Biodistribution and radiation dosimetry of LMI1195: first-in-human study of a novel 18F-labeled tracer for imaging myocardial innervation

Automated Radiolabeling and Evaluation of [18F]FPMBBG: A Novel Cardiac Neuronal PET Imaging Agent

This study reports the automated radiosynthesis and evaluation of [18F]FPMBBG, a radiopharmaceutical designed to target the norepinephrine transporter (NET). A newly developed fully protected benzylguanidine precursor, which prevents interference from non-protected benzylguanidine part during the nucleophilic process, has enabled a one-pot two-step fully automated cassette-based synthesis of [18F]FPMBBG. This advancement enhances the feasibility of the synthesis, ensures reproducibility, and allows for the production of substantial quantities of the radiotracer, paving the way for future clinical applications. [18F]FPMBBG was prepared in radiochemical yield of ~ 23% (n = 6, decay-corrected) within 70 min, with a radiochemical purity exceeding 98%, and molar activity of > 2 GBq/μmol. In studies using miniature Bama pigs, [18F]FPMBBG showed favorable distribution, providing high-contrast cardiac images at an early stage. Moreover, desipramine inhibition studies confirmed the high NET specificity of [18F]FPMBBG. The efficient automated synthesis, robust heart uptake, and minimal background signal highlight [18F]FPMBBG as a promising PET tracer for assessing cardiac sympathetic neuronal function.

Novel F-18-labeled Tracers of Sympathetic Function for Improved Risk Stratification and Clinical Outcomes

PURPOSE OF REVIEW

This review summarizes the role of the novel 18F-labeled positron emission tomography (PET) sympathetic radiotracers for risk stratification in patients with ischemic heart disease. PET tracers have demonstrated prognostic value by characterizing myocardial sympathetic nerve density and by extension the extent of myocardial sympathetic denervation. The unique features of these PET radiotracers are discussed in relation to clinical application.

RECENT FINDINGS

Absolute quantification of sympathetic denervation has been possible with 18F-labeled PET tracers which outperform low ejection fraction (<35%) in predicting sudden cardiac death (SCD) and allow for more optimal risk stratification in patients with heart failure. This underscores their utility in selecting patients for preventable strategies with implantable cardioverter defibrillators (ICD). Appropriate candidate selection for ICD placement is a major priority as at present 80% of patients that die from SCD do not receive an ICD (potentially preventable mortality) while only 1 in 8 patients with an ICD receive a life-saving shock. Furthermore, 1 in 3 patients with ICDs receive inappropriate shocks. Thus, there is a pressing need to more appropriately select and exclude patients who will and will not benefit from ICD placement, respectively, as both suffer poor outcomes. Despite the clear prognostic benefit offered by prior PET sympathetic radiotracers in imaging myocardial sympathetic denervation, their short half-lives necessitated costly onsite cyclotron synthesis obviating their pragmatic clinical use. 18F-labeled radiotracers have a longer half-life allowing centralized synthesis and transport to their point of use. As such, 18F-labeled sympathetic radiotracers define an innovation and may offer a more affordable and clinically practical approach for evaluation of risk in patients with cardiovascular disease. 18F-labeled sympathetic radiotracers are currently available for evaluation and risk stratification of patients with ischemic heart disease and heart failure. These radiotracers may offer a more practical approach for selection of ICD placement and consequent prevention of SCD; a major, yet unmet need, in heart failure patients and those that suffer SCD at large. However, further development and clinical testing of these 18F-labeled sympathetic radiotracers is required.

Molecular imaging of cardiovascular disease: Current status and future perspective

Advancements in knowledge of cardiovascular disease, pharmacology, and chemistry have led to the development of newer radiopharmaceuticals and targets for new and more suitable molecules. Molecular imaging encompasses multiple imaging techniques for identifying the characteristics of key components involved in disease. Despite its limitations in spatial resolution, the affinity for key molecules compensates for disadvantages in diagnosing diseases and elucidating their pathophysiology. This review introduce established molecular tracers involved in clinical practice and emerging tracers already applied in clinical studies, classifying the key component in A: artery, specifically those vulnerable plaque (A-I) inflammatory cells [18F-FDG]; A-II) lipid/fatty acid; A-III) hypoxia; A-IV) angiogenesis; A-V) protease [18F/68Ga-FAPI]; A-VI) thrombus/hemorrhage; A-VII) apoptosis and A-VIII) microcalcification [18F-NaF]) and B: myocardium, including myocardial ischemia, infarction and myocardiopathy (B-I) myocardial ischemia; B-II) myocardial infarction (myocardial damage and fibrosis); B-III) myocarditis and endocarditis; B-IV) sarcoidosis; B-V) amyloidosis; B-VI) metabolism; B-VII) innervation imaging). In addition to cardiovascular-specific tracers tested in animal models, many radiotracers may have been developed in other areas, such as oncology imaging or neuroimaging. While this review does not cover all available tracers, some of them hold potential for future use assessing cardiovascular disease. Advances in molecular biology, pharmaceuticals, and imaging sciences will facilitate the identification of precise disease mechanisms, enabling precise diagnoses, better assessment of disease status, and enhanced therapeutic evaluation in this multi-modality era.

Molecular Imaging of Heart Failure: An Update and Future Trends

Molecular imaging can detect and quantify pathophysiological processes underlying heart failure, complementing evaluation of cardiac structure and function with other imaging modalities. Targeted tracers have enabled assessment of various cellular and subcellular mechanisms of heart failure aiming for improved phenotyping, risk stratification, and personalized therapy. This review outlines the current status of molecular imaging in heart failure, accompanied with discussion on novel developments. The focus is on radionuclide methods with data from clinical studies. Imaging of myocardial metabolism can identify left ventricle dysfunction caused by myocardial ischemia that may be reversible after revascularization in the presence of viable myocardium. In vivo imaging of active inflammation and amyloid deposition have an established role in the detection of cardiac sarcoidosis and transthyretin amyloidosis. Innervation imaging has well documented prognostic value in predicting heart failure progression and arrhythmias. Tracers specific for inflammation, angiogenesis and myocardial fibrotic activity are in earlier stages of development, but have demonstrated potential value in early characterization of the response to myocardial injury and prediction of cardiac function over time. Early detection of disease activity is a key for transition from medical treatment of clinically overt heart failure towards a personalized approach aimed at supporting repair and preventing progressive cardiac dysfunction.

Multi-modality imaging for assessment of the microcirculation in peripheral artery disease: Bench to clinical practice

Peripheral artery disease (PAD) is a highly prevalent disorder with a high risk of mortality and amputation despite the introduction of novel medical and procedural treatments. Microvascular disease (MVD) is common among patients with PAD, and despite the established role as a predictor of amputations and mortality, MVD is not routinely assessed as part of current standard practice. Recent pre-clinical and clinical perfusion and molecular imaging studies have confirmed the important role of MVD in the pathogenesis and outcomes of PAD. The recent advancements in the imaging of the peripheral microcirculation could lead to a better understanding of the pathophysiology of PAD, and result in improved risk stratification, and our evaluation of response to therapies. In this review, we will discuss the current understanding of the anatomy and physiology of peripheral microcirculation, and the role of imaging for assessment of perfusion in PAD, and the latest advancements in molecular imaging. By highlighting the latest advancements in multi-modality imaging of the peripheral microcirculation, we aim to underscore the most promising imaging approaches and highlight potential research opportunities, with the goal of translating these approaches for improved and personalized management of PAD in the future.

Clinical Characterization of [18F]T-008, a Cholesterol 24-Hydroxylase PET Ligand: Dosimetry, Kinetic Modeling, Variability, and Soticlestat Occupancy

This series of studies characterized [18F]T-008, a PET radiotracer for imaging cholesterol 24-hydroxylase (CH24H), in healthy volunteers (ClinicalTrials.gov identifier NCT02497235). Assessments included radiation dosimetry, kinetic modeling, test-retest variability (TRT) evaluation, and a dose occupancy evaluation using soticlestat, a selective CH24H inhibitor. Soticlestat is currently in phase 3 development for the treatment of seizures in Dravet syndrome and Lennox-Gastaut syndrome. Methods: In the dosimetry study, 5 participants (3 men) underwent serial whole-body scans to estimate organ-absorbed doses and effective doses of [18F]T-008 using OLINDA/EXM 1.1. For the kinetic modeling and TRT study, 6 participants (all men) underwent two 210-min dynamic [18F]T-008 PET scans with arterial blood sampling. The regional total volume of distribution was estimated using a 1-tissue-compartment model, a 2-tissue-compartment model, and Logan graphic analysis. In the dose occupancy study, 11 participants (all men) underwent 120-min scans at baseline and 2 time points (peak and trough) after receiving single oral doses of soticlestat (50-600 mg). The relationship between effect-site soticlestat concentration and brain occupancy was evaluated with a specially developed pharmacokinetic model and a saturable maximal occupancy model. Results: The estimated mean whole-body effective dose was 0.0292 mSv/MBq (SD, 0.00147 mSv/MBq). [18F]T-008 entered the brain rapidly, with a distribution consistent with known CH24H distribution densities. The 2-tissue-compartment model and Logan graphic analysis best described the tracer kinetics. The mean TRT for estimating total volume of distribution was 7%-15%. Single doses of soticlestat in the range 50-600 mg resulted in occupancies of 64%-96% at 2 h and 11%-79% at 24 h. The estimated half-maximal effect-site concentration of soticlestat was 5.52 ng/mL. Conclusion: [18F]T-008 is a suitable PET radiotracer for quantitatively analyzing CH24H in the human brain. Using [18F]T-008 and PET, we demonstrated that soticlestat was brain-penetrant and established target engagement by displacing [18F]T-008 in a dose-dependent manner in the brain.

Synthesis and preliminary evaluation of benzylaminoimidazoline derivatives as novel norepinephrine transporter ligands

A series of benzylaminoimidazoline derivatives was synthesized and evaluated for norepinephrine transporter (NET) targeting. Among them, N-(3-iodobenzyl)-4,5-dihydro-1H-imidazol-2-amine (Compound 9) displayed the highest affinity for NET (IC50  = 5.65 ± 0.97 μM). The corresponding radiotracer [125 I]9 was further prepared by copper-mediated radioiodination and evaluated both in vitro and in vivo. The cellular uptake results suggested that [125 I]9 was specifically taken up by the NET-expressing SK-N-SH cell line. Biodistribution studies showed that [125 I]9 accumulated in the heart (5.54 ± 1.24 %ID/g at 5 min p.i. and 0.79 ± 0.08 %ID/g at 2 h p.i.) and adrenal gland (14.83 ± 3.47 %ID/g at 5 min p.i. and 3.87 ± 0.24 %ID/g at 2 h p.i.). The uptake in the heart and adrenal gland could be significantly inhibited by preinjection of desipramine (DMI). These results indicated that the benzylaminoimidazoline derivatives retained affinity for NET, which could provide structure-activity relationship data for further studies.

Pharmacological and metabolic parameters of [18F]flubrobenguane in clinical imaging populations

BACKGROUND

Cardiac sympathetic nervous system molecular imaging has demonstrated prognostic value. Compared with meta-[11C]hydroxyephedrine, [18F]flubrobenguane (FBBG) facilitates reliable estimation of SNS innervation using similar analytical methods and possesses a more convenient physical half-life. The aim of this study was to evaluate pharmacokinetic and metabolic properties of FBBG in target clinical cohorts.

METHODS

Blood sampling was performed on 20 participants concurrent to FBBG PET imaging (healthy = NORM, non-ischemic cardiomyopathy = NICM, ischemic cardiomyopathy = ICM, post-traumatic stress disorder = PTSD). Image-derived blood time-activity curves were transformed to plasma input functions using cohort-specific corrections for plasma protein binding, plasma-to-whole blood distribution, and metabolism.

RESULTS

The plasma-to-whole blood ratio was 0.78 ± 0.06 for NORM, 0.64 ± 0.06 for PTSD and 0.60 ± 0.14 for (N)ICM after 20 minutes. 22 ± 4% of FBBG was bound to plasma proteins. Metabolism of FBBG in (N)ICM was delayed, with a parent fraction of 0.71 ± 0.05 at 10 minutes post-injection compared to 0.53 ± 0.03 for PTSD/NORM. While there were variations in metabolic rate, metabolite-corrected plasma input functions were similar across all cohorts.

CONCLUSIONS

Rapid plasma clearance of FBBG limits the impact of disease-specific corrections of the blood input function for tracer kinetic modeling.

Phenotyping heart failure by nuclear imaging of myocardial perfusion, metabolism, and molecular targets

Nuclear imaging techniques can detect and quantify pathophysiological processes underlying heart failure, complementing evaluation of cardiac structure and function with other imaging modalities. Combined imaging of myocardial perfusion and metabolism can identify left ventricle dysfunction caused by myocardial ischaemia that may be reversible after revascularization in the presence of viable myocardium. High sensitivity of nuclear imaging to detect targeted tracers has enabled assessment of various cellular and subcellular mechanisms of heart failure. Nuclear imaging of active inflammation and amyloid deposition is incorporated into clinical management algorithms of cardiac sarcoidosis and amyloidosis. Innervation imaging has well-documented prognostic value with respect to heart failure progression and arrhythmias. Emerging tracers specific for inflammation and myocardial fibrotic activity are in earlier stages of development but have demonstrated potential value in early characterization of the response to myocardial injury and prediction of adverse left ventricular remodelling. Early detection of disease activity is a key for transition from broad medical treatment of clinically overt heart failure towards a personalized approach aimed at supporting repair and preventing progressive failure. This review outlines the current status of nuclear imaging in phenotyping heart failure and combines it with discussion on novel developments.

Cardiac innervations in diabetes mellitus-Anatomical evidence of neuropathy

The extensive innervations of the heart include a complex network of sympathetic, parasympathetic, and sensory nerves connected in loops that serve to regulate cardiac output. Metabolic dysfunction in diabetes affects many different organ systems, including the cardiovascular system; it causes cardiac arrhythmias, silent myocardial ischemia, and sudden cardiac death, among others. These conditions are associated with damage to the nerves that innervate the heart, cardiac autonomic neuropathy (CAN), which is caused by various pathophysiological mechanisms. In this review, the main facts about the anatomy of cardiac innervations and the current knowledge of CAN, its pathophysiological mechanisms, and its diagnostic approach are discussed. In addition, anatomical evidence for CAN from human and animal studies has been summarized.

Emerging PET Tracers in Cardiac Molecular Imaging

¹⁸F-fluorodeoxyglucose (FDG) and ¹⁸F-sodium fluoride (NaF) represent emerging PET tracers used to assess atherosclerosis-related inflammation and molecular calcification, respectively. By localizing to sites with high glucose utilization, FDG has been used to assess myocardial viability for decades, and its role in evaluating cardiac sarcoidosis has come to represent a major application. In addition to determining late-stage changes such as loss of perfusion or viability, by targeting mechanisms present in atherosclerosis, PET-based techniques have the ability to characterize atherogenesis in the early stages to guide intervention. Although it was once thought that FDG would be a reliable indicator of ongoing plaque formation, micro-calcification as portrayed by NaF-PET/CT appears to be a superior method of monitoring disease progression. PET imaging with NaF has the additional advantage of being able to determine abnormal uptake due to coronary artery disease, which is obscured by physiologic myocardial activity on FDG-PET/CT. In this review, we discuss the evolving roles of FDG, NaF, and other PET tracers in cardiac molecular imaging.

Nerve regeneration in transplanted organs and tracer imaging studies: A review

The technique of organ transplantation is well established and after transplantation the patient might be faced with the problem of nerve regeneration of the transplanted organ. Transplanted organs are innervated by the sympathetic, parasympathetic, and visceral sensory plexuses, but there is a lack of clarity regarding the neural influences on the heart, liver and kidneys and the mechanisms of their innervation. Although there has been considerable recent work exploring the potential mechanisms of nerve regeneration in organ transplantation, there remains much that is unknown about the heterogeneity and individual variability in the reinnervation of organ transplantation. The widespread availability of radioactive nerve tracers has also made a significant contribution to organ transplantation and has helped to investigate nerve recovery after transplantation, as well as providing a direction for future organ transplantation research. In this review we focused on neural tracer imaging techniques in humans and provide some conceptual insights into theories that can effectively support our choice of radionuclide tracers. This also facilitates the development of nuclear medicine techniques and promotes the development of modern medical technologies and computer tools. We described the knowledge of neural regeneration after heart transplantation, liver transplantation and kidney transplantation and apply them to various imaging techniques to quantify the uptake of radionuclide tracers to assess the prognosis of organ transplantation. We noted that the aim of this review is both to provide clinicians and nuclear medicine researchers with theories and insights into nerve regeneration in organ transplantation and to advance imaging techniques and radiotracers as a major step forward in clinical research. Moreover, we aimed to further promote the clinical and research applications of imaging techniques and provide clinicians and research technology developers with the theory and knowledge of the nerve.

Prediction of appropriate ICD therapy in patients with ischemic heart failure

BACKGROUND

Previous studies show inconsistent results on the role of innervation imaging (with I-123-mIBG) and perfusion imaging in predicting appropriate ICD therapy (aICDth). These studies included patients with both dilated and ischemic cardiomyopathy. This study compared the ability of 123I-mIBG imaging along with perfusion imaging (using thallium-199) to predict aICDth in patients with ischemic heart failure (IHF) in relation to indication for ICD implantation (primary vs. secondary prevention of sudden cardiac death (SCD)).

METHODS

mIBG/thallium SPECT imaging were performed before ICD implantation in 80 patients with IHF: 49 candidates for primary and 31 for secondary SCD prevention.

RESULTS

During a mean follow-up of 18 months, the imaging results could not predict patients with appropriate ICD therapy among patients with ICD implants for primary SCD prevention. While in the secondary SCD prevention group, those who received a ICDth had significantly larger summed scores of regional perfusion and innervation impairment, but not higher heart-to-mediastinal mIBG ratio. The best results to predict aICDth were using mIBG summed score (cut-off point > 34%, sensitivity 72%, specificity 100%, AUC 0.909, P < 0.0001).

CONCLUSION

The prognostic value of innervation and perfusion imaging in patients with IHF differ based on indication for ICD implantation (primary vs. secondary prevention of SCD).

Prototype device for endoventricular beta-emitting radiotracer detection and molecularly-guided intervention

BACKGROUND

We have set out to develop a catheter-based theranostic system that: (a) identifies diseased and at-risk myocardium via endocardial detection of systemically delivered β-emitting radiotracers and (b) utilizes molecular signals to guide delivery of therapeutics to appropriate tissue via direct intramyocardial injection.

METHODS

Our prototype device consists of a miniature β-radiation detector contained within the tip of a flexible intravascular catheter. The catheter can be adapted to incorporate an injection port and retractable needle for therapeutic delivery. The performance of the β-detection catheter was assessed in vitro with various β-emitting radionuclides and ex vivo in hearts of pigs following systemic injection of 18F-fluorodeoxyglucose (18F-FDG) at 1-week post-myocardial infarction. Regional catheter-based endocardial measurements of 18F activity were compared to regional tissue activity from PET/CT images and gamma counting.

RESULTS

The β-detection catheter demonstrated sensitive in vitro detection of β-radiation from 22Na (β+), 18F (β+), and 204Tl (β-), with minimal sensitivity to γ-radiation. For 18F, the catheter demonstrated a sensitivity of 4067 counts/s/μCi in contact and a spatial resolution of 1.1 mm FWHM. Ex vivo measurements of endocardial 18F activity with the β-detection catheter in the chronic pig infarct model demonstrated good qualitative and quantitative correlation with regional tissue activity from PET/CT images and gamma counting.

CONCLUSION

The prototype β-detection catheter demonstrates sensitive and selective detection of β- and β+ emissions over a wide range of energies and enables high-fidelity ex vivo characterization of endocardial activity from systemically delivered 18F-FDG.

PET Tracers for Imaging Cardiac Function in Cardio-oncology

PURPOSE OF REVIEW

Successful treatment of cancer can be hampered by the attendant risk of cardiotoxicity, manifesting as cardiomyopathy, left ventricle systolic dysfunction and, in some cases, heart failure. This risk can be mitigated if the injury to the heart is detected before the onset to irreversible cardiac impairment. The gold standard for cardiac imaging in cardio-oncology is echocardiography. Despite improvements in the application of this modality, it is not typically sensitive to sub-clinical or early-stage dysfunction. We identify in this review some emerging tracers for detecting incipient cardiotoxicity by positron emission tomography (PET).

RECENT FINDINGS

Vectors labeled with positron-emitting radionuclides (e.g., carbon-11, fluorine-18, gallium-68) are now available to study cardiac function, metabolism, and tissue repair in preclinical models. Many of these probes are highly sensitive to early damage, thereby potentially addressing the limitations of current imaging approaches, and show promise in preliminary clinical evaluations. The overlapping pathophysiology between cardiotoxicity and heart failure significantly expands the number of imaging tools available to cardio-oncology. This is highlighted by the emergence of radiolabeled probes targeting fibroblast activation protein (FAP) for sensitive detection of dysregulated healing process that underpins adverse cardiac remodeling. The growth of PET scanner technology also creates an opportunity for a renaissance in metabolic imaging in cardio-oncology research.

Machine learning-based risk model using 123I-metaiodobenzylguanidine to differentially predict modes of cardiac death in heart failure

BACKGROUND

Cardiac sympathetic dysfunction is closely associated with cardiac mortality in patients with chronic heart failure (CHF). We analyzed the ability of machine learning incorporating 123I-metaiodobenzylguanidine (MIBG) to differentially predict risk of life-threatening arrhythmic events (ArE) and heart failure death (HFD).

METHODS AND RESULTS

A model was created based on patients with documented 2-year outcomes of CHF (n = 526; age, 66 ± 14 years). Classifiers were trained using 13 variables including age, gender, NYHA functional class, left ventricular ejection fraction and planar 123I-MIBG heart-to-mediastinum ratio (HMR). ArE comprised arrhythmic death and appropriate therapy with an implantable cardioverter defibrillator. The probability of ArE and HFD at 2 years was separately calculated based on appropriate classifiers. The probability of HFD significantly increased as HMR decreased when any variables were combined. However, the probability of arrhythmic events was maximal when HMR was intermediate (1.5-2.0 for patients with NYHA class III). Actual rates of ArE were 3% (10/379) and 18% (27/147) in patients at low- (≤ 11%) and high- (> 11%) risk of developing ArE (P < .0001), respectively, whereas those of HFD were 2% (6/328) and 49% (98/198) in patients at low-(≤ 15%) and high-(> 15%) risk of HFD (P < .0001).

CONCLUSION

A risk model based on machine learning using clinical variables and 123I-MIBG differentially predicted ArE and HFD as causes of cardiac death.

Dynamic planar scintigraphy for the rapid kinetic measurement of myocardial 123I-MIBG turnover can identify Lewy body disease

BACKGROUND

Using two static scans for ¹²³I-meta-iodobenzyl-guanidine (¹²³I-MIBG) myocardial scintigraphy ignores the dynamic response from vesicular trapping in nerve terminals. Moreover, the long pause between scans is impractical for patients with Lewy body diseases (LBDs). Here, we optimized indices that capture norepinephrine kinetics, tested their diagnostic performance, and determined the differences in ¹²³I-MIBG performance among disease groups.

METHODS

We developed a new 30-min protocol for ¹²³I-MIBG dynamic planar imaging for suspected LBD patients. Pharmacokinetic modelling of time-activity curves (TACs) was used to calculate three new indices: unidirectional uptake of ¹²³I-MIBG to vesicular trapping (iUp), rate of myocardial ¹²³I-MIBG loss (iLoss), and non-specific fractional distribution of ¹²³I-MIBG in the interstitial space. We compared the performance of the new and existing indices with regard to discrimination of patients with or without LBDs. Subgroup analysis was performed to examine differences in ¹²³I-MIBG turnover between patients in a dementia with Lewy bodies (DLB) group and two Parkinson's disease (PD) groups, one with and the other without REM sleep behaviour disorder (RBD).

RESULTS

iLoss was highly discriminative, particularly for patients with low myocardial ¹²³I-MIBG trapping, and the new indices outperformed existing ones. ROC analysis revealed that the AUC of iLoss (0.903) was significantly higher than that of early HMR (0.863), while comparable to that of delayed HMR (0.892). The RBD-positive PD group and the DLB group had higher turnover rates than the RBD-negative PD group, indicating a potential association between prognosis and iLoss.

CONCLUSION

¹²³I-MIBG turnover can be quantified in 30 min using a three-parameter model based on ¹²³I-MIBG TACs. The discriminatory performance of the new model-based indices might help explain the neurotoxicity or neurodegeneration that occurs in LBD patients.

Imaging pheochromocytoma in small animals: preclinical models to improve diagnosis and treatment

Pheochromocytomas (PCCs) and paragangliomas (PGLs), together referred to as PPGLs, are rare chromaffin cell-derived tumors. They require timely diagnosis as this is the only way to achieve a cure through surgery and because of the potentially serious cardiovascular complications and sometimes life-threatening comorbidities that can occur if left untreated. The biochemical diagnosis of PPGLs has improved over the last decades, and the knowledge of the underlying genetics has dramatically increased. In addition to conventional anatomical imaging by CT and MRI for PPGL detection, new functional imaging modalities have emerged as very useful for patient surveillance and stratification for therapy. The availability of validated and predictive animal models of cancer is essential for translating molecular, imaging and therapy response findings from the bench to the bedside. This is especially true for rare tumors, such as PPGLs, for which access to large cohorts of patients is limited. There are few animal models of PPGLs that have been instrumental in refining imaging modalities for early tumor detection, as well as in identifying and evaluating novel imaging tracers holding promise for the detection and/or treatment of human PPGLs. The in vivo PPGL models mainly include xenografts/allografts generated by engrafting rat or mouse cell lines, as no representative human cell line is available. In addition, there is a model of endogenous PCCs (i.e., MENX rats) that was characterized in our laboratory. In this review, we will summarize the contribution that various representative models of PPGL have given to the visualization of these tumors in vivo and we present an example of a tracer first evaluated in MENX rats, and then translated to the detection of these tumors in human patients. In addition, we will illustrate briefly the potential of ex vivo biological imaging of intact adrenal glands in MENX rats.

Cardiac Sympathetic Positron Emission Tomography Imaging with Meta-[18F]Fluorobenzylguanidine is Sensitive to Uptake-1 in Rats

Dysfunction of the cardiac sympathetic nervous system contributes to the development of cardiovascular diseases including ischemia, heart failure, and arrhythmias. Molecular imaging probes such as meta-[¹²³I]iodobenzylguanidine have demonstrated the utility of assessing neuronal integrity by targeting norepinephrine transporter (NET, uptake-1). However, current radiotracers can report only on innervation due to suboptimal kinetics and lack sensitivity to NET in rodents, precluding mechanistic studies in these species. The objective of this work was to characterize myocardial sympathetic neuronal uptake mechanisms and kinetics of the positron emission tomography (PET) radiotracer meta-[¹⁸F]fluorobenzylguanidine ([¹⁸F]mFBG) in rats. Automated synthesis using spirocyclic iodonium(III) ylide radiofluorination produces [¹⁸F]mFBG in 24 ± 1% isolated radiochemical yield and 30-95 GBq/μmol molar activity. PET imaging in healthy rats delineated the left ventricle, with monoexponential washout kinetics (k_mono = 0.027 ± 0.0026 min^-1, A_mono = 3.08 ± 0.33 SUV). Ex vivo biodistribution studies revealed tracer retention in the myocardium, while pharmacological treatment with selective NET inhibitor desipramine, nonselective neuronal and extraneuronal uptake-2 inhibitor phenoxybenzamine, and neuronal ablation with neurotoxin 6-hydroxydopamine reduced myocardial retention by 33, 76, and 36%, respectively. Clearance of [¹⁸F]mFBG from the myocardium was unaffected by treatment with uptake-1 and uptake-2 inhibitors following peak myocardial activity. These results suggest that myocardial distribution of [¹⁸F]mFBG in rats is dependent on both NET and extraneuronal transporters and that limited reuptake to the myocardium occurs. [¹⁸F]mFBG may therefore prove useful for imaging intraneuronal dysfunction in small animals.

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.

40 Years Anniversary of Cardiac 123I-mIBG Imaging: State of the Heart

Purpose of Review

This narrative review reflects on the body of evidence on cardiac 123I-mIBG imaging that has accumulated since the introduction in the late 1970s and focusses on to what extent cardiac 123I-mIBG imaging has fulfilled its potential in cardiology especially.

Recent Findings

In contrast to the linear relationship between 123I-mIBG-derived parameters and overall prognosis in heart failure, there seems a “bell-shape” curve for 123I-mIBG-derived parameters and arrhythmic events. In addition, there is a potential clinical role for cardiac 123I-mIBG in optimizing patient selection for expensive devices (i.e., ICD and CRT). This needs of course to be established in future trials.

Summary

Cardiac 123I-mIBG imaging is, despite the numerous of studies, sometimes mistakenly seen as a nice to have technique rather than a must have imaging modality. Although cardiac 123I-mIBG imaging has grown and matured over the years, its full clinical potential has still not been tested to the maximum.

The role of myocardial innervation imaging in different clinical scenarios: an expert document of the European Association of Cardiovascular Imaging and Cardiovascular Committee of the European Association of Nuclear Medicine

Cardiac sympathetic activity plays a key role in supporting cardiac function in both health and disease conditions, and nuclear cardiac imaging has always represented the only way for the non-invasive evaluation of the functional integrity of cardiac sympathetic terminals, mainly through the use of radiopharmaceuticals that are analogues of norepinephrine and, in particular, with the use of 123I-mIBG imaging. This technique demonstrates the presence of cardiac sympathetic dysfunction in different cardiac pathologies, linking the severity of sympathetic nervous system impairment to adverse patient's prognosis. This article will outline the state-of-the-art of cardiac 123I-mIBG imaging and define the value and clinical applications in the different fields of cardiovascular diseases.

Molecular Imaging Using Cardiac PET/CT: Opportunities to Harmonize Diagnosis and Therapy

PURPOSE OF REVIEW

Current therapeutic strategies to mitigate heart failure progression after myocardial infarction involve support of endogenous repair through molecular targets. The capacity for repair varies greatly between individuals. In this review, we will assess how cardiac PET/CT enables precise characterization of early pathogenetic processes which govern ventricle remodeling and progression to heart failure.

RECENT FINDINGS

Inflammation in the first days after myocardial infarction predicts subsequent functional decline and can influence therapy decisions. The expansion of anti-inflammatory approaches to improve outcomes after myocardial infarction may benefit from noninvasive characterization using imaging. Novel probes also allow visualization of fibroblast transdifferentiation and activation, as a precursor to ventricle remodeling. The expanding arsenal of molecular imaging agents in parallel with new treatment options provides opportunity to harmonize diagnostic imaging with precision therapy.

Procedural recommendations of cardiac PET/CT imaging: standardization in inflammatory-, infective-, infiltrative-, and innervation- (4Is) related cardiovascular diseases: a joint collaboration of the EACVI and the EANM: summary

With this summarized document we share the standard for positron emission tomography (PET)/(diagnostic)computed tomography (CT) imaging procedures in cardiovascular diseases that are inflammatory, infective, infiltrative, or associated with dysfunctional innervation (4Is) as recently published in the European Journal of Nuclear Medicine and Molecular Imaging. This standard should be applied in clinical practice and integrated in clinical (multicentre) trials for optimal standardization of the procedurals and interpretations. A major focus is put on procedures using [¹⁸F]-2-fluoro-2-deoxyglucose ([¹⁸F]FDG), but 4Is PET radiopharmaceuticals beyond [¹⁸F]FDG are also described in this summarized document. Whilst these novel tracers are currently mainly applied in early clinical trials, some multicentre trials are underway and we foresee in the near future their use in clinical care and inclusion in the clinical guidelines. Diagnosis and management of 4Is related cardiovascular diseases are generally complex and often require a multidisciplinary approach by a team of experts. The new standards described herein should be applied when using PET/CT and PET/magnetic resonance, within a multimodality imaging framework both in clinical practice and in clinical trials for 4Is cardiovascular indications.

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.

Carbon-11: Radiochemistry and Target-Based PET Molecular Imaging Applications in Oncology, Cardiology, and Neurology

The positron emission tomography (PET) molecular imaging technique has gained its universal value as a remarkable tool for medical diagnosis and biomedical research. Carbon-11 is one of the promising radiotracers that can report target-specific information related to its pharmacology and physiology to understand the disease status. Currently, many of the available carbon-11 (t_1/2 = 20.4 min) PET radiotracers are heterocyclic derivatives that have been synthesized using carbon-11 inserted different functional groups obtained from primary and secondary carbon-11 precursors. A spectrum of carbon-11 PET radiotracers has been developed against many of the upregulated and emerging targets for the diagnosis, prognosis, prediction, and therapy in the fields of oncology, cardiology, and neurology. This review focuses on the carbon-11 radiochemistry and various target-specific PET molecular imaging agents used in tumor, heart, brain, and neuroinflammatory disease imaging along with its associated pathology.

Molecular imaging in nuclear cardiology: Pathways to individual precision medicine

Growth of molecular imaging bears potential to transform nuclear cardiology from a primarily diagnostic method to a precision medicine tool. Molecular targets amenable for imaging and therapeutic intervention are particularly promising to facilitate risk stratification, patient selection and exquisite guidance of novel therapies, and interrogation of systems-based interorgan communication. Non-invasive visualization of pathobiology provides valuable insights into the progression of disease and response to treatment. Specifically, inflammation, fibrosis, and neurohormonal signaling, central to the progression of cardiovascular disease and emerging therapeutic strategies, have been investigated by molecular imaging. As the number of radioligands grows, careful investigation of the binding properties and added-value of imaging should be prioritized to identify high-potential probes and facilitate translation to clinical applications. In this review, we discuss the current state of molecular imaging in cardiovascular medicine, and the challenges and opportunities ahead for cardiovascular molecular imaging to navigate the path from diagnosis to prognosis to personalized medicine.

Regional Distribution of Fluorine-18-Flubrobenguane and Carbon-11-Hydroxyephedrine for Cardiac PET Imaging of Sympathetic Innervation

OBJECTIVES

The aim of this study was to investigate the regional distribution of novel ¹⁸F-labeled positron emission tomographic (PET) tracer flubrobenguane (FBBG) (whose longer half-life could enable more widespread use) to assess myocardial presynaptic sympathetic nerve function in humans in comparison to [¹¹C]meta-hydroxyephedrine (HED).

BACKGROUND

The sympathetic nervous system (SNS) is vitally linked to cardiovascular regulation and disease. SNS imaging has shown prognostic value. HED is the most commonly used PET tracer for evaluation of sympathetic function in humans, but widespread clinical use is limited because of the short half-life of ¹¹C.

METHODS

A total of 25 participants (n = 6 healthy; n = 14 ischemic cardiomyopathy, left ventricular [LV] ejection fraction [EF] = 34 ± 5%; and n = 5 nonischemic cardiomyopathy, EF = 33 ± 3%) underwent 2 separate PET imaging visits 8.7 ± 7.6 days apart. On 1 visit, participants underwent dynamic HED PET imaging. On a different visit, participants underwent dynamic FBBG PET imaging. The order of testing was random. HED and FBBG global innervation (retention index [RI] and distribution volume [DV]) and regional denervation (% nonuniformity) were quantified to assess regional presynaptic sympathetic innervations.

RESULTS

FBBG RI (r² = 0.72; ICC = 0.79; p < 0.0001), DV (r² = 0.62; ICC = 0.78; p < 0.0001), and regional denervation (r² = 0.97; ICC = 0.98; p < 0.0001) correlated highly with HED. Average LV RI values were highly similar between HED (7.3 ± 2.4%/min) and FBBG (7.0 ± 1.7%/min; p = 0.33). Post-hoc analysis did not reveal any between-tracer differences on a regional level (17-segment), suggesting equivalent regional distributions in both patients with and without ischemic cardiomyopathy.

CONCLUSIONS

FBBG and HED yield equivalent global and regional distributions in both patients with and without ischemic cardiomyopathy. ¹⁸F-labeled PET tracers, such as FBBG, are critical for widespread distribution necessary for multicenter clinical trials and to maximize patient impact.

Procedural recommendations of cardiac PET/CT imaging: standardization in inflammatory-, infective-, infiltrative-, and innervation (4Is)-related cardiovascular diseases: a joint collaboration of the EACVI and the EANM

With this document, we provide a standard for PET/(diagnostic) CT imaging procedures in cardiovascular diseases that are inflammatory, infective, infiltrative, or associated with dysfunctional innervation (4Is). This standard should be applied in clinical practice and integrated in clinical (multicenter) trials for optimal procedural standardization. A major focus is put on procedures using [¹⁸F]FDG, but 4Is PET radiopharmaceuticals beyond [¹⁸F]FDG are also described in this document. Whilst these novel tracers are currently mainly applied in early clinical trials, some multicenter trials are underway and we foresee in the near future their use in clinical care and inclusion in the clinical guidelines. Finally, PET/MR applications in 4Is cardiovascular diseases are also briefly described. Diagnosis and management of 4Is-related cardiovascular diseases are generally complex and often require a multidisciplinary approach by a team of experts. The new standards described herein should be applied when using PET/CT and PET/MR, within a multimodality imaging framework both in clinical practice and in clinical trials for 4Is cardiovascular indications.

First Experience Using 18F-Flubrobenguane PET Imaging in Patients with Suspected Pheochromocytoma or Paraganglioma

Pheochromocytomas and paragangliomas are a rare tumor entity originating from adrenomedullary chromaffin cells in the adrenal medulla or in sympathetic, paravertebral ganglia outside the medulla. Small lesions are especially difficult to detect by conventional CT or MRI and even by SPECT with the currently available radiotracers (e.g., metaiodobenzylguanidine [MIBG]). The novel PET radiotracer ¹⁸F-flubrobenguane could change the diagnostic paradigm in suspected pheochromocytomas and paragangliomas because of its homology with MIBG and the general advantages of PET imaging. The aim of this retrospective analysis was to evaluate ¹⁸F-flubrobenguane in pheochromocytomas and paragangliomas and to investigate the biodistribution in patients. Methods: Twenty-three patients with suspected pheochromocytoma or paraganglioma underwent PET/CT or PET/MRI at 63 ± 24 min after injection of 256 ± 33 MBq of ¹⁸F-flubrobenguane. The SUV_mean and SUV_max of organs were measured with spheric volumes of interest. Threshold-segmented volumes of interest were used to measure the SUV_mean or SUV_max of the tumor lesions. One reader evaluated all cross-sectional imaging datasets (CT or MRI) separately, as well as the PET hybrid datasets, and reported the lesion number and size. The diagnostic certainty for a positive lesion was scored on a 3-point scale. Results: ¹⁸F-flubrobenguane showed a reproducible, stable biodistribution, with the highest SUV_max and SUV_mean being in the thyroid gland (30.3 ± 2.2 and 22.5 ± 1.6, respectively), pancreas (12.2 ± 0.8 and 9.5 ± 0.7, respectively), and tumor lesions (16.8 ± 1.7 and 10.1 ± 1.1, respectively) and the lowest SUV_max and SUV_mean being in muscle (1.1 ± 0.06 and 0.7 ± 0.04, respectively) and the lung (2.5 ± 0.17 and 1.85 ± 0.13, respectively). In a subgroup analysis, a significantly higher average SUV_mean was seen for both pheochromocytoma and paraganglioma than for healthy adrenal glands (11.9 ± 2.0 vs. 9.9 ± 1.5 vs. 3.7 ± 0.2, respectively). In total, 47 lesions were detected. The reader reported more and smaller lesions with higher certainty in PET hybrid imaging than in conventional imaging; however, statistical significance was not reached. Of the 23 (23/47, 49%) lesions smaller than 1 cm, 61% (14/23) were found on hybrid imaging only. Conclusion: Our preliminary data suggest ¹⁸F-flubrobenguane PET to be a new, effective staging tool for patients with suspected pheochromocytoma or paraganglioma. Major advantages are the fast acquisition and high spatial resolution of PET imaging and the intense uptake in tumor lesions, facilitating detection. Further studies are warranted to define the role of ¹⁸F-flubrobenguane PET, particularly in comparison to standard diagnostic procedures such as MRI or ¹²³I-MIBG SPECT/CT.

Molecular imaging of norepinephrine transporter-expressing tumors: current status and future prospects

The human norepinephrine transporter (hNET) is a transmembrane protein responsible for reuptake of norepinephrine in presynaptic sympathetic nerve terminals and adrenal chromaffin cells. Neural crest tumors, such as neuroblastoma, paraganglioma and pheochromocytoma often show high hNET expression. Molecular imaging of these tumors can be done using radiolabeled norepinephrine analogs that target hNET. Currently, the most commonly used radiopharmaceutical for hNET imaging is meta-[123I]iodobenzylguanidine ([123I]MIBG) and this has been the case since its development several decades ago. The γ-emitter, iodine-123 only allows for planar scintigraphy and single photon emission computed tomography imaging. These modalities typically have a poorer spatial resolution and lower sensitivity than positron emission tomography (PET). Additional practical disadvantages include the fact that a two-day imaging protocol is required and the need for thyroid blockade. Therefore, several PET alternatives for hNET imaging are actively being explored. This review gives an in-depth overview of the current status and recent developments in clinical trials leading to the next generation of clinical PET ligands for imaging of hNET-expressing tumors.

Recent advances in radiotracers targeting norepinephrine transporter: structural development and radiolabeling improvements

The norepinephrine transporter (NET) is a major target for the evaluation of the cardiac sympathetic nerve system in patients with heart failure and Parkinson's disease. It is also used in the therapeutic applications against certain types of neuroendocrine tumors, as exemplified by the clinically used ^123/131I-MIBG as theranostic single-photon emission computed tomography (SPECT) agent. With the development of more advanced positron emission tomography (PET) technology, more radiotracers targeting NET have been reported, with superior temporal and spatial resolutions, along with the possibility of functional and kinetic analysis. More recently, fluorine-18-labelled NET tracers have drawn increasing attentions from researchers, due to their longer radiological half-life relative to carbon-11 (110 min vs. 20 min), reduced dependence on on-site cyclotrons, and flexibility in the design of novel tracer structures. In the heart, certain NET tracers provide integral diagnostic information on sympathetic innervation and the nerve status. In the central nervous system, such radiotracers can reveal NET distribution and density in pathological conditions. Most radiotracers targeting cardiac NET-function for the cardiac application consistent of derivatives of either norepinephrine or MIBG with its benzylguanidine core structure, e.g. ¹¹C-HED and ¹⁸F-LMI1195. In contrast, all NET tracers used in central nervous system applications are derived from clinically used antidepressants. Lastly, possible applications of NET as selective tracers over organic cation transporters (OCTs) in the kidneys and other organs controlled by sympathetic nervous system will also be discussed.

Cardiac Imaging With 123I-meta-iodobenzylguanidine and Analogous PET Tracers: Current Status and Future Perspectives

Autonomic innervation plays an important role in proper functioning of the cardiovascular system. Altered cardiac sympathetic function is present in a variety of diseases, and can be assessed with radionuclide imaging using sympathetic neurotransmitter analogues. The most studied adrenergic radiotracer is cardiac ¹²³I-meta-iodobenzylguanidine (¹²³I-mIBG). Cardiac ¹²³I-mIBG uptake can be evaluated using both planar and tomographic imaging, thereby providing insight into global and regional sympathetic innervation. Standardly assessed imaging parameters are the heart-to-mediastinum ratio and washout rate, customarily derived from planar images. Focal tracer deficits on tomographic imaging also show prognostic utility, with some data suggesting that the best approach to tomographic image interpretation may differ from conventional methods. Cardiac ¹²³I-mIBG image findings strongly correlate with the severity and prognosis of many cardiovascular diseases, especially heart failure and ventricular arrhythmias. Cardiac ¹²³I-mIBG imaging in heart failure is FDA approved for prognostic purposes. With the robustly demonstrated ability to predict occurrence of potentially fatal arrhythmias, cardiac ¹²³I-mIBG imaging shows promise for better selecting patients who will benefit from an implantable cardioverter defibrillator, but clinical use has been hampered by lack of the randomized trial needed for incorporation into societal guidelines. In patients with ischemic heart disease, cardiac ¹²³I-mIBG imaging aids in assessing the extent of damage and in identifying arrhythmogenic regions. There have also been studies using cardiac ¹²³I-mIBG for other conditions, including patients following heart transplantation, diabetic related cardiac abnormalities and chemotherapy induced cardiotoxicity. Positron emission tomographic adrenergic radiotracers, that improve image quality, have been investigated, especially ¹¹C-meta-hydroxyephedrine, and most recently ¹⁸F-fluorbenguan. Cadmium-zinc-telluride cameras also improve image quality. With better spatial resolution and quantification, PET tracers and advanced camera technologies promise to expand the clinical utility of cardiac sympathetic imaging.

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.

First-in-Human Studies of [18F] Fluorohydroxyphenethylguanidines

BACKGROUND

Disease-induced damage to cardiac autonomic nerve populations is associated with an increased risk of sudden cardiac death. The extent of cardiac sympathetic denervation, assessed using planar scintigraphy or positron emission tomography, has been shown to predict the risk of arrhythmic events in heart failure patients staged for implantable cardioverter defibrillator therapy. The goal of this study was to perform first-in-human evaluations of 4-[¹⁸F]fluoro-meta-hydroxyphenethylguanidine and 3-[¹⁸F]fluoro-para-hydroxyphenethylguanidine, 2 new positron emission tomography radiotracers developed for quantifying regional cardiac sympathetic nerve density.

METHODS AND RESULTS

Cardiac positron emission tomography studies with 4-[¹⁸F]fluoro-meta-hydroxyphenethylguanidine and 3-[¹⁸F]fluoro-para-hydroxyphenethylguanidine were performed in normal subjects (n=4 each) to assess their imaging properties and organ kinetics. Patlak graphical analysis of their myocardial kinetics was evaluated as a technique for generating nerve density metrics. Whole-body biodistribution studies (n=4 each) were acquired and used to calculate human radiation dosimetry estimates. Patlak analysis proved to be an effective approach for quantifying regional nerve density. Using 960 left ventricular volumes of interest, across-subject Patlak slopes averaged 0.107±0.010 mL/min per gram for 4-[¹⁸F]fluoro-meta-hydroxyphenethylguanidine and 0.116±0.010 mL/min per gram for 3-[¹⁸F]fluoro-para-hydroxyphenethylguanidine. Tracer uptake was highest in heart, liver, kidneys, and salivary glands. Urinary excretion was the main elimination pathway.

CONCLUSIONS

4-[¹⁸F]fluoro-meta-hydroxyphenethylguanidine and 3-[¹⁸F]fluoro-para-hydroxyphenethylguanidine each produce high-quality positron emission tomography images of the distribution of sympathetic nerves in human heart. Patlak analysis provides reproducible measurements of regional cardiac sympathetic nerve density at high spatial resolution. Further studies of these tracers in heart failure patients will be performed to identify the best agent for clinical development.

CLINICAL TRIAL REGISTRATION

URL: https://www.clinicaltrials.gov . Unique identifier: NCT02385877.

How do we establish cardiac sympathetic nervous system imaging with 123I-mIBG in clinical practice? Perspectives and lessons from Japan and the US

Cardiac denervation is associated with progressive left ventricular (LV) dysfunction, ventricular arrhythmias, and sudden cardiac death (SCD) in heart failure (HF). In this regard, it is important to evaluate cardiac-specific sympathetic nervous system (SNS) function. The radiotracer Iodine-123 meta-iodobenzylguanidine (123I-mIBG) can noninvasively evaluate pre-synaptic SNS function. Recent multicenter trials have shown 123I-mIBG to have strong predictive value for fatal arrhythmias and cardiac death in HF. 123I-mIBG was initially developed in the USA in the 1970s. In 1992, the Japanese Ministry of Health and Labour approved 123I-mIBG for the assessment of cardiac function. Following approval, the Japanese nuclear cardiology community developed 123I-mIBG imaging services in various medical centers. Japanese groups have been trying to establish the clinical utility of 123I-mIBG and standardize parameters for data acquisition and image analysis. The US Food and Drug Administration (FDA) has approved clinical use of 123I-mIBG for cardiac and non-cardiac imaging. However, clinical use of 123I-mIBG in the US has been very limited. The number of 123I-mIBG studies in Japan has also been limited. There are similarities and differences between the two countries. To establish the clinical utility of 123I-mIBG in both countries, it is important to characterize the situations of 123I-mIBG in each.

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.

Molecular imaging of diabetes and diabetic complications: Beyond pancreatic β-cell targeting

Diabetes is a chronic non-communicable disease affecting over 400 million people worldwide. Diabetic patients are at a high risk of various complications, such as cardiovascular, renal, and other diseases. The pathogenesis of diabetes (both type 1 and type 2 diabetes) is associated with a functional impairment of pancreatic β-cells. Consequently, most efforts to manage and prevent diabetes have focused on preserving β-cells and their function. Advances in imaging techniques, such as magnetic resonance imaging, magnetic resonance spectroscopy, positron emission tomography, and single-photon-emission computed tomography, have enabled noninvasive and quantitative detection and characterization of the population and function of β-cells in vivo. These advantages aid in defining and monitoring the progress of diabetes and determining the efficacy of anti-diabetic therapies. Beyond β-cell targeting, molecular imaging of biomarkers associated with the development of diabetes, e.g., lymphocyte infiltration, insulitis, and metabolic changes, may also be a promising strategy for early detection of diabetes, monitoring its progression, and occurrence of complications, as well as facilitating exploration of new therapeutic interventions. Moreover, molecular imaging of glucose uptake, production and excretion in specified tissues is critical for understanding the pathogenesis of diabetes. In the current review, we summarize and discuss recent advances in noninvasive imaging technologies for imaging of biomarkers beyond β-cells for early diagnosis of diabetes, investigation of glucose metabolism, and precise diagnosis and monitoring of diabetic complications for better management of diabetic patients.

Radiopharmaceutical tracers for cardiac imaging

Cardiovascular disease (CVD) is the leading cause of death and disease burden worldwide. Nuclear myocardial perfusion imaging with either single-photon emission computed tomography or positron emission tomography has been used extensively to perform diagnosis, monitor therapies, and predict cardiovascular events. Several radiopharmaceutical tracers have recently been developed to evaluate CVD by targeting myocardial perfusion, metabolism, innervation, and inflammation. This article reviews old and newer used in nuclear cardiac imaging.