IN-VIVO INSTABILITY OF REDUCTION-MEDIATED TC-99M-LABELED MONOCLONAL-ANTIBODY

A murine monoclonal antibody that reacts with human osteogenic sarcoma (OST7) was reduced and directly labeled with Tc-99m without any loss of immunoreactivity. No fragmentation of the antibody was detected by high performance liquid chromatography after the labeling. However, SDS PAGE analysis of the labeled antibody demonstrated the presence of low molecular weight species. Although more than 95% of the radioactivity remained bound at the antibody after incubation with human serum for 24 h, Tc-99m-labeled OST7 was cleared faster from the circulation than I-125-labeled OST7 or In-111-labeled OST7 in mice. Urinary and fecal excretion of Tc-99m were higher than those of I-125. When the I-125-labeled antibody was dual-labeled with Tc-99m, the blood clearance of Tc-99m was faster than that of I-125, suggesting release of Tc-99m from the antibody in vivo. Tc-99m-labeled OST7, however, gave a higher tumor-to-blood ratio than I-125- or In-111-labeled OST7 in mice bearing human osteogenic sarcoma. The Tc-99m-labeled antibody prepared by the direct method was unstable in vivo, but retained a good tumor targeting ability.