INVITRO SKIN PERMEATION TECHNIQUES

This paper reviews various techniques used to assess permeation of drugs into and through skin under in vitro conditions. There have been a wide variety of diffusion cells designed for in vitro measurement of skin permeation. These cells have generally been designed in one of two ways: side-by-side (bichambers) and vertical in vivo mimic diffusion cells. A primary goal of in vitro permeation studies is the prediction of skin permeation in vivo. In this regard, side-by-side diffusional cells are useful in delineating mechanisms of permeation under controlled conditions, but are of more limited usefulness in predicting skin permeation in vivo. Vertical cells are more versatile because a wide variety of experimental conditions can be used to gain information useful in the evaluation of formulations ultimately destined for clinical use. A number of diffusion cell designs are reviewed and some of their advantages and disadvantages are discussed. In addition to diffusion cells used to assess passive diffusion, iontophoretic- and phonophoretic-enhanced skin permeation in vitro is also considered. Until the use of differentiated keratinocytes in culture becomes inexpensive and the relationships between skin permeation in vitro and in vivo are established, skin permeability will be measured using excised animal or human skin in diffusion cells.