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

The study aimed to develop and validate an in vitro method for investigating the penetration of caffeine through human skin and to compare the results with previously obtained in vivo data. The Follicular Closing Technique (FCT) was adapted for use with the Franz diffusion cell to differentiate between follicular and intercellular penetration of caffeine. The in vitro results showed that the follicular penetration pathway contributed to approximately 50% of the total penetration, similar to in vivo findings. However, the kinetics of caffeine penetration were significantly different between the two methods. The study concluded that while the combination of FCT and the Franz diffusion cell is a valuable in vitro method for studying follicular penetration, in vivo experiments remain essential due to structural changes in skin, absence of blood flow, and metabolism in vitro, which could lead to reduced penetration rates compared to in vivo conditions.