Nail Penetration: Focus on Topical Delivery of Antifungal Drugs for Onychomycosis Treatment

Abstract

Although a nail plate is similar to the stratum corneum of the skin in that it is also derived from epidermis, it is mainly composed of hard keratin with high content of disulfide-linkages (cystine), and is approximately 100-fold thicker than the stratum corneum. Although both are composed of keratinized dead cells, there are marked differences between the nail plate and stratum corneum in their physical and chemical characteristics (Table 2). In comparison to the thin stratum corneum, a much thicker nail plate means a much longer diffusion pathway for drug delivery through the nail. In contrast with the elastic and pliable stratum corneum, a nail plate is dense and hard, especially the dorsal nail plate layer. Based on the biochemical composition of the nail, one would expect the permeation characteristics of the nail to be different from those of the stratum corneum. This turns out to be true from the evidences available to date. Some physical and chemical differences between a nail plate and stratum corneum are1. The total lipid content of the nail lies between 0.1 and 1%, as opposed to approximately 10% lipid content of stratum corneum (1), suggesting the role of the lipoidal pathway in the nail plate is probably much less important than that in the stratum corneum.