Cross resistance to esters of methotrexate in a doxorubicin-resistant subline of P388 murine leukemia

Resistance to methotrexate was developed by continuous exposure of P388 murine leukemia cells in vitro to increasing concentrations of methotrexate up to 1 X 10(-7) M. Once established, the resistance to methotrexate was stable. This was also found in methotrexate-resistant cells that were maintained in methotrexate-free medium for more than 4 months. The sensitivity of the methotrexate-resistant P388 cells to doxorubicin was comparable to the sensitivity measured in the parental cell line. Another methotrexate-resistant cell line was developed, in a similar way, from doxorubicin-resistant P388 cells. This methotrexate-resistant cell line maintained its original resistance to doxorubicin. In methotrexate-sensitive cells, the dimethyl and dibutyl esters of methotrexate were 18.3- and 2.7-fold less active, respectively, than the free methotrexate in inhibiting cell growth. In methotrexate-resistant cells, the inhibitory effect of the methotrexate dimethyl ester was similar to its effect on the methotrexate-sensitive cell line. The activity of the methotrexate dibutyl ester was 3.3-fold lower than its activity in the parental cell line. However, both esters of methotrexate were much more active than free methotrexate in the methotrexate-resistant cell line. In the doxorubicin-resistant cell line, the activity of free methotrexate was comparable to its activity in the doxorubicin-sensitive parent cell line. However, this cell line was remarkably resistant to the ester analogs of methotrexate. The clinical implications of these findings are discussed.