Pharmacokinetics of 99mTc(Sn)- and 131I-labeled anti-carcinoembryonic antigen monoclonal antibody fragments in nude mice

The biodistribution, radioimmunoimaging, and high pressure liquid chromatography activity profiles of 99mTc(Sn) and 131I-labeled anti-carcinoembryonic antigen monoclonal antibody fragments were compared. Nude mice, bearing specific (colon carcinoma, LS174T) and nonspecific (pancreatic carcinoma, MIA) xenografts were given injections of the respective radiolabeled antibody fragments and also of irrelevant 125I-labeled antibody fragments (MOPC-21). The animals were imaged at 24 h after being given injections, they were sacrificed, and biodistribution studies were performed. Results of the study showed high kidney uptake [48.6% injected dose (ID)/g +/- 8.1% (SD)] and low tumor uptake (1.5% ID/g +/- 0.6%) for 99mTc(Sn)-labeled fragments and higher uptake (4.4% ID/g +/- 0.6%) for 131I-labeled fragments, resulting in a higher localization index for the radioiodinated monoclonal antibody fragments. Imaging results showed good tumor visualization at 24 h after injection for the 131I-labeled fragments and poor tumor visualization with predominant kidney uptake for 99mTc(Sn)-labeled fragments. After radiolabeling, high pressure liquid chromatography analysis indicated that 131I was primarily associated with F(ab')2 fragments, whereas 99mTc was mostly associated with Fab' fragments.