Metalloporphyrins and functional analogues as MRI contrast agents

Following the footprint of porphyrin-mediated photodynamic therapy (PDT), paramagnetic metalloporphyrins were originated as tumor seeking contrast agents (CAs) for magnetic resonance imaging (MRI). However, serial research has disproved their tumor selectivity, identified nonviable tissues as their real targets, and eventually elicited new applications in myocardial infarction delineation, tissue viability evaluation, ablation therapy assessment, as well as first pass or dynamic perfusion MRI, multi-organ contrast enhancement (CE), atherosclerotic plaque imaging and stem cell labeling or tracking. Furthermore, nonporphyrin analogues have been developed to reduce porphyrin related toxicities. These porphyrin and nonporphyrin compounds have been termed as necrosis-avid contrast agents (NACAs) to denote their major discovered affinity. The present review aims to document the evolving research in this particular field, to discuss possible mechanisms, to promote further preclinical and clinical development of this unique and promising class of MRI CAs, and to implicate a novel stroma targeting strategy for diagnosis and treatment of malignant and benign disorders.