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Dissolution is defined as the transfer rate of individual drug molecules from the solid particles (usually crystalline) into solution as individual free drug molecules. It is often the dissolution rate that is of greater interest than solubility due to the dynamic nature of the absorption process.

Whilst solubility assessment is typically focussed on the drug substance; dissolution is a parameter more relevant for a formulated drug product (e.g. tablet or capsule). In fact, dissolution testing is commonly used as an analytical technique to measure the rate of drug release from a pharmaceutical product. Although it is the drug product under test during dissolution it is the rate of drug substance release that is measured within the test. For immediate release drug products, rapid and complete dissolution is the goal; whereas for extended release a slower and controlled rate is desirable. Further details on dissolution testing apparatus, including images, is presented in Chapter 6.

Within pharmaceutical product development dissolution testing is used in two distinct ways:

 In a ‘quality’ environment dissolution testing is used to ensure reproducibility in products and to provide assurance on batch to batch variability. When used for quality control purposes the dissolution testing conditions are designed to evaluate the robustness of the formulation and manufacturing process. Thus the test must be sufficiently sensitive to discriminate between formulations that are different based on their content or process for manufacture. This level of discrimination will depend upon the therapeutic window of the product under test and is further explored in Chapter 8.

 The second application of dissolution is to mimic the physiological conditions at the site of absorption to provide a biorelevant environment. In this format, the equipment will be adapted to reflect the relevant composition, volume and agitation. There are also efforts to mimic the timeframe within each environment and reflect the dynamic conditions within the body. Correlations between dissolution testing and in vivo data are frequently sought so that dissolution testing could be used as a surrogate for some in vivo preclinical and clinical testing.

There is growing evidence to combine these approaches to have a dissolution methods that are biorelevant and also provide information on the product robustness.

The primary goal of a dissolution test is to develop a discriminating, robust and reproducible dissolution method that can highlight significant clinically relevant changes in product performance due to changes in the formulation or manufacturing process.

The dissolution method(s) will change during the development of a new product. In the early stages, the dissolution is used to understand the factors that govern drug release and to ensure that there are no interactions within the dosage form that adversely affect drug release. The next phase of testing would seek to correlate the in vitro dissolution release to in vivo release, once this data is available. Further details are provided in Chapter 6.