Abstract

The contributions of cytochrome P450 3A (CYP3A) and P-glycoprotein to sirolimus oral bioavailability in rats were evaluated by coadministration of sirolimus (Rapamune) with the CYP3A inhibitor ketoconazole or the P-glycoprotein inhibitord-α-tocopheryl poly(ethylene glycol 1000) succinate (TPGS). Groups of six male Sprague-Dawley rats (250–300 g) were administered Rapamune (1 mg/kg) by oral gavage, alone and with ketoconazole (30 mg/kg) or TPGS (50 mg/kg). Sirolimus levels were measured in whole blood over a 6-h time course. SirolimusC_max (6.6 ± 1.6 versus 26 ± 7 ng/ml) and area under the concentration versus time curve from 0 to 6 h (AUC_0–6) (22 ± 7 versus 105 ± 27 ng · h/ml) were increased 3- to 5-fold by ketoconazole. MedianT_max (1.5–2 h) was unchanged. TPGS had no effect on sirolimus absorption. The interaction of sirolimus with P-glycoprotein was also evaluated in vitro using HCT-8 and Caco-2 cell monolayers. Consistent with published reports, sirolimus was a good inhibitor of P-glycoprotein, inhibiting polarized basolateral-to-apical flux of rhodamine 123 with an IC₅₀ of 0.625 to 1.25 μM (cyclosporine caused >80% inhibition at 5 μM). Sirolimus did not demonstrate significant polarized flux in either direction using the same monolayers (basolateral-to-apical flux was <2 times the apical-to-basolateral). Moreover, sirolimus flux was not impacted by cyclosporine, suggesting that it does not undergo P-glycoprotein-mediated transport in this system. The lack of significant sirolimus transport by P-glycoprotein may, in part, explain the lack of a TPGS effect on sirolimus absorption in rats.