Comparative dual label study of first and second generation antitumor-associated glycoprotein-72 monoclonal antibodies in colorectal cancer patients.

Radiolabeled first-generation anti-tumor-associated glycoprotein-72 (TAG-72) monoclonal antibody (MAb), B72.3, has proven useful in detecting primary and secondary colorectal carcinoma. It has been anticipated that the development of second-generation, higher affinity, anti-TAG-72 MAbs, CC49 and CC83, would be of greater use in cancer detection and of value in radioimmunotherapy of human cancer. We compared the pharmacokinetics, biodistribution, and immune responses of 131I-labeled CC49 and CC83 to 125I-labeled B72.3 by preoperatively coninjecting dual-labeled MAbs into 16 colorectal cancer patients. The imaging properties of radiolabeled CC49 and CC83 were also assessed. Pharmacokinetics of all three MAbs were identical, and there were no differences in the uptake of any of three MAbs in tumor and normal tissues. Maximum tumor uptake was 0.0041% of the injected dose/g for 125I-B72.3, 0.0024% for 131I-CC49, and 0.0029% for 131I-CC83. Radiolabeled CC49 and CC83 detected most known tumor sites on scintigrams without any clear advantage for either MAb. Nonspecific splenic and testicular uptake was frequently observed. Anti-idiotypic human anti-mouse antibody responses were seen more frequently with B72.3 than with CC49 or CC83. We conclude that higher affinity, radiolabeled anti-TAG-72 MAbs can detect colorectal cancer but do not penetrate these tumors more effectively than B72.3. Improvements in tumor detection and radioimmunotherapeutic strategies will likely require the administration of smaller fragments of MAb molecules or novel delivery systems rather than the continued development of higher affinity MAbs.