Application of anti-sialyl Lea monoclonal antibody, KM231, for immunotherapy of cancer.

A mouse monoclonal antibody (MoAb), KM231 raised against human gastric cancer was found to recognize sialyl Lea -epitope expressed on glycoprotein and glycolipid with high affinity. KM231 reacted with many human gastrointestinal cancer tissues and could detect the antigen shed in sera of cancer patients. The present study was designed to evaluate competence of KM231 for immunotherapy of cancer. We first confirmed that KM231 could probe the cancer cells in vivo by injecting biotinylated KM231 into nude mice bearing human colorectal carcinoma cell, SW1116. Light- and electron-microscopic examination showed that the MoAb was localized in the tumor tissues and bound to the plasma membrane and cytoplasmic endosomes. Imaging studies with 125I-labeled KM231 revealed specific localization of the antibody in SW1116 tumors transplanted into nude mice. From Scatchard analysis of KM231 binding, the number of KM231 molecules bound to per SW1116 cell was calculated approximately 1.9 x 10(6) and the association constant was 1.3 x 10(8) liter/mol. We made KM231-ricin A chain immunotoxin for evaluating the tumoricidal effect of KM231. The immunotoxin exerted strong cytotoxicity toward sialyl Lea-expressing tumor cells specifically in vitro, but not toward sialyl Lea non-expressing cells. The in vivo tumoricidal effect of the immunotoxin was examined on ascites and subcutaneous xenograft tumors in nude mice. Three intraperitoneal injections of the immunotoxin (1.6 x 10(-6) mol) into nude mice bearing SW1116 ascites tumor resulted in extension of survival by 204% compared with controls. Further, repeated intraperitoneal administration of the immunotoxin (1.4 - 2 x 10(-6) mol) significantly inhibited the growth of established subcutaneous tumor (ratio of tumor inhibition = 0.7 - 0.54). These results indicated that KM231 has the ability to probe sialyl Lea-expressing tumor cells in vivo with high efficiency and to become tumoricidal drug when it conjugated with cytotoxic reagents like ricin A chain.