The labeling of DTPA-coupled proteins with 99mTc.

We have determined that reduced 99mTc will often bind to proteins even in the presence of DTPA at reasonable concentrations. The extent of this competition varies with the protein but many possess this property. We have shown that, despite this competition, DTPA-coupled proteins can be labeled with 99mTc and that this labeling can be achieved at near-neutral pH using stannous ion for reduction. Both lysosyme and IgG antibody were labeled with 99mTc after coupling with DTPA and their biodistributions determined at 1-2 h in mice along with the 111In-labeled proteins as controls. The results of these animal studies show that the proteins are largely labeled at the DTPA sites and that they exhibit in vivo stabilities comparable to that of the 111In labeled proteins.