Biological activity of 24,24-difluoro-25-hydroxycholecalciferol in chicks

The biological activity of subcutaneously injected 24,24-difluoro-25-hydroxycholecalciferol was compared with that of 25-hydroxycholecalciferol in the vitamin D-deficient growing chick. 24,24-Difluoro-25-hydroxycholecalciferol is equal to 25-hydroxycholecalciferol in the stimulation of: 1) growth, 2) intestinal calcium absorption, 3) elevation of serum calcium and serum phosphorus, 4) healing of rachitic cartilage (radiography), and 5) mineralization of rachitic bone (bone ash). The response appears to be linear in the range of 13.0 to 325 pmol daily. Since 24,24-difluorocholecaliferol cannot be 24-hydroxylated to produce either 24,25-dihydroxycholecalciferol or 1,24,25-trihydroxycholecalciferol, while it can be 1 alpha-hydroxylated to produce 24,24-difluoro-1,25-dihydroxycholecalciferol, these results demonstrate that 24-hydroxylation is not required for the known functions of cholecalciferol in the chick.