In vitro percutaneous absorption of alpha hydroxy acids in human skin

Abstract

Synopsis Alpha hydroxy acids (AHAs) are used in many cosmetic products as exfoliants, moisturizers, and emollients. The activity of AHAs on skin is likely influenced by their ability to be absorbed into the different layers of skin. The absorption of a homologous series of AHAs was measured through hunnan skin by using in vitro diffusion cell techniques. The [4C] radiolabeled compounds were applied to the skin in an oil-in-water emulsion vehicle. The absorption of the AHAs was measured at pH 3.0, to simulate the pH of the most acidic cosmetic formulations, and at pH 7.0, to observe the effect of complete ionization of AHAs on skin penetration. Much greater absorption of the AHAs was seen at pH 3.0. We also observed substantial absorption into the various skin layers (stratum corneum, viable epidermis and dermis) as well as the receptor fluid. Total absorption of glycolic acid and lactic acid was similar (27-30%). Absorption of the longer-chain AHAs decreased to 21.0% and 19.3%, for 2-hydroxyoctanoic and 2-hydroxydecanoic acids, respectively. At the end of the 24-h studies, these longer-chain AHAs did not form a depot in the skin. The stratum corneum was shown to have a pH gradient with an average pH near 7 at the viable epidermal layer. Therefore, the AHAs ionize to polar molecules as they enter and diffuse through the stratum corneum.