Creams, ointments, lotions and gels are topical drugs normally described as semisolid dosage forms. Unlike transdermal drug systems which are applied to the skin for systemic effects (the drug penetrates the skin whereupon it is then absorbed into the drug stream), topical drugs are applied to the skin for local action principally to counter dermatological problems.
At the present time, there are no compendial methods relating to the in vitro release testing of topical semisolids such as creams, ointments and gels or for skin permeation studies on the effects of transdermal products.
However, there does appear to be a growing body of opinion in support of the vertical diffusion cell system (sometimes called the “Franz Cell” after the name of its originator, Dr. T. J. Franz) to fulfil these roles.
Current USP thinking suggests two types of test to characterise semisolids, product quality tests (USP Chapter <3>) and product performance tests (the proposed USP Chapter <1724>).
USP Chapter <1724> SemiSolid Products – Performance Tests currently under revision suggests 3 apparatuses that may be used to determine drug release from semisolids, the Vertical Diffusion Cell, the Immersion Cell and the Flow Through Cell (Apparatus 4).
1) Vertical Diffusion or Franz Cell
The Vertical Diffusion or Franz Cell is a simple, reproducible test for measuring the drug release from creams, ointments and gels that is rapidly emerging as the apparatus of choice for the in vitro testing of topical semisolid dose forms and skin permeation studies.
The cell comprises two parts (a) the sample holder containing either 250 mg or 450 mg of the sample to be tested and (b) the reservoir of the diffusion cell itself containing the receptor medium.
The two parts are separated by a membrane designed to act as a conduit for diffusion to take place and which serves to contain the test sample whilst ensuring that it remains in contact with the receptor medium.
The receptor temperature is usually set to 32 degrees C to approximate normal skin conditions. Normally, no fewer than 6 samples are taken over a six hour period, say 0.5, 1, 2, 4, 5 and 6 hours and analysed using HPLC or similar technique.
An alternative to the Vertical Diffusion or Franz Cell for testing semisolids, the Immersion Cell is used with a special flat bottomed version of the 200 mL Small Volume Conversion Kit and a mini version of the conventional USP Apparatus 2 Paddle Method.
The PTFE Immersion Cell is designed to accommodate a 25 mm diameter membrane. It comprises four main parts:
The cell is provided with an alignment tool and an adjustment tool that allows the user to vary the volume of the reservoir within the cell.