Description of the intrafollicular delivery of large molecular weight molecules to follicles of human scalp skin in vitro

The objective of this study was to ascertain the elements that govern the intrafollicular delivery of large molecules to follicles of human scalp skin in vitro. The experiments were designed to assess the intrafollicular disposition of drug as a function of size, charge, and formulation. First, fluorescein covalently linked to antisense oligonucleotides and rhodamine-conjugated dextrans were topically applied to fresh human scalp skin in vitro. The drug position in the follicle was qualitatively determined by examining histologic sections of hair-bearing skin taken parallel to the skin surface at various depths and dissected hair follicles via fluorescent microscopy. Using radiolabeled antisense oligonucleotides we demonstrated that these cross sections can also be used to quantitatively localize the intrafollicular delivery of large molecules to follicles of human scalp skin in vitro. Experimental results showed that cationic lipid-based formulations enhanced delivery of oligonucleotides within the follicle. The qualitative analysis also illustrated that cationic lipid-based formulations directed the intrafollicular permeation along the junction of the internal and external root sheath. The charged, lower molecular weight (MW) dextrans permeated into all components of the hair follicle, including the hair shaft. The higher MW dextrans were confined to the follicular structures immediately surrounding the hair shaft. The demonstration of quantitation showed that similar to 0.5% of the applied dose was delivered to the hair bulbs and the deeper skin strata within 24 h of a single application. We conclude that topically applied agents of relatively large MW, in properly formulated delivery vehicles, have the potential to reach pharmacologically active concentrations at the hair bulb. It also should be noted that delivery takes place via the junction of the internal and external root sheath.