Teniposide (VM-26)- and Etoposide (VP- 16–213)-induced Augmentation of Methotrexate Transport and Polyglutamylation in Ehrlich Ascites Tumor Cells in Vitro

Teniposide (VM-26) and etoposide (VP- 16–213) were evaluated for their effects on methotrexate transport and metabolism in Ehrlich ascites tumor cells in vitro. VM-26 (10 µM) does not alter influx of methotrexate but slows efflux of the drug which leads to an increase in the steady-state level of exchangeable antifolate. VM-26 stimulation of net methotrexate transport occurs within 5 min of exposure of cells to VM-26 concentrations of from 10 to 50 µm; removal of extracellular VM-26 results in a rapid and complete reversal of this effect. As extracellular VM-26 is raised to 50 µm, the progressive rise in net methotrexate uptake observed is due not only to elevated exchangeable antifolate but to an enlarged nonexchangeable pool of intracellular drug as well, a major portion of which is methotrexate polyglutamyl derivatives. VM-26 does not alter the intracellular water or the chloride distribution ratio. Glucose partially reverses the stimulatory effect of VM-26 but to a lesser extent than glucose reversal of the vincristine-induced stimulation of net methotrexate transport. VP- 16–213 also stimulates net methotrexate transport in a manner qualitatively similar to that of VM-26. However, 100 µm VP- 16–213 is required to yield the same quantitative stimulation as seen for 10 µm VM-26. VP- 16–213-induced augmentation of intracellular methotrexate polyglutamate levels is also demonstrated. These results indicate that VM-26 and VP- 16–213 enhance net cell accumulation of methotrexate and methotrexate polyglutamyl derivatives in the Ehrlich ascites tumor in vitro at concentrations of these agents which are achieved in vivo. Potential therapeutic synergism between methotrexate and epipodophyllotoxins on this basis is therefore possible and is currently under evaluation in animal tumor systems.