ABSTRACT
INTRODUCTION Skin is the most accessible organ of the body (surface area of*2 m2), making it an attractive route to deliver topical as well as transdermal drugs. In fact, there are reports in the ancient Egyptian literature about the use of agents to treat skin conditions 4000 years ago (1). Yet, only a dozen transdermal delivery systems are on the market at present. The primary reason for limited exploitation of this delivery route is the excellent barrier function that the skin possesses to (i) prevent ingress of xenobiotics, (ii) withstand exposure to several environmental challenges, and (iii) prevent excessive loss of body fluids. This barrier is attributed primarily to the 15 to 20 mm thick stratum corneum that is composed of dead, flattened, keratin-rich corneocytes surrounded by a complex array of intercellular lipids organized into structured bilayers (2) (see chaps. 1 and 2). Passively diffusing drug molecules have to follow the tortuous pathway through the intercellular lipids and pass through lipophilic and hydrophilic domains, the epidermal layers, and basement membrane before reaching the capillaries in the dermal papillary layer for systemic absorption.
