Magnetic resonance imaging (MRI) offers the ability to visualize a number of biological processes at the molecular and cellular level. The quest for MRI methods that enable the monitoring of these processes with improved specificity and spatiotemporal resolution has induced a considerable amount of research into the chemistry of contrast agents. A novel class of agents has been developed that is able to report a change in its magnetic properties as a function of a specific parameter in the surrounding microenvironment. The vast majority consist of paramagnetic Gd3+ complexes that enhance the contrast in the MR image upon a change in the local concentration of a biologically relevant ion, such as H+, Ca2+, Zn2+, or Cu2+. This review summarizes their coordination chemistry and further aspects of these responsive paramagnetic complexes and discusses the most recent examples that are described in the literature.
Keywords: Biologically active cations, gadolinium, lanthanide complexes, longitudinal relaxivity, magnetic resonance imaging, metal-ion chelators, responsive contrast agents, Ligands, Relaxivity, Contrast Agents