Convective Solvent Transport Pathways for Absorption of Drugs from Topical Formulation

Physicochemical and formulation factors influencing penetration of drugs from topical products into the skin and mechanisms of drug permeation are well investigated and reported in the literature. However, mechanisms of drug absorption during short-term exposure have not been given sufficient importance. In this project, the extent of absorption of drug molecules into the skin from aqueous and ethanolic solutions following a 5-min application period was investigated. The experiments demonstrated measurable magnitude of absorption into the skin for all the molecules tested despite the duration of exposure being only few minutes. Among the two solvents used, absorption was greater from aqueous than ethanolic solution. The results suggest that an alternative penetration pathway, herein referred to as the convective transport pathway, is likely responsible for the rapid, significant uptake of drug molecules during initial few minutes of exposure. Additionally, absorption through the convective transport pathways is a function of the physicochemical nature of the formulation vehicle rather than the API.