Iron(III) Contrast Agent Candidates for MRI: a Survey of the Structure–Effect Relationship in the Last 15 Years of Studies

Iron(III) complexes are under investigation as MRI contrast agents, and new challenges have delineated the way they are being developed. This review article describes the enhanced features that have been develolped in the last 15 years of research. The consequences of relaxation theory impose kinetic, thermodynamic, and structural requirements on the potential contrast agent. Iron, as an endogenous metal, might fulfil these expectations, so its specificity is discussed here. The progress that has been made in achieving higher efficiency in accelerating proton relaxation with in vivo effects is also described. The structural relationships of iron(III) complexes as the key features of contrast agent candidates are particularly emphasized. Because iron is a redox‐active metal, its inertness must be ensured by the ligand. This problem, together with thermodynamic stability and the resulting toxicity, is discussed.