Dexamethasone Metabolism in Species Differences Vitro

Department of Pharmacology and Therapeutics, University of Liverpool, P.O. Box 147, Liverpool L69 3GE, U.K. Dexamethasone (DEX) is extensively metabolized to 6-hydroxyDEX (6OH-DEX) and side-chain cleaved metabolites in human liver both in vitro and in vivo with CYP3A4 responsible for the for- marion of 6-hydroxylated products. In the present study, the metabolism of [3H]DEX has been examined in the liver fractions from various mammalian species and metabolite profiles compared with those obtained with human fiver microsomes. Metabolites were quantified by radiometric high-pressure liquid chromatography (HPLC) and characterized by liquid chromatography-mass spectrometry (LC-MS) and co-chromatography with chemical standards, where available. 6OH- DEX formation was quantified for each species and the inhibitory potency of ketoconazole at 1 and 20 pM determined. Glycyrrhetinic acid, a specific inhibitor of l l-dehydrogenase, was also used to determine the extent of reductive DEX metabolism. Species differences in metabolite profiles obtained from microsomal incubations were both quantitative and qualitative. 6-Hydroxylation was variable (highest in the hamster) and was not always the major route of metabolism, and formation was sex-specific in the rat (male >> female). The inhibition of 6-hydroxylation (CYP3A) by ketoco- nazole was variable, and indicates that ketoconazole cannot be regarded as a selective inhibitor of CYP3A proteins in all species. Cytosolic incubations produced similar profiles in different species with the formation of a metabolite (M5) which was inhibited by glycyrrhetinic acid and tentatively identified in this study as ll-dehydro-side-chain cleaved DEX (llDH-9~F-A). In conclusion, the male rat gave a metabolite profile which was closest to that seen in the human. However, 6-hydrox- ylatlon was most extensive in the hamster which may therefore be a suitable model to use for further studies on DEX metabolism by CYP3A. © 1997 Elsevier Science Ltd. J. Steroid Biochem. Moh,c. BioL, Vol. 62, No. 4, pp. 345-352, 1997 INTRODUCTION Studies on the endogenous corticosteroid cortisol have shown its metabolism in human liver micro- somes to be complex [1]. In comparison, the meta- bolic profile of l~he synthetic glucocorticoid dexamethasone (DEX) (structures shown in Fig. 1), is simpler with 6-hydroxylation, a relatively minor bio- transformation pathway of cortisol being the major route of metabolism [2]. Both 6c~- and 6]3- hydroxyDEX isomers ..are formed in vitro and minor metabolic pathways irLclude side-chain cleavage/oxi- dation of both DEX and the 6-hydroxylated metab- olite [2]. Studies carried out using various cytochrome P450 (CYP) inhibitors and livers blotted for CYP iso- zymes have led to the identification of CYP3A4 as the

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