Elsevier

Vascular Pharmacology

Volume 58, Issues 1–2, January–February 2013, Pages 31-38
Vascular Pharmacology

Review
Targeted molecular imaging of vascular inflammation in cardiovascular disease using nano- and micro-sized agents

https://doi.org/10.1016/j.vph.2012.10.005Get rights and content

Abstract

Molecular imaging is emerging as a key experimental tool for the identification of inflammatory cellular and molecular processes involved in the development of cardiovascular disease. This review summarises current molecular imaging approaches for the detection of vascular inflammation using a range of nano- and micro-sized contrast agents. We highlight strategies for detection of cell adhesion molecules, which are key regulators of endothelial activation and leukocyte recruitment in atherogenesis and ischaemia–reperfusion in jury. In particular, we address the properties of targeted microparticles of iron oxide (MPIO) for MRI detection of endothelial cell-specific activation of adhesion molecules in experimental models of atherosclerosis, acute vascular inflammation and ischaemia–reperfusion injury, which are otherwise undetectable by conventional imaging modalities. The ability of targeted MPIO to detect endothelial activation could enable early subclinical disease detection and development of novel therapeutic strategies. We discuss opportunities for further development and potential translation of targeted MPIO for clinical imaging of cardiovascular disease.

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,

References (86)

  • R. Virmani et al.

    Pathology of the vulnerable plaque

    J. Am. Coll. Cardiol.

    (2006)
  • O. Will et al.

    Diagnostic precision of nanoparticle-enhanced MRI for lymph-node metastases: a meta-analysis

    Lancet Oncol.

    (2006)
  • A.M. Akhtar et al.

    In vivo quantification of VCAM-1 expression in renal ischemia reperfusion injury using non-invasive magnetic resonance molecular imaging

    PLoS One

    (2010)
  • V. Amirbekian et al.

    Detecting and assessing macrophages in vivo to evaluate atherosclerosis noninvasively using molecular MRI

    Proc. Natl. Acad. Sci. U. S. A.

    (2007)
  • K.C. Briley-Saebo et al.

    Clearance of iron oxide particles in rat liver: effect of hydrated particle size and coating material on liver metabolism

    Invest. Radiol.

    (2006)
  • K.C. Briley-Saebo et al.

    Fractionated Feridex and positive contrast: in vivo MR imaging of atherosclerosis

    Magn. Reson. Med.

    (2008)
  • A. Broisat et al.

    Nanobodies targeting mouse/human VCAM1 for the nuclear imaging of atherosclerotic lesions

    Circ. Res.

    (2012)
  • C. Burtea et al.

    Development of a magnetic resonance imaging protocol for the characterization of atherosclerotic plaque by using vascular cell adhesion molecule-1 and apoptosis-targeted ultrasmall superparamagnetic iron oxide derivatives

    Arterioscler. Thromb. Vasc. Biol.

    (2012)
  • H.H. Chen et al.

    MR imaging of biodegradable polymeric microparticles: a potential method of monitoring local drug delivery

    Magn. Reson. Med.

    (2005)
  • A. Chigaev et al.

    Regulation of cell adhesion by affinity and conformational unbending of alpha4beta1 integrin

    J. Immunol.

    (2007)
  • R.P. Choudhury et al.

    Molecular, cellular and functional imaging of atherothrombosis

    Nat. Rev. Drug Discov.

    (2004)
  • M.I. Cybulsky et al.

    A major role for VCAM-1, but not ICAM-1, in early atherosclerosis

    J. Clin. Invest.

    (2001)
  • H.M. Dansky et al.

    Adhesion of monocytes to arterial endothelium and initiation of atherosclerosis are critically dependent on vascular cell adhesion molecule-1 gene dosage

    Arterioscler. Thromb. Vasc. Biol.

    (2001)
  • M.J. Davies et al.

    The expression of the adhesion molecules ICAM-1, VCAM-1, PECAM, and E-selectin in human atherosclerosis

    J. Pathol.

    (1993)
  • J.C. Frias et al.

    Recombinant HDL-like nanoparticles: a specific contrast agent for MRI of atherosclerotic plaques

    J. Am. Chem. Soc.

    (2004)
  • J.D. Glickson et al.

    Lipoprotein nanoplatform for targeted delivery of diagnostic and therapeutic agents

    Mol. Imaging

    (2008)
  • Grieve, S.M., Lønborg, J., Mazhar, J., Tan, T.C., Ho, E., Liu, C.C., Lay, W., Gill, A.J., Kuchel, P., Bhindi, R.,...
  • S. Ha et al.

    Detection and monitoring of the multiple inflammatory responses by photoacoustic molecular imaging using selectively targeted gold nanorods

    Biomed. Opt. Express

    (2011)
  • M. Hartmann et al.

    Serial intravascular ultrasound assessment of changes in coronary atherosclerotic plaque dimensions and composition: an update

    Eur. J. Echocardiogr.

    (2011)
  • A. Hemmingsson et al.

    Relaxation enhancement of the dog liver and spleen by biodegradable superparamagnetic particles in proton magnetic resonance imaging

    Acta Radiol.

    (1987)
  • C. Heyn et al.

    In vivo magnetic resonance imaging of single cells in mouse brain with optical validation

    Magn. Reson. Med.

    (2006)
  • L.C. Hoyte et al.

    Molecular magnetic resonance imaging of acute vascular cell adhesion molecule-1 expression in a mouse model of cerebral ischemia

    J. Cereb. Blood Flow Metab.

    (2010)
  • B.A. Kaufmann et al.

    Detection of recent myocardial ischaemia by molecular imaging of P-selectin with targeted contrast echocardiography

    Eur. Heart J.

    (2007)
  • B.A. Kaufmann et al.

    Molecular imaging of inflammation in atherosclerosis with targeted ultrasound detection of vascular cell adhesion molecule-1

    Circulation

    (2007)
  • B.A. Kaufmann et al.

    Molecular imaging of the initial inflammatory response in atherosclerosis: implications for early detection of disease

    Arterioscler. Thromb. Vasc. Biol.

    (2010)
  • K.A. Kelly et al.

    Detection of vascular adhesion molecule-1 expression using a novel multimodal nanoparticle

    Circ. Res.

    (2005)
  • M.E. Kooi et al.

    Accumulation of ultrasmall superparamagnetic particles of iron oxide in human atherosclerotic plaques can be detected by in vivo magnetic resonance imaging

    Circulation

    (2003)
  • K. Ley et al.

    Monocyte and macrophage dynamics during atherogenesis

    Arterioscler. Thromb. Vasc. Biol.

    (2011)
  • H. Li et al.

    Inducible expression of vascular cell adhesion molecule-1 by vascular smooth muscle cells in vitro and within rabbit atheroma

    Am. J. Pathol.

    (1993)
  • P. Libby et al.

    Vascular cell adhesion molecule-1 and smooth muscle cell activation during atherogenesis

    J. Clin. Invest.

    (1993)
  • Y.L. Lim et al.

    Possible role of gadolinium in nephrogenic systemic fibrosis: report of two cases and review of the literature

    Clin. Exp. Dermatol.

    (2007)
  • J.R. Lindner et al.

    Ultrasound assessment of inflammation and renal tissue injury with microbubbles targeted to P-selectin

    Circulation

    (2001)
  • M.J. Lipinski et al.

    MRI to detect atherosclerosis with gadolinium-containing immunomicelles targeting the macrophage scavenger receptor

    Magn. Reson. Med.

    (2006)
  • Cited by (25)

    • Molecular imaging of the extracellular matrix in the context of atherosclerosis

      2017, Advanced Drug Delivery Reviews
      Citation Excerpt :

      Iron-oxide-based magnetic nanoparticles are available in variable sizes from nano-sized ultrasmall (20–50 nm) superparamagnetic particles of iron oxide (USPIO), superparamagnetic (60–250 nm) particles of iron oxide (SPIO) and micro-sized (0.9–4.5 μm) particles of iron oxide (MPIO). These types of particles offer different features for molecular imaging [109]. Gd-based probes cause a bright or positive signal effect, as a result of the shortening of the T1 relaxation time [110].

    • MRI-based assessment of endothelial function in mice in vivo

      2015, Pharmacological Reports
      Citation Excerpt :

      This technique enhances MRI sensitivity by using high relaxivity contrast agents with enormous impact on surrounding protons and targeted to biomarkers of the pathological processes taking place in the tissue. Numerous review articles discuss the different aspects of molecular MRI in detail [36], including MRI of endothelium and vascular wall [35,37,38] as well as the use of other imaging modalities [34]. One of the most explored fields of vascular imaging is the assessment and diagnosis of atherosclerosis; many various techniques have been developed to target markers appearing at consecutive stages of the disease's development, from early endothelial inflammation through the later stages of plaque formation, progression, and destabilization related to angiogenesis, extracellular protein expression or apoptosis [38].

    • Small animal cardiovascular MR imaging and spectroscopy

      2015, Progress in Nuclear Magnetic Resonance Spectroscopy
      Citation Excerpt :

      Finally, the very strong T2∗ shortening effect of MPIO enables the detection of even very sparse targets. A comprehensive review of the capabilities of MPIO-enhanced MRI is presented in [509]. McAteer and colleagues have widely explored MPIO-enhanced MRI for the detection of endothelial cell surface markers in relation to several inflammatory processes, including early stages of plaque formation.

    • Molecular targeting of imaging and drug delivery probes in Atherosclerosis

      2013, Annual Reports in Medicinal Chemistry
      Citation Excerpt :

      Covalent binding was successfully achieved by direct reaction of a VCAM antibody to a MPIO carrying tosyl groups. The construct was suitable to detect cardiac and cerebral vascular inflammation in mouse models.41 MPIO dually labeled with P-selectin and VCAM-1 antibodies were more effective than single-label particles in the imaging of plaques in mice,40 whereas MPIO labeled with an antibody of integrin αIIbβ3 detected platelet deposition at sites of vascular injury.52,53

    View all citing articles on Scopus
    View full text