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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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Such examples—i.e., correlation of physical chemistry, percutaneous penetration and pharmacodynamic relationships—are relatively limited and the authors believe that greater exploration in this area will eventually enhance the efficacy of active cosmetic ingredients and consumer satisfaction.
Clinical Relevance of In Vitro Data
The methods employed for studying percutaneous absorption range from in vivo systems, in which the full range of physiological processes are operative, to simple in vitro methods and modeling approaches. Percutaneous absorption can be studied with in vitro methods since it is possible to maintain the barrier properties of the stratum corneum (SC) in excised skin.
There are advantages of using in vitro methods, not in the least that human skin can be used and that procedures are not conducted with animals. Disadvantages include the lack of provision of pharmacokinetic data, and the difficulty of obtaining human skin in sufficient quantities. Attempts have been made to produce simple models of these processes, which have resulted in the development of several of in vitro systems.13-15 An appreciation of the limitations of such processes has enabled the application of the data generated by in vitro methods to be applied with increasing confidence for predicting the likely effects of chemicals in vivo.
Wester and Maibach16 compared in vitro and in vivo methods studying percutaneous absorption and suggested that there were notable differences such that in vitro methods alone would not always be a reliable or accurate predictor of percutaneous absorption in living humans. On the other hand, there is also some evidence that in vitro data can be predictive for in vivo percutaneous absorption in both animals and humans.17,18
However, the current database is limited and the variance often exceeds 1log. In short, only human skin should be used to provide data for risk assessment purposes or for the prediction of therapeutic effects, because there are major differences in the skin permeability properties of animal and human skin17,19 and the use of animal skin often results in an over-prediction of likely human percutaneous absorption.