Relaxometric charaterization of proteins tagged with Gd(III) complexes as potential MRI theranostic agents

Introduction Field-Cycling NMR Relaxometry is a widely used technique that enables one to study the molecular dynamics of macromolecules as proteins by measuring relaxation rates R1 in different magnetic fields. The characterization of paramagnetic compounds potentially useful as contrast agents in MRI is an important application of 1H FFC-NMR Relaxometry, providing structural and dynamic properties important for the characterization of their mechanism of action. Contrast agents based on gadolinium(III) complexes are widely used in clinical diagnostics, but due to the high toxicity of that metal ion, there are concerns about their safety. Aims Striving towards a decrease of the injected Gd(III) complex, several studies aim to enhance the relaxivity and thus the efficacy of the MRI agents. To this aim, low molecular weight Gd(III) complexes have been developed to be covalently bound to macromolecules with molecular reorientation times in the nanosecond time scale. Here, we investigate human transthyretin (TTR), which is a carrier protein for the delivery of cytotoxic drugs, as a potential theranostic agent. The paramagnetic Gd-C4- IA complex is used as a tag. Methods The analysis of the acquired relaxometry (or Nuclear Magnetic Relaxation Dispersion, NMRD) profiles is performed through the application of relaxation models able to describe interactions and motional processes occurring in the system by taking into account defined parameters and conditions. A fitting software with a graphical interface is under development, that will easily allow for the analysis of the relaxation profiles of paramagnetic molecules, proteins and nanoparticles. Results and Conclusions The relaxometry profile of TTR tagged with the paramagnetic Gd-C4-IA complex shows that this adduct represents a promising theranostic agent. The analysis of the profile with the available relaxation models provides information on the parameters describing the dynamic processes responsible for relaxation