We sought to understand when and how hydration enhances the percutaneous absorption of salicylate esters. Human epidermal membrane fluxes and stratum corneum solubilities of neat and diluted solutions of three esters were determined under hydrated and dehydrated conditions. Hydration doubled the human epidermal flux seen for methyl and ethyl salicylate under dehydrated conditions and increased the flux of neat glycol salicylate 10-fold. Mechanistic analyses showed that this hydration-induced enhancement arises mainly from an increase in the stratum corneum diffusivity of the three esters. Further, we showed that unlike methyl and ethyl salicylate, glycol salicylate is hygroscopic and the ∼10-fold hydration-induced flux enhancement seen with neat glycol salicylate may be due to its ability to hydrate the stratum corneum to a greater extent. The hydration-induced enhancements in in vitro epidermal flux seen here for glycol and ethyl salicylate were similar to those reported for their percutaneous absorption rates in a comparable in vivo study, whilst somewhat higher enhancement was seen for methyl salicylate in vivo. This may be explained by a physiologically induced self enhancement of neat methyl salicylate absorption in vivo which is not applicable in vitro.
Keywords: diffusivity; hydration; in vitro human skin permeation; in vivo comparison; topical salicylate esters.