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Skin Lightening Challenges
By: Zoe Diana Draelos, MD
Posted: February 3, 2009, from the February 2009 issue of GCI Magazine.
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Retinoids, such as retinol or retinaldehyde, can also be used as penetration enhancers for other skin lightening agents. Ingredient combinations where each ingredient affects a different portion of the skin pigmentation pathway may be more beneficial for skin lightening than single ingredient formulations. Since there are many steps in pigment formation, skin lighteners can interrupt melanin production and transfer at multiple sites. Formulations that have activity at multiple sites have a better chance of stopping pigment production. For further reading, see Ingredient Combinations for Skin Lightening.
Ascorbic acid, also known as vitamin C, is used in OTC pigment lightening formulations. It interrupts the production of melanin by interacting with copper ions to reduce dopaquinone, an important intermediate in pigment production.7 It is a potent antioxidant sometimes combined with hydroquinone to prevent oxidation of the hydroquinone.
By itself, low concentration ascorbic acid is a poor skin lightening agent. In higher concentrations, ascorbic acid can be a strong skin irritant due to its low pH, but may induce pigment lightening by inducing skin peeling in lighter-skinned individuals. High concentrations of ascorbic acid must be avoided in persons of color, as the irritation paradoxically will darken the skin.
Licorice extracts are used in many OTC products to lighten skin. The active agents are liquiritin and isoliquertin, which are glycosides containing flavonoids.8 Liquiritin induces skin lightening by dispersing melanin pigment and enhancing more even pigment distribution. It must be applied to the skin in a dose of 1 g/day for four weeks to see a clinical result. This may not be practical in OTC formulations due to the expense of such a high concentration. Since the liquiritin is an anti-inflammatory, irritation is not a side effect with licorice extract, whereas irritation is the dose-limiting problem with hydroquinone, retinoids and ascorbic acid.
Alpha Lipoic Acid
Alpha lipoic acid is found in some OTC pigment lightening preparations. It is a disulfide derivative of octanoic acid, which is able to inhibit tyrosinase, a key enzyme in pigment production. However, its ability to lighten skin is doubtful, as it is a large molecule with questionable penetration to the level of the melanocyte where tyrosinase is present. Effective pigment lightening agents must reach their target in order to induce the desired effect. Molecular size and molecular charge are two key considerations in pigment lightening efficacy.
Kojic acid, chemically known as 5-hydroxymethyl-4H-pyrane-4-one, is one of the most popular skin lightening agents found globally in OTC skin lightening creams. It is a hydrophilic fungal derivative obtained from the Aspergillus and Penicillium species. It is frequently employed in Asia for the treatment of melasma.9