ReviewTargeted molecular imaging of vascular inflammation in cardiovascular disease using nano- and micro-sized agents
Graphical abstract
Introduction
Atherosclerosis is a chronic inflammatory disease of medium to large arteries, characterised by endothelial activation and accumulation of inflammatory cells within the vessel wall. Conventional clinical imaging of cardiovascular disease by X-ray angiography, can detect the severity of coronary luminal stenosis and carotid intima-media thickening, and can guide interventions such as surgical coronary revascularization and primary percutaneous coronary intervention (PPCI). However, autopsy studies show that most fatal myocardial infarctions (MI) are due to atherosclerotic lesions that do not cause flow-limiting stenosis (Virmani et al., 2006) and therefore would not be detectable using standard X-ray angiography. Techniques such as intravascular ultrasonography (IVUS) (Hartmann et al., 2011) and optical coherence tomography (OCT) (Rieber et al., 2006) can enable characterisation of atherosclerotic plaque composition but cannot report specifically on the inflammatory processes that drive atherosclerotic lesion development. Therefore, imaging approaches are urgently needed for improved detection of inflammatory lesions and for assessment of novel targeted therapeutics in patients at future risk of clinical events, such as MI or ischemic stroke.
Molecular imaging is emerging as a novel approach to identify inflammatory processes in atherosclerosis at the molecular and cellular level. Key steps in the inflammatory cascade include vascular endothelial dysfunction and activation of cell adhesion molecules, monocyte recruitment and differentiation into macrophages, proteolysis and extracellular matrix degradation, apoptosis and angiogenesis (Choudhury et al., 2004). Endothelial activation is a key event in early atherogenesis, characterised by the up-regulation of adhesion molecules, vascular cell adhesion molecule-1, (VCAM-1), intercellular adhesion molecule-1 (ICAM-1), P- and E-selectin, promoting monocyte recruitment to the vascular wall and subsequent lesion development. Arterial regions exposed to low shear stress and disturbed flow, such as the inner curvature of the aortic arch and bifurcations, show increased endothelial activation and susceptibility to developing atherosclerotic lesions (Davies et al., 1993, Nakashima et al., 1998, Ramos et al., 1999). Initial monocyte rolling along activated endothelium is mediated by P-selectin and its interaction with integrin P-selectin glycoprotein ligand-1 (PSGL-1) expressed on monocytes, while firm adhesion of monocytes is mediated by VCAM-1 (CD106) and engagement of the integrin very late antigen-4, VLA-4 (also known as α4β1 integrin) expressed on monocytes (Dansky et al., 2001). VCAM-1 is a promising marker for molecular imaging of vascular inflammation in atherosclerosis, since it is not constitutively expressed in normal vessels but is rapidly up-regulated on vascular endothelial cells in both early and advanced lesions (Cybulsky et al., 2001, Davies et al., 1993) and is readily accessible to blood-borne, targeted contrast agents. VCAM-1 is also up-regulated by macrophages and smooth muscle cells in atherosclerotic plaques (Li et al., 1993, Libby and Li, 1993).
Approaches to in vivo molecular imaging of vascular adhesion molecules in cardiovascular disease have included monoclonal antibody and peptide ligands covalently linked to (i) magnetofluorescent nanoparticles for magnetic resonance imaging (MRI) and intravital microscopy/fluorescent molecular tomography (FMT) (Nahrendorf et al., 2006), (ii) microbubbles for contrast enhanced ultrasound (CEU) (Barreiro et al., 2009, Kaufmann et al., 2010, Kaufmann et al., 2007b, Lindner et al., 2001, Villanueva et al., 2007) or (iii) radiolabels (fluorine-18 (18F) or technetium-99m (99mTc)) for hybrid positron emission tomography (PET)–computed tomography (CT) (Nahrendorf et al., 2009) or single positron emission computed tomography (SPECT) imaging, respectively (Broisat et al., 2012). We have adopted a micro-sized approach for in vivo molecular MRI of vascular endothelial inflammatory responses using microparticles of iron oxide (MPIO) (McAteer et al., 2010, McAteer and Choudhury, 2009, McAteer et al., 2012). Due to their confinement to the intravascular compartment, we have designed MPIO that specifically target endothelial cell activation and leukocyte adhesion, the key regulatory events in atherogenesis. In this review, we discuss nano- and micro-sized contrast agent approaches for cellular and molecular imaging of vascular inflammation in cardiovascular disease by nuclear imaging, MRI and CEU imaging. In particular, we highlight the properties of ligand-targeted MPIO for molecular MRI of endothelial inflammation in experimental models of acute vascular inflammation (McAteer et al., 2007, Serres et al., 2011, Serres et al., 2012), atherosclerosis (McAteer et al., 2008, McAteer et al., 2010, McAteer et al., 2012) and ischaemia–reperfusion injury (Akhtar et al., 2010, Hoyte et al., 2010). For atherosclerosis imaging, we discuss the ability of leukocyte-mimetic MPIO to discriminate endothelial cell-specific activation during early atherosclerotic lesion development using in vivo MRI and the cellular specificity of MPIO binding to atherosclerosis-susceptible sites (McAteer et al., 2012). The limitations and translational potential of ligand-targeted MPIO for clinical imaging of cardiovascular disease are also discussed.
Section snippets
Nuclear imaging
Nuclear imaging techniques such as PET or SPECT allow quantitative measurements of atherosclerotic plaques with high sensitivity. PET has higher temporal resolution and sensitivity relative to SPECT, and can detect radiotracers such as 18F in the nanomolar to picomolar (10− 9 M to 10− 12 M) range. For in vivo vascular imaging using PET, radiotracer 18F-fluorodeoxyglucose (FDG) has been shown to accumulate in macrophage-rich atherosclerotic plaques (Rudd et al., 2009, Tahara et al., 2006). However,
MRI of vascular inflammation using nano-sized iron oxide agents
Nano-sized iron oxide agents, given their small diameter and long blood clearance time (> 24 h) are readily phagocytosed by inflammatory cells. Experimental in vivo MRI studies in hyperlipidemic rabbits (Briley-Saebo et al., 2008, Ruehm et al., 2001) and in humans (Kooi et al., 2003, Trivedi et al., 2006) have shown non-targeted USPIO accumulation in macrophage-rich atherosclerotic plaques. In apoE−/− mice, administered angiotensin II to accelerate vascular inflammation, USPIO were shown to
Properties of microparticles of iron oxide for endothelial cell-specific MRI
Microparticles of iron oxide (MPIO) offer a number of unique properties for endothelial cell-specific molecular imaging. (1) Due to their micron size range, MPIO retain endovascular specificity unlike nano-sized iron oxide agents which are susceptible to passive accumulation in atherosclerotic lesions and non-specific macrophage uptake (Briley-Saebo et al., 2006). (2) The high iron core content and “contrast blooming effect” of MPIO create conspicuous hypo-intense MRI contrast effects on T2
VCAM-1 targeted MPIO imaging of acute vascular inflammation
We have developed MPIO (1 μm diameter) conjugated to monoclonal VCAM-1 antibodies (VCAM-MPIO) and applied them for in vivo detection of acute endothelial cerebral activation in mice using in vivo MRI, at a time that is otherwise undetectable using conventional MRI (McAteer et al., 2007, McAteer et al., 2011). To induce acute inflammation, mice received a microinjection of proinflammatory cytokine interleukin 1β (IL-1β) into the left cerebral hemisphere, while the right hemisphere was not
Targeted imaging of endothelial cell inflammation in atherosclerosis
The ability to image inflammatory changes in atherosclerotic lesions may provide unique information to assess risk in atherosclerosis. Agents that target endothelial adhesion molecule upregulation may be particularly suited to the detection of early atherosclerotic lesions. However, targeted imaging in large arteries, such as the aorta, poses specific challenges since the contrast agent must bind with high affinity and specificity to the endothelial monolayer in sufficient quantity, under
Future directions and translation
The goal of molecular imaging of cardiovascular disease is to develop sensitive, specific, safe and economic methods for detection of atherosclerotic plaques vulnerable to thrombotic complications (Nahrendorf et al., 2012, Sanz and Fayad, 2008). Novel nano- and micro-sized agents are rapidly evolving that specifically target vascular inflammation in cardiovascular disease using a range of targeting ligands and imaging modalities. Targeted MPIO, due to their size and potent contrast effect,
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