MISCELLANEOUS TOPICAL AGENTS
Azelaic acid (Azelex) is a naturally occurring dicar-boxylic acid produced by the yeast Malassezia furfur. Azelaic acid inhibits tyrosinase, a rate-limiting enzyme in the synthesis of the pigment melanin. This may ex-plain why diminution of melanin pigmentation occurs in the skin of some patients with pityriasis versicolor, a dis-ease caused by M. furfur. Azelaic acid is bacteriostatic against a number of species thought to participate in the pathogenesis of acne, including Propionibacterium ac-nes. The drug may also reduce microcomedo formation by promoting normalization of epidermal keratino-cytes. Azelaic acid is used for the treatment of mild to moderate acne, particularly in cases characterized by marked inflammation-associated hyperpigmentation.
Topically administered local anesthetics are useful in dermatology for preparation of the skin prior to minor surgical procedures, such as skin biopsies, laser treat-ment of vascular malformations, and curettage of mol-luscum contagiosum lesions, particularly in young chil-dren and needle-phobic adults. The topical anesthetic may be used alone or may be applied prior to intrader-mal injection of a local anesthetic to reduce the pain caused by the needle. Two recently approved drugs in
this group are ELA-Max, a topical formulation of lido-caine, and EMLA, which contains a mixture of lido-caine and prilocaine.
Capsaicin (Zostrix) is approved for the relief of pain following herpes zoster infection (postherpetic neural-gia). The drug depletes neurons of substance P, an en-dogenous neuropeptide that may mediate cutaneous pain. It is applied to affected skin after open lesions have healed. Local irritation is common.
Anthralin (Anthra-Derm) is a potent reducing agent whose mechanism of action is unknown. It is approved for the treatment of psoriasis and also may be helpful in alopecia areata. The major toxicities are discoloration of skin, hair, and nails and irritant dermatitis.
Benzoyl peroxide is a potent oxidizing agent that has both antimicrobial and comedolytic properties; its pri-mary use is in treating acne vulgaris. It is converted in the skin to benzoic acid; clearance of absorbed drug is rapid, and no systemic toxicity has been observed. The major toxicities are irritation and contact allergy. Outgrowth of bacteria resistant to topical antibiotics used to treat acne can be reduced by the addition of ben-zoyl peroxide in combination products such as erythro-mycin (Benzamycin) and clindamycin (Benzaclin).
Calcipotriene (Dovonex), a synthetic vitamin D3 deriva-tive, is indicated for the treatment of moderate plaque psoriasis. Its mechanism of action is unknown, although it competes for calcitriol receptors on keratinocytes and normalizes differentiation. It also has a variety of im-munomodulatory effects in the skin. Although the drug can cause local irritation, the most serious toxicities are hypercalciuria and hypercalcemia, which are usually re-versible.