Synthesis and biological activity of 3β-fluorovitamin D₃: Comparison of the biological activity of 3β-fluorovitamin D₃ and 3-deoxyvitamin D₃

The alteration in the biologic activity of the vitamin D₃ molecule resulting from the replacement of a hydrogen atom with a fluorine atom is a subject of fundamental interest. To investigate this problem we synthesized 3β-fluorovitamin D₃ and its hydrogen analog, 3-deoxyvitamin D₃, and tested the biologic activity of each by in vitro and in vivo methods. Contrary to previous reports which showed that 3β-fluorovitamin D₃ was as active as vitamin D₃ in vivo, we found that the fluoro-analog was less active than vitamin D₃. With regard to stimulation of intestinal calcium transport and bone calcium mobilization in the D-deficient hypocalcemic rat, 3β-fluorovitamin D₃ showed significantly greater biologic activity than its hydrogen analog, 3-deoxyvitamin D₃. In the organ-cultured, embryonic chick duodenum, 3β-fluorovitamin D₃ was approx 1/1000th as active as the native hormone, 1,25-dihydroxyvitamin D₃, while 3-deoxyvitamin D₃ was inactive even at μM concentrations, in the induction of the vitamin D-dependent, calcium-binding protein. With regard to in vitro activity in displacing radiolabeled 25-hydroxyvitamin D₃ from vitamin D binding protein and radiolabelled 1,25-dihydroxyvitamin D₃ from a chick intestinal cytosol receptor, 3β-fluorovitamin D₃ and 3β-deoxyvitamin D₃ both showed very poor binding efficiencies when compared with vitamin D₃. Our results show that the substitution of a fluorine atom for a hydrogen atom at the C-3 position of the vitamin D₃ molecule results in a fluorovitamin with significantly more biological activity than its hydrogen analog, 3-deoxyvitamin D₃.