Despite the small diffusional area of hair follicles compared to that of the skin (about 0.1% of the total area), diffusion of drugs through these shunt routes is well established and described in the literature [72]. Moreover, the rate of diffusion can be several orders of magnitude higher than transepidermal throughput in the early phase following appli-cation, whereas during the steady-state phase the bulk of diffusion appears to be no longer intra-appendageal but occurs through the ma-trix of the stratum corneum [73].
However, for follicular disorders such as acne or alopecia, it would be valuable to as-sess the local drug bioavailability after topical application, but in vivo methodology on hu-man subjects is still lacking, since conven-tional invasive techniques used on animals are obviously not applicable. Indeed, one possible and direct approach is to sample the follicles after topical drug application.
Although this technique could be of inter-est, it remains somewhat marginal, since standard methods have not been yet devel-oped for quantitative follicular removal in human subjects. However, Marks et al. [56]
have developed a casting technique, that con-sists of applying a cyanoacrylate adhesive on the skin and subsequently covering it with a glass slide. After polymerization of the glue, the slide is removed, harvesting clusters of stratum corneum mixed with hair follicles.
This technique was also adopted by Mills and Kligman [74] and by Bojar et al. [75]. The latter studied the penetration of azelaic acid at 20% in a cream formulation on the back and the forehead of volunteers. Higher drug con-centrations were found in back casts than in forehead casts, suggesting that circulatory access to the follicle-rich forehead may have resulted in rapid removal from the hair folli-cle.
Conclusion
The methods described in this paper need further validation in humans, since even if a number of investigations have already been performed on animals, the obvious differ-ences between species mean that only ap-proximative conclusions can be drawn. This is not to say that experiments on animals are not useful, but the recent advances of the tech-niques, and particularly their limited inva-siveness, will certainly encourage more ex-tensive developments, in those countries where it is freely authorized.
Furthermore, earlier methods for bioavail-ability/bioequivalence assessment such as the blanching assay for corticosteroids have their limitations, and regulatory agencies need new methods to reduce the expensive and time-consuming clinical trials needed before filing of generic drugs, since these will gain greater importance in the future.
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