Effect of chronic administration of ritonavir on function of cytochrome P450 3A and P-glycoprotein in rats.

Ritonavir (RTV) is well known as an inhibitor of many drugs that are metabolized by cytochrome P450 (CYP) 3A or fluxed via P-glycoprotein (Pgp), although it is also reported that RTV is a potent inducer for them. In this study, to elucidate these contradictory phenomena, functional changes of CYP3A or Pgp during chronic administration of RTV were examined in rats. After pretreatment with RTV for indicated days (day 3-day 14), rats were used in the experiments. The area under the plasma drug concentration vs. time curve (AUC(0-infinity)) after oral administration of RTV (20 mg/kg) to these rats showed an RTV-treatment period-dependent decrease, and the mean AUC(0-infinity) of RTV in Day 14 rats decreased significantly by 57% as compared to the control. The AUC(0-infinity) after intravenous (i.v.) administration of RTV to Day 3 and Day 5 rats increased significantly by 28% and 22%, respectively, while there were no significant changes in the AUC(0-infinity) in Day 7 and Day 14 rats as compared to the control. As for i.v. administration of erythromycin (EM) or midazolam (MDZ) to RTV-treated rats, the AUC(0-infinity)in Day 3 and Day 5 rats increased significantly as compared to the control, while in Day 7 rats and rifampicin-treated rats, the AUC(0-infinity) of EM decreased significantly by 82% and 42%, respectively, as compared to the control. For MDZ, there were no significant changes in the AUC(0-infinity) in Day 7 or Day 14 rats. After i.v. administration of rhodamine123 (Rho123), the excretion clearances from blood circulation to the intestinal lumen and the biliary excretion clearances in Day 14 rats increased markedly by 2.2-fold and 2.6-fold as compared to the control. It has been confirmed that RTV is not only a potent inhibitor but also a potent inducer of CYP3A, and that RTV is a potent inducer of intestinal Pgp. This property of RTV is responsible for regulating the oral bioavailability of drugs that are mediated by CYP3A and Pgp.