In vitro and pilot in vivo imaging of 18 kDa translocator protein (TSPO) in inflammatory vascular disease

Imaging Inflammation Past, Present, and Future: Focus on Cardioimmunology

Growing evidence implicates the immune system as a critical mediator of cardiovascular disease progression and a viable therapeutic target. Increased inflammatory cell activity is seen in the full spectrum of disorders from early-stage atherosclerosis through myocardial infarction, cardiomyopathy, and chronic heart failure. Although therapeutic strategies to modulate inflammation have shown promise in preclinical animal models, efficacy in patients has been modest owing in part to the variable severity of inflammation across individuals. The diverse leukocyte subpopulations involved in different aspects of heart disease pose a challenge to effective therapy, wherein adverse and beneficial aspects of inflammation require appropriate balance. Noninvasive molecular imaging enables tissue-level interrogation of inflammatory cells in the heart and vasculature to provide mechanistic and temporal insights into disease progression. Although clinical imaging has relied on 18F-FDG as a nonselective and crude marker of inflammatory cell activity, new imaging probes targeting cell surface markers of different leukocyte subpopulations present the opportunity to visualize and quantify distinct phases of cardiac and vessel wall inflammation. Similarly, therapies are evolving to more effectively isolate adverse from beneficial cell populations. This parallel development of immunocardiology and molecular imaging provides the opportunity to refine treatments using imaging guidance, building toward mechanism-based precision medicine. Here, we discuss progress in molecular imaging of immune cells in cardiology from use of 18F-FDG in the past to the present expansion of the radiotracer arsenal and then to a future theranostic paradigm of tracer-therapy compound pairs with shared targets. We then highlight the critical experiments required to advance the field from preclinical concept to clinical reality.

Visualization of Thromboinflammation by 18F-DPA-714 PET in a Stroke Patient

ABSTRACT

A 44-year-old man who presented with progressive right limb weakness was diagnosed with ischemic stroke. He was referred for 18F-DPA-714 PET/CT for evaluation of the disease. 18F-DPA-714 PET/CT showed increased uptake of the intracranial thrombus. This DPA-714-avid thrombus highly suggested the involvement of immune cells in the extension of the clot resulting in neurological deterioration. This present case suggested that 18F-DPA-714 PET might be a promising tracer in visualizing thromboinflammation in vivo.

Positron Emission Tomography Imaging in Vasculitis

Systemic vasculitides comprise a group of autoimmune diseases affecting blood vessels. [18F]-fluoro-2-deoxy-d-glucose positron emission tomography/computed tomography (FDG-PET/CT) plays an important role in the diagnosis and therapeutic monitoring of vasculitides affecting large-sized and medium-sized vessels. FDG-PET/CT also provides complementary information to other vascular imaging tools. The resolution and sensitivity of newer generation scanners continues to increase, hereby improving the ability of FDG-PET/CT to accurately assess the full disease extent in patients with vasculitis. Novel tracers targeting specific immune cells will allow for more detailed detection of vascular infiltrates.

Novel Radiotracers for Molecular Imaging of Myocardial Inflammation: an Update Focused on Clinical Translation of Non-18F-FDG Radiotracers

Purpose of Review

The purpose of this paper is to provide a focused update on recent advances in non-18F-FDG radiotracers for myocardial inflammatory diseases, with a focus on cardiac sarcoidosis and myocarditis.

Recent Findings

Novel radiotracers targeting molecular features of inflammation have the potential to visualize underlying molecular mechanisms key to the pathogenesis of inflammatory cardiomyopathies such as sarcoidosis and myocarditis. These radiotracers may provide unique opportunities for improved mechanistic insight, higher specificity, and better quantification of disease activity, as well as potential for guidance and monitoring of immunomodulatory therapies.

Summary

Novel radiotracers provide unique possibilities in diagnosis, prognostic performance, and therapy guidance for cardiac sarcoidosis and myocarditis.

Emerging Nuclear Medicine Imaging of Atherosclerotic Plaque Formation

Atherosclerosis is a chronic widespread cardiovascular disease and a major predisposing factor for cardiovascular events, among which there are myocardial infarction and ischemic stroke. Atherosclerotic plaque formation is a process that involves different mechanisms, of which inflammation is the most common. Plenty of radiopharmaceuticals were developed to elucidate the process of plaque formation at different stages, some of which were highly specific for atherosclerotic plaque. This review summarizes the current nuclear medicine imaging landscape of preclinical and small-scale clinical studies of these specific RPs, which are not as widespread as labeled FDG, sodium fluoride, and choline. These include oxidation-specific epitope imaging, macrophage, and other cell receptors visualization, neoangiogenesis, and macrophage death imaging. It is shown that specific radiopharmaceuticals have strength in pathophysiologically sound imaging of the atherosclerotic plaques at different stages, but this also may induce problems with the signal registration for low-volume plaques in the vascular wall.

The role of PET/CT in large vessel vasculitis and related disorders: diagnosis, extent evaluation and assessment of therapy response

Large vessel vasculitides (LVV) are defined as chronic inflammatory disorders that affect the arteries with two major variants being distinguished: giant cell arteritis (GCA) and Takayasu's arteritis (TAK). These often present with nonspecific constitutional symptoms which makes an accurate diagnosis often challenging. Nevertheless, timely diagnosis is of utmost importance to initiate treatment and to avoid potential life-threatening complications. [¹⁸F]FDG-PET/CT is nowadays widely accepted as useful tool to aid in the diagnosis of large vessel vasculitis. However, its role to monitor disease activity and to predict disease relapse during follow-up is less obvious since vascular [¹⁸F]FDG uptake can be detected in the absence of clinical or biochemical signs of disease activity. In addition to the two major variants, [¹⁸F]FDG-PET/CT has shown promise in (peri-)aortitis and related disorders. This article aims to provide an update on the current knowledge and limitations of [¹⁸F]FDG-PET/CT for the diagnosis and assessment of treatment response in LVV. Furthermore, other radiopharmaceuticals targeting key components of the vascular immune system are being discussed which could provide an interesting alternative to image vascular inflammation in LVV.

Radiotracers to Address Unmet Clinical Needs in Cardiovascular Imaging, Part 2: Inflammation, Fibrosis, Thrombosis, Calcification, and Amyloidosis Imaging

Cardiovascular imaging is evolving in response to systemwide trends toward molecular characterization and personalized therapies. The development of new radiotracers for PET and SPECT imaging is central to addressing the numerous unmet diagnostic needs that relate to these changes. In this 2-part review, we discuss select radiotracers that may help address key unmet clinical diagnostic needs in cardiovascular medicine. Part 1 examined key technical considerations pertaining to cardiovascular radiotracer development and reviewed emerging radiotracers for perfusion and neuronal imaging. Part 2 covers radiotracers for imaging cardiovascular inflammation, thrombosis, fibrosis, calcification, and amyloidosis. These radiotracers have the potential to address several unmet needs related to the risk stratification of atheroma, detection of thrombi, and the diagnosis, characterization, and risk stratification of cardiomyopathies. In the first section, we discuss radiotracers targeting various aspects of inflammatory responses in pathologies such as myocardial infarction, myocarditis, sarcoidosis, atherosclerosis, and vasculitis. In a subsequent section, we discuss radiotracers for the detection of systemic and device-related thrombi, such as those targeting fibrin (e.g., ⁶⁴Cu-labeled fibrin-binding probe 8). We also cover emerging radiotracers for the imaging of cardiovascular fibrosis, such as those targeting fibroblast activation protein (e.g., ⁶⁸Ga-fibroblast activation protein inhibitor). Lastly, we briefly review radiotracers for imaging of cardiovascular calcification (¹⁸F-NaF) and amyloidosis (e.g., ^99mTc-pyrophosphate and ¹⁸F-florbetapir).

Novel PET Imaging of Inflammatory Targets and Cells for the Diagnosis and Monitoring of Giant Cell Arteritis and Polymyalgia Rheumatica

Giant cell arteritis (GCA) and polymyalgia rheumatica (PMR) are two interrelated inflammatory diseases affecting patients above 50 years of age. Patients with GCA suffer from granulomatous inflammation of medium- to large-sized arteries. This inflammation can lead to severe ischemic complications (e.g., irreversible vision loss and stroke) and aneurysm-related complications (such as aortic dissection). On the other hand, patients suffering from PMR present with proximal stiffness and pain due to inflammation of the shoulder and pelvic girdles. PMR is observed in 40-60% of patients with GCA, while up to 21% of patients suffering from PMR are also affected by GCA. Due to the risk of ischemic complications, GCA has to be promptly treated upon clinical suspicion. The treatment of both GCA and PMR still heavily relies on glucocorticoids (GCs), although novel targeted therapies are emerging. Imaging has a central position in the diagnosis of GCA and PMR. While [18F]fluorodeoxyglucose (FDG)-positron emission tomography (PET) has proven to be a valuable tool for diagnosis of GCA and PMR, it possesses major drawbacks such as unspecific uptake in cells with high glucose metabolism, high background activity in several non-target organs and a decrease of diagnostic accuracy already after a short course of GC treatment. In recent years, our understanding of the immunopathogenesis of GCA and, to some extent, PMR has advanced. In this review, we summarize the current knowledge on the cellular heterogeneity in the immunopathology of GCA/PMR and discuss how recent advances in specific tissue infiltrating leukocyte and stromal cell profiles may be exploited as a source of novel targets for imaging. Finally, we discuss prospective novel PET radiotracers that may be useful for the diagnosis and treatment monitoring in GCA and PMR.