POLYETHYLENE GLYCOL-8/SMDI COPOLYMER 135 1% (v/v) aqueous dishwashing liquid, rinsed with distilled water, and patted dry with a paper towel. A 250-300 pm thick layer of the skin was prepared with a Padgett Electrodermatome (Padgett Dermatome, Division of Kansas City Assemblage Co., Kan- sas City, MO). The dermatomed skin was refrigerated until used. Two hours before each experiment, the skin was placed at room temperature to equilibrate. Circular pieces of the dermatomed skin (about 12 mm in diameter) were cut with a brass punch and placed epidermis-side up on the diffusion cells. Methodology. The skin was cut into discs 12 mm in diameter that were mounted on flow-through (8) diffusion cells (Amie Systems, Riegelsville, PA). The diffusion cells were clamped and the receptor fluid, phosphate-buffered saline (PBS), was pumped through at a rate of 3.57 ml/hour. The membrane was left in place one hour to equili- brate before application of the material to be tested. The cells' temperature was main- tained at 32øC throughout the experiment by means of a water bath/circulator (Haake, Paramus, N J). Fraction collection took place at specified intervals (every hour for SA and every four hours for LA) throughout the experiment by means of a fraction collector (Isco, Inc., Lincoln, NE). Samples were collected directly into scintillation vials. All samples were tested in quadruplicate. Unless otherwise indicated, a 50-pl (low-dose) sample of the formula was dispensed and spread evenly on the skin surface using a micropipette. The cells were left uncovered throughout the experiment. In infinite-dose experiments (500 pl), the cells were covered during the experiment. Skin uptake. The skin was examined for uptake at set intervals: 2, 4, 6, and 8 hours for SA and 4, 8, 12, and 20 hours for LA. Four replicates were tested in each experiment. Before measuring uptake, the skin was washed thoroughly while mounted on the dif- fusion cells. Washing consisted of wiping the skin surface with a 1-cm 2 piece of tissue followed by addition of 100 pl of a 70/30 (v/v) ethyl alcohol/water solution to the surface of the skin and wiping it with another piece of tissue of the same size. Finally, the skin was wiped a third time with a 1-cm 2 piece of tissue, and the three pieces of tissue were set aside for analysis. The skin was then removed from the diffusion cells and tape- stripped. Twenty-seven consecutive tape-strippings were performed on each piece of skin. The first two strippings were added to the three pieces of tissue collected in the wash and were assayed together to determine the amount of active remaining on the skin. The other twenty-five strippings were bundled in groups of five and analyzed for drug content by means of scintillation counter (Beckman Instruments Inc., Fullerton, CA). The twenty-five strippings collected represented the amount of active in the stratum corneum (SC). When the stripping was completed, each piece of skin was digested and assayed for drug content. Digestion was performed by adding 2 ml of skin-digesting fluid and incubating the skin for 48 hours in a 40øC incubator (Precision Scientific Co., Chicago IL). The samples were then removed and brought to room temperature, and 0.1 ml of glacial acetic acid was added to each sample. Drug content was then measured using the scintillation counter. Analysis Radiolabeling. Salicylic acid and LA formulations were spiked with 1 l•l/ml of [•4C]SA 14 . ß - 14 and 4 l•l/ml of [ C]LA, respectively. Each m•crohter of [ C]SA or [•4C]LA contained 0.1 l•Ci (2.2 x 105 DPM). The receptor fluid from all permeation experiments was collected directly into scintillation vials. Ten milliliters of scintillation fluid was added to each vial, and all samples were analyzed in the scintillation counter.

136 JOURNAL OF COSMETIC SCIENCE High-performance liquid chromatography (HPLC). The assay used in this study for the quantification of SA is a variation of the method recommended by the United States Pharmacopeia (USP) for the simultaneous analysis of benzoic acid and SA from oint- ments (US Pharmacopeia XXlII, 1995). A Waters © (Franklin, MA) HPLC equipped with an ultraviolet (UV) detector set at 280 nm was used. The mobile phase was composed of 150 ml of methanol, 150 ml of acetonitrile, and 700 ml of water that contained 225 mg of tetramethylammonium hydroxide pentahydrate, 6.8 gm of mono- basic potassium phosphate, and 0.224 gm of sodium hydroxide. The pH of the mobile phase was adjusted to range between 5 and 6 by the addition of glacial acetic acid. By means of a 4-mm x 15-cm C18 column and a flow rate of 0.7 ml/min, the SA eluted at 2.1 minutes. Under these conditions the coefficient of variation (CV) was under 3% for an injection volume of 20 lal. EFFECT OF PP-15 ON THE APPARENT SOLUBILITY OF SALICYLIC ACID The saturation solubilities of SA in aqueous solutions containing 1%, 2%, 3%, 6%, and 10% w/w PP-15 were measured and compared to a control (without polymer). Since SA can salt-out the polymer at room temperature, its solubility was measured at a low temperatures (4øC). To each system, excess solid SA was added. The suspensions were placed in individual flasks in a 4øC refrigerator and shaken manually for 15 minutes twice a day. The supernatant collected from each mixture was centrifuged, filtered, and assayed for SA content weekly by HPLC. Saturation solubilities were reached when no further increase in SA solubility was obtained. BINDING EXPERIMENTS Equilibrium dialysis was used to measure polymer/active binding at 32øC. In this experiment, side-by-side diffusion chambers were used. The donor compartment con- tained the polymer and the drug dissolved in a buffer (pH 2.4 for SA and 2.0 for LA). The receptor compartment contained the buffer alone. Four concentrations of drug (SA: 0.00375, 0.0025, 0.00075, and 0.0005 M LA: 0.05, 0.25, 0.5, and 1 M) and one polymer concentration (0.05 M) were studied. The two chambers were separated by a porous cellulose acetate membrane with a cutoff of 500 daltons. Equilibrium was reached after 72 hours. At the end of the experiment, both compartments were sampled and the amount of drug in each compartment was assayed. Two controls were studied. In the first, no polymer was added to the donor chamber, and the concentrations of the actives were 0.0005 1 M and 1 M for SA and LA, respectively. The receptor compart- ment was monitored for drug appearance over time until equilibrium was reached. In the second control, the same concentration of polymer (0.05 M) was added to the two chambers, and either 0.00375 M SA or 0.05 M LA was added to the donor compartment. The receptor compartment was monitored for drug concentration over time until equi- librium was reached. EFFECT OF THE IONIC STRENGTH OF THE FORMULATION ON THE EFFICACY OF PP-15 In this experiment, the ionic strength of a solution containing 0.1% w/w SA and 3% w/w PP-15 was changed by the addition of salts. The two salts studied were calcium