The following article has been authored by John Heaney.
While Immersion/Enhancer Cells and Vertical Diffusion/Franz cells are both used to test semi-solid dosage forms; the capabilities of each apparatus are important to understand before deciding which is best for your lab. Testing of semi-solid dosage forms is broadly categorized into In-Vitro Permeation Testing (IVPT), and In-Vitro Release Testing (IVRT). The types of testing serve very different purposes and have different requirements.
IVPT is primarily used to simulate how a semi-solid dosage form will behave in-vivo or when a patient makes use of it. This typically uses bio-relevant media and has excised skin as the preferred membrane. The amount of dosage used for the test is typically quite small and is intended to be used up during the testing process. The amount released during this test is typically in picograms or nanograms per milliliter of solution, so a good analytical method is a requirement.
IVRT is primarily used to compare two similar products and determine if they are the same. These could be a generic product and an innovator, or the same product before and after a production change. The media is often an organic and aqueous mix, such as 30% ethanol in water. Membranes are meant to have as little restriction as possible and are typically 0.45 µm synthetic membrane filters such as PVDF or PTFE. Typically, more dosage is used than will possibly be exhausted during the test and this is referred to as an infinite dose. The amount released during the test typically is in milligrams per milliliter so most analysis will be done by HPLC.
Vertical Diffusion/Franz cells can vary widely in terms of the available surface area for testing and the receptor volume which holds the sample. Typical areas are 0.2 cm2 to 1.767 cm2, but manufacturers may offer cells with larger or smaller surface areas. Typical receptor volumes for Franz cells range from around 5mL up to 15mL, but again some manufacturers may offer different volumes if requested. They are suitable for both IVPT and IVRT testing but can be difficult to automate and tend to be somewhat operator intensive, requiring more training than a typical dissolution test.
Immersion/Enhancer cells are used in conjunction with dissolution testers. The cells hold the dosage from in a chamber separated from the receptor media by the membrane. Typical orifice diameters are 15mm and the amount of dosage present can vary from about 200 µL to over 1 mL. Receptor media is typically about 50 mL as enough is needed to submerge the cell in a small volume dissolution vessel.
Vertical Diffusion cells are best when the lab will be doing both IVRT and IVPT testing. They provide a lot of flexibility in how they can be used and are a great general-purpose device. If the lab is only interested in IVRT and has existing dissolution testers then Immersion and Enhancer cells may be the best solution. However, if testing needs to go beyond IVRT then a different type of apparatus will be required.
For more information it’s recommended to consult:
- USP Chapter <1724>
- In Vitro Permeation Test Studies for Topical Drug Products Submitted in ANDAs
- In Vitro Release Test Studies for Topical Drug Products Submitted in ANDAs