Vitamin D: A New Era in Dermatology

Reprinted from the April 1994 issue of Medical Sciences Bulletin, published by Pharmaceutical Information Associates, Ltd.

Vitamin D is generated in the skin from the action of sunlight (ultraviolet radiation). Because it is produced by all vertebrate species exposed to sunlight, including humans, it is technically a hormone rather than a vitamin. The major physi-ologic function of vitamin D is to maintain normal serum levels of calcium and phosphorus. It does this by enhancing intestinal absorption of dietary calcium and phosphorus and mobilizing calcium from the bone when dietary calcium is insufficient to meet needs. After forming in the skin, vitamin D undergoes two successive hydroxylations, first in the liver to 25- hydroxyvitamin D, and then in the kidneys to 1,25-dihydroxyvitamin D3 (calcitriol). Calcitriol has been used pharmacologically to reverse the disorder of calcium metabolism seen in patients with chronic renal failure, and to treat renal osteodystrophy, hypocalcemia associated with hypopara-thyroidism, vitamin D- dependent rickets, and osteoporosis.

Receptors for 1,25-(OH)2D3 have been found in the bone and intestines, and also in the pancreas, breast, pituitary, gonads, mononuclear cells, activated T-lymphocytes, and skin. Discovery in the 1980s of epidermal 1,25-(OH)2D3 receptors led to trials of oral calcitriol in patients with a number of skin disorders, including atopic dermatitis, acne, scleroderma, and psoriasis. Clinical efficacy was marginal and vitamin D intoxi-cation was fairly common, so researchers tried the topical application of calcitriol and its analogs. Topical administration was shown to inhibit the proliferation of keratinocytes and to induce terminal differentiation. Recently an active vitamin D3 analog, calcipotriene, was approved for the topical treatment of psoriasis (see Calcipotriene Approved for Topical Treatment of Psoriasis). The preparation has proved to be at least as effective as topical steroids, without the "skin-thinning" side effect. According to Michael Holick at the Vitamin D, Skin, and Bone Research Laboratory at Boston University School of Medicine, topical preparations of 1,25-(OH)2D3 and calcipotriene "are likely to replace corticosteroid cream for patients with mild limited psoriasis." Also, said Holick, calcitriol administered orally offers a therapeutic option for patients with extensive psoriasis and may be of value for patients with psoriatic arthritis. (Holick MF. Mayo Clin Proc. 1993; 68: 925-927.)

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