Sifting through the surfeit of neuroinflammation tracers

Alterations of the blood-brain barrier during aging

The blood-brain barrier (BBB) is a complex and dynamic interface that regulates the exchange of molecules and cells between the blood and the central nervous system. It undergoes structural and functional changes during aging, which may compromise its integrity and contribute to the pathogenesis of neurodegenerative diseases. In recent years, advances in microscopy and high-throughput bioinformatics have allowed a more in-depth investigation of the aging mechanisms of BBB. This review summarizes age-related alterations of the BBB structure and function from six perspectives: endothelial cells, astrocytes, pericytes, basement membrane, microglia and perivascular macrophages, and fibroblasts, ranging from the molecular level to the human multi-system level. These basic components are essential for the proper functioning of the BBB. Recent imaging methods of BBB were also reviewed. Elucidation of age-associated BBB changes may offer insights into BBB homeostasis and may provide effective therapeutic strategies to protect it during aging.

Western diet increases brain metabolism and adaptive immune responses in a mouse model of amyloidosis

Diet-induced increase in body weight is a growing health concern worldwide. Often accompanied by a low-grade metabolic inflammation that changes systemic functions, diet-induced alterations may contribute to neurodegenerative disorder progression as well. This study aims to non-invasively investigate diet-induced metabolic and inflammatory effects in the brain of an APPPS1 mouse model of Alzheimer's disease. [18F]FDG, [18F]FTHA, and [18F]GE-180 were used for in vivo PET imaging in wild-type and APPPS1 mice. Ex vivo flow cytometry and histology in brains complemented the in vivo findings. 1H- magnetic resonance spectroscopy in the liver, plasma metabolomics and flow cytometry of the white adipose tissue were used to confirm metaflammatory condition in the periphery. We found disrupted glucose and fatty acid metabolism after Western diet consumption, with only small regional changes in glial-dependent neuroinflammation in the brains of APPPS1 mice. Further ex vivo investigations revealed cytotoxic T cell involvement in the brains of Western diet-fed mice and a disrupted plasma metabolome. 1H-magentic resonance spectroscopy and immunological results revealed diet-dependent inflammatory-like misbalance in livers and fatty tissue. Our multimodal imaging study highlights the role of the brain-liver-fat axis and the adaptive immune system in the disruption of brain homeostasis in amyloid models of Alzheimer's disease.

Recent developments and challenges in positron emission tomography imaging of gliosis in chronic neuropathic pain

ABSTRACT

Understanding the mechanisms that underpin the transition from acute to chronic pain is critical for the development of more effective and targeted treatments. There is growing interest in the contribution of glial cells to this process, with cross-sectional preclinical studies demonstrating specific changes in these cell types capturing targeted timepoints from the acute phase and the chronic phase. In vivo longitudinal assessment of the development and evolution of these changes in experimental animals and humans has presented a significant challenge. Recent technological advances in preclinical and clinical positron emission tomography, including the development of specific radiotracers for gliosis, offer great promise for the field. These advances now permit tracking of glial changes over time and provide the ability to relate these changes to pain-relevant symptomology, comorbid psychiatric conditions, and treatment outcomes at both a group and an individual level. In this article, we summarize evidence for gliosis in the transition from acute to chronic pain and provide an overview of the specific radiotracers available to measure this process, highlighting their potential, particularly when combined with ex vivo/in vitro techniques, to understand the pathophysiology of chronic neuropathic pain. These complementary investigations can be used to bridge the existing gap in the field concerning the contribution of gliosis to neuropathic pain and identify potential targets for interventions.

Deciphering sources of PET signals in the tumor microenvironment of glioblastoma at cellular resolution

Various cellular sources hamper interpretation of positron emission tomography (PET) biomarkers in the tumor microenvironment (TME). We developed an approach of immunomagnetic cell sorting after in vivo radiotracer injection (scRadiotracing) with three-dimensional (3D) histology to dissect the cellular allocation of PET signals in the TME. In mice with implanted glioblastoma, translocator protein (TSPO) radiotracer uptake per tumor cell was higher compared to tumor-associated microglia/macrophages (TAMs), validated by protein levels. Translation of in vitro scRadiotracing to patients with glioma immediately after tumor resection confirmed higher single-cell TSPO tracer uptake of tumor cells compared to immune cells. Across species, cellular radiotracer uptake explained the heterogeneity of individual TSPO-PET signals. In consideration of cellular tracer uptake and cell type abundance, tumor cells were the main contributor to TSPO enrichment in glioblastoma; however, proteomics identified potential PET targets highly specific for TAMs. Combining cellular tracer uptake measures with 3D histology facilitates precise allocation of PET signals and serves to validate emerging novel TAM-specific radioligands.

Neuroinflammation in the Evolution of Motor Function in Stroke and Trauma Patients: Treatment and Potential Biomarkers

Neuroinflammation has a significant impact on different pathologies, such as stroke or spinal cord injury, intervening in their pathophysiology: expansion, progression, and resolution. Neuroinflammation involves oxidative stress, damage, and cell death, playing an important role in neuroplasticity and motor dysfunction by affecting the neuronal connection responsible for motor control. The diagnosis of this pathology is performed using neuroimaging techniques and molecular diagnostics based on identifying and measuring signaling molecules or specific markers. In parallel, new therapeutic targets are being investigated via the use of bionanomaterials and electrostimulation to modulate the neuroinflammatory response. These novel diagnostic and therapeutic strategies have the potential to facilitate the development of anticipatory patterns and deliver the most beneficial treatment to improve patients' quality of life and directly impact their motor skills. However, important challenges remain to be solved. Hence, the goal of this study was to review the implication of neuroinflammation in the evolution of motor function in stroke and trauma patients, with a particular focus on novel methods and potential biomarkers to aid clinicians in diagnosis, treatment, and therapy. A specific analysis of the strengths, weaknesses, threats, and opportunities was conducted, highlighting the key challenges to be faced in the coming years.

Individual regional associations between Aβ-, tau- and neurodegeneration (ATN) with microglial activation in patients with primary and secondary tauopathies

β-amyloid (Aβ) and tau aggregation as well as neuronal injury and atrophy (ATN) are the major hallmarks of Alzheimer's disease (AD), and biomarkers for these hallmarks have been linked to neuroinflammation. However, the detailed regional associations of these biomarkers with microglial activation in individual patients remain to be elucidated. We investigated a cohort of 55 patients with AD and primary tauopathies and 10 healthy controls that underwent TSPO-, Aβ-, tau-, and perfusion-surrogate-PET, as well as structural MRI. Z-score deviations for 246 brain regions were calculated and biomarker contributions of Aβ (A), tau (T), perfusion (N1), and gray matter atrophy (N2) to microglial activation (TSPO, I) were calculated for each individual subject. Individual ATN-related microglial activation was correlated with clinical performance and CSF soluble TREM2 (sTREM2) concentrations. In typical and atypical AD, regional tau was stronger and more frequently associated with microglial activation when compared to regional Aβ (AD: βT = 0.412 ± 0.196 vs. βA = 0.142 ± 0.123, p < 0.001; AD-CBS: βT = 0.385 ± 0.176 vs. βA = 0.131 ± 0.186, p = 0.031). The strong association between regional tau and microglia reproduced well in primary tauopathies (βT = 0.418 ± 0.154). Stronger individual associations between tau and microglial activation were associated with poorer clinical performance. In patients with 4RT, sTREM2 levels showed a positive association with tau-related microglial activation. Tau pathology has strong regional associations with microglial activation in primary and secondary tauopathies. Tau and Aβ related microglial response indices may serve as a two-dimensional in vivo assessment of neuroinflammation in neurodegenerative diseases.

Visualization of Thromboinflammation by 18F-DPA-714 PET in a Stroke Patient

ABSTRACT

A 44-year-old man who presented with progressive right limb weakness was diagnosed with ischemic stroke. He was referred for 18F-DPA-714 PET/CT for evaluation of the disease. 18F-DPA-714 PET/CT showed increased uptake of the intracranial thrombus. This DPA-714-avid thrombus highly suggested the involvement of immune cells in the extension of the clot resulting in neurological deterioration. This present case suggested that 18F-DPA-714 PET might be a promising tracer in visualizing thromboinflammation in vivo.

TSPO PET brain inflammation imaging: A transdiagnostic systematic review and meta-analysis of 156 case-control studies

INTRODUCTION

The 18-kDa translocator protein (TSPO) is increasingly recognized as a molecular target for PET imaging of inflammatory responses in various central nervous system (CNS) disorders. However, the reported sensitivity and specificity of TSPO PET to identify brain inflammatory processes appears to vary greatly across disorders, disease stages, and applied quantification methods. To advance TSPO PET as a potential biomarker to evaluate brain inflammation and anti-inflammatory therapies, a better understanding of its applicability across disorders is needed. We conducted a transdiagnostic systematic review and meta-analysis of all in vivo human TSPO PET imaging case-control studies in the CNS. Specifically, we investigated the direction, strength, and heterogeneity associated with the TSPO PET signal across disorders in pre-specified brain regions, and explored the demographic and methodological sources of heterogeneity.

METHODS

We searched for English peer-reviewed articles that reported in vivo human case-control TSPO PET differences. We extracted the demographic details, TSPO PET outcomes, and technical variables of the PET procedure. A random-effects meta-analysis was applied to estimate case-control standardized mean differences (SMD) of the TSPO PET signal in the lobar/whole-brain cortical grey matter (cGM), thalamus, and cortico-limbic circuitry between different illness categories. Heterogeneity was evaluated with the I2 statistic and explored using subgroup and meta-regression analyses for radioligand generation, PET quantification method, age, sex, and publication year. Significance was set at the False Discovery Rate (FDR)-corrected P < 0.05.

RESULTS

156 individual case-control studies were included in the systematic review, incorporating data for 2381 healthy controls and 2626 patients. 139 studies documented meta-analysable data and were grouped into 11 illness categories. Across all the illness categories, we observed a significantly higher TSPO PET signal in cases compared to controls for the cGM (n = 121 studies, SMD = 0.358, PFDR < 0.001, I2 = 68%), with a significant difference between the illness categories (P = 0.004). cGM increases were only significant for Alzheimer's disease (SMD = 0.693, PFDR < 0.001, I2 = 64%) and other neurodegenerative disorders (SMD = 0.929, PFDR < 0.001, I2 = 73%). Cortico-limbic increases (n = 97 studies, SMD = 0.541, P < 0.001, I2 = 67%) were most prominent for Alzheimer's disease, mild cognitive impairment, other neurodegenerative disorders, mood disorders and multiple sclerosis. Thalamic involvement (n = 79 studies, SMD = 0.393, P < 0.001, I2 = 71%) was observed for Alzheimer's disease, other neurodegenerative disorders, multiple sclerosis, and chronic pain and functional disorders (all PFDR < 0.05). Main outcomes for systemic immunological disorders, viral infections, substance use disorders, schizophrenia and traumatic brain injury were not significant. We identified multiple sources of between-study variance to the TSPO PET signal including a strong transdiagnostic effect of the quantification method (explaining 25% of between-study variance; VT-based SMD = 0.000 versus reference tissue-based studies SMD = 0.630; F = 20.49, df = 1;103, P < 0.001), patient age (9% of variance), and radioligand generation (5% of variance).

CONCLUSION

This study is the first overarching transdiagnostic meta-analysis of case-control TSPO PET findings in humans across several brain regions. We observed robust increases in the TSPO signal for specific types of disorders, which were widespread or focal depending on illness category. We also found a large and transdiagnostic horizontal (positive) shift of the effect estimates of reference tissue-based compared to VT-based studies. Our results can support future studies to optimize experimental design and power calculations, by taking into account the type of disorder, brain region-of-interest, radioligand, and quantification method.

Spirocyclic Iodonium Ylides for Fluorine-18 Radiolabeling of Non-Activated Arenes: From Concept to Clinical Research

Positron emission tomography (PET) is a powerful imaging tool for drug discovery, clinical diagnosis, and monitoring of disease progression. Fluorine-18 is the most common radionuclide used for PET, but advances in radiotracer development have been limited by the historical lack of methodologies and precursors amenable to radiolabeling with fluorine-18. Radiolabeling of electron-rich (hetero)aromatic rings remains a long-standing challenge in the production of PET radiopharmaceuticals. In this personal account, we discuss the history of spirocyclic iodonium ylide precursors, from inception to applications in clinical research, for the incorporation of fluorine-18 into complex non-activated (hetero)aromatic rings.

Concussion: Beyond the Cascade

Sport concussion affects millions of athletes each year at all levels of sport. Increasing evidence demonstrates clinical and physiological recovery are becoming more divergent definitions, as evidenced by several studies examining blood-based biomarkers of inflammation and imaging studies of the central nervous system (CNS). Recent studies have shown elevated microglial activation in the CNS in active and retired American football players, as well as in active collegiate athletes who were diagnosed with a concussion and returned to sport. These data are supportive of discordance in clinical symptomology and the inflammatory response in the CNS upon symptom resolution. In this review, we will summarize recent advances in the understanding of the inflammatory response associated with sport concussion and broader mild traumatic brain injury, as well as provide an outlook for important research questions to better align clinical and physiological recovery.

Pharmacological modulation of TSPO in microglia/macrophages and neurons in a chronic neurodegenerative model of prion disease

Neuroinflammation is an important component of many neurodegenerative diseases, whether as a primary cause or a secondary outcome. For that reason, either as diagnostic tools or to monitor progression and/or pharmacological interventions, there is a need for robust biomarkers of neuroinflammation in the brain. Mitochondrial TSPO (18 kDa Translocator protein) is one of few available biomarkers of neuroinflammation for which there are clinically available PET imaging agents. In this study, we further characterised neuroinflammation in a mouse model of prion-induced chronic neurodegeneration (ME7) including a pharmacological intervention via a CSF1R inhibitor. This was achieved by autoradiographic binding of the second-generation TSPO tracer, [3H]PBR28, along with a more comprehensive examination of the cellular contributors to the TSPO signal changes by immunohistochemistry. We observed regional increases of TSPO in the ME7 mouse brains, particularly in the hippocampus, cortex and thalamus. This increased TSPO signal was detected in the cells of microglia/macrophage lineage as well as in astrocytes, endothelial cells and neurons. Importantly, we show that the selective CSF1R inhibitor, JNJ-40346527 (JNJ527), attenuated the disease-dependent increase in TSPO signal, particularly in the dentate gyrus of the hippocampus, where JNJ527 attenuated the number of Iba1+ microglia and neurons, but not GFAP+ astrocytes or endothelial cells. These findings suggest that [3H]PBR28 quantitative autoradiography in combination with immunohistochemistry are important translational tools for detecting and quantifying neuroinflammation, and its treatments, in neurodegenerative disease. Furthermore, we demonstrate that although TSPO overexpression in the ME7 brains was driven by various cell types, the therapeutic effect of the CSF1R inhibitor was primarily to modulate TSPO expression in microglia and neurons, which identifies an important route of biological action of this particular CSF1R inhibitor and provides an example of a cell-specific effect of this type of therapeutic agent on the neuroinflammatory process.

Quantitative assessment of translocator protein (TSPO) in the non-human primate brain and clinical translation of [18F]LW223 as a TSPO-targeted PET radioligand

OBJECTIVES

Translocator protein 18 kDa (TSPO) positron emission tomography (PET) can be harnessed for the non-invasive detection of macrophage-driven inflammation. [¹⁸F]LW223, a newly reported TSPO PET tracer which was insensitive to rs6971 polymorphism, showed favorable performance characteristics in a recent imaging study involving a rat myocardial infarction model. To enable quantitative neuroimaging with [¹⁸F]LW223, we conducted kinetic analysis in the non-human primate (NHP) brain. Further, we sought to assess the utility of [¹⁸F]LW223-based TSPO imaging in a first-in-human study.

METHODS

Radiosynthesis of [¹⁸F]LW223 was accomplished on an automated module, whereas molar activities, stability in formulation, lipophilicity and unbound free fraction (f_u) of the probe were measured. Brain penetration and target specificity of [¹⁸F]LW223 in NHPs were corroborated by PET-MR imaging under baseline and pre-blocking conditions using the validated TSPO inhibitor, (R)-PK11195, at doses ranging from 5 to 10 mg/kg. Kinetic modeling was performed using one-tissue compartment model (1TCM), two-tissue compartment model (2TCM) and Logan graphical analyses, using dynamic PET data acquisition, arterial blood collection and metabolic stability testing. Clinical PET scans were performed in two healthy volunteers (HVs). Regional brain standard uptake value ratio (SUVr) was assessed for different time intervals.

RESULTS

[¹⁸F]LW223 was synthesized in non-decay corrected radiochemical yields (n.d.c. RCYs) of 33.3 ± 6.5% with molar activities ranging from 1.8 ± 0.7 Ci/µmol (n = 11). [¹⁸F]LW223 was stable in formulation for up to 4 h and LogD_7.4 of 2.31 ± 0.13 (n = 6) and f_u of 5.80 ± 1.42% (n = 6) were determined. [¹⁸F]LW223 exhibited good brain penetration in NHPs, with a peak SUV value of ca. 1.79 in the whole brain. Pre-treatment with (R)-PK11195 substantially accelerated the washout and attenuated the area under the time-activity curve, indicating in vivo specificity of [¹⁸F]LW223 towards TSPO. Kinetic modeling demonstrated that 2TCM was the most suitable model for [¹⁸F]LW223-based neuroimaging. Global transfer rate constants (K₁) and total volumes of distribution (V_T) were found to be 0.10 ± 0.01 mL/cm³/min and 2.30 ± 0.17 mL/cm³, respectively. Dynamic PET data analyses across distinct time windows revealed that the V_T values were relatively stable after 60 min post-injection. In a preliminary clinical study with two healthy volunteers, [¹⁸F]LW223 exhibited good brain uptake and considerable tracer retention across all analyzed brain regions. Of note, an excellent correlation between SUVr with V_T was obtained when assessing the time interval from 20 to 40 min post tracer injection (SUVr_(20-40 min), R² = 0.94, p < 0.0001), suggesting this time window may be suitable to estimate specific binding to TSPO in human brain.

CONCLUSION

Our findings indicate that [¹⁸F]LW223 is suitable for quantitative TSPO-targeted PET imaging in higher species. Employing state-of-the-art kinetic modeling, we found that [¹⁸F]LW223 was effective in mapping TSPO throughout the NHP brain, with best model fits obtained from 2TCM and Logan graphical analyses. Overall, our results indicate that [¹⁸F]LW223 exhibits favorable tracer performance characteristics in higher species, and this novel imaging tool may hold promise to provide effective neuroinflammation imaging in patients with neurological disease.

The Sensitivity of Tau Tracers for the Discrimination of Alzheimer's Disease Patients and Healthy Controls by PET

Hyperphosphorylated tau aggregates, also known as neurofibrillary tangles, are a hallmark neuropathological feature of Alzheimer's disease (AD). Molecular imaging of tau by positron emission tomography (PET) began with the development of [18F]FDDNP, an amyloid β tracer with off-target binding to tau, which obtained regional specificity through the differing distributions of amyloid β and tau in AD brains. A concerted search for more selective and affine tau PET tracers yielded compounds belonging to at least eight structural categories; 18F-flortaucipir, known variously as [18F]-T807, AV-1451, and Tauvid®, emerged as the first tau tracer approved by the American Food and Drug Administration. The various tau tracers differ concerning their selectivity over amyloid β, off-target binding at sites such as monoamine oxidase and neuromelanin, and degree of uptake in white matter. While there have been many reviews of molecular imaging of tau in AD and other conditions, there has been no systematic comparison of the fitness of the various tracers for discriminating between AD patient and healthy control (HC) groups. In this narrative review, we endeavored to compare the binding properties of the various tau tracers in vitro and the effect size (Cohen's d) for the contrast by PET between AD patients and age-matched HC groups. The available tracers all gave good discrimination, with Cohen's d generally in the range of two-three in culprit brain regions. Overall, Cohen's d was higher for AD patient groups with more severe illness. Second-generation tracers, while superior concerning off-target binding, do not have conspicuously higher sensitivity for the discrimination of AD and HC groups. We suppose that available pharmacophores may have converged on a maximal affinity for tau fibrils, which may limit the specific signal imparted in PET studies.

Beyond monoamines: I. Novel targets and radiotracers for Positron emission tomography imaging in psychiatric disorders

With the emergence of positron emission tomography (PET) in the late 1970s, psychiatry had access to a tool capable of non-invasive assessment of human brain function. Early applications in psychiatry focused on identifying characteristic brain blood flow and metabolic derangements using radiotracers such as [¹⁵ O]H₂ O and [¹⁸ F]FDG. Despite the success of these techniques, it became apparent that more specific probes were needed to understand the neurochemical bases of psychiatric disorders. The first neurochemical PET imaging probes targeted sites of action of neuroleptic (dopamine D₂ receptors) and psychoactive (serotonin receptors) drugs. Based on the centrality of monoamine dysfunction in psychiatric disorders and the measured success of monoamine-enhancing drugs in treating them, the next 30 years witnessed the development of an armamentarium of PET radiopharmaceuticals and imaging methodologies for studying monoamines. Continued development of monoamine-enhancing drugs over this time however was less successful, realizing only modest gains in efficacy and tolerability. As patent protection for many widely prescribed and profitable psychiatric drugs lapsed, drug development pipelines shifted away from monoamines in search of novel targets with the promises of improved efficacy, or abandoned altogether. Over this period, PET radiopharmaceutical development activities closely paralleled drug development priorities resulting in the development of new PET imaging agents for non-monoamine targets. Part one of this review will briefly survey novel PET imaging targets with relevance to the field of psychiatry, which include the metabotropic glutamate receptor type 5 (mGluR5), purinergic P₂ X₇ receptor, type 1 cannabinoid receptor (CB₁ ), phosphodiesterase 10A (PDE10A), and describe radiotracers developed for these and other targets that have matured to human subject investigations. Current limitations of the targets and techniques will also be discussed.

Molecular Imaging Studies of Alcohol Use Disorder

Alcohol use disorder (AUD) is a serious public health problem in many countries, bringing a gamut of health risks and impairments to individuals and a great burden to society. Despite the prevalence of a disease model of AUD, the current pharmacopeia does not present reliable treatments for AUD; approved treatments are confined to a narrow spectrum of medications engaging inhibitory γ-aminobutyric acid (GABA) neurotransmission and possibly excitatory N-methyl-D-aspartate (NMDA) receptors, and opioid receptor antagonists. Molecular imaging with positron emission tomography (PET) and single-photon emission computed tomography (SPECT) can open a window into the living brain and has provided diverse insights into the pathology of AUD. In this narrative review, we summarize the state of molecular imaging findings on the pharmacological action of ethanol and the neuropathological changes associated with AUD. Laboratory and preclinical imaging results highlight the interactions between ethanol and GABA A-type receptors (GABAAR), but the interpretation of such results is complicated by subtype specificity. An abundance of studies with the glucose metabolism tracer fluorodeoxyglucose (FDG) concur in showing cerebral hypometabolism after ethanol challenge, but there is relatively little data on long-term changes in AUD. Alcohol toxicity evokes neuroinflammation, which can be tracked using PET with ligands for the microglial marker translocator protein (TSPO). Several PET studies show reversible increases in TSPO binding in AUD individuals, and preclinical results suggest that opioid-antagonists can rescue from these inflammatory responses. There are numerous PET/SPECT studies showing changes in dopaminergic markers, generally consistent with an impairment in dopamine synthesis and release among AUD patients, as seen in a number of other addictions; this may reflect the composite of an underlying deficiency in reward mechanisms that predisposes to AUD, in conjunction with acquired alterations in dopamine signaling. There is little evidence for altered serotonin markers in AUD, but studies with opioid receptor ligands suggest a specific up-regulation of the μ-opioid receptor subtype. Considerable heterogeneity in drinking patterns, gender differences, and the variable contributions of genetics and pre-existing vulnerability traits present great challenges for charting the landscape of molecular imaging in AUD.

Identification and Prioritization of PET Neuroimaging Targets for Microglial Phenotypes Associated with Microglial Activity in Alzheimer's Disease

Activation of microglial cells accompanies the progression of many neurodegenerative disorders, including Alzheimer's disease (AD). Development of molecular imaging tools specific to microglia can help elucidate the mechanism through which microglia contribute to the pathogenesis and progression of neurodegenerative disorders. Through analysis of published genetic, transcriptomic, and proteomic data sets, we identified 19 genes with microglia-specific expression that we then ranked based on association with the AD characteristics, change in expression, and potential druggability of the target. We believe that the process we used to identify and rank microglia-specific genes is broadly applicable to the identification and evaluation of targets in other disease areas and for applications beyond molecular imaging.

Brain sex-dependent alterations after prolonged high fat diet exposure in mice

We examined effects of exposing female and male mice for 33 weeks to 45% or 60% high fat diet (HFD). Males fed with either diet were more vulnerable than females, displaying higher and faster increase in body weight and more elevated cholesterol and liver enzymes levels. Higher glucose metabolism was revealed by PET in the olfactory bulbs of both sexes. However, males also displayed altered anterior cortex and cerebellum metabolism, accompanied by a more prominent brain inflammation relative to females. Although both sexes displayed reduced transcripts of neuronal and synaptic genes in anterior cortex, only males had decreased protein levels of AMPA and NMDA receptors. Oppositely, to anterior cortex, cerebellum of HFD-exposed mice displayed hypometabolism and transcriptional up-regulation of neuronal and synaptic genes. These results indicate that male brain is more susceptible to metabolic changes induced by HFD and that the anterior cortex versus cerebellum display inverse susceptibility to HFD.

Assessing organ-level immunoreactivity in a rat model of sepsis using TSPO PET imaging

There is current need for new approaches to assess/measure organ-level immunoreactivity and ensuing dysfunction in systemic inflammatory response syndrome (SIRS) and sepsis, in order to protect or recover organ function. Using a rat model of systemic sterile inflammatory shock (intravenous LPS administration), we performed PET imaging with a translocator protein (TSPO) tracer, [¹⁸F]DPA-714, as a biomarker for reactive immunoreactive changes in the brain and peripheral organs. In vivo dynamic PET/CT scans showed increased [¹⁸F]DPA-714 binding in the brain, lungs, liver and bone marrow, 4 hours after LPS injection. Post-LPS mean standard uptake values (SUV_mean) at equilibrium were significantly higher in those organs compared to baseline. Changes in spleen [¹⁸F]DPA-714 binding were variable but generally decreased after LPS. SUV_mean values in all organs, except the spleen, positively correlated with several serum cytokines/chemokines. In vitro measures of TSPO expression and immunofluorescent staining validated the imaging results. Noninvasive molecular imaging with [¹⁸F]DPA-714 PET in a rat model of systemic sterile inflammatory shock, along with in vitro measures of TSPO expression, showed brain, liver and lung inflammation, spleen monocytic efflux/lymphocytic activation and suggested increased bone marrow hematopoiesis. TSPO PET imaging can potentially be used to quantify SIRS and sepsis-associated organ-level immunoreactivity and assess the effectiveness of therapeutic and preventative approaches for associated organ failures, in vivo.

Meta-analysis of molecular imaging of translocator protein in major depression

Molecular neuroimaging studies provide mounting evidence that neuroinflammation plays a contributory role in the pathogenesis of major depressive disorder (MDD). This has been the focus of a number of positron emission tomography (PET) studies of the 17-kDa translocator protein (TSPO), which is expressed by microglia and serves as a marker of neuroinflammation. In this meta-analysis, we compiled and analyzed all available molecular imaging studies comparing cerebral TSPO binding in MDD patients with healthy controls. Our systematic literature search yielded eight PET studies encompassing 238 MDD patients and 164 healthy subjects. The meta-analysis revealed relatively increased TSPO binding in several cortical regions (anterior cingulate cortex: Hedges’ g = 0.6, 95% CI: 0.36, 0.84; hippocampus: g = 0.54, 95% CI: 0.26, 0.81; insula: g = 0.43, 95% CI: 0.17, 0.69; prefrontal cortex: g = 0.36, 95% CI: 0.14, 0.59; temporal cortex: g = 0.39, 95% CI: –0.04, 0.81). While the high range of effect size in the temporal cortex might reflect group-differences in body mass index (BMI), exploratory analyses failed to reveal any relationship between elevated TSPO availability in the other four brain regions and depression severity, age, BMI, radioligand, or the binding endpoint used, or with treatment status at the time of scanning. Taken together, this meta-analysis indicates a widespread ∼18% increase of TSPO availability in the brain of MDD patients, with effect sizes comparable to those in earlier molecular imaging studies of serotonin transporter availability and monoamine oxidase A binding.

The role of PET/CT in large vessel vasculitis and related disorders: diagnosis, extent evaluation and assessment of therapy response

Large vessel vasculitides (LVV) are defined as chronic inflammatory disorders that affect the arteries with two major variants being distinguished: giant cell arteritis (GCA) and Takayasu's arteritis (TAK). These often present with nonspecific constitutional symptoms which makes an accurate diagnosis often challenging. Nevertheless, timely diagnosis is of utmost importance to initiate treatment and to avoid potential life-threatening complications. [¹⁸F]FDG-PET/CT is nowadays widely accepted as useful tool to aid in the diagnosis of large vessel vasculitis. However, its role to monitor disease activity and to predict disease relapse during follow-up is less obvious since vascular [¹⁸F]FDG uptake can be detected in the absence of clinical or biochemical signs of disease activity. In addition to the two major variants, [¹⁸F]FDG-PET/CT has shown promise in (peri-)aortitis and related disorders. This article aims to provide an update on the current knowledge and limitations of [¹⁸F]FDG-PET/CT for the diagnosis and assessment of treatment response in LVV. Furthermore, other radiopharmaceuticals targeting key components of the vascular immune system are being discussed which could provide an interesting alternative to image vascular inflammation in LVV.

Neuroinflammation, Stroke, Blood-Brain Barrier Dysfunction, and Imaging Modalities

Maintaining blood-brain barrier (BBB) integrity is crucial for the homeostasis of the central nervous system. Structurally comprising the BBB, brain endothelial cells interact with pericytes, astrocytes, neurons, microglia, and perivascular macrophages in the neurovascular unit. Brain ischemia unleashes a profound neuroinflammatory response to remove the damaged tissue and prepare the brain for repair. However, the intense neuroinflammation occurring during the acute phase of stroke is associated with BBB breakdown, neuronal injury, and worse neurological outcomes. Here, we critically discuss the role of neuroinflammation in ischemic stroke pathology, focusing on the BBB and the interactions between central nervous system and peripheral immune responses. We highlight inflammation-driven injury mechanisms in stroke, including oxidative stress, increased MMP (matrix metalloproteinase) production, microglial activation, and infiltration of peripheral immune cells into the ischemic tissue. We provide an updated overview of imaging techniques for in vivo detection of BBB permeability, leukocyte infiltration, microglial activation, and upregulation of cell adhesion molecules following ischemic brain injury. We discuss the possibility of clinical implementation of imaging modalities to assess stroke-associated neuroinflammation with the potential to provide image-guided diagnosis and treatment. We summarize the results from several clinical studies evaluating the efficacy of anti-inflammatory interventions in stroke. Although convincing preclinical evidence suggests that neuroinflammation is a promising target for ischemic stroke, thus far, translating these results into the clinical setting has proved difficult. Due to the dual role of inflammation in the progression of ischemic damage, more research is needed to mechanistically understand when the neuroinflammatory response begins the transition from injury to repair. This could have important implications for ischemic stroke treatment by informing time- and context-specific therapeutic interventions.

New Horizons-Cognitive Dysfunction Associated With Type 2 Diabetes

The prevalence of type 2 diabetes (T2D) and cognitive dysfunction increases with age. As society ages, clinicians will be increasingly tasked with managing older people who have both T2D and cognitive dysfunction. T2D is associated with an increased risk of cognitive dysfunction and hence there is increasing interest in whether T2D is a causal factor in the pathogenesis of cognitive decline and dementia. Recent advances in the use of sensitive measures of in vivo brain dysfunction in life-course studies can help understand potential mechanistic pathways and also help guide recommendations for clinical practice. In this article we will describe new horizons in the understanding of cognitive dysfunction associated with T2D. Coming from a clinical perspective, we discuss potential mechanisms and pathways linking the 2 conditions and the contribution of multimodal neuroimaging and study designs to advancing understanding in the field. We also highlight the important issues on the horizon that will need addressing in clinical identification, management, and risk reduction for people with coexistent T2D and cognitive dysfunction.

Characterization of neuroinflammatory positron emission tomography biomarkers in chronic traumatic encephalopathy

Chronic traumatic encephalopathy is a neurological disorder associated with head trauma and is confirmed upon autopsy. PET imaging of chronic traumatic encephalopathy may provide a means to move towards ante-mortem diagnosis and therapeutic intervention following brain injuries. Characterization of the neuroinflammatory PET biomarkers, 18 kDa translocator protein and monoamine oxidase-B was conducted using [3H]PBR-28 and [3H]L-deprenyl, respectively, in post-mortem chronic traumatic encephalopathy brain tissue. [3H]PBR-28 displayed high specific binding in both chronic traumatic encephalopathy (95.40 ± 1.87%; n = 11 cases) and healthy controls (89.89 ± 8.52%, n = 3 cases). Cell-type expression of the 18 kDa translocator protein was confirmed by immunofluorescence to microglia, astrocyte and macrophage markers. [3H]L-deprenyl also displayed high specific binding in chronic traumatic encephalopathy (96.95 ± 1.43%; n = 12 cases) and healthy controls (93.24 ± 0.43%; n = 2 cases), with the distribution co-localized to astrocytes by immunofluorescence. Saturation analysis was performed to quantify the target density of the 18 kDa translocator protein and monoamine oxidase-B in both chronic traumatic encephalopathy and healthy control tissue. Using [3H]PBR-28, the target density of the 18 kDa translocator protein in healthy controls was 177.91 ± 56.96 nM (n = 7 cases; mean ± standard deviation); however, a highly variable target density (345.84 ± 372.42 nM; n = 11 cases; mean ± standard deviation) was measured in chronic traumatic encephalopathy. [3H]L-deprenyl quantified a monoamine oxidase-B target density of 304.23 ± 115.93 nM (n = 8 cases; mean ± standard deviation) in healthy control tissue and is similar to the target density in chronic traumatic encephalopathy tissues (365.80 ± 128.55 nM; n = 12 cases; mean ± standard deviation). A two-sample t-test determined no significant difference in the target density values of the 18 kDa translocator protein and monoamine oxidase-B between healthy controls and chronic traumatic encephalopathy (P > 0.05), albeit a trend towards increased expression of both targets was observed in chronic traumatic encephalopathy. To our knowledge, this work represents the first in vitro characterization of 18 kDa translocator protein and monoamine oxidase-B in chronic traumatic encephalopathy and reveals the variability in neuroinflammatory pathology following brain injuries. These preliminary findings will be considered when designing PET imaging studies after brain injury and for the ultimate goal of imaging chronic traumatic encephalopathy in vivo.

Neuroinflammation PET imaging of the translocator protein (TSPO) in Alzheimer's disease: an update

Neuroinflammation is a significant contributor to Alzheimer's disease (AD). Until now, PET imaging of the translocator protein (TSPO) has been widely used to depict the neuroimmune endophenotype of AD. The aim of this review was to provide an update to the results from 2018 and to advance the characterization of the biological basis of TSPO imaging in AD by re-examining TSPO function and expression and the methodological aspects of interest. Although the biological basis of the TSPO PET signal is obviously related to microglia and astrocytes in AD, the observed process remains uncertain and might not be directly related to neuroinflammation. Further studies are required to re-examine the cellular significance underlying a variation in the PET signal in AD and how it can be impacted by a disease-modifying treatment.

A single dose of ketamine cannot prevent protracted stress-induced anhedonia and neuroinflammation in rats

Worldwide, millions of people suffer from treatment-resistant depression. Ketamine, a glutamatergic receptor antagonist, can have a rapid antidepressant effect even in treatment-resistant patients. A proposed mechanism for the antidepressant effect of ketamine is the reduction of neuroinflammation. To further explore this hypothesis, we investigated whether a single dose of ketamine can modulate protracted neuroinflammation in a repeated social defeat (RSD) stress rat model, which resembles features of depression. To this end, male animals exposed to RSD were injected with ketamine (20 mg/kg) or vehicle. A combination of behavioral analyses and PET scans of the inflammatory marker TSPO in the brain were performed. Rats submitted to RSD showed anhedonia-like behavior in the sucrose preference test, decreased weight gain, and increased TSPO levels in the insular and entorhinal cortices, as observed by [¹¹C]-PK11195 PET. Whole brain TSPO levels correlated with corticosterone levels in several brain regions of RSD exposed animals, but not in controls. Ketamine injection 1 day after RSD disrupted the correlation between TSPO levels and serum corticosterone levels, but had no effect on depressive-like symptoms, weight gain or the protracted RSD-induced increase in TSPO expression in male rats. These results suggest that ketamine does not exert its effect on the hypothalamic-pituitary-adrenal axis by modulation of neuroinflammation.

In Vivo Cerebral Translocator Protein (TSPO) Binding and Its Relationship with Blood Adiponectin Levels in Treatment-Naïve Young Adults with Major Depression: A [11C]PK11195 PET Study

Adiponectin is an adipokine that mediates cellular cholesterol efflux and plays important roles in neuroinflammatory processes. In this study, we undertook positron emission tomography (PET) with the translocator protein (TSPO) ligand [¹¹C]PK11195 and measured serum adiponectin levels in groups of treatment-naïve young adult patients with major depressive disorder (MDD) and matched healthy controls. Thirty treatment-naïve MDD patients (median age: 24 years) and twenty-three healthy controls underwent [¹¹C]PK11195 PET. We quantified TSPO availability in brain as the [¹¹C]PK11195 binding potential (BP_ND) using a reference tissue model in conjunction with the supervised cluster analysis (SVCA4) algorithm. Age, sex distribution, body mass index, and serum adiponectin levels did not differ between the groups. Between-group analysis using a region-of-interest approach showed significantly higher [¹¹C]PK11195 BP_ND in the left anterior and right posterior cingulate cortices in MDD patients than in controls. Serum adiponectin levels had significant negative correlations with [¹¹C]PK11195 BP_ND in the bilateral hippocampus in MDD patients, but significant positive correlations in the bilateral hippocampus in the control group. Our results indicate significantly higher TSPO binding in the anterior and posterior cingulate cortices in treatment-naïve young MDD patients, suggesting microglial activation in these limbic regions, which are involved in cognitive and emotional processing. The opposite correlations between [¹¹C]PK11195 BP_ND in the hippocampus with serum adiponectin levels in MDD and control groups suggest that microglial activation in the hippocampus may respond differentially to adiponectin signaling in MDD and healthy subjects, possibly with respect to microglial phenotype.

Mitochondrial Translocator Protein (TSPO) Expression in the Brain After Whole Body Gamma Irradiation

The brain’s early response to low dose ionizing radiation, as may be encountered during diagnostic procedures and space exploration, is not yet fully characterized. In the brain parenchyma, the mitochondrial translocator protein (TSPO) is constitutively expressed at low levels by endothelial cells, and can therefore be used to assess the integrity of the brain’s vasculature. At the same time, the inducible expression of TSPO in activated microglia, the brain’s intrinsic immune cells, is a regularly observed early indicator of subtle or incipient brain pathology. Here, we explored the use of TSPO as a biomarker of brain tissue injury following whole body irradiation. Post-radiation responses were measured in C57BL/6 wild type (Tspo^+/+) and TSPO knockout (Tspo^–/–) mice 48 h after single whole body gamma irradiations with low doses 0, 0.01, and 0.1 Gy and a high dose of 2 Gy. Additionally, post-radiation responses of primary microglial cell cultures were measured at 1, 4, 24, and 48 h at an irradiation dose range of 0 Gy-2 Gy. TSPO mRNA and protein expression in the brain showed a decreased trend after 0.01 Gy relative to sham-irradiated controls, but remained unchanged after higher doses. Immunohistochemistry confirmed subtle decreases in TSPO expression after 0.01 Gy in vascular endothelial cells of the hippocampal region and in ependymal cells, with no detectable changes following higher doses. Cytokine concentrations in plasma after whole body irradiation showed differential changes in IL-6 and IL-10 with some variations between Tspo^–/– and Tspo^+/+ animals. The in vitro measurements of TSPO in primary microglial cell cultures showed a significant reduction 1 h after low dose irradiation (0.01 Gy). In summary, acute low and high doses of gamma irradiation up to 2 Gy reduced TSPO expression in the brain’s vascular compartment without de novo induction of TSPO expression in parenchymal microglia, while TSPO expression in directly irradiated, isolated, and thus highly activated microglia, too, was reduced after low dose irradiation. The potential link between TSPO, its role in mitochondrial energy metabolism and the selective radiation sensitivity, notably of cells with constitutive TSPO expression such as vascular endothelial cells, merits further exploration.

Neuroinflammation in the Cerebellum and Brainstem in Friedreich Ataxia: An [18 F]-FEMPA PET Study

BACKGROUND

Neuroinflammation is proposed to accompany, or even contribute to, neuropathology in Friedreich ataxia (FRDA), with implications for disease treatment and tracking.

OBJECTIVES

To examine brain glial activation and systemic immune dysfunction in people with FRDA and quantify their relationship with symptom severity, duration, and onset age.

METHODS

Fifteen individuals with FRDA and 13 healthy controls underwent brain positron emission tomography using the translocator protein (TSPO) radioligand [¹⁸ F]-FEMPA, a marker of glial activation, together with the quantification of blood plasma inflammatory cytokines.

RESULTS

[¹⁸ F]-FEMPA binding was significantly increased in the dentate nuclei (d = 0.67), superior cerebellar peduncles (d = 0.74), and midbrain (d = 0.87), alongside increased plasma interleukin-6 (IL-6) (d = 0.73), in individuals with FRDA compared to controls. Increased [¹⁸ F]-FEMPA binding in the dentate nuclei, brainstem, and cerebellar anterior lobe correlated with earlier age of symptom onset (controlling for the genetic triplet repeat expansion length; all r_part  < -0.6), and in the pons and anterior lobe with shorter disease duration (r = -0.66; -0.73).

CONCLUSIONS

Neuroinflammation is evident in brain regions implicated in FRDA neuropathology. Increased neuroimmune activity may be related to earlier disease onset and attenuate over the course of the illness. © 2021 International Parkinson and Movement Disorder Society.

The aryl hydrocarbon receptor: A diagnostic and therapeutic target in glioma

Glioblastoma multiforme (GBM) is a deadly disease; 5-year survival rates have shown little improvement over the past 30 years. In vivo positron emission tomography (PET) imaging is an important method of identifying potential diagnostic and therapeutic molecular targets non-invasively. The aryl hydrocarbon receptor (AhR) is a transcription factor that regulates multiple genes involved in immune response modulation and tumorigenesis. The AhR is an attractive potential drug target and studies have shown that its activation by small molecules can modulate innate and adaptive immunity beneficially and prevent AhR-mediated tumour promotion in several cancer types. In this review, we provide an overview of the role of the AhR in glioma tumorigenesis and highlight its potential as an emerging biomarker for glioma therapies targeting the tumour immune response and PET diagnostics.

Direct Comparison of [18F]F-DPA with [18F]DPA-714 and [11C]PBR28 for Neuroinflammation Imaging in the same Alzheimer's Disease Model Mice and Healthy Controls

PURPOSE

In this study we compared the recently developed TSPO tracer [¹⁸F]F-DPA, with [¹⁸F]DPA-714 and [¹¹C]PBR28 by performing in vivo PET imaging on the same Alzheimer's disease mouse model APP/PS1-21 (TG) and wild-type (WT) mice with all three radiotracers.

PROCEDURES

To compare the radiotracer uptake, percentage of injected dose/mL (%ID/mL), standardized uptake value ratios to cerebellum (SUVR_CB), and voxel-wise analyses were performed.

RESULTS

The peak uptake of [¹⁸F]F-DPA was higher than 4.3% ID/mL, while [¹⁸F]DPA-714 reached just over 3% ID/mL, and [¹¹C]PBR28 was over 4% ID/mL in only one brain region in the WT mice. The peak/60-min uptake ratios of [¹⁸F]F-DPA were significantly higher (p < 0.001) than those of [¹⁸F]DPA-714 and [¹¹C]PBR28. The differences in [¹⁸F]F-DPA SUVR_CB between WT and TG mice were highly significant (p < 0.001) in the three studied time periods after injection. [¹⁸F]DPA-714 uptake was significantly higher in TG mice starting in the 20-40-min timeframe and increased thereafter, whereas [¹¹C]PBR28 uptake became significant at 10-20 min (p < 0.05). The voxel-wise analysis confirmed the differences between the radiotracers.

CONCLUSIONS

[¹⁸F]F-DPA displays higher brain uptake, higher TG-to-WT SUVR_CB ratios, and faster clearance than [¹⁸F]DPA-714 and [¹¹C]PBR28, and could prove useful for detecting low levels of inflammation and allow for shorter dynamic PET scans.

Pre-therapeutic microglia activation and sex determine therapy effects of chronic immunomodulation

Modulation of the innate immune system is emerging as a promising therapeutic strategy against Alzheimer's disease (AD). However, determinants of a beneficial therapeutic effect are ill-understood. Thus, we investigated the potential of 18 kDa translocator protein positron-emission-tomography (TSPO-PET) for assessment of microglial activation in mouse brain before and during chronic immunomodulation. Methods: Serial TSPO-PET was performed during five months of chronic microglia modulation by stimulation of the peroxisome proliferator-activated receptor (PPAR)-γ with pioglitazone in two different mouse models of AD (PS2APP, AppNL-G-F ). Using mixed statistical models on longitudinal TSPO-PET data, we tested for effects of therapy and sex on treatment response. We tested correlations of baseline with longitudinal measures of TSPO-PET, and correlations between PET results with spatial learning performance and β-amyloid accumulation of individual mice. Immunohistochemistry was used to determine the molecular source of the TSPO-PET signal. Results: Pioglitazone-treated female PS2APP and AppNL-G-F mice showed attenuation of the longitudinal increases in TSPO-PET signal when compared to vehicle controls, whereas treated male AppNL-G-F mice showed the opposite effect. Baseline TSPO-PET strongly predicted changes in microglial activation in treated mice (R = -0.874, p < 0.0001) but not in vehicle controls (R = -0.356, p = 0.081). Reduced TSPO-PET signal upon pharmacological treatment was associated with better spatial learning despite higher fibrillar β-amyloid accumulation. Immunohistochemistry confirmed activated microglia to be the source of the TSPO-PET signal (R = 0.952, p < 0.0001). Conclusion: TSPO-PET represents a sensitive biomarker for monitoring of immunomodulation and closely reflects activated microglia. Sex and pre-therapeutic assessment of baseline microglial activation predict individual immunomodulation effects and may serve for responder stratification.

Preclinical evaluation of (S)-[18F]GE387, a novel 18-kDa translocator protein (TSPO) PET radioligand with low binding sensitivity to human polymorphism rs6971

Purpose

Positron emission tomography (PET) studies with radioligands for 18-kDa translocator protein (TSPO) have been instrumental in increasing our understanding of the complex role neuroinflammation plays in disorders affecting the brain. However, (R)-[¹¹C]PK11195, the first and most widely used TSPO radioligand has limitations, while the next-generation TSPO radioligands have suffered from high interindividual variability in binding due to a genetic polymorphism in the TSPO gene (rs6971). Herein, we present the biological evaluation of the two enantiomers of [¹⁸F]GE387, which we have previously shown to have low sensitivity to this polymorphism.

Methods

Dynamic PET scans were conducted in male Wistar rats and female rhesus macaques to investigate the in vivo behaviour of (S)-[¹⁸F]GE387 and (R)-[¹⁸F]GE387. The specific binding of (S)-[¹⁸F]GE387 to TSPO was investigated by pre-treatment with (R)-PK11195. (S)-[¹⁸F]GE387 was further evaluated in a rat model of lipopolysaccharide (LPS)-induced neuroinflammation. Sensitivity to polymorphism of (S)-GE387 was evaluated in genotyped human brain tissue.

Results

(S)-[¹⁸F]GE387 and (R)-[¹⁸F]GE387 entered the brain in both rats and rhesus macaques. (R)-PK11195 blocked the uptake of (S)-[¹⁸F]GE387 in healthy olfactory bulb and peripheral tissues constitutively expressing TSPO. A 2.7-fold higher uptake of (S)-[¹⁸F]GE387 was found in the inflamed striatum of LPS-treated rodents. In genotyped human brain tissue, (S)-GE387 was shown to bind similarly in low affinity binders (LABs) and high affinity binders (HABs) with a LAB to HAB ratio of 1.8.

Conclusion

We established that (S)-[¹⁸F]GE387 has favourable kinetics in healthy rats and non-human primates and that it can distinguish inflamed from normal brain regions in the LPS model of neuroinflammation. Crucially, we have reconfirmed its low sensitivity to the TSPO polymorphism on genotyped human brain tissue. Based on these factors, we conclude that (S)-[¹⁸F]GE387 warrants further evaluation with studies on human subjects to assess its suitability as a TSPO PET radioligand for assessing neuroinflammation.

Supplementary Information

The online version contains supplementary material available at 10.1007/s00259-021-05495-w.

Have (R)-[11C]PK11195 challengers fulfilled the promise? A scoping review of clinical TSPO PET studies

PURPOSE

The prototypical TSPO radiotracer (R)-[¹¹C]PK11195 has been used in humans for more than thirty years to visualize neuroinflammation in several pathologies. Alternative radiotracers have been developed to improve signal-to-noise ratio and started to be tested clinically in 2008. Here we examined the scientific value of these "(R)-[¹¹C]PK11195 challengers" in clinical research to determine if they could supersede (R)-[¹¹C]PK11195.

METHODS

A systematic MEDLINE (PubMed) search was performed (up to end of year 2020) to extract publications reporting TSPO PET in patients with identified pathologies, excluding studies in healthy subjects and methodological studies.

RESULTS

Of the 288 publications selected, 152 used 13 challengers, and 142 used (R)-[¹¹C]PK11195. Over the last 20 years, the number of (R)-[¹¹C]PK11195 studies remained stable (6 ± 3 per year), but was surpassed by the total number of challenger studies for the last 6 years. In total, 3914 patients underwent a TSPO PET scan, and 47% (1851 patients) received (R)-[¹¹C]PK11195. The 2 main challengers were [¹¹C]PBR28 (24%-938 patients) and [¹⁸F]FEPPA (11%-429 patients). Only one-in-ten patients (11%-447) underwent 2 TSPO scans, among whom 40 (1%) were scanned with 2 different TSPO radiotracers.

CONCLUSIONS

Generally, challengers confirmed disease-specific initial (R)-[¹¹C]PK11195 findings. However, while their better signal-to-noise ratio seems particularly useful in diseases with moderate and widespread neuroinflammation, most challengers present an allelic-dependent (Ala147Thr polymorphism) TSPO binding and genetic stratification is hindering their clinical implementation. As new challengers, insensitive to TSPO human polymorphism, are about to enter clinical evaluation, we propose this systematic review to be regularly updated (living review).

TSPO: an emerging role in appetite for a therapeutically promising biomarker

There is accumulating evidence that an obesogenic Western diet causes neuroinflammatory damage to the brain, which then promotes further appetitive behaviour. Neuroinflammation has been extensively studied by analysing the translocator protein of 18 kDa (TSPO), a protein that is upregulated in the inflamed brain following a damaging stimulus. As a result, there is a rich supply of TSPO-specific agonists, antagonists and positron emission tomography ligands. One TSPO ligand, etifoxine, is also currently used clinically for the treatment of anxiety with a minimal side-effect profile. Despite the neuroinflammatory pathogenesis of diet-induced obesity, and the translational potential of targeting TSPO, there is sparse literature characterizing the effect of TSPO on appetite. Therefore, in this review, the influence of TSPO on appetite is discussed. Three putative mechanisms for TSPO's appetite-modulatory effect are then characterized: the TSPO–allopregnanolone–GABA_AR signalling axis, glucosensing in tanycytes and association with the synaptic protein RIM-BP1. We highlight that, in addition to its plethora of functions, TSPO is a regulator of appetite. This review ultimately suggests that the appetite-modulating function of TSPO should be further explored due to its potential therapeutic promise.

Detection of Alzheimer's disease-related neuroinflammation by a PET ligand selective for glial versus vascular translocator protein

A substantial and constitutive expression of translocator protein (TSPO) in cerebral blood vessels hampers the sensitive detection of neuroinflammation characterized by greatly induced TSPO expression in activated glia. Here, we conducted in vivo positron emission tomography (PET) and in vitro autoradiographic imaging of normal and TSPO-deficient mouse brains to compare the binding properties of ¹⁸F-FEBMP, a relatively novel TSPO radioligand developed for human studies based on its insensitivity to a common polymorphism, with ¹¹C-PK11195, as well as other commonly used TSPO radioligands including ¹¹C-PBR28, ¹¹C-Ac5216 and ¹⁸F-FEDAA1106. TSPO in cerebral vessels of normal mice was found to provide a major binding site for ¹¹C-PK11195, ¹¹C-PBR28 and ¹⁸F-FEDAA1106, in contrast to no overt specific binding of ¹⁸F-FEBMP and ¹¹C-Ac5216 to this vascular component. In addition, ¹⁸F-FEBMP yielded PET images of microglial TSPO with a higher contrast than ¹¹C-PK11195 in a tau transgenic mouse modeling Alzheimer's disease (AD) and allied neurodegenerative tauopathies. Moreover, TSPO expression examined by immunoblotting was significantly increased in AD brains compared with healthy controls, and was well correlated with the autoradiographic binding of ¹⁸F-FEBMP but not ¹¹C-PK11195. Our findings support the potential advantage of comparatively glial TSPO-selective radioligands such as ¹⁸F-FEBMP for PET imaging of inflammatory glial cells.

Parametric Mapping for TSPO PET Imaging with Spectral Analysis Impulsive Response Function

PURPOSE

The aim of this study was to investigate the use of spectral analysis (SA) for voxel-wise analysis of TSPO PET imaging studies. TSPO PET quantification is methodologically complicated by the heterogeneity of TSPO expression and its cell-dependent modulation during neuroinflammatory response. Compartmental models to account for this complexity exist, but they are unreliable at the high noise typical of voxel data. On the contrary, SA is noise-robust for parametric mapping and provides useful information about tracer kinetics with a free compartmental structure.

PROCEDURES

SA impulse response function (IRF) calculated at 90 min after tracer injection was used as main parameter of interest in 3 independent PET imaging studies to investigate its sensitivity to (1) a TSPO genetic polymorphism (rs6971) known to affect tracer binding in a cross-sectional analysis of healthy controls scanned with [11C]PBR28 PET; (2) TSPO density with [11C]PBR28 in a competitive blocking study with a TSPO blocker, XBD173; and (3) the higher affinity of a second radiotracer for TSPO, by using data from a head-to-head comparison between [11C]PBR28 and [11C]ER176 scans.

RESULTS

SA-IRF produced parametric maps of visually good quality. These were sensitive to TSPO genotype (mean relative difference between high- and mixed-affinity binders = 25 %) and TSPO availability (mean signal displacement after 90 mg oral administration of XBD173 = 39 %). Regional averages of voxel-wise IRF estimates were strongly associated with regional total distribution volume (V_T) estimated with a 2-tissue compartmental model with vascular compartment (Pearson's r = 0.86 ± 0.11) but less strongly with standard 2TCM-V_T (Pearson's r = 0.76 ± 0.32). Finally, SA-IRF estimates for [11C]ER176 were significantly higher than [11C]PBR28 ones, consistent with the higher amount of specific binding of the former tracer.

CONCLUSIONS

SA-IRF can be used for voxel-wise quantification of TSPO PET data because it generates high-quality parametric maps, it is sensitive to TSPO availability and genotype, and it accounts for the complexity of TSPO tracer kinetics with no additional assumptions.

MRI and PET of Brain Tumor Neuroinflammation in the Era of Immunotherapy, From the AJR Special Series on Inflammation

With the emergence of immune-modulating therapies, brain tumors present significant diagnostic imaging challenges. These challenges include planning personalized treatment and adjudicating accurate monitoring approaches and therapeutically specific response criteria. This has been due, in part, to the reliance on nonspecific imaging metrics, such as gadolinium-contrast-enhanced MRI or FDG PET, and rapidly evolving biologic understanding of neuroinflammation. The importance of the tumor-immune interaction and ability to therapeutically augment inflammation to improve clinical outcomes necessitates that the radiologist develop a working knowledge of the immune system and its role in clinical neuroimaging. In this article, we review relevant biologic concepts of the tumor microenvironment of primary and metastatic brain tumors, these tumors' interactions with the immune system, and MRI and PET methods for imaging inflammatory elements associated with these malignancies. Recognizing the growing fields of immunotherapeutics and precision oncology, we highlight clinically translatable imaging metrics for the diagnosis and monitoring of brain tumor neuroinflammation. Practical guidance is provided for implementing iron nanoparticle imaging, including imaging indications, protocol, interpretation, and pitfalls. A comprehensive understanding of the inflammatory mechanisms within brain tumors and their imaging features will facilitate the development of innovative non-invasive prognostic and predictive imaging strategies for precision oncology.

Exploring Translocator Protein (TSPO) Medicinal Chemistry: An Approach for Targeting Radionuclides and Boron Atoms to Mitochondria

Translocator protein 18 kDa [TSPO or peripheral-type benzodiazepine receptor (PBR)] was identified in the search of binding sites for benzodiazepine anxiolytic drugs in peripheral regions. In these areas, binding sites for TSPO ligands were recognized in steroid-producing tissues. TSPO plays an important role in many cellular functions, and its coding sequence is highly conserved across species. TSPO is located predominantly on the membrane of mitochondria and is overexpressed in several solid cancers. TSPO basal expression in the CNS is low, but it becomes high in neurodegenerative conditions. Thus, TSPO constitutes not only as an outstanding drug target but also as a valuable marker for the diagnosis of a number of diseases. The aim of the present article is to show the lesson we have learned from our activity in TSPO medicinal chemistry and in approaching the targeted delivery to mitochondria by means of TSPO ligands.

A binge high sucrose diet provokes systemic and cerebral inflammation in rats without inducing obesity

While the dire cardiometabolic consequences of the hypercaloric modern 'Western' diet are well known, there is not much information on the health impact of a high sucrose diet not inducing weight gain. Here, we tested the hypothesis that rats reared with intermittent binge access to sucrose in addition to normal chow would develop an inflammatory response in brain. To test this hypothesis, we undertook serial PET/MRI scans with the TSPO ligand [18F]DPA714 in a group of (n=9) rats at baseline and again after voluntarily consuming 5% sucrose solution three days a week for three months. Compared to a control group fed with normal chow (n=9), the sucrose rats indeed showed widespread increases in the availability of cerebral binding sites for the microglial marker, despite normal weight gain compared to the control diet group. Subsequent immunofluorescence staining of the brains confirmed the PET findings, showing a widespread 20% increase in the abundance of IBA-1-positive microglia with characteristic 'semi-activated' morphology in the binge sucrose rats, which had 23% lower density of microglial endpoints and 25% lower mean process length compared to microglia in the control rats with ordinary feeding. GFAP immunofluorescence showed no difference in astroglial coverage in the sucrose rats, except for a slight reduction in hypothalamus. The binge sucrose diet-induced neuroinflammation was associated with a significant elevation of white blood cell counts. Taking these results together, we find that long-term intake of sucrose in a binge paradigm, similar in sucrose content to the contemporary Western diet, triggered a low-grade systemic and central inflammation in non-obese rats. The molecular mechanism of this phenomenon remains to be established.

Potential Effects of Leukotriene Receptor Antagonist Montelukast in Treatment of Neuroinflammation in Parkinson's Disease

Parkinson’s disease (PD) is a neurodegenerative disorder where misfolded alpha-synuclein-enriched aggregates called Lewy bodies are central in pathogenesis. No neuroprotective or disease-modifying treatments are currently available. Parkinson’s disease is considered a multifactorial disease and evidence from multiple patient studies and animal models has shown a significant immune component during the course of the disease, highlighting immunomodulation as a potential treatment strategy. The immune changes occur centrally, involving microglia and astrocytes but also peripherally with changes to the innate and adaptive immune system. Here, we review current understanding of different components of the PD immune response with a particular emphasis on the leukotriene pathway. We will also describe evidence of montelukast, a leukotriene receptor antagonist, as a possible anti-inflammatory treatment for PD.

Reduced microglia activity in patients with long-term immunosuppressive therapy after liver transplantation

Purpose

Calcineurin inhibitors (CNI) can cause long-term impairment of brain function. Possible pathomechanisms include alterations of the cerebral immune system. This study used positron emission tomography (PET) imaging with the translocator protein (TSPO) ligand ¹⁸F-GE-180 to evaluate microglial activation in liver-transplanted patients under different regimens of immunosuppression.

Methods

PET was performed in 22 liver-transplanted patients (3 CNI free, 9 with low-dose CNI, 10 with standard-dose CNI immunosuppression) and 9 healthy controls. The total distribution volume (V_T) estimated in 12 volumes-of-interest was analyzed regarding TSPO genotype, CNI therapy, and cognitive performance.

Results

In controls, V_T was about 80% higher in high affinity binders (n = 5) compared to mixed affinity binders (n = 3). Mean V_T corrected for TSPO genotype was significantly lower in patients compared to controls, especially in patients in whom CNI dose had been reduced because of nephrotoxic side effect.

Conclusion

Our results provide evidence of chronic suppression of microglial activity in liver-transplanted patients under CNI therapy especially in patients with high sensitivity to CNI toxicity.

Supplementary Information

The online version contains supplementary material available at 10.1007/s00259-021-05398-w.

In vitro and pilot in vivo imaging of 18 kDa translocator protein (TSPO) in inflammatory vascular disease

Background

Inflammatory vascular disease of the arteries, such as inflamed atheromatous plaques or arteritis, may cause aneurysms or ischemic strokes. In this context, using positron emission tomography (PET) to image inflammation may help select patients who would benefit from appropriate therapeutic interventions. This study sought to assess the usefulness of the 18 kDa translocator protein (TSPO) tracers [¹¹C]-PBR28 and [¹⁸F]-PBR06 for imaging inflammatory vascular disease in vitro and in vivo. Immunohistochemistry for macrophage infiltration as well as autoradiography with [¹⁸F]-PBR06 were performed on eight paraffin-embedded, formalin-fixed atherosclerosis plaques prospectively collected after carotid endarterectomy of eight patients affected by ischemic stroke. Six different patients, one of whom was also included in the in vitro study, underwent PET imaging. Two patients with carotid stenosis associated with ischemic stroke were imaged with [¹⁸F]-PBR06 PET/CT, and four other patients (three with large vessel vasculitis and one with bilateral carotid stenosis but without stroke) were imaged with [¹¹C]-PBR28.

Results

All in vitro sections showed specific binding of [¹⁸F]-PBR06, which co-localized with immunohistochemistry markers for inflammation. However, in vivo TSPO imaging with either [¹¹C]-PBR28 or [¹⁸F]-PBR06 was negative in all participants.

Conclusion

Despite good uptake on surgical samples in vitro, [¹¹C]-PBR28 and [¹⁸F]-PBR06 are not viable clinical tools for imaging inflammatory vascular disease.

Trial registration: NCT02513589, registered 31 July 2015 and NCT00547976, registered 23 October 2007. https://clinicaltrials.gov.

Population-based input function for TSPO quantification and kinetic modeling with [11C]-DPA-713

INTRODUCTION

Quantitative positron emission tomography (PET) studies of neurodegenerative diseases typically require the measurement of arterial input functions (AIF), an invasive and risky procedure. This study aims to assess the reproducibility of [11C]DPA-713 PET kinetic analysis using population-based input function (PBIF). The final goal is to possibly eliminate the need for AIF.

MATERIALS AND METHODS

Eighteen subjects including six healthy volunteers (HV) and twelve Parkinson disease (PD) subjects from two [11C]-DPA-713 PET studies were included. Each subject underwent 90 min of dynamic PET imaging. Five healthy volunteers underwent a test-retest scan within the same day to assess the repeatability of the kinetic parameters. Kinetic modeling was carried out using the Logan total volume of distribution (VT) model. For each data set, kinetic analysis was performed using a patient-specific AIF (PSAIF, ground-truth standard) and then repeated using the PBIF. PBIF was generated using the leave-one-out method for each subject from the remaining 17 subjects and after normalizing the PSAIFs by 3 techniques: (a) Weightsubject×DoseInjected, (b) area under AIF curve (AUC), and (c) Weightsubject×AUC. The variability in the VT measured with PSAIF, in the test-retest study, was determined for selected brain regions (white matter, cerebellum, thalamus, caudate, putamen, pallidum, brainstem, hippocampus, and amygdala) using the Bland-Altman analysis and for each of the 3 normalization techniques. Similarly, for all subjects, the variabilities due to the use of PBIF were assessed.

RESULTS

Bland-Altman analysis showed systematic bias between test and retest studies. The corresponding mean bias and 95% limits of agreement (LOA) for the studied brain regions were 30% and ± 70%. Comparing PBIF- and PSAIF-based VT estimate for all subjects and all brain regions, a significant difference between the results generated by the three normalization techniques existed for all brain structures except for the brainstem (P-value = 0.095). The mean % difference and 95% LOA is -10% and ±45% for Weightsubject×DoseInjected; +8% and ±50% for AUC; and +2% and ± 38% for Weightsubject×AUC. In all cases, normalizing by Weightsubject×AUC yielded the smallest % bias and variability (% bias = ±2%; LOA = ±38% for all brain regions). Estimating the reproducibility of PBIF-kinetics to PSAIF based on disease groups (HV/PD) and genotype (MAB/HAB), the average VT values for all regions obtained from PBIF is insignificantly higher than PSAIF (%difference = 4.53%, P-value = 0.73 for HAB; and %difference = 0.73%, P-value = 0.96 for MAB). PBIF also tends to overestimate the difference between PD and HV for HAB (% difference = 32.33% versus 13.28%) and underestimate it in MAB (%difference = 6.84% versus 20.92%).

CONCLUSIONS

PSAIF kinetic results are reproducible with PBIF, with variability in VT within that obtained for the test-retest studies. Therefore, VT assessed using PBIF-based kinetic modeling is clinically feasible and can be an alternative to PSAIF.

Microglia Activation in Basal Ganglia Is a Late Event in Huntington Disease Pathophysiology

OBJECTIVE

To define the role played by microglia in different stages of Huntington disease (HD), we used the TSPO radioligand [11C]-ER176 and PET to evaluate microglial activation in relation to neurodegeneration and in relation to the clinical features seen at premanifest and manifest stages of the disease.

METHODS

This is a cross-sectional study in which 18 subjects (6 controls, 6 premanifest, and 6 manifest HD gene carriers) underwent a [11C]-ER176 PET scan and an MRI for anatomic localization. Segmentation of regions of interest (ROIs) was performed, and group differences in [11C]-ER176 binding (used to evaluate the extent of microglial activation) were assessed by the standardized uptake value ratio (SUVR). Microglial activation was correlated with ROIs volumes, disease burden, and the scores obtained in the clinical scales. As an exploratory aim, we evaluated the dynamic functions of microglia in vitro, by using induced microglia-like (iMG) cells from peripheral blood monocytes.

RESULTS

Individuals with manifest HD present higher [11C]-ER176 SUVR in both globi pallidi and putamina in comparison with controls. No differences were observed when we compared premanifest HD with controls or with manifest HD. We also found a significant correlation between increased microglial activation and cumulative disease burden, and with reduced volumes. iMG from controls, premanifest HD, and manifest HD patients showed similar phagocytic capacity.

CONCLUSIONS

Altogether, our data demonstrate that microglial activation is involved in HD pathophysiology and is associated with disease progression.

TSPO PET imaging of natalizumab-associated progressive multifocal leukoencephalopathy

Progressive multifocal leukoencephalopathy (PML) is a severe infection of the central nervous system caused by the polyomavirus JC (JCV) that can occur in multiple sclerosis (MS) patients treated with natalizumab. Clinical management of patients with natalizumab-associated PML is challenging not the least because current imaging tools for the early detection, longitudinal monitoring and differential diagnosis of PML lesions are limited. Here we evaluate whether TSPO positron emission tomography (PET) imaging can be applied to monitor the inflammatory activity of PML lesions over time and differentiate them from MS lesions. For this monocenter pilot study we followed 8 patients with natalizumab-associated PML with PET imaging using the TSPO radioligand [18F]GE-180 combined with frequent 3 T MRI imaging. In addition we compared TSPO PET signals in PML lesions with the signal pattern of MS lesions from 17 independent MS patients. We evaluated the standardized uptake value ratio (SUVR) as well as the morphometry of the TSPO uptake for putative PML and MS lesions areas compared to a radiologically unaffected pseudo-reference region in the cerebrum. Furthermore TSPO expression in situ was immunohistochemically verified by determining the density and cellular identity of TSPO-expressing cells in brain sections from four patients with early natalizumab-associated PML as well as five patients with other forms of PML and six patients with inflammatory demyelinating CNS lesions (clinically isolated syndrome/MS). Histological analysis revealed a reticular accumulation of TSPO expressing phagocytes in PML lesions, while such phagocytes showed a more homogenous distribution in putative MS lesions. TSPO PET imaging showed an enhanced tracer uptake in natalizumab-associated PML lesions that was present from the early to the chronic stages (up to 52 months after PML diagnosis). While gadolinium enhancement on MRI rapidly declined to baseline levels, TSPO tracer uptake followed a slow one phase decay curve. A TSPO-based 3-dimensional diagnostic matrix taking into account the uptake levels as well as the shape and texture of the TSPO signal differentiated more than 96% of PML and MS lesions. Indeed, treatment with rituximab after natalizumab-associated PML in three patients did not affect tracer uptake in the assigned PML lesions but reverted tracer uptake to baseline in the assigned active MS lesions. Taken together our study suggests that TSPO PET imaging can reveal CNS inflammation in natalizumab-associated PML. TSPO PET may facilitate longitudinal monitoring of disease activity and help to distinguish recurrent MS activity from PML progression.

In Vitro Evaluation of [3H]CPPC as a Tool Radioligand for CSF-1R

Microglia play a role in several central nervous system (CNS) diseases and are a highly sought target for positron emission tomography (PET) imaging and therapeutic intervention. 5-Cyano-N-(4-(4-[¹¹C]methylpiperazin-1-yl)-2-(piperidin-1-yl)phenyl)furan-2-carboxamide ([¹¹C]CPPC) is a radiopharmaceutical designed to selectively target microglia via macrophage colony stimulating factor-1 receptor (CSF-1R) in the CNS. Herein, we report the first preclinical evaluation of [³H]CPPC using radioligand binding methods for the evaluation of putative CSF-1R inhibitors in rodent models of neuroinflammation. The distribution of [³H]CPPC by autoradiography did not align with 18 kDa translocator protein (TSPO) distribution using [³H]PBR28 and IBA-1 staining for microglia. In the CNS, [³H]CPPC had considerable nonspecific binding, as indicated by a low displacement of the tritiated ligand by unlabeled CPPC and the known CSF1R inhibitors BLZ-945 and PLX3397. Spleen was identified as a tissue that provided an adequate signal-to-noise ratio to enable screening with [³H]CPPC and a library of 20 novel PLX3397 derivatives. However, unlabeled CPPC lacked selectivity and showed off-target binding to a substantial number of kinase targets (204 out of 403 tested) at a concentration relevant to in vitro radioligand binding assays (10 μM). These findings suggest that, while [³H]CPPC may have utility as a radioligand tool for the evaluation of peripheral targets and screening of CSF-1R inhibitors, it may have limited utility as an in vivo CNS imaging probe on the basis of the current evaluation.

Kinetic modeling and parameter estimation of TSPO PET imaging in the human brain

Purpose

Translocator protein 18-kDa (TSPO) imaging with positron emission tomography (PET) is widely used in research studies of brain diseases that have a neuro-immune component. Quantification of TSPO PET images, however, is associated with several challenges, such as the lack of a reference region, a genetic polymorphism affecting the affinity of the ligand for TSPO, and a strong TSPO signal in the endothelium of the brain vessels. These challenges have created an ongoing debate in the field about which type of quantification is most useful and whether there is an appropriate simplified model.

Methods

This review focuses on the quantification of TSPO radioligands in the human brain. The various methods of quantification are summarized, including the gold standard of compartmental modeling with metabolite-corrected input function as well as various alternative models and non-invasive approaches. Their advantages and drawbacks are critically assessed.

Results and conclusions

Researchers employing quantification methods for TSPO should understand the advantages and limitations associated with each method. Suggestions are given to help researchers choose between these viable alternative methods.

Preclinical Evaluation of TSPO and MAO-B PET Radiotracers in an LPS Model of Neuroinflammation

Discovery of novel PET radiotracers targeting neuroinflammation (microglia and astrocytes) is actively pursued. Employing a lipopolysaccharide (LPS) rat model, this longitudinal study evaluated the translocator protein 18-kDa radiotracer [¹⁸F]FEPPA (primarily microglia) and monoamine oxidase B radiotracers [¹¹C]L-deprenyl and [¹¹C]SL25.1188 (astrocytes preferred). Increased [¹⁸F]FEPPA binding peaked at 1 week in LPS-injected striatum whereas increased lazabemide-sensitive [¹¹C]L-deprenyl binding developed later. No increase in radiotracer uptake was observed for [¹¹C]SL25.1188. The unilateral intrastriatal LPS rat model may serve as a useful tool for benchmarking PET tracers targeted toward distinct phases of neuroinflammatory reactions involving both microglia and astrocytes.