Skin and Nail: Barrier Function, Structure, and Anatomy Considerations for Drug Delivery

Drug delivery to and through both skin and nails is an area receiving ever-increasing attention. In order to effectively deliver active pharmaceutical ingredients (APIs) across the skin (transdermal delivery) or nail (transungual delivery) it is necessary to consider the anatomy and physiology of these barriers. With this information in hand, one can more effectively utilize drug delivery approaches to maximize the effectiveness of the API – getting the right amount to the right place at the right time. The advantages of local delivery include, among others, minimized systemic toxicity, high local drug concentrations, avoidance of fi rst pass issues and cost. Topical delivery of systemic therapeutics also offers benefi ts but presents a greater technical challenge. Among the benefi ts, fi rst pass avoidance, convenience and sustained release are most often sited. Skin: The skin is the largest organ in the body, making up 12 – 15% of body weight and with a surface area of 1 – 2m2. Not surprisingly, the skin has a complex architecture that can vary depending on body location (Figure 1). There are 3 distinct regions in the skin. From outermost inward, they are the epidermis, dermis and subcutis (hypodermis). The epidermis itself has five regions and can range in total thickness from 0.5mm (eyelid) to 1.5mm on the palms and soles (Figure 2). The most important function of the skin is to provide a selectively permeable barrier. The outermost region of this barrier is called the stratum corneum (SC), a dynamic structure with cells migrating in a deep to superficial direction as they mature. The SC, also called the horny layer, consists of cells called corneocytes that are emSkin and Nail: Barrier Function, Structure, and Anatomy Considerations for Drug Delivery