indication

Used to treat vitamin D deficiency or insufficiency, refractory rickets (vitamin D resistant rickets), familial hypophosphatemia and hypoparathyroidism, and in the management of hypocalcemia and renal osteodystrophy in patients with chronic renal failure undergoing dialysis. Also used in conjunction with calcium in the management and prevention of primary or corticosteroid-induced osteoporosis.

pharmacology

Calcitriol, a pharmaceutical form of vitamin D, has anti-osteoporotic, immunomodulatory, anticarcinogenic, antipsoriatic, antioxidant, and mood-modulatory activities. Calcitriol has been found to be effective in the treatment of psoriasis when applied topically. Calcitriol has been found to induce differentiation and/or inhibit cell proliferation in a number of malignant cell lines including human prostate cancer cells. Vitamin D deficiency has long been suspected to increase the susceptibility to tuberculosis. The active form of calcitriol, 1,25-(OH)₂-D3, has been found to enhance the ability of mononuclear phagocytes to suppress the intracellular growth of Mycobacterium tuberculosis. 1,25-(OH)₂-D3 has demonstrated beneficial effects in animal models of such autoimmune diseases as rheumatoid arthritis. It has also been found to induce monocyte differentiation and to inhibit lymphocyte proliferation and production of cytokines, including interleukin IL-1 and IL-2, as well as to suppress immunoglobulin secretion by B lymphocytes. Vitamin D appears to demonstrate both immune-enhancing and immunosuppressive effects.

mechanism of action

The mechanism of action of calcitriol in the treatment of psoriasis is accounted for by their antiproliferative activity for keratinocytes and their stimulation of epidermal cell differentiation. The anticarcinogenic activity of the active form of Calcitriol appears to be correlated with cellular vitamin D receptor (VDR) levels. Vitamin D receptors belong to the superfamily of steroid-hormone zinc-finger receptors. VDRs selectively bind 1,25-(OH)₂-D3 and retinoic acid X receptor (RXR) to form a heterodimeric complex that interacts with specific DNA sequences known as vitamin D-responsive elements. VDRs are ligand-activated transcription factors. The receptors activate or repress the transcription of target genes upon binding their respective ligands. It is thought that the anticarcinogenic effect of Calcitriol is mediated via VDRs in cancer cells. The immunomodulatory activity of calcitriol is thought to be mediated by vitamin D receptors (VDRs) which are expressed constitutively in monocytes but induced upon activation of T and B lymphocytes. 1,25-(OH)₂-D3 has also been found to enhance the activity of some vitamin D-receptor positive immune cells and to enhance the sensitivity of certain target cells to various cytokines secreted by immune cells.

toxicity

LD₅₀ (oral, rat) = 620 μg/kg; LD₅₀ (intraperitoneal, rat) > 5 mg/kg; Overdose evident in elevated blood calcium levels causing symptoms of anorexia, nausea and vomiting, polyuria, polydipsia, weakness, pruritus, and nervousness, potentially with irreversible calcification of soft tissue in the kidney and liver.

biotransformation

The first pathway involves 24-hydroxylase activity in the kidney; this enzyme is also present in many target tissues which possess the vitamin D receptor such as the intestine. The end product of this pathway is a side chain shortened metabolite, calcitroic acid. The second pathway involves the conversion of calcitriol via the stepwise hydroxylation of carbon-26 and carbon-23, and cyclization to yield ultimately 1a,25R(OH)₂-26,23S-lactone D3. The lactone appears to be the major metabolite circulating in humans.

absorption

Rapidly absorbed from the intestine.

half life

5-8 hours

route of elimination

Enterohepatic recycling and biliary excretion of calcitriol occur. The metabolites of calcitriol are excreted primarily in feces. Cumulative excretion of radioactivity on the sixth day following intravenous administration of radiolabeled calcitriol averaged 16% in urine and 49% in feces.

drug interactions

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.

Telithromycin: Telithromycin may reduce clearance of Calcitriol. Consider alternate therapy or monitor for changes in the therapeutic/adverse effects of Calcitriol if Telithromycin is initiated, discontinued or dose changed.

Voriconazole: Voriconazole, a strong CYP3A4 inhibitor, may increase the serum concentration of calcitriol by decreasing its metabolism. Monitor for changes in the therapeutic and adverse effects of calcitriol if voriconazole is initiated, discontinued or dose changed.