Radionuclide imaging of neurohormonal system of the heart

A Molecular Toolbox of Positron Emission Tomography Tracers for General Anesthesia Mechanism Research

With appropriate radiotracers, positron emission tomography (PET) allows direct or indirect monitoring of the spatial and temporal distribution of anesthetics, neurotransmitters, and biomarkers, making it an indispensable tool for studying the general anesthesia mechanism. In this Perspective, PET tracers that have been recruited in general anesthesia research are introduced in the following order: 1) ¹¹C/¹⁸F-labeled anesthetics, i.e., PET tracers made from inhaled and intravenous anesthetics; 2) PET tracers targeting anesthesia-related receptors, e.g., neurotransmitters and voltage-gated ion channels; and 3) PET tracers for studying anesthesia-related neurophysiological effects and neurotoxicity. The radiosynthesis, pharmacodynamics, and pharmacokinetics of the above PET tracers are mainly discussed to provide a practical molecular toolbox for radiochemists, anesthesiologists, and those who are interested in general anesthesia.

Protective Effect of Natural Medicinal Plants on Cardiomyocyte Injury in Heart Failure: Targeting the Dysregulation of Mitochondrial Homeostasis and Mitophagy

Heart failure occurs because of various cardiovascular pathologies, such as coronary artery disease or cardiorenal syndrome, eventually reaching end-stage disease. Various factors contribute to cardiac structural or functional changes that result in systolic or diastolic dysfunction. Several studies have confirmed that the key factor in heart failure progression is myocardial cell death, and mitophagy is the major mechanism regulating myocardial cell death in heart failure. The clinical mechanisms of heart failure are well understood in practice. However, the essential role of mitophagic regulation in heart failure has only recently received widespread attention. Receptor-mediated mitophagy is involved in various mitochondrial processes like oxidative stress injury, energy metabolism disorders, and calcium homeostasis, which are also the main causes of heart failure. Understanding of the diverse regulatory mechanisms in mitophagy and the complexity of its pathophysiology in heart failure remains incomplete. Related studies have found that various natural medicinal plants and active ingredients, such as flavonoids and saponins, can regulate mitophagy to a certain extent, improve myocardial function, and protect myocardial cells. This review comprehensively covers the relevant mechanisms of different types of mitophagy in regulating heart failure pathology and controlling mitochondrial adaptability to stress injury. Further, it explores the relationship between mitophagy and cardiac ejection dysfunction. Natural medicinal plant-targeted regulation strategies and scientific evidence on mitophagy were provided to elucidate current and potential strategies to apply mitophagy-targeted therapy for heart failure.

Cardiac hybrid imaging: novel tracers for novel targets

Non-invasive cardiac imaging has explored enormous advances in the last few decades. In particular, hybrid imaging represents the fusion of information from multiple imaging modalities, allowing to provide a more comprehensive dataset compared to traditional imaging techniques in patients with cardiovascular diseases. The complementary anatomical, functional and molecular information provided by hybrid systems are able to simplify the evaluation procedure of various pathologies in a routine clinical setting. The diagnostic capability of hybrid imaging modalities can be further enhanced by introducing novel and specific imaging biomarkers. The aim of this review is to cover the most recent advancements in radiotracers development for SPECT/CT, PET/CT, and PET/MRI for cardiovascular diseases.

Initial Evaluation of AF78: a Rationally Designed Fluorine-18-Labelled PET Radiotracer Targeting Norepinephrine Transporter

Purpose

Taking full advantage of positron emission tomography (PET) technology, fluorine-18-labelled radiotracers targeting norepinephrine transporter (NET) have potential applications in the diagnosis and assessment of cardiac sympathetic nerve conditions as well as the delineation of neuroendocrine tumours. However, to date, none have been used clinically. Drawbacks of currently reported radiotracers include suboptimal kinetics and challenging radiolabelling procedures.

Procedures

We developed a novel fluorine-18-labelled radiotracer targeting NET, AF78, with efficient one-step radiolabelling based on the phenethylguanidine structure. Radiosynthesis of AF78 was undertaken, followed by validation in cell uptake studies, autoradiography, and in vivo imaging in rats.

Results

[¹⁸F]AF78 was successfully synthesized with 27.9 ± 3.1 % radiochemical yield, > 97 % radiochemical purity and > 53.8 GBq/mmol molar activity. Cell uptake studies demonstrated essentially identical affinity for NET as norepinephrine and meta-iodobenzylgaunidine. Both ex vivo autoradiography and in vivo imaging in rats showed homogeneous and specific cardiac uptake.

Conclusions

The new PET radiotracer [¹⁸F]AF78 demonstrated high affinity for NET and favourable biodistribution in rats. A structure-activity relationship between radiotracer structures and affinity for NET was revealed, which may serve as the basis for the further design of NET targeting radiotracers with favourable features.

Electronic supplementary material

The online version of this article (10.1007/s11307-019-01407-5) contains supplementary material, which is available to authorized users.

Neurohormonal modulation in pulmonary arterial hypertension

Pulmonary hypertension is a fatal condition of elevated pulmonary pressures, complicated by right heart failure. Pulmonary hypertension appears in various forms; one of those is pulmonary arterial hypertension (PAH) and is particularly characterised by progressive remodelling and obstruction of the smaller pulmonary vessels. Neurohormonal imbalance in PAH patients is associated with worse prognosis and survival. In this back-to-basics article on neurohormonal modulation in PAH, we provide an overview of the pharmacological and nonpharmacological strategies that have been tested pre-clinically and clinically. The benefit of neurohormonal modulation strategies in PAH patients has been limited by lack of insight into how the neurohormonal system is changed throughout the disease and difficulties in translation from animal models to human trials. We propose that longitudinal and individual assessments of neurohormonal status are required to improve the timing and specificity of neurohormonal modulation strategies. Ongoing developments in imaging techniques such as positron emission tomography may become helpful to determine neurohormonal status in PAH patients in different disease stages and optimise individual treatment responses.

Noninvasive Radionuclide Molecular Imaging of the CD4-Positive T Lymphocytes in Acute Cardiac Rejection

Heart transplantation (HT) is an effective treatment for end-stage heart disease. However, acute rejection (AR) is still the main cause of death within one year after HT. AR is an acute immune response mediated by T lymphocytes, mainly CD4+ T lymphocytes. This study innovatively develops a radiolabeled probe ^99mTc-HYNIC-mAb_CD4 for noninvasive visualization of CD4+ T lymphocyte infiltration and detection of AR. The ^99mTc-HYNIC-mAb_CD4 and its isotype control ^99mTc-HYNIC-IgG were successfully prepared and characterized. The specificity and affinity of the probe in vitro were assessed by cell-binding experiments. Binding of ^99mTc-HYNIC-mAb_CD4 to CD4+ T lymphocytes was higher than that of the macrophages and IgG probe groups, and mAb_CD4 was effective in the blockade of the binding reaction. The biodistribution data confirmed the SPECT/CT images, with significantly higher levels of ^99mTc-HYNIC-mAb_CD4 observed in allografts compared to allograft treatment (10 mg/kg/d Cyclosporin A subcutaneously for 5 consecutive days after surgery), isografts, or in rats which received allografts injected with ^99mTc-HYNIC-IgG. Histological examination confirmed more CD4+ T lymphocyte infiltration in the allograft hearts than other groups. In summary, ^99mTc-HYNIC-mAb_CD4 achieved high affinity and specificity of binding to CD4+ T lymphocytes and accumulation in the transplanted heart. Radionuclide molecular imaging with ^99mTc-HYNIC-mAb_CD4 may be a potential diagnostic method for acute cardiac rejection.

PET Radiotracers for CNS-Adrenergic Receptors: Developments and Perspectives

Epinephrine (E) and norepinephrine (NE) play diverse roles in our body’s physiology. In addition to their role in the peripheral nervous system (PNS), E/NE systems including their receptors are critical to the central nervous system (CNS) and to mental health. Various antipsychotics, antidepressants, and psychostimulants exert their influence partially through different subtypes of adrenergic receptors (ARs). Despite the potential of pharmacological applications and long history of research related to E/NE systems, research efforts to identify the roles of ARs in the human brain taking advantage of imaging have been limited by the lack of subtype specific ligands for ARs and brain penetrability issues. This review provides an overview of the development of positron emission tomography (PET) radiotracers for in vivo imaging of AR system in the brain.

Ventricular Distribution Pattern of the Novel Sympathetic Nerve PET Radiotracer 18F-LMI1195 in Rabbit Hearts

We aimed to determine a detailed regional ventricular distribution pattern of the novel cardiac nerve PET radiotracer ¹⁸F-LMI1195 in healthy rabbits. Ex-vivo high resolution autoradiographic imaging was conducted to identify accurate ventricular distribution of ¹⁸F-LMI1195. In healthy rabbits, ¹⁸F-LMI1195 was administered followed by the reference perfusion marker ²⁰¹Tl for a dual-radiotracer analysis. After 20 min of ¹⁸F-LMI1195 distribution time, the rabbits were euthanized, the hearts were extracted, frozen, and cut into 20-μm short axis slices. Subsequently, the short axis sections were exposed to a phosphor imaging plate to determine ¹⁸F-LMI1195 distribution (exposure for 3 h). After complete ¹⁸F decay, sections were re-exposed to determine ²⁰¹Tl distribution (exposure for 7 days). For quantitative analysis, segmental regions of Interest (ROIs) were divided into four left ventricular (LV) and a right ventricular (RV) segment on mid-ventricular short axis sections. Subendocardial, mid-portion, and subepicardial ROIs were placed on the LV lateral wall. ¹⁸F-LMI1195 distribution was almost homogeneous throughout the LV wall without any significant differences in all four LV ROIs (anterior, posterior, septal and lateral wall, 99 ± 2, 94 ± 5, 94 ± 4 and 97 ± 3%LV, respectively, n.s.). Subepicardial ²⁰¹Tl uptake was significantly lower compared to the subendocardial portion (subendocardial, mid-portion, and subepicardial activity: 90 ± 3, 96 ± 2 and *80 ± 5%LV, respectively, *p < 0.01 vs. mid-portion). This was in contradistinction to the transmural wall profile of ¹⁸F-LMI1195 (90 ± 4, 96 ± 5 and 84 ± 4%LV, n.s.). A slight but significant discrepant transmural radiotracer distribution pattern of ²⁰¹Tl in comparison to ¹⁸F-LMI1195 may be a reflection of physiological sympathetic innervation and perfusion in rabbit hearts.

[123 I-MIBG scintigraphy in the assessment of heart failure prognosis and effectiveness of cardiac resynchronization therapy]

Cardiac resynchronization therapy (CRT) is one of the methods of treating patients with chronic heart failure, which can reduce the mortality rate of this group. Scintigraphic assessment of sympathetic myocardial innervation allows us to evaluate the heart failure prognosis and the effectiveness of interventional treatment. The method is based on use of the radiopharmaceutical 123 I-methiodiobenzylguanidine (123 I-MIBG), which is a structural analogue of norepinephrine and is able to selectively accumulate in the sympathetic nerve endings. This review includes a brief description of norepinephrine metabolism and pharmacokinetics of 123 I-MIBG in the sympathetic nerve ending, a brief description of the study methodology and the clinical significance of this method in patients with heart failure. Particular attention is paid to the possibilities of using this method in patients with severe chronic heart failure before and after CRT.

Less Exploited GPCRs in Precision Medicine: Targets for Molecular Imaging and Theranostics

Precision medicine relies on individually tailored therapeutic intervention taking into account individual variability. It is strongly dependent on the availability of target-specific drugs and/or imaging agents that recognize molecular targets and patient-specific disease mechanisms. The most sensitive molecular imaging modalities, Single Photon Emission Computed Tomography (SPECT) and Positron Emission Tomography (PET), rely on the interaction between an imaging radioprobe and a target. Moreover, the use of target-specific molecular tools for both diagnostics and therapy, theranostic agents, represent an established methodology in nuclear medicine that is assuming an increasingly important role in precision medicine. The design of innovative imaging and/or theranostic agents is key for further accomplishments in the field. G-protein-coupled receptors (GPCRs), apart from being highly relevant drug targets, have also been largely exploited as molecular targets for non-invasive imaging and/or systemic radiotherapy of various diseases. Herein, we will discuss recent efforts towards the development of innovative imaging and/or theranostic agents targeting selected emergent GPCRs, namely the Frizzled receptor (FZD), Ghrelin receptor (GHSR-1a), G protein-coupled estrogen receptor (GPER), and Sphingosine-1-phosphate receptor (S1PR). The pharmacological and clinical relevance will be highlighted, giving particular attention to the studies on the synthesis and characterization of targeted molecular imaging agents, biological evaluation, and potential clinical applications in oncology and non-oncology diseases. Whenever relevant, supporting computational studies will be also discussed.

Novel 18F-Labeled PET Imaging Agent FV45 Targeting the Renin-Angiotensin System

Renin-angiotensin system (RAS) plays an important role in the regulation of blood pressure and hormonal balance. Using positron emission tomography (PET) technology, it is possible to monitor the physiological and pathological distribution of angiotensin II type 1 receptors (AT₁), which reflects the functionality of RAS. A new ¹⁸F-labeled PET tracer derived from the clinically used AT₁ antagonist valsartan showing the least possible chemical alteration from the valsartan structure has been designed and synthesized with several strategies, which can be applied for the syntheses of further derivatives. Radioligand binding study showed that the cold reference FV45 (K _i 14.6 nM) has almost equivalent binding affinity as its lead valsartan (K _i 11.8 nM) and angiotensin II (K _i 1.7 nM). Successful radiolabeling of FV45 in a one-pot radiofluorination followed by the deprotection procedure with 21.8 ± 8.5% radiochemical yield and >99% radiochemical purity (n = 5) enabled a distribution study in rats and opened a path to straightforward large-scale production. A fast and clear kidney uptake could be observed, and this renal uptake could be selectively blocked by pretreatment with AT₁-selective antagonist valsartan. Overall, as the first ¹⁸F-labeled PET tracer based on a derivation from clinically used drug valsartan with almost identical chemical structure, [¹⁸F]FV45 will be a new tool for assessing the RAS function by visualizing AT₁ receptor distributions and providing further information regarding cardiovascular system malfunction as well as possible applications in inflammation research and cancer diagnosis.

The Impact of Ageing on 11C-Hydroxyephedrine Uptake in the Rat Heart

We aimed to explore the impact of ageing on 11C-hydroxyephedrine (11C-HED) uptake in the healthy rat heart in a longitudinal setting. To investigate a potential cold mass effect, the influence of specific activity on cardiac 11C-HED uptake was evaluated: 11C-HED was synthesized by N-methylation of (−)-metaraminol as the free base (radiochemical purity >95%) and a wide range of specific activities (0.2–141.9 GBq/μmol) were prepared. ¹¹C-HED (48.7 ± 9.7MBq, ranged 0.2–60.4 μg/kg cold mass) was injected in healthy Wistar Rats. Dynamic 23-frame PET images were obtained over 30 min. Time activity curves were generated for the blood input function and myocardial tissue. Cardiac 11C-HED retention index (%/min) was calculated as myocardial tissue activity at 20–30 min divided by the integral of the blood activity curves. Additionally, the impact of ageing on myocardial 11C-HED uptake was investigated longitudinally by PET studies at different ages of healthy Wistar Rats. A dose-dependent reduction of cardiac 11C-HED uptake was observed: The estimated retention index as a marker of norepinephrine function decreased at a lower specific activity (higher amount of cold mass). This observed high affinity of 11C-HED to the neural norepinephrine transporter triggered a subsequent study: In a longitudinal setting, the 11C-HED retention index decreased with increasing age. An age-related decline of cardiac sympathetic innervation could be demonstrated. The herein observed cold mass effect might increase in succeeding scans and therefore, 11C-HED microPET studies should be planned with extreme caution if one single radiosynthesis is scheduled for multiple animals.

SPECT vs. PET in cardiac innervation imaging: clash of the titans

PURPOSE

We aim to provide an overview of the conventional single photon emission computed tomography (SPECT) and emerging positron emission tomography (PET) catecholamine analogue tracers for assessing myocardial nerve integrity, in particular focusing on 18F-labeled tracers.

RESULTS

Increasingly, the cardiac sympathetic nervous system (SNS) is being studied by non-invasive molecular imaging approaches. Forming the backbone of myocardial SNS imaging, the norepinephrine (NE) transporter at the sympathetic nerve terminal plays a crucial role for visualizing denervated myocardium: in particular, the single-photon-emitting NE analogue 123I-meta-Iodobenzylguanidine (123I-mIBG) has demonstrated favorable results in the identification of patients at a high risk for cardiac death. However, cardiac neuronal PET agents offer several advantages including improved spatio-temporal resolution and intrinsic quantifiability. Compared to their 11C-labeled counterparts with a short half-life (20.4 min), novel 18F-labeled PET imaging agents to assess myocardial nerve integrity have the potential to revolutionize the field of SNS molecular imaging. The longer half-life of 18F (109.8 min) allows for more flexibility in the study design and delivery from central cyclotron facilities to smaller hospitals may lead to further cost reduction. A great deal of progress has been made by the first in-human studies of such 18F-labeled SNS imaging agents. Moreover, dedicated animal platforms open avenues for further insights into the handling of radiolabeled catecholamine analogues at the sympathetic nerve terminal.

CONCLUSIONS

18F-labeled imaging agents demonstrate key properties for mapping cardiac sympathetic nerve integrity and might outperform current SPECT-based or 11C-labeled tracers in the long run.

Impact of Novel Antidepressants on Cardiac 123I-Metaiodobenzylguanidine Uptake: Experimental Studies on SK-N-SH Cells and Healthy Rabbits

¹²³I-metaiodobenzylguanidine (¹²³I-MIBG) has independent prognostic value for risk stratification among heart failure patients, but the use of concomitant medication should not affect its quantitative information. We evaluated whether the 4 classes of antidepressants currently most prescribed as first-line treatment for major depressive disorder (MDD) have the potential to alter ¹²³I-MIBG imaging results. Methods: The inhibition effect of desipramine, escitalopram, venlafaxine, and bupropion on ¹³¹I-MIBG uptake was assessed by in vitro uptake assays using human neuroblastoma SK-N-SH cells. The half-maximal inhibitory concentration of tracer uptake was determined from dose-response curves. To evaluate the effect of intravenous pretreatment with desipramine (1.5 mg/kg) and escitalopram (2.5 or 15 mg/kg) on ¹²³I-MIBG cardiac uptake, in vivo planar ¹²³I-MIBG scanning of healthy New Zealand White rabbits was performed. Results: The half-maximal inhibitory concentrations of desipramine, escitalopram, venlafaxine, and bupropion on ¹³¹I-MIBG cellular uptake were 11.9 nM, 7.5 μM, 4.92 μM, and 12.9 μM, respectively. At the maximum serum concentration (as derived by previous clinical trials), the inhibition rates of ¹³¹I-MIBG uptake were 90.6% for desipramine, 25.5% for venlafaxine, 11.7% for bupropion, and 0.72% for escitalopram. A low inhibition rate for escitalopram in the cell uptake study triggered investigation of an in vivo rabbit model: with a dosage considerably higher than used in clinical practice, the noninhibitory effect of escitalopram was confirmed. Furthermore, pretreatment with desipramine markedly reduced cardiac ¹²³I-MIBG uptake. Conclusion: In the present in vitro binding assay and in vivo rabbit study, the selective serotonin reuptake inhibitor escitalopram had no major impact on neuronal cardiac ¹²³I-MIBG uptake within therapeutic dose ranges, whereas other types of first-line antidepressants for MDD treatment led to a significant decrease. These preliminary results warrant further confirmatory clinical trials regarding the reliability of cardiac ¹²³I-MIBG imaging, in particular, if the patient's neuropsychiatric status would not tolerate withdrawal of a potentially norepinephrine-interfering antidepressant.

Subcellular storage and release mode of the novel 18F-labeled sympathetic nerve PET tracer LMI1195

BACKGROUND

¹⁸F-N-[3-bromo-4-(3-fluoro-propoxy)-benzyl]-guanidine (¹⁸F-LMI1195) is a new class of PET tracer designed for sympathetic nervous imaging of the heart. The favorable image quality with high and specific neural uptake has been previously demonstrated in animals and humans, but intracellular behavior is not yet fully understood. The aim of the present study is to verify whether it is taken up in storage vesicles and released in company with vesicle turnover.

RESULTS

Both vesicle-rich (PC12) and vesicle-poor (SK-N-SH) norepinephrine-expressing cell lines were used for in vitro tracer uptake studies. After 2 h of ¹⁸F-LMI1195 preloading into both cell lines, effects of stimulants for storage vesicle turnover (high concentration KCl (100 mM) or reserpine treatment) were measured at 10, 20, and 30 min. ¹³¹I-meta-iodobenzylguanidine (¹³¹I-MIBG) served as a reference. Both high concentration KCl and reserpine enhanced ¹⁸F-LMI1195 washout from PC12 cells, while tracer retention remained stable in the SK-N-SH cells. After 30 min of treatment, ¹⁸F-LMI1195 releasing index (percentage of tracer released from cells) from vesicle-rich PC12 cells achieved significant differences compared to cells without treatment condition. In contrast, such effect could not be observed using vesicle-poor SK-N-SH cell lines. Similar tracer kinetics after KCl or reserpine treatment were also observed using ¹³¹I-MIBG. In case of KCl exposure, Ca²+-free buffer with the calcium chelator, ethylenediaminetetracetic acid (EDTA), could suppress the tracer washout from PC12 cells. This finding is consistent with the tracer release being mediated by Ca²+ influx resulting from membrane depolarization.

CONCLUSIONS

Analogous to ¹³¹I-MIBG, the current in vitro tracer uptake study confirmed that ¹⁸F-LMI1195 is also stored in vesicles in PC12 cells and released along with vesicle turnover. Understanding the basic kinetics of ¹⁸F-LMI1195 at a subcellular level is important for the design of clinical imaging protocols and imaging interpretation.

Impact of presynaptic sympathetic imbalance in long-QT syndrome by positron emission tomography

OBJECTIVE

We investigated the impact of cardiac presynaptic norepinephrine recycling in patients with long-QT syndrome (LQTS) using positron emission tomography (PET) with ¹¹C-meta-hydroxyephedrine ([¹¹C]mHED-PET).

METHODS

[¹¹C]mHED-PET was performed in 25 patients with LQTS (LQT1: n=14; LQT2: n=11) and 20 healthy controls and correlated with clinical parameters. [¹¹C]mHED-PET images were analysed for global and regional retention indices (RI) and washout rates (WO) reflecting dynamic parameters of the tracer activity.

RESULTS

Global and regional RI values were similar between patients with LQTS and controls. Although the global WO rates were similar between these groups, regional WO rates were on average higher in the lateral left ventricle (LV) wall in patients with LQTS (dose, mean ±SD; 0.08±0.14 vs 0.00%±0.09% min^-1; p=0.033). In addition, patients with LQTS with a longer QTc interval showed a higher global WO rate. Clinical symptoms correlated with higher global WO rates. In the presence of normal global WO rates, asymptomatic LQTS patients showed higher global RI values.

CONCLUSION

The increased regional WO rate of [¹¹C]mHED in the lateral LV suggests an imbalance of presynaptic catecholamine reuptake and release, resulting in a higher synaptic catecholamine concentration, in particular in LQT1 patients. This might enhance β-adrenoceptor signalling and thereby aggravate inherited ion channel dysfunction and may facilitate occurrence of ventricular tachyarrhythmias. Detection of regional differences in LV sympathetic nervous function may modify disease expression and potentially serve as a non-invasive risk marker in congenital LQTS.

TRIAL REGISTRATION NUMBER

2006-002767-41;Results.