Verapamil enhances the toxicity of conjugates of epidermal growth factor with Pseudomonas exotoxin and antitransferrin receptor with pseudomonas exotoxin

Abstract
Verapamil, a clinically important calcium channel blocker, has been found to cause a 40‐fold enhancement of killing of the human KB cell line by a cytotoxic conjugate of epidermal growth factor with Pseudomonas exotoxin (EGF‐PE). Synergistic effects of verapamil and EGF‐PE are also seen on HeLa D98 cells and a human epidermal carcinoma cell line, A431. Verapamil also potentiates the effect of a toxic conjugate formed between Pseudomonas exotoxin and a monoclonal antibody to the human transferrin receptor (anti‐TFR‐PE) and enhances the effect of Pseudomonas exotoxin (PE) alone. Two other calcium antagonists were tested. Diltiazem enhances the cytotoxic effect of EGF‐PE, but nifedipine does not. Verapamil does not affect the binding and uptake of 125I‐EGF by KB cells, but it significantly delays the disappearance of internalized 125I‐EGF from the cells. Density gradient fractionation studies using cell homogenates suggest that 125I‐EGF accumulates in an undegraded form in lysosomes when cells are treated with verapamil. By immunofluorescence microscopy using an antibody to PE, EGF‐PE was found to accumulate in lysosomes; by electron microscopy the lysosomes had an abnormal appearance. The effects of verapamil on toxicity of EGF‐PE and lysosomal function appear to be related. However, it is not known whether the enhanced toxicity of EGF‐PE in the presence of verapamil is due to its delayed degradation in lysosomes or some more general effect of verapamil on membrane permeability.