Clinical Relevance of Topical Active Delivery Systems in Cosmetics

By: Haw-Yueh Thong, MD, MS, Dnational Taiwan University Hospital and Howard I. Maibach, MD, University of California

Posted: May 4, 2009, from the May 2009 issue of Cosmetics & Toiletries.

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Presently there are no internationally approved guidelines to conduct in vitro tests to measure percutaneous absorption. Draft guidelines for in vivo and in vitro percutaneous absorption studies have been proposed by the Organization for Economic Cooperation and Development (OECD).²⁰ Discussion documents also have been produced by the US Environmental Protection Agency (EPA) and the US Food and Drug Administration (FDA), prompting much discussion for the principles of conducting percutaneous absorption measurements.^14,21 In spite of several national and international standardization efforts by the EPA, the European Centre for the Validation of Alternative Methods (ECVAM) and the OECD,¹⁴ the industry still lacks a complete in vivo validation method that is relevant in humans.

Percutaneous Enhancers for Cosmetic Actives: An Overview
The difficulty in validating clinically relevant in vitro percutaneous absorption systems lies in the complexity of percutaneous penetration. Wester and Maibach¹⁶ proposed that the process of percutaneous absorption can be examined through ten steps. These steps start with vehicle release of chemical to the skin and proceed into the various kinetics and factors affecting percutaneous absorption (see Ten Steps to Percutaneous Absorption).

Given the many complex aspects of percutaneous absorption, with the greatest hindrance to it being the SC, extensive research has been conducted and innovations designed in the field of penetration enhancement. The literature is abundant with articles, patents and reviews on this topic.⁵ In the sidebar, Common Penetration Enhancement Techniques, and Table 1, common penetration enhancement techniques and examples of chemical penetration enhancers are listed.

Yet despite the plethora of candidate penetration enhancement techniques, few have enjoyed wide scale medical acceptance. Hydration by occlusive systems or topical vehicles remains the most facile way to obtain a reduction in barrier potential of the SC. To date, clinicians, cosmetic chemists and pharmaceutical scientists still regard chemical induced occlusion as a convenient and safe method of enhancing CAID.²²

Conclusion
Taken together, extensive experimentation has led to some medical acceptance of methods of enhancing flux, such as the solubilization of active cosmetic ingredients with propylene glycol. However, much remains to be achieved and there is little data available on the penetration of cosmetic actives.