Permeation of topically applied caffeine through human skin – a comparison of in vivo and in vitro data

Due to ethical reasons, in vivo penetration studies are not applicable at all stages of development of new substances. Therefore, the development of appropriate in vitro methods is essential, as well as the comparison of the obtained in vivo and in vitro data, in order to identify their transferability. The aim of the present study was to investigate the follicular penetration of caffeine in vitro and to compare the data with the in vivo results determined previously under similar conditions.The Follicular Closing Technique (FCT) represents a method to investigate the follicular penetration selectively. In the present study, FCT was combined with the Franz diffusion cell in order to differentiate between follicular and intercellular penetration of caffeine into the receptor medium in vitro. Subsequently, the results were compared with the data obtained in an earlier study investigating follicular and intercellular penetration of caffeine in vivo.The comparison of the data revealed that the in vitro experiments were valuable for the investigation of the follicular penetration pathway, which contributed in vivo as well as in vitro to approximately 50% of the total penetration, whereas the kinetics of caffeine penetration were shown to be significantly different.The combination of FCT with the Franz diffusion cell represents a valuable method to investigate follicular penetration in vitro. Nevertheless, in vivo experiments should not be abandoned as in vitro, structural changes of skin occur and blood flow and metabolism are absent, probably accounting for reduced penetration rates in vitro.