j. Soc. Cosmet. Chem., 48, 187-197 (July/August 1997) In vitro percutaneous absorption of alpha hydroxy acids in human skin MARGARET E. K. KRAELING and ROBERT L. BRONAUGH, Office of Cosmetics and Colors, Food and Drug Administration, Laurel, MD 20708. Accepted for publication Septewzber 3 O, 1997. Presented in part at the Annual Scientific Meeeting of the Society of Cosmetic Chemists, New York, Decewzber 12-13, 1996 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. INTRODUCTION Alpha hydroxy acids (AHAs) are widely used in cosmetic products. The small, short- chained compounds glycolic acid and lactic acid are most widely used, but longer-chain AHAs have been found to increase stratum corneum extensibility and flexibility (1) and have been used in some products. The effects of AHAs on skin structure are noticed in the stratum corneum (2), the viable epidermis (3), and even deeper in the dermal layer (3). The mechanism of AHA action is still unknown. Van Scot et al. have suggested that AHAs reduce stratum corneum corneocyte cohesion by interference with ionic bonding (4). But structural changes in the epidermis and dermis suggest that effects on the stratum corneum could originate from AHA activity in these deeper layers. Understand- ing the extent of absorption of AHAs is important, particularly with regard to the 187

188 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS localization in the various layers of skin. Also, the effect of AHA chemical structure on absorption was examined by studying the absorption of a homologous series of com- pounds. The percutaneous absorption of five AHAs was measured through viable excised human skin in diffusion cells: glycolic acid (GA), lactic acid, 2-hydroxyhexanoic acid (2- hydroxycaproic acid), 2-hydroxyoctanoic acid (2-hydroxycaprylic acid), and 2- hydroxydecanoic acid (2-hydroxycapric acid). AHA absorption was assessed by deter- mining levels of absorbed material in skin layers and in the receptor fluid beneath the skin. MATERIALS AND METHODS MATERIALS [1-•4C]Glycolic acid (specific activity, 55 mCi/mmol 99% purity) was obtained from American Radiolabeled Chemicals, Inc. (St. Louis, Me), and [1-•4C]DL-lactic acid (specific activity, 50 mCi/mmol 98% purity) was obtained from Sigma Chemical Co. (St. Louis, Me). [1-•4C]2-Hydroxyhexanoic acid (specific activity, 17.6 mCi/mmol 96% purity), [1-14C]2 - hydroxyoctanoic acid (specific activity, 19.2 mCi/mmol 97% purity), and [1-•4C]2 - hydroxydecanoic acid (specific activity, 16.4 92% purity) were synthesized by Research Triangle Institute (Research Triangle Park, NC). [3H]Water (specific activity, 55.5 mCi/mmol 97% purity) was purchased from New England Nuclear Corp. (Boston, MA). Nonlabeled glycolic acid, lactic acid, 2-hydroxyoctanoic acid, and 2- hydroxydecanoic acid were obtained from Sigma Chemical Co. Nonlabeled 2- hydroxyhexanoic acid was obtained from Aldrich Chemical Co. (Milwaukee, WI). Com- mercial product 1 (5% GA, pH 2.5) and commercial product 2 (10% GA, pH 3.5) were obtained from a local cosmetics supplier. OIL-IN-WATER EMULSION FORMULATIONS Percutaneous absorption of glycolic acid was studied by using two oil-in-water emulsion formulations (Formulations A and B). The composition of Formulation A is given in Table I. It contained two non-ionic emulsifying agents: polyethylene glycol (PEG) 100 stearate (2%) and PEG-4 lauryl ether (Laureth-4) (1%). Formulation B had the same composition as Formulation A, except that 1% ammonium laureth sulfate (ALS), an ionic surfactant, was used in place of the Laureth-4. Formulation A was the vehicle used in most of the percutaneous absorption studies. Emulsions containing 5 % AHAs were prepared by dissolving the acid in either the pH 3 or pH 7 buffer, readjusting the buffer to the proper pH, and then mixing with the other ingredients in phase B. Phases A and B were heated separately to 75-80øC, and then phase B was added to phase A and mixed at high shear in an Omni-Mixer Homogenizer (Omni International, Warrenton, ¾A) for 1 h. Mixing was continued at a lower shear until the temperature of the emulsion reached room temperature. Phase C, the preservative, was then added, and the emulsion was stirred for an additional 30 min. For the 0.5 % emulsions, a stock emulsion (containing no AHA) was prepared, and then