co-administration of drugs activating or inhibiting this metabolic pathway may affect tacrolimus pharmacokinetics. We report a case of dramatic increase in tacrolimus blood concentration that could be attributed, at least in part, to enzymatic inhibition by protease inhibitors and diltiazem.
