One of the requirements for dissolution testing is a mixing mechanism to provide efficient product (tablet/capsule) and medium interactions. Within the GI tract, such a mixing mechanism is provided by peristaltic compressions and motions.
To evaluate dissolution characteristics in vitro, one needs to provide a mixing mechanism as well. The question is should the in vitro environment have this mixing/stirring based on the peristaltic mechanism as well? The answer is not necessarily yes. For any in vitro testing, one tries to simulate an in vivo environment or process but not to duplicate it. This is one of the basic underlying principles of conducting in vitro testing. There are numerous examples of such practices.
For example, in vitro cell growths (cultures) are usually achieved in simple media not in body fluids. Dissolution tests are conducted using simple media (e.g. buffers). Similarly, controlled temperature environments (baths or cabinets) for testing are maintained, including for dissolution testing, using electronic thermostats with heating elements with or without circulating gasses or water. None of these reflect in vivo environments. The mechanisms to simulate the in vivo environments do not require duplication of the physiological (feed-back) processes where such controls are achieved based on enzymatic-based chemical reactions and circulating physiological fluids. It is important to note that arguments (suggestions) of conducting tests by duplicating physiological environments usually reflect a lack of appreciation and experience in the physiological aspect of dissolution testing.
The reason for presenting such weak arguments is to support the continued use of paddle and basket apparatuses, which do not provide the needed stirring and mixing, thus product-medium interactions. One may argue that stirring and mixing based on peristaltic motion may be preferred or used, which does not mean that one needs to keep using a knowingly flawed approach in the absence of such a mechanism. Obviously, an irrelevant testing environment will result in irrelevant dissolution results no matter how such studies, and results thus obtained are presented. One should be watchful of such erroneous suggestions and practices.
On the other hand, it is possible to modify currently used stirrers (paddle/basket) with others, such as crescent-shaped (brush-based), to address the flaws to achieve a better product-medium interaction representative of the physiological process. A number of studies have demonstrated that the use of modified (crescent-shape) sinpdle can provide improved and physiologically relevant dissolution results compared to paddle and basket apparatuses.
One of the criteria to judge the physiological relevancy is that the apparatus must provide testing using a single or common set of experimental conditions, as in vivo products are tested using a single set of experimental conditions. The use of the crescent-shape spindle fulfills this requirement.