VCAM-1 targeted MRI for imaging of inflammation in mouse atherosclerosis using paramagnetic and superparamagnetic lipid-based contrast agents

Introduction: The recruitment of leukocytes into the vessel wall intima plays a significant role in the onset and progression of atherosclerotic lesions. Imaging of VCAM-1 expression in atherosclerotic plaques may serve as a surrogate marker of inflammatory cell recruitment into plaques. The purpose of this study was to image VCAM-1 expression in carotid artery lesions of apoE-/mice, to compare the efficacy of a lipidbased paramagnetic and a lipid-based superparamagnetic contrast agent, and to relate contrast changes to plaque location. Methods: Paramagnetic liposomes and superparamagnetic micellar iron oxides were prepared as previously described [1] and antibodies were conjugated to the distal ends of the PEG chains. Particles were either non-conjugated, conjugated to a non-binding control IgG, or an anti mouse VCAM-1 antibody. The contrast agents were incubated on mouse endothelioma cells over-expressing VCAM-1 upon TNFα stimulation, to verify their binding specificity. T1 and T2 values of cell pellets were measured at 6.3T. A tapered cast was placed around the right carotid artery of apoE-/mice on ‘western’ type diet 9 weeks before the MRI experiment to induce a more vulnerable lesion upstream and a stabilized lesion downstream of the cast [2]. In vivo T1w and T2w spin echo imaging with fat saturation was performed at 6.3T before and 24 hours after injection of contrast agent. Mice were injected with control IgG coupled liposomes (n=4), anti-VCAM-1 liposomes (n=4), control IgG coupled micellar iron oxides (n=3) or anti VCAM-1 micellar iron oxides (n=3). The injected dose was 100 μmol Gd / kg or 10 mg Fe / kg body weight. Parameters for T1w imaging were: TR=800 ms, TE=10.3 ms, FOV=2.56 cm, matrix=256, slice thickness=0.5 mm, NA=8, scan time=27 min, and for T2w imaging: TR=2000 ms, TE=20 ms, NA=4, scan time=34 min, and otherwise identical parameters.