How to Develop Bioresponsive MRI Probes Based on Paramagnetic Gd(III) for in vivo Applications

Among the many biological imaging techniques, magnetic resonance imaging (MRI) has been widely adopted for biomedical and clinical diagnostic applications, because of its ability to image deep tissues with high spatiotemporal resolution. Bioresponsive contrast agents are the key to expanding the diagnostic potential of MRI by providing anatomical information and discerning biochemical activity. Recent developments in the field of responsive and gadolinium-based agents have resulted in novel complexes that can sense their chemical microenvironments and thus study various functional processes in the tissue. Herein, we discuss the design and use of Gd(III)-based and bioresponsive MRI contrast agents for specific biological markers such as Ca(II) and Zn(II) cations and zwitterionic amino acid neurotransmitters. Combining their basic physicochemical characteristics with aspects that should be considered for their use in vivo would achieve the desired sensing features and enable their applications in functional molecular imaging to visualize essential biological processes.