The skin, or integument, of the human body both provides protection and receives sensory stimuli from the environment. The skin is the most extensive and readily accessible organ of the body. In an average adult, it covers a surface area of over 2m2 and receives about one-third of the blood circulation; this blood drains into the venous circulation and so avoids first-pass metabolism. It is normally self-regenerating.
Although it is well known that drugs can be applied to the skin for local treatment of dermatological conditions, the advantages of accessibility and the avoidance of first-pass metabolism make it attractive for the systemic delivery of drugs. The objective of a transdermal delivery system is to provide a sustained concentration of drug for absorption, without breaching the barrier function of the skin, and avoiding local irritation. However, the slow transport of many drugs across the skin limits this technique to potent drugs which require plasma concentrations of only a few pg per ml.
Generally, the higher the lipid solubility of a drug and the lower its melting point, the faster it will penetrate the skin. Several drugs can be successfully administered by this route, notably scopolamine, glycerine trinitrate, clonidine and oestradiol. Nicotine patches are becoming common in smoking cessation programs. Nonsteroidal anti-inflammatory drugs (NSAIDs) are also being administered increasingly by transdermal delivery for the treatment of local muscle inflammation. Recently some attention has been focused on the delivery of acetylcholinesterase inhibitors for the treatment of Alzheimer's disease1. Other potential candidates are E-blockers, antihistamines and testosterone.
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