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j. Soc. Cosmet. Chem., 38, 397-417 (November/December 1987) An original predictive method for in vivo percutaneous absorption studies ANDR]2 ROUGIER, CLAIRE LOTTE, and DIDIER DUPUIS, Ddpartement de Biologie, Centre de Recherche Fondamentale de l'Oreal, 1 Avenue Eugene Schueller, 93 600 Aulnay sous Bols, France. Received March 2, 1987. Award for best paper presented at the Annual ScientijS'c Meeting of the Society of Cosmetic Chemists, New York, December 4-5, 1986. Synopsis Barrier function is doubtless one of the most important skin functions. It restricts entry by obstructing the inward uptake of molecules and participates in the homeostasis of the organism, in particular by limiting the outward movement of water. It is well established that the main barrier is constituted by the stratum corneum which also acts as a "reservoir" for molecules topically applied. Our findings in animal and in man clearly show that these two principal functions of the horny layer are intimately linked. Thus, there exists a linear relationship between the amount of chemical pres•ent in the stratum corneum at the end of a 30-min application period and the total amount penetrated within four days. Moreover, it is of note that this relationship has been found to be independent of the main factors likely to modify the absorption level of a chemical, i.e., application time, dose applied, vehicle used, anatomical site involved, and species chosen. The overall absorption of a substance can therefore be predicted by simply stripping the treated area and measuring the amount of product present in the horny layer at the end of a 30-min application period. Moreover, due to the relatively large amounts of chemical found in the stratum corneum at the end of application, this rapid and mildly invasive method allows percutaneous absorption measurements to be performed on animals or on humans, either by using appropriate analytical techniques or by reducing the radioactivity applied when labeled compounds are used. INTRODUCTION From a practical viewpoint, it remains difficult to draw valid conclusions from the literature concerning the absorption level of a given compound. This is essentially due to the diversity of techniques used, animal species (1,2), anatomical location (3,4), duration of application (1), dose applied (5,6), and vehicle used (1,7). Furthermore, since this kind of research has interested scientists from widely differing disciplines, each worker has chosen or adapted the methodology in order to elucidate his own problem. From a theoretical viewpoint, over the two past decades considerable attention has been paid to developing an understanding of the mechanisms and routes by which chemical 397