The quantitative distribution of percutaneously applied caffeine in the human skin.

The distribution coefficient of caffeine (water/n-Octanol) and the estimation of caffeine in urine after local application indicate to a high permeation rate of caffeine through the skin. This could be confirmed by using different vehicles in vivo and in vitro. 14C labeled caffeine penetrates rapidly the epidermis and corium. The maximum of absorption is reached at 100 min after local application in vivo. In vitro by absence of the transport possibilities of blood and lymph vessels, the concentration at 1,000 min after local application is 450 X higher than in vivo. Therefore, after 1,000 min in vivo the concentration of caffeine in the different skin layers is very low.