A methotrexate-resistant human breast cancer cell line with multiple defects, including diminished formation of methotrexate polyglutamates.

Methotrexate (MTX)-resistant human breast cancer cells (MTXR ZR-75) were obtained following serial passage of the wild-type ZR-75-1 cells (wild-type ZR-75) in MTX. The resistant cell line contains neither quantitative nor qualitative changes in dihydrofolate reductase compared to the parental line. Resistance is associated with a 3-fold decrease in MTX transport into MTXR ZR-75 cells as well as a 3-fold decrease in the activity of thymidylate synthetase in the resistant subline. Moreover, marked differences were observed between the wild-type and MTXR ZR-75 cells in their ability to convert MTX to its polyglutamate derivatives. Wild-type ZR-75 cells accumulate significant intracellular levels of antifolates during prolonged (24 h) exposure to 2 microM MTX, due to the formation of MTX polyglutamates. In contrast, essentially no polyglutamates are formed in the MTXR cells even during conditions which result in a vast excess of free intracellular drug in these cells. This defect is not associated with any apparent change in the activity of the enzyme folylpolyglutamyl synthetase, nor is there any alteration in the apparent Km of this enzyme for MTX in the resistant cells. Further studies demonstrate that the MTXR ZR-75 cells are cross-resistant to antifolate analogues which can be converted to polyglutamate derivatives (aminopterin and dichloromethotrexate), yet they are relatively sensitive to antifolate analogues such as 2,4-diamino-5-(3',4'-dichlorophenyl)-6-methylpyrimidine, triazinate, and trimetrexate, which cannot be converted to polyglutamate forms. These studies identify a new mechanism (diminished accumulation of MTX polyglutamates) associated with resistance to MTX and lend additional support to the hypothesis that the formation of these derivatives is an important determinant of MTX cytotoxicity.