Solute-Vehicle-Skin Interactions and Their Contribution to Pharmacokinetics of Skin Delivery

Human skin provides an effective route of delivery for selected drugs. Topical penetration of molecules is largely attributed to passive diffusion, and the degree of penetration can be represented by in silico, in vitro, and ex vivo models. Percutaneous absorption of pharmaceutical ingredients is a delicate balance between the molecular properties of the drug, the skin properties of the patients, and the formulation properties. Understanding this interplay can aid in the development of products applied to the skin. The kinetics of percutaneous absorption and an understanding of the rate-limiting steps involved can facilitate the optimization of these systems and enhance the degree to which skin drug delivery can be achieved. Solute-vehicle, vehicle-skin, and solute-skin interactions contribute notably to product release as well as the rate of absorption and diffusion across skin layers. These interactions alter the degree of permeation by interfering with the skin barrier or solubility and thermodynamic activity of the active pharmaceutical ingredient. This article aims to provide a concise understanding of some of the factors involved in the skin absorption of topical products, i.e., the pharmacokinetics of percutaneous absorption as well as the solute-vehicle-skin interactions that determine the rate of release of products and the degree of drug diffusion across the skin.