Human hepatic metabolism of the anti-osteoporosis drug eldecalcitol involves sterol C4-methyl oxidase

The metabolism of eldecalcitol (ED-71), a 2β-hydroxypropoxylated analog of the active form of vitamin D3 was investigated by using in vitro systems. ED-71 was metabolized to 1α,2β,25-trihydroxyvitamin D3 (1α,2β,25(OH)3D3) in human small intestine and liver microsomes. To identify the enzymes involved in this metabolism, we examined NADPH-dependent metabolism by recombinant P450 isoforms belonging to the CYP1, 2, and 3 families, and revealed that CYP3A4 had the activity. However, the CYP3A4 -specific inhibitor, ketoconazole, decreased the activity in human liver microsomes by only 36%, suggesting that other enzymes could be involved in ED-71 metabolism. Because metabolism was dramatically inhibited by cyanide, we assumed that sterol C4-methyl oxidase like gene product (SC4MOL) might contribute to the metabolism of ED-71. It is noted that SC4MOL is physiologically essential for cholesterol synthesis. Recombinant human SC4MOL expressed in COS7, Saccharomyces cerevisiae, or Escherichia coli cells converted ED-71 to 1α,2β,25(OH)3D3. Furthermore, we evaluated the metabolism of ED-71 by recombinant CYP24A1, which plays an important role in the metabolism of the active form of vitamin D3 (1α,25(OH)2D3) and its analogs. The k cat/K m value for 24- or 23-hydroxylation of ED-71 was only 3% of that for 1α,25(OH)2D3, indicating that ED-71 was resistant to CYP24A1-dependent catabolism. Among the three enzymes catalyzing ED-71, SC4MOL appears to be most important in the metabolism of ED-71. To the best of our knowledge, this is the first study showing that SC4MOL can function as a drug-metabolizing enzyme. The yeast and E. coli expression systems for SC4MOL could be useful for structure-function analyses of SC4MOL.