The following article has been authored by John Heaney.
Transdermal systems, or TDS, are often patches which when placed on the skin will deliver a drug over an extended period of time. Like any drug product, these need to be tested to ensure they are providing therapeutic benefit to the patients. In many cases, the best way to do this is performing a dissolution test using Apparatus 5, paddle over disk or watch glass, or Apparatus 6, the rotating cylinder.
The primary way to choose between Apparatus 5 and 6 comes down to TDS size, where Apparatus 6 has a clear advantage. Generally, TDS cannot be cut to fit the apparatus, as in many cases this might render the patch functionally useless. The larger surface area of using Apparatus 6 with an extension helps ensure it can work with the largest patches.
However, Apparatus 6 does have some disadvantages. The extension cylinder, which provides the larger surface area, is deceptively delicate and must be handled very carefully making it somewhat operator intensive. Potentially single piece Apparatus 6, which have the same dimensions as the standard apparatus with the extension installed, could be used but as of this writing are not mentioned in USP chapter <724>.
Apparatus 5 tends to be easier to use as the screens are not as delicate as the cylinder extension for Apparatus 6. The primary concern with Apparatus 5 is the potential for a lower concentration located under the disc as compared to above it, as the patch itself, if not the apparatus would restrict the flow of media. Generally, the apparatus are designed to minimize this by both use of screens, and different sizes available so that the apparatus is only as big as required to hold the TDS.
Deciding which apparatus works best for a product may be easy if the patch is large enough, but for smaller patches, the decision may require more thought; taking into account the drug behavior and operator.