Iron-based compounds coordinated with phospho-polymers as biocompatible probes for dual 31P/1H magnetic resonance imaging and spectroscopy

In this work, we present the synthesis and evaluation of magnetic resonance (MR) properties of novel phosphorus/iron-containing probes for dual P-31 and H-1 MR imaging and spectroscopy (MRI and MRS). The presented probes are composed of biocompatible semitelechelic and multivalent phospho-polymers based on poly(2-methacryloyloxyethyl phosphorylcholine) (pMPC) coordinated with small paramagnetic Fe3+ ions or superparamagnetic maghemite (gamma-Fe2O3) nanoparticles via deferoxamine group linked to the end or along the polymer chains. All probes provided very short H-1 T-1 and T-2 relaxation times even at low iron concentrations. The presence of iron had a significant impact on the shortening of P-31 relaxation, with the effect being more pronounced for probes based on gamma-Fe2O3 and multivalent polymer. While the water-soluble probe having one Fe3+ ion per polymer chain was satisfactorily visualized by both P-31-MRS and P-31-MRI, the probe with multiple Fe3+ ions could only be detected by P-31-MRS, and the probes consisting of gamma-Fe2O3 nanoparticles could not be imaged by either technique due to their ultra-short P-31 relaxations. In this proof-of-principle study performed on phantoms at a clinically relevant magnetic fields, we demonstrated how the different forms and concentrations of iron affect both the H-1 MR signal of the surrounding water molecules and the P-31 MR signal of the phospho-polymer probe. Thus, this double contrast can be exploited to simultaneously visualize body anatomy and monitor probe biodistribution.