The following article has been authored by John Heaney.
While determining if a sinker is needed is easy; determining what type of sinker needed may be less obvious. USP <711> does allow for sinkers made from twists of inert wire. This can be especially handy for R&D where the method is being developed and an experiment is being run to see if the sinkers provide more consistency to the results. For use in other environments, such as QC, it would require the users to be trained on the correct length of inert wire to cut, as well as the spacing between the coils to ensure the dosages are sharing the same conditions.
Making sinkers also has another distinct advantage. If the dosage is an unusual shape or size, it may be far better to focus on making appropriate sinkers from short lengths of inert wire than to try to force fit it into a manufactured sinker. This means training staff and documenting the basic configuration of the sinker but this can often be done fairly easily.
For QC departments, having pre-made sinkers that have nearly identical shapes and dimensions can be helpful to reduce variability and ensure that the data obtained is representative of the batch rather than representative of the equipment and sinkers used for testing. There are a number of factors that go into choosing a suitable sinker.
Size is the first and most obvious factor. Choosing a sinker that is too small means that the dosage form will not fit into it without damage. A sinker that is too large, depending on the design, has the possibility of losing the tablet with it floating out. Or it may allow the dosage form to move around in the sinker which could cause additional variability in the results.
The next big question is the shape or design of the sinker. Sinkers have different spacing between the wires, some have a few millimeters, some are closer to a 40 mesh screen or even finer, some hold the dosage along its length. The most important thing to know is how the dosage form dissolves. What size are the particles or if there is swelling, how big does the dosage form get?
Users need to avoid picking a design with a mesh or spacing between the wires that is so small that it presents a barrier to the API releasing from a dosage in much the same way as a large group of people leaving through a doorway that is too small. If the flow of the dissolution media is significantly reduced, it in turn will affect the dissolution rate, usually negatively, as there would not be sink conditions present around the dosage.
Gelcaps can be of some concern with this as the gel can potentially block the openings of the sinker, preventing release of the drug. If the sinker is acting as a brace rather than simply holding the dosage form in the bottom of the vessel, it could affect the dissolution results by slowing the opening of the gelcap if not outright preventing it.
The weight of the sinker as well as the material used should have some consideration as well. If the dosage is inclined to float then a heavier sinker may be required to ensure it stays at the bottom of the dissolution vessel.
It’s also good to compare sinkers to help prove the method has been properly validated. This can be fairly simple by using sinkers of different styles (2-3 vessels per type of sinker) and then evaluating the results over a short series of tests. Ideally, vessels would be alternated between sinker types in the series of tests to avoid any kind of bias. This would provide documented proof of the best sinker to use for a given method, ensuring any audit of the method or data would stand up to scrutiny.
Determining whether or not a sinker should be used is often quite easy. Determining the best sinker to use requires some thought about the dosage form itself and ideally a series of tests to evaluate how sinkers may compare to each other.
Recommended Reading: Investigation of Dissolution Performance of Hard Gelatin Capsule Products Using Various Sinkers by Yaser Mansuroglu and Jennifer Dressman DT202008_A02.pdf (dissolutiontech.com)