Transdermal absorption of drugs, i.e. nitroglycerin and scoporamine, from marketed transdermal therapeutic systems (TTS) has been evaluated mainly according to T. Higuchi's theory. This theory, at first, was reviewed from the thermodynamic point of view. Much attention is paid to the enhanced transdermal absorption to expand the utility of TTS to many drugs and it becomes realistic by use of prodrugs as esters of viderabine, an antivirus drug, by the application of penetration-enhancers such as Azone, and/or by appearance of iontophoresis. Secondly, such enhancing systems were summarized theoretically and the differences between them and Higuchi's theory on the absorption rates were discussed. The route for transdermal absorption and its kinetic model might be modified in such enhancing systems, since the systems affect the skin barrier function. Reasonable absorption routes and skin model were discussed, thirdly. These three considerations could be useful for the development and evaluation of new TTS.
