In vivo evaluation of copper-64-labeled monooxo-tetraazamacrocyclic ligands

Copper-64 (T_1/2=12.7 h; β⁺: 0.653 MeV, 17.4%; β⁻: 0.578 MeV, 39%) has applications in positron emission tomography (PET) imaging and radiotherapy, and is conveniently produced on a biomedical cyclotron. Tetraazamacrocyclic ligands are the most widely used bifunctional chelators (BFCs) for attaching copper radionuclides to antibodies and peptides due to their relatively high kinetic stability. In this paper, we evaluated three monooxo-tetraazamacrocyclic ligands with different ring sizes and oxo group positions. H1 1,4,7,10-tetraazacyclotridecan–11-one], H2 1,4,8,11-tetraazacyclotetradecan-5-one] and H3 1,4,7,10-tetraazacyclotridecan-2-one] were radiolabeled with ⁶⁴Cu in high radiochemical yields under mild conditions. The three ⁶⁴Cu-labeled complexes are all +1 charged, as determined by their electrophoretic mobility. While they demonstrated >95% stability in rat serum out to 24 h, both biodistribution and microPET imaging studies revealed high uptake and long retention of the compounds in major clearance organs (e.g., blood, liver and kidney), which suggests that ⁶⁴Cu dissociated from the complexes in vivo. Of the three complexes, ⁶⁴Cu-2⁺, which has a cyclam backbone (1,4,8,11-tetraazacyclotetradecane), exhibited the lowest nontarget organ accumulation. The data from these studies may invalidate the candidacy of the monooxo-tetraazamacrocyclics as BFCs for copper radiopharmaceuticals. However, the data presented here suggest that neutral or negatively charged Cu(II) complexes of tetraazamacrocyclic ligands with a cyclam backbone (tetradecane) are optimal for copper radiopharmaceutical applications.