The following article has been authored by John Heaney.
Coning of disintegrating dosage forms is fairly common when using Apparatus 2 (Paddles) to perform dissolution testing. The reason for this is the spinning of the paddle causes a cone shaped zone to form directly beneath it where there is low hydrodynamic activity. In other words, it’s a bit of a dead zone where the stirring has little effect, and the particles are allowed to settle.
Depending on the formulation this coning can be problematic. It can prevent drug from dissolving in the media due to a reduction in the effective surface area, as particles inside the cone cannot reach the media as easily as the particles on the surface of the cone can. This can lead to lower amounts of drug dissolved in solution, as well as higher variability.
Typically, if coning is believed to be a problem, either due to being unable to match a dissolution profile or high variability, then an investigation should be performed. The investigation would start with trying to resolve the issue with official apparatus. This could mean attempting the method with baskets or increasing the paddle speed to help minimize the coning. If these do not yield the desired results, Apex vessels can be tried.
PEAKTM Vessels trademarked by VanKel (now Agilent) are now more commonly referred to as Apex vessels. They are distinct from the standard USP defined dissolution vessels in that instead of a hemispherical bottom there is a point. The primary purpose of this point is to prevent coning that may occur with certain disintegrating dosage forms. Typically results using Apex vessels as compared to the standard USP vessels will be higher due to the increased surface area for dissolution and pushing material outside of the dead zone located directly beneath the paddle. However, with more material exposed to the dissolution media, the results are also likely to be more consistent.
Apex vessels are not official vessels or Apparatus but are mentioned in USP chapter <1092> which is advisory. Apex vessels should be used with proper scientific justification where other methods have been tried using official apparatus but have failed to provide a discriminating dissolution test.