The use of a fluorescent methotrexate probe to monitor the effects of three vinca alkaloids on a mixed population of parental L1210 and gene-amplified methotrexate-resistant cells by flow cytometry.

Cells resistant to methotrexate (L1210/R7A) and possessing an increased level of dihydrofolate reductase due to gene amplification can be detected by the technique of flow cytofluorimetry using a new fluorescent derivative of methotrexate (F-MTX) based on a putrescine linker. Comparative studies of dihydrofolate reductase enzyme and cell growth inhibition following treatment with methotrexate and F-MTX suggest that the two agents possess similar modes of action. In an artificially mixed population of cells sensitive and resistant to methotrexate it is possible, using F-MTX, to recognise and separate distinct cell subpopulations showing differential fluorescence using a fluorescence-activated cell sorter (FACS IV). The selective removal of the resistant cells within a mixed population of sensitive and resistant cells has been demonstrated for 5 X 10(-8) M vinblastine by means of flow cytometry. The effectiveness of the vinca alkaloids decreases in the order vinblastine greater than vindesine greater than vincristine, which previously was shown to be the order of effectiveness in producing collateral sensitivity.