Enhancement of the activity of immunotoxins by analogues of verapamil.

Verapamil has been shown to enhance immunotoxin activity but only at concentrations that are too high for in vivo use. Therefore, four structural analogues of verapamil (D792, D595, D528, and Sz45) were evaluated for their ability to enhance the in vitro activity of immunotoxins made with either ricin A chain or Pseudomonas exotoxin. The following immunotoxins were used: HB21-PE and 454A12-rRTA which recognize the human transferrin receptor; and 260F9-rRTA which reacts with human ovarian carcinoma and breast carcinoma cells. The activities of the immunotoxins were determined in ovarian carcinoma cells and in KB cells using inhibition of either protein synthesis or colony formation as a measure for the cytotoxicity of the immunotoxins. Each of the four analogues enhanced the activity of ricin A-immunotoxins in a dose-dependent manner. D792 and D595 also increased the activity of IIB21-PE. Low concentrations of either Sz45 or D528 enhanced the activity of HB21-PE, but high concentrations of these two analogues either had less enhancing potency than low concentrations or even decreased the activity of HB21-PE. Specificity of enhancement by the analogues was shown by competition of the activity of the immunotoxins by the corresponding antibody and by inactivity of an irrelevant immunotoxin. The amount of enhancement ranged from 2-fold to greater than 60-fold and was dependent on the cell line and on the experimental conditions. The enhancing ability of the drugs did not correlate with their calcium-antagonistic activity. When compared with verapamil, D792 and D595 had greater enhancing potency with regard to both ricin A-immunotoxins and Pseudomonas exotoxin-immunotoxins. Greater enhancing potency and less in vivo toxicity makes D792 a candidate for use in the enhancement of immunotoxins in vivo.