142 JOURNAL OF COSMETIC SCIENCE 0.018 0.016 0.014 0.012 o.olo 0.008 0.006 0.004 0.002 0.000 i i i 0 2 4 6 8 10 Time (Hours) Figure 5. Effect of ionic strength on the efficacy of PP-15 in 0.1% w/w salicylic acid formulations. ¸, Control O, 3% PP-15 I, 3% PP-15 + 0.1M NaC1 &, 3% PP-15 + 0.1 M CaC12. EFFECT OF PP-15 ON THE APPARENT SOLUBILITY OF SA The addition of 1%, 2%, 3%, 6%, and 10% w/w PP-15 to the formula increased SA apparent solubility from 0.00113 (control) to 0.00159, 0.00244, 0.00311, 0.00614, and 0.00891 gm/ml, respectively. These results are presented in Figure 6. The increase in apparent solubility was linear with the amount of PP-15 added. The addition of 3% PP-15 increased the apparent solubility 2.8 times. DRUG/POLYMER BINDING A plot of the fraction of drug bound as a function of drug concentration is displayed in Figure 7. In the case of PP-15, the data showed that the fraction of SA bound (= 0.55) did not change markedly throughout the range of concentrations studied. It was also found that LA did not bind to PP-15 significantly. EFFECT OF SA AND LA ON THE CLOUD POINT OF PP-15 The effect of the drug concentration on the cloud point of PP-15 is displayed in Figure 8. An increase in the concentration of SA decreased the cloud point of PP-15. This decrease was linear with concentration. It was also found that the presence of LA to a level of 10% w/w did not affect the cloud point of PP-15. DISCUSSION For treatment of acne and other conditions in which exfoliation of the cells at the skin

POLYETHYLENE GLYCOL-8/SMDI COPOLYMER 143 0.010 0.00g 0.006 0.004 0.002 0.000 ,[[ , [ i -2 0 2 4 6 $ 10 12 14 Percent polymer added (w/w) Figure 6. Effect of PP-15 on the apparent solubility of salicylic acid at 4øC. o 1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0 -0.1 1.00e-4 1.00e-3 1.00e- 1 1.00e+0 Drug concentration 0Violes/lite0 Figure 7. PP-15 binding to salicylic acid and lactic acid. [•, Salicylic acid ß lactic acid. surface is desirable, the target tissue for SA is assumed to be the SC it is therefore advantageous to maintain significant SC levels for an extended period of time. However, SA is rapidly transported through the skin from simple solutions and suspensions (Figures 1,3), allowing most of the applied dose to permeate the skin within an eight hour period. Levels within both the SC and the viable tissues reach a peak at an early