Expanding the Scope of Pyclen-Picolinate Lanthanide Chelates to Potential Theranostic Applications

A family of three picolinate pyclen-based ligands, previously investigated for the complexation of Y³⁺ and some lanthanide ions (Gd³⁺, Eu³⁺), was studied with ¹⁶¹Tb and ¹⁷⁷Lu in view of potential radiotherapeutic applications. The set of six Tb³⁺ and Lu³⁺ complexes was synthesized and fully characterized. The coordination properties in the solid state and in solution were thoroughly studied. Potentiometric titrations, supported by density functional theory (DFT) calculations, showed the very high stability constants of the Tb³⁺ and Lu³⁺ complexes, associated with remarkable kinetic inertness for up to 30 days in 1 M HCl. A complete radiolabeling study performed with both ¹⁶¹Tb and ¹⁷⁷Lu radionuclides, including experiments with regard to the stability with and without scavengers and kinetic inertness using challenging agents, proved that the ligands could reasonably compete with the DOTA analogue. To conclude, the potential of using the same ligand for both radiotherapy and optical imaging was highlighted for the first time.