Anomalies in reduction-mediated technetium-99m labelling of monoclonal antibodies

A reduction-mediated technetium-99m labelling method has been evaluated with a range of tumour-specific monoclonal antibodies. Antibodies reduced with 2-mercaptoethanol (2-ME) had free sulphydryl groups, but their number was much higher than could be accounted for by only limited intra-chain reduction of disulphide bonds. Reduced antibody could be labelled efficiently with 99mTc using an methylene diphosphonate (MDP) bone-scanning kit, although this seemed to depend on the presence of residual 2-ME in the preparations. With a carcinoembryonic antigen (CEA)-specific antibody, immunoreactivity of labelled antibody was confirmed, and after injection into nude with CEA-producing xenografts there was localisation into the tumours. Sephacryl S300 gel filtration showed the radiolabel eluting at a single discrete peak at the expected 150 kDa. However, examination of all of the labelled antibodies by polyacrylamide gel electrophoresis (PAGE) and autoradiography showed the presence of a large number of radiolabelled low molecular weight degradation products of the antibodies. These degradation products seemed to be formed in previously reduced antibodies during processing for PAGE, indicating some fragility of the reduced antibody.