ABSTRACT
INTRODUCTION When one speaks of targeting of skin appendages using topically applied agents/ products, one is referring to one or another (or both) of two skin structures-hair follicles (pilosebaceous units) and/or eccrine sweat glands. Very little information has been published concerning drug delivery to/through eccrine sweat glands, whereas follicular drug delivery has received a considerable amount of research attention in recent years. This review is mainly concerned with the body of literature covering targeted follicular drug delivery. Successful targeting, as the phrase is used here, simply means getting more drug molecules into the specified appendage of action than can be delivered with conventional dosage forms (delivery systems). At the same time, it implies restricting the amount of drug that reaches therapeutically uninvolved sites, most particularly, the systemic circulation. Targeted follicular delivery would be useful to stem hair loss or promote new hair growth. In principle, topically applied agents can reach the hair bulb by either transfollicular or transepidermal route, with the former involving drug diffusion through the upper reaches of the pilosebaceous gland and the latter involving secondary local/systemic distribution into hair follicles. In fact, both pathways occur simultaneously in most cases, but the relative contribution of each pathway to the overall delivery seemingly should vary substantially depending upon the physicochemical properties of the therapeutic agent, the nature of its formulation, the specific site of topical application, and the elapsed time after topical dosing.
