P-glycoprotein-mediated efflux of indinavir metabolites in Caco-2 cells expressing cytochrome P450 3A4

The role of P-glycoprotein in secretion of indinavir metabolites produced by CYP3A4 was evaluated in Caco-2 cells expressing CYP3A4. Metabolism of indinavir by CYP3A4 expressing Caco-2 cells grown on filters resulted in the formation of N-dealkylation products (M5 and M6) and hydroxylation of indinavir, which were preferentially secreted into the apical compartment. Apical secretion of the metabolites was inhibited by cyclosporin A (CsA) with all three classes of metabolites showing similar sensitivity to CsA, suggesting that they are all secreted by the same pathway. M6 stimulated P-glycoprotein (Pgp)-ATPase activity in a concentration-dependent manner. This stimulation was inhibited by the Pgp-specific monoclonal antibody C219. A method was developed to specifically inhibit Pgp using the monoclonal antibody UIC2 to determine whether Pgp efflux accounts for a significant proportion of the apical secretion of indinavir metabolites. UIC2 recognizes an extracellular transient conformational epitope that is stabilized by some Pgp substrates or by ATP depletion. Incubation of Caco-2 cells with UIC2 in the presence of 1 microM CsA resulted in 50 to 80% inhibition of Pgp-mediated vinblastine efflux, with no significant inhibition observed by UIC2 or CsA alone. Inhibition of Pgp in CYP3A4-expressing Caco-2 cells by UIC2 and 1 microM CsA resulted in a significant decrease in the apical secretion of M6, M5, and OH-indinavir and an increase in the amount of the metabolites secreted in the basolateral compartment and retained in the cytosol. These results are consistent with a role of Pgp in elimination of CYP3A4-generated metabolites and indicate that even relatively polar metabolites may be secreted from the cell by Pgp.