“Discriminatory testing” terminology requires clarity

Often in literature and discussion, terminology of a discriminatory test is used to describe that a dissolution test is capable of differentiating or discriminating between products based on formulation and/or manufacturing differences. However, implied understanding of this terminology is that these differences may reflect products’ in vivo differences, thus their quality in humans. The underlying implied assumption of in vivo relevance is emphasized by suggestions that dissolution testing be conducted using in vivo relevant experimental conditions, e.g., dissolution medium be aqueous having pH in the range of 1 to 7. Interestingly, it is also very well documented in the literature that dissolution results with formulation/manufacturing differences seldom reflect corresponding in vivo behavior.

It is, therefore, safe to consider that the use of terminology of “discriminatory test” as commonly used does not appear to be correct. To be correct, the discriminatory test terminology should clearly identify a test as “in vitro discriminatory test” or “in vivo discriminatory test” also known as bio-relevant test.

An in vitro discriminatory test would be the test to reflect differences in physical characteristics of the test products (formulation/manufacturing) with no direct or definite consequences in vivo. Such tests may be conducted using any of the experimental conditions necessary concerning apparatuses (paddle/basket, Erlenmeyer flask with magnetic stir etc.) and media (organic or aqueous solvents having any pH), etc. In this respect, disintegration test may be considered as a discriminating test, if formulation/manufacturing differences be linked to disintegrating time. It may be important to note that although often in vitro discriminatory dissolution tests are developed and used but their usefulness is limited and may be an unnecessary burden on the pharmaceutical industry and the regulatory agencies.

An in vivo discriminatory test or a bio-relevant test, on the other hand, would be a test which would relate differences in formulation/manufacturing of products to corresponding differences in vivo such as bioavailability/bioequivalence characteristics. For an in vivo discriminatory test, an essential requirement is that the test must be conducted using physiologically relevant experimental conditions. For example, an apparatus must provide gentle but efficient stirring and mixing environment, medium must be aqueous with pH in the range of 5-7 and maintained at 37C, and the medium must not be de-aerated but equilibrated with dissolved gasses. In addition, as the testing environment is linked to the GI tract physiology, which does not change from product to product (e.g. IR to ER), the experimental conditions should not be changed from product to product as well.

Therefore, it is prudent that one should clearly indicate the nature of the test described whether it is an in vitro or in vivo discriminatory type so that proper evaluation and use of the test be considered.

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