Transdermal patch systems

Transdermal delivery is the term that is confined to a situation in which the drug diffuses through different layers of the skin into systemic circulation to elicit the therapeutic response (Brown et al., 2006). An example would be management of hypertension using a transdermal clonidine patch. In a broader sense transdermal delivery also includes local anesthetic patches in which the drug is intended to diffuse regionally in the skin to elicit the pharmacological action only in the treated area of the skin. Often, delivery of local anesthesia has been classified under topical drug delivery. An overview of cutaneous drug delivery system is shown in Figure 1.2.

Transdermal drug delivery systems also termed as "patches" are self-contained discrete dosage forms designed to deliver a therapeutically effective amount of drug through intact skin (Wokovich et al., 2006). Most commercially available transdermal drug delivery systems are of three different types, namely reservoir systems, matrix systems with rate-controlling membrane, and matrix systems without rate-controlling membrane. The reservoir system is made up of three major components, namely the drug

reservoir, the rate-controlling membrane, and the adhesive. The drug present in the reservoir, along with the other excipients, has to permeate through the rate-controlling membrane before reaching the skin. The adhesive that holds the system in place on the skin can completely cover the drug release area or only the perimeter around the non-adhering drug release surface. In the matrix type, the drug may be embedded in the adhesive matrix. A ratecontrolling membrane may be present between the drug-loaded matrix and the adhesive or sometimes the matrix itself can control the rate of release of the actives from the system.

The drugs that have made it into the transdermal market include sco-polamine, nitroglycerine, nicotine, clonidine, fantanyl, estradiol, testosterone, lidocaine, and oxybutinin (Langer, 2004). Recent additions to this list include lidocaine-tetracaine, selegiline, methyl phenidate, and rotigotine. However, the future focus is production of transdermal systems capable of delivering peptides and proteins including insulin, growth hormone, and vaccine across the skin.

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