ATC Group: A11C Vitamin A and D, incl. combinations of the two

Alfacalcidol is Vitamin D-hormone analog (an active metabolite of Vitamin D), which performs important functions in regulation of the calcium balance and the bone metabolism and is activated by the enzyme 25-hydroxylase (converted rapidly in the liver to 1,25-dihydroxyvitamin D) for systemic and in osteoblasts for local D-hormone actions.

Vitamin D₃ must undergo a two-step metabolic process to be active; the first step occurs in the microsomal fraction of the liver where Vitamin D is hydroxylated at position 25 (25-hydroxycholecalciferol or calcifediol); the second step takes place in the kidney where 1,25-dihydroxycholecalciferol or calcitriol is formed due to the activity of enzyme 25-hydroxycholecalciferol 1-hydroxylase. Vitamin D increases absorption of calcium and phosphorus in the intestine and improves normal bone formation and mineralization.

Calcitriol is the most active known form of vitamin D₃ in stimulating intestinal calcium transport. In physiological amounts it augments the intestinal absorption of calcium and phosphate and plays a significant part in the regulation of bone mineralisation.

Doxercalciferol is a synthetic vitamin D₂ analog that requires metabolic activation to form the active 1α,25-(OH)₂D₂ metabolite, which binds to the vitamin D receptor (VDR) to result in the selective activation of vitamin D responsive pathways. Vitamin D and doxercalciferol have been shown to reduce PTH levels by inhibiting PTH synthesis and secretion.

Vitamin A plays an important role in the visual process. It is isomerised to the 11-cis isomer and subsequently bound to the opsin to form the photoreceptor for vision under subdued light. Vitamin A also participates in the formation and maintenance of the integrity of epithelial tissues and mucous membranes.

Vitamin D₂ (ergocalciferol) is a fat soluble vitamin. In conjunction with parathyroid hormone and calcitonin, it regulates calcium haemostasis. Ergocalciferol metabolites promote active absorption of calcium and phosphorous by the small intestine, increase rate of excretion and resorption of minerals in bone and promote resorption of minerals in bone and promote resorption of phosphate by renal tubules.

In its biologically active form vitamin D₃ stimulates intestinal calcium absorption, incorporation of calcium into the osteoid, and release of calcium from bone tissue. In the small intestine it promotes rapid and delayed calcium uptake. The passive and active transport of phosphate is also stimulated.