RESUMO
An endogenous probe for CYP3A activity would be useful for early identification of in vivo cytochrome P450 (CYP) 3A4 inhibitors. The aim of this study was to determine whether formation clearance (CL(f)) of the sum of 6ß-hydroxycortisol and 6ß-hydroxycortisone is a useful probe of CYP3A4 inhibition in vivo. In human liver microsomes (HLMs), the formation of 6ß-hydroxycortisol and 6ß-hydroxycortisone was catalyzed by CYP3A4, and itraconazole inhibited these reactions with half maximal inhibitory concentration (IC(50))(,u) values of 3.1 nmol/l and 3.4 nmol/l, respectively. The in vivo IC(50,u) value of itraconazole for the combined CL(f) of 6ß-hydroxycortisone and 6ß-hydroxycortisol was 1.6 nmol/l. The greater inhibitory potency in vivo is probably due to circulating inhibitory itraconazole metabolites. The maximum in vivo inhibition was 59%, suggesting that f(m,CYP3A4) for cortisol and cortisone 6ß-hydroxylation is ~60%. Given the significant decrease in CL(f) of 6ß-hydroxycortisone and 6ß-hydroxycortisol after 200-mg and 400-mg single doses of itraconazole, this endogenous probe can be used to detect moderate and potent CYP3A4 inhibition in vivo.
Assuntos
Cortisona/análogos & derivados , Inibidores do Citocromo P-450 CYP3A , Citocromo P-450 CYP3A/biossíntese , Hidrocortisona/análogos & derivados , Sondas Moleculares/metabolismo , Cortisona/antagonistas & inibidores , Cortisona/metabolismo , Citocromo P-450 CYP3A/metabolismo , Combinação de Medicamentos , Avaliação Pré-Clínica de Medicamentos/métodos , Humanos , Hidrocortisona/antagonistas & inibidores , Hidrocortisona/biossíntese , Hidrocortisona/metabolismo , Itraconazol/metabolismo , Itraconazol/farmacologia , Microssomos Hepáticos/efeitos dos fármacos , Microssomos Hepáticos/enzimologia , Microssomos Hepáticos/metabolismo , Reprodutibilidade dos TestesRESUMO
The antifungal agent fluconazole was found to be a potent inhibitor of cytochrome P450 (P450) 2C9 (Ki = 7-8 microM), the principal enzyme responsible for the clearance (85%) of the more potent anticoagulant (S)-warfarin to the inactive (S)-7- and (S)-6-hydroxywarfarin metabolites in vivo. Fluconazole was also found to be a potent inhibitor of the P4503A4-catalyzed formation of (R)-10-hydroxywarfarin (Ki = 15-18 microM) as well as the low KM P450 enzymes responsible for the formation of (R)-6-, (R)-7-, and (R)-8-hydroxywarfarin (Ki = 2-6 microM). By contrast, experiments with the P4501A2 inhibitor furafylline and cDNA-expressed P4501A2 indicate that fluconazole is a weak inhibitor of this enzyme (Ki > 800 microM), as measured by the inability of fluconazole to significantly suppress the P4501A2-dependent 6-hydroxylation of (R)-warfarin. The prediction generated from these studies, that fluconazole is a potent in vivo inhibitor of warfarin metabolism, , is tested in complementary studies reported in the accompanying article, "Warfarin-Fluconazole II".