Modeling transdermal drug release.

The stratum corneum forms the outermost layer of the skin and is essentially a multilamellar lipid milieu punctuated by protein-filled corneocytes that augment membrane integrity and significantly increase membrane tortuosity. The lipophilic character of the stratum corneum, coupled with its intrinsic tortuosity, ensure that it almost always provides the principal barrier to the entry of drug molecules into the organism; the only exceptions being highly lipophilic species which might encounter problems at the stratum corneum-viable epidermis interface where they must partition into a predominantly aqueous environment. Drugs can be administered either as suspensions or as solutions and the formulation can range in complexity from a gel or an ointment to a multilayer transdermal patch. In this review we describe the theoretical principles used to describe transdermal release and we show that relatively simple membrane transport models based on the appropriate solution to Fick's second law of diffusion can be used to explain drug release kinetics into this complex biological membrane.