9-Methoxy-N2-methylellipticinium
acetate (MMEA) is representative of a series of quaternized
ellipticines that exhibited selective cytotoxicity for human
brain tumor cell lines of glial origin in the in vitro primary screen of the U.S. National Cancer Institute. The present investigation was initiated to determine whether membrane potential contributes to the cellular accumulation of this lipophilic
cation by selected
brain tumor and non-
brain tumor cell lines. The results indicate that accumulation of MMEA by
drug-sensitive cell lines, but not
drug-resistant cell lines, is reduced by experimental conditions that depolarize the plasma membrane, e.g., stepped increases in the extracellular
potassium concentration. These experimental conditions result in increased cellular fluorescence of cells stained with the voltage-sensitive anionic
dye bis(1,3-dibutylbarbituric acid)trimethine oxonol, suggesting that decreased accumulation of MMEA is the result of decreased membrane potential. Membrane potential measurements using the null point method indicated that the mean membrane potential of selected MMEA-sensitive cell lines (-39.4 +/- 6.8 mV) was significantly lower (P < .005) than MMEA-resistant cell lines (-17 +/- 3.8 mV). Ultrastructural studies with the MMEA-sensitive U-251
glioblastoma indicated that the first morphological effects of MMEA occurred in mitochondria, where dissolution of cristae was observed, followed by engulfment of mitochondria in multilamellar phagocytic vesicles. Electron microscopic autoradiographic studies with
tritium-labeled MMEA revealed that the
drug was localized in mitochondria and nuclei.