ABSTRACT
The skin is a seemingly impermeable barrier with its primary function to protect against entry of foreign agents into the body. Nevertheless, intact skin may be used as a route of administration for systemic delivery of simple potent drug molecules through a transdermal patch. In fact, recent literature suggests that the transdermal route now competes with oral administration as the most successful innovative research area in drug delivery, as 40% of current drug products under clinical evaluation are transdermal (Barry, 2001). The first U.S.-approved transdermal patch was introduced in 1981 for scopolamine. In the past two decades, another nine drugs have been introduced to the U.S. market, namely nitroglycerin and clonidine for cardiovascular disease, nicotine for smoking cessation, fentanyl for chronic pain, estradiol with or without levonorgesterel or norethisterone for hormone replacement, estradiol with norelgestromin for hormonal contraception, testosterone for hypogonadism (Hogan and Cottam, 1991), lidocaine for postherpetic neuralgia, and oxybutynin for detrusor hyperactivity, and most recently selegiline to treat major depressive disorder. Many more transdermal drug delivery systems (TDDS) are currently under investigation, including products to treat Parkinson’s disease, Alzheimer’s disease, and skin cancer (Benson, 2005) (Table 11.1).
