Potential of α7 nicotinic acetylcholine receptor PET imaging in atherosclerosis

18F-ASEM PET/MRI targeting alpha7-nicotinic acetylcholine receptor can reveal skeletal muscle denervation

BACKGROUND

The increased expression of the nicotinic acetylcholine receptor (nAChR) in muscle denervation is thought to be associated with electrophysiological acetylcholine supersensitivity after nerve injury. Hence, we investigated the utility of the 18F-ASEM alpha7-nAChR targeting radiotracer as a new diagnostic method by visualizing skeletal muscle denervation in mouse models of sciatic nerve injury.

METHODS

Ten-week-old C57BL/6 male mice were utilized. The mice were anesthetized, and the left sciatic nerve was resected after splitting the gluteal muscle. One week (n = 11) and three weeks (n = 6) after the denervation, 18F-ASEM positron emission tomography/magnetic resonance imaging (PET/MRI) was acquired. Maximum standardized uptake values (SUVmax) of the tibialis anterior muscle were measured for the denervated side and the control side. Autoradiographic evaluation was performed to measure the mean counts of the denervated and control tibialis anterior muscles at one week. In addition, immunohistochemistry was used to identify alpha7-nAChR-positive areas in denervated and control tibialis anterior muscles at one week (n = 6). Furthermore, a blocking study was conducted with methyllycaconitine (MLA, n = 5).

RESULTS

18F-ASEM PET/MRI showed significantly increased 18F-ASEM uptake in the denervated tibialis anterior muscle relative to the control side one week and three weeks post-denervation. SUVmax of the denervated muscles at one week and three weeks showed significantly higher uptake than the control (P = 0.0033 and 0.0277, respectively). The relative uptake by autoradiography for the denervated muscle was significantly higher than in the control, and immunohistochemistry revealed significantly greater alpha7-nAChR expression in the denervated muscle (P = 0.0277). In addition, the blocking study showed no significant 18F-ASEM uptake in the denervated side when compared to the control (P = 0.0796).

CONCLUSIONS

Our results suggest that nAChR imaging with 18F-ASEM has potential as a noninvasive diagnostic method for peripheral nervous system disorders.

The Influence of Nicotine on Trophoblast-Derived Exosomes in a Mouse Model of Pathogenic Preeclampsia

Preeclampsia (PE) is a serious complication of pregnancy with a pathogenesis that is not fully understood, though it involves the impaired invasion of extravillous trophoblasts (EVTs) into the decidual layer during implantation. Because the risk of PE is actually decreased by cigarette smoking, we considered the possibility that nicotine, a critical component of tobacco smoke, might protect against PE by modifying the content of exosomes from EVTs. We investigated the effects of nicotine on our PE model mouse and evaluated blood pressure. Next, exosomes were extracted from nicotine-treated extravillous trophoblasts (HTR-8/SVneo), and the peptide samples were evaluated by DIA (Data Independent Acquisition) proteomic analysis following nano LC-MS/MS. Hub proteins were identified using bioinformatic analysis. We found that nicotine significantly reduced blood pressure in a PE mouse model. Furthermore, we identified many proteins whose abundance in exosomes was modified by nicotine treatment of EVTs, and we used bioinformatic annotation and network analysis to select five key hub proteins with potential roles in the pathogenesis or prevention of PE. EVT-derived exosomes might influence the pathogenesis of PE because the cargo delivered by exosomes can signal to and modify the receiving cells and their environment.

Positron Emission Tomography Techniques to Measure Active Inflammation, Fibrosis and Angiogenesis: Potential for Non-invasive Imaging of Hypertensive Heart Failure

Heart failure, which is responsible for a high number of deaths worldwide, can develop due to chronic hypertension. Heart failure can involve and progress through several different pathways, including: fibrosis, inflammation, and angiogenesis. Early and specific detection of changes in the myocardium during the transition to heart failure can be made via the use of molecular imaging techniques, including positron emission tomography (PET). Traditional cardiovascular PET techniques, such as myocardial perfusion imaging and sympathetic innervation imaging, have been established at the clinical level but are often lacking in pathway and target specificity that is important for assessment of heart failure. Therefore, there is a need to identify new PET imaging markers of inflammation, fibrosis and angiogenesis that could aid diagnosis, staging and treatment of hypertensive heart failure. This review will provide an overview of key mechanisms underlying hypertensive heart failure and will present the latest developments in PET probes for detection of cardiovascular inflammation, fibrosis and angiogenesis. Currently, selective PET probes for detection of angiogenesis remain elusive but promising PET probes for specific targeting of inflammation and fibrosis are rapidly progressing into clinical use.

18F-ASEM Imaging for Evaluating Atherosclerotic Plaques Linked to α7-Nicotinic Acetylcholine Receptor

Background

Atherosclerosis is a chronic vascular inflammatory procedure alongside with lipid efflux disorder and foam cell formation. α7-Nicotinic acetylcholine receptor (α7nAChR) is a gated-calcium transmembrane channel widely expressed in neuron and non-neuron cells, such as monocytes and macrophages, activated T cells, dendritic cells, and mast cells. ¹⁸F-ASEM is an inhibitor targeted to α7nAChR that had been successfully applied in nervous system diseases. Previous studies had highlighted that α7nAChR was related to the emergency of vulnerable atherosclerotic plaques with excess inflammation cells. Thus, ¹⁸F-ASEM could be a complementary diagnostic approach to atherosclerotic plaques.

Materials and Methods

The synthesis of ASEM precursor and ¹⁸F-labeling had been performed successfully. We had established the ApoE^–/– mice atherosclerotic plaques model (fed with western diet) and New Zealand rabbits atherosclerotic models (balloon-sprained experiment and western diet). After damage of endothelial cells and primary plaque formation, ¹⁸F-ASEM imaging of atherosclerotic plaques linked to α7nAChR had been conducted. In vivo micro-PET/CT imaging of ApoE^–/– mice and the control group was performed 1 h after injection of ¹⁸F-ASEM (100–150 μCi); PET/CT imaging for rabbits with atherosclerotic plaques and control ones was also performed. Meanwhile, we also conducted CT scan on the abdominal aorta of these rabbits. After that, the animals were sacrificed, and the carotid and abdominal aorta were separately taken out for circular sections. The paraffin-embedded specimens were sectioned with 5 μm thickness and stained with hematoxylin–eosin (H&E) and oil red.

Results

In vivo vessel binding of ¹⁸F-ASEM and α7nAChR expression in the model group with atherosclerosis plaques was significantly higher than that in the control group. PET/CT imaging successfully identified the atherosclerotic plaques in ApoE^–/– mice and model rabbits, whereas no obvious signals were detected in normal mice or rabbits. Compared with ¹⁸F-FDG, ¹⁸F-ASEM had more significant effect on the early monitoring of inflammation in carotid atherosclerotic plaques of ApoE^–/– mice and model rabbits. ¹⁸F-ASEM had relatively more palpable effect on the imaging of abdominal aorta with atherosclerosis in rabbits. H&E and oil red staining identified the formation of atherosclerotic plaques in model animals, which provided pathological basis for the evaluation of imaging effects.

Conclusion

We first confirmed ¹⁸F-ASEM as radiotracer with good imaging properties for precise identification of atherosclerotic diseases.

Atherosclerosis imaging using PET: Insights and applications

PET imaging is able to harness biological processes to characterise high-risk features of atherosclerotic plaque prone to rupture. Current radiotracers are able to track inflammation, microcalcification, hypoxia, and neoangiogenesis within vulnerable plaque. 18 F-fluorodeoxyglucose (18 F-FDG) is the most commonly used radiotracer in vascular studies and is employed as a surrogate marker of plaque inflammation. Increasingly, 18 F-FDG and other PET tracers are also being used to provide imaging endpoints in cardiovascular interventional trials. The evolution of novel PET radiotracers, imaging protocols, and hybrid scanners are likely to enable more efficient and accurate characterisation of high-risk plaque. This review explores the role of PET imaging in atherosclerosis with a focus on PET tracers utilised in clinical research and the applications of PET imaging to cardiovascular drug development.

Nicotine and vascular dysfunction

Cigarette smoking is the single most important risk factor for the development of cardiovascular diseases (CVDs). However, the role of nicotine, the addictive component of all tobacco products, in the development of CVD is incompletely understood. Although increased public awareness of the harms of cigarette smoking has successfully led to a decline in its prevalence, the use of electronic cigarettes (e-cig) or electronic nicotine delivery system has increased dramatically in recent years because of the perception that these products are safe. This review summarizes our current knowledge of the expression and function of the nicotinic acetylcholine receptors in the cardiovascular system and the impact of nicotine exposure on cardiovascular health, with a focus on nicotine-induced vascular dysfunction. Nicotine alters vasoreactivity through endothelium-dependent and/or endothelium-independent mechanisms, leading to clinical manifestations in both cigarette smokers and e-cig users. In addition, nicotine induces vascular remodelling through its effects on proliferation, migration and matrix production of both vascular endothelial and vascular smooth muscle cells. The purpose of this review is to identify critical knowledge gaps regarding the effects of nicotine on the vasculature and to stimulate continued nicotine research.

The Availability of the α7-Nicotinic Acetylcholine Receptor in Early Identification of Vulnerable Atherosclerotic Plaques: A Study Using a Novel 18F-Label Radioligand PET

Background: It has been confirmed that the α7-nicotinic acetylcholine receptor (α7nAChR) is an important target for identifying vulnerable atherosclerotic plaques. Previously, we successfully designed and synthesized a series of ¹⁸F-labeled PET molecular probes targeting α7nAChR, which are mainly used in the diagnosis of Alzheimer's disease. Based on the characteristics of α7nAChR in blood vessels, we have firstly screened for a suitable novel ¹⁸F-labeled PET molecular probe ([¹⁸F]YLF-DW), with high selectivity for α7nAChR over α4β2nAChR and a good effect for the imaging of atherosclerotic animal models, to effectively identify vulnerable atherosclerotic plaques at an early stage. Meanwhile, we compared it with the “gold standard” pathological examination of atherosclerosis, to verify the reliability of [¹⁸F]YLF-DW in early diagnosis of atherosclerosis.

Methods: The vulnerable atherosclerotic plaques model of ApoE-/-mice were successfully established. Then based on the methods of 3D-QSAR and molecular docking, we designed oxazolo[4,5-b] pyridines and fluorenone compounds, which are targeted at α7nAChR. Through further screening, a novel alpha7 nicotinic acetylcholine receptor radioligand ([¹⁸F]YLF-DW) was synthesized and automatically ¹⁸F-labeled using a Stynthra RNplus module. Subsequently, we employed [¹⁸F]YLF-DW for the targeting of α7nAChR in atherosclerotic plaques and control group, using a micro-PET/CT respectively. After imaging, the mice were sacrificed by air embolism and the carotid arteries taken out for making circular sections. The paraffin embedded specimens were sectioned with 5 μm thickness and stained with oil red. After staining, immunohistochemistry experiment was carried out to verify the effect of micro-PET/CT imaging.

Results: The micro-PET/CT imaging successfully identified the vulnerable atherosclerotic plaques in the carotid arteries of ApoE-/-mice; whereas, no signal was observed in normal control mice. In addition, compared with the traditional imaging agent [¹⁸F]FDG, [¹⁸F]YLF-DW had a significant effect on the early plaques imaging of carotid atherosclerosis. The results of oil red staining and immunohistochemistry also showed early formations of carotid plaques in ApoE-/-mice and provided pathological bases for the evaluation of imaging effect.

Conclusion: We innovated to apply the novel molecular probe ([¹⁸F]YLF-DW) to the identification of vulnerable atherosclerotic plaques in carotid arteries, to detect atherosclerosis early inflammatory response and provide powerful input for the early diagnosis of atherosclerotic lesions, which may play an early warning role in cardiovascular acute events.

Frontiers in positron emission tomography imaging of the vulnerable atherosclerotic plaque

Rupture of vulnerable atherosclerotic plaques leading to an atherothrombotic event is the primary driver of myocardial infarction and stroke. The ability to detect non-invasively the presence and evolution of vulnerable plaques could have a huge impact on the future identification and management of atherosclerotic cardiovascular disease. Positron emission tomography (PET) imaging with an appropriate radiotracer has the potential to achieve this goal. This review will discuss the biological hallmarks of plaque vulnerability before going on to evaluate and to present PET imaging approaches which target these processes. The focus of this review will be on techniques beyond [18F]FDG imaging, some of which are clinically advanced, and others which are on the horizon. As inflammation is the primary driving force behind atherosclerotic plaque development, we will predominantly focus on approaches which either directly, or indirectly, target this process.

Clinical imaging of cardiovascular inflammation

Cardiovascular disease due to atherosclerosis is the number one cause of morbidity and mortality worldwide. In the past twenty years, compelling preclinical and clinical data have indicated that a maladaptive inflammatory response plays a crucial role in the development of atherosclerosis initiation and progression in the vasculature, all the way to the onset of life-threatening cardiovascular events. Furthermore, inflammation is key to heart and brain damage and healing after myocardial infarction or stroke. Recent evidence indicates that this interplay between the vasculature, organs target of ischemia and the immune system is mediated by the activation of hematopoietic organs (bone marrow and spleen). In this evolving landscape, non-invasive imaging is becoming more and more essential to support either mechanistic preclinical studies to investigate the role of inflammation in cardiovascular disease (CVD), or as a translational tool to quantify inflammation in the cardiovascular system and hematopoietic organs in patients. In this review paper, we will describe the clinical applications of non-invasive imaging to quantify inflammation in the vasculature, infarcted heart and brain, and hematopoietic organs in patients with cardiovascular disease, with specific focus on [18F]FDG PET and other novel inflammation-specific radiotracers. Furthermore, we will briefly describe the most recent clinical applications of other imaging techniques such as MRI, SPECT, CT, CEUS and OCT in this arena.

Short-term discontinuation of vagal nerve stimulation alters 18F-FDG blood pool activity: an exploratory interventional study in epilepsy patients

Background

Vagus nerve activation impacts inflammation. Therefore, we hypothesized that vagal nerve stimulation (VNS) influenced arterial wall inflammation as measured by ¹⁸F-FDG uptake.

Results

Ten patients with left-sided VNS for refractory epilepsy were studied during stimulation (VNS-on) and in the hours after stimulation was switched off (VNS-off). In nine patients, ¹⁸F-FDG uptake was measured in the right carotid artery, aorta, bone marrow, spleen, and adipose tissue. Target-to-background ratios (TBRs) were calculated to normalize the respective standardized uptake values (SUVs) for venous blood pool activity. Median values are shown with interquartile range and compared using the Wilcoxon signed-rank test. Arterial SUVs tended to be higher during VNS-off than VNS-on [SUV_max all vessels 1.8 (1.5–2.2) vs. 1.7 (1.2–2.0), p = 0.051]. However, a larger difference was found for the venous blood pool at this time point, reaching statistical significance in the vena cava superior [_meanSUV_mean 1.3 (1.1–1.4) vs. 1.0 (0.8–1.1); p = 0.011], resulting in non-significant lower arterial TBRs during VNS-off than VNS-on. Differences in the remaining tissues were not significant. Insulin levels increased after VNS was switched off [55.0 pmol/L (45.9–96.8) vs. 48.1 pmol/L (36.9–61.8); p = 0.047]. The concurrent increase in glucose levels was not statistically significant [4.8 mmol/L (4.7–5.3) vs. 4.6 mmol/L (4.5–5.2); p = 0.075].

Conclusions

Short-term discontinuation of VNS did not show a consistent change in arterial wall ¹⁸F-FDG-uptake. However, VNS did alter insulin and ¹⁸F-FDG blood levels, possibly as a result of sympathetic activation.

New SPECT and PET Radiopharmaceuticals for Imaging Inflammatory Diseases: A Meta-analysis of the Last 10 Years

Modern molecular nuclear medicine is rapidly developing in the field of imaging of chronic inflammatory diseases, and many new radiopharmaceuticals have been recently described and tested in animals and man. These can detect early pathophysiological changes before the development of anatomical changes and, often, before clinical onset of symptoms. This field includes new radiopharmaceuticals for SPECT and PET use to define new strategies for imaging immune cells as well as tissue modifications induced by the inflammatory process. In this review, we present the results of a meta-analysis based on radiopharmaceuticals (for SPECT or PET) that are not commercially available and that have been used, at least once, in humans in the last 10 years.