Effect of methotrexate on the cell cycle of L1210 leukemia

The influence of methotrexate (MTX) on the proliferative activity of cells in different phases of cell cycle has been studied. MTX (5 mg/kg) was injected i.p. 3 days after the inoculation of 5 X 10(6) leukemia cells, into F1 (DBA X C57 BL) mice. It was shown that MTX causes degeneration of cells, being in G1- as well as in S-phase at the time of drug injection. Incorporation of 3H-TdR was suppressed for a period ranging from 2 to 12 hr after MTX administration, which is demonstrated by the decrease in the number of grains per cell. The number of cells labeled after 3H-TdR injection was also sharply decreased during this period. For a period of 3 until 15 hr after MTX administration the mitotic index decreased significantly as a result of inhibition of DNA synthesis. The blocking of the G1-S transition was evident during 4 hr after MTX. Thereafter the G1-S transition proceeds at a rate which is practically equal to that for nontreated controls. MTX did not inhibit transition to mitosis of cells being in G2-phase and in a very late S-phase at the time of drug injection. The sensitivity of G1-cells to the cytocidal effect of MTX shows that for L1210 leukemia cells MTX can be classified as a cycle-specific drug killing both G1-and S-cells rather than S-phase specific agent with self-limitation.