[In vitro permeability studies as a substitute for in vivo studies--which requirements have to be met?]

In order to reduce costs and shorten time-to-market, the permeability of drug substances can be characterized by in vitro techniques including the use of cell and tissue models. It is required to apply appropriate quality standards similar to those used in animal or clinical studies. The Caco-2 cell model is a well-established and recognized in vitro technique in research, industry and in the drug discovery sector. However, the lack of standardized operating is reflected in the heterogeneity of the data acquired using this model. Using the Caco-2 cell model as an example, this paper demonstrates how to test the suitability of a cell culture model for conducting drug permeability studies. The procedures involve not only the characterization of cell morphology but also functional assessment of the model and the periodic testing of monolayer permeability. Both the useful comparison of permeability data and the reliable prediction of in vivo permeability require prior classification of the in vitro permeability in accordance with the FDA"s Biopharmaceutics Classification System (BCS).