Dermal Absorption: Considerations on Risk Assessment, Drug Administration, and the Human Skin Microbiome

The following chapter describes the human skin and its complexity. On the one hand, the skin protects against external influences; on the other, it provides a large surface area for the absorption of substances. In this context, the process of dermal absorption and its influencing factors are presented. Different legal jurisdictions are outlined in the context of dermal absorption and health risk assessment of substances. Several types of drug delivery systems – the dermal system, transdermal system, film-forming system, minimally invasive transdermal drug delivery, and in silico approaches – are explained in detail by presenting respective case studies. Furthermore, the important role of human skin microbiota in barrier function is explained. Complying with the 3Rs rules, in vitro studies on dermal absorption have been established. Moreover, efforts have been made in developing in silico approaches but have revealed difficulties by modeling dermal absorption process of formulations over single compounds. Drug delivery systems might be promising alternatives over conventional methods due to the avoidance of the first-pass metabolic effect, lowering development of potential negative side effects and enhancing patient compliance. The skin microbiome has been highlighted for its influence on skin barrier function. However, large-scale studies are still needed to apply this existing knowledge to product development. Overall, the skin is one of the largest organs in the human body – hence it deserves special attention.