Methods for quantifying intrafollicular drug delivery: a critical appraisal.

IMPORTANCE OF THE FIELD: In recent years there has been increasing awareness that the hair follicles and their associated pilosebaceous structures may act as significant permeation pathways and/or reservoirs for topically applied drugs. This has implications in terms of dermatological therapy for acne, hirsutism, alopecias or certain skin cancers as well as systemic drug delivery. As the processes modulating follicular drug penetration are poorly understood at present, there is an emergent need for methodologies that can quantify follicular drug penetration and deposition. So far, a review article specifically dedicated to these methodological aspects has not yet been written. AREAS COVERED IN THIS REVIEW: This paper reviews the available quantitative follicular methodologies that have been developed over the years, describing the advantages and disadvantages of each approach. This review covers comparative techniques that are based on measuring drug flux through 'follicle-free' and 'follicle-containing' integuments, the skin sandwich, differential stripping and optical imaging-based technologies. Techniques for measuring drug-sebum interactions are also discussed. WHAT THE READER WILL GAIN: The reader will develop an understanding of the complexities involved in quantifying drug delivery through follicles and pilosebaceous units. The Expert opinion section will give the reader insights into how more broad-ranging future research could allow identification of the most useful methods for quantifying follicular drug transport. TAKE HOME MESSAGE: This is still a poorly understood field. It clearly warrants much larger scale studies than have been performed so far involving multiple techniques and multiple drugs.