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Liposomes containing high-spin Fe(iii ) coordination complexes were prepared towards the production of T 1 MRI probes with improved relaxivity. The amphiphilic Fe( iii) complexes were anchored into the liposome with two alkyl chains to give a coordination sphere containing mixed amide and hydroxypropyl pendant groups. The encapsulated complex contains a macrocyclic ligand with three phosphonate pendants, [Fe(NOTP)] 3−, which was chosen for its good aqueous solubility. Four types of MRI probes were prepared including those with intraliposomal Fe(iii) complex (LipoA) alone, amphiphilic Fe(iii) complex (LipoB), both intraliposomal and amphiphilic complex (LipoC) or micelles formed with amphiphilic complex. Water proton relaxivities r 1 and r 2 were measured and compared to a small molecule macrocyclic Fe(iii) complex containing similar donor groups. Micelles of the amphiphilic Fe( iii) complex had proton relaxivity values ( r 1 = 2.6 mM−1 s −1 ) that were four times higher than the small hydrophilic analog. Liposomes with amphiphilic Fe(iii) complex (LipoB) have a per iron relaxivity of 2.6 mM −1 s −1 at pH 7.2, 34 °C at 1.4 T whereas liposomes containing both amphiphilic and intraliposomal Fe(iii) complexes (lipoC) have r 1 of 0.58 mM −1 s −1 on a per iron basis consistent with quenching of the interior Fe(iii) complex relaxivity. Liposomes containing only encapsulated [Fe(NOTP)]3− have a lowered r 1 of 0.65 mM−1s −1 per iron complex. Studies show that the biodistribution and clearance of the different types liposomal nanoparticles differ greatly. LipoB is a blood pool agent with a long circulation time whereas lipoC is cleared more rapidly through both renal and hepatobiliary pathways. These clearance differences are consistent with lower stability of LipoC compared to LipoB. 

Abstract Early studies suggested that Fe^IIIcomplexes cannot compete with Gd^IIIcomplexes as T₁MRI contrast agents. Now it is shown that one member of a class of high‐spin macrocyclic Fe^IIIcomplexes produces more intense contrast in mice kidneys and liver at 30 minutes post‐injection than does a commercially used Gd^IIIagent and also produces similar T₁relaxivity in serum phantoms at 4.7 T and 37 °C. Comparison of four different Fe^IIImacrocyclic complexes elucidates the factors that contribute to relaxivity in vivo including solution speciation. Variable‐temperature¹⁷O NMR studies suggest that none of the complexes has a single, integral inner‐sphere water that exchanges rapidly on the NMR timescale. MRI studies in mice show large in vivo differences of three of the Fe^IIIcomplexes that correspond, in part, to their r₁relaxivity in phantoms. Changes in overall charge of the complex modulate contrast enhancement, especially of the kidneys.