NCIt definition : A folic acid antagonist and mammalian dihydrofolate reductase inhibitor with antineoplastic
activity. Ketotrexate inhibits dihydrofolate reductase, an enzyme that reduces dihydrofolic
acid to tetrahydrofolic acid which is essential for the synthesis of purine nucleotides
and thymidylate. By depleting tetrahydrofolic acid availability, DNA synthesis is
halted.;