Kinetic correlates of methotrexate transport and therapeutic responsiveness in murine tumors

Net accumulation of methotrexate by carrier-mediated transport in different murine tumor cells in vitro exhibits a positive correlation with the relative drug pharmacokinetics and therapeutic responsiveness in these tumors in vivo. The transport of methotrexate by Sarcoma 180, Ehrlich carcinoma, P388, P288, and L1210 leukemia cells is qualitatively similar. Influx of drug exhibits saturation kinetics and is highly temperature dependent (Q10, 6.1 to 9.4). Efflux of exchangeable methotrexate from all of the different tumor cells exhibited first-order kinetics and the same high temperature dependence seen for influx (Q10, 6.1 to 8.0). The major kinetic determinant of responsiveness is the Km for influx. Values vary from 3.1 to 11.2 X 10(-6) M and are highest in cells from a nonresponsive Sarcoma 180 tumor, somewhat lower in the poorly responsive Ehrlich tumor, lower in moderately responsive P388 and P288 leukemias, and lowest in the highly responsive L1210 leukemia. Values for the influx Vmax differ to some extent, but in a manner not correlatable with responsiveness. The level of responsiveness of the P388 leukemia in vivo can also be partially attributed to an efflux rate that is lower than that measured for the other tumor cells. Steady-state levels of drug accumulation in vitro reflected influx and efflux rates and were consistently correlatable with therapeutic responsiveness. There was no significant difference in the extent to which folate and reduced 5-substituted folate derivatives compete with methotrexate for uptake in cells from all five tumors. The average value for Ki measured with folate for each tumor cell type was 50- and 80-fold higher than for 5-formyltetrahydrofolate and 5-methyltetrahydrofolate.