DRUGS USED TO TREAT DISORDERS OF PIGMENTATION
Hydroquinone interferes with the production of the pigment melanin by epidermal melanocytes through at least two mechanisms: it competitively inhibits tyrosi-nase, one of the principal enzymes responsible for con-verting tyrosine to melanin, and it selectively damages melanocytes and melanosomes (the organelles within which melanin is stored).
Hydroquinone is applied topically to treat disorders characterized by excessive melanin in the epidermis, such as melasma. In the United States, nonprescription skin-lightening products contain hydroquinone at con-centrations of 2% or less; higher concentrations are available by prescription.
The incidence of adverse effects with hydroquinone increases in proportion to its concentration. A relatively common side effect is local irritation, which may actu-ally exacerbate the discoloration of the skin being treated. Allergic contact dermatitis occurs less com-monly. A rare but more serious complication is exoge-nous ochronosis, in which a yellow-brown pigment de-posited in the dermis results in blue-black pigmentation of the skin that may be permanent.
Monobenzone (Benoquin) potently inhibits melanin production and destroys melanocytes. Like hydro-quinone, monobenzone was originally introduced for the topical treatment of disorders of excess melanin pig-mentation, including melasma. It is now used only to permanently depigment the remaining normally pig-mented skin in patients with extensive vitiligo. Irritant and allergic contact dermatitis are common side effects.