Follicular transport route--research progress and future perspectives.

The important role of hair follicles as penetration pathways and reservoir structures for topically applied compounds has been validated in numerous animal models as well as in humans. Follicular penetration rates are modulated by regional variations in size and proportions and the functional status. Advances have especially been made in the targeting of hair follicle-associated cell populations including antigen-presenting cells and stem cells. Improved investigative methods based on differential stripping, spectrophotometry and confocal laser scanning microscopy have led to the determination of the penetration profiles and kinetics for a multiplicity of drugs and drug delivery systems. The observation that particulate delivery systems aggregate and remain in hair follicle openings and their penetration along the follicular duct occurs in a size-dependent manner, which has led to advanced concepts of targeted drug delivery of bioactive compounds in the field of solid particles, as well as semi-solid particles, such as liposomes. This review summarizes the recent progress in this field, and underlines the necessity for pilot studies in human volunteers to further the development of clinical applications for follicular targeting.