Is the use of sinkers in drug dissolution testing valid – Maybe not!

Often it is suggested that if a product (tablet or capsule) floats or moves randomly during dissolution testing, it may produce variable results, therefore, one should use a sinker to avoid mobility to reduce the variability in dissolution results. However, it should be noted that use of sinkers may invalidate dissolution testing/results and their relevance, because:

  1. Dissolution tests are conducted to determine potential drug release characteristics in vivo, where mobility of the products is natural and expected. Therefore, use of a sinker makes drug dissolution testing physiologically not relevant.
  2. When a sinker is used, it forces the product to settle at the bottom of the vessel, where interaction between product and dissolution medium is minimal, resulting in inefficient dissolution. In addition, the caging effect of sinkers may further reduce the dissolution rate. In fact, the use of a sinker may exaggerate effect of poor hydrodynamics within a vessel. Thus, its use will provide inaccurate results even for in vitro dissolution characteristics of the test product. 
  3. It would not be possible to accurately compare the dissolution characteristics of two products in which one would float, and the other not. In such cases, how would one subtract the effect of the sinker from the product which requires one and other which does not, so that a proper comparison of dissolution characteristics of the two products can be made?

Therefore, one should be careful when using a sinker as the tests/results may provide inaccurate reflection product characteristics.

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