Integrin-targeted imaging of inflammation in vascular remodeling

Construction of PLGA nanoparticles modified with RWrNM and DLPC and their application in acute rhinosinusitis

In an effort to overcome the nasal mucus barrier and epithelial barrier, as well as reduce entry into the bloodstream, we designed RWrNM and DLPC-modified PLGA nanoparticles (PDR-NPs). These nanoparticles were further encapsulated with dexamethasone acetate (Dexac) to form Dexac/PDR-NPs. Transmission electron microscopy (TEM) analysis revealed their spherical shape with an outer lipid layer. Dynamic light scattering (DLS) determined their particle size to be 125.77 ± 2.01 nm, with a polydispersity index (PDI) of 0.139 ± 0.029. The experimental results demonstrate that DLPC-modified PLGA nanoparticles can effectively reduce interactions with mucin at different concentrations, decrease aggregation, and facilitate their crossing of the mucus barrier. Additionally, results from the cellular uptake assay revealed a significantly greater uptake of PDR-NPs by inflammatory RAW 264.7 cells (2.99-fold higher than that of free C6, p < 0.0001) and inflammatory HUVECs (7.20-fold higher than that of free C6, p < 0.0001). Furthermore, Dexac/PDR-NPs effectively reduced the levels of inflammatory factors nitric oxide (NO) (p < 0.001) and interleukin-6 (IL-6) (p < 0.05) in the supernatant of inflammatory RAW 264.7 cells. Intravital imaging of rats revealed that PDR-NPs had a longer residence time in inflamed nasal tissue compared to PD-NPs. Furthermore, in vivo pharmacodynamic experiments showed that Dexac/PDR-NPs effectively reduced the symptoms of nasal inflammation, lowered the pH of nasal secretions, decreased serum inflammatory factor levels (TNF-α and IL-6), and reduced nasal mucosal inflammatory factor levels (IL-1β), while also reducing the degree of inflammation in the nasal mucosa. Both cytotoxicity assays and in vivo results indicate that PDR-NPs have a good safety profile. PDR-NPs not only overcome the nasal mucus barrier but also reduce the systemic toxicities associated with drug entry into the circulation by enhancing the targeting of inflammatory macrophages and inflammatory vascular endothelial cells. PDR-NPs allow for an "open sources and cut costs" treatment strategy to increase drug retention in the inflamed nasal tissues, reducing toxicity and increasing efficacy. In conclusion, PDR-NPs can be a promising drug delivery system for the local treatment of acute rhinosinusitis.

Nonsteroidal Anti-Inflammatory Drug Conjugated with Gadolinium (III) Complex as an Anti-Inflammatory MRI Agent

Studies have been actively conducted to ensure that gadolinium-based contrast agents for magnetic resonance imaging (MRI) are accompanied by various biological functions. A new example is the anti-inflammatory theragnostic MRI agent to target inflammatory mediators for imaging diagnosis and to treat inflammatory diseases simultaneously. We designed, synthesized, and characterized a Gd complex of 1,4,7-tris(carboxymethylaza) cyclododecane-10-azaacetylamide (DO3A) conjugated with a nonsteroidal anti-inflammatory drug (NSAID) that exerts the innate therapeutic effect of NSAIDs and is also applicable in MRI diagnostics. Gd-DO3A-fen (0.1 mmol/kg) was intravenously injected into the turpentine oil-induced mouse model, with Gd-DO3A-BT as a control group. In the in vivo MRI experiment, the contrast-to-noise ratio (CNR) was higher and persisted longer than that with Gd-DO3A-BT; specifically, the CNR difference was almost five times at 2 h after injection. Gd-DO3A-fen had a binding affinity (K_a) of 6.68 × 10⁶ M^-1 for the COX-2 enzyme, which was 2.1-fold higher than that of fenbufen, the original NSAID. In vivo evaluation of anti-inflammatory activity was performed in two animal models. In the turpentine oil-induced model, the mRNA expression levels of inflammatory parameters such as COX-2, TNF-α, IL-1β, and IL-6 were reduced, and in the carrageenan-induced edema model, swelling was suppressed by 72% and there was a 2.88-fold inhibition compared with the saline group. Correlation analysis between in vitro, in silico, and in vivo studies revealed that Gd-DO3A-fen acts as an anti-inflammatory theragnostic agent by directly binding to COX-2.

Targeted Diagnosis, Therapeutic Monitoring, and Assessment of Atherosclerosis Based on Mesoporous Silica Nanoparticles Coated with cRGD-Platelets

Objective

The off-target effects and severe side effects of PPARα and LXRα agonists greatly limit their application in atherosclerosis (AS). Therefore, this study intended to use mesoporous silica nanoparticles as carriers to generate MnO nanoparticles in situ with T1WI-MRI in mesoporous pores and simultaneously load PPARα and LXRα agonists. Afterward, cRGD-chelated platelet membranes can be used for coating to construct a new nanotheranostic agent.

Methods

cRGD-platelet@MnO/MSN@PPARα/LXRα nanoparticles were synthesized by a chemical method. Dynamic light scattering (DLS) was utilized to detect the size distribution and polydispersity index (PDI) of the nanoparticles. The safety of the nanoparticles was detected by CCK8 in vitro and HE staining and kidney function in vivo. Cell apoptosis was detected by flow cytometry detection and TUNEL staining. Oxidative stress responses (ROS, SOD, MDA, and NOX levels) were tested via a DCFH-DA assay and commercial kits. Immunofluorescence and phagocytosis experiments were used to detect the targeting of nanoparticles. Magnetic resonance imaging (MRI) was used to detect the imaging performance of cRGD-platelet@MnO/MSN@PPARα/LXRα nanoparticles. Using western blotting, the expression changes in LXRα and ABCA1 were identified.

Results

cRGD-platelet@MnO/MSN@PPARα/LXRα nanoparticles were successfully established, with a particle size of approximately 150 nm and PDI less than 0.3, and showed high safety both in vitro and in vivo. cRGD-platelet@MnO/MSN@PPARα/LXRα nanoparticles showed good targeting properties and better MRI imaging performance in AS. cRGD-platelet@MnO/MSN@PPARα/LXRα nanoparticles showed better antioxidative capacities, MRI imaging performance, and diagnostic and therapeutic effects on AS by regulating the expression of LXRα and ABCA1.

Conclusion

In the present study, cRGD-platelet@MnO/MSN@PPARα/LXRα nanoparticles with high safety and the capacity to target vulnerable plaques of AS were successfully established. They showed better performance on MRI images and treatment effects on AS by promoting cholesterol efflux through the regulation of ABCA1. These findings might address the problems of off-target effects and side effects of nanoparticle-mediated drug delivery, which will enhance the efficiency of AS treatment and provide new ideas for the clinical treatment of AS.

Molecular Imaging of Aortic Aneurysm and Its Translational Power for Clinical Risk Assessment

Aortic aneurysms (AAs) are dilations of the aorta, that are often fatal upon rupture. Diagnostic radiological techniques such as ultrasound (US), magnetic resonance imaging (MRI), and computed tomography (CT) are currently used in clinical practice for early diagnosis as well as clinical follow-up for preemptive surgery of AA and prevention of rupture. However, the contemporary imaging-based risk prediction of aneurysm enlargement or life-threatening aneurysm-rupture remains limited as these are restricted to visual parameters which fail to provide a personalized risk assessment. Therefore, new insights into early diagnostic approaches to detect AA and therefore to prevent aneurysm-rupture are crucial. Multiple new techniques are developed to obtain a more accurate understanding of the biological processes and pathological alterations at a (micro)structural and molecular level of aortic degeneration. Advanced anatomical imaging combined with molecular imaging, such as molecular MRI, or positron emission tomography (PET)/CT provides novel diagnostic approaches for in vivo visualization of targeted biomarkers. This will aid in the understanding of aortic aneurysm disease pathogenesis and insight into the pathways involved, and will thus facilitate early diagnostic analysis of aneurysmal disease. In this study, we reviewed these molecular imaging modalities and their association with aneurysm growth and/or rupture risk and their limitations. Furthermore, we outline recent pre-clinical and clinical developments in molecular imaging of AA and provide future perspectives based on the advancements made within the field. Within the vastness of pre-clinical markers that have been studied in mice, molecular imaging targets such as elastin/collagen, albumin, matrix metalloproteinases and immune cells demonstrate promising results regarding rupture risk assessment within the pre-clinical setting. Subsequently, these markers hold potential as a future diagnosticum of clinical AA assessment. However currently, clinical translation of molecular imaging is still at the onset. Future human trials are required to assess the effectivity of potentially viable molecular markers with various imaging modalities for clinical rupture risk assessment.

Imaging angiogenesis in atherosclerosis in large arteries with 68Ga-NODAGA-RGD PET/CT: relationship with clinical atherosclerotic cardiovascular disease

BACKGROUND

Integrin alpha-V-beta-3 (αvβ3) pathway is involved in intraplaque angiogenesis and inflammation and represents a promising target for molecular imaging in cardiovascular diseases such as atherosclerosis. The aim of this study was to assess the clinical correlates of arterial wall accumulation of ⁶⁸Ga-NODAGA-RGD, a specific α_vβ₃ integrin ligand for PET.

MATERIALS AND METHODS

The data of 44 patients who underwent ⁶⁸Ga-NODAGA-RGD PET/CT scans were retrospectively analyzed. Tracer accumulation in the vessel wall of major arteries was analyzed semi-quantitatively by blood-pool-corrected target-to-background ratios. Tracer uptake was compared with clinically documented atherosclerotic cardiovascular disease, cardiovascular risk factors and calcified plaque burden. Data were compared using the Mann-Whitney U test, Pearson correlation and Spearman correlation.

RESULTS

⁶⁸Ga-NODAGA-RGD arterial uptake was significantly higher in patients with previous clinically documented atherosclerotic cardiovascular disease (mean TBR 2.44 [2.03-2.55] vs. 1.81 [1.56-1.96], p = 0.001) and showed a significant correlation with prior cardiovascular or cerebrovascular event (r = 0.33, p = 0.027), BMI (ρ = 0.38, p = 0.01), plaque burden (ρ = 0.31, p = 0.04) and hypercholesterolemia (r = 0.31, p = 0.04).

CONCLUSIONS

⁶⁸Ga-NODAGA-RGD holds promise as a non-invasive marker of disease activity in atherosclerosis, providing information about intraplaque angiogenesis.

Nanoparticle-Assisted Diagnosis and Treatment for Abdominal Aortic Aneurysm

An abdominal aortic aneurysm (AAA) is a localized dilatation of the aorta related to the regional weakening of the wall structure, resulting in substantial morbidity and mortality with the aortic ruptures as complications. Ruptured AAA is a dramatic catastrophe, and aortic emergencies constitute one of the leading causes of acute death in older adults. AAA management has been centered on surgical repair of larger aneurysms to mitigate the risks of rupture, and curative early diagnosis and effective pharmacological treatments for this condition are still lacking. Nanoscience provided a possibility of more targeted imaging and drug delivery system. Multifunctional nanoparticles (NPs) may be modified with ligands or biomembranes to target agents' delivery to the lesion site, thus reducing systemic toxicity. Furthermore, NPs can improve drug solubility, circulation time, bioavailability, and efficacy after systemic administration. The varied judiciously engineered nano-biomaterials can exist stably in the blood vessels for a long time without being taken up by cells. Here, in this review, we focused on the NP application in the imaging and treatment of AAA. We hope to make an overview of NP-assisted diagnoses and therapy in AAA and discussed the potential of NP-assisted treatment.

Targeting the Extracellular Matrix in Abdominal Aortic Aneurysms Using Molecular Imaging Insights

This review outlines recent preclinical and clinical advances in molecular imaging of abdominal aortic aneurysms (AAA) with a focus on molecular magnetic resonance imaging (MRI) of the extracellular matrix (ECM). In addition, developments in pharmacologic treatment of AAA targeting the ECM will be discussed and results from animal studies will be contrasted with clinical trials. Abdominal aortic aneurysm (AAA) is an often fatal disease without non-invasive pharmacologic treatment options. The ECM, with collagen type I and elastin as major components, is the key structural component of the aortic wall and is recognized as a target tissue for both initiation and the progression of AAA. Molecular imaging allows in vivo measurement and characterization of biological processes at the cellular and molecular level and sets forth to visualize molecular abnormalities at an early stage of disease, facilitating novel diagnostic and therapeutic pathways. By providing surrogate criteria for the in vivo evaluation of the effects of pharmacological therapies, molecular imaging techniques targeting the ECM can facilitate pharmacological drug development. In addition, molecular targets can also be used in theranostic approaches that have the potential for timely diagnosis and concurrent medical therapy. Recent successes in preclinical studies suggest future opportunities for clinical translation. However, further clinical studies are needed to validate the most promising molecular targets for human application.

Recent progress on nanoparticles for targeted aneurysm treatment and imaging

An abdominal aortic aneurysm (AAA) is a localized dilatation of the aorta that plagues millions. Its rupture incurs high mortality rates (~80-90%), pressing an urgent need for therapeutic methods to prevent this deadly outcome. Judiciously designed nanoparticles (NPs) have displayed a unique potential to fulfill this need. Aneurysms feature excessive inflammation and extracellular matrix (ECM) degradation. As such, typically inflammatory cells and exposed ECM proteins have been targeted with NPs for therapeutic, diagnostic, or theranostic purposes in experimental models. NPs have been used not only for encapsulation and delivery of drugs and biomolecules in preclinical tests, but also for enhanced imaging to monitor aneurysm progression in patients. Moreover, they can be readily modified with various molecules to improve lesion targeting, detectability, biocompatibility, and circulation time. This review updates on the progress, limitations, and prospects of NP applications in the context of AAA.

Non-oncological applications of RGD-based single-photon emission tomography and positron emission tomography agents

INTRODUCTION

Non-invasive imaging techniques (especially single-photon emission tomography and positron emission tomography) apply several RGD-based imaging ligands developed during a vast number of preclinical and clinical investigations. The RGD (Arg-Gly-Asp) sequence is a binding moiety for a large selection of adhesive extracellular matrix and cell surface proteins. Since the first identification of this sequence as the shortest sequence required for recognition in fibronectin during the 1980s, fundamental research regarding the molecular mechanisms of integrin action have paved the way for development of several pharmaceuticals and radiopharmaceuticals with clinical applications. Ligands recognizing RGD may be developed for use in the monitoring of these interactions (benign or pathological). Although RGD-based molecular imaging has been actively investigated for oncological purposes, their utilization towards non-oncology applications remains relatively under-exploited.

METHODS AND SCOPE

This review highlights the new non-oncologic applications of RGD-based tracers (with the focus on single-photon emission tomography and positron emission tomography). The focus is on the last 10 years of scientific literature (2009-2020). It is proposed that these imaging agents will be used for off-label indications that may provide options for disease monitoring where there are no approved tracers available, for instance Crohn's disease or osteoporosis. Fundamental science investigations have made progress in elucidating the involvement of integrin in various diseases not pertaining to oncology. Furthermore, RGD-based radiopharmaceuticals have been evaluated extensively for safety during clinical evaluations of various natures.

CONCLUSION

Clinical translation of non-oncological applications for RGD-based radiopharmaceuticals and other imaging tracers without going through time-consuming extensive development is therefore highly plausible. Graphical abstract.

Infection and Inflammation Imaging: Beyond FDG

This review discusses nuclear imaging of inflammation using molecular probes beyond fluoro-d-glucose, is structured by cellular targets, and focuses on those tracers that have been successfully applied clinically.

Current and Emerging Preclinical Approaches for Imaging-Based Characterization of Atherosclerosis

Atherosclerotic plaques can remain quiescent for years, but become life threatening upon rupture or disruption, initiating clot formation in the vessel lumen and causing acute myocardial infarction and ischemic stroke. Whether and how a plaque ruptures is determined by its macroscopic structure and microscopic composition. Rupture-prone plaques usually consist of a thin fibrous cap with few smooth muscle cells, a large lipid core, a dense infiltrate of inflammatory cells, and neovessels. Such lesions, termed high-risk plaques, can remain asymptomatic until the thrombotic event. Various imaging technologies currently allow visualization of morphological and biological characteristics of high-risk atherosclerotic plaques. Conventional protocols are often complex and lack specificity for high-risk plaque. Conversely, new imaging approaches are emerging which may overcome these limitations. Validation of these novel imaging techniques in preclinical models of atherosclerosis is essential for effective translational to clinical practice. Imaging the vessel wall, as well as its biological milieu in small animal models, is challenging because the vessel wall is a small structure that undergoes continuous movements imposed by the cardiac cycle as it is adjacent to circulating blood. The focus of this paper is to provide a state-of-the-art review on techniques currently available for preclinical imaging of atherosclerosis in small animal models and to discuss the advantages and limitations of each approach.

Perspectives on Small Animal Radionuclide Imaging; Considerations and Advances in Atherosclerosis

This review addresses nuclear SPECT and PET imaging in small animals in relation to the atherosclerotic disease process, one of our research topics of interest. Imaging of atherosclerosis in small animal models is challenging, as it operates at the limits of current imaging possibilities regarding sensitivity, and spatial resolution. Several topics are discussed, including technical considerations that apply to image acquisition, reconstruction, and analysis. Moreover, molecules developed for or applied in these small animal nuclear imaging studies are listed, including target-directed molecules, useful for imaging organs or tissues that have elevated expression of the target compared to other tissues, and molecules that serve as substrates for metabolic processes. Differences between animal models and human pathophysiology that should be taken into account during translation from animal to patient as well as differences in tracer behavior in animal vs. man are also described. Finally, we give a future outlook on small animal radionuclide imaging in atherosclerosis, followed by recommendations. The challenges and solutions described might be applicable to other research fields of health and disease as well.

Molecular Ultrasound Imaging of αvβ3-Integrin Expression in Carotid Arteries of Pigs After Vessel Injury

OBJECTIVES

Interventions such as balloon angioplasty can cause vascular injury leading to platelet activation, thrombus formation, and inflammatory response. This induces vascular smooth muscle cell activation and subsequent re-endothelialization with expression of αvβ3-integrin by endothelial cells and vascular smooth muscle cell. Thus, poly-N-butylcyanoacrylate microbubbles (MBs) targeted to αvβ3-integrin were evaluated for monitoring vascular healing after vessel injury in pigs using molecular ultrasound imaging.

MATERIALS AND METHODS

Approval for animal experiments was obtained. The binding specificity of αvβ3-integrin-targeted MB to human umbilical vein endothelial cells was tested with fluorescence microscopy. In vivo imaging was performed using a clinical ultrasound system and an 8-MHz probe. Six mini pigs were examined after vessel injury in the left carotid artery. The right carotid served as control. Uncoated MB, cDRG-coated MB, and αvβ3-integrin-specific cRGD-coated MB were injected sequentially. Bound MBs were assessed 8 minutes after injection using ultrasound replenishment analysis. Measurements were performed 2 hours, 1 and 5 weeks, and 3 and 6 months after injury. In vivo data were validated by immunohistochemistry.

RESULTS

Significantly stronger binding of cRGD-MB than MB and cDRG-MB to human umbilical vein endothelial cells was found (P < 0.01). As vessel injury leads to upregulation of αvβ3-integrin, cRGD-MBs bound significantly stronger (P < 0.05) in injured carotid arteries than at the counter side 1 week after vessel injury and significant differences could also be observed after 5 weeks. After 3 months, αvβ3-integrin expression decreased to baseline and binding of cRGD-MB was comparable in both vessels. Values remained at baseline also after 6 months.

CONCLUSIONS

Ultrasound imaging with RGD-MB is promising for monitoring vascular healing after vessel injury. This may open new perspectives to assess vascular damage after radiological interventions.

Multi-modality molecular imaging of aortic aneurysms

Aneurysms of the thoracic and abdominal aorta are common and can be associated with significant morbidity and mortality when complications, including dissection, rupture, or thrombosis, occur. Current approaches to diagnosis and risk stratification rely on measurements of aneurysm size and rate of growth, often using various imaging modalities, which may be suboptimal in identifying patients at the highest and lowest risk of complications. Targeting the biological processes underlying aneurysm formation and expansion with molecular imaging offers an exciting opportunity to characterize aortic aneurysms beyond size and address current gaps in our approach to diagnosis and treatment. In this review, we summarize the epidemiology and biology of aortic aneurysms and highlight the role of molecular imaging in furthering our understanding of aneurysm pathogenesis and its potential future role in guiding management.

Cardiovascular PET/MR imaging: Quo Vadis?

With the recent advent of PET/MRI scanners, the combination of molecular imaging with a variety of known and novel PET radiotracers, the high spatial resolution of MRI, and its potential for multi-parametric imaging are anticipated to increase the diagnostic accuracy in cardiovascular disease detection, while providing novel mechanistic insights into the initiation and progression of the disease state. For the time being, cardiac PET/MRI emerges as potential clinical tool in the identification and characterization of infiltrative cardiac diseases, such as sarcoidosis, acute or chronic myocarditis, and cardiac tumors, respectively. The application of PET/MRI in conjunction with various radiotracer probes in the identification of the vulnerable atherosclerotic plaque also holds much promise but needs further translation and validation in clinical investigations. The combination of molecular imaging and creation of multi-parametric imaging maps with PET/MRI, however, are likely to set new horizons to develop predictive parameters for myocardial recovery and treatment response in ischemic and non-ischemic cardiomyopathy patients. Molecular imaging and multi-parametric imaging in cardiovascular disease with PET/MRI at current stage are at its infancy but bear a bright future.

Role of molecular imaging with positron emission tomographic in aortic aneurysms

Aortic aneurysms (AA) are often asymptomatic before the occurrence of acute, potentially fatal complications including dissection and/or rupture. Beyond aortic size, the ability to assess aortic wall characteristics and processes contributing to aneurysm development may allow improved selection of patients who may benefit from prophylactic surgical intervention. Current risk stratification for aneurysms relies upon routine noninvasive imaging of aortic size without assessing the underlying pathophysiologic processes, including features such as inflammation, which may be associated with aneurysm development and progression. The use of molecular imaging modalities with positron emission tomographic (PET) scan allows characterization of aortic wall inflammatory activity. Elevated uptake of Fuorine-2-deoxy-D-glucose (FDG), a radiotracer with elevated avidity in highly-metabolic cells, has been correlated with the development and progression of both abdominal and thoracic AA in a number of animal models and clinical studies. Other novel PET radiotracers targeting matrix metalloproteinases (MMPs), mitochondrial translocator proteins (TSPO) and endothelial cell adhesion molecules are being investigated for clinical utility in identifying progression of disease in AA. By further defining the activation of molecular pathways in assessing aortic regions at risk for dilatation, this imaging modality can be integrated into future clinical decision-making models.

RGD targeting of human ferritin iron oxide nanoparticles enhances in vivo MRI of vascular inflammation and angiogenesis in experimental carotid disease and abdominal aortic aneurysm

Purpose

To evaluate Arg‐Gly‐Asp (RGD)‐conjugated human ferritin (HFn) iron oxide nanoparticles for in vivo magnetic resonance imaging (MRI) of vascular inflammation and angiogenesis in experimental carotid disease and abdominal aortic aneurysm (AAA).

Materials and Methods

HFn was genetically engineered to express the RGD peptide and Fe₃O₄ nanoparticles were chemically synthesized inside the engineered HFn (RGD‐HFn). Macrophage‐rich left carotid lesions were induced by ligation in FVB mice made hyperlipidemic and diabetic (n = 14), with the contralateral right carotid serving as control. Murine AAAs were created by continuous angiotensin II infusion in ApoE‐deficient mice (n = 12), while control mice underwent saline infusion (n = 8). All mice were imaged before and after intravenous injection with either RGD‐HFn‐Fe₃O₄ or HFn‐Fe₃O₄ using a gradient‐echo sequence on a whole‐body 3T clinical scanner, followed by histological analysis. The nanoparticle accumulation was assessed by the extent of T2*‐induced carotid lumen reduction (% lumen loss) or aortic T2*‐weighted signal intensity reduction (% SI [signal intensity] loss).

Results

RGD‐HFn‐Fe₃O₄ was taken up more than HFn‐Fe₃O₄ in both the ligated left carotid arteries (% lumen loss; 69 ± 9% vs. 36 ± 7%, P = 0.01) and AAAs (% SI loss; 47 ± 6% vs. 20 ± 5%, P = 0.01). The AAA % SI loss correlated positively with AAA size (r = 0.89, P < 0.001). Histology confirmed the greater accumulation and colocalization of RGD‐HFn‐Fe₃O₄ to both vascular macrophages and endothelial cells.

Conclusion

RGD‐HFn‐Fe₃O₄ enhances in vivo MRI by targeting both vascular inflammation and angiogenesis, and provides a promising translatable MRI approach to detect high‐risk atherosclerotic and aneurysmal vascular diseases.

Level of Evidence: 1

J. Magn. Reson. Imaging 2017;45:1144–1153

Molecular Imaging of Angiogenesis and Vascular Remodeling in Cardiovascular Pathology

Angiogenesis and vascular remodeling are involved in a wide array of cardiovascular diseases, from myocardial ischemia and peripheral arterial disease, to atherosclerosis and aortic aneurysm. Molecular imaging techniques to detect and quantify key molecular and cellular players in angiogenesis and vascular remodeling (e.g., vascular endothelial growth factor and its receptors, αvβ3 integrin, and matrix metalloproteinases) can advance vascular biology research and serve as clinical tools for early diagnosis, risk stratification, and selection of patients who would benefit most from therapeutic interventions. To target these key mediators, a number of molecular imaging techniques have been developed and evaluated in animal models of angiogenesis and vascular remodeling. This review of the state of the art molecular imaging of angiogenesis and vascular (and valvular) remodeling, will focus mostly on nuclear imaging techniques (positron emission tomography and single photon emission tomography) that offer high potential for clinical translation.

Novel Molecular Imaging Approaches to Abdominal Aortic Aneurysm Risk Stratification

Selection of patients for abdominal aortic aneurysm repair is currently based on aneurysm size, growth rate, and symptoms. Molecular imaging of biological processes associated with aneurysm growth and rupture, for example, inflammation and matrix remodeling, could improve patient risk stratification and lead to a reduction in abdominal aortic aneurysm morbidity and mortality. (18)F-fluorodeoxyglucose-positron emission tomography and ultrasmall superparamagnetic particles of iron oxide magnetic resonance imaging are 2 novel approaches to abdominal aortic aneurysm imaging evaluated in clinical trials. A variety of other tracers, including those that target inflammatory cells and proteolytic enzymes (eg, integrin αvβ3 and matrix metalloproteinases), have proven effective in preclinical models of abdominal aortic aneurysm and show great potential for clinical translation.

αVβ3 integrin-targeted microSPECT/CT imaging of inflamed atherosclerotic plaques in mice

Background

α_Vβ₃-integrin is expressed by activated endothelial cells and macrophages in atherosclerotic plaques and may represent a valuable marker of high-risk plaques. We evaluated ^99mTc-maraciclatide, an integrin-specific tracer, for imaging vascular inflammation in atherosclerotic lesions in mice.

Methods

Apolipoprotein E-negative (ApoE^−/−) mice on a Western diet (n = 10) and normally fed adult C57BL/6 control mice (n = 4) were injected with ^99mTc-maraciclatide (51.8 ± 3.7 MBq). A blocking peptide was infused in three ApoE^−/− mice; this condition served as another control. After 90 min, the animals were imaged via single-photon emission computed tomography (SPECT). While maintained in the same position, the mice were transferred to computed tomography (CT) to obtain contrast-enhanced images of the aortic arch. Images from both modalities were fused, and signal was quantified in the aortic arch and in the vena cava for subtraction of blood-pool activity. The aorta was carefully dissected after imaging for gamma counting, autoradiography, and histology.

Results

Tracer uptake was significantly higher in ApoE^−/− mice than in both groups of control mice (1.56 ± 0.33 vs. 0.82 ± 0.24 vs. 0.98 ± 0.11, respectively; P = 0.006). Furthermore, higher tracer activity was detected via gamma counting in the aorta of hypercholesterolemic mice than in both groups of control mice (1.52 ± 0.43 vs. 0.78 ± 0.19 vs. 0.47 ± 0.31 ^99mTc-maraciclatide %ID/g, respectively; P = 0.018). Autoradiography showed significantly higher tracer uptake in the atherosclerotic aorta than in the control aorta (P = 0.026). Finally, in the atherosclerotic aorta, immunostaining indicated that the integrin signal came predominantly from macrophages and was correlated with the macrophage CD68 immunomarker (r = 0.73).

Conclusions

^99mTc-maraciclatide allows in vivo detection of inflamed atherosclerotic plaques in mice and may represent a non-invasive approach for identifying high-risk plaques in patients.

RGD-Targeted Ultrasound Contrast Agent for Longitudinal Assessment of Hep-2 Tumor Angiogenesis In Vivo

OBJECTIVE

To prepare arginine-glycine-aspartate (RGD)-targeted ultrasound contrast microbubbles (MBs) and explore the feasibility of their use in assessing dynamic changes in αvβ3 integrin expression in a murine model of tumor angiogenesis.

METHODS

RGD peptides were conjugated to the surfaces of microbubbles via biotin-avidin linkage. Microbubbles bearing RADfK peptides were prepared as controls. The RGD-MBs were characterized using an Accusizer 780 and optical microscopy. The binding specificity of the RGD-MBs for ανβ3-expressing endothelial cells (bEnd.3) was demonstrated in vitro by a competitive inhibition experiment. In an in vivo study, mice bearing tumors of three different stages were intravenously injected with RGD-MBs and subjected to targeted, contrast-enhanced, high-frequency ultrasound. Subsequently, tumors were harvested and sectioned for immunofluorescence analysis of ανβ3 expression.

RESULTS

The mean size of the RGD-MBs was 2.36 ± 1.7 μm. The RGD-MBs showed significantly higher adhesion levels to bEnd.3 cells compared to control MBs (P < 0.01). There was rarely binding of RGD-MBs to αvβ3-negative MCF-7 cells. Adhesion of the RGD-MBs to the bEnd.3 cells was significantly inhibited following treatment with anti-alpha(v) antibodies. The quantitative acoustic video intensity for high-frequency, contrast-enhanced ultrasound imaging of subcutaneous human laryngeal carcinoma (Hep-2) tumor xenografts was significantly higher in small tumors (19.89 ± 2.49) than in medium tumors (11.25 ± 2.23) and large tumors (3.38 ± 0.67) (P < 0.01).

CONCLUSIONS

RGD-MBs enable noninvasive in vivo visualization of changes in tumor angiogenesis during tumor growth in subcutaneous cancer xenografts.

Feasibility of [18F]-RGD for ex vivo imaging of atherosclerosis in detection of αvβ3 integrin expression

BACKGROUND

Inflammation and angiogenesis play an important role in atherosclerotic plaque rupture. Therefore, molecular imaging of these processes could be used for determination of rupture-prone atherosclerotic plaques. αvβ3 integrin is involved in the process of angiogenesis. Targeted imaging of αvβ3 integrin has been shown to be possible in previous studies on tumor models, using radiolabeled arginine-glycine-aspartate (RGD). Our aim was to investigate feasibility of ex vivo detection of αvβ3 integrin in carotid endarterectomy (CEA) specimens.

METHODS AND RESULTS

Nineteen CEA specimens were incubated in 5 MBq [18F]-RGD-K5 for 1 hour followed by 1 hour emission microPET scan. The results were quantified in 4 mm wide segments as percent incubation dose per gram (%Inc/g). Segmental-to-total ratio was calculated and presence of αvβ3 integrin and endothelial cells in each segment was confirmed by immunohistochemical staining for CD31 and αvβ3 integrin, respectively. [18F]-RGD-K5 uptake was heterogeneously distributed across CEA specimens and was localized within the vessel wall. Significant correlations were observed between segmental-to-total ratio with αvβ3 integrin staining score (r = 0.58, P = .038) and CD31 staining score (ρ = 0.67, P < .002).

CONCLUSION

This study showed the feasibility of integrin imaging by determination of αvβ3 integrin expression in human atherosclerotic plaques.

Nuclear Molecular Imaging for Vulnerable Atherosclerotic Plaques

Atherosclerosis is an inflammatory disease as well as a lipid disorder. Atherosclerotic plaque formed in vessel walls may cause ischemia, and the rupture of vulnerable plaque may result in fatal events, like myocardial infarction or stroke. Because morphological imaging has limitations in diagnosing vulnerable plaque, molecular imaging has been developed, in particular, the use of nuclear imaging probes. Molecular imaging targets various aspects of vulnerable plaque, such as inflammatory cell accumulation, endothelial activation, proteolysis, neoangiogenesis, hypoxia, apoptosis, and calcification. Many preclinical and clinical studies have been conducted with various imaging probes and some of them have exhibited promising results. Despite some limitations in imaging technology, molecular imaging is expected to be used both in the research and clinical fields as imaging instruments become more advanced.

Radiolabeled Cyclic RGD Peptide Bioconjugates as Radiotracers Targeting Multiple Integrins

Angiogenesis is a requirement for tumor growth and metastasis. The angiogenic process depends on vascular endothelial cell migration and invasion, and is regulated by various cell adhesion receptors. Integrins are such a family of receptors that facilitate the cellular adhesion to and migration on extracellular matrix proteins in the intercellular spaces and basement membranes. Among 24 members of the integrin family, αvβ3 is studied most extensively for its role in tumor angiogenesis and metastasis. The αvβ3 is expressed at relatively low levels on epithelial cells and mature endothelial cells, but it is highly expressed on the activated endothelial cells of tumor neovasculature and some tumor cells. This restricted expression makes αvβ3 an excellent target to develop antiangiogenic drugs and diagnostic molecular imaging probes. Since αvβ3 is a receptor for extracellular matrix proteins with one or more RGD tripeptide sequence, many radiolabeled cyclic RGD peptides have been evaluated as "αvβ3-targeted" radiotracers for tumor imaging over the past decade. This article will use the dimeric and tetrameric cyclic RGD peptides developed in our laboratories as examples to illustrate basic principles for development of αvβ3-targeted radiotracers. It will focus on different approaches to maximize the radiotracer tumor uptake and tumor/background ratios. This article will also discuss some important assays for preclinical evaluations of integrin-targeted radiotracers. In general, multimerization of cyclic RGD peptides increases their integrin binding affinity and the tumor uptake and retention times of their radiotracers. Regardless of their multiplicity, the capability of cyclic RGD peptides to bind other integrins (namely, αvβ5, α5β1, α6β4, α4β1, and αvβ6) is expected to enhance the radiotracer tumor uptake due to the increased integrin population. The results from preclinical and clinical studies clearly show that radiolabeled cyclic RGD peptides (such as (99m)Tc-3P-RGD2, (18)F-Alfatide-I, and (18)F-Alfatide-II) are useful as the molecular imaging probes for early cancer detection and noninvasive monitoring of the tumor response to antiangiogenic therapy.

Molecular imaging of plaque vulnerability

Over the past decade, significant progress has been made in the development of novel imaging strategies focusing on the biology of the vessel wall for identification of vulnerable plaques. While the majority of these studies are still in the pre-clinical stage, few techniques (e.g., (18)F-FDG and (18)F-NaF PET imaging) have already been evaluated in clinical studies with promising results. Here, we will briefly review the pathobiology of atherosclerosis and discuss molecular imaging strategies that have been developed to target these events, with an emphasis on mechanisms that are associated with atherosclerotic plaque vulnerability.

PET imaging of cardiac wound healing using a novel [68Ga]-labeled NGR probe in rat myocardial infarction

PURPOSE

Peptides containing the asparagine-glycine-arginine (NGR) motif bind to aminopeptidase N (CD13), which is expressed on inflammatory cells, endothelial cells, and fibroblasts. It is unclear whether radiolabeled NGR-containing tracers could be used for in vivo imaging of the early wound-healing phase after myocardial infarction (MI) using positron emission tomography (PET).

PROCEDURES

Uptake of novel tracer [(68)Ga]NGR was assessed together with [(68)Ga]arginine-glycine-aspartic acid ([(68)Ga]RGD) and 2-deoxy-2-[(18) F]fluoro-D-glucose after myocardial ischemia/reperfusion (MI/R) injury using μ-PET and autoradiography, and relative expressions of CD13 and integrin β3 were assessed in fibroblasts, inflammatory cells, and endothelial cells by immunohistochemistry.

RESULTS

In the infarcted myocardium, uptake of [(68)Ga]NGR was maximal from days 3 to 7 after MI/R, and correlated with fibroblast and inflammatory cell infiltration as well as [(68)Ga]RGD uptake.

CONCLUSIONS

[(68)Ga]NGR allows noninvasive and sequential determination of CD13 expression in fibroblasts and inflammatory cells by PET. This will facilitate monitoring of CD13 in the individual wound healing processes, allowing patient-specific therapies to improve outcome after MI.

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.

Emergence of molecular imaging of aortic aneurysm: implications for risk stratification and management

Imaging cellular and molecular processes associated with aneurysm expansion, dissection, and rupture can potentially transform the management of patients with thoracic and abdominal aortic aneurysm. Here, we review recent advances in molecular imaging of aortic aneurysm, focusing on imaging modalities with the greatest potential for clinical translation and application, PET, SPECT, and MRI. Inflammation (e.g., with (18)F-FDG, nanoparticles) and matrix remodeling (e.g., with matrix metalloproteinase-targeted tracers) are highlighted as promising targets for molecular imaging of aneurysm. Potential alternative or complementary approaches to molecular imaging for aneurysm risk stratification are briefly discussed.

PET/CT imaging of integrin αvβ3 expression in human carotid atherosclerosis

OBJECTIVES

The goal of this study was to evaluate the feasibility of [(18)F]Galacto-RGD positron emission tomography (PET)/computed tomography (CT) imaging of αvβ3 expression in human carotid plaques.

BACKGROUND

The integrin αvβ3 is expressed by macrophages and angiogenic endothelial cells in atherosclerotic lesions and thus is a marker of plaque inflammation and, potentially, of plaque vulnerability. [(18)F]Galacto-RGD is a PET tracer binding specifically to αvβ3. Therefore, [(18)F]Galacto-RGD PET/CT imaging of αvβ3 expression in human carotid plaques might provide a novel noninvasive biomarker of plaque vulnerability.

METHODS

[(18)F]Galacto-RGD PET/CT imaging was performed in 10 patients with high-grade carotid artery stenosis scheduled for carotid endarterectomy. Tracer uptake was measured in the stenotic areas of the carotid arteries, as well as on the contralateral side, and was corrected for blood pool activity, measured in the distal common carotid artery (target-to-background [TB] ratio). TB ratio was correlated with immunohistochemistry of αvβ3 expression (LM609), macrophage density (CD68), and microvessel density (CD31) of the surgical specimen. In addition, ex vivo autoradiography of the surgical specimen with [(18)F]Galacto-RGD and competition experiments with an unlabeled αvβ3-specific RGD peptide were performed.

RESULTS

[(18)F]Galacto-RGD PET/CT showed significantly higher TB ratios in stenotic areas compared with nonstenotic areas (p = 0.01). TB ratios correlated significantly with αvβ3 expression (R = 0.787, p = 0.026) and intensity of ex vivo autoradiography (R = 0.733, p = 0.038). Binding to atherosclerotic plaques was efficiently blocked in ex vivo competition experiments. A weak-to-moderate correlation was found with macrophage density (R = 0.367, p = 0.299) and microvessel density (R = 0.479, p = 0.176), which did not reach statistical significance.

CONCLUSIONS

[(18)F]Galacto-RGD PET/CT shows specific tracer accumulation in human atherosclerotic carotid plaques, which correlates with αvβ3 expression. Based on these initial data, larger prospective studies are now warranted to evaluate the potential of molecular imaging of αvβ3 expression for assessment of plaque inflammation in patients.

Diet intervention reduces uptake of αvβ3 integrin-targeted PET tracer 18F-galacto-RGD in mouse atherosclerotic plaques

BACKGROUND

Expression of α(v)β(3) integrin has been proposed as a marker for atherosclerotic lesion inflammation. We studied whether diet intervention reduces uptake of α(v)β(3) integrin-targeted positron emission tomography tracer (18)F-galacto-RGD in mouse atherosclerotic plaques.

METHODS AND RESULTS

Hypercholesterolemic LDLR(-/-) ApoB(100/100) mice on high-fat diet for 4 months were randomized to further 3 months on high-fat diet (high-fat group, n = 8) or regular mouse chow (intervention group, n = 7). Intima-media ratio describing plaque burden was comparable between intervention and high-fat groups (2.0 ± 0.5 vs 2.3 ± 0.8, P = .5). Uptake of (18)F-galacto-RGD in the aorta was lower in the intervention than high-fat group (%ID/g 0.16 vs 0.23, P < .01). Autoradiography showed 35% lower uptake of (18)F-galacto-RGD in the atherosclerotic plaques in the intervention than high-fat group (P = .007). Uptake of (18)F-galacto-RGD in plaques correlated with uptake of (3)H-deoxyglucose and nuclear density, which was lower in the intervention than high-fat group (P = .01). Flow cytometry demonstrated macrophages expressing α(v) and β(3) integrins in the aorta.

CONCLUSIONS

Uptake of (18)F-galacto-RGD in mouse atherosclerotic lesions was reduced by lipid-lowering diet intervention. Expression of α(v)β(3) integrin is a potential target for evaluation of therapy response in atherosclerosis.

Targeted molecular imaging of angiogenesis in PET and SPECT: a review

Over the past few decades, there have been significant advancements in the imaging techniques of positron emission tomography (PET) and single photon emission tomography (SPECT). These changes have allowed for the targeted imaging of cellular processes and the development of hybrid imaging systems (e.g., SPECT/CT and PET/CT), which provide both functional and structural images of biological systems. One area that has garnered particular attention is angiogenesis as it relates to ischemic heart disease and limb ischemia. Though the aforementioned techniques have benefits and consequences, they enable scientists and clinicians to identify regions that are vulnerable to or have been exposed to ischemic injury via non-invasive means. This literature review highlights the advancements in molecular imaging techniques and specific probes as they pertain to the process of angiogenesis in cardiovascular disease.

PET imaging of angiogenesis after myocardial infarction/reperfusion using a one-step labeled integrin-targeted tracer 18F-AlF-NOTA-PRGD2

PURPOSE

The α(v)β(3) integrin represents a potential target for noninvasive imaging of angiogenesis. The purpose of this study was to evaluate a novel one-step labeled integrin α(v)β(3)-targeting positron emission tomography (PET) probe, (18)F-AlF-NOTA-PRGD2, for angiogenesis imaging in a myocardial infarction/reperfusion (MI/R) animal model.

METHODS

Male Sprague-Dawley rats underwent 45-min transient left coronary artery occlusion followed by reperfusion. The myocardial infarction was confirmed by ECG, (18)F-fluorodeoxyglucose (FDG) imaging, and cardiac ultrasound. In vivo PET imaging was used to determine myocardial uptake of (18)F-AlF-NOTA-PRGD2 at different time points following reperfusion. The control peptide RAD was labeled with a similar procedure and used to confirm the specificity. Ex vivo autoradiographic analysis and CD31/CD61 double immunofluorescence staining were performed to validate the PET results.

RESULTS

Myocardial origin of the (18)F-AlF-NOTA-PRGD2 accumulation was confirmed by (18)F-FDG and autoradiography. PET imaging demonstrated increased focal accumulation of (18)F-AlF-NOTA-PRGD2 in the infarcted area which started at day 3 (0.28 ± 0.03%ID/g, p < 0.05) and peaked between 1 and 3 weeks (0.59 ± 0.16 and 0.55 ± 0.13%ID/g, respectively). The focal accumulation decreased but still kept at a higher level than the sham group after 4 months of reperfusion (0.31 ± 0.01%ID/g, p < 0.05). Pretreatment with unlabeled arginine-glycine-aspartic acid (RGD) peptide significantly decreased tracer uptake, indicating integrin specificity of this tracer. At 1 week after MI/R, uptake of the control tracer (18)F-AlF-NOTA-RAD that does not bind to integrin, in the infarcted area, was only 0.21 ± 0.01%ID/g. Autoradiographic imaging showed the same trend of uptake in the myocardial infarction area. The time course of focal tracer uptake was consistent with the pattern of vascular density and integrin β(3) expression as measured by CD31 and CD61 immunostaining analysis.

CONCLUSION

PET imaging using one-step labeled (18)F-AlF-NOTA-PRGD2 allows noninvasive visualization of ischemia/reperfusion-induced myocardial angiogenesis longitudinally. The favorable in vivo kinetics and easy production method of this integrin-targeted PET tracer facilitates its future clinical translation for lesion evaluation and therapy response monitoring in patients with occlusive cardiovascular diseases.