Comparative in vitro-in vivo skin permeation of cosmetic ingredients

The potential EU ban on the use of animals in the testing of cosmetic products and their ingredients from 2009 together with the ethical reasons for avoiding tests on human beings have recently prompted the development of guidelines on in vitro alternative methods and an extensive search for novel validated in vitro permeation models. The prediction of the percutaneous absorption by means of in vitro methodologies requires good in vitro-in vivo correlation. Moreover, due to differences in the type of vehicle (conventional vehicles or particulate carriers) and in the physicochemical properties of the substance, such as its solubility, the selection of appropriate in vitro conditions is crucial. In particular, the validation of an in vitro skin penetration model appears quite difficult for highly lipophilic substances, because of their limited rate of transfer between stratum corneum and the viable epidermis, due to their low solubility in most physiological receptor fluids. This chapter focuses on comparative in vitro-in vivo skin permeation of butyl methoxydibenzoylmethane, a highly lipophilic UV-A absorber (log Po/w = 5.6), methylbenzylidene camphor (log Po/w = 4.8) a less lipophilic sunscreen agent, and glycolic acid, a hydrophilic active substance (log Po/w < 0). Special attention is paid to the influence of the in vitro receptor medium on the in vitro-in vivo correlation. Skin absorption