General Introduction and Conceptual Differentiation of Topical and Transdermal Drug Delivery Systems

Abstract

Since percutaneous absorption is pivotal to the effectiveness of both topical and transdermal systems, there is a tendency to view these systems as being functionally closely related. This is an unbefitting generalization for the simple reason that the microenvironments from which transdermal and dermatological systems operate are extremely different. For the most part, transdermal delivery involves delivery from compositionally stable matrices tailored such that the thermodynamic activity of the drug is at, or is set close to, its neat state activity (unit activity) over practically the full course of delivery. Where solubilizing and/or enhancing agents are used, they also have an even function for the greater part of the time the systems are in use. Packaging is built around the systems to keep them pristine until put into use, and impermeable backing materials are built in to keep them functioning as intended while worn. Thus, release tends to be zero-order, or something very close to this, for the duration of patch wear. Best of all, the processes of delivery lend themselves to careful definition in the laboratory. In contrast, topical delivery systems operate in totally uncontrolled environments after being applied. The spread film may be as little as 20 µm thin, covering areas measured in hundreds of square centimeters. Such thin, unprotected films experience rapid and profound compositional alterations as components evaporate and/or are absorbed into the skin. Wide swings in the thermodynamic activity of the drug are to be expected.