Transdermal drug delivery
A transdermal patch is a medicated adhesive patch that is placed on the skin to deliver a specific dose of medication through the skin and into the bloodstream. Percutaneous absorption of a drug generally results from direct penetration of the drug through the stratum corneum, deeper epidermal tissues, and the dermis. When the drug reaches the vascularized dermal layer, it becomes available for absorption into the general circulation.
Among the factors influencing percutaneous absorption are the physicochemical properties of the drug, including its molecular weight, solubility, partition coefficient, nature of the vehicle, and condition of the skin. Chemical permeation enhancers, iontophoresis, or both are often used to enhance the percutaneous absorption of a drug.
In general, patches are composed of three key compartments:
- a protective seal that forms the external surface and protects it from damage,
- a compartment that holds the medication itself and has an adhesive backing to hold the entire patch on the skin surface, and
- a release liner that protects the adhesive layer during storage and is removed just prior to application.
Most patches belong to one of two general types—the reservoir system and the matrix system. The reservoir system incorporates the drug in a compartment of the patch, which is separated from the adhesion surface. Drug transport from the patch to the skin is channelized and controlled through a rate-limiting surface layer. The matrix system, on the other hand, incorporates the drug uniformly across the patch in a polymer matrix. Diffusion of the drug through the polymer matrix and the bioadhesive properties of the polymer determines the rate of drug absorption.
Marketed transdermal patches are exemplified by Estraderm® (estradiol), Testoderm® (testosterone), Alora® (estradiol), Androderm® (testosterone), and Transderm-Scop® (scopolamine). Nicoderm® is a nicotine patch, which releases nicotine over 16 h, continuously suppressing the smoker’s craving for a cigarette. In addition, occlusive dressings are available, which have low water vapor permeability. These dressings help prevent water loss from the skin surface, resulting in increased hydration of the stratum corneum.