The percutaneous absorption studies were performed using a flow-through diffusion cell system with skin specimens from 24 healthy women to assess the penetration of glycolic acid (GA). Percentages of GA, based on 14C-labelled activity, found in the skin after application of 4% GA at pH 2.0 or pH 3.8, after 24 h were as follows: stratum corneum (SC)= 2.65+/-1.80 versus 1.13+/-1.14 (P<0.05); viable skin (VS)= 13.46+/-7.44 versus 2.23+/-1.51 (P< 0.05) and effluent fraction (EF) = 12.22+/-9.03 versus 1.42+/-0.77 (P < 0.05), respectively. The applications of 4-60%, GA at their native pH resulted in an increased penetration of GA through the skin. For example, application of 20% GA, pH 1.9, resulted in the following values: SC = 2.69+/-2.26 (P > 0.05); VS = 4.88+/-4.05 (P > 0.05) and EF = 30.69+/-13.25 (P < 0.05). Duration of application also affected the extent of penetration of drug. For example, application of 20% GA, pH 1.9, for 6 h resulted in the following levels: SC = 1.16+/-0.80 (P < 0.05); VS = 4.07+/-1.78 (P > 0.05) and EF = 6.12+/-4.95 (P < 0.05).
In conclusion: (i) absorption of GA in human skin are pH-, strength- and time-dependent; and (ii) the in vitro method appears to provide an appropriate model to reflect in vivo absorption of GA through human skin.