Molecular imaging of macrophages in atherosclerotic plaques using bimodal PEG-micelles

Pegylated, fluorescent, and paramagnetic micelles were developed. The micelles were conjugated with macrophage scavenger receptor (MSR)-specific antibodies. The abdominal aortas of atherosclerotic apoE-KO mice were imaged with T₁-weighted high-resolution MRI before and 24 h after intravenous administration of the contrast agent (CA). Pronounced signal enhancement (SE) (up to 200%) was observed for apolipoprotein E knockout (apoE-KO) mice that were injected with MSR-targeted micelles, while the aortic vessel wall of mice injected with nontargeted micelles showed little SE. To allow fluorescence microscopy and optical imaging of the excised aorta, the micelles were made fluorescent by incorporating either a quantum dot (QD) in the micelle corona or rhodamine lipids in the micelle. Ultraviolet (UV) illumination of the aorta allowed the identification of regions with high macrophage content, while MSR-targeted rhodamine micelles could be detected with fluorescence microscopy and were found to be associated with macrophages. In conclusion, this study demonstrates that macrophages in apoE-KO mice can be effectively and specifically detected by molecular MRI and optical methods upon administration of a pegylated micellar CA.