specific forms of vitamin d Calcitriol (1,25-dihydroxycholecalciferol; calcijex, rocaltrol) is available for oral and parenteral use. A number of derivatives are also used therapeutically (Figure 61-7). Doxercalciferol (1 «-hydroxyvitamin D2, hectoral), a prodrug that must be activated by hepatic 25-hydroxylation, is approved for treating secondary hyperparathy-roidism. Dihydrotachysterol (dht, roxane) is a reduced form of vitamin D2 that is converted in the liver to its active form, 25-hydroxydihydrotachysterol. it is active at high doses in mobilizing bone Ca2+ and therefore can be used to maintain plasma Ca2+ in hypoparathyroidism. 1a-hydroxycholecalciferol (1-OHD3, alphacalcidol; one-alpha) is a synthetic vitamin D3 analog that is already hydroxylated in the 1a position and is rapidly converted by 25-hydroxylase to 1,25-(OH)2D3. Because it does not require renal activation, it has been used to treat renal osteodystrophy. Ergocalciferol (calciferol, drisdol) is vitamin D2; it is available for oral, intramuscular, or intravenous administration. It is indicated for the prevention and treatment of vitamin D deficiency and for the treatment of hypoparathyroidism and the genetic disorders of vitamin D synthesis or action described above.
Calcitriol Analogs Several vitamin D analogs suppress PTH secretion by the parathyroid glands but have less or negligible hypercalcemic activity (Figure 61-7). They therefore have advantages in treating secondary hyperparathyroidism. Calcipotriol (calcipotriene) is a synthetic derivative of calcitriol with a modified side chain that is used topically for psoriasis (see Chapter 62). Paricalcitol (1,25-dihydrox-19-norvitamin D2, zemplar) is a synthetic D2 derivative that is FDA-approved for intravenous treatment of secondary hyperparathyroidism in patients with chronic renal failure. 22-oxacalcitriol (1,25-dihydroxy-22-oxavitamin D3, OCT, maxacalcitol, oxarol) has a low affinity for vitamin D-binding protein and thus has a shorter t1/2. it is used in patients with secondary hyperparathyroidism due to renal failure.
Indications for Therapy with Vitamin D
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