MECHANISM OF SYNERGISTIC CELL KILLING WHEN METHOTREXATE PRECEDES CYTOSINE-ARABINOSIDE - STUDY OF L1210 AND HUMAN-LEUKEMIC CELLS

Synergistic killing of L1210 cells occurs when methotrexate (MTX) is administered just before 1-β-D-arabinofuranosylcytosine (Ara-C). This phenomenon is dependent upon both the dose and time of exposure to MTX. Such increased killing of cells can be explained by the enhanced intracellular accumulation of Ara-C in cells exposed to MTX. This enhancement of Ara-C entry into cells was only observed when the dose of MTX was high enough (1, 10, and 100 μM) to result in free intracellular non-dihydrofolate reductase-bound MTX. At the highest doses of MTX (10 and 100 μM) Ara-C triphosphate was increased eightfold and deoxycytidine triphosphate was decreased by 50%. Therefore, the maximum synergistic cell kill when MTX precedes Ara-C may be the consequence of greater inhibition of DNA polymerase by the increased Ara-C triphosphate in the presence of the decreasing natural substrate of this enzyme, deoxycytidine triphosphate. Enhanced Ara-C accumulation after administratin of MTX was also observed in human acute myelogenous leukemia cells.