Iron Nanoparticles as Magnetic Resonance Imaging Contrast Agents

Abstract

Magnetic nanoparticles are effective in a range of biomedical applications including magneticresonance imaging (MRI) contrast enhancement. The efficacy of nanoparticles ascontrast agents depends mainly on the surface chemistry and magnetic properties of theparticles, with a large magnetic moment inducing efficient transverse (T₂) relaxation ofprotons. This results in improved negative enhancement of MRI contrast on T₂ weightedsequences. Iron oxide nanoparticles (FeOx NPs) have been used in MRI for 20 years andare the only commercially available T₂ contrast agents. A significantly larger magneticmoment can potentially be achieved with iron nanoparticles (Fe NPs), but developmenthas been hampered by difficulty in preparing stable particles. In this study, stable Fe NPwere prepared by a novel, simple, synthesis and compared with FeOx NP as T₂ contrastagents in a range of MRI-based biomedical applications.The effectiveness of Fe NPs versus FeOx NPs to negatively enhance MRI contrast onT₂ weighted sequences was first examined in vitro. The Fe NPs and FeOx NPs werecharacterised by electron microscopy and found to be of similar size (16nm). The Fe NPspossessed a core of highly magnetic α-Fe inside a 3nm shell of FeOx of the same crystalstructure as the pure FeOx NPs. Both types of NP were coated with the same molecule,DMSA, to produce aqueous dispersions with similar hydrodynamic particle sizes andpharmacokinetics. When dispersed in gels and examined by MRI, the Fe NPs were foundto produce more than twice the amount of T₂ contrast change per unit concentrationrelative to FeOx NPs. When cells were labelled in vitro, Fe NPs produced greater T₂contrast enhancement in all cell types tested, whilst there was no significant difference in the uptake of iron or the cytotoxicity between cells labelled with Fe or FeOx NPs.To assess the clinical applicability of the nanoparticles in vivo, FeOx NPs and Fe NPswere administered to mice and MRI experiments were performed at 1.5 T. Contrast effectsof the NPs were examined in the liver, spleen and lymph nodes, as tissues in theseorgans are rich in phagocytic cells and have a strong tendency to take up circulatingNPs. In all three organs studied, the Fe NPs produced noticeably darker contrast thanthe FeOx NPs, providing twice the contrast improvement.One of the most intensely researched applications of magnetic nanoparticles in MRI is improving detection of cancer in the lymph nodes. To model the size and NP uptake ofsmall lymph node metastases in humans, a mouse model was developed by injecting 4T1breast cancer cells directly into the mouse spleen. Analysis of mice bearing 4T1 tumoursperformed at 1.5 T showed that Fe NPs produced better contrast than FeOx NPs andimproved the detection of small tumours in the spleen as determined by two blindedradiologists. Indeed, the heightened sensitivity and specificity improved the threshold ofcancer detection on previous studies performed at 1.5 T.It was then examined whether the improved T₂ contrast could enable new MRI applicationsin vivo. A novel assay to detect induced immune responses following dendriticcell-based vaccination using MRI was developed. By tracking cells labelled with ironnanoparticles, a difference in contrast could be detected between nave mice and thosethat had developed a strong immune response after vaccination. This assay only reachedstatistical significance with Fe NPs and not with FeOx NPs.As a consequence of these studies, another MRI-based technique for assessing inductionof an immune response was developed, based on the simple observation that lymph nodesdraining the injection site became enlarged. This enlargement was seen as early as 12 hours after vaccination and was caused by a cellular in filtrate dominated by lymphoidcells. In experiments where vaccination was performed multiple times using different tumoursas a source of antigen, incremental increases in lymph node size were detectableby MRI, which was shown to be a highly antigen-specific response. In the vaccine modelstudied, the increase in lymph node size was associated with protection from a tumour challenge. Thus, Fe NPs produce a significant improvement of T₂ contrast over FeOx NPs in a rangeof applications without any differences found in uptake or cytotoxicity. These findingsare substantial enough to justify further investigations into the application of Fe NPs ina variety of clinical settings.