[The effect of methotrexate on intracellular nucleotide pools]

In a previous study, we showed that methotrexate (MTX) enhanced the intracellular production of ara-CTP. The study described in the present paper has elucidated the mechanism of this MTX-enhanced ara-CTP production, which occurs as a result of MTX reducing the intracellular dCTP pool, and the decreased dCTP pool then allowing activation of deoxycytidine Kinase. One of the mechanisms of synergistic interaction between 1-beta-D-arabinofuranosylcytosine (ara-C) and MTX might therefore be the stimulated phosphorylation of ara-C with MTX-activated deoxy-cytidine kinase. Using high-pressure liquid chromatography, the sequential changes occurring in the acid-soluble intracellular nucleotide pools of L1210 mouse leukemic cells were analysed after treatment with MTX (12mg/kg). At 3 hr after treatment with MTX a reduction of the dTTP pool to 46% of the control level was observed. The dCTP pool was also reduced to 36% of the control level after treatment with MTX. The levels of the dATP and dGTP pools were not significantly changed, at least during the observation period. Pyrimidine ribonucleotide pools were almost unchanged at 3 hr, but in the sequential changes observed in purine ribonucleotide pools after treatment with MTX, both diphosphate and triphosphate pools were seen to be on the decline, the reduction of triphosphate pools being especially marked. A decline in ATP and GPT to 24-30% of control levels was observed at 3 hr after treatment with MTX.