P1.4 Hair follicles - Traps for lateral spreading

U. Jacobi, H.-J. Weigmann, W. Sterry, J. Lademann

Center of Experimental and Applied Cutaneous Physiology, Department of Dermatology and Allergology, Charité - Universitätsmedizin, Berlin, Germany

The lateral spreading of topically applied substances is a competitive process to penetration into the stratum corneum (SC). The aim of the present study was to investigate the penetration and the lateral spreading of topically applied emulsions containing UV filter substances on different body sites. In addition, the pathways of the lateral spreading were visualized using dyes. In these investigations, tape stripping in combination with spectroscopic and microscopic measurements and photography were performed inside and outside the application area. The amount of removed corneocytes as well as the concentration of the topically applied UV filter substances was determined in vivo using UV/VIS spectroscopy. The lateral spreading of topically applied dyes was determined in vivo using photography, while laser scanning microscopy was used to visualize this process on the removed tape strips. A spreading of the topically applied substances from the treated to the untreated areas was observed. This lateral spreading depended on the body site, the emulsion used for application and the size of the treated skin area. Significant amounts of topically applied substances were found adjoining the application area, due to the lateral spreading which takes place on the skin surface. Using dyes, it was seen that furrows acted as pathways for the spreading emulsion. In addition, the hair follicles functioned as traps for the dye, where the dye still remained even 24 h after the experiment. The lateral spreading of a topically applied substance varies depending on the volunteer and the body site. This is caused by differences in the skin surface topography - the network of furrows and the density and size of follicles. The primary furrows function as pathways for the lateral spreading, whereas the follicles are traps forming a long-term reservoir for the topically applied substance. In general, the lateral spreading must be considered to be a competitive process when studying penetration processes of topically applied substances. It has to be considered during drug treatment of small limited skin areas to avoid side effects and for the interpretation of recovery rates obtained in penetration studies.