Selection of topically applied chemical candidates: transdermal drug delivery systems

Drugs have been applied to the skin to treat superficial disorders and for cosmetic purposes since ancient times. Although early records suggest that the skin has also been used as a portal of entry for the systemic delivery of drugs, it was not until the latter third of the 20th century that transdermal delivery, particularly through the use of transdermal patches, became a common practice. Today, nearly 20 drugs have been successfully incorporated into transdermal drug delivery systems (TDDS) and delivered through the skin to treat general illnesses. The limited number of drugs available on the market when compared to conventional dosage forms (e.g. oral and intravenous products) can be mainly explained by the remarkable barrier property of the skin. This results in the selection of chemical candidates possessing adequate physicochemical properties to penetrate into and permeate through the skin. Moreover, therapeutically relevant levels of the candidate chemical need to reach the target site and TDDS have to demonstrate an unmet clinical need compared to already available dosage forms in terms of safety, patient compliance, and cost. This chapter first gives a perspective history of TDDS and their advantages and limitations. The three main requirements that a chemical candidate must possess in order to be successfully delivered using TDDS are then presented. Finally, the future prospects for TDDS for the management of general conditions are discussed