Targeted delivery to pilosebaceous structures: analysis of study methods.

Follicular delivery involves depositing drugs in the hair follicle, hair shaft and sebaceous glands, all components of the pilosebaceous unit. There are a number of dermatological disorders involving the pilosebaceous structures, including ache, seborrhea, androgenetic alopecia, alopecia arcata and some skin cancers. Such conditions are often clinical manifestations of excessive sebum production and an accumulation of keratinized material in the follicular openings and sebaceous ducts. Androgens that influence hair growth and sebum release may also mediate these conditions.[12,30]

Since the follicular route is important for drug penetration as well as localized action, targeted delivery of active compounds to the pilosebaceous unit or its components can help treat follicular diseases.

Targeting refers to the selective deposition and/or increase in concentration of a particular compound at the target site, which is the pilosebaceous unit. Several methods can be adopted to study such targeting, using either human or animal models. The follicular ducts in people are relatively long compared with those of most experimental animals, and human sebaceous secretion also differs from animal secretion in that it contains squalene and other fatty acids.[3]

In Vivo Methods - Human

Skin biopsy: Introduced by Marks and Dawber, the skin-surface biopsy technique uses clear, nontoxic cyanoacrylates as tissue adhesives to examine the horny layer of the skin and its surface squames.[23] It is a good method to observe hair follicles and sweat glands. A researcher places a drop of the adhesive on a clean region of the skin and then gently presses a slightly moist glass slide on top of the area. After 30 to 40 sec, the slide is removed, along with a superficial layer of stratum corneum (SC) and follicular casts. The superficial horny layer may be examined histologically or histochemically, and the in vivo arrangement of the squames may be studied with a scanning electron microscope.

Follicular biopsy: The follicular biopsy technique can generate both qualitative data (histological observation) and quantitative data (measurement of follicular drug concentrations) in the study of pilosebaceous targeted delivery. In 1980, Mills and Kligman reported the usefulness of the follicular biopsy technique in sampling the sebaceous follicle contents in a non-invasive manner.[26]

In addition to morphologic studies, they found that the follicular casts could be analyzed bacteriologically or chemically using chromatographic methods. They used this technique to assay the comedolytic activity of retinoic acid (tretinoin) and salicylic acid by topically applying the solutions to the backs of African-American men with microcomedones (induced by a two-week occlusive exposure to 10% crude coal tar).[27]

After two weeks of applying the test agents, researchers collected samples of the follicular contents using the cyanoacrylate follicular biopsy technique. Using a stereoscope to count, they discovered that retinoic acid is more effective than salicylic acid in reducing the microcomedone density.

Azelaic acid: Follicular biopsies can also determine the follicular concentrations of azelaic acid (AzA), which has been shown to have antimicrobial effects in the treatment of acne.[2] In a recent study, researchers applied AzA cream (20% w/w) to the foreheads and backs of subjects and took samples during a 5-hr period. Researchers removed the surface AzA by washing with acetone, and then collected the follicular casts using the follicular biopsy method. Samples were processed in a centrifuge, and the supernatants were analyzed by high-performance liquid chromatography (HPLC) after derivatization.

Data showed successful AzA detection in the pilosebaceous units, and the in vivo concentrations of AzA attained in the follicular casts were comparable to the concentrations required to elicit antimicrobial effects in vitro. The follicular casts from the subjects' backs showed higher concentrations of AzA than did those from the forehead samples. The authors state that the peak follicular concentrations attained depend on several factors, including the number and density of the pilosebaceous units, blood and sebum flow, and drug metabolism.

This method can also help screen for antiacne formulations when researchers have found a correlation between follicular delivery and clinical activity. Mills et al analyzed salicylic acid levels in sebaceous follicles after topically applying salicylic acid (2%) in a hydroalcoholic vehicle or gel base.25 Using the follicular biopsy technique to sample the sebaceous contents, the follicular concentrations of salicylic acid (as determined by HPLC) were higher with the hydroalcoholic vehicle. The authors point out the usefulness of this technique for assessing the antiacne efficacy of formulations using different vehicles.

In Vitro Methods

A long-term histoculture of human hair follicles growing in intact scalp skin at the air/liquid interface has been developed based on a gel-supported, three-dimensional histoculture system of mouse skin and …