Cytotoxicity of Compounds That Interfere with the Regulation of Intracellular pH: A Potential New Class of Anticancer Drugs1

The extracellular pH (pi I,.) in many solid tumors is often lower than in normal tissues. Cells may survive conditions of acid pi 1, because antiports in their membrane exchange Na* for 11+,or 11CX)., for Cl~, and thus regulate the intracellular pH (pH,). We have therefore assessed the effects of drugs which interfere with regulation of pi I; on survival of Chinese hamster ovary and human bladder cancer MGH-U1 cells in tissue culture. Nigericin, an ionophore which acidifies the cytoplasm when cells are placed in medium at low pi I,.,was not toxic at pi I, 6.5 or above but became very toxic as pi I, was reduced below this value. Amiloride and 4,4'-diisothiocyanostilbene 2,2-disulfonic acid, inhibitors of the Na*/H* and HCO3~/Cr exchangers, respectively, decreased pH, in the presence of nigericin at low pi I...These drugs showed little or no toxicity in the pi I. range of 6.0-7.0 but added greatly to the toxicity of nigericin. A combination of all three drugs led to toxicity in the pi I,. range of 6.5-6.8, well within the measured range of tumor pH, but not at pH, 7.0 or above. A combination of low pH and hypoxia, two conditions likely to be found in regions distant from tumor blood vessels, caused cell mortality in the absence of drugs, and this effect was increased by nigericin used alone or in combination with amiloride and 4,4'-diisothiocyanostilbene 2,2-disulfonic acid. These drugs may be regarded as pro totypes for potential new anticancer agents that might achieve selective killing of tumor cells by interfering with the regulation of intracellular pH.