Dissolution Performance Testing Of Transdermal Systems

Abstract

-Introduction The "transdermal system" product concept has enjoyed a fast paced development in recent history. The term "transdermal system" applies to products that provide controlled delivery of drugs for systemic circulation rather than simple topical application of some pharmacologically active compound to the skin, especiall y for local effects. A transdermal system is characterized by some level of "system" control of drug delivery, and well defined "unit dose" product presenta-tion as well as the systemic derived efficacy. Testing transdermal systems, then, must not only address the traditional pharmaceutical dosage properties, such as potency, uniformity and purity, but also chemical and physical performance. This discussion will focus on physicochemical testing of transdermal systems. In such a system, the drug is dispersed and/or dissolved in the monolith (generally some form of polymer) and diffuses out of the system. The amount of drug released is governed by surface area, drug diffusion coefficients, monolith thickness and concen-tration of the drug in the polymer. Quantitative treatment of this process is defined by Fickian diffusion laws. As such, the driving force for the diffusion is the difference between drug concentra-tions in the polymer and the receptor solution. The next level of complexity is the addition of a rate-controlling membrane in the system, as in Figure 2. This system also has an added adhesive layer because the rate-controlling membrane often does not have adhesive properties. Drug release is controlled when the membrane diffusion is slower than diffusion within the reservoir.