New macrobicyclic chelator for the development of ultrastable 64Cu-radiolabeled bioconjugate

Ethylene cross-bridged cyclam with two acetate pendant arms, ECB-TE2A, is known to form the most kinetically stable (64)Cu complexes. However, its usefulness as a bifunctional chelator is limited because of its harsh radiolabeling conditions. Herein, we report new cross-bridged cyclam chelator for the development of ultrastable (64)Cu-radiolabeled bioconjugates. Propylene cross-bridged TE2A (PCB-TE2A) was successfully synthesized in an efficient way. The Cu(II) complex of PCB-TE2A exhibited much higher kinetic stability than ECB-TE2A in acid decomplexation studies, and also showed high resistance to reduction-mediated demetalation. Furthermore, the quantitative radiolabeling of PCB-TE2A with (64)Cu was achieved under milder conditions compared to ECB-TE2A. Biodistribution studies strongly indicate that the (64)Cu complexes of PCB-TE2A cleared out rapidly from the body with minimum decomplexation.