The determination of penetration pathways of topically applied substances into the skin is the subject of several investigations. Recently, follicular penetration has become a major focus of interest. To date, a direct, non-invasive quantification of the amount of topically applied substance penetrated into the follicles had not been possible. The development of such a method was the aim of this study. Therefore, the advantages of both stripping techniques, tape stripping and cyanoacrylate skin surface biopsy, were combined and evaluated. Tape stripping was used to remove the part of the stratum corneum that contained the topically applied dye. Subsequently, the follicular contents were ripped off by cyanoacrylate skin surface biopsy. The combined method termed "differential stripping" was evaluated in vitro and in vivo, and the amount of topically applied fluorescent dye penetrated into the hair follicles was quantified after different penetration times. After 30 min, 5% of the recovered concentration of sodium fluorescein was found in the follicular infundibula, where it was still detectable after 48 h. Altogether, the results of this investigation revealed that differential stripping is a new method that can be used to study the penetration of topically applied substances into the follicular infundibula non-invasively and selectively.
Recently, hair follicles were shown to be of great importance concerning the penetration and reservoir behavior of topically applied substances. Especially microparticles are known to be efficient drug carriers into the hair follicles. It has been shown in various investigations that, depending on their size, microparticles can be utilized to enhance the penetration of substances into the hair follicles. In the present study, it was investigated whether they could also be used to block the hair follicles to stop penetration. For this purpose, the follicular penetration of a sodium fluorescein-containing hydrogel and follicular reservoir depletion were investigated by means of differential stripping in vivo. In a second study design, the skin areas were pretreated with microparticles 5 µm in diameter. Subsequently, the follicular penetration of sodium fluorescein-containing hydrogel was likewise investigated by differential stripping. The results showed that the hair follicle infundibula contained a significant fluorescent signal when sodium fluorescein in hydrogel was applied. In contrast, in the case of pretreatment with microparticles, only a very low fluorescence signal was measurable in the hair follicle infundibula. The microparticles clearly blocked the follicular orifices so that a penetration of sodium fluorescein was almost completely inhibited. This was confirmed by histological investigations. Thus, depending on their size, microparticles act as efficient drug carriers or can be utilized as follicle blockers to stop the penetration of topically applied substances. This could be very useful in skin protection to prevent the storage of harmful topically applied substances in the hair follicles, known to represent long-term reservoirs.
Investigations concerning the penetration of topically applied drugs are the subject of a multiplicity of research, as the exact knowledge of these mechanisms is the prerequisite for the optimization of such substances. As the hair follicles represent a good long-term reservoir for topically applied substances, it can be assumed that they also represent a reservoir for microorganisms residing on and in the skin. Therefore, the hair follicles must be seen as one main site of action for antimicrobial substances and a simple non-invasive in vivo method for the determination of penetration of antimicrobial substances into the hair follicles. The aim of the present study was to show that differential stripping, by removing the hair follicle content selectively, represents a suitable method to demonstrate the penetration of antifungal substances into the hair follicles. The follicular casts, removed from skin areas of human volunteers, which were partially pre-treated with brilliant green (well-known antifungal properties), were applied on agar plates inoculated with Candida albicans. The results showed inhibited growth of C. albicans, if the follicular casts were removed from skin areas pre-treated with brilliant green. This indicates clear evidence that brilliant green had penetrated into the hair follicles. Therefore, differential stripping represents a suitable method to determine the penetration of antifungal substances into the hair follicles, which is of clinical importance for the optimization of topical antifungal therapeutics.
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