Second antibody clearance of radiolabeled antibody in cancer radioimmunodetection.

The imaging of tumors using radiolabeled antibodies previously has required the implementation of computer-assisted subtraction techniques to reduce background radioactivity. A decrease in radioactivity in the blood of hamsters bearing human colonic tumor xenografts has been achieved by administering a second antibody directed against a radiolabeled primary antibody to carcinoembryonic antigen (CEA). This method was found to reduce the level of blood radioactivity by a factor of 4 within 2 hr after injection of the second antibody and to enhance tumor/nontumor ratios within 24 hr. Unlike liposomally entrapped second antibody, the primary anti-CEA antibody did not show increased accretion of radioactivity in the liver, spleen, or other major organs. These results suggest that administration of a second antibody alone may improve tumor imaging with a radiolabeled antitumor antibody.