Methotrexate resistant cells as targets for selective chemotherapy.

Strategies that are selective for eradicating methotrexate resistant cells are described. These strategies have been developed based on knowledge of the mechanism of drug resistance encountered in experimental systems and in the clinic. Drug resistance to methotrexate in experimental tumors is commonly due to either gene amplification (dihydrofolate reductase) or to impaired transport of methotrexate. While no effective drugs or methods to prevent gene amplification have been described, the concept of developing "pro drugs", i.e. a drug that is selectively reduced by dihydrofolate reductase to an inhibitor of another critical folate enzyme (thymidylate synthase, methionine synthetase, folylpolyglutamate synthetase) remains worthwhile. Second generation antifolates such as trimetrexate which are effective vs methotrexate transport resistant cells have already been developed and are in clinical trial.