99mTc-Annexin A5 for noninvasive characterization of atherosclerotic lesions: imaging and histological studies in myocardial infarction-prone Watanabe heritable hyperlipidemic rabbits

Recent Advances in the Development of PET/SPECT Probes for Atherosclerosis Imaging

The rupture of vulnerable atherosclerotic plaques and subsequent thrombus formation are the major causes of myocardial and cerebral infarction. Accordingly, the detection of vulnerable plaques is important for risk stratification and to provide appropriate treatment. Inflammation imaging using 2-deoxy-2-[¹⁸F]fluoro-D-glucose (¹⁸F-FDG) has been most extensively studied for detecting vulnerable atherosclerotic plaques. It is of great importance to develop PET/SPECT probes capable of specifically visualizing the biological molecules involved in atherosclerotic plaque formation and/or progression. In this article, we review recent advances in the development of PET/SPECT probes for visualizing atherosclerotic plaques and their application to therapy monitoring, mainly focusing on experimental studies.

Increased B-type-natriuretic peptide promotes myocardial cell apoptosis via the B-type-natriuretic peptide/long non-coding RNA LSINCT5/caspase-1/interleukin 1β signaling pathway

Chronic heart failure (CHF) is the final stage of various heart diseases, and is increasingly recognized as a major health problem in the elderly. Previous studies demonstrated that B-type-natriuretic peptide (BNP) is an established biomarker of CHF. Furthermore, BNP also regulates cell proliferation, differentiation and apoptosis. Recent evidence has revealed that BNP affects myocardial cell apoptosis during myocardial ischemia-reperfusion injury. Long non-coding RNAs (lncRNAs) are emerging as novel molecular compounds involved in gene regulation, and have important roles in numerous human diseases. However, the mechanism underlying the BNP and lncRNA-induced regulation of myocardial cell apoptosis remains to be elucidated. The present study reported that lncRNA LSINCT5, upregulated by BNP, is able to regulate myocardial cell apoptosis via the activation of the caspase-1/interleukin (IL)-1β signaling pathway. BNP-induced apoptosis of HCM cells was observed using flow cytometry, and involved caspase-1. In addition, expression profiling using a human lncRNA polymerase chain reaction array revealed that LSINCT5 was highly expressed in BNP-treated myocardial cells, as compared with untreated cells. The role of lncRNA LSINCT5 in HCM cell apoptosis was also investigated. The results of the present study indicated that LSINCT5 silencing by small interfering RNA inhibits caspase-1/IL-1β signaling, and suppresses apoptosis in BNP-treated HCM cells. Therefore, high expression levels of BNP promote the apoptosis of myocardial cells through the lncRNA LSINCT5 mediator, which activates the caspase-1/IL-1β signaling pathway. These findings uncovered a novel pathogenic mechanism, and provided a potential therapeutic target for CHF.

In vivo and in vitro evidence that ⁹⁹mTc-HYNIC-interleukin-2 is able to detect T lymphocytes in vulnerable atherosclerotic plaques of the carotid artery

PURPOSE

Recent advances in basic science have established that inflammation plays a pivotal role in the pathogenesis of atherosclerosis. Inflammatory cells are thought to be responsible for the transformation of a stable plaque into a vulnerable one. Lymphocytes constitute at least 20 % of infiltrating cells in these vulnerable plaques. Therefore, the interleukin-2 (IL-2) receptor, being overexpressed on activated T lymphocytes, may represent an attractive biomarker for plaque vulnerability. The aim of this study was to evaluate the specificity of radiolabelled IL-2 [(99m)Tc-hydrazinonicotinamide (HYNIC)-IL-2] for imaging the lymphocytic infiltration in carotid plaques in vivo by planar and single photon emission computed tomography (SPECT)/CT imaging and ex vivo by microSPECT and autoradiography.

METHODS

For the in vivo study, ten symptomatic patients with advanced plaques at ultrasound who were scheduled for carotid endarterectomy underwent (99m)Tc-HYNIC-IL-2 scintigraphy. The images were analysed visually on planar and SPECT images and semi-quantitatively on SPECT images by calculating target to background (T/B) ratios. After endarterectomy, immunomorphological evaluation and immunophenotyping were performed on plaque slices. For the ex vivo studies, four additional patients were included and, after in vitro incubation of removed plaques with (99m)Tc-HYNIC-IL-2, autoradiography was performed and microSPECT images were acquired.

RESULTS

Visual analysis defined clear (99m)Tc-HYNIC-IL-2 uptake in seven of the ten symptomatic plaques. SPECT/CT allowed visualization in eight of ten. A significant correlation was found between the number of CD25+ lymphocytes and the total number of CD25+ cells in the plaque and the T/B ratio with adjacent carotid artery as background (Pearson's r = 0.89, p = 0.003 and r = 0.87, p = 0.005, respectively). MicroSPECT imaging showed clear (99m)Tc-HYNIC-IL-2 uptake within the plaque wall and not in the lipidic core. With autoradiography, only CD3+ lymphocytes were found to be labelled.

CONCLUSION

These in vivo and ex vivo studies confirm the specificity of (99m)Tc-HYNIC-IL-2 for imaging activated T lymphocytes in carotid plaques. (99m)Tc-HYNIC-IL-2 is a true marker for the inflamed plaque and therefore of plaque instability.

Development of a radioiodinated triazolopyrimidine probe for nuclear medical imaging of fatty acid binding protein 4

Fatty acid binding protein 4 (FABP4) is the most well-characterized FABP isoform. FABP4 regulates inflammatory pathways in adipocytes and macrophages and is involved in both inflammatory diseases and tumor formation. FABP4 expression was recently reported for glioblastoma, where it may participate in disease malignancy. While FABP4 is a potential molecular imaging target, with the exception of a tritium labeled probe there are no reports of other nuclear imaging probes that target this protein. Here we designed and synthesized a nuclear imaging probe, [¹²³I]TAP1, and evaluated its potential as a FABP4 targeting probe in in vitro and in vivo assays. We focused on the unique structure of a triazolopyrimidine scaffold that lacks a carboxylic acid to design the TAP1 probe that can undergo facilitated delivery across cell membranes. The affinity of synthesized TAP1 was measured using FABP4 and 8-anilino-1-naphthalene sulfonic acid. [¹²⁵I]TAP1 was synthesized by iododestannylation of a precursor, followed by affinity and selectivity measurements using immobilized FABPs. Biodistributions in normal and C6 glioblastoma-bearing mice were evaluated, and excised tumors were subjected to autoradiography and immunohistochemistry. TAP1 and [¹²⁵I]TAP1 showed high affinity for FABP4 (K_i = 44.5±9.8 nM, K_d = 69.1±12.3 nM). The FABP4 binding affinity of [¹²⁵I]TAP1 was 11.5- and 35.5-fold higher than for FABP3 and FABP5, respectively. In an in vivo study [¹²⁵I]TAP1 displayed high stability against deiodination and degradation, and moderate radioactivity accumulation in C6 tumors (1.37±0.24% dose/g 3 hr after injection). The radioactivity distribution profile in tumors partially corresponded to the FABP4 positive area and was also affected by perfusion. The results indicate that [¹²⁵I]TAP1 could detect FABP4 in vitro and partly in vivo. As such, [¹²⁵I]TAP1 is a promising lead compound for further refinement for use in in vivo FABP4 imaging.

Molecular imaging of inflammation in atherosclerosis

Acute rupture of vulnerable plaques frequently leads to myocardial infarction and stroke. Within the last decades, several cellular and molecular players have been identified that promote atherosclerotic lesion formation, maturation and plaque rupture. It is now widely recognized that inflammation of the vessel wall and distinct leukocyte subsets are involved throughout all phases of atherosclerotic lesion development. The mechanisms that render a stable plaque unstable and prone to rupture, however, remain unknown and the identification of the vulnerable plaque remains a major challenge in cardiovascular medicine. Imaging technologies used in the clinic offer minimal information about the underlying biology and potential risk for rupture. New imaging technologies are therefore being developed, and in the preclinical setting have enabled new and dynamic insights into the vessel wall for a better understanding of this complex disease. Molecular imaging has the potential to track biological processes, such as the activity of cellular and molecular biomarkers in vivo and over time. Similarly, novel imaging technologies specifically detect effects of therapies that aim to stabilize vulnerable plaques and silence vascular inflammation. Here we will review the potential of established and new molecular imaging technologies in the setting of atherosclerosis, and discuss the cumbersome steps required for translating molecular imaging approaches into the clinic.

The vulnerable coronary plaque: update on imaging technologies

Several studies have been carried out on vulnerable plaque as the main culprit for ischaemic cardiac events. Historically, the most important diagnostic technique for studying coronary atherosclerotic disease was to determine the residual luminal diameter by angiographic measurement of the stenosis. However, it has become clear that vulnerable plaque rupture as well as thrombosis, rather than stenosis, triggers most acute ischaemic events and that the quantification of risk based merely on severity of the arterial stenosis is not sufficient. In the last decades, substantial progresses have been made on optimisation of techniques detecting the arterial wall morphology, plaque composition and inflammation. To date, the use of a single technique is not recommended to precisely identify the progression of the atherosclerotic process in human beings. In contrast, the integration of data that can be derived from multiple methods might improve our knowledge about plaque destabilisation. The aim of this narrative review is to update evidence on the accuracy of the currently available non-invasive and invasive imaging techniques in identifying components and morphologic characteristics associated with coronary plaque vulnerability.

Radioiodinated peptide probe for selective detection of oxidized low density lipoprotein in atherosclerotic plaques

INTRODUCTION

Despite the significant effort in developing radioprobes for atherosclerosis, few have low molecular weight. Oxidized LDL (OxLDL), a highly proinflammatory and proatherogenic factor that is abundant in atherosclerotic plaques, plays a pivotal role in plaque destabilization, which makes OxLDL a relevant probe target. We developed a radioiodinated short peptide, AHP7, as a low molecular weight probe for specific OxLDL imaging and evaluated its utility using myocardial infarction-prone Watanabe heritable hyperlipidemic rabbits (WHHLMI).

METHODS

[¹²⁵I]AHP7 was designed and synthesized based on the sequence of Asp-hemolysin, an OxLDL binding protein extracted from Aspergillus fumigatus. In vitro binding studies with OxLDL having varying degrees of oxidation were performed. Radioactivity accumulation in the aorta was measured 30 min post-administration in rabbits. Autoradiography and histological studies were performed using serial aorta sections. A radioiodinated scrambled peptide ([¹²⁵I]AHP scramble) was used as a negative control.

RESULTS

[¹²⁵I]AHP7 bound to OxLDL in proportion to the degree of oxidation (R=0.91, P<0.0001) and was inhibited by unlabeled AHP7 in a concentration-dependent manner. The aorta accumulation level and aorta/blood and aorta/muscle ratios of [¹²⁵I]AHP7 in WHHLMI were 2.8-, 1.3- and 1.8-fold higher, respectively, than those in control rabbits (P<0.001). Co-administration of AHP7 significantly reduced [¹²⁵I]AHP7 radioactivity in aorta sections (P<0.0001). Regional radioactivity levels in the aorta sections showed nonuniformity but similarity to the immunohistochemical OxLDL density.

CONCLUSIONS

The potential of radioiodinated AHP7 for selectively imaging OxLDL was demonstrated both in vitro and in vivo.

Molecular imaging of atherosclerosis for improving diagnostic and therapeutic development

Despite recent progress, cardiovascular and allied metabolic disorders remain a worldwide health challenge. We must identify new targets for therapy, develop new agents for clinical use, and deploy them in a clinically effective and cost-effective manner. Molecular imaging of atherosclerotic lesions has become a major experimental tool in the last decade, notably by providing a direct gateway to the processes involved in atherogenesis and its complications. This review summarizes the current status of molecular imaging approaches that target the key processes implicated in plaque formation, development, and disruption and highlights how the refinement and application of such tools might aid the development and evaluation of novel therapeutics.

Quantitative determination of apoptosis of pancreatic β-cells in a murine model of type 1 diabetes mellitus

Type 1 diabetes mellitus is characterized by a significant deficit in pancreatic β-cell mass, presumably caused by β-cell apoptosis. We investigated the incidence of β-cell apoptosis in streptozotocin-treated mice and nonobese diabetic (NOD) mice with (99m)Tc-annexin A5.

METHODS

Vehicle-treated mice, streptozotocin-treated mice, and NOD mice at the ages of 5, 9, 16, and 20 wk (5-8 mice per group) were injected with (99m)Tc-annexin A5 and sacrificed 6 h later for autoradiography, and the regional (99m)Tc-annexin A5 level in the pancreas was evaluated. Pancreatic islets were identified by insulin immunohistochemical staining, and apoptotic cells were determined by terminal deoxynucleotidyl transferase-mediated dUTP nick-end labeling (TUNEL) staining. The (99m)Tc-annexin A5 level in pancreatic islets was expressed as the percentage injected dose per area of pancreatic islets and normalized by animal body weight (%ID × 10(6)/mm(2)/kg). The level of apoptotic cells in pancreatic islets was expressed as the number of TUNEL-positive cells per area of pancreatic islets (cells/mm(2)).

RESULTS

The (99m)Tc-annexin A5 accumulation level was significantly higher (2.5 ± 0.7 vs. 0.7 ± 0.1 %ID × 10(6)/mm(2)/kg, P < 0.05) and the number of TUNEL-positive cells was significantly higher (1,170 ± 535 vs. 5 ± 6 cells/mm(2), P < 0.05) in the pancreatic islets of the streptozotocin-treated mice than in those of the vehicle-treated mice. The (99m)Tc-annexin A5 accumulation level was significantly higher (1.1 ± 0.4 vs. 0.5 ± 0.1 %ID × 10(6)/mm(2)/kg, P < 0.05) and the number of TUNEL-positive cells was significantly higher (152 ± 82 vs. 4 ± 9 cells/mm(2), P < 0.05) in the pancreatic islets of 16-wk-old NOD mice than in those of 5-wk-old NOD mice. In addition, the level of (99m)Tc-annexin A5 correlated with the number of TUNEL-positive cells in the pancreatic islets of the streptozotocin-treated mice (r = 0.821, P < 0.001) and NOD mice (r = 0.721, P < 0.001).

CONCLUSION

There is significant islet cell apoptosis with (99m)Tc-annexin A5 accumulation in the pancreas of both streptozotocin and NOD mice.

Localization of deoxyglucose and annexin A5 in experimental atheroma correlates with macrophage infiltration but not lipid deposition in the lesion

PURPOSE

The aim of this study was to understand the relationship of lipid deposition to the macrophage content, macrophage metabolism, and apoptosis in plaque. We compared the uptake of 2-deoxy-2-fluoro-D-[(14)C]glucose ([(14)C]FDG) and [(99m)Tc]HYNIC-annexin V ([(99m)Tc]annexin A5) with the lesion histology in apolipoprotein E knockout (apoE(-/-)) mice.

PROCEDURES

Male apoE(-/-) mice (n = 9) were injected with [(14)C]FDG and [(99m)Tc]annexin A5. Cryostat sections of aorta samples (n = 49) were used for dual-tracer autoradiography, and regional tracer uptake levels were evaluated. Lesions were identified histologically with Movat's pentachrome (AHA lesion phenotypes), Mac-2 staining (macrophage infiltration) and Oil Red O staining (lipid deposition).

RESULTS

The highest uptakes of [(14)C]FDG (3.10 ± 1.50 %ID × kilogram per square millimeter) and [(99m)Tc]annexin A5 (0.49 ± 0.20 %ID × kilogram per square millimeter) were shown in atheromatous lesions (types III and IV). Each tracer uptake showed better correlation with macrophage infiltration than lipid deposition ([(14)C]FDG, r = 0.44 vs. r = 0.14; [(99m)Tc]annexin A5, r = 0.65 vs. r = 0.48).

CONCLUSIONS

Both tracers were concentrated in type III and IV atheromatous lesions which corresponded to macrophage infiltration rather than lipid deposition.

Atherosclerosis plaque heterogeneity and response to therapy detected by in vivo molecular imaging of matrix metalloproteinase activation

Matrix metalloproteinases (MMPs) play a key role in the development of atherosclerosis and its complications. In vivo detection and quantification of MMP activation can help track the propensity to complications and response to therapy. We sought to establish an in vivo imaging approach for monitoring MMP activation in atherosclerotic mouse aorta and use it to assess the response to dietary modification.

METHOD

Apolipoprotein-deficient mice were fed normal chow or a high-fat diet (HFD) for up to 3 mo or a HFD for 2 mo, followed by 1 mo on normal chow. Then they underwent micro-SPECT/CT, along with autoradiography and oil red O staining of tissues.

RESULTS

After 3 mo of HFD, there was considerable atherosclerosis in the aorta. In vivo micro-SPECT/CT using RP782 (an (111)In-labeled tracer targeting activated MMPs) showed a heterogeneous pattern of tracer uptake along the aorta. Heterogeneity of RP782 uptake was confirmed by autoradiography, and specificity was demonstrated using excess unlabeled precursor. Tracer uptake quantified by micro-SPECT significantly correlated with uptake quantified by autoradiography. Comparison of oil red O staining with autoradiography demonstrated areas of discordance between plaque presence and tracer uptake. HFD withdrawal led to significant reduction in RP782 uptake beyond the effect on plaque area. MMP expression and macrophage infiltration were similarly heterogeneous along the aorta and significantly reduced after withdrawal from the HFD. Finally, RP782 uptake significantly correlated with aortic macrophage content.

CONCLUSION

Molecular imaging of MMP activation reveals the heterogeneity of atherosclerotic plaques and is a useful tool for tracking plaque biology and response to therapy.

Molecular imaging of macrophage protease activity in cardiovascular inflammation in vivo

Macrophages contribute pivotally to cardiovascular diseases (CVD), notably to atherosclerosis. Imaging of macrophages in vivo could furnish new tools to advance evaluation of disease and therapies. Proteolytic enzymes serve as key effectors of many macrophage contributions to CVD. Therefore, intravital imaging of protease activity could aid evaluation of the progress and outcome of atherosclerosis, aortic aneurysm formation, or rejection of cardiac allografts. Among the large families of proteases, matrix metalloproteinases (MMPs) and cysteinyl cathepsins have garnered the most interest because of their participation in extracellular matrix remodelling. These considerations have spurred the development of dedicated imaging agents for protease activity detection. Activatable fluorescent probes, radiolabelled inhibitors, and nanoparticles are currently under exploration for this purpose. While some agents and technologies may soon see clinical use, others will require further refinement. Imaging of macrophages and protease activity should provide an important adjunct to understanding pathophysiology in vivo, evaluating the effects of interventions, and ultimately aiding clinical care.

Tissue factor detection for selectively discriminating unstable plaques in an atherosclerotic rabbit model

Tissue factor (TF), a transmembrane glycoprotein that acts as an essential cofactor to factor VII/VIIa, initiates the exogenous blood coagulation cascade leading to thrombin generation and subsequent thrombus formation in vivo. TF expression is closely related to plaque vulnerability, and high TF expression is shown in macrophage-rich atheromatous lesions, making TF a potential target for detecting atheromatous lesions in vivo. Thus, we prepared (99m)Tc-labeled anti-TF-monoclonal antibody (TF-mAb) IgG as a molecular probe and evaluated its usefulness to achieve TF-specific imaging using myocardial infarction-prone Watanabe heritable hyperlipidemic (WHHLMI) rabbits.

METHODS

Anti-TF-mAb was created using a standard hybridoma technique and was labeled by (99m)Tc with 6-hydrazinonicotinic acid (HYNIC) as a chelating agent to obtain (99m)Tc-TF-mAb. The immunoreactivity of HYNIC-TF-mAb was estimated by flow cytometry. WHHLMI and control rabbits were injected intravenously with (99m)Tc-TF-mAb. Twenty-four hours after the injection, the aorta was removed and radioactivity was measured. Autoradiography and histologic studies were performed using serial aorta sections. Subclass matched antibody (IgG(1)) was used as a negative control.

RESULTS

HYNIC-TF-mAb showed 93% immunoreactivity of the anti-TF-mAb. The radioactivity accumulation in WHHLMI aortas was 6.1-fold higher than that of control rabbits. Autoradiograms showed a heterogeneous distribution of radioactivity in the intima of WHHLMI aortas. Regional radioactivity accumulation was positively correlated with TF expression density (R = 0.64, P < 0.0001). The highest radioactivity accumulation in percentage injected dose × body weight/mm(2) × 10(2) was found in atheromatous lesions (5.2 ± 1.9) followed by fibroatheromatous (2.1 ± 0.7), collagen-rich (1.8 ± 0.7), and neointimal lesions (1.8 ± 0.6). In contrast, (99m)Tc-IgG(1) showed low radioactivity accumulation in WHHLMI aortas that was independent of the histologic grade of lesions.

CONCLUSION

The TF-detecting ability and preferential accumulation in atheromatous lesions of (99m)Tc-TF-mAb were demonstrated, indicating its potential for selective imaging of macrophage-rich atheromatous lesions in vivo.

Rapid detection of hypoxia-inducible factor-1-active tumours: pretargeted imaging with a protein degrading in a mechanism similar to hypoxia-inducible factor-1alpha

PURPOSE

Hypoxia-inducible factor-1 (HIF-1) plays an important role in malignant tumour progression. For the imaging of HIF-1-active tumours, we previously developed a protein, POS, which is effectively delivered to and selectively stabilized in HIF-1-active cells, and a radioiodinated biotin derivative, (3-(123)I-iodobenzoyl)norbiotinamide ((123)I-IBB), which can bind to the streptavidin moiety of POS. In this study, we aimed to investigate the feasibility of the pretargeting method using POS and (123)I-IBB for rapid imaging of HIF-1-active tumours.

METHODS

Tumour-implanted mice were pretargeted with POS. After 24 h, (125)I-IBB was administered and subsequently, the biodistribution of radioactivity was investigated at several time points. In vivo planar imaging, comparison between (125)I-IBB accumulation and HIF-1 transcriptional activity, and autoradiography were performed at 6 h after the administration of (125)I-IBB. The same sections that were used in autoradiographic analysis were subjected to HIF-1alpha immunohistochemistry.

RESULTS

(125)I-IBB accumulation was observed in tumours of mice pretargeted with POS (1.6%ID/g at 6 h). This result is comparable to the data derived from (125)I-IBB-conjugated POS-treated mice (1.4%ID/g at 24 h). In vivo planar imaging provided clear tumour images. The tumoral accumulation of (125)I-IBB significantly correlated with HIF-1-dependent luciferase bioluminescence (R=0.84, p<0.01). The intratumoral distribution of (125)I-IBB was heterogeneous and was significantly correlated with HIF-1alpha-positive regions (R=0.58, p<0.0001).

CONCLUSION

POS pretargeting with (123)I-IBB is a useful technique in the rapid imaging and detection of HIF-1-active regions in tumours.

Molecular imaging in atherosclerosis

Atherosclerosis is the major cause of cardiovascular disease, which still has the leading position in morbidity and mortality in the Western world. Many risk factors and pathobiological processes are acting together in the development of atherosclerosis. This leads to different remodelling stages (positive and negative) which are both associated with plaque physiology and clinical presentation. The different remodelling stages of atherosclerosis are explained with their clinical relevance. Recent advances in basic science have established that atherosclerosis is not only a lipid storage disease, but that also inflammation has a fundamental role in all stages of the disease. The molecular events leading to atherosclerosis will be extensively reviewed and described. Further on in this review different modalities and their role in the different stages of atherosclerosis will be discussed. Non-nuclear invasive imaging techniques (intravascular ultrasound, intravascular MRI, intracoronary angioscopy and intravascular optical coherence tomography) and non-nuclear non-invasive imaging techniques (ultrasound with Doppler flow, electron-bean computed tomography, coronary computed tomography angiography, MRI and coronary artery MR angiography) will be reviewed. After that we focus on nuclear imaging techniques for detecting atherosclerotic plaques, divided into three groups: atherosclerotic lesion components, inflammation and thrombosis. This emerging area of nuclear imaging techniques can provide measures of biological activity of atherosclerotic plaques, thereby improving the prediction of clinical events. As we will see in the future perspectives, at present, there is no special tracer that can be called the diagnostic tool to diagnose prospective stroke or infarction in patients. Nevertheless, we expect such a tracer to be developed in the next few years and maybe, theoretically, it could even be used for targeted therapy (in the form of a beta-emitter) to combat cardiovascular disease.

Pre-clinical and clinical evaluation of nuclear tracers for the molecular imaging of vulnerable atherosclerosis: an overview

Cardiovascular diseases (CVD) are the leading cause of mortality worldwide. Despite major advances in the treatment of CVD, a high proportion of CVD victims die suddenly while being apparently healthy, the great majority of these accidents being due to the rupture or erosion of a vulnerable coronary atherosclerotic plaque. A non-invasive imaging methodology allowing the early detection of vulnerable atherosclerotic plaques in selected individuals prior to the occurrence of any symptom would therefore be of great public health benefit. Nuclear imaging could allow the identification of vulnerable patients by non-invasive in vivo scintigraphic imaging following administration of a radiolabeled tracer. The purpose of this review is to provide an overview of radiotracers that have been recently evaluated for the detection of vulnerable plaques together with the biological rationale that initiated their development. Radiotracers targeted at the inflammatory process seem particularly relevant and promising. Recently, macrophage targeting allowed the experimental in vivo detection of atherosclerosis using either SPECT or PET. A few tracers have also been evaluated clinically. Targeting of apoptosis and macrophage metabolism both allowed the imaging of vulnerable plaques in carotid vessels of patients. However, nuclear imaging of vulnerable plaques at the level of coronary arteries remains challenging, mostly because of their small size and their vicinity with unbound circulating tracer. The experimental and pilot clinical studies reviewed in the present paper represent a fundamental step prior to the evaluation of the efficacy of any selected tracer for the early, non-invasive detection of vulnerable patients.

Radiotracer imaging of atherosclerotic plaque biology

Traditional imaging modalities used in the assessment of atherosclerotic plaque have focused on anatomic characteristics of size, location and luminal encroachment. The ability to identify plaques that are at risk for rupture, and thus may go on to cause clinical events, remains limited, however. By labeling tracer compounds capable of identifying important cellular or molecular processes involved in plaque vulnerability with radioactive isotopes, there is now potential for the noninvasive identification of vulnerable plaques. This article discusses several radiotracers that can report on high-risk plaque pathophysiology.

Imaging atherosclerotic plaque inflammation

Inflammation within atherosclerotic plaques is one of the main drivers of atherosclerotic plaque rupture, which frequently leads to clinical events such as myocardial infarction and stroke. Current gold standard techniques such as X-ray angiography and ultrasound can rapidly report on luminal encroachment but give no readout on inflammatory state of the plaque. We summarize several alternative imaging techniques--CT, MRI, and nuclear imaging--that are close to the clinical arena, and we provide the relative advantages of each.

Prominent lectin-like oxidized low density lipoprotein (LDL) receptor-1 (LOX-1) expression in atherosclerotic lesions is associated with tissue factor expression and apoptosis in hypercholesterolemic rabbits

BACKGROUND

Despite increasing in vitro evidence that lectin-like oxidized low density lipoprotein (LDL) receptor-1 (LOX-1), a cell-surface receptor for oxidized LDL, is implicated in the atherogenesis and thrombus formation, its in vivo participation to the atherosclerotic plaque destabilization, rupture and thrombus formation remains unclear. Here, we compared the in vivo expression of LOX-1, with tissue factor (TF) expression and cell apoptosis, in atherosclerotic lesions of myocardial infarction-prone Watanabe heritable hyperlipidemic (WHHLMI) rabbits.

METHODS AND RESULTS

We prepared sixty series of cross sections in the aortic arch and the thoracic aorta from four WHHLMI rabbits. LOX-1 and TF expression, as well as apoptotic events were determined by immunohistochemical staining and TUNEL methods, respectively. LOX-1 expression was mainly observed in the macrophage-rich lipid areas of vulnerable plaque-like atheromatous lesions where TF expression and apoptotic events were prominent. LOX-1 expression was positively correlated with TF expression (r=0.53, p<0.0001), apoptotic events (r=0.52, p<0.0001) and morphological vulnerability (r=0.63, p<0.0001).

CONCLUSIONS

LOX-1 expression appears to be closely associated with TF expression, apoptotic events and the morphological vulnerability, suggesting the in vivo involvement of LOX-1 in the destabilization and rupture of atherosclerotic lesions and the subsequent thrombus formation. The present findings in hypercholesterolemic rabbits should help advance our understanding of the pathophysiology of atherosclerosis.

Distribution Profiles of Membrane Type-1 Matrix Metalloproteinase (MT1-MMP), Matrix Metalloproteinase-2 (MMP-2) and Cyclooxygenase-2 (COX-2) in Rabbit Atherosclerosis: Comparison with Plaque Instability Analysis

BACKGROUND

Despite increasing evidence that membrane type 1 matrix metalloproteinase (MT1-MMP), matrix metalloproteinase-2 (MMP-2), and cyclooxygenase-2 (COX-2) are involved in the pathogenesis of atherosclerosis, the possible links among these enzymes remain unclear. Accordingly, we investigated the distribution of MT1-MMP, MMP-2, and COX-2 immunohistologically in the atherosclerotic lesions of hypercholesterolemic (WHHLMI) rabbits.

METHODS AND RESULTS

Distribution of MT1-MMP, MMP-2, and COX-2 was examined by immunohistochemical staining using sixty cross sections of the ascending-arch and thoracic aortas prepared from 4 WHHLMI rabbits. MT1-MMP and MMP-2 staining was prominently observed in the macrophage-rich regions of the atheromatous lesions, and was positively correlated with morphological vulnerability (r=0.63 for MT1-MMP; r=0.60 for MMP-2; p<0.0001). MT1-MMP staining was positively correlated with MMP-2 staining (r=0.61, p<0.0001). COX-2 staining was also the highest in the macrophage-rich regions of the atheromatous lesions, with relatively high staining levels in other more stable lesions.

CONCLUSIONS

Co-distribution of MT1-MMP, MMP-2, and COX-2 was demonstrated in grade IV atheroma, indicating a possible link among these enzymes in the destabilization of atherosclerotic plaques. The relatively high COX-2 distribution in other more stable lesions may indicate its additional roles in the stabilization of atherosclerotic lesions. The present findings in hypercholesterolemic rabbits should help advance our understanding of the pathophysiology of atherosclerosis and provide useful information for the development of new therapeutic and diagnostic (imaging) agents that target MMPs and COX-2 in atherosclerosis.