Effect of [3H]methotrexate impurities on apparent transport of methotrexate by a sensitive and resistant L1210 cell line

Transport of methotrexate by L1210 sensitive and resistant cell lines was studied using [3H]methotrexate and methotrexate. The intracellular and cellular efflux of drug was analyzed by radioligand binding assay and high performance liquid chromatography. It was shown that the initial, rapid uptake of [3H]methotrexate was not methotrexate but rather [3H]p-aminobenzoylglutamate, even when the [3H]methotrexate was greater than 97% homogeneous. In the mutant cell line, the impurity could account for all of the apparent methotrexate uptake at 1 to 10 micro M extracellular drug. Similarly, the rapid efflux of [3H]methotrexate was also shown to be all, or in part, 3H-impurities, in both the mutant and sensitive cell lines. These results do not conflict with the currently accepted model of a carrier-mediated process for reduced folate and antifolate transport but do suggest that the quantitative interpretation of the early time points of transport experiments be more critically evaluated, especially when mutant cell lines are being analyzed.