The following article has been authored by John Heaney.
Not all tablets, capsules, and gelcaps behave the same in dissolution media. In some cases, a dosage may be buoyant enough to move far more than normal when subjected to the turbulence caused by a paddle (Apparatus 2) if not float all the way to the surface. Either of these would be considered unacceptable if they were observed during a test.
The case of a dosage form floating to the surface is obvious. It’s far out of position from the bottom of the dissolution vessel and is not subject to the same hydrodynamic forces as a dosage which stays in place.
In the case of a dosage form that is moving around when subjected to the forces beneath the paddle, the issues may be less obvious. Yes, the dosage is still at the bottom of a dissolution vessel, but it’s moving, and each vessel may have a dosage that is moving differently from another. More specifically, the hydrodynamic forces that each dosage is subject to is likely to be different; perhaps significantly. This in turn could lead to more variability with the results for each vessel which would make it more difficult to discern if there are problems with that batch.
As with any dissolution test, the initial method development of a test for a new tablet, capsule, or gelcap should include observation. The analyst should be present and recording observations for each vessel. In some cases, if the dosage is sufficiently buoyant, placing it in a small beaker of media would show that a sinker may be required. If possible, or practical, calculating the density of the dosage prior to performing a dissolution test may be sufficient to determine if a sinker is needed if the density is sufficiently lower than that of the media.
In a QC environment, the dissolution method should be well defined. If a dosage form suddenly needs a sinker and one is not called for in the method, it may be indicative of a problem at best with that individual dosage form, or at worst, the batch as a whole. While a dosage suddenly floating that did not float before is indicative of a serious problem, a dosage moving around more than usual at the bottom of the vessel while using Apparatus 2 (paddles) can also be indicative of poor deaeration. If the media has been poorly deaerated then it is most likely that bubbles will appear on the surface of the dosage. If enough of them adhere to the surface of the dosage, they can cause the tablet to become more buoyant as well as serve as a barrier against the diffusion of the drug away from the surface of the tablet. Sinkers are not a solution to poorly deaerated media.
Generally speaking, it’s not difficult to determine whether or not you need a sinker. Determining how to pick a sinker will be covered in a future blog.