ABSTRACT
To ensure reliable results, transdermal experiments must be carefully designed considering several factors that might inšuence in vitro and in vivo studies. For in vitro studies, these include considerations of the diffusion cells, skin source and type, solvents, buffer and other inactive excipients in formulation, cell design, and analysis, among others. Factors affecting delivery mediated by microporation, iontophoresis, electroporation, or sonophoresis are discussed in individual chapters in this book and are not further discussed here. For in vivo studies, additional considerations involve assessment of skin damage or irritation, sensitivity of the analytical method, and patch design considerations. Problems relating to assay sensitivity arise because the amount of drug absorbed from the skin is small and is often below assay sensitivity following dilution in the body šuids. Tracers are often used in animal studies, but the appearance of radioactivity in urine does not account for metabolism of the drug in skin. Thus, plasma concentrations need to be directly measured, and biological response can be measured in some cases. Radioimmunoassay or enzyme-linked immunosorbent assay (ELISA) kits can be used, as these are very sensitive and are rapidly becoming available for many drugs. For a good correlation of in vitro and in vivo studies, the principal barrier to transport should be the same.
