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Economic development and aesthetic needs have driven demand for skin-brightening products around the world, especially in non-white cultures. 1 Skin brightening is defined as, “the practice of using chemical substances or any other products with depigmenting potential in an attempt to lighten the skin tone or improve skin complexion by lessening the concentration of melanin to obtain a reduction of the physiological skin pigmentation.”2, 3
The key mechanism of action for skin-brightening agents is to reduce melanin, which is the main source of skin color. Melanin is responsible for pigment deposition in the human skin, eyes and hair, and is produced by epidermal melanocytes in a ratio of 1:36 with basal keratinocytes.4 Melanin is synthesized through the process of melanogenesis, which is mediated by melanocytes once the skin is exposed to UV irradiation.5 Under normal physiological conditions, pigmentation has a beneficial effect for the photoprotection of human skin against harmful UV damage; however, excessive production of melanin can lead to skin blemishes such as freckles and melasma,6, 7 as well as cancer and melanoderma.8
To date, a variety of cosmetic brightening agents targeting melanin have been developed and their corresponding brightening mechanisms have been emphasized; these are examined here. First, however, given its complexity and array of potential targets for brightening agents, the biological process of melanin production is reviewed here.
Melanogenesis is a complex pathway involving a combination of enzymatic and chemical catalyzed reactions. Melanocytes produce two types of melanin—eumelanin, which appears as a brownish/blackish color; and pheomelanin, which appears as a reddish/yellowish color—formed by the conjugation of glutathione or cysteine.9-11 The melanogenesis process is initiated with the oxidation of L-tyrosine to dopaquinone by the key enzyme tyrosinase. The resulting quinone will serve as a substrate for the synthesis of eumelanin and pheomelanin.12, 13
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