Labeling of antibodies

A radiolabeled binding protein comprising:

• (i) a single domain antibody fragment VHH,
• (ii) a polyethylene glycol (PEG) of a molecular weight of at least 15 kDa,
• (iii) a radionuclide, wherein the radionuclide is rubidium-82, copper-61, copper-62, copper-64, yttrium-86, gallium-68 or zirconium-89, and
• (iv) a chelating moiety, wherein the chelating moiety is 1,4,7-triazacyclononane-triacetic acid (NOTA), 1,4,7,10-tetraazacyclododecane-tetraacetic acid (DOTA), triazacyclononane-phosphinate (TRAP), or desferrioxamine (DFO), wherein the radionuclide is bound by the chelating moiety;
• the chelating moiety and the antibody fragment VHH are joined by a first linker, wherein the first linker comprises an amino acid sequence comprising a sortase recognition motif;
• the PEG of a molecular weight of at least 15 kDa and the first linker are joined together by a second linker;
• the first linker and the second linker together comprise the structure:
• wherein the chelating moiety is attached at the cysteine (Cys), the PEG of a molecular weight of at least 15 kDa is attached through the position indicated by an x, and the leucine (Leu) is at the C-terminus of the antibody fragment VHH;
• the ratio of the antibody fragment VHH to the chelating moiety is 1:1;
• and the ratio of the antibody fragment VHH to the PEG of a molecular weight of at least 15 kDa is 1:1.