Synthesis of 1-(p-isothiocyanatobenzyl) derivatives of DTPA and EDTA. Antibody labeling and tumor-imaging studies

To investigate the /sup 111/In labeling of tumor-localizing monoclonal antibodies (MoAb), the chelate 1-(p-isothiocyanatobenzyl)diethylenetriaminepentaacetic acid (p-SCN-Bz-DPTA) (1) and its EDTA analogue (2) have been synthesized. By the use of a MoAb (B72.3) specific for a high molecular weight antigen (TAG-72) on cells of a colorectal carcinoma grown in nude mice, optimal chemical conditions for MoAb conjugation of those ligands and of the dicyclic and isobutylcarboxy carbonic anhydrides of DTPA and subsequent /sup 111/In labeling were determined. All conjugates were shown by a competitive binding assay to retain their specificity and activity in vitro when less than one ligand is protein coupled both prior to and after /sup 111/In labeling. Chemical methods for purification of the MoAb were systematically investigated by injection of purified immunoprotein into athymic mice bearing LS-174T tumors that express the TAG-72 antigen. Tissue distribution studies revealed that simple addition of EDTA to labeled immunoglobulins was ineffective at complexing indium not linked to protein by chelates. Similarly, gel chromatography (Sephadex G-50) was not sufficient; rather, size exclusion HPLC had to be employed to remove unreacted /sup 111/In and aggregated antibody. To compare the relative utility of the four chelates for /sup 111/In diagnostic radioimmunoimaging, scintigraphic images of tumor-bearingmore » mice were obtained and evaluated along with tissue distributions. Results showed that clear images of these solid tissue tumors free of extraneous radiation could be obtained only by using p-SCN-Bz-DTPA purified by HPLC. Methods developed are now being employed in clinical trials for diagnosis of human colorectal cancer. 71 references, 5 figures, 1 table.« less