ABSTRACT

The skin is the largest organ of the body and is a complex, layered structure that forms a formidable barrier to the outside environment. Many factors govern the delivery of drugs and cosmetics into the skin from topically applied formulations. These include the size of the molecule, the lipophilicity of the component, type of formulation, presence of penetration enhancers, and physical state of the stratum corneum (SC) (1). In dermatology, the drug is rarely applied to the skin in the form of a pure chemical but, instead, is normally incorporated into a suitable carrier system-the vehicle. The term ‘‘vehicle’’ in this context is relatively new and was developed only when it became possible to differentiate the specific (therapeutic) effect of a chemical substance from the ancillary effects of ‘‘inactive’’ ingredients in a formulation. In crude terms, the vehicle or base may be regarded as the sum of the ingredients in which the drug is presented to the skin. In more sophisticated terms, we are now beginning to understand how to design and engineer a topical vehicle to deliver the active ingredient in a specific, dynamic pattern of penetration enhancement or retardation.