indication

Alfacalcidol is an active metabolite of Vitamin D, which performs important functions in regulation of the calcium balance and the bone metabolism.

pharmacology

Alfacalcidol is Vitamin D-hormone analog which is activated by the enzyme 25-hydroxylase in the liver for systemic and in osteoblasts for local D-hormone actions.

mechanism of action

The first step involved in the activation of vitamin D3 is a 25-hydroxylation which is catalysed by the 25-hydroxylase in the liver and then by other enzymes. The mitochondrial sterol 27-hydroxylase catalyses the first reaction in the oxidation of the side chain of sterol intermediates. The active form of vitamin D3 (calcitriol) binds to intracellular receptors that then function as transcription factors to modulate gene expression. Like the receptors for other steroid hormones and thyroid hormones, the vitamin D receptor has hormone-binding and DNA-binding domains. The vitamin D receptor forms a complex with another intracellular receptor, the retinoid-X receptor, and that heterodimer is what binds to DNA. In most cases studied, the effect is to activate transcription, but situations are also known in which vitamin D suppresses transcription. Calcitriol increases the serum calcium concentrations by: increasing GI absorption of phosphorus and calcium, increasing osteoclastic resorption, and increasing distal renal tubular reabsorption of calcium. Calcitriol appears to promote intestinal absorption of calcium through binding to the vitamin D receptor in the mucosal cytoplasm of the intestine. Subsequently, calcium is absorbed through formation of a calcium-binding protein.

toxicity

Hypercalcemia - Early symptoms of hypercalcemia, include nausea and vomiting, weakness, headache, somnolence, dry mouth, constipation, metallic taste, muscle pain and bone pain. Late symptoms and signs of hypercalcemia, include polyuria, polydipsia, anorexia, weight loss, nocturia, conjunctivitis, pancreatitis, photophobia, rhinorrhea, pruritis, hyperthermia, decreased libido, elevated BUN, albuminuria, hypercholesterolemia, elevated ALT (SGPT) and AST (SGOT), ectopic calcification, nephrocalcinosis, hypertension and cardiac arrhythmias.

drug interactions

Calcipotriol: Vitamin D Analogs may enhance the adverse/toxic effect of other Vitamin D Analogs. Avoid combined use of multiple vitamin D analogs (at pharmacologic doses). Prescribing information for calcitriol, doxercalciferol, paricalcitol, and alfacalcidol each specifically cautions against such combined use.

Cholecalciferol: Vitamin D analogs may enhance the adverse/toxic effect of other Vitamin D analogs. Avoid combined use of multiple vitamin D analogs (at pharmacologic doses). Prescribing information for calcitriol, doxercalciferol, paricalcitol, and alfacalcidol each specifically cautions against such combined use. Though not specified in the prescribing information for calcipotriene, cholecalciferol, and ergocalciferol, each contains warnings regarding the potential for vitamin D toxicity.

Colesevelam: Bile acid sequestrants such as colesevelam may decrease the serum concentration of Vitamin D Analogs. More specifically, bile acid sequestrants may impair absorption of Vitamin D Analogs. Avoid concomitant administration of vitamin D analogs and bile acid sequestrants (e.g., cholestyramine). Monitor plasma calcium concentrations in patients receiving combined therapy with these agents. This is particularly important in patients receiving higher doses of a bile acid sequestant (i.e., 30 g/day or more of cholestyramine or equivalent) or in patients experiencing bile acid sequestrant-induced steatorrhea. Specific recommendations regarding the separation of administration of these agents are not defined; however, it would seem prudent to separate the administration of these agents by several hours to minimize the potential risk of interaction. Similar precautions do not apply to parenterally administered vitamin D analogs.