Imaging angiogenesis in an intracerebrally induced model of brain macrometastasis using αv β3 -targeted iron oxide microparticles.

Brain metastasis is responsible for a large proportion of cancer mortality, and there are currently no effective treatments. Moreover, the impact of treatments, particularly antiangiogenic therapeutics, is difficult to ascertain using current magnetic resonance imaging (MRI) methods. Imaging of the angiogenic vasculature has been successfully carried out in solid tumours using microparticles of iron oxide (MPIO) conjugated to a Arg-Gly-Asp peptide (RGD) targeting integrin αv β3 . The aim of this study was to determine whether RGD-MPIO could be used to identify angiogenic blood vessels in brain metastases in vivo. A mouse model of intracerebrally implanted brain macrometastasis was established through intracerebral injection of 4T1-GFP cells. T2 *-weighted imaging was used to visualise MPIO-induced hypointense voxels in vivo, and Prussian blue staining was used to visualise MPIO and endogenous iron histologically ex vivo. The RGD-MPIO showed target-specific binding in vivo, but the sensitivity of the agent for visualising angiogenic vessels per se was reduced by the presence of endogenous iron-laden macrophages in larger metastases, resulting in pre-existing hypointense areas within the tumour. Further, our data suggest that peptide-targeted MPIO, but not antibody-targeted MPIO, are taken up by perivascular macrophages within the macrometastatic microenvironment, resulting in additional nonspecific contrast. While pre-MPIO imaging will circumvent the issues surrounding pre-existing hypointensities and enable detection of specific contrast, our preliminary findings suggest that the use of antibodies rather than peptides as the targeting ligand may represent a preferable route forward for new angiogenesis-targeted molecular MRI agents.