Tumor models in drug development.

In this short essay, we have taken the opportunity to review briefly the history of anticancer drug screening, consider the changes that have been made throughout that history, and reflect on the suitability of current screening practices and the models employed. A major change in emphasis in drug discovery has influenced the development and selection of new model tumor systems as well as screening practices. This new direction, a search for drugs that are selective for particular tumor histotypes, especially solid tumors, was stimulated by the paucity of drugs that have clinical solid tumor activity. The new approach to drug discovery and screening is in itself an experiment. Only time will tell if this approach is successful.