Secondary amide derivatives of Fe(III)-CDTA: Impact of ligand substitution on relaxivity, stability, and kinetic inertness

The substitution of carboxylate donors with neutral amide groups is a well-established approach for tuning charge distribution and hydration properties in metal chelates; however, its influence on Fe(III) systems has remained largely unexplored. Here, we report two CDTA-derived ligands incorporating one or two secondary butylamide substituents (CD3A-BA and CD2A-BA₂) and comprehensively characterize their Fe(III) complexes using potentiometry, capillary zone electrophoresis, UV-Vis spectrophotometry, transchelation assays (with HBED and human serum transferrin), ¹H NMRD, variable-temperature ¹⁷O NMR, and in vitro cytotoxicity tests. Amide incorporation decreases ligand basicity and leads to reduced complex stability and kinetic inertness compared to the parent CDTA framework. Nevertheless, the new Fe(III) complexes retain strong metal affinity and remain monohydrated under physiologically relevant conditions. Progressive amide substitution systematically slows inner-sphere water exchange, consistent with the higher residual positive charge of the complexes. Despite this, relaxivity in media mimicking physiological conditions remains unchanged, with negligible protein binding and no detectable Fe(III) release. Cellular assays confirm an absence of cytotoxicity at the tested concentrations. Collectively, these findings elucidate how secondary-amide substitution governs stability, reactivity, and hydration dynamics in Fe(III)-CDTA derivatives, offering valuable insights for the rational design of Fe(III)-based MRI contrast agents.