Rescue therapy

High doses of methotrexate may be given for 36–48 h and terminated before severe toxicity occurs to cells of the gastrointestinal tract and bone marrow. Leucovorin, a form of tetrahydrofolate that is accumulated more readily by normal than by neoplastic cells, is then administered. This results in rescue of the normal cells because leucovorin bypasses the dihydrofolate reductase step in folic acid synthesis.