Cyclam-Based Chelators Bearing Phosphonated Pyridine Pendants for 64Cu-PET Imaging : Synthesis, Physico-Chemical Studies, Radiolabeling and Bioimaging.

Herein we present the preparation of two novel cyclam-based macrocycles (te1pyp and cb-te1pyp), bearing phosphonate-appended pyridine side-arms for the coordination of copper(II) ions in the context of 64Cu PET imaging. The two ligands have been prepared through conventional protection-alkylation sequences on cyclam, and their coordination properties have been thoroughly investigated. The corresponding copper complexes have been fully characterized in the solid-state (X-Ray diffraction analysis) and in solution (EPR and UV-Vis spectroscopies). Potentiometric studies, combined with spectrometry, have also allowed us to determine their thermodynamic stability constants, confirming their high affinity for copper(II) cations. The kinetic inertness of the complexes has also been verified by acid-assisted dissociation experiments, enabling their use in 64Cu-PET imaging in mice for the first time. Indeed, the two ligands could be quantitatively radiolabeled under mild conditions, and the resulting 64Cu complexes have demonstrated excellent stability in serum. PET imaging demon-strated a set of features emerging from the combination of picolinates and phosphonate units: high stability in vivo, fast clear-ance from the body via renal elimination, and most interestingly, very low fixation in the liver. The latter is in contrast with what was observed for monopicolinate cyclam (te1pa), that had a non-negligible accumulation in the liver, owing probably to its different charge and lipophillicity. These results thus pave the way for the use of such phosphonated pyridine chelators for in vivo 64Cu-PET imaging.