The measurement of the stratum corneum reservoir: a simple method to predict the influence of vehicles on in vivo percutaneous absorption.

The influence of nine vehicles on in vivo percutaneous absorption of benzoic acid has been studied in the hairless rat. Although the vehicles used were simple in composition, the total amount of benzoic acid which penetrated varied by a factor of 50. A weak relationship was found between penetration of benzoic acid and its maximum solubility in the different vehicles. A linear relationship was demonstrated between the amount of benzoic acid present in the stratum corneum at the end of a 30 min application period and the total amount absorbed over 4 days. We suggest that the influence of a vehicle on the overall absorption of a substance can therefore be predicted by simply measuring the amount present in the stratum corneum at the end of a 30 min application period.