Synthesis and pharmacokinetics of 1 alpha-hydroxyvitamin D3 tritiated at 22 and 23 positions showing high specific radioactivity.

A novel synthesis of a radioactive compound of 1 alpha-hydroxyvitamin D3 (1 alpha OHD3) (1) and its pharmacokinetics are described. Radioactive 1 alpha OHD3 tritiated at 22 and 23 positions ([22,23-(3)H4]1 alpha OHD3) (5) was prepared via key reactions of the reduction of acetylenic side chain in the ketone (12) with tritium gas in the presence of palladium-charcoal and the subsequent Wittig reaction with the A-ring synthon (16). [22,23-(3)H4]1 alpha OHD3 (5) showed high specific radioactivity (111.5 Ci/mmol) and was used successfully in pharmacokinetics studies with rats. In the pharmacokinetics studies, the plasma concentration level of the active form of vitamin D3, 1 alpha,25-dihydroxy-vitamin D3 [1 alpha,25(OH)2D3], after oral or intravenous administration of [22,23-(3)H4]1 alpha OHD3 (5), showed longer half-life, lower maximum concentration, and lower area under the curve than those after treatment of 1 alpha,25(OH)2D3 tritiated at 26 and 27 positions (4). These results might suggest a beneficial therapeutic utility of 1 alpha OHD3 (1) over the treatment of 1 alpha,25(OH)2D3 (2).