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Vol. 298, Issue 1, 323-330, July 2001
Department of Drug Metabolism, Merck Research Laboratories, West
Point, Pennsylvania
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 µM 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 µM
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.
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