Terfenadine t-butyl hydroxylation catalyzed by human and marmoset cytochrome P450 3A and 4F enzymes in livers and small intestines.

1. Roles of human cytochrome P450 (P450) 3A4 in oxidation of an antihistaminic drug terfenadine have been previously investigated in association with terfenadine-ketoconazole interaction. Several antihistamine drugs have been recently identified as substrates for multiple P450 enzymes. In this study, overall roles of P450 3A4, 2J2, and 4F12 enzymes in terfenadine t-butyl hydroxylation were investigated in small intestines and livers from humans, marmosets, and/or cynomolgus monkeys. 2. Human liver microsomes and liver and small intestine microsomes from marmosets and cynomolgus monkeys effectively mediated terfenadine t-butyl hydroxylation. Ketoconazole and N-hydroxy-N'-(4-butyl-2-methylphenyl)-formamidine (a P450 4A/F inhibitor) almost completely and moderately inhibited these activities, respectively, in human liver microsomes; however, these chemicals did not show substantially suppression in marmoset liver. Anti-human P450 3A and 4F antibodies showed the roughly supportive inhibitory effects. 3. Recombinant P450 3A4/90 and 4F12 showed high terfenadine t-butyl hydroxylation activities with substrate inhibition constants of 84-144 μM (under 26-76 μM of K_m values), in similar manners to liver and intestine microsomes. 4. These results suggest that human and marmoset P450 3A4/90 and 4F12 in livers or small intestines played important roles in terfenadine t-butyl hydroxylation. Marmosets could be a model for humans during first pass extraction of terfenadine and related substrates.

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