Skin Permeation and Transdermal Delivery Systems of Drugs: History to Overcome Barrier Function in the Stratum Corneum

Transdermal Drug Delivery Systems (TDDS), where active drugs must be absorbed into the systemic circulation after penetrating the skin barrier, were first launched in 1979, and about 10 TDDS containing different kinds of drugs were developed during the initial decade. Interestingly, a developmental rush has come again in the present century. Various penetration-enhancing approaches to improve drug permeation of the skin (stratum corneum) have been attempted. These approaches are of two types: chemical and physical. Examples of the chemical approach are enhancers such as alcohol, monoterpenes and fatty acid esters, as well as chemical modification of prodrugs. In contrast, physical approaches include the use of electrical-, thermal- and mechanical-energy, as well as microneedles, needle-free injectors or electroporation to completely or partially evade the barrier function in the stratum corneum. The chemical approaches are mainly effective in increasing the skin permeation of low-molecular chemicals, whereas physical means are effective for these chemicals but also high-molecules like peptides, proteins and nucleotides (DNA or RNA). Marked development has been observed in these physical means in the past decade. In addition, recent developments in tissue engineering technologies enables the use of cultured skin containing keratinocytes and fibroblasts as a TDDS. An effective "cell delivery system" may be a reality in the near future. This paper will look back on the 30-year history of TDDS and evaluate the feasibility of a new generation of these systems.