Stable and Inert Yttrium(III) Complexes with Pyclen-Based Ligands Bearing Pendant Picolinate Arms: Toward New Pharmaceuticals for β-Radiotherapy.

We report the synthesis of two azaligands based on the pyclen macrocyclic platform containing two picolinate and one acetate pendant arms. The two ligands differ in the relative positions of the pendant functions, which are arranged either in a symmetrical (L3) or nonsymmetrical (L4) fashion. The complexation properties of the ligands toward nat Y3+ and 90 Y3+ were investigated to assess their potential as chelating units for radiopharmaceutical applications. The X-ray structures of the YL3 and YL4 complexes show nonadentate binding of the ligand to the metal ions. A multinuclear 1 H, 13 C, and 89 Y NMR study shows that the complexes present a structure in solution similar to that observed in the solid state. The two complexes present very high thermodynamic stability constants (log KYL = 23.36(2) and 23.07(2) for YL3 and YL4, respectively). The complexes also show a remarkable inertness with respect to their proton-assisted dissociation, especially YL4. 90 Y radiolabeling was proved to be efficient under mild conditions. The 90 YL3 and 90 YL4 radiochelates were found to be more stable than 90 Y(DOTA).