Metabolism of 2-methyl analogs of 1α,25-dihydroxyvitamin D ₃ in rat osteosarcoma cells (UMR 106)

Several novel A-ring modified analogs of 1α,25-dihydroxyvitamin D₃ [1α,25(OH)₂D₃] have been synthesized in order to investigate the structure-function relationships of 1α,25(OH)₂D₃. We synthesized A-ring modified analogs which contain a methyl group on C-2 of the A-ring. There are eight 2-methyl diastereomers, which differ in the stereochemistry of the methyl group on C-2 and the hydroxyl groups on C-1 and C-3. Further our biological activity studies of the 2-methyl diastereomers indicated that the potency of each analog is highly dependent on the stereochemistry of the A-ring substituents [Konno et al., Biorg. Med. Chem. Letts. 8(2), 151-156 (1998); Nakagawa et al., Biochem. Pharmacol. 60(12), 1937-1947 (2000)]. For example, the VDR binding affinities exhibited by the 1α-isomers are significantly higher than those exhibited by the 1β-isomers. Furthermore, out of all the 1α-isomers, the 2α-methyl isomers, when compared to the corresponding 2β-methyl isomers, showed much higher potency in inducing cell differentiation of HL-60 cells, but failed to stimulate apoptosis. In contrast the 2β-methyl isomers strongly stimulated apoptosis. At present it is unknown how the addition of the 2-methyl modification to the hormone, 1α,25(OH)₂D₃ alters its metabolism in target tissues. Previously, we reported that 1α,25(OH)₂D₃ is metabolized in rat osteosarcoma (UMR 106) cells via both the C-24 oxidation and the C-3 epimerization pathways. Therefore, we studied the metabolism of the four 1α,2-methyl diastereomers in UMR 106 cells. Our results indicated that in UMR 106 cells, all four diastereomers were metabolized into several polar metabolites via the C-24 oxidation pathway. Thus, the presence of the 2-methyl group on the A-ring did not inhibit the metabolism of the analogs via the C-24 oxidation pathway. However, it is significant to note that the 2-methyl group prevented the metabolism of the analogs via the C-3 epimerization pathway. In summary, we report that the 2-methyl group interferes with the action of the enzyme(s) involved in C-3 epimerization, but not with the enzyme 1α,25(OH) ₂D₃-24-hydroxylase, which is responsible for C-24 oxidation pathway.