Calcitriol

Overview
Calcitriol (INN) or 1,25-dihydroxycholecalciferol (abbreviated 1,25-(OH)2D3) is the active form of vitamin D found in the body (vitamin D3). It increases the absorption of calcium and phosphate from the gastrointestinal tract and kidneys and inhibits release of PTH.

Calcitriol is marketed under various trade names including Rocaltrol (Roche), Calcijex (Abbott) and Decostriol (Mibe, Jesalis).

Production and function
It is produced in the kidneys via 25-Hydroxyvitamin D3 1-alpha-Hydroxylase by conversion from 25-hydroxycholecalciferol (calcidiol).

This is stimulated by a decrease in serum calcium and/or phosphate (PO43−), and an increase in parathyroid hormone (PTH) levels. It increases blood calcium levels by increasing the absorption of calcium and phosphate from the gastrointestinal tract, increasing calcium and phosphate reabsorption in the kidneys and inhibiting the release of PTH.

Calcitriol is also commonly used as a medication in the treatment of hypocalcemia and osteoporosis.

Metabolism
Calcitriol becomes calcitroic acid through the action of 24-hydroxylase. Calcitroic acid is excreted in the urine.

Indications
Calcitriol is indicated for:
 * Treatment of hypocalcaemia – hypoparathyroidism, osteomalacia (adults), rickets (infants, children), renal osteodystrophy, chronic renal dialysis
 * Treatment of osteoporosis
 * Prevention of corticosteroid-induced osteoporosis

Calcitriol is also sometimes used topically in the treatment of psoriasis, however the evidence to support its efficacy is inconclusive. The vitamin D analogue calcipotriol is more commonly used for psoriasis.

Adverse effects
The main adverse drug reaction associated with calcitriol therapy is hypercalcaemia – early symptoms include: nausea, vomiting, constipation, anorexia, apathy, headache, thirst, sweating, and/or polyuria). Compared to other vitamin D compounds in clinical use (cholecalciferol, ergocalciferol), calcitriol has a higher risk of inducing hypercalcaemia. However, such episodes may be shorter and easier to treat due to its relatively short half-life.