The skin condition psoriasis develops when keratinocytes, the primary type of skin cells, form too rapidly due to an underlying malfunction of the immune system. As the overabundant keratinocytes migrate to the skin's surface, they build up into the scaly, silvery thick patches of skin characteristic of psoriasis. Although psoriasis usually causes only minor discomfort, the skin's appearance can cause serious damage to the patient's self-image. Vitamin D3 analogs offer one line of treatment to minimize the overproduction of skin cells in psoriasis, but they do not cure the condition.
Analogs
Patients cannot use vitamin D itself because it would have severe toxic effects, raising blood levels of calcium and phosphate. The vitamin D3 analogs calcipotriene---also known as calcipotriol---and calcitriol are very strong metabolites of vitamin D3 that slow skin cell turnover while minimizing effects on bone. Formulated into a topical ointment, they are useful to treat mild to moderate cases of psoriasis. More severe cases require systemic drugs like immunomodulators or methotrexate.
Mechanism
Calcipotriene and calcitriol act in the body like powerful versions of the naturally occurring vitamin D. When absorbed into the skin cells, through a mechanism that remains unclear, these drugs normalize the production time of keratinocytes. Skin cells build up less quickly, easing the development of thick, scaly patches of skin. However, vitamin D3 analogs do not cure psoriasis because they do not address the underlying immune malfunction that triggers a flare-up of psoriasis.
Dosage
Calcitriol comes at a strength of 3 micrograms of analog per 100 grams of ointment, according to the Merck Manual. Patients using calcitriol spread the ointment over the skin lesions twice per day, to a maximum dosage of 200 grams per week. Calcipotriene is less potent than calcitriol and comes in an ointment with 50 micrograms of analog per gram. Using more than 100 grams of calcipotriene ointment per week can cause an alarming increase in calcium levels, according to the National Institute of Arthritis and Musculoskeletal and Skin Diseases. Some doctors encourage their patients to alternate the use of vitamin D3 analog ointments and topical corticosteroids during the week to avoid the worst side effects of either treatment.
Effectiveness
The Merck Manual recommends vitamin D3 analogs, often along with topical corticosteroids, for the treatment of several forms of psoriasis. Plaque psoriasis, the most common type; inverse psoriasis, in which the lesions attack skin folds; and acrodermatitis continua of Hallopeau, a pustular psoriasis on the fingers or toes, all respond to vitamin D3 analogs.
Side Effects
Doctors should monitor patients using vitamin D3 analogs for the levels of vitamin D and calcium in their blood because too much of either can suppress the release of parathyroid hormone, which regulates the body's levels of calcium for optimal heart and bone health. Absorption of high levels of vitamin D can also provoke excessive levels of phosphate in the blood. Patients using vitamin D3 analogs should limit their exposure to the sun and should not use phototherapy to treat psoriasis. The formula should not be used on any lesions on the face because it can irritate the skin.
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