Testing Novel Dosage Forms and Non-Compendial Modifications
to Dissolution Apparatus: Apparatus 2 Sinkers

In Apparatus 2, the dosage form is allowed to sink to the bottom of the vessel before the blade begins rotation. However, some dosage forms may be too light and will require a 'weight' (sinker) to enable them to sink. This may be accomplished by winding a few turns of a short, loose piece of wire, made of a suitably non-reactive material, around the dosage form to form a helix.

Alternative sinker devices may be required during method development. However, the performance of the various sinkers is generally not equivalent and the sinkers are not interchangeable unless validated.

Dosage forms which require sinkers are not solely limited to capsules that do not sink. Any dosage form that exhibits the following characteristics may require a sinker to obtain the best dissolution results.

Sinker
Required 1 Dosage form is too light
and does not immerse

Sinker
Required 2 Dosage form initially sinks,
then floats (hydrogels)

Sinker
Required 3 Dosage form disintegrates,
then particles float

Sinkers should exhibit a number of desirable characteristics including the following:

1 There should be little
hydrodynamic interference Minimal
Impact on
Test Results

2 Sinker material should be
stable, inert, and robust Minimal
Impact on
Test Results

3 It should have minimal
contact with the dosage form Minimal
Impact on
Test Results

There are a number of disadvantages associated with the use of sinkers which may have far-reaching implications during dissolution testing. These include the points shown below.

All these problems will result in an unacceptable variation in dissolution performance.

Helical glass sinkers may break

Random variations in position
and gyration of dosage

Metallic wire reacts with dissolution medium

Gummy excipients

Lack of precise definition of helix