Nature‐inspired nanoformulations for contrast‐enhanced in vivo MR imaging of macrophages

Magnetic resonance imaging (MRI) of macrophages in atherosclerosis requires the use of contrast‐enhancing agents. Reconstituted lipoprotein particles that mimic native high‐density lipoproteins (HDL) are a versatile delivery platform for Gd‐based contrast agents (GBCA) but require targeting moieties to direct the particles to macrophages. In this study, a naturally occurring methionine oxidation in the major HDL protein, apolipoprotein (apo) A‐I, was exploited as a novel way to target HDL to macrophages. We also tested if fully functional GBCA–HDL can be generated using synthetic apo A‐I peptides. The fluorescence and MRI studies reveal that specific oxidation of apo A‐I or its peptides increases the in vitro macrophage uptake of GBCA–HDL by 2–3 times. The in vivo imaging studies using an apo E‐deficient mouse model of atherosclerosis and a 3.0 T MRI system demonstrate that this modification significantly improves atherosclerotic plaque detection using GBCA–HDL. At 24 h post‐injection of 0.05 mmol Gd kg −1 GBCA–HDL containing oxidized apo A‐I or its peptides, the atherosclerotic wall/muscle normalized enhancement ratios were 90 and 120%, respectively, while those of GBCA–HDL containing their unmodified counterparts were 35 and 45%, respectively. Confocal fluorescence microscopy confirms the accumulation of GBCA–HDL containing oxidized apo A‐I or its peptides in intraplaque macrophages. Together, the results of this study confirm the hypothesis that specific oxidation of apo A‐I targets GBCA–HDL to macrophages in vitro and in vivo . Furthermore, our observation that synthetic peptides can functionally replace the native apo A‐I protein in HDL further encourages the development of these contrast agents for macrophage imaging. Copyright © 2014 John Wiley & Sons, Ltd.