Reversal of methotrexate inhibition of colony growth of L1210 leukemia cells in semisolid medium.

L1210 mouse leukemia cells were grown in a methylcellulose-based medium, and the inhibitory effect of methotrexate (MTX) on colony formation and its reversal were examined. The effect on colony formation was studied in order to compare the results with those obtained with normal mouse bone marrow cells grown in a similar manner in previous studies and in additional experiments presented. Light microscopy could not be used for colony counting of L1210 cells because MTX did not inhibit colony formation and only affected further colony growth. Therefore, it was necessary to evaluate the toxic effect of MTX by analysis of colony size distribution using an electronic image analyzer. Results show that the reversal of MTX toxicity to L1210 cells with leucovorin is competitive and is similar to that with normal mouse bone marrow cells. Thymidine in combination with a purine prevents MTX toxicity as well. The optimal concentration of thymidine is 10(-5) M, whereas at least 10(-4) M purine is required. Reversal of MTX toxicity by thymidine and purines is independent of the MTX concentration and is possible at drug concentrations as high as 10(-4) M. Compared to mouse bone marrow cells, L1210 cells appear to require more purines to prevent the toxic effects of MTX. MTX toxicity towards bone marrow myeloid precursor cells can be reversed by 10(-4) M inosine alone. These cells are better protected against MTX toxicity when 10(-6) to 10(-5) M thymidine is added. The results suggest that the use of a high-purine-low-thymidine combination has advantages over the use of leucovorin in controlling toxicity over a wide range of MTX concentrations and in providing some degree of selective protection to normal proliferating cells.