1. Assessment of Drug Release and Permeation across Skin

2. The percutaneous/dermal absorption process is a global term which describes the passage of compounds across the skin. This process can be divided into three steps: Penetration, which is the entry of a substance into a particular layer or structure such as the entrance of a compound into the SC Permeation, which is the penetration through one layer into another, which is both functionally and structurally different from the first Resorption which is the uptake of a substance into the vas- cular system (lymph and/or blood vessel), which acts as the central compartment

4. Evaluation of transdermal drug delivery systems In vitro drug release kinetics The release kinetics of drug from the TDDS technology can be evaluated using a two compartment diffusion cell assembly. Cellophan membrane is mounted on a vertical diffusion cell such as Franz diffusion cell. The skin permeation profile is followed by sampling the receptor solution at predetermined time intervals until steady state is reached. In vitro skin permeation kinetics The release and skin permeation kinetics of drug from the TDDS technology can be evaluated using a two compartment diffusion cell assembly. Skin is mounted on a vertical Franz diffusion

6. Choice of Skin The choice of skin depends on the purpose of the test and the availability of skin samples For risk assessment purposes, human skin is preferred. The use of human skin is subject to national and international ethical considerations. Rat skin is often used for in vivo toxicological studies Human skin (abdomen or breast) and pig skin are recommended for cosmetic testing. Checking for skin integrity and viability is important. Excised skin has to be used promptly or refrigerated for short time

7. Use of Reconstructed Skin for Transdermal Testing Several types of reconstructed skin model exist, which can be categorized as either epidermal models or full thickness models Epidermal models were made by seeding normal human keratinocytes on the surface of a suitable membrane such as cellulose acetate or polycarbonate. Under submerged conditions, proliferating keratinocytes form a multilayer matrix covering the surface of the membrane. After proliferation, the keratinocytes are exposed to the air (air--liquid interface) in order to induce their differentiation, which then gives rise to a multilayered epidermis

8. Full-thickness skin equivalents First, a de-epidermized dermis (DED) derived from native human skin may be used. The acellular DED is then populated with fibroblasts and cultured under submerged conditions before seeding of keratinocytes. Seeding of the keratinocytes on top of the DED, differentiation is induced by exposure of the DED to the air.

11. In vivo studies A. Animal models B. Human studies