The in vitro use of the hair follicle closure technique to study the follicular and percutaneous permeation of topically applied drugs

Recent studies on follicular permeation emphasise the importance of hair follicles as diffusion pathways, but only a limited amount of data are available about the follicular permeation of topically applied drugs. This study examines the use of a hair follicle closure technique in vitro, to determine the participation of hair follicles in transdermal drug penetration. Various substances, with different lipophilicities, were tested: caffeine, diclofenac, flufenamic acid, ibuprofen, paracetamol, salicylic acid and testosterone. Diffusion experiments were conducted with porcine skin, the most common replacement material for human skin, in Franz-type diffusion cells over 28 hours. Different experimental settings allowed the differentiation between interfollicular and follicular permeation after topical application of the test compounds. A comparison of the apparent permeability coefficients of the drugs demonstrates that the percutaneous permeations of caffeine and flufenamic acid were significantly higher along the hair follicles. In the cases of paracetamol and testosterone, the follicular pathway appears to be of importance, while no difference was found between interfollicular and follicular permeation for diclofenac, ibuprofen and salicylic acid. Thus, the hair follicle closure technique represents an adequate in vitro method for gaining information about follicular or percutaneous permeation, and can replace in vivo testing in animals or humans.