Human cytochrome P450-dependent differential metabolism among three 2α-substituted-1α,25-dihydroxyvitamin D3 analogs

Abstract

Our previous studies revealed that C2α-substituted-1α,25(OH)2D3 analogs had unique biological activities. For example, 19-nor-2α-(3-hydroxypropyl)-1α,25(OH)2D3 (MART-10), which has a high affinity for vitamin D receptor (VDR), is more bioavailable and more potent than 1α,25(OH)2D3 in inhibiting cancer cell growth and invasion because of its weaker binding to vitamin D binding protein (DBP), and more resistance to CYP24A1-dependent metabolism. In this study, we examined the metabolism of MART-10 and two other 2α-substituted analogs, 2α-(3-hydroxypropoxy)-1α,25(OH)2D3 (O2C3) and 2α-(3-hydroxypropyl)-1α,25(OH)2D3 (O1C3) by using human liver microsomes and human P450s. We demonstrated that O2C3 was converted to 1α,2α,25(OH)3D3 in human liver microsomes, whereas both O1C3 and MART-10 were hardly metabolized. The metabolism of O2C3 was significantly inhibited by ketoconazole, and the recombinant human CYP3A4 converted O2C3 to 1α,2α,25(OH)3D3, which suggests that CYP3A4 is responsible for the metabolism of O2C3 in human liver. The kcat/Km values of CYP3A4 for O1C3 and MART-10 are much smaller than that for O2C3. The kcat/Km values of human CYP24A1 for the three analogs are 1% (MART-10), 3% (O2C3), and 4% (O1C3) of that for 1α,25(OH)2D3, indicating that MART-10 is the most resistant to CYP24A1 hydroxylation. On the other hand, 1α,2α,25(OH)3D3, the metabolite of O2C3 by CYP3A4, was metabolized by CYP24A1 via multiple pathways similar to 1α,25(OH)2D3, which suggests that O2C3 can be metabolized by two sequential hydroxylations, first by CYP3A4 and then by CYP24A1 in human body. These results suggest that modification at C-2α position and C-19 demethylenation markedly change metabolic profiles and biological activities of vitamin D analogs.