The following article has been authored by John Heaney.
Intrinsic dissolution is a specific niche of dissolution testing often reserved for early R&D. The purpose is to provide scientists with an idea of the solubility of the API they’re studying. This data can be invaluable as it will help determine the excipients required to develop an oral dosage form.
There are 2 types of methods for this. The first is a Rotating Disk (often called the Wood’s Apparatus) and the second is Stationary Disk. In both cases a pellet of the pure drug is pressed into a drug compact in a die within the apparatus using a hydraulic press. This provides a constant surface area when performing the test.
In the case of the Rotating Disk, the compact is suspended in a standard round bottom dissolution vessel from the spindle of the dissolution tester and is spun between 60 and 500 rpm depending on the method requirements.
For the Stationary Disk, the compact is placed in the bottom of a flat bottom dissolution vessel, and a paddle provides circulation, in a configuration similar to Apparatus 5, paddle over disk for transdermal products.
Dissolution media for the test is typically biorelevant and media temperature is held at 37°C. Deaerating the media is recommended as any bubbles that lodge on the compact will affect the surface area and the results. Typically, 5 samples are pulled during the test to evaluate the flux.
Ideally the dissolution curve should be a straight line with this test, but curvature can occur. If the curvature goes up, it indicates more drug is being released over time than at the start of the test and often indicates there is an issue with the experiment. For example, the compact has cracked creating more surface area exposed to the dissolution media. If the curvature goes down, it may indicate an issue with sink conditions or may indicate there has been a transformative change in the compact at the surface which would require reevaluating the dissolution media and compaction pressure.
Intrinsic dissolution testing is a valuable source of information for R&D teams developing new products. More information can be found in USP 〈1087〉 Intrinsic Dissolution—Dissolution Testing Procedures for Rotating Disk and Stationary Disk.