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Home > Medical Professionals > Research > Index > Nanotechnology and Molecular Imaging in Atherosclerosis Lanza et al

Nanotechnology and Molecular Imaging in Atherosclerosis

Gregory M. Lanza, MD, PhD, Patrick M. Winter, PhD,
Anne M. Morawski, BS, Shelton D. Caruthers, PhD,
Ralph W. Fuhrhop, Huiying Zhang, MD, Todd A. Williams,
BS,Grace Hu, MS, John S. Allen, BS, Elizabeth K. Lacy, BS,
Samuel A. Wickline, MD
Department of Medicine, Washington University School of Medicine, St. Louis, MO and Philips Medical Systems, Cleveland, OH

In atherosclerotic disease, angiogenic vessels primarily develop from the vaso vasorum in the adventitial layer of the plaque and extend into the thickening intimal layer of the atheroma rather than originating from the primary arterial lumen 1. Extensive neovascular proliferation has been spatially localized to atherosclerotic plaque, and in particular, to “culprit” lesions clinically associated with unstable angina, myocardial infarction and stroke 2. As recently reviewed and suggested, plaque angiogenesis promotes plaque growth, intraplaque hemorrhage, and lesion instability and antiangiogenic therapeutic strategies may interrupt the progression of atherosclerotic disease, which could avert stroke or myocardial infarctions associated with unstable plaque rupture 3.

A number of traditional magnetic resonance imaging procedures have been employed for noninvasive imaging of advanced atherosclerotic plaques including: lumenal angiography 4, T2-weighted plaque characterization 5, high-resolution black-blood techniques for visualization of thinning fibrous caps 6, and delayed and dynamic contrast-enhanced MRI to estimate vascularity in late carotid disease 7,8. We have recently reported a new MRI technique for molecular imaging of angiogenesis associated with early atherosclerotic plaque growth.

Paramagnetic nanoparticles targeted to anb3-integrins, an angiogenic biomarker expressed by proliferating neovasculature 9, provides sensitive and specific detection and characterization of early atherosclerosis in hypercholesterolemic rabbits 10. These imaging data reaffirmed the diffuse nature of the early atherosclerotic process and illustrated the considerable heterogeneity of disease found within individual aortic segments. Although the precise role of anb3-integrin in the expansion of the vasa vasorum was not addressed, its detection and localization by noninvasive imaging methods may serve as a useful biochemical signature to demarcate lesion-prone sites of focal plaque growth. We have also demonstrated in vitro that this molecular imaging platform can be used to deliver therapeutic agents through a novel mechanism called “contact facilitated lipid exchange” 11. We showed how the unique combination of molecular imaging and targeted therapy features of this nanoparticulate platform could provide confirmation, spatial localization and quantification of site-specific drug delivery.
Few studies have addressed the efficacy of antiangiogenic therapies directed against atherosclerotic disease.  The majority focus instead upon the pathophysiologic mechanisms of angiogenesis. The most notable study treated Apo E -/- mice for 4 months (20 to 36 weeks) with TNP-470 or endostatin (30 mg/kg QOD, 1.68 g/kg total dose) 12, which reduced plaque angiogenesis and diminished atheroma growth despite persistent elevation of total cholesterol levels. Neither endostatin nor TNP-470 altered foam cell deposition or fibromuscular lesion development during early atherogenesis, and they were both less effective during later periods of treatment (32 to 48 weeks).   Unfortunately, high dose TNP-470 has serious side effects (i.e., neurocognitive toxicity13).

Recently, we demonstrated that nanoparticles can also specifically and locally deliver potent pharmaceutical agents to elicit a marked antiangiogenic effect with a single dose. Therefore, anb3-targeted paramagnetic nanoparticles can serve as a platform to diagnose, treat, and monitor antiangiogenic therapy in early atherosclerosis.  anb3-targeted nanoparticles with fumagillin significantly reduced aortic angiogenesis in atherosclerotic rabbits as determined by MR molecular imaging of anb3-integrin, which was further corroborated by immunohistochemistry. In contradistinction to the previous study in Apo E -/- mice, early atherosclerosis was imaged, quantified and treated with a single injection of anb3-targeted paramagnetic nanoparticles incorporating fumagillin. Moreover, our total dose of fumagillin was 10,000-fold lower, which could be clinically relevant given the neurocognitive side-effects of orally administered TNP-470 13. Clearly, the use of targeted drug delivery could substantially reduce the total fumagillin requirement versus prolonged oral administration of TNP-470.

Fumagillin was delivered to targeted cells through “contact facilitated drug delivery”, a mechanism in which ligand-directed binding promotes the direct transfer of lipids and drug from the nanoparticle’s surfactant monolayer to the targeted cell membrane 11. Ordinarily, this is a slow, inefficient process, but ligand binding of the nanoparticle minimizes the equilibrium separation of the lipid surfaces and substantially increases the frequency and duration of lipid surface interactions. The requirement of this novel delivery mechanism is confirmed by the lack of therapeutic effect with nontargeted fumagillin nanoparticles. Despite producing ~60% of the targeted MRI signal enhancement, they lacked adequate membrane surface interactions to effectively deliver the drug. Additionally, simple antagonism of the endothelial anb3-integrin with high-avidity, anb3-targeted nanoparticles, produced no discernable effect on neovasculature expansion, indicating that the therapeutic effect of anb3-targeted nanoparticles was dependent upon delivery of fumagillin.

With incorporation of a therapeutic agent, anb3-targeted paramagnetic nanoparticles were used to confirm and qualitatively assess drug delivery based upon MR contrast enhancement. Quantifying the local drug concentration allows rational drug dosing and prognostication of the antiangiogenic response. Moreover, follow-up imaging with anb3-targeted paramagnetic nanoparticles (without drug) provided sensitive and specific assessment of therapeutic efficacy noninvasively.

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