194 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS 50 45- 40- 35- 30- 25- 20 0.05 0.10 0.15 0.20 0.25 0.30 0.35 % Tritiated Water Penetration (20 min. Test) Figure 2. Human skin variability: correlation between barrier integrity and glycolic acid absorption. The values are the mean of two to six determinations in each of five subjects. Table IV Percent Applied Dose Absorbed of 0.5% AHA in Formulation A 0.5% 2-OH-hexanoic 0.5% 2-OH-octanoic 0.5% 2-OH-decanoic acid acid acid Location pH3 pH3 pH3 Receptor fluid 10.1 _+ 2.7 15.4 _+ 3.1 8.8 _+ 2.5 Stratum corneum 3.2 _+ 0.9 1.4 _+ 0.3 2.6 + 0.6 Epidermis 8.4 + 1.i n 2.8 + 0.4 a'b 5.8 _+ 0.9 b Dermis 6.7 _+ 0.7 a'• 1.4 _+ 0.2 • 2.1 + 0.3 • Total in skin 18.3 _+ 2.6 •'• 5.5 _+ 0.9 • 10.5 _+ 1.0 • Total absorption 28.4 _+ 3.9 21.0 _+ 2.5 19.3 + 3.1 Values are the mean _+ SEM of two to five determinations in each of three subjects. Values in each location with similar superscripts are significantly different from each other (ANOVA, p 0.05). other AHAs at pH 3.0 to simulate the acidic pH of some commercial cosmetic products containing these ingredients. The effect of pH is clearly seen in Table II on both receptor fluid and skin levels of the three lower-chain AHAs. The magnitude of reduction in absorption at pH 7.0 differed among the AHAs in some locations. Less of a pH differ- ence was seen in the skin levels obtained with lactic acid and 2-hydroxyhexanoic acid. Even at pH 7.0, between 9% and t0% of the applied lactic and 2-hydroxyhexanoic acid was absorbed. The differing GA absorption profiles with Formulations A and B (Tables II and III) illustrate the potential effects of cosmetic vehicles on AHA absorption. Differences in

ABSORPTION OF AHAs IN SKIN 195 Human skin 1 Human skin 2 7 - 5 - 0 2 4 6 8 10 12 14 16 Number of Tape Strips Figure 3. The pH profile of human skin in flow-through diffusion cells 24 h after application of an O/W emulsion, pH 3.0. The values are the mean + SEM of three determinations in each of two subjects. lOO * 60 40 20 3 4 5 6 7 pH Figure 4. The effect of pH on glycolic acid ionization as determined by the Henderson-Hasselbach equation. skin response to similar AHA products (based on concentration and pH) may be due to differences in absorption due to vehicle effects. However, neither Formulation A nor Formulation B differed from two commercial products in their effects on hairless guinea pig barrier integrity (Figure 1). The variability in absorption properties of normal human skin is illustrated in Figure 2. Absorption values obtained from the skin of one or two donors can be misleading,