Rapid pharmacokinetic evaluation of topical drug formulations

A new in vitro test system was developed to efficiently determine the effect of formulation on topical drug delivery. Sheets of viable, excised pig skin were sandwiched betwween two standard 24-well plates. The lower wells contained receptor fluid and a magnetic stirrer. The upper wells were opened to the atmosphere for formulation application. Using 14C-salicylic acid as a model compound, eight different formulations were evaluated representing hydrophilic and lipophilic solutions, a suspension and o/w and w/o emulsions. Formulations were applied to the skin surface in six different wells on three sets of plates. Twenty-four hours after application, excess drug was wiped from the skin surface and assayed for radiolabel. The stratum corneum was removed by tape stripping. Radiolabel contained in the remaining epidermis, dermis and receptor fluid was also determined. Statistical analysis (ANOVA, Student-Newman-Keuls multiple-range test, p=0.05) of radiolabel penetrating into the dermis and receptor fluid revealed the following order of formulations: ethanol= aqueous surfactant less than o/w emulsion = w/o emulsion less than lipophilic solution. These results demonstrate the importance of vehicle in directing drug delivery using a test system capable of simultaneously evaluating a large number of formulations.