Recent advances in metabolic imaging

Advances of research in diabetic cardiomyopathy: diagnosis and the emerging application of sequencing

Diabetic cardiomyopathy (DCM) is one of the most prevalent and severe complications associated with diabetes mellitus (DM). The onset of DCM is insidious, with the symptoms being obvious only in the late stage. Consequently, the early diagnosis of DCM is a formidable challenge which significantly influences the treatment and prognosis of DCM. Thus, it becomes imperative to uncover innovative approaches to facilitate the prompt identification and diagnosis of DCM. On the traditional clinical side, we tend to use serum biomarkers as well as imaging as the most common means of diagnosing diseases because of their convenience as well as affordability. As we delve deeper into the mechanisms of DCM, a wide variety of biomarkers are becoming competitive diagnostic indicators. Meanwhile, the application of multiple imaging techniques has also made efforts to promote the diagnosis of DCM. Besides, the spurt in sequencing technology has made it possible to give hints about disease diagnosis from the genome as well as the transcriptome, making diagnosis less difficult, more sensitive, and more predictive. Overall, sequencing technology is expected to be the superior choice of plasma biomarkers for detecting lesions at an earlier stage than imaging, and its judicious utilization combined with imaging technologies will lead to a more sensitive diagnosis of DCM in the future. Therefore, this review meticulously consolidates the progress and utilization of various biomarkers, imaging methods, and sequencing technologies in the realm of DCM diagnosis, with the aim of furnishing novel theoretical foundation and guide future research endeavors towards enhancing the diagnostic and therapeutic landscape of DCM.

Human cardiac metabolism

The heart is the most metabolically active organ in the human body, and cardiac metabolism has been studied for decades. However, the bulk of studies have focused on animal models. The objective of this review is to summarize specifically what is known about cardiac metabolism in humans. Techniques available to study human cardiac metabolism are first discussed, followed by a review of human cardiac metabolism in health and in heart failure. Mechanistic insights, where available, are reviewed, and the evidence for the contribution of metabolic insufficiency to heart failure, as well as past and current attempts at metabolism-based therapies, is also discussed.

Diagnostic importance of lung perfusion/ventilation scans in the evaluation of pulmonary embolism in COVID-19 patients: systematic review of the literature

OBJECTIVE

To investigate the outcomes of ventilation/perfusion scintigraphy on the diagnosis of pulmonary embolism in coronavirus disease 2019 (COVID-19) patients, we performed a systematic review of the available literature.

MATERIALS AND METHODS

PubMed and Scopus were systematically searched up to 4 June 2022, for relevant studies. We included studies on patients with COVID-19 who have performed ventilation/perfusion scintigraphy for diagnosis of pulmonary embolism to describe any diagnosis outcome. Irrelevant and non-English articles were excluded.

RESULTS

Overall, 27 articles were included in our review. The database search yielded studies from PubMed, Scopus, and studies identified through reviewing the reference list of included studies. Extracted information from the included studies could be categorized into several aspects: Diagnosis of pulmonary embolism with Q single-photon emission computed tomography (SPECT) CT, Tracheobronchial uptake, Diagnostic value of V/Q rather than Q at diagnosis pulmonary embolism, Different characteristics (morphological alterations) of COVID-19 in ventilation orperfusion scan, the prevalence of pulmonary embolism with Q or V/Q criteria, and Design of radiotherapy planning in lung cancer patients with COVID-19.

CONCLUSION

Different perfusion patterns in COVID-19 are challenging but can be alleviated by adding SPECT/computed tomography (CT) to lung perfusion scans. Although perfusion only SPECT/CT can rule out or rule in others in considerable number of patients, ventilation scan is still needed in certain patients.

Role of Echocardiography in Diabetic Cardiomyopathy: From Mechanisms to Clinical Practice

It has been well established that diabetes mellitus (DM) is considered as a core risk factor for the development of cardiovascular diseases. However, what is less appreciated is the fact that DM may affect cardiac function irrespective of cardiac pathologies to which it contributes, such as coronary artery disease and hypertension. Although echocardiography provides accurate and reproducible diagnostic and prognostic data in patients with DM, its use in these patients is still underappreciated, resulting in progression of DM-related heart failure in many patients. Hence, in the present review, we aimed to discuss the role of echocardiography in the contemporary management of diabetic cardiomyopathy (DCM), as well as the role of emerging echocardiographic techniques, which may contribute to earlier diagnosis and more appropriate management of this complication of DM. In order to improve outcomes, focus must be placed on early diagnosis of this condition using a combination of echocardiography and emerging biomarkers, but perhaps the more important thing is to change perspective when it comes to the clinical importance of DCM.

Strategies for Imaging Metabolic Remodeling of the Heart in Obesity and Heart Failure

PURPOSE OF REVIEW

Define early myocardial metabolic changes among patients with obesity and heart failure, and to describe noninvasive methods and their applications for imaging cardiac metabolic remodeling.

RECENT FINDINGS

Metabolic remodeling precedes, triggers, and sustains functional and structural remodeling in the stressed heart. Alterations in cardiac metabolism can be assessed by using a variety of molecular probes. The glucose tracer analog, 18F-FDG, and the labeled tracer 11C-palmitate are still the most commonly used tracers to assess glucose and fatty acid metabolism, respectively. The development of new tracer analogs and imaging agents, including those targeting the peroxisome proliferator-activated receptor (PPAR), provides new opportunities for imaging metabolic activities at a molecular level. While the use of cardiac magnetic resonance spectroscopy in the clinical setting is limited to the assessment of intramyocardial and epicardial fat, new technical improvements are likely to increase its usage in the setting of heart failure. Noninvasive imaging methods are an effective tool for the serial assessment of alterations in cardiac metabolism, either during disease progression, or in response to treatment.

Cardiac Adiposity and Arrhythmias: The Role of Imaging

Increased cardiac fat depots are metabolically active tissues that have a pronounced pro-inflammatory nature. Increasing evidence supports a potential role of cardiac adiposity as a determinant of the substrate of atrial fibrillation and ventricular arrhythmias. The underlying mechanism appears to be multifactorial with local inflammation, fibrosis, adipocyte infiltration, electrical remodeling, autonomic nervous system modulation, oxidative stress and gene expression playing interrelating roles. Current imaging modalities, such as echocardiography, computed tomography and cardiac magnetic resonance, have provided valuable insight into the relationship between cardiac adiposity and arrhythmogenesis, in order to better understand the pathophysiology and improve risk prediction of the patients, over the presence of obesity and traditional risk factors. However, at present, given the insufficient data for the additive value of imaging biomarkers on commonly used risk algorithms, the use of different screening modalities currently is indicated for personalized risk stratification and prognostication in this setting.

Metabolic and Molecular Imaging of the Diabetic Cardiomyopathy

The term diabetic cardiomyopathy is defined as the presence of abnormalities in myocardial structure and function that occur in the absence of, or in addition to, well-established cardiovascular risk factors. A key contributor to this abnormal structural-functional relation is the complex interplay of myocardial metabolic remodeling, defined as the loss the flexibility in myocardial substrate metabolism and its downstream detrimental effects, such as mitochondrial dysfunction, inflammation, and fibrosis. In parallel with the growth in understanding of these biological underpinnings has been developmental advances in imaging tools such as positron emission tomography and magnetic resonance imaging and spectroscopy that permit the detection and in many cases quantification, of the processes that typifies the myocardial metabolic remodeling in diabetic cardiomyopathy. The imaging readouts can be obtained in both preclinical models of diabetes mellitus and patients with diabetes mellitus facilitating the bi-directional movement of information between bench and bedside. Moreover, imaging biomarkers provided by these tools are now being used to enhance discovery and development of therapies designed to reduce the myocardial effects of diabetes mellitus through metabolic modulation. In this review, the use of these imaging tools in the patient with diabetes mellitus from a mechanistic, therapeutic effect, and clinical management perspective will be discussed.

21st Century Advances in Multimodality Imaging of Obesity for Care of the Cardiovascular Patient

Although obesity is typically defined by body mass index criteria, this does not differentiate true body fatness, as this includes both body fat and muscle. Therefore, other fat depots may better define cardiometabolic and cardiovascular disease (CVD) risk imposed by obesity. Data from translational, epidemiological, and clinical studies over the past 3 decades have clearly demonstrated that accumulation of adiposity in the abdominal viscera and within tissue depots lacking physiological adipose tissue storage capacity (termed "ectopic fat") is strongly associated with the development of a clinical syndrome characterized by atherogenic dyslipidemia, hyperinsulinemia/glucose intolerance/type 2 diabetes mellitus, hypertension, atherosclerosis, and abnormal cardiac remodeling and heart failure. This state-of-the-art paper discusses the impact of various body fat depots on cardiometabolic parameters and CVD risk. Specifically, it reviews novel and emerging imaging techniques to evaluate adiposity and the risk of cardiometabolic diseases and CVD.

Molecular Imaging of the Heart

Multimodality cardiovascular imaging is routinely used to assess cardiac function, structure, and physiological parameters to facilitate the diagnosis, characterization, and phenotyping of numerous cardiovascular diseases (CVD), as well as allows for risk stratification and guidance in medical therapy decision-making. Although useful, these imaging strategies are unable to assess the underlying cellular and molecular processes that modulate pathophysiological changes. Over the last decade, there have been great advancements in imaging instrumentation and technology that have been paralleled by breakthroughs in probe development and image analysis. These advancements have been merged with discoveries in cellular/molecular cardiovascular biology to burgeon the field of cardiovascular molecular imaging. Cardiovascular molecular imaging aims to noninvasively detect and characterize underlying disease processes to facilitate early diagnosis, improve prognostication, and guide targeted therapy across the continuum of CVD. The most-widely used approaches for preclinical and clinical molecular imaging include radiotracers that allow for high-sensitivity in vivo detection and quantification of molecular processes with single photon emission computed tomography and positron emission tomography. This review will describe multimodality molecular imaging instrumentation along with established and novel molecular imaging targets and probes. We will highlight how molecular imaging has provided valuable insights in determining the underlying fundamental biology of a wide variety of CVDs, including: myocardial infarction, cardiac arrhythmias, and nonischemic and ischemic heart failure with reduced and preserved ejection fraction. In addition, the potential of molecular imaging to assist in the characterization and risk stratification of systemic diseases, such as amyloidosis and sarcoidosis will be discussed. © 2019 American Physiological Society. Compr Physiol 9:477-533, 2019.

Experimental Study of a Radiopharmaceutical Agent Based on Modified Fatty Acid Labeled with Technetium-99m

Using rat model of coronary occlusion, we studied pharmacokinetics and the efficiency of a new radiopharmaceutical agent ^99mTc-PDA-DTPA intended for diagnostics of changes in myocardial metabolism and its analogue ¹²³I-PMPDA. ^99mTc-PDA-DTPA was eliminated mostly by the kidneys and maximal concentration in the heart was attained within 60 min after intravenous injection; no accumulation in the area of myocardial infarction was observed. The studied substance was inferior to its analogue 123I-PMPDA by the quality of scintigraphic visualization of the heart.

Tactics for preclinical validation of receptor-binding radiotracers

INTRODUCTION

Aspects of radiopharmaceutical development are illustrated through preclinical studies of [125I]-(E)-1-(2-(2,3-dihydrobenzofuran-5-yl)ethyl)-4-(iodoallyl)piperazine ([125I]-E-IA-BF-PE-PIPZE), a radioligand for sigma-1 (σ1) receptors, coupled with examples from the recent literature. Findings are compared to those previously observed for [125I]-(E)-1-(2-(2,3-dimethoxy-5-yl)ethyl)-4-(iodoallyl)piperazine ([125I]-E-IA-DM-PE-PIPZE).

METHODS

Syntheses of E-IA-BF-PE-PIPZE and [125I]-E-IA-BF-PE-PIPZE were accomplished by standard methods. In vitro receptor binding studies and autoradiography were performed, and binding potential was predicted. Measurements of lipophilicity and protein binding were obtained. In vivo studies were conducted in mice to evaluate radioligand stability, as well as specific binding to σ1 sites in brain, brain regions and peripheral organs in the presence and absence of potential blockers.

RESULTS

E-IA-BF-PE-PIPZE exhibited high affinity and selectivity for σ1 receptors (Ki = 0.43 ± 0.03 nM, σ2/σ1 = 173). [125I]-E-IA-BF-PE-PIPZE was prepared in good yield and purity, with high specific activity. Radioligand binding provided dissociation (koff) and association (kon) rate constants, along with a measured Kd of 0.24 ± 0.01 nM and Bmax of 472 ± 13 fmol/mg protein. The radioligand proved suitable for quantitative autoradiography in vitro using brain sections. Moderate lipophilicity, Log D7.4 2.69 ± 0.28, was determined, and protein binding was 71 ± 0.3%. In vivo, high initial whole brain uptake, >6% injected dose/g, cleared slowly over 24 h. Specific binding represented 75% to 93% of total binding from 15 min to 24 h. Findings were confirmed and extended by regional brain biodistribution. Radiometabolites were not observed in brain (1%).

CONCLUSIONS

Substitution of dihydrobenzofuranylethyl for dimethoxyphenethyl increased radioligand affinity for σ1 receptors by 16-fold. While high specific binding to σ1 receptors was observed for both radioligands in vivo, [125I]-E-IA-BF-PE-PIPZE displayed much slower clearance kinetics than [125I]-E-IA-DM-PE-PIPZE. Thus, minor structural modifications of σ1 receptor radioligands lead to major differences in binding properties in vitro and in vivo.

Cardiac Metabolism in Perspective

The heart is a biological pump that converts chemical to mechanical energy. This process of energy conversion is highly regulated to the extent that energy substrate metabolism matches energy use for contraction on a beat-to-beat basis. The biochemistry of cardiac metabolism includes the biochemistry of energy transfer, metabolic regulation, and transcriptional, translational as well as posttranslational control of enzymatic activities. Pathways of energy substrate metabolism in the heart are complex and dynamic, but all of them conform to the First Law of Thermodynamics. The perspectives expand on the overall idea that cardiac metabolism is inextricably linked to both physiology and molecular biology of the heart. The article ends with an outlook on emerging concepts of cardiac metabolism based on new molecular models and new analytical tools. © 2016 American Physiological Society. Compr Physiol 6:1675-1699, 2016.

Assessing Cardiac Metabolism: A Scientific Statement From the American Heart Association

In a complex system of interrelated reactions, the heart converts chemical energy to mechanical energy. Energy transfer is achieved through coordinated activation of enzymes, ion channels, and contractile elements, as well as structural and membrane proteins. The heart's needs for energy are difficult to overestimate. At a time when the cardiovascular research community is discovering a plethora of new molecular methods to assess cardiac metabolism, the methods remain scattered in the literature. The present statement on "Assessing Cardiac Metabolism" seeks to provide a collective and curated resource on methods and models used to investigate established and emerging aspects of cardiac metabolism. Some of those methods are refinements of classic biochemical tools, whereas most others are recent additions from the powerful tools of molecular biology. The aim of this statement is to be useful to many and to do justice to a dynamic field of great complexity.

Noninvasive Detection of Early Metabolic Left Ventricular Remodeling in Systemic Hypertension

OBJECTIVES

Hypertension (HTN) is a common cause of left ventricular hypertrophy (LVH). Sustained pressure overload induces a permanent myocardial switch from fatty-acid to glucose metabolism. In this study, we tested the hypothesis that metabolic remodeling, characterized by increased myocardial glucose uptake, precedes structural and functional remodeling in HTN-induced LVH.

METHODS

We recruited 31 patients: 11 with HTN only, 9 with HTN and LVH and 11 normotensive controls without LVH. Transthoracic echocardiography was performed to assess the function, mass, wall thickness and diastolic function of the left ventricle. Positron emission tomography imaging was performed, and the rate of myocardial 2-deoxy-2-[18F]fluoro-D-glucose uptake, Ki, was determined using a 3-compartment kinetic model.

RESULTS

The mean Ki values were significantly higher in HTN patients than in those with HTN and LVH (p < 0.001) and in controls (p = 0.003). The unexpected decrease in Ki with LVH may be secondary to a decreased Ki with diastolic dysfunction (DD), 0.039 ± 0.032 versus 0.072 ± 0.013 (p = 0.004). There was also a significant stepwise decrease in Ki with increasing DD grade (p = 0.04).

CONCLUSION

Glucose metabolic remodeling is detectable in hypertensive patients before the development of LVH. Furthermore, lower glucose uptake rates are observed in patients with DD. The mechanism for this last finding requires further investigation.

Remodeling of glucose metabolism precedes pressure overload-induced left ventricular hypertrophy: review of a hypothesis

When subjected to pressure overload, the ventricular myocardium shifts from fatty acids to glucose as its main source for energy provision and frequently increases its mass. Here, we review the evidence in support of the concept that metabolic remodeling, measured as an increased myocardial glucose uptake using dynamic positron emission tomography (PET) with the glucose analogue 2-deoxy-2-[(18)F]fluoro-D-glucose (FDG), precedes the onset of left ventricular hypertrophy (LVH) and heart failure. Consistent with this, early intervention with propranolol, which attenuates glucose uptake, prevents the maladaptive metabolic response and preserves cardiac function in vivo. We also review ex vivo studies suggesting a link between dysregulated myocardial glucose metabolism, intracellular accumulation of glucose 6-phosphate (G6P) and contractile dysfunction of the heart. G6P levels correlate with activation of mTOR (mechanistic target of rapamycin) and endoplasmic reticulum stress. This sequence of events could be prevented by pretreatment with rapamycin (mTOR inhibition) or metformin (enzyme 5'-AMP-activated protein kinase activation). In conclusion, we propose that metabolic imaging with FDG PET may provide a novel approach to guide the treatment of patients with hypertension-induced LVH.

Rethinking cardiac metabolism: metabolic cycles to refuel and rebuild the failing heart

The heart is a self-renewing biological pump that converts chemical energy into mechanical energy. The entire process of energy conversion is subject to complex regulation at the transcriptional, translational and post-translational levels. Within this system, energy transfer occurs with high efficiency, facilitated by a series of compound-conserved cycles. At the same time, the constituent myocardial proteins themselves are continuously made and degraded in order to adjust to changes in energy demand and changes in the extracellular environment. We recently have identified signals arising from intermediary metabolism that regulate the cycle of myocardial protein turnover. Using a new conceptual framework, we discuss the principle of metabolic cycles and their importance for refueling and for rebuilding the failing heart.