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

ABSORPTION OF AHAs IN SKIN 189 Table I Composition of the Oil-in-Water Emulsions for 5% Alpha Hydroxy Acids Formulation A Grams per 100 grams emulsion Phase A Polyoxyethylene (100) glycerol stearate (ICI Surfactants, Wilmington, DE) 2.0 Mineral oil (light) (Penreco, Karns City, PA) 10.0 Cetearyl alcohol (Henkel Corp., Hoboken, NJ) 3.0 Phase B Laureth-4 (Lipo Chemicals, Paterson, NJ) 1.0 Propylene glycol (Aldrich Chemical Co., Milwaukee, WI) 5.0 Alpha hydroxy acid 5.0 Phthalate-HC1 buffer* Potassium phosphate-NaOH** 73.0 Phase C Preservative Methyl-p-hydroxybenzoate (Pfaltz & Bauer, Inc., Stamford, CT) 0.5 Propyl-p-hydroxybenzoate (Pfaltz & Bauer, Inc., Stamford, CT) 0.5 * Phthalate-HC1 buffer (pH = 3): 50 ml of 0.1 M potassium bi- phthalate + 22.3 ml of 0.1 M HC1 diluted with water to 100 mi. ** Potassium phosphate-NaOH buffer (pH = 7): 50 ml of 0.1 M potassium phosphate + 29.1 ml 0.1 M NaOH diluted with water to 100 mi. appropriate amounts of AHA were added to aliquots of the stock emulsion to give the desired concentration of AHA. PERCUTANEOUS ABSORPTION EXPERIMENTS Skin absorption studies were conducted by using human skin freshly obtained from abdominoplasty procedures. The skin was placed in a saline solution at the clinic and kept in cool packing as it was transported to the laboratory and transferred to Hepes buffered Hanks' balanced salt solution (HHBSS). Subcutaneous fat was removed from the skin, and the surface was gently cleaned with a 10% soap solution and rinsed with distilled water. The skin was mounted on a Styrofoam block and cut with a Padgett dermatome (Padgett Instruments, Dermatome Division, Kansas City, MO) to a thick- ness of 200-340 pm. Skin discs were prepared with a punch and placed epidermis-side up in Teflon flow-through diffusion cells (5). Prior to assembly, the flow-through diffusion cell system was disinfected with 70% ethanol and rinsed with receptor fluid. The diffusion cells were maintained at 35øC in an aluminum holding block heated by a circulating water bath this maintained the surface temperature of the stratum corneum at 32øC. The skin was perfused with HHBSS, pH 7.4, receptor fluid at a flow rate of 1.5 ml/h to maintain the viability of the skin in the diffusion cells for the duration of the 24-h study (6).