TRANSPORT OF THE ANTITUMOR ANTIBIOTIC CL-920 INTO L1210 LEUKEMIA-CELLS BY THE REDUCED FOLATE CARRIER SYSTEM

  • 1 January 1984
    • journal article
    • research article
    • Vol. 44  (8) , 3366-3370
Abstract
CI-920 is a structurally novel antitumor antibiotic which has activity against a wide spectrum of tumor cells in vitro and is curative in [mouse] L1210 leukemia in vivo. Several lines of evidence indicate that this drug penetrates L1210 cells via the reduced folate carrier system. Reduced folates (100 .mu.M) including leucovorin and 5-methyltetrahydrofolate completely protected L1210 cells from growth inhibition by CI-920. Protective effects were not observed with folic acid, a compound which is transported by a process distinct from that for reduced folates. CI-920 was a potent inhibitor of methotrexate influx exhibiting a mixture of competitive and noncompetitive inhibition and having a Ki (slope) of 30.0 .mu.M and a Ki (intercept) of 58.8 .mu.M. The inhibition appeared to be irreversible since, after cells were preincubated with drug, the inhibitory effects persisted after cells were washed in drug-free media. The irreversibility could be eliminated by dithiothreitol, suggesting that CI-920 may interact with a thiol which is essential to this transport system. Cells made 71-fold resistant to CI-920 by continuous exposure to increasing concentrations of this drug were 245-fold cross-resistant to methotrexate but were collaterally sensitive to the lipophilic antifolate trimetrexate and contained normal levels of dihydrofolate reductase. This mutant cell line had a severely impaired reduced folate carrier system exhibiting methotrexate influx rates of < 1% of control cells. Inhibition of methotrexate influx by a number of CI-920 analogs showed that the intact lactone ring and the presence of the phosphate ester were required for maximum interaction with the carrier system and that the degree of inhibition correlated with relative antitumor potency. These observations are compatible with the concept that CI-920 utilizes the folate carrier system and could be of fundamental importance for understanding the cytotoxicity and selectivity of CI-920.