ABSTRACT
Topical delivery is confined to a situation in which the drug is intended for localized treatment of certain dermatological conditions, e.g., eczema, psoriasis, etc. In such case, the active agent should penetrate into the skin to reach the site of action but not reaching the systemic circulation, thus avoiding systemic side effects. In transdermal delivery, however, the active agent permeates through the whole skin thickness with the different skin layers reaching the systemic circulation to elicit a therapeutic response. Release of a therapeutic agent from the applied formulation to the skin surface and its transport across the skin layers to the systemic circulation is a multistep process which involves: (a) release from the formulation, (b) partitioning into the SC, (c) penetration through the SC, principally via the lipidic intercellular pathway (Other potential routes of transport across the SC include the intracellular pathway and transport through skin appendages, mainly the hair follicles and sebaceous glands) (d) partitioning from the SC into the viable epidermis, (e) diffusion through the viable epidermis into the upper dermis, and (f) uptake into the local capillary network eventually reaching the systemic circulation. Accordingly, an ideal permeant should have a sufficient lipophilicity to partition into the SC, but also sufficient hydrophilicity to enable further partitioning into the viable epidermis [2-4].Transdermal drug delivery offers great advantages over the oral route. By delivering a steady flow of drugs into the bloodstream over an extended period of time, transdermal systems can avoid the “peak and valley” effect of the oral therapy and can enable a more controlled and effective treatment. Away from controlled delivery, other advantages include minimizing the first-pass metabolism, avoidance of drug degradation under the influence of the harsh acidity of the stomach, and prevention of possible erratic delivery due to food interactions. However, there are several factors that limit the success of transdermal technology. This includes the following: • Local skin irritation associated with the application of certain drugs and formulations.
