Enhancement of the specific cytotoxicity of a breast cancer-associated antigen immunotoxin by the carboxylic ionophore monensin

Monensin is a carboxylic ionophore which dissipates proton gradients across cell membranes. Monensin is known to potentiate the cytotoxic activity of immunotoxins (antibody-toxin conjugates) directed against several human tumor-associated antigens. We have investigated the effect of monensin on an immunotoxin cytotoxic to the human breast cancer cell line MCF-7. This immunotoxin is composed of an antibody directed against a human breast cancer membrane antigen, and ricin A chain, which has been produced by recombinant DNA techniques. In a 16-hour cytotoxicity assay, monensin reduced 34-fold the median inhibitory dose, from 1.4 X 10(-8)M (without monensin) to 4.1 X 10(-10)M (with monensin). In timed cytotoxicity assays, 50% of control protein synthesis was reached in immunotoxin treated cells 8-fold faster in the presence of monensin (0.5 hours) than in its absence (4 hours). Monensin produced no enhancement of immunotoxin effect on a control cell line, nor on a control immunotoxin on MCF-7 cells, demonstrating specificity of monensin effect. In addition, specific immunotoxin alone or with monensin produced no toxicity on MCF-7 cells maintained at 23 degrees C. These results suggest that both binding and internalization of immunotoxin are necessary for the monensin effect. Monensin was a potent enhancer of immunotoxin effect on human breast cancer cells. This effect occurs without the presence of ricin B chain in the conjugate.