High radiolabeling efficiency, preferably to high specific activity, and good stability of the radioimmunoconjugate are essential features for a successful immunoconjugate for imaging or therapy. In this study, the radiolabeling efficiency, in vitro stability and biodistribution of immunoconjugates with eight different bifunctional chelators labeled with 64 Cu were compared. The anti-CD20 antibody, rituximab, was conjugated to four macrocyclic bifunctional chelators (p-SCN-Bn-DOTA, p- , three DTPA derivatives (p-SCN-Bn-DTPA, p-SCN-CHX-A"-DTPA and ITC-2B3M-DTPA) and a macrobicyclic hexamine ("sarcophagine") chelator (sar-CO 2 H) = (1-NH 2 -8-NHCO(CH 2 ) 3 CO 2 H)sar where sar = sarcophagine = 3, 6,10,13,16,19-hexaazabicyclo[6.6.6]icosane). Radiolabeling efficiency under various conditions, in vitro stability in serum at 37°C and in vivo biodistribution and imaging in normal mice over 48 h were studied. All chelators except sar-CO 2 H were conjugated to rituximab by thiourea bond formation with an average of 4.9 +/− 0.9 chelators per antibody molecule. Sar-CO 2 H was conjugated to rituximab by amide bond formation with 0.5 chelators per antibody molecule. Efficiencies of 64 Cu radiolabeling were dependent on the concentration of immunoconjugate. Notably, the 64 Cu-NOTA-rituximab conjugate demonstrated highest radiochemical yield (95%) under very dilute conditions (31 nM NOTA-rituximab conjugate). Similarly, sar-CO-rituximab, containing 1/10 th the number of chelators per antibody compared to other conjugates retained high labeling efficiency (98 %) at an antibody concentration of 250 nM. In contrast to the radioimmunoconjugates containing DTPA derivatives, which demonstrated poor serum stability, all macrocyclic radioimmunoconjugates were very stable in serum with <6 % dissociation of 64 Cu over 48 h. In vivo biodistribution profiles in normal female Balb/C mice were similar for all the macrocyclic radioimmunoconjugates with most of the activity remaining in the blood pool up to 48 h. Whilst all the macrocyclic bifunctional chelators are suitable for molecular imaging using 64 Cu-labeled antibody conjugates, NOTA and sar-CO 2 H show significant advantages over the others in that they can be radiolabeled rapidly at room temperature, under dilute conditions resulting in high specific activity. Europe PMC Funders Group

A tris(hydroxypyridinone) chelator coordinates the PET imaging isotope, 89Zr4+, rapidly and quantitatively under ambient conditions, but a 89Zr-labelled tris(hydroxypyridinone)-immunoconjugate is not stable to in vivo demetallation.

Peptides and proteins may be tagged with metallic elements in order to use them as imaging reporters or for other applications. The polypeptide of interest is first conjugated to a suitable chelating agent that forms stable complexes with the element of interest. This conjugation step is undertaken either in aqueous or in non-aqueous conditions depending on the solubility of the substrate. For polypeptides of greater than approximately 10 kDa in size, this is normally done in aqueous medium. Most commonly the chelators are reacted with lysine amino groups. The protein is first desalted into a suitable buffer at pH 8-9 and a molar excess of a bifunctional chelating agent is added. After a suitable period of incubation, excess, unreacted or hydrolyzed chelator is removed and the protein conjugate is desalted into an acidic buffer. The conjugate can then be tagged by addition of a suitable metal salt followed, if necessary, by removal of unchelated metal. As described in the protocol that follows, the entire conjugation, purification and labeling procedure takes about 2 d.