Tumor scintigraphy by the method for subtracting the initial image with technetium-99m labeled antibody

The method for subtracting the initial image from the localization image wa s evaluated for radioimmunoscintigraphy of tumors with technetium-99m (Tc-9 9m) labeled antibodies. Monoclonal antibodies were parental mouse and mouse -human chimeric antibodies to carcinoembryonic antigen (CEA), designated F1 1-39 and ChF11-39, respectively, both of which have been found to discrimin ate CEA in tumor tissues from the CEA-related antigens. After reduction of the intrinsic disulfide bonds, these antibodies were labeled with Tc-99m. I n vivo studies were performed on athymic nude mice bearing the human CEA-pr oducing gastric carcinoma xenografts. Though biodistribution results showed selective and progressive accumulation of Tc-99m labeled antibodies at the tumor site, high radioactivity in blood was inappropriate for scintigraphi c visualization of the tumors within a few hours. We examined the subtracti on of the initial Tc-99m image from the Tc-99m localization image after a f ew hours. Subtracted images of the same count reflected the in vivo behavio r of the Tc-99m radioactivity. The subtracted scintigrams revealed excellen t tumor images with no significant extrarenal background. Visualization of the tumor site was dependent on antigen-specific binding and nonspecific ex udation. These results demonstrate that a method of subtraction of the init ial image may serve as a potentially useful diagnostic method for an abnorm al site for agents with a low pharmacokinetic value.