Intestinal and hepatic CYP3A4 catalyze hydroxylation of 1α,25-dihydroxyvitamin D3:: Implications for drug-induced osteomalacia

The decline in bone mineral density that occurs after long-term treatment with some antiepileptic drugs is thought to be mediated by increased vitamin D-3 metabolism. In this study, we show that the inducible enzyme CYP3A4 is a major source of oxidative metabolism of 1 alpha, 25-dihydroxyvitamin D 3 [ 1,25( OH)(2)D-3] in human liver and small intestine and could contribute to this adverse effect. Heterologously-expressed CYP3A4 catalyzed the 23- and 24-hydroxylation of 1,25( OH)(2)D-3. No human microsomal cytochrome P450 enzyme tested, other than CYP3A5, supported these reactions. CYP3A4 exhibited opposite product stereochemical preference compared with that of CYP24A1, a known 1,25( OH)(2)D-3 hydroxylase. The three major metabolites generated by CYP3A4 were 1,23R, 25( OH) 3 D 3, 1,24S, 25( OH) 3 D 3, and 1,23S, 25( OH) D-3(3). Although the metabolic clearance of CYP3A4 was less than that of CYP24A1, comparison of metabolite profiles and experiments using CYP3A-specific inhibitors indicated that CYP3A4 was the dominant source of 1,25( OH)(2)D-3 23- and 24-hydroxylase activity in both human small intestine and liver. Consistent with this observation, analysis of mRNA isolated from human intestine and liver ( including samples from donors treated with phenytoin) revealed a general absence of CYP24A1 mRNA. In addition, expression of CYP3A4 mRNA in a panel of duodenal samples was significantly correlated with the mRNA level of a known vitamin D receptor gene target, calbindin-D9K. These and other data suggest that induction of CYP3A4-dependent 1,25( OH)(2)D-3 metabolism by antiepileptic drugs and other PXR ligands may diminish intestinal effects of the hormone and contribute to osteomalacia.