Summary Marker Transport across biological barriers in vitro: Comparison of cell culture models for the gastrointestinal barrier, the blood-brain barrier and the alveolar epithelium of the lung

In order to respond to the flood of new active ingredients currently being generated by combinatorial chemistry or molecular biological synthesis, sel ection procedures able to filter out rapidly and economically those drug ca ndidates with the highest development potential are required. This necessit ates the measurement of fundamental biopharmaceutical parameters very early in the drug development process. Any pharmaceutically active agent must be able to overcome the body's natural protective mechanisms. A broad variety of biological barriers can be simulated in the laboratory by cell monolaye r models. Apart from ethical aspects, the advantage of these in vitro test systems is that permeability studies can tie performed at high throughput r ates under controlled and reproducible conditions. The validity of such a m odel is ultimately reflected in its ability to accurately predict the behav iour of an active ingredient at the corresponding in vivo barrier.