Transfollicular drug delivery:: Penetration of drugs through human scalp skin and comparison of penetration between scalp and abdominal skins in vitro

In order to quantitatively investigate the importance of transfollicular pathway for drug delivery, drug penetration through human scalp skin was investigated using liquid formulations containing lipophilic and hydrophilic drugs in vitro. The penetration pathway for drugs through the scalp skin was examined using fluorescent probes, Additionally, the drug penetration through the scalp skin was compared with that via human abdominal skin to clarify the usefulness of intrafollicular delivery. Lipophilic melatonin (MT) and ketoprofen (KP) showed high permeabilities through the scalp skin, although the flux of KP was much higher. Absorption enhancers, N-methyl-2-pyrrolidone and isopropylmyristate, only slightly increased the fluxes. Hydrophilic fluorouracil (5FU) and acyclovir (ACV) penetrated through the scalp skin with relatively large fluxes. However, there was large variability in the fluxes of these drugs across scalp skin from different sources. When the relationship between the flux and hair follicle density was estimated, there was good correlation between the two (r = 0.651 for MT and r = 0.666 for ACV, P < 0.05). The histologic examination of the scalp skin, following application of the formulation with nile red or sodium fluorescein, indicated that the probes permeated into the junction of the internal and external root sheath of follicles and diffused into the dermis via the outer root sheath at the initial times. The penetration of nile red, a lipophilic probe, via the stratum corneum of scalp skin was later than that via the follicles. The permeation of MT and 5FU through the scalp skin was much higher than that via the abdominal skin, being 27 and 48 times as high as the abdominal skin, respectively. These results indicate that the drug delivery through the scalp skin will offer an available delivery means for drugs, particularly for hydrophilic drugs.