Molecular Imaging of the Chemokine Receptor CXCR4 After Acute Myocardial Infarction

Molecular imaging of inflammation crosstalk along the cardio-renal axis following acute myocardial infarction

Rationale: Acute myocardial infarction (MI) triggers a systemic inflammatory response including crosstalk along the heart-kidney axis. We employed radionuclide-based inflammation-targeted whole-body molecular imaging to identify potential cardio-renal crosstalk after MI in a translational setup. Methods: Serial whole-body positron emission tomography (PET) with the specific CXCR4 ligand 68Ga-Pentixafor was performed after MI in mice. Tracer retention in kidneys and heart was compared to hematopoietic organs to evaluate systemic inflammation, validated by ex vivo analysis and correlated with progressive contractile dysfunction. Additionally, 96 patients underwent 68Ga-Pentixafor PET within the first week after MI, for systems-based image analysis and to determine prognostic value for adverse renal outcome. Results: In mice, transient myocardial CXCR4 upregulation occurred early after MI. Cardiac and renal PET signal directly correlated over the time course (r = 0.62, p < 0.0001), suggesting an inflammatory link between organs. Ex-vivo autoradiography (r = 0.9, p < 0.01) and CD68 immunostaining indicated signal localization to inflammatory cell content. Renal signal at 7d was inversely proportional to left ventricular ejection fraction at 6 weeks after MI (r = -0.79, p < 0.01). In patients, renal CXCR4 signal also correlated with signal from infarct (r = 0.25, p < 0.05) and remote myocardium (r = 0.39, p < 0.0001). Glomerular filtration rate (GFR) was available in 48/96 (50%) during follow-up. Worsening of renal function (GFR loss >5 mL/min/1.73m2), occurred a mean 80.5 days after MI in 16/48 (33.3%). Kaplan-Meier analysis revealed adverse renal outcome for patients with elevated remote myocardial CXCR4 signal (p < 0.05). Multivariate Cox analysis confirmed an independent predictive value (relative to baseline GFR, LVEF, infarct size; HR, 5.27). Conclusion: Systems-based CXCR4-targeted molecular imaging identifies inflammatory crosstalk along the cardio-renal axis early after MI.

Emerging Preclinical and Clinical Applications of Theranostics for Nononcological Disorders

Studies in nuclear medicine have shed light on molecular imaging and therapeutic approaches for oncological and nononcological conditions. Using the same radiopharmaceuticals for diagnosis and therapeutics of malignancies, the theranostics approach, has improved clinical management of patients. Theranostic approaches for nononcological conditions are recognized as emerging topics of research. This review focuses on preclinical and clinical studies of nononcological disorders that include theranostic strategies. Theranostic approaches are demonstrated as possible in the clinical management of infections and inflammations. There is an emerging need for randomized trials to specify the factors affecting validity and efficacy of theranostic approaches in nononcological diseases.

PET Imaging of Post-infarct Myocardial Inflammation

PURPOSE OF REVIEW

To examine the use of positron emission tomography (PET) for imaging post-infarct myocardial inflammation and repair.

RECENT FINDINGS

Dysregulated immune responses after myocardial infarction are associated with adverse cardiac remodelling and an increased likelihood of ischaemic heart failure. PET imaging utilising novel tracers can be applied to visualise different components of the post-infarction inflammatory and repair processes. This approach could offer unique pathophysiological insights that could prove useful for the identification and risk-stratification of individuals who would ultimately benefit most from emerging immune-modulating therapies. PET imaging could also bridge the clinical translational gap as a surrogate measure of drug efficacy in early-stage clinical trials in patients with myocardial infarction. The use of hybrid PET/MR imaging, in particular, offers the additional advantage of simultaneous in vivo molecular imaging and detailed assessment of myocardial function, viability and tissue characterisation. Further research is needed to realise the true clinical translational value of PET imaging after myocardial infarction.

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.

Molecular imaging of fibroblast activation protein after myocardial infarction using the novel radiotracer [68Ga]MHLL1

Background: Myocardial infarction (MI) evokes an organized remodeling process characterized by the activation and transdifferentiation of quiescent cardiac fibroblasts to generate a stable collagen rich scar. Early fibroblast activation may be amenable to targeted therapy, but is challenging to identify in vivo. We aimed to non-invasively image active fibrosis by targeting the fibroblast activation protein (FAP) expressed by activated (myo)fibroblasts, using a novel positron emission tomography (PET) radioligand [⁶⁸Ga]MHLL1 after acute MI.

Methods: One-step chemical synthesis and manual as well as module-based radiolabeling yielded [⁶⁸Ga]MHLL1. Binding characteristics were evaluated in murine and human FAP-transfected cells, and stability tested in human serum. Biodistribution in healthy animals was interrogated by dynamic PET imaging, and metabolites were measured in blood and urine. The temporal pattern of FAP expression was determined by serial PET imaging at 7 d and 21 d after coronary artery ligation in mice as percent injected dose per gram (%ID/g). PET measurements were validated by ex vivo autoradiography and immunostaining for FAP and inflammatory macrophages.

Results: [⁶⁸Ga]MHLL1 displayed specific uptake in murine and human FAP-positive cells (p = 0.0208). In healthy mice the tracer exhibited favorable imaging characteristics, with low blood pool retention and dominantly renal clearance. At 7 d after coronary artery ligation, [⁶⁸Ga]MHLL1 uptake was elevated in the infarct relative to the non-infarcted remote myocardium (1.3 ± 0.3 vs. 1.0 ± 0.2 %ID/g, p < 0.001) which persisted to 21 d after MI (1.3 ± 0.4 vs. 1.1 ± 0.4 %ID/g, p = 0.013). Excess unlabeled compound blocked tracer accumulation in both infarct and non-infarct remote myocardium regions (p < 0.001). Autoradiography and histology confirmed the regional uptake of [⁶⁸Ga]MHLL1 in the infarct and especially border zone regions, as identified by Masson trichrome collagen staining. Immunostaining further delineated persistent FAP expression at 7 d and 21 d post-MI in the border zone, consistent with tracer distribution in vivo.

Conclusion: The simplified synthesis of [⁶⁸Ga]MHLL1 bears promise for non-invasive characterization of fibroblast activation protein early in remodeling after MI.

Molecular Imaging of Inflammation and Fibrosis in Pressure Overload Heart Failure

[Figure: see text].

Molecular imaging-guided repair after acute myocardial infarction by targeting the chemokine receptor CXCR4

AIMS

Balance between inflammatory and reparative leucocytes allows optimal healing after myocardial infarction (MI). Interindividual heterogeneity evokes variable functional outcome complicating targeted therapy. We aimed to characterize infarct chemokine CXC-motif receptor 4 (CXCR4) expression using positron emission tomography (PET) and establish its relationship to cardiac outcome. We tested whether image-guided early CXCR4 directed therapy attenuates chronic dysfunction.

METHODS AND RESULTS

Mice (n = 180) underwent coronary ligation or sham surgery and serial PET imaging over 7 days. Infarct CXCR4 content was elevated over 3 days after MI compared with sham (%ID/g, Day 1:1.1 ± 0.2; Day 3:0.9 ± 0.2 vs. 0.6 ± 0.1, P < 0.001), confirmed by flow cytometry and histopathology. Mice that died of left ventricle (LV) rupture exhibited persistent inflammation at 3 days compared with survivors (1.2 ± 0.3 vs. 0.9 ± 0.2% ID/g, P < 0.001). Cardiac magnetic resonance measured cardiac function. Higher CXCR4 signal at 1 and 3 days independently predicted worse functional outcome at 6 weeks (rpartial = -0.4, P = 0.04). Mice were treated with CXCR4 blocker AMD3100 following the imaging timecourse. On-peak CXCR4 blockade at 3 days lowered LV rupture incidence vs. untreated MI (8% vs. 25%), and improved contractile function at 6 weeks (+24%, P = 0.01). Off-peak CXCR4 blockade at 7 days did not improve outcome. Flow cytometry analysis revealed lower LV neutrophil and Ly6Chigh monocyte content after on-peak treatment. Patients (n = 50) early after MI underwent CXCR4 PET imaging and functional assessment. Infarct CXCR4 expression in acute MI patients correlated with contractile function at time of PET and on follow-up.

CONCLUSION

Positron emission tomography imaging identifies early CXCR4 up-regulation which predicts acute rupture and chronic contractile dysfunction. Imaging-guided CXCR4 inhibition accelerates inflammatory resolution and improves outcome. This supports a molecular imaging-based theranostic approach to guide therapy after MI.

Imaging of T-cell Responses in the Context of Cancer Immunotherapy

Immunotherapy, which promotes the induction of cytotoxic T lymphocytes and enhances their infiltration into and function within tumors, is a rapidly expanding and evolving approach to treating cancer. However, many of the critical denominators for inducing effective anticancer immune responses remain unknown. Efforts are underway to develop comprehensive ex vivo assessments of the immune landscape of patients prior to and during response to immunotherapy. An important complementary approach to these efforts involves the development of noninvasive imaging approaches to detect immune targets, assess delivery of immune-based therapeutics, and evaluate responses to immunotherapy. Herein, we review the merits and limitations of various noninvasive imaging modalities (MRI, PET, and single-photon emission tomography) and discuss candidate targets for cellular and molecular imaging for visualization of T-cell responses at various stages along the cancer-immunity cycle in the context of immunotherapy. We also discuss the potential use of these imaging strategies in monitoring treatment responses and predicting prognosis for patients treated with immunotherapy.

Quantification of Macrophage-Driven Inflammation During Myocardial Infarction with 18F-LW223, a Novel TSPO Radiotracer with Binding Independent of the rs6971 Human Polymorphism

Myocardial infarction (MI) is one of the leading causes of death worldwide, and inflammation is central to tissue response and patient outcomes. The 18-kDa translocator protein (TSPO) has been used in PET as an inflammatory biomarker. The aims of this study were to screen novel, fluorinated, TSPO radiotracers for susceptibility to the rs6971 genetic polymorphism using in vitro competition binding assays in human brain and heart; assess whether the in vivo characteristics of our lead radiotracer, 18F-LW223, are suitable for clinical translation; and validate whether 18F-LW223 can detect macrophage-driven inflammation in a rat MI model. Methods: Fifty-one human brain and 29 human heart tissue samples were screened for the rs6971 polymorphism. Competition binding assays were conducted with 3H-PK11195 and the following ligands: PK11195, PBR28, and our novel compounds (AB5186 and LW223). Naïve rats and mice were used for in vivo PET kinetic studies, radiometabolite studies, and dosimetry experiments. Rats underwent permanent coronary artery ligation and were scanned using PET/CT with an invasive input function at 7 d after MI. For quantification of PET signal in the hypoperfused myocardium, K1 (rate constant for transfer from arterial plasma to tissues) was used as a surrogate marker of perfusion to correct the binding potential for impaired radiotracer transfer from plasma to tissue (BPTC). Results: LW223 binding to TSPO was not susceptible to the rs6971 genetic polymorphism in human brain and heart samples. In rodents, 18F-LW223 displayed a specific uptake consistent with TSPO expression, a slow metabolism in blood (69% of parent at 120 min), a high plasma free fraction of 38.5%, and a suitable dosimetry profile (effective dose of 20.5-24.5 μSv/MBq). 18F-LW223 BPTC was significantly higher in the MI cohort within the infarct territory of the anterior wall relative to the anterior wall of naïve animals (32.7 ± 5.0 vs. 10.0 ± 2.4 cm3/mL/min, P ≤ 0.001). Ex vivo immunofluorescent staining for TSPO and CD68 (macrophage marker) resulted in the same pattern seen with in vivo BPTC analysis. Conclusion:18F-LW223 is not susceptible to the rs6971 genetic polymorphism in in vitro assays, has favorable in vivo characteristics, and is able to accurately map macrophage-driven inflammation after MI.

Emerging methods for the characterization of ischemic heart disease: ultrafast Doppler angiography, micro-CT, photon-counting CT, novel MRI and PET techniques, and artificial intelligence

After an ischemic event, disruptive changes in the healthy myocardium may gradually develop and may ultimately turn into fibrotic scar. While these structural changes have been described by conventional imaging modalities mostly on a macroscopic scale-i.e., late gadolinium enhancement at magnetic resonance imaging (MRI)-in recent years, novel imaging methods have shown the potential to unveil an even more detailed picture of the postischemic myocardial phenomena. These new methods may bring advances in the understanding of ischemic heart disease with potential major changes in the current clinical practice. In this review article, we provide an overview of the emerging methods for the non-invasive characterization of ischemic heart disease, including coronary ultrafast Doppler angiography, photon-counting computed tomography (CT), micro-CT (for preclinical studies), low-field and ultrahigh-field MRI, and ¹¹C-methionine positron emission tomography. In addition, we discuss new opportunities brought by artificial intelligence, while addressing promising future scenarios and the challenges for the application of artificial intelligence in the field of cardiac imaging.

Autoradiographical assessment of inflammation-targeting radioligands for atherosclerosis imaging: potential for plaque phenotype identification

PURPOSE

Many radioligands have been developed for the visualization of atherosclerosis by targeting inflammation. However, interpretation of in vivo signals is often limited to plaque identification. We evaluated binding of some promising radioligands in an in vitro approach in atherosclerotic plaques with different phenotypes.

METHODS

Tissue sections of carotid endarterectomy tissue were characterized as early plaque, fibro-calcific plaque, or phenotypically vulnerable plaque. In vitro binding assays for the radioligands [111In]In-DOTATATE; [111In]In-DOTA-JR11; [67Ga]Ga-Pentixafor; [111In]In-DANBIRT; and [111In]In-EC0800 were conducted, the expression of the radioligand targets was assessed via immunohistochemistry. Radioligand binding and expression of radioligand targets was investigated and compared.

RESULTS

In sections characterized as vulnerable plaque, binding was highest for [111In]In-EC0800; followed by [111In]In-DANBIRT; [67Ga]Ga-Pentixafor; [111In]In-DOTA-JR11; and [111In]In-DOTATATE (0.064 ± 0.036; 0.052 ± 0.029; 0.011 ± 0.003; 0.0066 ± 0.0021; 0.00064 ± 0.00014 %Added activity/mm2, respectively). Binding of [111In]In-DANBIRT and [111In]In-EC0800 was highest across plaque phenotypes, binding of [111In]In-DOTA-JR11 and [67Ga]Ga-Pentixafor differed most between plaque phenotypes. Binding of [111In]In-DOTATATE was the lowest across plaque phenotypes. The areas positive for cells expressing the radioligand's target differed between plaque phenotypes for all targets, with lowest percentage area of expression in early plaque sections and highest in phenotypically vulnerable plaque sections.

CONCLUSIONS

Radioligands targeting inflammatory cell markers showed different levels of binding in atherosclerotic plaques and among plaque phenotypes. Different radioligands might be used for plaque detection and discerning early from vulnerable plaque. [111In]In-EC0800 and [111In]In-DANBIRT appear most suitable for plaque detection, while [67Ga]Ga-Pentixafor and [111In]In-DOTA-JR11 might be best suited for differentiation between plaque phenotypes.

Imaging Inflammation with Positron Emission Tomography

The impact of inflammation on the outcome of many medical conditions such as cardiovascular diseases, neurological disorders, infections, cancer, and autoimmune diseases has been widely acknowledged. However, in contrast to neurological, oncologic, and cardiovascular disorders, imaging plays a minor role in research and management of inflammation. Imaging can provide insights into individual and temporospatial biology and grade of inflammation which can be of diagnostic, therapeutic, and prognostic value. There is therefore an urgent need to evaluate and understand current approaches and potential applications for imaging of inflammation. This review discusses radiotracers for positron emission tomography (PET) that have been used to image inflammation in cardiovascular diseases and other inflammatory conditions with a special emphasis on radiotracers that have already been successfully applied in clinical settings.

Clinical results of fibroblast activation protein (FAP) specific PET for non-malignant indications: systematic review

PURPOSE

Small molecules targeting fibroblast activation protein (FAP) have emerged as a new group of tracers for positron emission tomography (PET) in 2018. While most of the existing literature has been focussed on the application of FAP-specific PET in various kinds of cancers, some researchers have, both intentionally or unintentionally, used FAP-specific PET in patients with non-cancerous diseases. The purpose of this systematic review is therefore to summarize the available evidence of FAP-specific PET for non-malignant indications.

METHODS

The MEDLINE database was searched for studies presenting the clinical use of FAP-specific PET, the records were screened according to PRISMA guidelines and articles containing patients suffering from non-malignant diseases were included.

RESULTS

Sixteen studies with 303 patients were included. FAP-specific PET has been used in cardiac imaging, IgG₄-related disease, benign tumors as well as various kinds of inflammation. Two prospective studies on FAP-specific PET for IgG₄-related disease show its potential to differentiate inflammatory from fibrotic lesions, which could be used to determine the management of these patients.

CONCLUSION

While publications on FAP-specific PET for non-malignant indications are mostly limited to case reports and incidental findings, the first retrospective and prospective studies present promising results for IgG₄-related as well as cardiovascular disease that warrant further research. Several currently recruiting trials will add to the body evidence in the next few years.

Imaging Cardiovascular and Lung Macrophages With the Positron Emission Tomography Sensor 64Cu-Macrin in Mice, Rabbits, and Pigs

BACKGROUND

Macrophages, innate immune cells that reside in all organs, defend the host against infection and injury. In the heart and vasculature, inflammatory macrophages also enhance tissue damage and propel cardiovascular diseases.

METHODS

We here use in vivo positron emission tomography (PET) imaging, flow cytometry, and confocal microscopy to evaluate quantitative noninvasive assessment of cardiac, arterial, and pulmonary macrophages using the nanotracer ⁶⁴Cu-Macrin-a 20-nm spherical dextran nanoparticle assembled from nontoxic polyglucose.

RESULTS

PET imaging using ⁶⁴Cu-Macrin faithfully reported accumulation of macrophages in the heart and lung of mice with myocardial infarction, sepsis, or pneumonia. Flow cytometry and confocal microscopy detected the near-infrared fluorescent version of the nanoparticle (^VT680Macrin) primarily in tissue macrophages. In 5-day-old mice, ⁶⁴Cu-Macrin PET imaging quantified physiologically more numerous cardiac macrophages. Upon intravenous administration of ⁶⁴Cu-Macrin in rabbits and pigs, we detected heightened macrophage numbers in the infarcted myocardium, inflamed lung regions, and atherosclerotic plaques using a clinical PET/magnetic resonance imaging scanner. Toxicity studies in rats and human dosimetry estimates suggest that ⁶⁴Cu-Macrin is safe for use in humans.

CONCLUSIONS

Taken together, these results indicate ⁶⁴Cu-Macrin could serve as a facile PET nanotracer to survey spatiotemporal macrophage dynamics during various physiological and pathological conditions. ⁶⁴Cu-Macrin PET imaging could stage inflammatory cardiovascular disease activity, assist disease management, and serve as an imaging biomarker for emerging macrophage-targeted therapeutics.

Circular RNA Involved in the Protective Effect of Malva sylvestris L. on Myocardial Ischemic/Re-Perfused Injury

Ischemic heart disease has become a major health challenge worldwide. Malva sylvestris L. (MS) is a traditional herbal medicine with anti-inflammatory properties and have been used as antioxidant and anti- inflammatory agent in infectious diseases and inflammatory diseases.In this study, we aimed at elucidating the mechanism of MS against ischemia-reperfusion (I/R)–induced injury in vivo and in vitro. The I/R animal model in rats and oxygen glucose deprivation/re-oxygenation (OGD/Re) model in H9c2 cells were used in this study. MS was used to pre-treat the rats and cells. Electrocardiogram, histology staining, qPCR, ELISA, CCK-8, and circRNA microarray were performed. We found that pre-treatment with MS extract attenuate OGD/Re-induced cell apoptosis and cell viability inhibition in H9c2 cells. In addition, pre-treatment with MS protected against I/R injury in vivo. The protective effects of MS pre-treatment were associated with inflammatory genes expression and cytokines release. Further mechanistic investigation revealed that MS protected cardiomyocytes through regulating circular RNA (circRNA). We identified a novel circRNA circ003593 that mediated the protective role of MS in vitro through NLRP3 complex, which was associated with reperfusion injury salvage kinase (RISK) signaling pathway. Conclusion: this study is the first time to demonstrate the protective role of MS on I/R injury. Our findings reveal a novel circRNA circ003593-mediated the protective role of MS through NLRP3 inflammasome. Circ003593 may serve as a potential therapeutic target for ischemic heart diseases.

Applications of PET-MR Imaging in Cardiovascular Disorders

Cardiac PET/MR imaging is an integrated imaging approach that requires less radiation than PET/computed tomography and combines the high spatial resolution and morphologic data from MR imaging with the physiologic information from PET. This hybrid approach has the potential to improve the diagnostic and prognostic evaluation of several cardiovascular conditions, such as ischemic heart disease, infiltrative diseases such as sarcoidosis, acute and chronic myocarditis, and cardiac masses. Herein, the authors discuss the strengths of PET and MR imaging in several cardiovascular conditions; the challenges and potential; and the current data on the application of this powerful hybrid imaging modality.

Clinical Molecular Imaging of Pulmonary CXCR4 Expression to Predict Outcome of Pirfenidone Treatment in Idiopathic Pulmonary Fibrosis

BACKGROUND

Idiopathic pulmonary fibrosis (IPF) is a progressive disease for which two antifibrotic drugs recently were approved. However, an unmet need exists to predict responses to antifibrotic treatment, such as pirfenidone. Recent data suggest that upregulated expression of CXCR4 is indicative of outcomes in IPF.

RESEARCH QUESTION

Can quantitative, molecular imaging of pulmonary CXCR4 expression as a biomarker for disease activity predict response to the targeted treatment pirfenidone and prognosis in patients with IPF?

STUDY DESIGN AND METHODS

CXCR4 expression was analyzed by immunohistochemistry examination of lung tissues and reverse-transcriptase polymerase chain reaction analysis of BAL. PET-CT scanning with the specific CXCR4 ligand ⁶⁸Ga-pentixafor was performed in 28 IPF patients and compared with baseline clinical characteristics. In 16 patients, a follow-up scan was obtained 6 to 12 weeks after initiation of treatment with pirfenidone. Patients were followed up in our outpatient clinic for ≥ 12 months.

RESULTS

Immunohistochemistry analysis showed high CXCR4 staining of epithelial cells and macrophages in areas with vast fibrotic remodeling. Targeted PET scanning revealed CXCR4 upregulation in fibrotic areas of the lungs, particularly in zones with subpleural honeycombing. Baseline CXCR4 signal demonstrated a significant correlation with Gender Age Physiology stage (r = 0.44; P = .02) and with high-resolution CT scan score (r = 0.38; P = .04). Early changes in CXCR4 signal after initiation of pirfenidone treatment correlated with the long-term course of FVC after 12 months (r = -0.75; P = .0008). Moreover, patients with a high pulmonary CXCR4 signal on follow-up PET scan after 6 weeks into treatment demonstrated a statistically significant worse outcome at 12 months (P = .002). In multiple regression analysis, pulmonary CXCR4 signal on follow-up PET scan emerged as the only independent predictor of long-term outcome (P = .0226).

INTERPRETATION

CXCR4-targeted PET imaging identified disease activity and predicted outcome of IPF patients treated with pirfenidone. It may serve as a future biomarker for personalized guidance of antifibrotic treatment.

Accuracy of cardiac functional parameters measured from gated radionuclide myocardial perfusion imaging in mice

BACKGROUND

Quantitative cardiac contractile function assessment is the primary indicator of disease progression and therapeutic efficacy in small animals. Operator dependency is a major challenge with commonly used echocardiography. Simultaneous assessment of cardiac perfusion and function in nuclear scans would reduce burden on the animal and facilitate longitudinal studies. We evaluated the accuracy of contractile function measurements obtained from electrocardiogram-gated nuclear perfusion imaging compared with anatomic imaging.

METHODS AND RESULTS

In healthy C57Bl/6N mice (n = 11), 99mTc-sestamibi SPECT and 13N-ammonia PET underestimated left ventricular volumes (23 to 28%, P = 0.02) compared to matched anatomic images, though ejection fraction (LVEF) was comparable (%, SPECT: 73 ± 8 vs CMR: 72 ± 6, P = 0.1). At 1 week after myocardial infarction (n = 13), LV volumes were significantly lower in perfusion images compared to CMR and contrast CT (P = 0.003), and LVEF was modestly overestimated (%, SPECT: 37 ± 8, vs CMR: 27 ± 7, P = 0.003). Nuclear images exhibited good intra- and inter-reader agreement. Perfusion SPECT accurately calculated infarct size compared to histology (r = 0.95, P < 0.001).

CONCLUSIONS

Cardiac function can be calculated by gated nuclear perfusion imaging in healthy mice. After infarction, perfusion imaging overestimates LVEF, which should be considered for comparison to other modalities. Combined functional and infarct size analysis may optimize imaging protocols and reduce anaesthesia duration for longitudinal studies.

Imaging the injured beating heart intravitally and the vasculoprotection afforded by haematopoietic stem cells

Aims

Adequate microcirculatory perfusion, and not just opening of occluded arteries, is critical to salvage heart tissue following myocardial infarction. However, the degree of microvascular perfusion taking place is not known, limited primarily by an inability to directly image coronary microcirculation in a beating heart in vivo. Haematopoietic stem/progenitor cells (HSPCs) offer a potential therapy but little is known about their homing dynamics at a cellular level and whether they protect coronary microvessels. This study used intravital microscopy to image the anaesthetized mouse beating heart microcirculation following stabilization.

Methods and results

A 3D-printed stabilizer was attached to the ischaemia–reperfusion injured (IRI) beating heart. The kinetics of neutrophil, platelet and HSPC recruitment, as well as functional capillary density (FCD), was imaged post-reperfusion. Laser speckle contrast imaging (LSCI) was used for the first time to monitor ventricular blood flow in beating hearts. Sustained hyperaemic responses were measured throughout reperfusion, initially indicating adequate flow resumption. Intravital microscopy confirmed large vessel perfusion but demonstrated poor transmission of flow to downstream coronary microvessels. Significant neutrophil adhesion and microthrombus formation occurred within capillaries with the latter occluding them, resulting in patchy perfusion and reduced FCD. Interestingly, ‘patrolling’ neutrophils were also observed in capillaries. Haematopoietic stem/progenitor cells readily trafficked through the heart but local retention was poor. Despite this, remarkable anti-thromboinflammatory effects were observed, consequently improving microvascular perfusion.

Conclusion

We present a novel approach for imaging multiple microcirculatory perturbations in the beating heart with LSCI assessment of blood flow. Despite deceptive hyperaemic responses, increased microcirculatory flow heterogeneity was seen, with non-perfused areas interspersed with perfused areas. Microthrombi, rather than neutrophils, appeared to be the major causative factor. We further applied this technique to demonstrate local stem cell presence is not a pre-requisite to confer vasculoprotection. This is the first detailed in vivo characterization of coronary microcirculatory responses post-reperfusion injury.

Small animal models of heart failure

Heart disease is a major cause of death worldwide with increasing prevalence, which urges the development of new therapeutic strategies. Over the last few decades, numerous small animal models have been generated to mimic various pathomechanisms contributing to heart failure (HF). Despite some limitations, these animal models have greatly advanced our understanding of the pathogenesis of the different aetiologies of HF and paved the way to understanding the underlying mechanisms and development of successful treatments. These models utilize surgical techniques, genetic modifications, and pharmacological approaches. The present review discusses the strengths and limitations of commonly used small animal HF models, which continue to provide crucial insight and facilitate the development of new treatment strategies for patients with HF.

A new class of PentixaFor- and PentixaTher-based theranostic agents with enhanced CXCR4-targeting efficiency

Non-invasive PET imaging of CXCR4 expression in cancer and inflammation as well as CXCR4-targeted radioligand therapy (RLT) have recently found their way into clinical research by the development of the theranostic agents [⁶⁸Ga]PentixaFor (cyclo(D-Tyr¹-D-[NMe]Orn²(AMBS-[⁶⁸Ga]DOTA)-Arg³-Nal⁴-Gly⁵) = [⁶⁸Ga]DOTA-AMBS-CPCR4) and [¹⁷⁷Lu/⁹⁰Y]PentixaTher (cyclo(D-3-iodo-Tyr¹-D-[NMe]Orn²(AMBS-[¹⁷⁷Lu/⁹⁰Y]DOTA)-Arg³-Nal⁴-Gly⁵) = [¹⁷⁷Lu/⁹⁰Y]DOTA-AMBS-iodoCPCR4). Although convincing clinical results have already been obtained with both agents, this study was designed to further investigate the required structural elements for improved ligand-receptor interaction for both peptide cores (CPCR4 and iodoCPCR4). To this aim, a series of DOTA-conjugated CPCR4- and iodoCPCR4-based ligands with new linker structures, replacing the AMBA-linker in PentixaFor and PentixaTher, were synthesized and evaluated. Methods: The in vitro investigation of the novel compounds alongside with the reference peptides PentixaFor and PentixaTher encompassed the determination of hCXCR4 and mCXCR4 affinity (IC₅₀) of the respective ^natGa-, ^natLu-, ^natY- and ^natBi-complexes in Jurkat and Eμ-myc 1080 cells using [¹²⁵I]FC-131 and [¹²⁵I]CPCR4.3 as radioligands, respectively, as well as the evaluation of the internalization and externalization kinetics of selected ⁶⁸Ga- and ¹⁷⁷Lu-labeled compounds in hCXCR4-transfected Chem-1 cells. Comparative small animal PET imaging studies (1h p.i.) as well as in vivo biodistribution studies (1, 6 and 48h p.i.) were performed in Daudi (human B cell lymphoma) xenograft bearing CB17 SCID mice. Results: Based on the affinity data and cellular uptake studies, [⁶⁸Ga/¹⁷⁷Lu]DOTA-r-a-ABA-CPCR4 and [⁶⁸Ga/¹⁷⁷Lu]DOTA-r-a-ABA-iodoCPCR4 (with r-a-ABA = D-Arg-D-Ala-4-aminobenzoyl-) were selected for further evaluation. Both analogs show app. 10-fold enhanced hCXCR4 affinity compared to the respective references [⁶⁸Ga]PentixaFor and [¹⁷⁷Lu]PentixaTher, four times higher cellular uptake in hCXCR4 expressing cells and improved cellular retention. Unfortunately, the improved in vitro binding and uptake characteristics of [⁶⁸Ga]DOTA-r-a-ABA-CPCR4 and -iodoCPCR4 could not be recapitulated in initial PET imaging studies; both compounds showed similar uptake in the Daudi xenografts as [⁶⁸Ga]PentixaFor, alongside with higher background accumulation, especially in the kidneys. However, the subsequent biodistribution studies performed for the corresponding ¹⁷⁷Lu-labeled analogs revealed a clear superiority of [¹⁷⁷Lu]DOTA-r-a-ABA-CPCR4 and [¹⁷⁷Lu]DOTA-r-a-ABA-iodoCPCR4 over [¹⁷⁷Lu]PentixaTher with respect to tumor uptake (18.3±3.7 and 17.2±2.0 %iD/g, respectively, at 1h p.i. vs 12.4±3.7%iD/g for [¹⁷⁷Lu]PentixaTher) as well as activity retention in tumor up to 48h. Especially for [¹⁷⁷Lu]DOTA-r-a-ABA-CPCR4 with its low background accumulation, tumor/organ ratios at 48h were 2- to 4-fold higher than those obtained for [¹⁷⁷Lu]PentixaTher (except for kidney). Conclusions: The in-depth evaluation of a series of novel CPCR4- and iodoCPCR4 analogs with modified linker structure has yielded reliable structure-activity relationships. It was generally observed that a) AMBA-by-ABA-substitution leads to enhanced ligand internalization, b) the extension of the ABA-linker by two additional amino acids (DOTA-Xaa₂-Xaa₁-ABA-) provides sufficient linker length to minimize the interaction of the [M³⁺]DOTA-chelate with the receptor, and that c) introduction of a cationic side chain (Xaa₂) greatly enhances receptor affinity of the constructs, obliterating the necessity for Tyr¹-iodination of the pentapeptide core to maintain high receptor affinity (such as in [¹⁷⁷Lu]PentixaTher). As a result, [¹⁷⁷Lu]DOTA-r-a-ABA-CPCR4 has emerged from this study as a powerful second-generation therapeutic CXCR4 ligand with greatly improved targeting efficiency and tumor retention and will be further evaluated in preclinical and clinical CXCR4-targeted dosimetry and RLT studies.

Positron emission tomography imaging in cardiovascular disease

Positron emission tomography (PET) imaging is useful in cardiovascular disease across several areas, from assessment of myocardial perfusion and viability, to highlighting atherosclerotic plaque activity and measuring the extent of cardiac innervation in heart failure. Other important roles of PET have emerged in prosthetic valve endocarditis, implanted device infection, infiltrative cardiomyopathies, aortic stenosis and cardio-oncology. Advances in scanner technology, including hybrid PET/MRI and total body PET imaging, as well as the development of novel PET tracers and cardiac-specific postprocessing techniques using artificial intelligence will undoubtedly continue to progress the field.

Targeted PET Imaging of Chemokine Receptor 2-Positive Monocytes and Macrophages in the Injured Heart

Proinflammatory macrophages are important mediators of inflammation after myocardial infarction and of allograft injury after heart transplantation. The aim of this study was to image the recruitment of proinflammatory chemokine receptor 2-positive (CCR2+) cells in multiple heart injury models. Methods: ⁶⁴Cu-DOTA-extracellular loop 1 inverso (ECL1i) PET was used to image CCR2+ monocytes and macrophages in a heart transplantation mouse model. Flow cytometry was performed to characterize CCR2+ cells. Autoradiography on a human heart specimen was conducted to confirm binding specificity. ⁶⁴Cu- and ⁶⁸Ga-DOTA-ECL1i were compared in an ischemia-reperfusion injury mouse model. Results: ⁶⁴Cu-DOTA-ECL1i showed sensitive and specific detection of CCR2+ cells in all tested mouse models, with efficacy comparable to that of ⁶⁸Ga-DOTA-ECL1i. Flow cytometry demonstrated specific expression of CCR2 on monocytes and macrophages. The tracer binds to human CCR2. Conclusion: This work establishes the utility of ⁶⁴Cu-DOTA-ECL1i to image CCR2+ monocytes and macrophages in mouse models and provides the requisite preclinical information to translate the targeted clinical-grade CCR2 imaging probe for clinical investigation of heart diseases.

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.

The Changing Face of Nuclear Cardiology: Guiding Cardiovascular Care Toward Molecular Medicine

Radionuclide imaging of myocardial perfusion, function, and viability has been established for decades and remains a robust, evidence-based and broadly available means for clinical workup and therapeutic guidance in ischemic heart disease. Yet, powerful alternative modalities have emerged for this purpose, and their growth has resulted in increasing competition. But the potential of the tracer principle goes beyond the assessment of physiology and function, toward the interrogation of biology and molecular pathways. This is a unique selling point of radionuclide imaging, which has been underrecognized in cardiovascular medicine until recently. Now, molecular imaging methods for the detection of myocardial infiltration, device infection, and cardiovascular inflammation are successfully gaining clinical acceptance. This is further strengthened by the symbiotic quest of cardiac imaging and therapy for an increasing implementation of molecule-targeted procedures, in which specific therapeutic interventions require specific diagnostic guidance toward the most suitable candidates. This review will summarize the current advent of clinical cardiovascular molecular imaging and highlight its transformative contribution to the evolution of cardiovascular therapy beyond mechanical interventions and broad blockbuster medication, toward a future of novel, individualized molecule-targeted and molecular imaging-guided therapies.

Current and novel radiopharmaceuticals for imaging cardiovascular inflammation

Cardiovascular disease (CVD) remains the leading cause of death worldwide despite advances in diagnostic technologies and treatment strategies. The underlying cause of most CVD is atherosclerosis, a chronic disease driven by inflammatory reactions. Atherosclerotic plaque rupture could cause arterial occlusion leading to ischemic tissue injuries such as myocardial infarction (MI) and stroke. Clinically, most imaging modalities are based on anatomy and provide limited information about the on-going molecular activities affecting the vulnerability of atherosclerotic lesion for risk stratification of patients. Thus, the ability to differentiate stable plaques from those that are vulnerable is an unmet clinical need. Of various imaging techniques, the radionuclide-based molecular imaging modalities including positron emission tomography and single-photon emission computerized tomography provide superior ability to noninvasively visualize molecular activities in vivo and may serve as a useful tool in tackling this challenge. Moreover, the well-established translational pathway of radiopharmaceuticals may also facilitate the translation of discoveries from benchtop to clinical investigation in contrast to other imaging modalities to fulfill the goal of precision medicine. The relationship between inflammation occurring within the plaque and its proneness to rupture has been well documented. Therefore, an active effort has been significantly devoted to develop radiopharmaceuticals specifically to measure CVD inflammatory status, and potentially elucidate those plaques which are prone to rupture. In the following review, molecular imaging of inflammatory biomarkers will be briefly discussed.

Imaging inflammation after myocardial infarction: implications for prognosis and therapeutic guidance

Inflammation after myocardial infarction (MI) has been in the focus of cardiovascular research for several years as it influences the remodeling process of the ischemic heart and thereby critically determines the clinical outcome of the patient. Today, it is well appreciated that inflammation is a crucial necessity for the initiation of the natural wound healing process; however, excessive inflammation can have detrimental effects and might result in adverse ventricular remodeling which is associated with an increased risk of heart failure. Newly emerged imaging techniques facilitate the non-invasive assessment of immune cell infiltration into the ischemic myocardium and can provide greater insight into the underlying complex and dynamic repair mechanisms. Molecular imaging of inflammation in the context of MI may help with stratification of patients at high risk of adverse ventricular remodeling post-MI which may be of diagnostic, therapeutic, and prognostic value. Novel radiopharmaceuticals may additionally provide a way to combine patient monitoring and therapy. In spite of great advances in recent years in the field of imaging sciences, clinicians still need to overcome some obstacles to a wider implementation of inflammation imaging post-MI. This review focuses on inflammation as a molecular imaging target and its potential implication in prognosis and therapeutic guidance.

Infection and Inflammation Imaging: Beyond FDG

This review discusses nuclear imaging of inflammation using molecular probes beyond fluoro-d-glucose, is structured by cellular targets, and focuses on those tracers that have been successfully applied clinically.

Molecular Imaging of Myocardial Inflammation With Positron Emission Tomography Post-Ischemia: A Determinant of Subsequent Remodeling or Recovery

Inflammation after myocardial ischemia influences ventricular remodeling and repair and has emerged as a therapeutic target. Conventional diagnostic measurements address systemic inflammation but cannot quantify local tissue changes. Molecular imaging facilitates noninvasive assessment of leukocyte infiltration into damaged myocardium. Preliminary experience with ¹⁸F-labeled fluorodeoxyglucose ([¹⁸F]FDG) demonstrates localized inflammatory cell signal within the infarct territory as an independent predictor of subsequent ventricular dysfunction. Novel targeted radiotracers may provide additional insight into the enrichment of specific leukocyte populations. Challenges to wider implementation of inflammation imaging after myocardial infarction include accurate and reproducible quantification, prognostic value, and capacity to monitor inflammation response to novel treatment. This review describes myocardial inflammation following ischemia as a molecular imaging target and evaluates established and emerging radiotracers for this application. Furthermore, the potential role of inflammation imaging to provide prognostic information, support novel drug and therapeutic research, and assess biological response to cardiac disease is discussed.

Multimodality Imaging of Inflammation and Ventricular Remodeling in Pressure-Overload Heart Failure

Inflammation contributes to ventricular remodeling after myocardial ischemia, but its role in nonischemic heart failure is poorly understood. Local tissue inflammation is difficult to assess serially during pathogenesis. Although ¹⁸F-FDG accumulates in inflammatory leukocytes and thus may identify inflammation in the myocardial microenvironment, it remains unclear whether this imaging technique can isolate diffuse leukocytes in pressure-overload heart failure. We aimed to evaluate whether inflammation with ¹⁸F-FDG can be serially imaged in the early stages of pressure-overload-induced heart failure and to compare the time course with functional impairment assessed by cardiac MRI. Methods: C57Bl6/N mice underwent transverse aortic constriction (TAC) (n = 22), sham surgery (n = 12), or coronary ligation as an inflammation-positive control (n = 5). MRI assessed ventricular geometry and contractile function at 2 and 8 d after TAC. Immunostaining identified the extent of inflammatory leukocyte infiltration early in pressure overload. ¹⁸F-FDG PET scans were acquired at 3 and 7 d after TAC, under ketamine-xylazine anesthesia to suppress cardiomyocyte glucose uptake. Results: Pressure overload evoked rapid left ventricular dilation compared with sham (end-systolic volume, day 2: 40.6 ± 10.2 μL vs. 23.8 ± 1.7 μL, P < 0.001). Contractile function was similarly impaired (ejection fraction, day 2: 40.9% ± 9.7% vs. 59.2% ± 4.4%, P < 0.001). The severity of contractile impairment was proportional to histology-defined myocardial macrophage density on day 8 (r = -0.669, P = 0.010). PET imaging identified significantly higher left ventricular ¹⁸F-FDG accumulation in TAC mice than in sham mice on day 3 (10.5 ± 4.1 percentage injected dose [%ID]/g vs. 3.8 ± 0.9 %ID/g, P < 0.001) and on day 7 (7.8 ± 3.7 %ID/g vs. 3.0 ± 0.8 %ID/g, P = 0.006), though the efficiency of cardiomyocyte suppression was variable among TAC mice. The ¹⁸F-FDG signal correlated with ejection fraction (r = -0.75, P = 0.01) and ventricular volume (r = 0.75, P < 0.01). Western immunoblotting demonstrated a 60% elevation of myocardial glucose transporter 4 expression in the left ventricle at 8 d after TAC, indicating altered glucose metabolism. Conclusion: TAC induces rapid changes in left ventricular geometry and contractile function, with a parallel modest infiltration of inflammatory macrophages. Metabolic remodeling overshadows inflammatory leukocyte signal using ¹⁸F-FDG PET imaging. More selective inflammatory tracers are requisite to identify the diffuse local inflammation in pressure overload.

Imaging Inflammation in Atherosclerosis with CXCR4-Directed 68Ga-Pentixafor PET/CT: Correlation with 18F-FDG PET/CT

C-X-C motif chemokine receptor 4 (CXCR4) is expressed on the surface of various cell types involved in atherosclerosis, with a particularly rich receptor expression on macrophages and T cells. First pilot studies with ⁶⁸Ga-pentixafor, a novel CXCR4-directed PET tracer, have shown promise to noninvasively image inflammation within atherosclerotic plaques. The aim of this retrospective study was to investigate the performance of ⁶⁸Ga-pentixafor PET/CT for imaging atherosclerosis in comparison to ¹⁸F-FDG PET/CT. Methods: Ninety-two patients (37 women and 55 men; mean age, 62 ± 10 y) underwent ⁶⁸Ga-pentixafor and ¹⁸F-FDG PET/CT for staging of oncologic diseases. In these subjects, lesions in the walls of large arteries were identified using morphologic and PET criteria for atherosclerosis (n = 652). Tracer uptake was measured and adjusted for vascular lumen (background) signal by calculation of target-to-background ratios (TBRs) by 2 investigators masked to the other PET scan. On a lesion-to-lesion and patient basis, the TBRs of both PET tracers were compared and additionally correlated to the degree of arterial calcification as quantified in CT. Results: On a lesion-to-lesion basis, ⁶⁸Ga-pentixafor and ¹⁸F-FDG uptake showed a weak correlation (r = 0.28; P < 0.01). ⁶⁸Ga-pentixafor PET identified more lesions (n = 290; TBR ≥ 1.6, P < 0.01) and demonstrated higher uptake than ¹⁸F-FDG PET (1.8 ± 0.5 vs. 1.4 ± 0.4; P < 0.01). The degree of plaque calcification correlated negatively with both ⁶⁸Ga-pentixafor and ¹⁸F-FDG uptake (r = -0.38 vs. -0.31, both P < 0.00001). Conclusion: CXCR4-directed imaging of the arterial wall with ⁶⁸Ga-pentixafor PET/CT identified more lesions than ¹⁸F-FDG PET/CT, with only a weak correlation between tracers. Further studies to elucidate the underlying biologic mechanisms and sources of CXCR4 positivity, and to investigate the clinical utility of chemokine receptor-directed imaging of atherosclerosis, are highly warranted.

CXCR4-Directed Imaging in Solid Tumors

Despite histological evidence in various solid tumor entities, available experience with CXCR4-directed diagnostics and endoradiotherapy mainly focuses on hematologic diseases. With the goal of expanding the application of CXCR4 theranostics to solid tumors, we aimed to elucidate the feasibility of CXCR4-targeted imaging in a variety of such neoplasms. Methods: Nineteen patients with newly diagnosed, treatment-naïve solid tumors including pancreatic adenocarcinoma or neuroendocrine tumor, cholangiocarcinoma, hepatocellular carcinoma, renal cell carcinoma, ovarian cancer, and prostate cancer underwent [⁶⁸Ga]Pentixafor PET/CT. CXCR4-mediated uptake was assessed both visually and semi-quantitatively by evaluation of maximum standardized uptake values (SUV_max) of both primary tumors and metastases. With physiologic liver uptake as reference, tumor-to-background ratios (TBR) were calculated. [⁶⁸Ga]Pentixafor findings were further compared to immunohistochemistry and [¹⁸F]FDG PET/CT. Results: On [⁶⁸Ga]Pentixafor PET/CT, 10/19 (52.6%) primary tumors were visually detectable with a median SUV_max of 5.4 (range, 1.7-16.0) and a median TBR of 2.6 (range, 0.8-7.4), respectively. The highest level of radiotracer uptake was identified in a patient with cholangiocarcinoma (SUV_max, 16.0; TBR, 7.4). The relatively low uptake on [⁶⁸Ga]Pentixafor was also noted in metastases, exhibiting a median SUV_max of 4.5 (range, 2.3-8.8; TBR, 1.7; range, 1.0-4.1). A good correlation between uptake on [⁶⁸Ga]Pentixafor and histological derived CXCR4 expression was noted (R = 0.62, P < 0.05). In the 3 patients in whom [¹⁸F]FDG PET/CT was available, [⁶⁸Ga]Pentixafor exhibited lower uptake in all lesions. Conclusions: In this cohort of newly diagnosed, treatment-naïve patients with solid malignancies, CXCR4 expression as detected by [⁶⁸Ga]Pentixafor-PET/CT and immunohistochemistry was rather moderate. Thus, CXCR4-directed imaging may not play a major role in the management of solid tumors in the majority of patients.

Validation of [125I]CPCR4.3 as an investigative tool for the sensitive and specific detection of hCXCR4 and mCXCR4 expression in vitro and in vivo

BACKGROUND

The development and clinical translation of [68Ga] Pentixafor has substantially promoted the relevance of non-invasive PET imaging of CXCR4 expression in a broad spectrum of diseases, including cancer and inflammation. Its pronounced selectivity for the human receptor (hCXCR4), however, precludes the use of [68Ga] Pentixafor for imaging receptor expression and dynamics in CXCR4-related diseases in endogenous mouse models. To overcome this restriction, [125I]CPCR4.3, a structurally related pentapeptide ligand, has been evaluated as a preclinical tool for efficient in vitro and in vivo targeting of hCXCR4 and mCXCR4.

RESULTS

Compared to the reference [68Ga] Pentixafor, [125I]CPCR4.3 showed 2.4- to 11-fold increased specific binding to human cancer cell lines with different hCXCR4 expression levels (Jurkat, Daudi, HT-29, SH-5YSY, MCF-7, LNCaP) as well as strong and highly specific binding to mCXCR4 expressing cells (mCXCR4-transfected CHO cells, Eμ-myc 1080, 4 T1), which was not detectable for [68Ga]Pentixafor. This is the consequence of the equally high affinity of iodo-CPCR4 to hCXCR4 and mCXCR4 (IC50 = 5.4 ± 1.5 and 4.9 ± 1.7 nM, respectively) as opposed to [natGa] Pentixafor (hCXCR4: 42.4 ± 11.6 nM, mCXCR4: > 1000 nM). Additionally, [125I]CPCR4.3 showed enhanced tracer internalization (factor of 1.5-2 compared to the reference). In vivo biodistribution studies in immunocompetent Black Six and immunocompromised CD-1 nude mice showed predominant hepatobiliary excretion of [125I]CPCR4.3 (logP = 0.51), leading to high activity levels in liver and intestines. However, [125I]CPCR4.3 also showed high and specific accumulation in organs with endogenous mCXCR4 expression (spleen, lung, adrenals), even at low receptor expression levels.

CONCLUSIONS

Due to its excellent hCXCR4 and mCXCR4 targeting efficiency, both in vitro and in vivo, [125I]CPCR4.3 represents a sensitive and reliable tool for the species-independent quantification of CXCR4 expression. Its suboptimal clearance properties will certainly restrict its use for in vivo imaging applications using 123I (for SPECT) or 124I (for PET), but due to its high and specific accumulation in mCXCR4 expressing tissues, [125I]CPCR4.3 holds promise as a powerful preclinical tool for the investigation and quantification of CXCR4 involvement and kinetics in various murine disease models via, e.g., biodistribution and autoradiography studies.

Myocardial Inflammation Predicts Remodeling and Neuroinflammation After Myocardial Infarction

BACKGROUND

The local inflammatory tissue response after acute myocardial infarction (MI) determines subsequent healing. Systemic interaction may induce neuroinflammation as a precursor to neurodegeneration.

OBJECTIVES

This study sought to assess the influence of MI on cardiac and brain inflammation using noninvasive positron emission tomography (PET) of the heart-brain axis.

METHODS

After coronary artery ligation or sham surgery, mice (n = 49) underwent serial whole-body PET imaging of the mitochondrial translocator protein (TSPO) as a marker of activated macrophages and microglia. Patients after acute MI (n = 3) were also compared to healthy controls (n = 9).

RESULTS

Infarct mice exhibited elevated myocardial TSPO signal at 1 week versus sham (percent injected dose per gram: 8.0 ± 1.6 vs. 4.8 ± 0.9; p < 0.001), localized to activated CD68⁺ inflammatory cells in the infarct. Early TSPO signal predicted subsequent left ventricular remodeling at 8 weeks (r_partial = -0.687; p = 0.001). In parallel, brain TSPO signal was elevated at 1 week (1.7 ± 0.2 vs. 1.4 ± 0.2 for sham; p = 0.017), localized to activated microglia. After interval decline at 4 weeks, progressive heart failure precipitated a second wave of neuroinflammation (1.8 ± 0.2; p = 0.005). TSPO was concurrently up-regulated in remote cardiomyocytes at 8 weeks (8.8 ± 1.7, p < 0.001) without inflammatory cell infiltration, suggesting mitochondrial impairment. Angiotensin-converting enzyme inhibitor treatment lowered acute inflammation in the heart (p = 0.003) and brain (p = 0.06) and improved late cardiac function (p = 0.05). Patients also demonstrated elevation of cardiac TSPO signal in the infarct territory, paralleled by neuroinflammation versus controls.

CONCLUSIONS

The brain is susceptible to acute MI and chronic heart failure. Immune activation may interconnect heart and brain dysfunction, a finding that provides a foundation for strategies to improve heart and brain outcomes.

[Positron emission tomography/computed tomography in rheumatology]

Combined positron emission tomography/computed tomography (PET/CT) is a whole-body imaging procedure, which enables sensitive detection of inflammatory changes. It may be used to simultaneously obtain both precise anatomical and molecular information in order to comprehensively characterize diseases. The glucose analogue ¹⁸F-fluorodeoxyglucose (FDG) represents a universally applicable radiotracer for imaging of inflammatory processes. Its accumulation in tissues can be semiquantitatively characterized by use of standardized uptake values (SUV). In principle, a broad spectrum of infectious and non-infectious inflammatory and malignant diseases can be imaged. ¹⁸F-FDG PET/CT has become a valuable modality and is increasingly being used for evaluation of large vessel vasculitis and for evaluation of elevated systemic inflammatory markers without known cause. Beside the radiotracer ¹⁸F-FDG, other radiopharmaceuticals enable a non-invasive analysis of additional parameters of inflammatory disorders, such as other metabolic pathways or the expression of surface receptors.

Molecular Imaging in Ischemic Heart Disease

Purpose of Review

The purpose of this paper is to review current and new modalities to image key biological processes in ischemic heart disease and after myocardial infarction non-invasively.

Recent Findings

New imaging targets have been developed to detect and quantify myocardial damage after ischemia. Although positron emission tomography (PET) has been leading the development of new probes in the past, continuous improvements of magnetic resonance imaging (MRI) together with the development of new novel MRI contrast agents opens new research avenues including the combination of both PET and MRI to obtain anatomic, functional, and molecular information simultaneously, which is not possible from a single imaging session.

Summary

This review summarizes the state of art of non-invasive molecular imaging of the myocardium during ischemia and after myocardial infarction using PET and MRI. We also describe the different contrast agents that have been developed to image the different phases of cardiac healing and the biological processes associated with each of those phases. Importantly, here we focus on imaging of inflammation as it is the key biological process that orchestrates clearance of dead cells, tissue remodeling, cardiac repair, and future outcome. We also focus on clinical translation of some of the novel contrast agents that have been tested in patients and discuss the need for larger, multi-center patient studies to fully validate the applicability of new imaging probes.

Assessment of Copper Nanoclusters for Accurate in Vivo Tumor Imaging and Potential for Translation

Nanoparticles have been widely used for preclinical cancer imaging. However, their successful clinical translation is largely hampered by potential toxicity, unsatisfactory detection of malignancy at early stages, inaccurate diagnosis of tumor biomarkers, and histology for imaging-guided treatment. Herein, a targeted copper nanocluster (CuNC) is reported with high potential to address these challenges for future translation. Its ultrasmall structure enables efficient renal/bowel clearance, minimized off-target effects in nontargeted organs, and low nonspecific tumor retention. The pH-dependent in vivo dissolution of CuNCs affords minimal toxicity and potentially selective drug delivery to tumors. The intrinsic radiolabeling through the direct addition of ⁶⁴Cu to CuNC (⁶⁴Cu-CuNCs-FC131) synthesis offers high specific activity for sensitive and accurate detection of CXCR4 via FC131-directed targeting in novel triple negative breast cancer (TNBC) patient-derived xenograft mouse models and human TNBC tissues. In summary, this study not only reveals the potential of CXCR4-targeted ⁶⁴Cu-CuNCs for TNBC imaging in clinical settings, but also provides a useful strategy to design and assess the translational potential of nanoparticles for cancer theranostics.

[18F]Fluoroethyltriazolyl Monocyclam Derivatives as Imaging Probes for the Chemokine Receptor CXCR4

Determining chemokine receptor CXCR4 expression is significant in multiple diseases due to its role in promoting inflammation, cell migration and tumorigenesis. [⁶⁸Ga]Pentixafor is a promising ligand for imaging CXCR4 expression in multiple tumor types, but its utility is limited by the physical properties of ⁶⁸Ga. We screened a library of >200 fluorine-containing structural derivatives of AMD-3465 to identify promising candidates for in vivo imaging of CXCR4 expression by positron emission tomography (PET). Compounds containing fluoroethyltriazoles consistently achieved higher docking scores. Six of these higher scoring compounds were radiolabeled by click chemistry and evaluated in PC3-CXCR4 cells and BALB/c mice bearing bilateral PC3-WT and PC3-CXCR4 xenograft tumors. The apparent CXCR4 affinity of the ligands was relatively low, but tumor uptake was CXCR4-specific. The tumor uptake of [¹⁸F]RPS-534 (7.2 ± 0.3 %ID/g) and [¹⁸F]RPS-547 (3.1 ± 0.5 %ID/g) at 1 h p.i. was highest, leading to high tumor-to-blood, tumor-to-muscle, and tumor-to-lung ratios. Total cell-associated activity better predicted in vivo tumor uptake than did the docking score or apparent CXCR4 affinity. By this metric, and on the basis of their high yielding radiosynthesis, high tumor uptake, and good contrast to background, [¹⁸F]RPS-547, and especially [¹⁸F]RPS-534, are promising ¹⁸F-labeled candidates for imaging CXCR4 expression.

Molecular Imaging Visualizes Recruitment of Inflammatory Monocytes and Macrophages to the Injured Heart

RATIONALE

Paradigm shifting studies have revealed that the heart contains functionally diverse populations of macrophages derived from distinct embryonic and adult hematopoietic progenitors. Under steady-state conditions, the heart is largely populated by CCR2- (C-C chemokine receptor type 2) macrophages of embryonic descent. After tissue injury, a dramatic shift in macrophage composition occurs whereby CCR2+ monocytes are recruited to the heart and differentiate into inflammatory CCR2+ macrophages that contribute to heart failure progression. Currently, there are no techniques to noninvasively detect CCR2+ monocyte recruitment into the heart and thus identify patients who may be candidates for immunomodulatory therapy.

OBJECTIVE

To develop a noninvasive molecular imaging strategy with high sensitivity and specificity to visualize inflammatory monocyte and macrophage accumulation in the heart.

METHODS AND RESULTS

We synthesized and tested the performance of a positron emission tomography radiotracer (68Ga-DOTA [1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid]-ECL1i [extracellular loop 1 inverso]) that allosterically binds to CCR2. In naive mice, the radiotracer was quickly cleared from the blood and displayed minimal retention in major organs. In contrast, biodistribution and positron emission tomography demonstrated strong myocardial tracer uptake in 2 models of cardiac injury (diphtheria toxin induced cardiomyocyte ablation and reperfused myocardial infarction). 68Ga-DOTA-ECL1i signal localized to sites of tissue injury and was independent of blood pool activity as assessed by quantitative positron emission tomography and ex vivo autoradiography. 68Ga-DOTA-ECL1i uptake was associated with CCR2+ monocyte and CCR2+ macrophage infiltration into the heart and was abrogated in CCR2-/- mice, demonstrating target specificity. Autoradiography demonstrated that 68Ga-DOTA-ECL1i specifically binds human heart failure specimens and with signal intensity associated with CCR2+ macrophage abundance.

CONCLUSIONS

These findings demonstrate the sensitivity and specificity of 68Ga-DOTA-ECL1i in the mouse heart and highlight the translational potential of this agent to noninvasively visualize CCR2+ monocyte recruitment and inflammatory macrophage accumulation in patients.

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.