Modification of drug penetration into human skin using microemulsions

The objective of this study was to investigate the penetration of the hydrophilic substance diphenhydramine hydrochloride from a W/O-microemulsion into human skin under ex vivo conditions. The focus of the study was to determine the amount of a model substance in the different skin layers, rather than measure the transdermal rate. Modifications of the vehicle components clarified the extent, to which it is possible to control the penetration of a hydrophilic drug incorporated in a microemulsion (ME) system. A standard ME showed an accumulation of penetrated drug in the dermis, indicating a potential following high absorption rate. Incorporation of cholesterol into the system leads to an even higher penetration rate and a shifting of the concentration profile further towards the epidermis. In comparison, addition of oleic acid had no effect. The results are in concordance with the assumption that diphenhydramine hydrochloride follows hydrophilic structures into the stratum corneum. Therefore, an alteration of the barrier properties can be achieved obviously only by influencing this hydrophilic pathway.