Effects of rifampin on tacrolimus pharmacokinetics in healthy volunteers.

Tacrolimus is a marketed immunosuppressant used in liver and kidney transplantation. It is subject to extensive metabolism by CYP3A4 and is a substrate for P-glycoprotein-mediated transport. A pharmacokinetic interaction with rifampin, an antituberculosis agent and potent inducer of CYP3A4 and P-glycoprotein, and tacrolimus was evaluated in six healthy male volunteers. Tacrolimus was administered at doses of 0.1 mg/kg orally and 0.025 mg/kg/4 hours intravenously. The pharmacokinetics of tacrolimus were obtained from serial blood samples collected over 96 hours, after single oral and intravenous administration prior to and during an 18-day concomitant rifampin dosing phase. Coadministration of rifampin significantly increased tacrolimus clearance (36.0 +/- 8.1 ml/hr/kg vs. 52.8 +/- 9.6 ml/hr/kg; p = 0.03) and decreased tacrolimus bioavailability (14.4% +/- 5.7% vs. 7.0% +/- 2.7%; p = 0.03). Rifampin appears to induce both intestinal and hepatic metabolism of tacrolimus, most likely through induction of CYP3A and P-glycoprotein in the liver and small bowel.