pH dependent cytotoxicity of N-dodecylimidazole: a compound that acquires detergent properties under acidic conditions

N-dodecylimidazole is a compound which acquires detergent properties under acidic conditions and might be useful in killing selectively cells in those regions of solid tumours which have a reduced extracellular pH (pHe). We have therefore studied the effects of N-dodecylimidazole against malignant cells in tissue culture. N-dodecylimidazole displayed pHe-dependent cytotoxicity against EMT-6 and MGH U1 cells; cell killing was dose dependent and was 100-fold greater at pHe 6.0 than pHe 7.0. Reduced toxicity of N-dodecylimidazole was observed at higher cell concentrations (> 10(6) cells ml-1), and only minor effects were observed against multicellular tumour spheroids. Potential mechanisms of action of N-dodecylimidazole include detergent-mediated lysis of the cell membrane at low pHe, and selective uptake into lysosomes where detergent activity leads to rupture of the lysosomal membrane and release of cytolytic enzymes. Inhibition of activity of cysteine proteases by the inhibitor E-64 did not protect cells against the toxicity of N-dodecylimidazole, suggesting that these lysosomal enzymes do not play a major role in the mechanism of action of this compound. Lysis of erythrocytes (which contain no lysosomes) was observed with low concentrations of N-dodecylimidazole. Dependence of Cell Lysis on cell concentration was similar to that observed for two other detergents that act on the plasma membrane, Triton X-100 and sodium dodecyl sulfate. We conclude that N-dodecylimidazole causes pHe dependent cell killing in two cultured tumour cell lines, and that its mechanism of action is probably due to acid mediated production of detergent activity which acts primarily on the cell plasma membrane.