A New Bifunctional Chelating Agent Conjugated with Monoclonal Antibody and Labelled with Technetium-99m for Targeted Scintigraphy: 6-(4-isothiocyanatobenzyl)-5,7-dioxo-1,11-(carboxymethyl)-1,4,8,11-tetraazacyclotridecane

Objective: The purpose of this study was to obtain the convenient, synthetically useful bifunctional chelating agent, 6-(4-isothiocyanatobenzyl)-5,7-dioxo-1,11-(carboxymethyl)-1,4,8,11-tetraazacyclotridecane, and to apply it to stable ^99mTc-labelling of monoclonal antibodies (mAbs). Methods: The chelate was synthesised by reaction of nitrobenzyl malonate and triethylenetetramine followed by alkylation by reacting with bromoacetic acid at pH 10. The amino group was converted to isothiocyanato derivative by reacting with thiophosgene at pH 2.0. Conjugation with mAbs [(anti-carcinoembryonic antigen (CEA) and anti-epidermal growth factor receptor (EGFr)] was performed at pH 8.4 using trisodium phosphate solution by incubating at 37°C for 1 h and subjected to purification on size exclusion chromatography. Results: When radioimmunoconjugates were labelled with ^99mTc, the specific activity of immunoconjugates was 20–30 mCi/mg of protein and their immunoreactivity exceeded 80%. The stability in serum indicated that the metal remained bound to antibodies. Biodistribution studies in athymic mice grafted with U-87 human glioblastoma multiforme and MDA-MB-468 human breast carcinoma tumours revealed significant localisation of ^99mTc-labelled antibodies in tumours and reduced accumulation in normal organs. Conclusion: This bifunctional chelating agent is promising for immunoscintigraphy because of good tumour-to-normal organ contrast.