Dinuclear Fe(III) Hydroxypropyl-Appended Macrocyclic Complexes as MRI Probes

Four high-spin Fe(III) macrocyclic complexes, including three dinuclear and one mononuclear complex, were prepared toward the development of more effective iron-based magnetic resonance imaging (MRI) contrast agents. All four complexes contain a 1,4,7-triazacyclononane macrocyclic backbone with two hydroxypropyl pendant groups, an ancillary aryl or biphenyl group, and a coordination site for a water ligand. The pH potentiometric titrations support one or two deprotonations of the complexes, most likely deprotonation of hydroxypropyl groups at near-neutral pH. Variable-temperature O-17 NMR studies suggest that the inner-sphere water ligand is slow to exchange with bulk water on the NMR time scale. Water proton T-1 relaxation times measured for solutions of the Fe(III) complexes at pH 7.2 showed that the dinuclear complexes have a 2- to 3-fold increase in r(1) relaxivity in comparison to the mononuclear complex per molecule at field strengths ranging from 1.4 T to 9.4 T. The most effective agent, a dinuclear complex with macrocycles linked through parasubstitution of an aryl group (Fe-2(PARA)), has an r1 of 6.7 mM(-1) s(-1) at 37 degrees C and 4.7 T or 3.3 mM(-1) s(-1) per iron center in the presence of serum albumin and shows enhanced blood pool and kidney contrast in mice MRI studies.

Automated summary

Four high-spin Fe(III) macrocyclic complexes were synthesized for potential applications as iron-based MRI contrast agents, with dinuclear complexes showing significantly increased relaxivity compared to mononuclear counterparts at appropriate field strengths.