
	Diffusion cells and study of percutaneous penetration

Transdermal systems are increasingly used to admin-ister drugs as these systems allow continuous deliv-ery into the bloodstream. The desire to reduce the number of animal experiments and to develop predic-tive models which are easier to use has encouraged R & D teams to develop in vitro models for study of percutaneous penetration by diffusion cells.

These cells consist of two glass components which constitute two compar-tments separated by a synthetic membrane or a fragment of skin. The formulation stu-died (transdermal system, suspen-sion, gel or solution) is placed in donor (epidermal) com-partment. The subs-tance studied diffuses through the skin and is collected in the receptor (dermal) compartment which represents the blood flow.

The studies undertaken on these in vitro models al-low, for example, the quantitative assessment of the diffusion of a substance and its precise kinetics. They also allow qualitative studies concerning the cutane-ous metabolism of a molecule. The interest of this type of methodology is the possibility of using either animal skin or human skin. The model offers the ad-vantage of flexibility of use and rapid implementation. On the other hand, like many models, it remains es-sentially indicative.

In using diffusion cells, for each molecule studied, highly-sensitive analytical techniques must be avail-able permitting the accurate and precise determina-tion of the small quantities used in such experiments. Therefore, liquid chromatography coupled with spectrometric detection, immunoassays and measurement of radioactive isotopes form part of the methods generally used.

There are many applications for this system for both systemically acting and locally acting products. In the case of pharmaceutical products, the matter is to check whether the percutaneous way is a possible route for drug administration. The concern is also to make comparison between different molecules in the same chemical group or to evaluate the effect of the enhancers on the penetration of the active drug. Fi-nally, for cosmetics, it is possible to determine the distribution of the product in various layers of the skin. X.M.

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