Magnetic resonance imaging of ruptured plaques in the rabbit with ultrasmall superparamagnetic particles of iron oxide

Accumulation of Iron Oxide-Based Contrast Agents in Rabbit Atherosclerotic Plaques in Relation to Plaque Age and Vulnerability Features

Purpose

In this study, a detailed characterization of a rabbit model of atherosclerosis was performed to assess the optimal time frame for evaluating plaque vulnerability using superparamagnetic iron oxide nanoparticle (SPION)-enhanced magnetic resonance imaging (MRI).

Methods

The progression of atherosclerosis induced by ballooning and a high-cholesterol diet was monitored using angiography, and the resulting plaques were characterized using immunohistochemistry and histology. Morphometric analyses were performed to evaluate plaque size and vulnerability features. The accumulation of SPIONs (novel dextran-coated SPIONDex and ferumoxytol) in atherosclerotic plaques was investigated by histology and MRI and correlated with plaque age and vulnerability. Toxicity of SPIONDex was evaluated in rats.

Results

Weak positive correlations were detected between plaque age and intima thickness, and total macrophage load. A strong negative correlation was observed between the minimum fibrous cap thickness and plaque age as well as the mean macrophage load. The accumulation of SPION in the atherosclerotic plaques was detected by MRI 24 h after administration and was subsequently confirmed by Prussian blue staining of histological specimens. Positive correlations between Prussian blue signal in atherosclerotic plaques, plaque age, and macrophage load were detected. Very little iron was observed in the histological sections of the heart and kidney, whereas strong staining of SPIONDex and ferumoxytol was detected in the spleen and liver. In contrast to ferumoxytol, SPIONDex administration in rabbits was well tolerated without inducing hypersensitivity. The maximum tolerated dose in rat model was higher than 100 mg Fe/kg.

Conclusion

Older atherosclerotic plaques with vulnerable features in rabbits are a useful tool for investigating iron oxide-based contrast agents for MRI. Based on the experimental data, SPIONDex particles constitute a promising candidate for further clinical translation as a safe formulation that offers the possibility of repeated administration free from the risks associated with other types of magnetic contrast agents.

Molecular Imaging in Ischemic Heart Disease

Purpose of Review

The purpose of this paper is to review current and new modalities to image key biological processes in ischemic heart disease and after myocardial infarction non-invasively.

Recent Findings

New imaging targets have been developed to detect and quantify myocardial damage after ischemia. Although positron emission tomography (PET) has been leading the development of new probes in the past, continuous improvements of magnetic resonance imaging (MRI) together with the development of new novel MRI contrast agents opens new research avenues including the combination of both PET and MRI to obtain anatomic, functional, and molecular information simultaneously, which is not possible from a single imaging session.

Summary

This review summarizes the state of art of non-invasive molecular imaging of the myocardium during ischemia and after myocardial infarction using PET and MRI. We also describe the different contrast agents that have been developed to image the different phases of cardiac healing and the biological processes associated with each of those phases. Importantly, here we focus on imaging of inflammation as it is the key biological process that orchestrates clearance of dead cells, tissue remodeling, cardiac repair, and future outcome. We also focus on clinical translation of some of the novel contrast agents that have been tested in patients and discuss the need for larger, multi-center patient studies to fully validate the applicability of new imaging probes.

Imaging vulnerable plaques by targeting inflammation in atherosclerosis using fluorescent-labeled dual-ligand microparticles of iron oxide and magnetic resonance imaging

OBJECTIVE

Identification of patients with high-risk asymptomatic carotid plaques remains an elusive but essential step in stroke prevention. Inflammation is a key process in plaque destabilization and a prelude to clinical sequelae. There are currently no clinical imaging tools to assess the inflammatory activity within plaques. This study characterized inflammation in atherosclerosis using dual-targeted microparticles of iron oxide (DT-MPIO) as a magnetic resonance imaging (MRI) probe.

METHODS

DT-MPIO were used to detect and characterize inflammatory markers, vascular cell adhesion molecule 1 (VCAM-1). and P-selectin on (1) tumor necrosis factor-α-treated cells by immunocytochemistry and (2) aortic root plaques of apolipoprotein-E deficient mice by in vivo MRI. Furthermore, apolipoprotein E-deficient mice with focal carotid plaques of different phenotypes were developed by means of periarterial cuff placement to allow in vivo molecular MRI using these probes. The association between biomarkers and the magnetic resonance signal in different contrast groups was assessed longitudinally in these models.

RESULTS

Immunocytochemistry confirmed specificity and efficacy of DT-MPIO to VCAM-1 and P-selectin. Using this in vivo molecular MRI strategy, we demonstrated (1) the DT-MPIO-induced magnetic resonance signal tracked with VCAM-1 (r = 0.69; P = .014), P-selectin (r = 0.65; P = .022), and macrophage content (r = 0.59; P = .045) within aortic root plaques and (2) high-risk inflamed plaques were distinguished from noninflamed plaques in the murine carotid artery within a practical clinical imaging time frame.

CONCLUSIONS

These molecular MRI probes constitute a novel imaging tool for in vivo characterization of plaque vulnerability and inflammatory activity in atherosclerosis. Further development and translation into the clinical arena will facilitate more accurate risk stratification in carotid atherosclerotic disease in the future.

Effects of Nanoprobe Morphology on Cellular Binding and Inflammatory Responses: Hyaluronan-Conjugated Magnetic Nanoworms for Magnetic Resonance Imaging of Atherosclerotic Plaques

Atherosclerosis is an inflammatory disease of arterial walls and the rupturing of atherosclerotic plaques is a major cause of heart attack and stroke. Imaging techniques that can enable the detection of atherosclerotic plaques before clinical manifestation are urgently needed. Magnetic resonance imaging (MRI) is a powerful technique to image the morphology of atherosclerotic plaques. In order to better analyze molecular processes in plaques, contrast agents that can selectively bind to plaque receptors will prove invaluable. CD44 is a cell surface protein overexpressed in plaque tissues, the level of which can be correlated with the risks of plaque rupture. Thus, targeting CD44 is an attractive strategy for detection of atherosclerotic plaques. Herein, we report the synthesis of hyaluronan-conjugated iron oxide nanoworms (HA-NWs). A new purification and gel electrophoresis protocol was developed to ensure the complete removal of free HA from HA-NWs. Compared to the more traditional spherical HA-bearing nanoparticles, HA-NWs had an elongated shape, which interacted much stronger with CD44-expressing cells in CD44- and HA-dependent manners. Furthermore, the HA-NWs did not induce much inflammatory response compared to the spherical HA nanoparticles. When assessed in vivo, HA-NWs enabled successful imaging of atherosclerotic plaques in a clinically relevant model of ApoE knockout transgenic mice for noninvasive plaque detection by MRI. Thus, nanoprobe shape engineering can be a useful strategy to significantly enhance their desired biological properties.

The Rabbit Model of Accelerated Atherosclerosis: A Methodological Perspective of the Iliac Artery Balloon Injury

Acute coronary syndrome resulting from coronary occlusion following atherosclerotic plaque development and rupture is the leading cause of death in the industrialized world. New Zealand White (NZW) rabbits are widely used as an animal model for the study of atherosclerosis. They develop spontaneous lesions when fed with atherogenic diet; however, this requires long time of 4 - 8 months. To further enhance and accelerate atherogenesis, a combination of atherogenic diet and mechanical endothelial injury is often employed. The presented procedure for inducing atherosclerotic plaques in rabbits uses a balloon catheter to disrupt the endothelium in the left iliac artery of NZW rabbits fed with atherogenic diet. Such mechanical damage caused by the balloon catheter induces a chain of inflammatory reactions initiating neointimal lipid accumulation in a time dependent fashion. Atherosclerotic plaque following balloon injury show neointimal thickening with extensive lipid infiltration, high smooth muscle cell content and presence of macrophage derived foam cells. This technique is simple, reproducible and produces plaque of controlled length within the iliac artery. The whole procedure is completed within 20 - 30 min. The procedure is safe with low mortality and also offers high success in obtaining substantial intimal lesions. The procedure of balloon catheter induced arterial injury results in atherosclerosis within two weeks. This model can be used for investigating the disease pathology, diagnostic imaging and to evaluate new therapeutic strategies.

Indirect magnetic resonance imaging lymphography identifies lymph node metastasis in rabbit pyriform sinus VX2 carcinoma using ultra-small super-paramagnetic iron oxide

Background

USPIO is a contrast agent for MRI that can generate T2W images with low signal intensities. After subcutaneous or intravenous injection of USPIO, normal lymph node tissues uptake these nano-particles, but tumor cells do not. Thus, tumor metastasis can be detected using this contrast agent.

Objective

The aim of this study was to access the feasibility of USPIO enhanced MRI for the detection of cervical lymph node metastasis in a pyriform sinus carcinoma animal model and to investigate the ability of USPIO to enhance images of cervical lymph node metastases.

Methods and Findings

Twenty New Zealand rabbits were randomly divided into tumor and inflammatory groups, and each group contained 10 rabbits. In the inflammatory group, a 0.5 ml egg yolk emulsion was injected into the sub-mandibular muscle of the rabbits to induce an inflammatory reaction in their cervical lymph nodes. In the tumor group, a VX2 tumor tissue suspension was transplanted into the pyriform sinus sub-mucosa of the rabbits using direct laryngoscope. Four weeks after the tumor or egg yolk injection, MRIs were performed before and after USPIO injection to observe the imaging enhancement features of USPIO. After that, a histo-pathological analysis was performed for all rabbits. We found the metastatic lymph nodes had no signal reduced intensity or irregular signal reduced intensity on T2-weighted image by using USPIO enhancement. In the tumor group,the sensitivity and specificity of plain MRI were 57.6% and 60.7%. The corresponding values of USPIO-enhanced MRl were 96.1% and 85.7%. (P<0.05)

Conclusion

The features and the extent of the lymph node metastases corresponded to those observed on USPIO-enhanced MR images. USPIO-enhanced MRI is useful for the detection and estimation of lymph node metastasis in this cervical carcinoma animal model.

Assessment of inflammation with a very small iron-oxide particle in a murine model of reperfused myocardial infarction

PURPOSE

To investigate a very small iron-oxide particle (VSOP) in a mouse model of acute ischemia-reperfusion to access the mechanism of such particles in areas of myocardial inflammation.

MATERIALS AND METHODS

Animals were injected with VSOP at several time points, in a mouse model of acute myocardial infarction (MI), before and after MI. MRI was used to localize areas of VSOP enhancement, evaluate VSOP areas extension, and determine the related T2* values. Histology, electron microscopy, macrophage counting, and Evan's Blue staining were also performed.

RESULTS

We found that areas of VSOP uptake decreased from 1 to 8 days post-MI while the related T2* values increased. T2* and VSOP areas, defined from MRI data, correlated well between 1 and 3 days post-MI but not at 7 days after injection. Histological analysis and electron microscopy showed colocalization of macrophages with areas of VSOP staining. However, there was no correlation between number of macrophages and the extension of the VSOP areas achieved by MR. We found that only areas of increased permeability (assessed by Evan's Blue staining) showed colocalization of macrophages and VSOP uptake.

CONCLUSION

This study demonstrates that VSOP allows the assessment of myocardial inflammation associated with increased permeability during infarct healing in a mouse model of ischemia-reperfusion.

CD44 targeting magnetic glyconanoparticles for atherosclerotic plaque imaging

PURPOSE

The cell surface adhesion molecule CD44 plays important roles in the initiation and development of atherosclerotic plaques. We aim to develop nanoparticles that can selectively target CD44 for the non-invasive detection of atherosclerotic plaques by magnetic resonance imaging.

METHODS

Magnetic glyconanoparticles with hyaluronan immobilized on the surface have been prepared. The binding of these nanoparticles with CD44 was evaluated in vitro by enzyme linked immunosorbent assay, flow cytometry and confocal microscopy. In vivo magnetic resonance imaging of plaques was performed on an atherosclerotic rabbit model.

RESULTS

The magnetic glyconanoparticles can selectively bind CD44. In T2* weighted magnetic resonance images acquired in vivo, significant contrast changes in aorta walls were observed with a very low dose of the magnetic nanoparticles, allowing the detection of atherosclerotic plaques. Furthermore, imaging could be performed without significant delay after probe administration. The selectivity of hyaluronan nanoparticles in plaque imaging was established by several control experiments.

CONCLUSIONS

Magnetic nanoparticles bearing surface hyaluronan enabled the imaging of atherosclerotic plaques in vivo by magnetic resonance imaging. The low dose of nanoparticles required, the possibility to image without much delay and the high biocompatibility are the advantages of these nanoparticles as contrast agents for plaque imaging.

Advances in molecular imaging of atherosclerosis and myocardial infarction: shedding new light on in vivo cardiovascular biology

Molecular imaging of the cardiovascular system heavily relies on the development of new imaging probes and technologies to facilitate visualization of biological processes underlying or preceding disease. Molecular imaging is a highly active research discipline that has seen tremendous growth over the past decade. It has broadened our understanding of oncologic, neurologic, and cardiovascular diseases by providing new insights into the in vivo biology of disease progression and therapeutic interventions. As it allows for the longitudinal evaluation of biological processes, it is ideally suited for monitoring treatment response. In this review, we will concentrate on the major accomplishments and advances in the field of molecular imaging of atherosclerosis and myocardial infarction with a special focus on magnetic resonance imaging.

Temporal and noninvasive monitoring of inflammatory-cell infiltration to myocardial infarction sites using micrometer-sized iron oxide particles

Micrometer-sized iron oxide particles (MPIO) are a more sensitive MRI contrast agent for tracking cell migration compared to ultrasmall iron oxide particles. This study investigated the temporal relationship between inflammation and tissue remodeling due to myocardial infarction (MI) using MPIO-enhanced MRI. C57Bl/6 mice received an intravenous MPIO injection for cell labeling, followed by a surgically induced MI seven days later (n=7). For controls, two groups underwent either sham-operated surgery without inducing an MI post-MPIO injection (n=7) or MI surgery without MPIO injection (n=6). The MRIs performed post-MI showed significant signal attenuation around the MI site for the mice that received an intravenous MPIO injection for cell labeling, followed by a surgically induced MI seven days later, compared to the two control groups (P<0.01). The findings suggested that the prelabeled inflammatory cells mobilized and infiltrated into the MI site. Furthermore, the linear regression of contrast-to-noise ratio at the MI site and left ventricular ejection function suggested a positive correlation between the labeled inflammatory cell infiltration and cardiac function attenuation during post-MI remodeling (r2=0.98). In conclusion, this study demonstrated an MRI technique for noninvasively and temporally monitoring inflammatory cell migration into the myocardium while potentially providing additional insight concerning the pathologic progression of a myocardial infarction.

Early identification of aortic valve sclerosis using iron oxide enhanced MRI

PURPOSE

To test the ability of MION-47 enhanced MRI to identify tissue macrophage infiltration in a rabbit model of aortic valve sclerosis (AVS).

MATERIALS AND METHODS

The aortic valves of control and cholesterol-fed New Zealand White rabbits were imaged in vivo pre- and 48 h post-intravenous administration of MION-47 using a 1.5 Tesla (T) MR clinical scanner and a CINE fSPGR sequence. MION-47 aortic valve cusps were imaged ex vivo on a 3.0T whole-body MR system with a custom gradient insert coil and a three-dimensional (3D) FIESTA sequence and compared with aortic valve cusps from control and cholesterol-fed contrast-free rabbits. Histopathological analysis was performed to determine the site of iron oxide uptake.

RESULTS

MION-47 enhanced the visibility of both control and cholesterol-fed rabbit valves in in vivo images. Ex vivo image analysis confirmed the presence of significant signal voids in contrast-administered aortic valves. Signal voids were not observed in contrast-free valve cusps. In MION-47 administered rabbits, histopathological analysis revealed iron staining not only in fibrosal macrophages of cholesterol-fed valves but also in myofibroblasts from control and cholesterol-fed valves.

CONCLUSION

Although iron oxide labeling of macrophage infiltration in AVS has the potential to detect the disease process early, a macrophage-specific iron compound rather than passive targeting may be required.

Assessment of macrophage infiltration in a murine model of abdominal aortic aneurysm

PURPOSE

To evaluate the use of an ultrasmall superparamagnetic iron oxide (USPIO) contrast agent as a marker for the detection of macrophage in a preclinical abdominal aortic aneurysm animal (AAA) model.

MATERIALS AND METHODS

Osmotic pumps were implanted subcutaneously in apoE(-/-) mice for continuous infusion of Angiotensin II (Ang-II). Weekly bright-blood gradient echo scans were performed on the suprarenal abdominal aorta to evaluate aneurysm development. Once an AAA was detected, animals were administered 1000 mumol/kg of the USPIO contrast agent ferumoxtran-10 (Combidex) followed by in vivo scanning 24 h post-USPIO administration. After in vivo imaging, aortas were harvested for ex vivo imaging, histology, iron quantification, and gene expression analysis.

RESULTS

Reduced signal intensity was evident in the post-USPIO transverse images of the abdominal aorta. The areas of reduced signal were primarily along the aneurysm shoulder and outer perianeurysm areas and corresponded to regions of macrophage infiltration and colocalized USPIO determination by means of histological staining. The absolute iron content measured significantly correlated to the area of signal reduction in the ex vivo images (r = 0.9; P < 0.01). In the AAA tissue, the macrophage-driven cytokine gene expression was up-regulated along with a matrix metalloproteinase known to mediate extracellular matrix breakdown in this disease model.

CONCLUSION

These results demonstrate the feasibility of using an USPIO contrast agent as a surrogate for detecting the acute inflammatory process involved in the development of abdominal aneurysms.

Atorvastatin and uptake of ultrasmall superparamagnetic iron oxide nanoparticles (Ferumoxtran-10) in human monocyte-macrophages: implications for magnetic resonance imaging

Ferumoxtran-10 is an ultrasmall superparamagnetic iron oxide nanoparticle potentially useful as a contrast material in magnetic resonance imaging for the diagnosis of inflammatory and degenerative disorders associated with high macrophage activity. In clinical trials, it is currently applied to monitor the effect of atorvastatin therapy on macrophage activity in human carotid plaques. A recent study reported the inhibition of iron oxide nanoparticle uptake in macrophages by lovastatin, an effect which could compromise the suitability of Ferumoxtran-10 as an MRI contrast material in patients on statin therapy. Therefore, we examined the effect of atorvastatin on human monocyte-macrophage uptake of Ferumoxtran-10 in vitro using biochemical assays, magnetic resonance imaging and transmission electron microscopy. Our study showed that non-toxic concentrations of atorvastatin did not affect the amount of Ferumoxtran-10 taken up by HMMs. Furthermore, the intracellular distribution of iron oxide nanoparticles and the resulting MRI signal intensities remained unchanged by statin treatment. These results were obtained using atorvastatin concentrations probably vastly exceeding those reached in patient plasma in vivo. Atorvastatin therapy itself is therefore unlikely to affect Ferumoxtran-10 based macrophage detection by MRI, a prerequisite for the use of this contrast material to monitor lesion macrophage burden during lipid-lowering therapy.