Optimization of in vitro skin permeation by lactic acid from gel formulations

The purpose of this study was to evaluate the effect of the concentration as well as the vehicle's pH on in vitro skin permeation by lactic acid from gel formulations, using an optimization technique. Nine gels containing 3%, 6%, and 9% (w/w) lactic acid in three different phosphate buffers, with pH values of 2.8, 3.8, and 4.8, were prepared and were applied in modified Franz diffusion cells. The pH of the vehicles and the lactic acid concentrations were used to create a mathematical model that correlates these factors with the cumulative amount of lactic acid permeated through human cadaver epidermis. For this purpose, the optimization technique 3(2) was applied. It was found that the correlation of the above factors can be adequately described with a polynomial equation, which can be used for predicting the cumulative amounts of lactic acid permeated. The results indicated that as the lactic acid's concentration increased, the cumulative amount permeated also increased after 24 h at all pH values. Moreover, the amount of lactic acid permeated decreased as the pH of the gels was increased. The greater amount permeated at all time intervals (6 h, 9 h, 12 h, and 24 h) was obtained when the concentration of lactic acid was 9% and the pH of the gel formulation was 2.8.