Optimum conditions for labeling of DTPA-coupled antibodies with technetium-99m

The covalent attachment of strong chelating groups such as DTPA to IgG antibodies may simplify the labeling of these proteins with 99mTc and may improve the stability of the label. Accordingly, we have investigated the labeling of DTPA-coupled antibodies by determining the effect of DTPA:tin molar ratio, pH, and DTPA concentration. We have determined that the optimum conditions are a molar ratio of 1:1.5, a pH of 4, a DTPA concentration as high as possible, and an antibody concentration as low as possible. Using these conditions, a DTPA-coupled antibody was labeled with 99mTc and its stability in 37 degrees C serum compared with that of the uncoupled antibody labeled in the identical fashion. High performance liquid chromatographic analysis of the incubates showed that the coupled antibody lost its label slowly compared to the uncoupled antibody. Both labeled antibodies were also administered to normal mice along with 111In-labeled coupled antibody as a further control. Biodistribution results obtained at 1 hr and 20 hr confirm the increased stability of the label in the case of the coupled antibody and provide evidence for redistribution of the 99mTc following catabolism at sites of localization. To obtain the above results, however, it was necessary to attach an average of two to five DTPA groups per antibody molecule. Furthermore, it was not possible to reduce to negligible levels nonspecific binding of 99mTc to the antibody.