Transchelation of 99mTc from low affinity sites to high affinity sites of antibody.
An exclusive labeling of high affinity sites of IgG and its F(ab')2 fragments with 99mTc was accomplished. Antibody was first labeled in 0.1 M acetate buffer at pH 4.5, using stannous chloride as a reducing agent. Thus, high capacity, low affinity sites and low capacity, high affinity sites were both labeled. These 99mTc complexes were stable at pH 4.5 and 7.0; however, they became destabilized at pH 8.2 and 9.0. Transchelation of 99mTc to DTPA took place at the higher pH values and leveled off at 54% 99mTc-F(ab')2 and 73% 99mTc-IgG. These results indicate that the majority of 99mTc bound to the low affinity sites was transchelated to the high affinity sites rather than to DTPA since low affinity sites account for 84% of total F(ab')2 sites and 76% of IgG sites. Biodistribution data in mice at 2.5 h postinjection were consistent with this hypothesis in that tissue concentrations of 111In-DTPA-F(ab')2 were similar to the reequilibrated 99mTc-F(ab')2 but were much higher than that of the unequilibrated 99mTc-F(ab')2.
D000818:Animals; D001667:Binding, Competitive; D006801:Humans; D007140:Immunoglobulin Fab Fragments; D007074:Immunoglobulin G; D007700:Kinetics; D051379:Mice; D004369:Pentetic Acid; D011485:Protein Binding; D012709:Serum Albumin / Q000276:immunology; D012710:Serum Albumin, Bovine / Q000276:immunology; D013667:Technetium; D014018:Tissue Distribution
D007140:Immunoglobulin Fab Fragments; D007074:Immunoglobulin G; D012709:Serum Albumin; D012710:Serum Albumin, Bovine; D013667:Technetium; D004369:Pentetic Acid