Detection of inflamed atherosclerotic lesions with diadenosine-5',5'''-P1,P4-tetraphosphate (Ap4A) and positron-emission tomography

Uncovering atherosclerotic cardiovascular disease by PET imaging

Assessing atherosclerosis severity is essential for precise patient stratification. Specifically, there is a need to identify patients with residual inflammation because these patients remain at high risk of cardiovascular events despite optimal management of cardiovascular risk factors. Molecular imaging techniques, such as PET, can have an essential role in this context. PET imaging can indicate tissue-based disease status, detect early molecular changes and provide whole-body information. Advances in molecular biology and bioinformatics continue to help to decipher the complex pathogenesis of atherosclerosis and inform the development of imaging tracers. Concomitant advances in tracer synthesis methods and PET imaging technology provide future possibilities for atherosclerosis imaging. In this Review, we summarize the latest developments in PET imaging techniques and technologies for assessment of atherosclerotic cardiovascular disease and discuss the relationship between imaging readouts and transcriptomics-based plaque phenotyping.

Assessment of Total-Body Atherosclerosis by PET/Computed Tomography

Atherosclerotic burden has become the focus of cardiovascular risk assessment. PET/computed tomography (CT) imaging with the tracers 18F-fluorodeoxyglucose and 18F-sodium fluoride shows arterial wall inflammation and microcalcification, respectively. Arterial uptake of both tracers is modestly age dependent. 18F-sodium fluoride uptake is consistently associated with risk factors and more easily measured in the heart. Because of extremely high sensitivity, ultrashort acquisition, and minimal radiation to the patient, total-body PET/CT provides unique opportunities for atherosclerosis imaging: disease screening and delayed and repeat imaging with global disease scoring and parametric imaging to better characterize the atherosclerosis of individual patients.

Recent Advances of Radionuclide-Based Molecular Imaging of Atherosclerosis

Atherosclerosis is a systemic disease characterized by the development of multifocal plaque lesions within vessel walls and extending into the vascular lumen. The disease takes decades to develop symptomatic lesions, affording opportunities for accurate detection of plaque progression, analysis of risk factors responsible for clinical events, and planning personalized treatment. Of the available molecular imaging modalities, radionuclidebased imaging strategies have been favored due to their sensitivity, quantitative detection and pathways for translational research. This review summarizes recent advances of radiolabeled small molecules, peptides, antibodies and nanoparticles for atherosclerotic plaque imaging during disease progression.

Imaging the Cellular Biology of the Carotid Plaque

Carotid atherosclerotic disease is a significant preventable cause of stroke. Clinical decision-making in current practice is based primarily on detection of the severity of luminal stenosis, as determined by ultrasound or conventional angiographic imaging modalities. New insights in the biology of atherosclerosis now suggests that the morphological characteristics of the carotid plaque as well as the molecular and cellular processes occurring within it may be more important markers of plaque vulnerability and stroke risk. This review summarizes emerging applications in the molecular imaging of atherosclerosis and detection of the vulnerable carotid plaque. We discuss how advances in imaging platforms and biochemical technology (e.g. targeted contrast agents) have driven some exciting and promising novel diagnostic imaging approaches from bench to bedside.

Utility of positron emission tomography for drug development for heart failure

Only about 1 in 5,000 investigational agents in a preclinical stage acquires Food and Drug Administration approval. Among many reasons for this includes an inefficient transition from preclinical to clinical phases, which exponentially increase the cost and the delays the process of drug development. Positron emission tomography (PET) is a nuclear imaging technique that has been used for the diagnosis, risk stratification, and guidance of therapy. However, lately with the advance of radiochemistry and of molecular imaging technology, it became evident that PET could help novel drug development process. By using a PET radioligand to report on receptor occupancy during novel agent therapy, it may help assess the effectiveness, efficacy, and safety of such a new medication in an early preclinical stage and help design successful clinical trials even at a later phase. In this article, we explore the potential implications of PET in the development of new heart failure therapies and review PET's application in the respective pathophysiologic pathways such as myocardial perfusion, metabolism, innervation, inflammation, apoptosis, and cardiac remodeling.

Preclinical models of atherosclerosis. The future of Hybrid PET/MR technology for the early detection of vulnerable plaque

Cardiovascular diseases are the leading cause of death in developed countries. The aetiology is currently multifactorial, thus making them very difficult to prevent. Preclinical models of atherothrombotic diseases, including vulnerable plaque-associated complications, are now providing significant insights into pathologies like atherosclerosis, and in combination with the most recent advances in new non-invasive imaging technologies, they have become essential tools to evaluate new therapeutic strategies, with which can forecast and prevent plaque rupture. Positron emission tomography (PET)/computed tomography imaging is currently used for plaque visualisation in clinical and pre-clinical cardiovascular research, albeit with significant limitations. However, the combination of PET and magnetic resonance imaging (MRI) technologies is still the best option available today, as combined PET/MRI scans provide simultaneous data acquisition together with high quality anatomical information, sensitivity and lower radiation exposure for the patient. The coming years may represent a new era for the implementation of PET/MRI in clinical practice, but first, clinically efficient attenuation correction algorithms and research towards multimodal reagents and safety issues should be validated at the preclinical level.

Noninvasive imaging of arterial inflammation using FDG-PET/CT

PURPOSE OF REVIEW

Noninvasive imaging of atherosclerotic plaques has substantially advanced over the past decade such that currently available imaging techniques allow for characterization of high-risk morphological features of the plaques and quantification of the biological activity within the atherosclerotic milieu. Vascular PET/CT imaging provides insights into the biological activity of atherosclerotic plaques and, in particular, plaque inflammation. Fluoro-deoxyglucose-PET/CT imaging is currently used to improve the understanding of atherosclerotic pathophysiology, facilitate the discovery of new treatments and improve clinical prognostication in humans.

RECENT FINDINGS

Several studies have evaluated the feasibility, validity and reproducibility of fluoro-deoxyglucose-PET/CT for imaging of atherosclerotic plaque inflammation. Fluoro-deoxyglucose-PET/CT imaging is demonstrated to have the potential to predict the efficacy of novel antiatherosclerotic therapeutics by using a relatively small sample size and within a relatively short time period in several multicenter trials.

SUMMARY

The currently feasible assessment of inflammation within the atherosclerotic plaques has been demonstrated to enhance assessment of clinical risk, provide a better understanding of therapeutic efficacy of novel drugs, and it may provide a window into inflammation within the coronary tree. Further technological advances in PET technology have the potential to catalyze further progress in imaging of atherosclerotic plaque biology.

Visualizing the atherosclerotic plaque: a chemical perspective

Atherosclerosis is the major underlying pathologic cause of coronary artery disease. An early detection of the disease can prevent clinical sequellae such as angina, myocardial infarction, and stroke. The different imaging techniques employed to visualize the atherosclerotic plaque provide information of diagnostic and prognostic value. Furthermore, the use of contrast agents helps to improve signal-to-noise ratio providing better images. For nuclear imaging techniques and optical imaging these agents are absolutely necessary. We report on the different contrast agents that have been used, are used or may be used in future in animals, humans, or excised tissues for the distinct imaging modalities for atherosclerotic plaque imaging.

Positron emission tomography imaging of atherosclerosis

Atherosclerosis-related cardiovascular events are the leading causes of death in the industrialized world. Atherosclerosis develops insidiously and the initial manifestation is usually sudden cardiac death, stroke, or myocardial infarction. Molecular imaging is a valuable tool to identify the disease at an early stage before fatal manifestations occur. Among the various molecular imaging techniques, this review mainly focuses on positron emission tomography (PET) imaging of atherosclerosis. The targets and pathways that have been investigated to date for PET imaging of atherosclerosis include: glycolysis, cell membrane metabolism (phosphatidylcholine synthesis), integrin α_vβ_3, low density lipoprotein (LDL) receptors (LDLr), natriuretic peptide clearance receptors (NPCRs), fatty acid synthesis, vascular cell adhesion molecule-1 (VCAM-1), macrophages, platelets, etc. Many PET tracers have been investigated clinically for imaging of atherosclerosis. Early diagnosis of atherosclerotic lesions by PET imaging can help to prevent the premature death caused by atherosclerosis, and smooth translation of promising PET tracers into the clinic is critical to the benefit of patients.

Small animal positron emission tomography imaging and in vivo studies of atherosclerosis

Atherosclerosis is a growing health challenge globally, and despite our knowledge of the disease has increased over the last couple of decades, many unanswered questions remain. As molecular imaging can be used to visualize, characterize and measure biological processes at the molecular and cellular levels in living systems, this technology represents an opportunity to investigate some of these questions in vivo. In addition, molecular imaging may be translated into clinical use and eventually pave the way for more personalized treatment regimes in patients. Here, we review the current knowledge obtained from in vivo positron emission tomography studies of atherosclerosis performed in small animals.

Radiolabelled probes for imaging of atherosclerotic plaques

Cardiovascular disease is the leading cause of death worldwide. Unstable atherosclerotic plaques are prone to rupture followed by thrombus formation, vessel stenosis, and occlusion and frequently lead to acute myocardial infarction and brain infarction. As such, unstable plaques represent an important diagnostic target in clinical settings and the specific diagnosis of unstable plaques would enable preventive treatments for cardiovascular disease. To date, various imaging methods such as computed tomography (CT), magnetic resonance imaging (MRI), ultrasound (US), and intravascular ultrasound (IVUS) have been widely used clinically. Although these methods have advantages in terms of spatial resolution and the ability to make detailed identification of morphological alterations such as calcifications and vessel stenosis, these techniques require skill or expertise to discriminate plaque instability, which is essential for early diagnosis and treatment and can present difficulties for quantitative estimation. On the other hand, nuclear imaging techniques such as positron emission tomography (PET) and single photon emission computed tomography (SPECT) can noninvasively collect quantitative information on the expression levels of functional molecules and metabolic activities in vivo and thus provide functional diagnoses of unstable plaques with high sensitivity. Specifically, unstable plaques are characterized by an abundance of invasive inflammatory cells (macrophages), increased oxidative stress that increases oxidized LDL and its receptor expressed on cells in the lesions, increased occurrence of apoptosis of macrophages and other cells involved in disease progression, increased protease expression and activity, and finally thrombus formation triggered by plaque rupture, which is the most important mechanism leading to the onset of infarctions and ischemic sudden death. Therefore, these characteristics can all be targets for molecular imaging by PET and SPECT. In this paper, we review the present state and future of radiolabelled probes that have been developed for detecting atherosclerotic unstable plaques with nuclear imaging techniques.

Quantification of diadenosine polyphosphates in blood plasma using a tandem boronate affinity-ion exchange chromatography system

Endogenous diadenosine polyphosphates (Ap(n)As) have been associated with a variety of biological effects but quantifying their concentration in blood is difficult. We report on the development of a tandem affinity-ion exchange high-performance liquid chromatography (HPLC) system that employs boronate affinity upstream of ion exchange chromatography for automated rapid (45-min) resolution and extraction of Ap(n)As from human plasma. This system obviates previous requirements for multiple column separations and handling steps, so it is ideally set up for time- and cost-efficient screening of blood samples for Ap(n)A pharmacokinetic and biodistribution studies.

Intravascular optical coherence tomography detection of atherosclerosis and inflammation in murine aorta

OBJECTIVE

The goal of this study was to evaluate the feasibility of imaging the aorta of apolipoprotein E-deficient (ApoE(-/-)) mice for the detection of atherosclerosis and macrophages using optical coherence tomography (OCT) compared with histology.

METHODS AND RESULTS

Atherosclerosis was induced by high-fat diet in 7-week-old ApoE(-/-) mice for 10 (n=7) and 22 (n=7) weeks. Nine-week-old ApoE(-/-) mice (n=7) fed a standard chow diet were used as controls. OCT images of a 10-mm descending aorta in situ were performed in 4 mice for each, and plaque and macrophages were determined at 0.5-mm intervals. Automated detection and quantification of macrophages were performed independently using a customized algorithm. Coregistered histological cross-sections were stained with hematoxylin-eosin, Mac-3, and von Kossa. Three mice in each group had en face OCT imaging to detect macrophages, which were compared with lipid-positive area with Sudan IV. OCT images were successfully acquired in all mice. OCT and histology were able to discriminate macrophages and plaque among the 3 groups and showed excellent correlation for (1) visual detection of plaque (r=0.98) and macrophages (r=0.93), (2) automated detection and quantification of macrophages by OCT versus Mac-3-positive area (r=0.92), and (3) en face OCT detection of macrophages versus Sudan IV-positive area (r=0.92).

CONCLUSIONS

Murine intra-aortic OCT is feasible and shows excellent correlation with histology for detection of atherosclerotic plaque and macrophages.

PET imaging of aortic atherosclerosis: Is combined imaging of plaque anatomy and function an amaranthine quest or conceivable reality?

Traditionally, blood vessels have been studied using contrast luminography to determine the site, extent and severity of luminal compromise by atherosclerotic deposits. Similar anatomical data can now be acquired non-invasively using ultrasound, computed tomography or magnetic resonance imaging. Plaque stability is an important determinant of subsequent vascular events and currently functional data on the stability of plaque is less well provided by these methods. The search for non-invasive techniques to image combined plaque anatomy and function has been pursued with visionary anticipation. This expectation may soon be realised as imaging with radionuclide-labelled atheroma-targeted contrast agents has demonstrated that plaque functional characteristics can now be shown. Increasingly positron emission tomography/computed tomography (PET/CT) imaging with (18)F fluorodexoyglucose (FDG) and other radionuclides is being used to determine culprit plaques in complex clinically scenarios. Clinically, this information may prove extremely valuable in the assessment of stable and unstable patients and its use in prime time medical practice is eagerly awaited. We will discuss the current clinical applications of functional atheroma imaging in the aorta and highlight the promising preclinical data on novel image biomarkers of plaque instability. If clinical science is able to successfully translate these advances in vascular imaging from the bench to the bedside, a new paradigm will be achieved in cardiovascular diagnostics.

Vascular imaging with positron emission tomography

Atherosclerosis is an inflammatory disease that causes most myocardial infarctions, strokes and acute coronary syndromes. Despite the identification of multiple risk factors and widespread use of drug therapies, it still remains a global health concern with associated costs. Although angiography is established as the gold standard means of detecting coronary artery stenosis, it does not image the vessel wall itself, reporting only on its consequences such as luminal narrowing and obstruction. MRI and computed tomography provide more information about the plaque structure, but recently positron emission tomography (PET) imaging using [(18) F]-fluorodeoxyglucose (FDG) has been advocated as a means of measuring arterial inflammation. This results from the ability of FDG-PET to highlight areas of high glucose metabolism, a feature of macrophages within atherosclerosis, particularly in high-risk plaques. It is suggested that the degree of FDG accumulation in the vessel wall reflects underlying inflammation levels and that tracking any changes in FDG uptake over time or with drug therapy might be a way of getting an early efficacy readout for novel anti-atherosclerotic drugs. Early reports also demonstrate that FDG uptake is correlated with the number of cardiovascular risk factors and possibly even the risk of future cardiovascular events. This review will outline the evidence base, shortcomings and emerging applications for FDG-PET in vascular imaging. Alternative PET tracers and other candidate imaging modalities for measuring vascular inflammation will also be discussed.

Noninvasive Positron Emission Tomography Imaging of Coronary Arterial Inflammation

The importance of inflammation to atherothrombosis has led to the pursuit of noninvasive imaging methods to measure inflammation within the arterial wall. There is substantial evidence supporting the use of (18)F-fluorodeoxyglucose positron emission tomography (FDG-PET) imaging for evaluation of atherosclerotic plaque inflammation. However, coronary imaging with this technique has been limited, due to several technical hurdles. Nonetheless, early experiences in coronary FDG-PET imaging have been encouraging. This review outlines the development of vascular PET imaging and its potential use for evaluation of coronary artery disease.

Imaging atherosclerotic plaque inflammation by fluorodeoxyglucose with positron emission tomography: ready for prime time?

Inflammation is a determinant of atherosclerotic plaque rupture, the event leading to most myocardial infarctions and strokes. Although conventional imaging techniques identify the site and severity of luminal stenosis, the inflammatory status of the plaque is not addressed. Positron emission tomography imaging of atherosclerosis using the metabolic marker fluorodeoxyglucose allows quantification of arterial inflammation across multiple vessels. This review sets out the background and current and potential future applications of this emerging biomarker of cardiovascular risk, along with its limitations.

18F-4V for PET-CT imaging of VCAM-1 expression in atherosclerosis

OBJECTIVES

The aim of this study was to iteratively develop and validate an (18)F-labeled small vascular cell adhesion molecule (VCAM)-1 affinity ligand and demonstrate the feasibility of imaging VCAM-1 expression by positron emission tomography-computed tomography (PET-CT) in murine atherosclerotic arteries.

BACKGROUND

Hybrid PET-CT imaging allows simultaneous assessment of atherosclerotic lesion morphology (CT) and may facilitate early risk assessment in individual patients. The early induction, confinement of expression to atherosclerotic lesions, and accessible position in proximity to the blood pool render the adhesion molecule VCAM-1 an attractive imaging biomarker for inflamed atheroma prone to complication.

METHODS

A cyclic, a linear, and an oligomer affinity peptide, internalized into endothelial cells by VCAM-1-mediated binding, were initially derivatized with DOTA to determine their binding profiles and pharmacokinetics. The lead compound was then (18)F-labeled and tested in atherosclerotic apoE(-/-) mice receiving a high-cholesterol diet as well as wild type murine models of myocardial infarction and heart transplant rejection.

RESULTS

The tetrameric peptide had the highest affinity and specificity for VCAM-1 (97% inhibition with soluble VCAM-1 in vitro). In vivo PET-CT imaging using (18)F-4V showed 0.31 +/- 0.02 SUV in murine atheroma (ex vivo %IDGT 5.9 +/- 1.5). (18)F-4V uptake colocalized with atherosclerotic plaques on Oil Red O staining and correlated to mRNA levels of VCAM-1 measured by quantitative reverse transcription polymerase chain reaction (R = 0.79, p = 0.03). Atherosclerotic mice receiving an atorvastatin-enriched diet had significantly lower lesional uptake (p < 0.05). Furthermore, (18)F-4V imaging in myocardial ischemia after coronary ligation and in transplanted cardiac allografts undergoing rejection showed high in vivo PET signal in inflamed myocardium and good correlation with ex vivo measurement of VCAM-1 mRNA by quantitative polymerase chain reaction.

CONCLUSIONS

(18)F-4V allows noninvasive PET-CT imaging of VCAM-1 in inflammatory atherosclerosis, has the dynamic range to quantify treatment effects, and correlates with inflammatory gene expression.

Nanoparticles for optical molecular imaging of atherosclerosis

Molecular imaging contributes to future personalized medicine dedicated to the treatment of cardiovascular disease, the leading cause of mortality in industrialized countries. Endoscope-compatible optical imaging techniques would offer a stand-alone alternative and high spatial resolution validation technique to clinically accepted imaging techniques in the (intravascular) assessment of vulnerable atherosclerotic lesions, which are predisposed to initiate acute clinical events. Efficient optical visualization of molecular epitopes specific for vulnerable atherosclerotic lesions requires targeting of high-quality optical-contrast-enhancing particles. In this review, we provide an overview of both current optical nanoparticles and targeting ligands for optical molecular imaging of atherosclerotic lesions and speculate on their applicability in the clinical setting.

Novel diadenosine polyphosphate analogs with oxymethylene bridges replacing oxygen in the polyphosphate chain: potential substrates and/or inhibitors of Ap4A hydrolases

Dinucleoside polyphosphates (Np(n)N's; where N and N' are nucleosides and n = 3-6 phosphate residues) are naturally occurring compounds that may act as signaling molecules. One of the most successful approaches to understand their biological functions has been through the use of Np(n)N' analogs. Here, we present the results of studies using novel diadenosine polyphosphate analogs, with an oxymethylene group replacing one or two bridging oxygen(s) in the polyphosphate chain. These have been tested as potential substrates and/or inhibitors of the symmetrically acting Ap(4)A hydrolase [bis(5'-nucleosyl)-tetraphosphatase (symmetrical); EC 3.6.1.41] from E. coli and of two asymmetrically acting Ap(4)A hydrolases [bis(5'-nucleosyl)-tetraphosphatase (asymmetrical); EC 3.6.1.17] from humans and narrow-leaved lupin. The six chemically synthesized analogs were: ApCH(2)OpOCH(2)pA (1), ApOCH(2)pCH(2)OpA (2), ApOpCH(2)OpOpA (3), ApCH(2)OpOpOCH(2)pA (4), ApOCH(2)pOpCH(2)OpA (5) and ApOpOCH(2)pCH(2)OpOpA (6). The eukaryotic asymmetrical Ap(4)A hydrolases degrade two compounds, 3 and 5, as anticipated in their design. Analog 3 was cleaved to AMP (pA) and beta,gamma-methyleneoxy-ATP (pOCH(2)pOpA), whereas hydrolysis of analog 5 gave two molecules of alpha,beta-oxymethylene ADP (pCH(2)OpA). The relative rates of hydrolysis of these analogs were estimated. Some of the novel nucleotides were moderately good inhibitors of the asymmetrical hydrolases, having K(i) values within the range of the K(m) for Ap(4)A. By contrast, none of the six analogs were good substrates or inhibitors of the bacterial symmetrical Ap(4)A hydrolase.

Noninvasive in vivo imaging of monocyte trafficking to atherosclerotic lesions

BACKGROUND

Monocytes play a key role in atherogenesis, but their participation has been discerned largely via ex vivo analyses of atherosclerotic lesions. We sought to establish a noninvasive technique to determine monocyte trafficking to atherosclerotic lesions in live animals.

METHODS AND RESULTS

Using a micro-single-photon emission computed tomography small-animal imaging system and a Food and Drug Administration-approved radiotracer ([indium 111] oxyquinoline, (111)In-oxine), we demonstrate here that monocyte recruitment to atherosclerotic lesions can be visualized in a noninvasive, dynamic, and 3-dimensional fashion in live animals. We show in vivo that monocytes are recruited avidly to plaques within days of adoptive transfer. Using micro-single-photon emission computed tomography imaging as a screening tool, we were able to investigate modulatory effects on monocyte recruitment in live animals. We found that 3-hydroxy-3-methylglutaryl coenzyme A reductase inhibitors rapidly and substantially reduce monocyte recruitment to existing atherosclerotic lesions, as imaged here in vivo.

CONCLUSIONS

This novel approach to track monocytes to atherosclerotic plaques in vivo should have broad applications and create new insights into the pathogenesis of atherosclerosis and other inflammatory diseases.

Intravascular detection of inflamed atherosclerotic plaques using a fluorescent photosensitizer targeted to the scavenger receptor

Inflammation plays an important role in the pathophysiology of atherosclerotic disease. We have previously shown that the targeted photosensitizer chlorin (e(6)) conjugated with maleylated albumin (MA-ce6) is taken up by macrophages via the scavenger receptor with high selectivity. In a rabbit model of inflamed plaque in New Zealand white rabbits via balloon injury of the aorto-iliac arteries and high cholesterol diet we showed that the targeted conjugate showed specificity towards plaques compared to free ce6. We now show that an intravascular fiber-based spectrofluorimeter advanced along the -iliac vessel through blood detects 24-fold higher fluorescence in atherosclerotic vessels compared to control rabbits (p < 0.001 ANOVA). Within the same animals, signal derived from the injured iliac artery was 16-fold higher than the contralateral uninjured iliac (p < 0.001). Arteries were removed and selective accumulation of MA-ce6 in plaques was confirmed using: (1) surface spectrofluorimetry, (2) fluorescence extraction of ce6 from aortic segments, and (3) confocal microscopy. Immunohistochemical analysis of the specimens showed a significant correlation between MA-ce6 uptake and RAM-11 macrophage staining (R = 0.83, p < 0.001) and an inverse correlation between MA-ce6 uptake and smooth muscle cell staining (R = -0.74, p < 0.001). MA-ce6 may function as a molecular imaging agent to detect and/or photodynamically treat inflamed plaques.