Synergistic Cytotoxic Effect on Gastric Cancer Cells of an Immunotoxin Cocktail in Which Antibodies Recognize Different Epitopes on CDH17

Cadherin-17 (CDH17) is highly expressed in gastric cancer and is thus considered to be a good target for antibody therapy. CDH17 is classified as a nonclassical cadherin, in that it is composed of seven extracellular cadherin domains. We generated anti-CDH17 monoclonal antibodies (mAbs) which recognize the extracellular domain of CDH17. Competitive assay using AGS, a gastric cancer cell line, cells revealed that five selected anti-CDH17 mAbs recognize different epitopes on CDH17. As AGS cells were shown to exhibit broad expression pattern of CDH17 by flow cytometry, we separated three clones with a low (10,000/cell), medium (50,000/cell), and high (200,000/cell) expression level, designating them as AGSlow, AGSmed, and AGShigh, respectively. The mAbs, coupled with saporin, exhibited effective cytotoxicity to AGShigh, but poor cytotoxicity to AGSlow. By contrast, the immunotoxin cocktail using the three clones D2101, D2005, and D2008, which recognize different epitopes, exhibited efficient cytotoxicity, even to the AGSlow group. The effect of the immunotoxin cocktail is synergistic, as the combination index was demonstrated to be below 1.0, as calculated by the method of Chou and Talalay using CalcuSyn software. These results suggest that the immunotoxin cocktail targeted to multiple epitopes has synergistic effects on low expression level cells, which expand the applicable range of immunotoxin therapy for cancer.