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Assaraf, Y.G. et al.

Cross-resistance to the lipid-soluble antifolate trimetrexate in human carcinoma cells with the multidrug-resistant phenotype.

We are studying mechanisms of resistance to hydrophilic and lipophilic antifolates in cultured mammalian cells. We determined the cytotoxicity of methotrexate and the lipid-soluble antifolate trimetrexate to various human carcinoma cells and their doxorubicin-resistant sublines. These multidrug-resistant cells were 17-fold to 26-fold more resistant to trimetrexate but as sensitive to methotrexate as their parental cells. Verapamil and quinidine, which are known to modulate the degree of pleiotropic drug resistance, reversed the cross-resistance to trimetrexate but did not alter methotrexate toxicity in multidrug-resistant cells. By flow cytometry, we show that multidrug-resistant Chinese hamster ovary cells retained eightfold more fluorescein-methotrexate bound to dihydrofolate reductase upon competition with trimetrexate than the drug-sensitive cells did. However, methotrexate displaced fluorescein-methotrexate equally well in sensitive and multidrug-resistant cells. Furthermore, verapamil produced dose-dependent displacement of fluorescein-methotrexate from the multidrug-resistant cells in the presence of low concentrations of trimetrexate but had no significant effect on displacement of fluorescein-methotrexate from sensitive cells. Hamster cells that overproduce dihydrofolate reductase by 93-fold were 145-fold and 96-fold more resistant to methotrexate and trimetrexate, respectively, than their sensitive parental cells. This form of antifolate resistance was not altered by verapamil or quinidine. We conclude that the cross-resistance to trimetrexate in cells that do not overproduce dihydrofolate reductase is associated with the multidrug-resistant phenotype. Possible implications of trimetrexate resistance in cancer chemotherapy are discussed.[1]

References

Cross-resistance to the lipid-soluble antifolate trimetrexate in human carcinoma cells with the multidrug-resistant phenotype. Assaraf, Y.G., Molina, A., Schimke, R.T. J. Natl. Cancer Inst. (1989) [Pubmed]

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