The Absolute Fundamentals of Transdermal Permeation (Drug Delivery for Dummies)

Abstract

There are many excellent papers (Table 1) available in the pharmaceutical and dermatological literature concerning the complexities of transdermal drug delivery. With the recent explosion of knowledge in this field, authors have attempted to analyse and describe, in ever-greater scientific detail, the myriad of effects that govern the rate at which an endogenous molecule is able to leave a topically applied delivery vehicle, partition into the stratum corneum and diffuse through the barrier layers of the skin. Furthermore, complex mathematical models have been proposed that adequately describe this kinetic process for a number of permeants. The novice investigator, however, is often lost in this complex pile of journal reprints in which the authors presume that the readers are calculus fanatics or are able to mentally solve differential equations. Industrial formulators, too, seldom concern themselves with the exponential model of the permeation rate constants or the elucidation of the bipolar diffusional pathways using sophisticated analytical systems. These readers are often more concerned with the fundamentals of the drug delivery process: answering the “how can I make more drug diffuse through the skin?”question. Novice investigators often wish to conduct some form of systematic isolation and investigation of the various drug diffusion/delivery parameters by the use of in vitro permeation cell systems. It is with this readership in mind that we present here a no-frills, basic text on the fundamental aspects affecting the permeation of molecules through the skin or through any in vitro diffusion membrane.