Molecular MR Imaging of Collagen in Mouse Atherosclerosis by Using Paramagnetic CNA35 Micelles

Magnetic resonance imaging (MRI) is increasingly used in biomedicine to visualize plaques in the walls of major arteries in relation to atherosclerosis, the prime cause of myocardial infarction and ischemic stroke. The present study aims to explore the utility of contrast-enhanced MRI for improving the specificity of the MRI evaluation of atherosclerotic plaques with the use of a Gd-based paramagnetic contrast agent that is targeted to collagen. Collagen is a major component of the extracellular matrix and as such plays an important role in the stability of atherosclerotic plaques. Micelles were made with lipid containing 45 mol-% Gd for MRI detection and a low mol fraction of fluorescent lipid for fluorescence microscopic analysis. Collagen-targeted, functional micelles were prepared by conjugation of the CNA35 protein, while nonfunctional control micelles were conjugated with a mutated version of the protein. The micelles were characterized with respect to their magnetic, biochemical, and biophysical properties. Atherosclerotic plaques were induced in the right carotid artery of apo-E knock-out mice by surgical placement of a tapered polymeric cast. In vivo MRI was performed at 6.3 Tesla before and up to 24 h after intravenous injection of paramagnetic micelles (50 mu mol Gdkg1). MRI revealed the strongest signal enhancements by CNA35 micelles. At early time points after injection of CNA35 micelles, contrast enhancement was higher in the collagen-richer lesions compared to that in the collagen-poorer lesions. Confocal laser scanning microscopy confirmed co-localization of CNA35 micelles and collagen in the plaques. We have demonstrated molecular MR imaging of collagen in experimental atherosclerosis by using a CNA35-functionalized micellar contrast agent.