Analysis of C-3 epimerization mechanism in vitamin D_3 to develop a new osteoporosis drug

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First, highly sensitive LC/MS methods for analysis of vitamin D_3 drug and metabolites were developed. Cookson-type reagents, which quantitatively react with vitamin D_3 compounds to form the Diets-Alder adducts, were employed to enhance their detection responses in APCI-MS. Pharmacokinetic study of 1α-hydroxyvitamin D_3 in humans was carried out by the LC/MS method combined with derivatization with a proton-affinitive Cookson-type reagent, DMEQTAD. Using this method, urinary vitamin D_3 metabolites in human under physiological conditions were also detected and characterized. Next, an electron-affinitive Cookson-type reagent, NPTAD, was applied to the assay of 25-hydroxyvitamin D_3 in human plasma using LC/electron capture APCI-MS. This method was superior to the commercially available RIA in sensitivity and sample throughput. Furthermore, a new derivatization reagent having the boronic acid as the reacting group was developed for the assay of 24,25-dihydroxyvitamin D_3 [24,25(OH)_2D_3].Second, studies on the C-3 epimerization in 24,25(OH)_2D_3 were performed. In consequence, we found the 3-epimer[3-epi-24,25(OH)_2D_3] is formed from 24,25(OH)_2D_3 by the catalysis of 3α-and β-hydroxysteroid dehydrogenase (HSD) and the HSD-catalyzed epimerization is reversible. In addition, 3-oxo-form was also isolated from the reaction mixture, which strongly suggested that the formation of the 3-epimer by HSD involves a dehydrogenation and reduction processes.In third study, osteoclastic and osteoblastic activities of some vitamin D_3 compounds were examined using a culture system of the goldfish scale. It was observed that 3-epi-24,25(OH)_2D_3 activated osteoblastic cell at 10^<-8>M, which was different from the effects of 24,25(OH)_2D_3 and active vitamin D_3, 1,25-dihydroxyvitamin D_3, in bone metabolism.