Optimized relaxivity and stability of [Gd(H(2,2)-1,2-HOPO)(H ₂O)]^- for use as an MRI contrast agent

Relaxometry and solution thermodynamic measurements show that Gd(H(2,2)-1,2-HOPO) is a good candidate as a contrast agent for magnetic resonance imaging (MRI-CA). Acidic, octadentate H(2,2)-1,2-HOPO forms a very stable Gd(III) complex [pGd = 21.2(2)]. The coordination sphere at the Gd(III) center is completed by one water molecule that is not replaced by common physiological anions. In addition, this ligand is highly selective for Gd(III) binding in the presence of Zn(II) or Ca(II). The symmetric charge distribution of the 1,2-HOPO chelates is associated with favorably long electronic relaxation time T_1,2e comparable to those of GdDOTA. This, in addition to the fast water exchange rate typical of HOPO chelates, improves the relaxivity to r_1p = 8.2 mM^-1 s^-1 (0.47 T). This remarkably high value is unprecedented for small-molecule, q = 1 MRI-CA.