Rigid and Compact Binuclear Bis-hydrated Gd-complexes as High Relaxivity MRI Agents

The first binuclear Gd-complex of the 12-membered pyridine-based polyaminocarboxylate macrocyclic ligand PCTA was synthesized by C-C connection of the pyridine units through two different synthetic procedures. A dimeric AAZTA-ligand was also synthesized with the aim to compare the relaxometric results or the two ditopic Gd-complexes. Thus, the H-1 relaxometric study on [Gd(2)PCTA(2)(H2O)(4)] and on [Gd(2)AAZTA(2)(H2O)(4)](2-) highlighted the remarkable rigidity and compactness of the two binuclear complexes, which results in molar relaxivities (per Gd), at 1.5 T and 298 K of ca. 12-12.6 mM(-1) s(-1) with an increase of ca. 80 % at 1.5 T and 298 K (+70 % at 310 K) with respect to the corresponding mononuclear complexes.