Comparison of divalent transition metal ion paraCEST MRI contrast agents

Transition-metal-ion-based paramagnetic chemical exchange saturation transfer (paraCEST) agents are a promising new class of compounds for magnetic resonance imaging (MRI) contrast. Members in this class of compounds include paramagnetic complexes of Fe^II, Co^II, and Ni^II. The development of the coordination chemistry for these paraCEST agents is presented with an emphasis on the choice of the azamacrocycle backbone and pendent groups with the goals of controlling the oxidation state, spin state, and stability of the complexes. Chemical exchange saturation transfer spectra and images are compared for different macrocyclic complexes containing amide or heterocyclic pendent groups. The potential of paraCEST agents that function as pH- and redox-activated MRI probes is discussed.