Microdialysis sampling for investigations of bioavailability and bioequivalence of topically administered drugs: current state and future perspectives.

Microdialysis (MD) in the skin is a unique technique for in vivo sampling of topically as well as systemically administered drugs at the site of action, e.g. sampling the unbound tissue concentrations in the dermis and subcutaneous tissue. MD as a research method has undergone significant development, improvement and validation during the last decade and has proved to be a versatile, safe and valuable tool for pharmacokinetic and pharmacodynamic studies. This review gives an overview of the current state and future perspectives of dermal MD sampling. Methodological issues such as choice of instrumentation, calibration and experimental procedures are discussed along with the analytical considerations necessary for successful sampling. Clinical MD studies in the skin are reviewed with emphasis on pharmacokinetic studies of topically applied drugs with or without impairment of skin barrier function by skin disease or barrier perturbation. A comparison between MD and other tissue sampling techniques reveals the advantages and limitations of the method. Subsequently, an in-depth discussion of the application of MD for the evaluation of bioavailability and bioequivalence of topical formulations is concluded by the current regulatory point of view. The future perspective includes further expansion and validation of the use of MD in the experimental and clinical setting as well as in the optimization of the method for regulatory purposes, i.e. the commercialization of bioequivalent, generic drug products.