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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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Hydroquinone is available in both the OTC and prescription U.S. markets. While controversy has arisen about its availability, no restrictions on hydr-quinone use have been issued by the U.S. Food and Drug Administration (FDA) as of this writing. Hydroquinone is a medication that received grandfather status from the FDA, as it was in common use when the FDA was founded. Thus, some hydroquinone formulations have never undergone the testing required for today’s drug approval.

Many prescription products have made submissions to the FDA and completed the required safety and efficacy testing, but OTC formulations and some generic formulations were never tested. It is these untested formulations that have created concern. The maximum concentration in OTC products is 2%, while most prescription formulations are 4%. In general, 4% hydroquinone is more effective than 2% hydroquinone, but neither concentration will lighten pigment that is deep in the skin due to penetration issues. Current hydroquinone preparations are unable to reach the middle to deep dermis where post-inflammatory acne pigmentation may lie; therefore, its pigment lightening abilities are limited. Deep penetration of hydroquinone also raises unresolved safety issues.

In all formulations, hydroquinone is highly unstable and turns brown on contact with air, which oxidizes the hyroquinone molecule. Once the hydroquinone has oxidized, it is no longer active and should be discarded. It would be advisable to label OTC products with a warning that brown, discolored cream is unsuitable for use.

Mequinol

With the regulatory issues surrounding hydroquinone, a new pigment lightener known as mequinol has been introduced. At present, mequinol is only available by prescription but has been approved for use in the U.S. and Europe. It is chemically known as 4-hydroxyanisole and is chemically synthesized. Other names include methoxyphenol, hydroquinone monomethyl ether, and p-hydroxyanisole.

Mequinol is available in the U.S. in a 2% concentration and is commercially marketed as a prescription skin lightener in combination with 0.01% tretinoin as a penetration enhancer; vitamin C in the form of ascorbic acid; and ascorbyl palmitate, to enhance skin lightening. These active agents are dissolved in an ethyl alcohol vehicle. It is unknown exactly how mequinol lightens skin, but it is a competitive inhibitor in the formation of melanin precursors. It does not damage the melanocyte, as hydroquinone does.

Retinoids

Many dermatologists combine hydroquinone with prescription vitamin A formulations such as tretinoin to improve skin lightening results. Tretinoin has an effect on skin pigmentation as seen by a decrease in freckling.4 Freckling is created by the irregular grouping of melanocytes,5 which can be normalized with retinoids through alterations in pigment transfer.6 While this effect is more dramatic with topical prescription tretinoin, topical OTC retinol has been thought to provide similar effects.