Synthesis and Biologic Activity of a C-Ring Analogue of Vitamin D₃: Biologic and Protein Binding Properties of 11α-Hydroxyvitamin D₃

The influence of C-ring substituents on the biologic activity and protein binding properties of vitamin D₃ has not been systematically investigated. To this end, we dehydro-genated cholesta-5,7-dien-3β-ol (1) to the 5,7,9(11)-triene (3). After protection of the 5,7-diene with a 4-phenyl-1,2,4-triazoline-3,5-dione Diels-Alder adduct, oxidation of the unprotected 9(11)-olefin gave epoxide 5. Reverse Diels-Alder and reductive ring opening of epoxide 5 gave cholesta-5,7-diene-3β,11α-diol (6). Photolysis of 6 to the previtamin followed by thermal rearrangement resulted in 11α-hydroxyvitamin D₃ (8). We found that vitamin 8 increased calcium transport at a dose of 500 pmol/rat but failed to increase bone calcium mobilization at a dose as high as 50000 pmol/rat. Under the same-conditions, corresponding doses of vitamin D₃ and 25-hydroxyvitamin D₃ increased bone calcium mobilization and intestinal calcium transport. The new vitamin analogue, 8, was slightly less efficient (B-50 = 6.8 × 10^-8 M) than 25-hydroxyvitamin D₃, 24(R),25-dihydroxyvitamin D₃, and 25-(S),26-dihydroxyvitamin D₃ (7.1 × 10^-9 M, 7.7 × 10^-9 M, and 7.9 × 10^-9 M, respectively) in displacing radiolabeled 25-hydroxyvitamin D₃ from rat plasma vitamin D binding protein. On the other hand, vitamin analogue 8 showed significantly greater binding efficiency than 1α-hydroxyvitamin D₃, 1,25-dihydroxyvitamin D₃, and vitamin D₃ (B-50 = 2.5 × 10^-6 M, 9.84 × 10^-8 M, and 5.46 × 10^-7 M, respectively), under these same conditions! Vitamin analogue 8 displayed approximately the same efficiency as vitamin D₃ in displacing radiolabeled 1,25-dihydroxyvitamin D₃ from a chick intestinal cytosol receptor but was less effective than 25-hydroxyvitamin D₃, 24-(R),25-dihydroxyvitamin D₃, 25(S),26-dihydroxyvitamin D₃, 1α-hydrbxyvitamin D₃, and 1,25-dihydroxyvitamin D₃- We conclude that introduction of an 11α-hydroxyl group into the C-ring of vitamin D₃ results in a vitamin analogue with moderate yitamin D₃ agonist activity in the intestine but no activity with respect to bone calcium mobilization at the levels tested. 11α-Hydroxyvitamin D₃ does not have improved binding affinity to the intestinal cytosol receptor when compared to vitamin D₃. The new vitamin analogue shows significantly greater binding affinity to plasma vitamin D binding protein than vitamin D₃ (6.79 × 10^-8 M vs. 5.46 × 10^-7 M) or 1α-hydroxyvitamin D₃ (6.79 × 10^-8 M vs. 2.5 × 10^-6 M), suggesting that the presence of an extra hydroxyl group sufficiently removed from the 3β-hydroxyl is important in the binding of vitamin D analogues to vitamin D binding protein.