Mechanism of action
· Dihydrotachysterol is a synthetic analogue of vit D2 that is much less active in antirachitic tests but after hydroxylated to 25-hydroxydihydrotachysterol, the modified drug binds to the vitamin D receptor.
· The bound form of the vitamin D receptor serves a transcriptional regulator of bone matrix proteins, inducing the expression of osteocalcin and suppressing synthesis of type I collagen.
· Vitamin D (when bound to the vitamin D receptor) also stimulates the expression of a number of proteins involved in transporting calcium from the lumen of the intestine, across the epithelial cells and into blood. This stimulates intestinal calcium absorption and increases renal phosphate excretion. These are functions that are normally carried out by the parathyroid hormone.
Reference: Tripathi K.D., "Essentials of medical pharmacology", Jaypee brothers medical publishers, Seventh edition, New Delhi, page no. 342-343