Abstract

Caco-2 cells grown in the presence of 1α,25-di-OH vitamin D₃ (di-OH vit D₃) were used as a model to evaluate the effects of P-glycoprotein (Pgp) efflux on CYP3A4-mediated metabolism of indinavir during intestinal absorption. Caco-2 cells grown under these conditions demonstrated significant CYP3A4 activity and maintained Pgp-mediated directional transport of indinavir. Metabolism of indinavir in the di-OH vit D₃-treated cells correlated with the level of CYP3A activity and generated metabolites consistent with CYP3A4-mediated metabolism. During transport experiments, indinavir metabolites are selectively secreted into the apical compartment, consistent with Pgp-mediated efflux. Using formation of the most abundant metabolite, M6, as a marker for indinavir metabolism, we observed that the extent of indinavir metabolism is not significantly affected by the direction of indinavir transport or by inhibition of Pgp with cyclosporin A. However, because Pgp efflux results in higher indinavir transport in the basolateral-to-apical direction than in the apical-to-basolateral direction, the ratio of M6 produced normalized to the amount of drug transported across the monolayer was higher for apical-to-basolateral transport. Thus, Pgp efflux in a direction opposite to absorptive transport results in more metabolite produced per mole of drug that is absorbed. In summary, the results support a role of Pgp in increasing intestinal presystemic metabolism and in removal of CYP3A4-generated metabolites from the intracellular compartment.