Surface recovery and stripping methods to quantify percutaneous absorption of caffeine in humans.

The percutaneous absorption of caffeine from two vehicles, an emulsion and an acetone solution, was quantified by in vivo techniques in humans. A surface recovery technique over a 6-h application and a stripping method after a 30-min application were performed on the volar aspect of the forearm on 12 volunteers. Caffeine was assessed by HPLC. Two phases were distinguished in the percutaneous absorption of caffeine: a higher filling up of the stratum corneum with the oil-in-water emulsion than with the acetone solution, which was then followed by a steady-state flux corresponding to the penetration in the living tissues. The permeability constants (Kp) with emulsion and acetone were 1.59 x 10(-4) and 9.53 x 10(-8) cm/h, respectively. The stripping method showed concentrations of caffeine in stratum corneum that were five times higher with emulsion (212 ng/cm2) than with acetone (37 ng/cm2). With acetone as a vehicle, approximately 40% of caffeine of the cornfield layer was found around the treated area. This sizeable lateral spread within the stratum corneum was not observed with the emulsion.