Hyperpigmented lesions, whether they are solar lentigos, freckles, or melasma, are the source of frequent complaints by cosmetic patients. In addition, some cosmetic patients develop postinflammatory hyperpigmentation after chemical peels, laser treatments, or even after a bout of acne. Melanin synthesis within melanosomes and their distribution to keratinocytes within the epidermal melanin unit determines skin pigmentation. Hyperpigmentation occurs when this system goes awry (see Chapter 13). But dark spots and patches are unacceptable to cosmetic patients. For this reason, there are hundreds of products on the market that are touted as “lightening creams.” Although there are many product choices available, the number of effective agents to treat hyperpigmentation disorders is relatively small. Unfortunately, most of these agents require months of use for improvement to be seen. Combination with retinoids (Chapter 30), sunscreens (Chapter 29), chemical peels (Chapter 20), and lights or lasers (Chapter 24) may enhance the effectiveness of these products. Currently available topical agents used to treat hyperpigmentation include tyrosinase inhibitors, melanosome-transfer inhibitors, melanocyte-cytotoxic agents, retinoids, peeling agents, and sunscreens. This chapter will discuss the ingredients commonly used for the treatment of pigmentary disorders.
Tyrosinase, the enzyme that controls the synthesis of melanin, is a unique product of melanocytes (Fig. 13-1). It is considered to be the rate-limiting enzyme for the biosynthesis of melanin in epidermal melanocytes. Therefore, tyrosinase activity is thought to be a major regulatory step in melanogenesis. Several products on the market contain ingredients that inhibit tyrosinase and thus decrease melanin formation.
Hydroquinone (HQ) (Fig. 33-1) is used in over-the-counter (OTC) products (2% concentration or less), prescription drugs (4%), and custom pharmacy formulations (2% to ≥10%) as an ingredient to inhibit melanin production and produce skin lightening. The cosmetic products are often labeled as “skin brighteners.” HQ also occurs naturally as an ingredient in various plant-derived food and beverage products, such as vegetables, fruits, grains, coffee, tea, beer, and wine.1 For many years, HQ has been the main treatment modality for postinflammatory hyperpigmentation and melasma. HQ exerts its depigmenting effect by inhibiting tyrosinase and by virtue of its cytotoxicity to melanocytes.2 It is known to cause reversible inhibition of cellular metabolism by affecting both DNA and RNA synthesis. Also, HQ is an efficient blocker of tyrosinase and has been shown to decrease its activity by 90%.3 Although useful as a sole agent, HQ is often combined with other agents such as tretinoin, glycolic acid, kojic acid, and azelaic acid.4
The chemical structure of hydroquinone.
HQ is currently available as OTC in 2% concentrations and by prescription in 4% concentrations. Although the 4% concentration is more effective than the more conventional 2% concentration, it ...