Gadolinium nicotinate clusters as potential MRI contrast agents

Three multinuclear gadolinium(III) clusters containing the nicotinate (nic) ligand, [ Gd-2(nic)(6)(H2O)(4)] (1), [ Gd-4(mu(3)-OH)(4)(Hnic)(5)(H2O)(12)](ClO4)(8)center dot 7H(2)O (2), and [ Gd-4(mu(3)-OH)(4)(nic)(6)(H2O)(6)](2)(ClO4)(4) center dot 4H(2)O (3) were synthesized and characterized. Potential applications of these Gd(III) clusters as high-field magnetic resonance imaging (MRI) contrast agents were evaluated. The longitudinal relaxivities (r(1)) of 1-3 in water at 7 T were determined to be 10.45 +/- 0.16, 9.28 +/- 0.06, and 2.04 +/- 0.29 mM(-1) s(-1) on a per Gd ion basis, respectively. In 1% agarose solution, the r(1) values increased slightly to 10.37 +/- 0.31, 10.74 +/- 0.11, and 4.60 +/- 0.29 mM(-1) s(-1) for 1-3, respectively. The ability to tune the number of inner-sphere water molecules, cluster sizes, organic ligands, and Gd coordination modes in such Gd(III) clusters provide interesting opportunities to further enhance the MR relaxivities for potential MRI applications.