TY - JOUR T1 - Kinetic Studies of 25-Hydroxy-19-nor-vitamin D<sub>3</sub> and 1α,25-Dihydroxy-19-nor-vitamin D<sub>3</sub> Hydroxylation by CYP27B1 and CYP24A1 JF - Drug Metabolism and Disposition JO - Drug Metab Dispos SP - 1482 LP - 1488 DO - 10.1124/dmd.107.015602 VL - 35 IS - 9 AU - Naoko Urushino AU - Sachie Nakabayashi AU - Midori A. Arai AU - Atsushi Kittaka AU - Tai C. Chen AU - Keiko Yamamoto AU - Keiko Hayashi AU - Shigeaki Kato AU - Miho Ohta AU - Masaki Kamakura AU - Shinichi Ikushiro AU - Toshiyuki Sakaki Y1 - 2007/09/01 UR - http://dmd.aspetjournals.org/content/35/9/1482.abstract N2 - Our previous study demonstrated that 25-hydroxy-19-nor-vitamin D3 [25(OH)-19-nor-D3] inhibited the proliferation of immortalized noncancerous PZ-HPV-7 prostate cells similar to 1α,25-dihydroxyvitamin D3 [1α,25(OH)2D3], suggesting that 25(OH)-19-nor-D3 might be converted to 1α,25-dihydroxy-19-nor-vitamin D3 [1α,25(OH)2-19-nor-D3] by CYP27B1 before exerting its antiproliferative activity. Using an in vitro cell-free model to study the kinetics of CYP27B1-dependent 1α-hydroxylation of 25(OH)-19-nor-D3 and 25-hydroxyvitamin D3 [25(OH)D3] and CYP24A1-dependent hydroxylation of 1α,25(OH)-19-nor-D3 and 1α,25(OH)2D3, we found that kcat/Km for 1α-hydroxylation of 25(OH)-19-nor-D3 was less than 0.1% of that for 25(OH)D3, and the kcat/Km value for 24-hydroxylation was not significantly different between 1α,25(OH)2-19-nor-D3 and 1α,25(OH)2D3. The data suggest a much slower formation and a similar rate of degradation of 1α,25(OH)2-19-nor-D3 compared with 1α,25(OH)2D3. We then analyzed the metabolites of 25(OH)D3 and 25(OH)-19-nor-D3 in PZ-HPV-7 cells by high-performance liquid chromatography. We found that a peak that comigrated with 1α,25(OH)2D3 was detected in cells incubated with 25(OH)D3, whereas no 1α,25(OH)2-19-nor-D3 was detected in cells incubated with 25(OH)-19-nor-D3. Thus, the present results do not support our previous hypothesis that 25(OH)-19-nor-D3 is converted to 1α,25(OH)2-19-nor-D3 by CYP27B1 in prostate cells to inhibit cell proliferation. We hypothesize that 25(OH)-19-nor-D3 by itself may have a novel mechanism to activate vitamin D receptor or it is metabolized in prostate cells to an unknown metabolite with antiproliferative activity without 1α-hydroxylation. Thus, the results suggest that 25(OH)-19-nor-D3 has potential as an attractive agent for prostate cancer therapy. The American Society for Pharmacology and Experimental Therapeutics ER -