Novel Positron Emission Tomography Tracers for Imaging Vascular Inflammation

PET Radiotracers in Atherosclerosis: A Review

Traditional atherosclerosis imaging modalities are limited to late stages of disease, prior to which patients are frequently asymptomatic. Positron emission tomography (PET) imaging allows for the visualization of metabolic processes underscoring disease progression via radioactive tracer, allowing earlier-stage disease to be identified. 2-deoxy-2-[fluorine-18]fluoro-D-glucose (18F-FDG) uptake largely reflects the metabolic activity of macrophages, but is unspecific and limited in its utility. By detecting areas of microcalcification, 18F-Sodium Fluoride (18F-NaF) uptake also provides insight into atherosclerosis pathogenesis. Gallium-68 DOTA-0-Tyr3-Octreotate (68Ga-DOTATATE) PET has also shown potential in identifying vulnerable atherosclerotic plaques with high somatostatin receptor expression. Finally, 11-carbon (11C)-choline and 18F-fluoromethylcholine (FMCH) tracers may identify high-risk atherosclerotic plaques by detecting increased choline metabolism. Together, these radiotracers quantify disease burden, assess treatment efficacy, and stratify risk for adverse cardiac events.

Recent Developments in PET and SPECT Radiotracers as Radiopharmaceuticals for Hypoxia Tumors

Hypoxia, a deficiency in the levels of oxygen, is a common feature of most solid tumors and induces many characteristics of cancer. Hypoxia is associated with metastases and strong resistance to radio- and chemotherapy, and can decrease the accuracy of cancer prognosis. Non-invasive imaging methods such as positron emission tomography (PET) and single-photon emission computed tomography (SPECT) using hypoxia-targeting radiopharmaceuticals have been used for the detection and therapy of tumor hypoxia. Nitroimidazoles are bioreducible moieties that can be selectively reduced under hypoxic conditions covalently bind to intracellular macromolecules, and are trapped within hypoxic cells and tissues. Recently, there has been a strong motivation to develop PET and SPECT radiotracers as radiopharmaceuticals containing nitroimidazole moieties for the visualization and treatment of hypoxic tumors. In this review, we summarize the development of some novel PET and SPECT radiotracers as radiopharmaceuticals containing nitroimidazoles, as well as their physicochemical properties, in vitro cellular uptake values, in vivo biodistribution, and PET/SPECT imaging results.

Imaging of atherosclerosis with [64Cu]Cu-DOTA-TATE in a translational head-to-head comparison study with [18F]FDG, and Na[18F]F in rabbits

Atherosclerosis is a chronic inflammatory disease of the larger arteries that may lead to cardiovascular events. Identification of patients at highest risk of cardiovascular events is challenging, but molecular imaging using positron emission tomography (PET) may prove useful. The aim of this study was to evaluate and compare head-to-head three different PET tracers. Furthermore, tracer uptake is compared to gene expression alterations of the arterial vessel wall. Male New Zealand White rabbits (control group; n = 10, atherosclerotic group; n = 11) were used for the study. Vessel wall uptake was assessed with the three different PET tracers: [¹⁸F]FDG (inflammation), Na[¹⁸F]F (microcalcification), and [⁶⁴Cu]Cu-DOTA-TATE (macrophages), using PET/computed tomography (CT). Tracer uptake was measured as standardized uptake value (SUV), and arteries from both groups were analyzed ex vivo by autoradiography, qPCR, histology, and immunohistochemistry. In rabbits, the atherosclerotic group showed significantly higher uptake of all three tracers compared to the control group [¹⁸F]FDG: SUV_mean 1.50 ± 0.11 versus 1.23 ± 0.09, p = 0.025; Na[¹⁸F]F: SUV_mean 1.54 ± 0.06 versus 1.18 ± 0.10, p = 0.006; and [⁶⁴Cu]Cu-DOTA-TATE: SUV_mean 2.30 ± 0.27 versus 1.65 ± 0.16; p = 0.047. Of the 102 genes analyzed, 52 were differentially expressed in the atherosclerotic group compared to the control group and several genes correlated with tracer uptake. In conclusion, we demonstrated the diagnostic value of [⁶⁴Cu]Cu-DOTA-TATE and Na[¹⁸F]F for identifying atherosclerosis in rabbits. The two PET tracers provided information distinct from that obtained with [¹⁸F]FDG. None of the three tracers correlated significantly to each other, but [⁶⁴Cu]Cu-DOTA-TATE and Na[¹⁸F]F uptake both correlated with markers of inflammation. [⁶⁴Cu]Cu-DOTA-TATE was higher in atherosclerotic rabbits compared to [¹⁸F]FDG and Na[¹⁸F]F.

A role of FDG PET/CT for Response Assessment in Large Vessel Disease?

Currently, a large amount of evidence-based data clearly demonstrates the usefulness of [¹⁸F]FDG PET/CT in the diagnosis of several infectious and inflammatory diseases, including those related to the large vessels. The aim of this article is to clarify whether, beyond initial diagnosis, [¹⁸F]FDG PET/CT may have a role in treatment response assessment in inflammatory or infectious diseases of the large vessels, including large vessel vasculitis, vascular graft infection, retroperitoneal fibrosis/chronic periaortitis and infective native aortic aneurysms. Rapidly accumulating data suggest that [¹⁸F]FDG PET/CT could be a valuable imaging method for therapy monitoring in some infectious and inflammatory diseases of large vessels. The available data, albeit preliminary, indicate that [¹⁸F]FDG PET/CT could even play a pivotal role in the management of these diseases, leading to better drug dosage, confirmation of the usefulness of the treatment, and early modification of the therapeutic strategy. However, to date, the role of [¹⁸F]FDG PET/CT for treatment assessment in large vessel diseases, in particular large vessel vasculitis, is not clearly defined and well-designed prospective studies are needed to confirm its possible role in treatment monitoring and treatment guidance.

Looking ahead: giant-cell arteritis in 10 years time

Although great improvements have been achieved in the fields of diagnosing and treating patients with giant-cell arteritis (GCA) in the last decades, several questions remain unanswered. The progressive increase in the number of older people, together with growing awareness of the disease and use of advanced diagnostic tools by healthcare professionals, foretells a possible increase in both prevalence and number of newly diagnosed patients with GCA in the coming years. A thorough clarification of pathogenetic mechanisms and a better definition of clinical subsets are the first steps toward a better understanding of the disease and, subsequently, toward a better use of existing and future therapeutic options. Examination of the role of different imaging techniques for GCA diagnosing and monitoring, optimization, and personalization of glucocorticoids and other immunosuppressive agents, further development and introduction of novel drugs, identification of prognostic factors for long-term outcomes and management of treatment discontinuation will be the central topics of the research agenda in years to come.

Targeting the CCL2-CCR2 axis for atheroprotection

Decades of research have established atherosclerosis as an inflammatory disease. Only recently though, clinical trials provided proof-of-concept evidence for the efficacy of anti-inflammatory strategies with respect to cardiovascular events, thus offering a new paradigm for lowering residual vascular risk. Efforts to target the inflammasome-interleukin-1β-interleukin-6 pathway have been highly successful, but inter-individual variations in drug response, a lack of reduction in all-cause mortality, and a higher rate of infections also highlight the need for a second generation of anti-inflammatory agents targeting atherosclerosis-specific immune mechanisms while minimizing systemic side effects. CC-motif chemokine ligand 2/monocyte-chemoattractant protein-1 (CCL2/MCP-1) orchestrates inflammatory monocyte trafficking between the bone marrow, circulation, and atherosclerotic plaques by binding to its cognate receptor CCR2. Adding to a strong body of data from experimental atherosclerosis models, a coherent series of recent large-scale genetic and observational epidemiological studies along with data from human atherosclerotic plaques highlight the relevance and therapeutic potential of the CCL2-CCR2 axis in human atherosclerosis. Here, we summarize experimental and human data pinpointing the CCL2-CCR2 pathway as an emerging drug target in cardiovascular disease. Furthermore, we contextualize previous efforts to interfere with this pathway, scrutinize approaches of ligand targeting vs. receptor targeting, and discuss possible pathway-intrinsic opportunities and challenges related to pharmacological targeting of the CCL2-CCR2 axis in human atherosclerotic disease.

Systematically evaluating DOTATATE and FDG as PET immuno-imaging tracers of cardiovascular inflammation

In recent years, cardiovascular immuno-imaging by positron emission tomography (PET) has undergone tremendous progress in preclinical settings. Clinically, two approved PET tracers hold great potential for inflammation imaging in cardiovascular patients, namely FDG and DOTATATE. While the former is a widely applied metabolic tracer, DOTATATE is a relatively new PET tracer targeting the somatostatin receptor 2 (SST2). In the current study, we performed a detailed, head-to-head comparison of DOTATATE-based radiotracers and [18F]F-FDG in mouse and rabbit models of cardiovascular inflammation. For mouse experiments, we labeled DOTATATE with the long-lived isotope [64Cu]Cu to enable studying the tracer's mode of action by complementing in vivo PET/CT experiments with thorough ex vivo immunological analyses. For translational PET/MRI rabbit studies, we employed the more widely clinically used [68Ga]Ga-labeled DOTATATE, which was approved by the FDA in 2016. DOTATATE's pharmacokinetics and timed biodistribution were determined in control and atherosclerotic mice and rabbits by ex vivo gamma counting of blood and organs. Additionally, we performed in vivo PET/CT experiments in mice with atherosclerosis, mice subjected to myocardial infarction and control animals, using both [64Cu]Cu-DOTATATE and [18F]F-FDG. To evaluate differences in the tracers' cellular specificity, we performed ensuing ex vivo flow cytometry and gamma counting. In mice subjected to myocardial infarction, in vivo [64Cu]Cu-DOTATATE PET showed higher differential uptake between infarcted (SUVmax 1.3, IQR, 1.2-1.4, N = 4) and remote myocardium (SUVmax 0.7, IQR, 0.5-0.8, N = 4, p = 0.0286), and with respect to controls (SUVmax 0.6, IQR, 0.5-0.7, N = 4, p = 0.0286), than [18F]F-FDG PET. In atherosclerotic mice, [64Cu]Cu-DOTATATE PET aortic signal, but not [18F]F-FDG PET, was higher compared to controls (SUVmax 1.1, IQR, 0.9-1.3 and 0.5, IQR, 0.5-0.6, respectively, N = 4, p = 0.0286). In both models, [64Cu]Cu-DOTATATE demonstrated preferential accumulation in macrophages with respect to other myeloid cells, while [18F]F-FDG was taken up by macrophages and other leukocytes. In a translational PET/MRI study in atherosclerotic rabbits, we then compared [68Ga]Ga-DOTATATE and [18F]F-FDG for the assessment of aortic inflammation, combined with ex vivo radiometric assays and near-infrared imaging of macrophage burden. Rabbit experiments showed significantly higher aortic accumulation of both [68Ga]Ga-DOTATATE and [18F]F-FDG in atherosclerotic (SUVmax 0.415, IQR, 0.338-0.499, N = 32 and 0.446, IQR, 0.387-0.536, N = 27, respectively) compared to control animals (SUVmax 0.253, IQR, 0.197-0.285, p = 0.0002, N = 10 and 0.349, IQR, 0.299-0.423, p = 0.0159, N = 11, respectively). In conclusion, we present a detailed, head-to-head comparison of the novel SST2-specific tracer DOTATATE and the validated metabolic tracer [18F]F-FDG for the evaluation of inflammation in small animal models of cardiovascular disease. Our results support further investigations on the use of DOTATATE to assess cardiovascular inflammation as a complementary readout to the widely used [18F]F-FDG.

Cardiovascular disturbances in COVID-19: an updated review of the pathophysiology and clinical evidence of cardiovascular damage induced by SARS-CoV-2

Severe acute respiratory coronavirus-2 (SARS-Co-2) is the causative agent of coronavirus disease-2019 (COVID-19). COVID-19 is a disease with highly variable phenotypes, being asymptomatic in most patients. In symptomatic patients, disease manifestation is variable, ranging from mild disease to severe and critical illness requiring treatment in the intensive care unit. The presence of underlying cardiovascular morbidities was identified early in the evolution of the disease to be a critical determinant of the severe disease phenotype. SARS-CoV-2, though a primarily respiratory virus, also causes severe damage to the cardiovascular system, contributing significantly to morbidity and mortality seen in COVID-19. Evidence on the impact of cardiovascular disorders in disease manifestation and outcome of treatment is rapidly emerging. The cardiovascular system expresses the angiotensin-converting enzyme-2, the receptor used by SARS-CoV-2 for binding, making it vulnerable to infection by the virus. Systemic perturbations including the so-called cytokine storm also impact on the normal functioning of the cardiovascular system. Imaging plays a prominent role not only in the detection of cardiovascular damage induced by SARS-CoV-2 infection but in the follow-up of patients' clinical progress while on treatment and in identifying long-term sequelae of the disease.

Therapeutic strategies targeting inflammation and immunity in atherosclerosis: how to proceed?

Atherosclerosis is a chronic inflammatory disease of the arterial wall, characterized by the formation of plaques containing lipid, connective tissue and immune cells in the intima of large and medium-sized arteries. Over the past three decades, a substantial reduction in cardiovascular mortality has been achieved largely through LDL-cholesterol-lowering regimes and therapies targeting other traditional risk factors for cardiovascular disease, such as hypertension, smoking, diabetes mellitus and obesity. However, the overall benefits of targeting these risk factors have stagnated, and a huge global burden of cardiovascular disease remains. The indispensable role of immunological components in the establishment and chronicity of atherosclerosis has come to the forefront as a clinical target, with proof-of-principle studies demonstrating the benefit and challenges of targeting inflammation and the immune system in cardiovascular disease. In this Review, we provide an overview of the role of the immune system in atherosclerosis by discussing findings from preclinical research and clinical trials. We also identify important challenges that need to be addressed to advance the field and for successful clinical translation, including patient selection, identification of responders and non-responders to immunotherapies, implementation of patient immunophenotyping and potential surrogate end points for vascular inflammation. Finally, we provide strategic guidance for the translation of novel targets of immunotherapy into improvements in patient outcomes.

In this Review, the authors provide an overview of the immune cells involved in atherosclerosis, discuss preclinical research and published and ongoing clinical trials assessing the therapeutic potential of targeting the immune system in atherosclerosis, highlight emerging therapeutic targets from preclinical studies and identify challenges for successful clinical translation.

Inflammation is an important component of the pathophysiology of cardiovascular disease; an imbalance between pro-inflammatory and anti-inflammatory processes drives chronic inflammation and the formation of atherosclerotic plaques in the vessel wall.

Clinical trials assessing canakinumab and colchicine therapies in atherosclerotic cardiovascular disease have provided proof-of-principle of the benefits associated with therapeutic targeting of the immune system in atherosclerosis.

The immunosuppressive adverse effects associated with the systemic use of anti-inflammatory drugs can be minimized through targeted delivery of anti-inflammatory drugs to the atherosclerotic plaque, defining the window of opportunity for treatment and identifying more specific targets for cardiovascular inflammation.

Implementing immunophenotyping in clinical trials in patients with atherosclerotic cardiovascular disease will allow the identification of immune signatures and the selection of patients with the highest probability of deriving benefit from a specific therapy.

Clinical stratification via novel risk factors and discovery of new surrogate markers of vascular inflammation are crucial for identifying new immunotherapeutic targets and their successful translation into the clinic.

Large-vessel vasculitis

Large-vessel vasculitis (LVV) manifests as inflammation of the aorta and its major branches and is the most common primary vasculitis in adults. LVV comprises two distinct conditions, giant cell arteritis and Takayasu arteritis, although the phenotypic spectrum of primary LVV is complex. Non-specific symptoms often predominate and so patients with LVV present to a range of health-care providers and settings. Rapid diagnosis, specialist referral and early treatment are key to good patient outcomes. Unfortunately, disease relapse remains common and chronic vascular complications are a source of considerable morbidity. Although accurate monitoring of disease activity is challenging, progress in vascular imaging techniques and the measurement of laboratory biomarkers may facilitate better matching of treatment intensity with disease activity. Further, advances in our understanding of disease pathophysiology have paved the way for novel biologic treatments that target important mediators of disease in both giant cell arteritis and Takayasu arteritis. This work has highlighted the substantial heterogeneity present within LVV and the importance of an individualized therapeutic approach. Future work will focus on understanding the mechanisms of persisting vascular inflammation, which will inform the development of increasingly sophisticated imaging technologies. Together, these will enable better disease prognostication, limit treatment-associated adverse effects, and facilitate targeted development and use of novel therapies.

Advances in Radiopharmaceutical Sciences for Vascular Inflammation Imaging: Focus on Clinical Applications

Biomedical imaging technologies offer identification of several anatomic and molecular features of disease pathogenesis. Molecular imaging techniques to assess cellular processes in vivo have been useful in advancing our understanding of several vascular inflammatory diseases. For the non-invasive molecular imaging of vascular inflammation, nuclear medicine constitutes one of the best imaging modalities, thanks to its high sensitivity for the detection of probes in tissues. 2-[¹⁸F]fluoro-2-deoxy-d-glucose ([¹⁸F]FDG) is currently the most widely used radiopharmaceutical for molecular imaging of vascular inflammatory diseases such as atherosclerosis and large-vessel vasculitis. The combination of [¹⁸F]FDG and positron emission tomography (PET) imaging has become a powerful tool to identify and monitor non-invasively inflammatory activities over time but suffers from several limitations including a lack of specificity and avid background in different localizations. The use of novel radiotracers may help to better understand the underlying pathophysiological processes and overcome some limitations of [¹⁸F]FDG PET for the imaging of vascular inflammation. This review examines how [¹⁸F]FDG PET has given us deeper insight into the role of inflammation in different vascular pathologies progression and discusses perspectives for alternative radiopharmaceuticals that could provide a more specific and simple identification of pathologies where vascular inflammation is implicated. Use of these novel PET tracers could lead to a better understanding of underlying disease mechanisms and help inform the identification and stratification of patients for newly emerging immune-modulatory therapies. Future research is needed to realize the true clinical translational value of PET imaging in vascular inflammatory diseases.

Novel Tracers and Radionuclides in PET Imaging

The use of PET imaging agents in oncology, cardiovascular disease, and neurodegenerative disease shows the power of this technique in evaluating the molecular and biological characteristics of numerous diseases. These agents provide crucial information for designing therapeutic strategies for individual patients. Novel PET tracers are in continual development and many have potential use in clinical and research settings. This article discusses the potential applications of tracers in diagnostics, the biological characteristics of diseases, the ability to provide prognostic indicators, and using this information to guide treatment strategies including monitoring treatment efficacy in real time to improve outcomes and survival.

Application progress of multiple imaging modalities in Takayasu arteritis

Takayasu arteritis (TA) is a chronic, idiopathic, granulomatous large vessel vasculitis of unknown etiology. The clinical manifestations of TA are incredibly variable, mainly depending on the location of the lesions. In the light of its insidious progress and the diversity of clinical manifestations, a substantial proportion of patients might experience a considerable delay in diagnosis, which leads to irreversible malignant complications, highlighting the importance of early diagnosis. There has been accumulating evidence that early identification of disease is pivotal to initiate timely therapy and ameliorate the prognosis. Therefore, this review discusses and summarizes the latest evidence on the application progress of multiple imaging modalities.

PET Imaging of Post-infarct Myocardial Inflammation

PURPOSE OF REVIEW

To examine the use of positron emission tomography (PET) for imaging post-infarct myocardial inflammation and repair.

RECENT FINDINGS

Dysregulated immune responses after myocardial infarction are associated with adverse cardiac remodelling and an increased likelihood of ischaemic heart failure. PET imaging utilising novel tracers can be applied to visualise different components of the post-infarction inflammatory and repair processes. This approach could offer unique pathophysiological insights that could prove useful for the identification and risk-stratification of individuals who would ultimately benefit most from emerging immune-modulating therapies. PET imaging could also bridge the clinical translational gap as a surrogate measure of drug efficacy in early-stage clinical trials in patients with myocardial infarction. The use of hybrid PET/MR imaging, in particular, offers the additional advantage of simultaneous in vivo molecular imaging and detailed assessment of myocardial function, viability and tissue characterisation. Further research is needed to realise the true clinical translational value of PET imaging after myocardial infarction.

New insights into the role of imaging in polymyalgia rheumatica

PMR is an inflammatory rheumatic disease of elderly people characterized by pain and stiffness in the neck, shoulder and pelvic girdles. No specific diagnostic confirmatory tests exist and clinical symptoms, as well as increased acute phase reactants, are unspecific. The diagnostic value of imaging including ultrasound, MRI and 18F-fluorodeoxyglucose positron emission tomography (FDG-PET) with/without CT for PMR is increasingly studied. These techniques, particularly FDG-PET/CT, may help to detect underlying GCA in PMR patients with an incomplete response to glucocorticoids and/or recurrent relapses. Recent imaging studies provide novel insights into the anatomical basis of inflammation in PMR, particularly at hip and spine, which may help to distinguish this disease from other mimicking conditions. In this review, we discuss novel insights into the pathoanatomy of PMR, compare the diagnostic values of different imaging techniques and summarize current data on the role of imaging for monitoring and outcome prediction.

Imaging Inflammation - From Whole Body Imaging to Cellular Resolution

Imaging techniques have evolved impressively lately, allowing whole new concepts like multimodal imaging, personal medicine, theranostic therapies, and molecular imaging to increase general awareness of possiblities of imaging to medicine field. Here, we have collected the selected (3D) imaging modalities and evaluated the recent findings on preclinical and clinical inflammation imaging. The focus has been on the feasibility of imaging to aid in inflammation precision medicine, and the key challenges and opportunities of the imaging modalities are presented. Some examples of the current usage in clinics/close to clinics have been brought out as an example. This review evaluates the future prospects of the imaging technologies for clinical applications in precision medicine from the pre-clinical development point of view.

Inflammation and cardiovascular disease: From mechanisms to therapeutics

Inflammation constitutes a complex, highly conserved cascade of molecular and cellular events. Inflammation has been labeled as “the fire within,” is highly regulated, and is critical to host defense and tissue repair. In general, inflammation is beneficial and has evolved to promote survival. However, inflammation can also be maladaptive when chronically activated and sustained, leading to progressive tissue injury and reduced survival. Examples of a maladaptive response include rheumatologic disease and atherosclerosis. Despite evidence gathered by Virchow over 100 years ago showing that inflammatory white cells play a role in atherogenesis, atherosclerosis was until recently viewed as a disease of passive cholesterol accumulation in the subendothelial space. This view has been supplanted by considerable basic scientific and clinical evidence demonstrating that every step of atherogenesis, from the development of endothelial cell dysfunction to foam cell formation, plaque formation and progression, and ultimately plaque rupture stemming from architectural instability, is driven by the cytokines, interleukins, and cellular constituents of the inflammatory response. Herein we provide an overview of the role of inflammation in atherosclerotic cardiovascular disease, discuss the predictive value of various biomarkers involved in inflammation, and summarize recent clinical trials that evaluated the capacity of various pharmacologic interventions to attenuate the intensity of inflammation and impact risk for acute cardiovascular events.