Head-to-Head Comparison of 11C-PBR28 and 18F-GE180 for Quantification of the Translocator Protein in the Human Brain

PET imaging of TREM2 in amyloid-beta induced neuroinflammation

PURPOSE

The triggering receptor expressed on myeloid cells 2 (TREM2) has become a promising target for biologics in both monitoring and treating neuroinflammation in Alzheimer's disease (AD). This study aimed to develop and compare bispecific anti-TREM2 antibodies featuring different transferrin receptor (TfR) binders to enhance brain delivery, identifying the most suitable format for in vivo PET imaging of TREM2 in transgenic AD mice.

METHODS

Three bispecific TREM2-antibody formats were produced and evaluated for their ability to cross the blood-brain barrier (BBB) via TfR-mediated transcytosis and bind TREM2. Blood concentration profiles up to 72 h post-injection (p.i.), and ex vivo brain uptake of iodine-125-labeled antibody constructs were quantified in AppNL-G-F and age-matched wild type (WT) mice using a γ-counter. The best-performing bispecific TREM2-antibody was radiolabeled with iodine-124 and used for in vivo PET imaging of brain TREM2 levels in AppNL-G-F mice at 72 h p.i. Brain TREM2 concentrations were subsequently quantified using ELISA.

RESULTS

The antibody format carrying two scFv8D3 TfR-binders (IgG-scFv2), demonstrated the highest brain concentrations of all tested bispecific constructs. This antibody also exhibited significantly higher brain concentrations in AppNL-G-F mice compared to WT mice at both 48 and 72 h p.i. This difference was further visualized and quantified through in vivo PET imaging. Moreover, brain concentrations of the antibody ligand correlated with elevated TREM2 levels in brain homogenates.

CONCLUSION

These findings highlight IgG-scFv2 as a promising radioligand for in vivo PET imaging of TREM2, advancing non-invasive neuroinflammation studies and supporting drug development for AD and other neurodegenerative diseases.

Cyclooxygenase-1 and cyclooxygenase-2 densities measured using positron emission tomography are not altered in the brains of individuals with stable multiple sclerosis

Multiple sclerosis (MS) is a chronic inflammatory disease affecting the central nervous system that involves immune-mediated demyelination and axonal degeneration. Clinical imaging techniques play a critical role in diagnosing and assessing the prognosis of MS. Magnetic resonance imaging has been most frequently used to visualize demyelination and detect acute and chronic active lesions, which are key indicators of clinical course of illness. Previous research has also highlighted the effectiveness of translocator protein 18-kDa (TSPO) positron emission tomography (PET) imaging for identifying chronic active lesions and progressive pathology. Building on this work, the present study used PET imaging to explore the role of cyclooxygenase-1 and -2 (COX-1 and COX-2)-key enzymes involved in neuroinflammation-in individuals with MS. Five participants with MS were recruited, and lesions were identified using 7 Tesla MRI. No significant differences in COX radioligand binding were observed in the co-registered PET images between lesioned areas and normal-appearing brain tissues, nor between individuals with MS and healthy volunteers. The negative findings underscore the complexity of MS pathology and raise several important considerations for planning future studies using COX PET for imaging in MS.

[18F]SF51, a novel 18F-labeled PET radioligand for translocator protein 18kDa (TSPO) in brain, works well in monkeys but fails in humans

[18F]SF51 is a novel radioligand for imaging translocator protein 18 kDa (TSPO) that previously displayed excellent imaging properties in nonhuman primates. This study assessed its performance in human brain and its dosimetry. Seven healthy participants underwent brain PET imaging to measure TSPO binding using a two-tissue compartment model (2TCM) to calculate total distribution volume (VT). This cohort included two high-affinity binders (HABs), three mixed-affinity binders (MABs), and two low-affinity binders (LABs). Two other participants received whole-body scans to assess radiation exposure. Peak brain radioactivity reached a standardized uptake value (SUV) of 1.4 at 3 minutes post-injection, diminishing to 30% of peak by 120 minutes. The average VT for all genotype groups was notably low (<1 mL·cm-3), emphasizing the radioligand's poor binding in brain. [18F]SF51 remained sensitive to the TSPO polymorphism in vivo, as shown by a two-fold difference in VT between HABs and LABs. VT stabilization by 80 minutes post-injection suggested minimal radiometabolite accumulation in brain. The average effective dose was 13.8 ± 0.9 µSv/MBq. Contrary to previously published animal data, [18F]SF51 showed low binding to human TSPO, with uptake remaining influenced by the rs6971 polymorphism. These findings highlight the challenges of developing TSPO radioligands and underscore the significant species differences that may influence translational outcomes.ClinicalTrials.gov identifier: NCT05564429; registered 10/03/2022.

Synthesis and evaluation of TSPO-targeting radioligand [18F]F-TFQC for PET neuroimaging in epileptic rats

The translocator protein (TSPO) positron emission tomography (PET) can noninvasively detect neuroinflammation associated with epileptogenesis and epilepsy. This study explored the role of the TSPO-targeting radioligand [18F]F-TFQC, an m-trifluoromethyl ER176 analog, in the PET neuroimaging of epileptic rats. Initially, [18F]F-TFQC was synthesized with a radiochemical yield of 8%-10% (EOS), a radiochemical purity of over 99%, and a specific activity of 38.21 ± 1.73 MBq/nmol (EOS). After determining that [18F]F-TFQC exhibited good biochemical properties, [18F]F-TFQC PET neuroimaging was performed in epileptic rats at multiple time points in various stages of disease progression. PET imaging showed specific [18F]F-TFQC uptake in the right hippocampus (KA-injected site, i.e., epileptogenic zone), which was most pronounced at 1 week (T/NT 1.63 ± 0.21) and 1 month (T/NT 1.66 ± 0.20). The PET results were further validated using autoradiography and pathological analysis. Thus, [18F]F-TFQC can reflect the TSPO levels and localize the epileptogenic zone, thereby offering the potential for monitoring neuroinflammation and guiding anti-inflammatory treatment in patients with epilepsy.

Brain inflammation co-localizes highly with tau in mild cognitive impairment due to early-onset Alzheimer's disease

Brain inflammation, with an increased density of microglia and macrophages, is an important component of Alzheimer's disease and a potential therapeutic target. However, it is incompletely characterized, particularly in patients whose disease begins before the age of 65 years and, thus, have few co-pathologies. Inflammation has been usefully imaged with translocator protein (TSPO) PET, but most inflammation PET tracers cannot image subjects with a low-binder TSPO rs6971 genotype. In an important development, participants with any TSPO genotype can be imaged with a novel tracer, 11C-ER176, that has a high binding potential and a more favourable metabolite profile than other TSPO tracers currently available. We applied 11C-ER176 to detect brain inflammation in mild cognitive impairment (MCI) caused by early-onset Alzheimer's disease. Furthermore, we sought to correlate the brain localization of inflammation, volume loss, elevated amyloid-β (Aβ)and tau. We studied brain inflammation in 25 patients with early-onset amnestic MCI (average age 59 ± 4.5 years, 10 female) and 23 healthy controls (average age 65 ± 6.0 years, 12 female), both groups with a similar proportion of all three TSPO-binding affinities. 11C-ER176 total distribution volume (VT), obtained with an arterial input function, was compared across patients and controls using voxel-wise and region-wise analyses. In addition to inflammation PET, most MCI patients had Aβ (n = 23) and tau PET (n = 21). For Aβ and tau tracers, standard uptake value ratios were calculated using cerebellar grey matter as region of reference. Regional correlations among the three tracers were determined. Data were corrected for partial volume effect. Cognitive performance was studied with standard neuropsychological tools. In MCI caused by early-onset Alzheimer's disease, there was inflammation in the default network, reaching statistical significance in precuneus and lateral temporal and parietal association cortex bilaterally, and in the right amygdala. Topographically, inflammation co-localized most strongly with tau (r = 0.63 ± 0.24). This correlation was higher than the co-localization of Aβ with tau (r = 0.55 ± 0.25) and of inflammation with Aβ (0.43 ± 0.22). Inflammation co-localized least with atrophy (-0.29 ± 0.26). These regional correlations could be detected in participants with any of the three rs6971 TSPO polymorphisms. Inflammation in Alzheimer's disease-related regions correlated with impaired cognitive scores. Our data highlight the importance of inflammation, a potential therapeutic target, in the Alzheimer's disease process. Furthermore, they support the notion that, as shown in experimental tissue and animal models, the propagation of tau in humans is associated with brain inflammation.

Emerging TSPO-PET Radiotracers for Imaging Neuroinflammation: A Critical Analysis

The translocator protein (TSPO) is a biomarker for imaging neuroinflammation via Positron Emission Tomography (PET) across a broad range of CNS conditions. Most clinically used PET ligands targeting TSPO have limitations, including high lipophilicity and off-target binding or poor binding to a mutated TSPO isoform present in up to 30% of the population. Research efforts over the past decade have focused on development of improved TSPO PET radiotracers that overcome these limitations. This review provides a critical analysis of the development and validation of these so-called "third-generation" radiotracers in clinical and preclinical settings. We also offer our perspective on the future directions of TSPO PET imaging, including recommendations for overcoming current challenges and capitalizing on emerging opportunities in molecular imaging for neuroinflammatory diseases.

TSPO Radioligands for Neuroinflammation: An Overview

The translocator protein (TSPO) is predominately localized on the outer mitochondrial membrane in steroidogenic cells. In the brain, TSPO expression, low under normal conditions, results upregulated in response to glial cell activation, that occurs in neuroinflammation. As a consequence, TSPO has been extensively studied as a biomarker of such conditions by means of TSPO-targeted radiotracers. Although [11C]-PK11195, the prototypical TSPO radioligand, is still widely used for in vivo studies, it is endowed with severe limitations, mainly low sensitivity and poor amenability to quantification. Consequently, several efforts have been focused on the design of new radiotracers for the in vivo imaging of TSPO. The present review will provide an outlook on the latest advances in TSPO radioligands for neuroinflammation imaging. The final goal is to pave the way for (radio)chemists in the future design and development of novel effective and sensitive radiopharmaceuticals targeting TSPO.

Structure-based drug design for TSPO: Challenges and opportunities

The translocator protein 18 kDa (TSPO) is an evolutionarily conserved mitochondrial transmembrane protein implicated in various neuropathologies and inflammatory conditions, making it a longstanding diagnostic and therapeutic target of interest. Despite the development of various classes of TSPO ligand chemotypes, and the elucidation of bacterial and non-human mammalian experimental structures, many unknowns exist surrounding its differential structural and functional features in health and disease. There are several limitations associated with currently used computational methodologies for modelling the native structure and ligand-binding behaviour of this enigmatic protein. In this perspective, we provide a critical analysis of the developments in the uses of these methods, outlining their uses, inherent limitations, and continuing challenges. We offer suggestions of unexplored opportunities that exist in the use of computational methodologies which offer promise for enhancing our understanding of the TSPO.

The development status of PET radiotracers for evaluating neuroinflammation

Neuroinflammation is associated with the pathophysiologies of neurodegenerative and psychiatric disorders. Evaluating neuroinflammation using positron emission tomography (PET) plays an important role in the early diagnosis and determination of proper treatment of brain diseases. To quantify neuroinflammatory responses in vivo, many PET tracers have been developed using translocator proteins, imidazole-2 binding site, cyclooxygenase, monoamine oxidase-B, adenosine, cannabinoid, purinergic P2X7, and CSF-1 receptors as biomarkers. In this review, we introduce the latest developments in PET tracers that can image neuroinflammation, focusing on clinical trials, and further consider their current implications.

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.

Preclinical Evaluation of Novel PET Probes for Dementia

The development of novel PET imaging agents that selectively bind specific dementia-related targets can contribute significantly to accurate, differential and early diagnosis of dementia causing diseases and support the development of therapeutic agents. Consequently, in recent years there has been a growing body of literature describing the development and evaluation of potential new promising PET tracers for dementia. This review article provides a comprehensive overview of novel dementia PET probes under development, classified by their target, and pinpoints their preclinical evaluation pathway, typically involving in silico, in vitro and ex/in vivo evaluation. Specific target-associated challenges and pitfalls, requiring extensive and well-designed preclinical experimental evaluation assays to enable successful clinical translation and avoid shortcomings observed for previously developed 'well-established' dementia PET tracers are highlighted in this review.

Neuroinflammation and anti-inflammatory therapy for ischemic stroke

Stroke remains one of the most devastating and challenging neurological diseases worldwide. Inflammation, as well as oxidative stress is one of the main contributors to post-stroke injuries, and oxidative stress can further induce inflammation. Moreover, the inflammatory response is closely related to immune modulation in ischemic stroke progression. Hence, major ischemic stroke treatment strategies include targeting inflammatory responses, immune modulation (especially immune cells), and inflammatory response to suppress stroke progression. To date, several drugs have demonstrated clinical efficacy, such as Etanercept and Fingolimod. However, only edaravone dexborneol has successfully passed the phase III clinical trial and been approved by the National Medical Products Administration (NMPA) to treat ischemic stroke in China, which can restore redox balance and regulate inflammatory immune responses, thus providing neuroprotection in ischemic stroke. In this review, we will comprehensively summarize the current advances in the application of inflammatory biomarkers, neuroinflammation and neuro-immunotherapeutic scenarios for ischemic stroke, thus aiming to provide a theoretical basis and new prospects and frontiers for clinical applications.

A Systematic Review on Dementia and Translocator Protein (TSPO): When Nuclear Medicine Highlights an Underlying Expression

Background: Translocator protein (TSPO) is a neuroinflammation hallmark. Different TSPO affinity compounds have been produced and over time, the techniques of radiolabeling have been refined. The aim of this systematic review is to summarize the development of new radiotracers for dementia and neuroinflammation imaging. Methods: An online search of the literature was conducted in the PubMed, Scopus, Medline, Cochrane Library, and Web of Science databases, selecting published studies from January 2004 to December 2022. The accepted studies considered the synthesis of TSPO tracers for nuclear medicine imaging in dementia and neuroinflammation. Results: A total of 50 articles was identified. Twelve papers were selected from the included studies’ bibliographies and 34 were excluded. Thus, 28 articles were ultimately selected for quality assessment. Conclusion: Huge efforts in developing specific and stable tracers for PET/SPECT imaging have been made. The long half-life of 18F makes this isotope a preferable choice to 11C. An emerging limitation to this however is that neuroinflammation involves all of the brain which inhibits the possibility of detecting a slight inflammation status change in patients. A partial solution to this is using the cerebellum as a reference region and developing higher TSPO affinity tracers. Moreover, it is necessary to consider the presence of distomers and racemic compounds interfering with pharmacological tracers’ effects and increasing the noise ratio in images.

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 (fu) 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 LogD7.4 of 2.31 ± 0.13 (n = 6) and fu 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 (VT) 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 VT 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 VT 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.

Microglial Imaging in Alzheimer's Disease and Its Relationship to Brain Amyloid: A Human 18F-GE180 PET Study

BACKGROUND

Emerging evidence suggests a potential causal role of neuroinflammation in Alzheimer's disease (AD). Using positron emission tomography (PET) to image overexpressed 18 kDA translocator protein (TSPO) by activated microglia has gained increasing interest. The uptake of 18F-GE180 TSPO PET was observed to co-localize with inflammatory markers and have a two-stage association with amyloid PET in mice. Very few studies evaluated the diagnostic power of 18F-GE180 PET in AD population and its interpretation in human remains controversial about whether it is a marker of microglial activation or merely reflects disrupted blood-brain barrier integrity in humans.

OBJECTIVE

The goal of this study was to study human GE180 from the perspective of the previous animal observations.

METHODS

With data from twenty-four participants having 18F-GE180 and 18F-AV45 PET scans, we evaluated the group differences of 18F-GE180 uptake between participants with and without cognitive impairment. An association analysis of 18F-GE180 and 18F-AV45 was then conducted to test if the relationship in humans is consistent with the two-stage association in AD mouse model.

RESULTS

Elevated 18F-GE180 was observed in participants with cognitive impairment compared to those with normal cognition. No regions showed reduced 18F-GE180 uptake. Consistent with mouse model, a two-stage association between 18F-GE180 and 18F-AV45 was observed.

CONCLUSIONS

18F-GE180 PET imaging showed promising utility in detecting pathological alterations in a symptomatic AD population. Consistent two-stage association between 18F-GE180 and amyloid PET in human and mouse suggested that 18F-GE180 uptake in human might be considerably influenced by microglial activation.

18F-Radiolabeled Translocator Protein (TSPO) PET Tracers: Recent Development of TSPO Radioligands and Their Application to PET Study

Translocator protein 18 kDa (TSPO) is a transmembrane protein in the mitochondrial membrane, which has been identified as a peripheral benzodiazepine receptor. TSPO is generally present at high concentrations in steroid-producing cells and plays an important role in steroid synthesis, apoptosis, and cell proliferation. In the central nervous system, TSPO expression is relatively modest under normal physiological circumstances. However, some pathological disorders can lead to changes in TSPO expression. Overexpression of TSPO is associated with several diseases, such as neurodegenerative diseases, neuroinflammation, brain injury, and cancers. TSPO has therefore become an effective biomarker of related diseases. Positron emission tomography (PET), a non-invasive molecular imaging technique used for the clinical diagnosis of numerous diseases, can detect diseases related to TSPO expression. Several radiolabeled TSPO ligands have been developed for PET. In this review, we describe recent advances in the development of TSPO ligands, and ¹⁸F-radiolabeled TSPO in particular, as PET tracers. This review covers pharmacokinetic studies, preclinical and clinical trials of ¹⁸F-labeled TSPO PET ligands, and the synthesis of TSPO ligands.

Essential Principles and Recent Progress in the Development of TSPO PET Ligands for Neuroinflammation Imaging

The translocator protein 18kDa (TSPO) is expressed in the outer mitochondrial membrane and is implicated in several functions, including cholesterol transport and steroidogenesis. Under normal physiological conditions, TSPO is present in very low concentrations in the human brain but is markedly upregulated in response to brain injury and inflammation. This upregulation is strongly associated with activated microglia. Therefore, TSPO is particularly suited for assessing active gliosis associated with brain lesions following injury or disease. For over three decades, TSPO has been studied as a biomarker. Numerous radioligands for positron emission tomography (PET) that target TSPO have been developed for imaging inflammatory progression in the brain. Although [11C]PK11195, the prototypical first-generation PET radioligand, is still widely used for in vivo studies, mainly now as its single more potent R-enantiomer, it has severe limitations, including low sensitivity and poor amenability to quantification. Second-generation radioligands are characterized by higher TSPO specific signals but suffer from other drawbacks, such as sensitivity to the TSPO single nucleotide polymorphism (SNP) rs6971. Therefore, their applications in human studies have the burden of needing to genotype subjects. Consequently, recent efforts are focused on developing improved radioligands that combine the optimal features of the second generation with the ability to overcome the differences in binding affinities across the population. This review presents essential principles in the design and development of TSPO PET ligands and discusses prominent examples among the main chemotypes.

Neuroinflammation in Low-Level PM2.5-Exposed Rats Illustrated by PET via an Improved Automated Produced [18F]FEPPA: A Feasibility Study

Background

[¹⁸F]FEPPA is a potent TSPO imaging agent that has been found to be a potential tracer for imaging neuroinflammation. In order to fulfill the demand of this tracer for preclinical and clinical studies, we have developed a one-pot automated synthesis with simplified HPLC purification of this tracer, which was then used for PET imaging of neuroinflammation in fine particulate matter- (PM2.5-) exposed rats.

Results

Using this automated synthesis method, the RCY of the [¹⁸F]FEPPA was 38 ± 4% (n = 17, EOB) in a synthesis time of 83 ± 8 min from EOB. The radiochemical purity and molar activities were greater than 99% and 209 ± 138 GBq/μmol (EOS, n = 15), respectively. The quality of the [¹⁸F]FEPPA synthesized by this method met the U.S. Pharmacopoeia (USP) criteria. The stability test showed that the [¹⁸F]FEPPA was stable at 21 ± 2°C for up to 4 hr after the end of synthesis (EOS). Moreover, microPET imaging showed that increased tracer activity of [¹⁸F]FEPPA in the brain of PM2.5-exposed rats (n = 6) were higher than that of normal controls (n = 6) and regional-specific.

Conclusions

Using the improved semipreparative HPLC purification, [¹⁸F]FEPPA has been produced in high quantity, high quality, and high reproducibility and, for the first time, used for PET imaging the effects of PM2.5 in the rat brain. It is ready to be used for imaging inflammation in various clinical or preclinical studies, especially for nearby PET centers without cyclotrons.

Novel PET Imaging of Inflammatory Targets and Cells for the Diagnosis and Monitoring of Giant Cell Arteritis and Polymyalgia Rheumatica

Giant cell arteritis (GCA) and polymyalgia rheumatica (PMR) are two interrelated inflammatory diseases affecting patients above 50 years of age. Patients with GCA suffer from granulomatous inflammation of medium- to large-sized arteries. This inflammation can lead to severe ischemic complications (e.g., irreversible vision loss and stroke) and aneurysm-related complications (such as aortic dissection). On the other hand, patients suffering from PMR present with proximal stiffness and pain due to inflammation of the shoulder and pelvic girdles. PMR is observed in 40-60% of patients with GCA, while up to 21% of patients suffering from PMR are also affected by GCA. Due to the risk of ischemic complications, GCA has to be promptly treated upon clinical suspicion. The treatment of both GCA and PMR still heavily relies on glucocorticoids (GCs), although novel targeted therapies are emerging. Imaging has a central position in the diagnosis of GCA and PMR. While [18F]fluorodeoxyglucose (FDG)-positron emission tomography (PET) has proven to be a valuable tool for diagnosis of GCA and PMR, it possesses major drawbacks such as unspecific uptake in cells with high glucose metabolism, high background activity in several non-target organs and a decrease of diagnostic accuracy already after a short course of GC treatment. In recent years, our understanding of the immunopathogenesis of GCA and, to some extent, PMR has advanced. In this review, we summarize the current knowledge on the cellular heterogeneity in the immunopathology of GCA/PMR and discuss how recent advances in specific tissue infiltrating leukocyte and stromal cell profiles may be exploited as a source of novel targets for imaging. Finally, we discuss prospective novel PET radiotracers that may be useful for the diagnosis and treatment monitoring in GCA and PMR.

Imaging Neuroinflammation in Neurodegenerative Disorders

Neuroinflammation plays a major role in the etiopathology of neurodegenerative diseases, including Alzheimer and Parkinson diseases, frontotemporal lobar degeneration, and amyotrophic lateral sclerosis. In vivo monitoring of neuroinflammation using PET is critical to understand this process, and data are accumulating in this regard, thus a review is useful. From PubMed, we retrieved publications using any of the available PET tracers to image neuroinflammation in humans as well as selected articles dealing with experimental animal models or the chemistry of currently used or potential radiotracers. We reviewed 280 articles. The most common PET neuroinflammation target, translocator protein (TSPO), has limitations, lacking cellular specificity and the ability to separate neuroprotective from neurotoxic inflammation. However, TSPO PET is useful to define the amount and location of inflammation in the brain of people with neurodegenerative disorders. We describe the characteristics of TSPO and other potential PET neuroinflammation targets and PET tracers available or in development. Despite target and tracer limitations, in recent years there has been a sharp increase in the number of reports of neuroinflammation PET in humans. The most studied has been Alzheimer disease, in which neuroinflammation seems initially neuroprotective and neurotoxic later in the progression of the disease. We describe the findings in all the major neurodegenerative disorders. Neuroinflammation PET is an indispensable tool to understand the process of neurodegeneration, particularly in humans, as well as to validate target engagement in therapeutic clinical trials.

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 (VT) estimated in 12 volumes-of-interest was analyzed regarding TSPO genotype, CNI therapy, and cognitive performance.

RESULTS

In controls, VT was about 80% higher in high affinity binders (n = 5) compared to mixed affinity binders (n = 3). Mean VT 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.

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.

Radiosynthesis and characterization of [18F]BS224: a next-generation TSPO PET ligand insensitive to the rs6971 polymorphism

PURPOSE

Translocator protein 18-kDa (TSPO) positron emission tomography (PET) is a valuable tool to detect neuroinflammed areas in a broad spectrum of neurodegenerative diseases. However, the clinical application of second-generation TSPO ligands as biomarkers is limited because of the presence of human rs6971 polymorphism that affects their binding. Here, we describe the ability of a new TSPO ligand, [18F]BS224, to identify abnormal TSPO expression in neuroinflammation independent of the rs6971 polymorphism.

METHODS

An in vitro competitive inhibition assay of BS224 was conducted with [3H]PK 11195 using membrane proteins isolated from 293FT cells expressing TSPO-wild type (WT) or TSPO-mutant A147T (Mut), corresponding to a high-affinity binder (HAB) and low-affinity binder (LAB), respectively. Molecular docking was performed to investigate the interaction of BS224 with the binding sites of rat TSPO-WT and TSPO-Mut. We synthesized a new 18F-labeled imidazopyridine acetamide ([18F]BS224) using boronic acid pinacol ester 6 or iodotoluene tosylate precursor 7, respectively, via aromatic 18F-fluorination. Dynamic PET scanning was performed up to 90 min after the injection of [18F]BS224 to healthy mice, and PET imaging data were obtained to estimate its absorbed doses in organs. To evaluate in vivo TSPO-specific uptake of [18F]BS224, lipopolysaccharide (LPS)-induced inflammatory and ischemic stroke rat models were used.

RESULTS

BS224 exhibited a high affinity (Ki = 0.51 nM) and selectivity for TSPO. The ratio of IC50 values of BS224 for LAB to that for HAB indicated that the TSPO binding affinity of BS224 has low binding sensitivity to the rs6971 polymorphism and it was comparable to that of PK 11195, which is not sensitive to the polymorphism. Docking simulations showed that the binding mode of BS224 is not affected by the A147T mutation and consequently supported the observed in vitro selectivity of [18F]BS224 regardless of polymorphisms. With optimal radiochemical yield (39 ± 6.8%, decay-corrected) and purity (> 99%), [18F]BS224 provided a clear visible image of the inflammatory lesion with a high signal-to-background ratio in both animal models (BPND = 1.43 ± 0.17 and 1.57 ± 0.37 in the LPS-induced inflammatory and ischemic stroke rat models, respectively) without skull uptake.

CONCLUSION

Our results suggest that [18F]BS224 may be a promising TSPO ligand to gauge neuroinflammatory disease-related areas in a broad range of patients irrespective of the common rs6971 polymorphism.

[11C]PBR28 radiotracer kinetics are not driven by alterations in cerebral blood flow

The positron emission tomography (PET) radiotracer [¹¹C]PBR28 has been increasingly used to image the translocator protein (TSPO) as a marker of neuroinflammation in a variety of brain disorders. Interrelatedly, similar clinical populations can also exhibit altered brain perfusion, as has been shown using arterial spin labelling in magnetic resonance imaging (MRI) studies. Hence, an unsolved debate has revolved around whether changes in perfusion could alter delivery, uptake, or washout of the radiotracer [¹¹C]PBR28, and thereby influence outcome measures that affect interpretation of TSPO upregulation. In this simultaneous PET/MRI study, we demonstrate that [¹¹C]PBR28 signal elevations in chronic low back pain patients are not accompanied, in the same regions, by increases in cerebral blood flow (CBF) compared to healthy controls, and that areas of marginal hypoperfusion are not accompanied by decreases in [¹¹C]PBR28 signal. In non-human primates, we show that hypercapnia-induced increases in CBF during radiotracer delivery or washout do not alter [¹¹C]PBR28 outcome measures. The combined results from two methodologically distinct experiments provide support from human data and direct experimental evidence from non-human primates that changes in CBF do not influence outcome measures reported by [¹¹C]PBR28 PET imaging studies and corresponding interpretations of the biological meaning of TSPO upregulation.

Comparison of Invasive and Non-invasive Estimation of [11C]PBR28 Binding in Non-human Primates

PURPOSE

To identify a reliable alternative to the full blood [¹¹C]PBR28 quantification method that would be easily replicated in multiple research and clinical settings.

PROCEDURES

Ten [¹¹C]PBR28 scans were acquired from 7 healthy non-human primates (NHP). Arterial input functions (AIFs) were averaged to create a population template input function (TIF). Population-based input functions were created by scaling the TIF with injected activity per body weight (PBIF) or unmetabolized tracer activity in blood at 15-,30-, and 60-min post-injection (PBIF15, PBIF30, and PBIF60). Two additional input functions were used: the native unmetabolized total plasma activity (Totals) and the Totals curve metabolite corrected by a scaled template parent fraction from a 30-min sample (TPF30-IF). Total distribution volumes (V_Ts) were calculated using PBIF, PBIF30, PBIF15, PBIF60, Totals, TPF30-IF, and the individual AIF (V_T^AIF). Distribution volume ratios (DVR) were computed using the cerebellum and the centrum semiovale (CSO), as pseudo-reference regions (DVR^Cereb, DVR^CSO). Results obtained with each method were compared to V_T^AIF. Applicability of these alternative methods was tested on an independent pharmacological challenge dataset of microglial activation and depletion. Evaluation was carried at baseline, immediately after intervention (acute), and weeks post-intervention (post-recovery).

RESULTS

V_Ts computed using PBIF15 and PBIF30 showed the best correlation to V_T^AIF (r > 0.90), while V_T derived from the blood-free-scaled PBIF showed poor correlation (r = 0.46) and DVR^CSO correlated the least (r = 0.26). In the pharmacological challenge study, most population-derived V_T values were comparable to V_T^AIF at baseline and showed varied sensitivity to challenges at acute and post-recovery evaluation. DVR values did not detect relevant changes.

CONCLUSIONS

Population-based input functions scaled with a single blood sample might be a useful alternative to using AIF to compute [¹¹C]PBR28 binding in healthy NHPs or animals with comparable metabolism and overall perform better than pseudo-reference regions approaches.

PET Imaging of Neuroinflammation in Alzheimer's Disease

Neuroinflammation play an important role in Alzheimer's disease pathogenesis. Advances in molecular imaging using positron emission tomography have provided insights into the time course of neuroinflammation and its relation with Alzheimer's disease central pathologies in patients and in animal disease models. Recent single-cell sequencing and transcriptomics indicate dynamic disease-associated microglia and astrocyte profiles in Alzheimer's disease. Mitochondrial 18-kDa translocator protein is the most widely investigated target for neuroinflammation imaging. New generation of translocator protein tracers with improved performance have been developed and evaluated along with tau and amyloid imaging for assessing the disease progression in Alzheimer's disease continuum. Given that translocator protein is not exclusively expressed in glia, alternative targets are under rapid development, such as monoamine oxidase B, matrix metalloproteinases, colony-stimulating factor 1 receptor, imidazoline-2 binding sites, cyclooxygenase, cannabinoid-2 receptor, purinergic P2X7 receptor, P2Y12 receptor, the fractalkine receptor, triggering receptor expressed on myeloid cells 2, and receptor for advanced glycation end products. Promising targets should demonstrate a higher specificity for cellular locations with exclusive expression in microglia or astrocyte and activation status (pro- or anti-inflammatory) with highly specific ligand to enable in vivo brain imaging. In this review, we summarised recent advances in the development of neuroinflammation imaging tracers and provided an outlook for promising targets in the future.

Interplay between immunity and amyotrophic lateral sclerosis: Clinical impact

Amyotrophic lateral sclerosis (ALS) is a debilitating and rapidly fatal neurodegenerative disease. Despite decades of research and many new insights into disease biology over the 150 years since the disease was first described, causative pathogenic mechanisms in ALS remain poorly understood, especially in sporadic cases. Our understanding of the role of the immune system in ALS pathophysiology, however, is rapidly expanding. The aim of this manuscript is to summarize the recent advances regarding the immune system involvement in ALS, with particular attention to clinical translation. We focus on the potential pathophysiologic mechanism of the immune system in ALS, discussing local and systemic factors (blood, cerebrospinal fluid, and microbiota) that influence ALS onset and progression in animal models and people. We also explore the potential of Positron Emission Tomography to detect neuroinflammation in vivo, and discuss ongoing clinical trials of therapies targeting the immune system. With validation in human patients, new evidence in this emerging field will serve to identify novel therapeutic targets and provide realistic hope for personalized treatment strategies.

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.

Impact of TSPO Receptor Polymorphism on [18F]GE-180 Binding in Healthy Brain and Pseudo-Reference Regions of Neurooncological and Neurodegenerative Disorders

TSPO-PET tracers are sensitive to a single-nucleotide polymorphism (rs6971-SNP), resulting in low-, medium- and high-affinity binders (LABs, MABs and HABS), but the clinical relevance of [18F]GE-180 is still unclear. We evaluated the impact of rs6971-SNP on in vivo [18F]GE-180 binding in a healthy brain and in pseudo-reference tissue in neuro-oncological and neurodegenerative diseases. Standardized uptake values (SUVs) of [18F]GE-180-PET were assessed using a manually drawn region of interest in the frontoparietal and cerebellar hemispheres. The SUVs were compared between the LABs, MABs and HABs in control, glioma, four-repeat tauopathy (4RT) and Alzheimer's disease (AD) subjects. Second, the SUVs were compared between the patients and controls within their rs6971-subgroups. After excluding patients with prior therapy, 24 LABs (7 control, 5 glioma, 6 4RT and 6 AD) were analyzed. Age- and sex-matched MABs (n = 38) and HABs (n = 50) were selected. The LABs had lower frontoparietal and cerebellar SUVs when compared with the MABs and HABs, but no significant difference was observed between the MABs and HABs. Within each rs6971 group, no SUV difference between the patients and controls was detected in the pseudo-reference tissues. The rs6971-SNP affects [18F]GE-180 quantification, revealing lower binding in the LABs when compared to the MABs and HABs. The frontoparietal and cerebellar ROIs were successfully validated as pseudo-reference regions.

Safety, Biodistribution, and Radiation Dosimetry of 18F-rhPSMA-7.3 in Healthy Adult Volunteers

This first-in-humans study investigated the safety, biodistribution, and radiation dosimetry of a novel ¹⁸F-labeled radiohybrid prostate-specific membrane antigen (rhPSMA) PET imaging agent, ¹⁸F-rhPSMA-7.3. Methods: Six healthy volunteers (3 men, 3 women) underwent multiple whole-body PET acquisitions at scheduled time points up to 248 min after the administration of ¹⁸F-rhPSMA-7.3 (mean activity, 220; range, 210–228 MBq). PET scans were conducted in 3 separate sessions, and subjects were encouraged to void between sessions. Blood and urine samples were collected for up to 4 h after injection to assess metabolite-corrected radioactivity in whole blood, plasma, and urine. Quantitative measurements of ¹⁸F radioactivity in volumes of interest over target organs were determined directly from the PET images at 8 time points, and normalized time–activity concentration curves were generated. These normalized cumulated activities were then inputted into the OLINDA/EXM package to calculate the internal radiation dosimetry and the subjects’ effective dose. Results:¹⁸F-rhPSMA-7.3 was well tolerated. One adverse event (mild headache, not requiring medication) was considered possibly related to ¹⁸F-rhPSMA-7.3. The calculated effective dose was 0.0141 mSv/MBq when using a 3.5-h voiding interval. The organs with the highest mean absorbed dose per unit of administered radioactivity were the adrenals (0.1835 mSv/MBq), the kidneys (0.1722 mSv/MBq), the submandibular glands (0.1479 mSv), and the parotid glands (0.1137 mSv/MBq). At the end of the first scanning session (mean time, 111 min after injection), an average of 7.2% (range, 4.4%–9.0%) of the injected radioactivity of ¹⁸F-rhPSMA-7.3 was excreted into urine. Conclusion: The safety, biodistribution, and internal radiation dosimetry of ¹⁸F-rhPSMA-7.3 are considered favorable for PET imaging.

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.

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.

Membrane-embedded TSPO: an NMR view

Translocator Protein (18 kDa) (TSPO) is a mitochondrial transmembrane protein commonly used as a biomarker for neuroinflammation and is also a potential therapeutic target in neurodegenerative diseases. Despite intensive research efforts, the function of TSPO is still largely enigmatic. Deciphering TSPO structure in the native lipid environment is essential to gain insight into its cellular activities and to design improved diagnostic and therapeutic ligands. Here, we discuss the influence of lipid composition on the structure of mammalian TSPO embedded into lipid bilayers on the basis of solid-state NMR experiments. We further highlight that cholesterol can influence both the tertiary and quaternary TSPO structure and also influence TSPO localization in mitochondria-associated endoplasmic reticulum membranes.

Recent developments on PET radiotracers for TSPO and their applications in neuroimaging

The 18 kDa translocator protein (TSPO), previously known as the peripheral benzodiazepine receptor, is predominately localized to the outer mitochondrial membrane in steroidogenic cells. Brain TSPO expression is relatively low under physiological conditions, but is upregulated in response to glial cell activation. As the primary index of neuroinflammation, TSPO is implicated in the pathogenesis and progression of numerous neuropsychiatric disorders and neurodegenerative diseases, including Alzheimer's disease (AD), amyotrophic lateral sclerosis (ALS), Parkinson's disease (PD), multiple sclerosis (MS), major depressive disorder (MDD) and obsessive compulsive disorder (OCD). In this context, numerous TSPO-targeted positron emission tomography (PET) tracers have been developed. Among them, several radioligands have advanced to clinical research studies. In this review, we will overview the recent development of TSPO PET tracers, focusing on the radioligand design, radioisotope labeling, pharmacokinetics, and PET imaging evaluation. Additionally, we will consider current limitations, as well as translational potential for future application of TSPO radiopharmaceuticals. This review aims to not only present the challenges in current TSPO PET imaging, but to also provide a new perspective on TSPO targeted PET tracer discovery efforts. Addressing these challenges will facilitate the translation of TSPO in clinical studies of neuroinflammation associated with central nervous system diseases.

PET Agents in Dementia: An Overview

This article presents an overview of imaging agents for PET that have been applied for research and diagnostic purposes in patients affected by dementia. Classified by the target which the agents visualize, seven groups of tracers can be distinguished, namely radiopharmaceuticals for: (1) Misfolded proteins (ß-amyloid, tau, α-synuclein), (2) Neuroinflammation (overexpression of translocator protein), (3) Elements of the cholinergic system, (4) Elements of monoamine neurotransmitter systems, (5) Synaptic density, (6) Cerebral energy metabolism (glucose transport/ hexokinase), and (7) Various other proteins. This last category contains proteins involved in mechanisms underlying neuroinflammation or cognitive impairment, which may also be potential therapeutic targets. Many receptors belong to this category: AMPA, cannabinoid, colony stimulating factor 1, metabotropic glutamate receptor 1 and 5 (mGluR1, mGluR5), opioid (kappa, mu), purinergic (P2X7, P2Y12), sigma-1, sigma-2, receptor for advanced glycation endproducts, and triggering receptor expressed on myeloid cells-1, besides several enzymes: cyclooxygenase-1 and 2 (COX-1, COX-2), phosphodiesterase-5 and 10 (PDE5, PDE10), and tropomyosin receptor kinase. Significant advances in neuroimaging have been made in the last 15 years. The use of 2-[¹⁸F]-fluoro-2-deoxy-D-glucose (FDG) for quantification of regional cerebral glucose metabolism is well-established. Three tracers for ß-amyloid plaques have been approved by the Food and Drug Administration and European Medicines Agency. Several tracers for tau neurofibrillary tangles are already applied in clinical research. Since many novel agents are in the preclinical or experimental stage of development, further advances in nuclear medicine imaging can be expected in the near future. PET studies with established tracers and tracers for novel targets may result in early diagnosis and better classification of neurodegenerative disorders and in accurate monitoring of therapy trials which involve these targets. PET data have prognostic value and may be used to assess the response of the human brain to interventions, or to select the appropriate treatment strategy for an individual patient.

An In Vivo Study of a Rat Fluid-Percussion-Induced Traumatic Brain Injury Model with [11C]PBR28 and [18F]flumazenil PET Imaging

Traumatic brain injury (TBI) modelled by lateral fluid percussion-induction (LFPI) in rats is a widely used experimental rodent model to explore and understand the underlying cellular and molecular alterations in the brain caused by TBI in humans. Current improvements in imaging with positron emission tomography (PET) have made it possible to map certain features of TBI-induced cellular and molecular changes equally in humans and animals. The PET imaging technique is an apt supplement to nanotheranostic-based treatment alternatives that are emerging to tackle TBI. The present study aims to investigate whether the two radioligands, [¹¹C]PBR28 and [¹⁸F]flumazenil, are able to accurately quantify in vivo molecular-cellular changes in a rodent TBI-model for two different biochemical targets of the processes. In addition, it serves to observe any palpable variations associated with primary and secondary injury sites, and in the affected versus the contralateral hemispheres. As [¹¹C]PBR28 is a radioligand of the 18 kD translocator protein, the up-regulation of which is coupled to the level of neuroinflammation in the brain, and [¹⁸F]flumazenil is a radioligand for GABA_A-benzodiazepine receptors, whose level mirrors interneuronal activity and eventually cell death, the use of the two radioligands may reveal two critical features of TBI. An up-regulation in the [¹¹C]PBR28 uptake triggered by the LFP in the injured (right) hemisphere was noted on day 14, while the uptake of [¹⁸F]flumazenil was down-regulated on day 14. When comparing the left (contralateral) and right (LFPI) hemispheres, the differences between the two in neuroinflammation were obvious. Our results demonstrate a potential way to measure the molecular alterations in a rodent-based TBI model using PET imaging with [¹¹C]PBR28 and [¹⁸F]flumazenil. These radioligands are promising options that can be eventually used in exploring the complex in vivo pharmacokinetics and delivery mechanisms of nanoparticles in TBI treatment.

PET imaging of neuroinflammation in neurological disorders

A growing need exists for reliable in-vivo measurement of neuroinflammation to better characterise the inflammatory processes underlying various diseases and to inform the development of novel therapeutics that target deleterious glial activity. PET is well suited to quantify neuroinflammation and has the potential to discriminate components of the neuroimmune response. However, there are several obstacles to the reliable quantification of neuroinflammation by PET imaging. Despite these challenges, PET studies have consistently identified associations between neuroimmune responses and pathophysiology in brain disorders such as Alzheimer's disease. Tissue studies have also begun to clarify the meaning of changes in PET signal in some diseases. Furthermore, although PET imaging of neuroinflammation does not have an established clinical application, novel targets are under investigation and a small but growing number of studies have suggested that this imaging modality could have a role in drug development. Future studies are needed to further improve our knowledge of the cellular mechanisms that underlie changes in PET signal, how immune response contributes to neurological disease, and how it might be therapeutically modified.

Tracing the History of the Human Translocator Protein to Recent Neurodegenerative and Psychiatric Imaging

The human 18 kDa translocator protein (TSPO) has been widely used as a measure of glial activation in health and disease. With the continuous progress of radiotracers with increased affinity and selectivity, associations between TSPO expression, disease severity, and progression have been examined, particularly in neurodegenerative disorders such as multiple sclerosis (MS), Parkinson's disease (PD), and amyotrophic lateral sclerosis (ALS). However, findings in psychiatric disorders have prompted reassessment of the interpretation of regional TSPO expression differences in the brain, specifically with respect to potential neuroinflammatory components. This "mini" Review aims to guide readers through the complexity of TSPO imaging research by identifying the successes, challenges, and promising new directions of the field. We will provide a brief history of how TSPO imaging has evolved over the last three decades and present lessons learned in the context of neurodegenerative and psychiatric disorders.

Confirmation of Specific Binding of the 18-kDa Translocator Protein (TSPO) Radioligand [18F]GE-180: a Blocking Study Using XBD173 in Multiple Sclerosis Normal Appearing White and Grey Matter

PURPOSE

Measurements of non-displaceable binding (V_ND) of positron emission tomography (PET) ligands are not often made in vivo in humans because they require ligands to displace binding to target receptors and there are few readily available, safe ones to use. A technique to measure V_ND for ligands for the 18-kDa translocator protein (TSPO) has recently been developed which compares the total volume of distribution (V_T) before and after administration of the TSPO ligand XBD173. Here, we used XBD173 with an occupancy plot to quantify V_ND for two TSPO radiotracers, [¹⁸F]GE-180 and [¹¹C]PBR28, in cohorts of people with multiple sclerosis (MS). Additionally, we compared plots of subjects carrying high (HAB) or mixed binding (MAB) affinity polymorphisms of TSPO to estimate V_ND without receptor blockade.

PROCEDURES

Twelve people with MS underwent baseline MRI and 90-min dynamic [¹⁸F]GE-180 PET or [¹¹C]PBR28 PET (n = 6; three HAB, three MAB each). Arterial blood sampling was used to generate plasma input functions for the two-tissue compartment model. V_ND was calculated using two independent methods: the occupancy plot (by modelling the differences in signal post XBD173) and the polymorphism plot (by modelling the differences in signal across presence and absence of rs6971 genotypes).

RESULTS

Whole brain V_T (mean ± standard deviation) was 0.29 ± 0.17 ml/cm³ for [¹⁸F]GE-180 and 5.01 ± 1.88 ml/cm³ for [¹¹C]PBR28. Using the occupancy and polymorphism plots respectively, V_ND for [¹⁸F]GE-180 was 0.11 ml/cm³ (95 % CI = 0.02, 0.16) and 0.20 ml/cm³ (0.16, 0.34), accounting for, on average, 55 % of V_T in the whole brain. For [¹¹C]PBR28, these values were 3.81 ml/cm³ (3.02, 4.21) and 3.49 ml/cm³ (1.38, 4.27), accounting for 67 % of average whole brain V_T.

CONCLUSIONS

Although V_T for [¹⁸F]GE-180 is low, indicating low brain penetration, half the signal shown by MS subjects reflected specific TSPO binding. V_T for [¹¹C]PBR28 was higher and two thirds of the binding was non-specific. No brain ROIs were devoid of specific signal, further confirming that true reference tissue approaches are potentially problematic for estimating TSPO levels.

Reliable quantification of 18F-GE-180 PET neuroinflammation studies using an individually scaled population-based input function or late tissue-to-blood ratio

Purpose

Tracer kinetic modeling of tissue time activity curves and the individual input function based on arterial blood sampling and metabolite correction is the gold standard for quantitative characterization of microglia activation by PET with the translocator protein (TSPO) ligand ¹⁸F-GE-180. This study tested simplified methods for quantification of ¹⁸F-GE-180 PET.

Methods

Dynamic ¹⁸F-GE-180 PET with arterial blood sampling and metabolite correction was performed in five healthy volunteers and 20 liver-transplanted patients. Population-based input function templates were generated by averaging individual input functions normalized to the total area under the input function using a leave-one-out approach. Individual population-based input functions were obtained by scaling the input function template with the individual parent activity concentration of ¹⁸F-GE-180 in arterial plasma in a blood sample drawn at 27.5 min or by the individual administered tracer activity, respectively. The total ¹⁸F-GE-180 distribution volume (V_T) was estimated in 12 regions-of-interest (ROIs) by the invasive Logan plot using the measured or the population-based input functions. Late ROI-to-whole-blood and ROI-to-cerebellum ratio were also computed.

Results

Correlation with the reference V_T (with individually measured input function) was very high for V_T with the population-based input function scaled with the blood sample and for the ROI-to-whole-blood ratio (Pearson correlation coefficient = 0.989 ± 0.006 and 0.970 ± 0.005). The correlation was only moderate for V_T with the population-based input function scaled with tracer activity dose and for the ROI-to-cerebellum ratio (0.653 ± 0.074 and 0.384 ± 0.177). Reference V_T, population-based V_T with scaling by the blood sample, and ROI-to-whole-blood ratio were sensitive to the TSPO gene polymorphism. Population-based V_T with scaling to the administered tracer activity and the ROI-to-cerebellum ratio failed to detect a polymorphism effect.

Conclusion

These results support the use of a population-based input function scaled with a single blood sample or the ROI-to-whole-blood ratio at a late time point for simplified quantitative analysis of ¹⁸F-GE-180 PET.

Electronic supplementary material

The online version of this article (10.1007/s00259-020-04810-1) contains supplementary material, which is available to authorized users.

Radiosynthesis of (R,S)-[18 F]GE387: A Potential PET Radiotracer for Imaging Translocator Protein 18 kDa (TSPO) with Low Binding Sensitivity to the Human Gene Polymorphism rs6971

Translocator protein (TSPO) is a biomarker of neuroinflammation, which is a hallmark of many neurodegenerative diseases and has been exploited as a positron emission tomography (PET) target. Carbon-11-labelled PK11195 remains the most applied agent for imaging TSPO, despite its short-lived isotope and low brain permeability. Second-generation radiotracers show variance in affinity amongst subjects (low-, mixed-, and high-affinity binders) caused by the genetic polymorphism (rs6971) of the TSPO gene. To overcome these limitations, a new structural scaffold was explored based on the TSPO pharmacophore, and the analogue with a low-affinity binder/high-affinity binder (LAB/HAB) ratio similar (1.2 vs. 1.3) to that of (R)-[¹¹ C]PK11195 was investigated. The synthesis of the reference compound was accomplished in six steps and 9 % overall yield, and the precursor was prepared in eight steps and 8 % overall yield. The chiral separation of the reference and precursor compounds was performed using supercritical fluid chromatography with >95 % ee. The absolute configuration was determined by circular dichroism. Optimisation of reaction conditions for manual radiolabelling revealed acetonitrile as a preferred solvent at 100 °C. Automation of this radiolabelling method provided R and S enantiomers in respective 21.3±16.7 and 25.6±7.1 % decay-corrected yields and molar activities of 55.8±35.6 and 63.5±39.5 GBq μmol^-1 (n=3). Injection of the racemic analogue into a healthy rat confirmed passage through the blood-brain barrier.

In vivo molecular imaging of neuroinflammation in Alzheimer's disease

It has become increasingly evident that neuroinflammation plays a critical role in the pathophysiology of Alzheimer's disease (AD) and other neurodegenerative disorders. Increased glial cell activation is consistently reported in both rodent models of AD and in AD patients. Moreover, recent genome wide association studies have revealed multiple genes associated with inflammation and immunity are significantly associated with an increased risk of AD development (e.g. TREM2). Non‐invasive in vivo detection and tracking of neuroinflammation is necessary to enhance our understanding of the contribution of neuroinflammation to the initiation and progression of AD. Importantly, accurate methods of quantifying neuroinflammation may aid early diagnosis and serve as an output for therapeutic monitoring and disease management. This review details current in vivo imaging biomarkers of neuroinflammation being explored and summarizes both pre‐clinical and clinical results from molecular imaging studies investigating the role of neuroinflammation in AD, with a focus on positron emission tomography and magnetic resonance spectroscopy (MRS).

Evaluation of the novel TSPO radiotracer 2-(7-butyl-2-(4-(2-([18F]fluoroethoxy)phenyl)-5-methylpyrazolo[1,5-a]pyrimidin-3-yl)-N,N-diethylacetamide in a preclinical model of neuroinflammation

Translocator Protein (18 kDa, TSPO) is regarded as a useful biomarker for neuroinflammation imaging. TSPO PET imaging could be used to understand the role of neuroinflammation in brain diseases and as a tool for evaluating novel therapeutic effects. As a promising TSPO probe, [¹⁸F]DPA-714 is highly specific and offers reliable quantification of TSPO in vivo. In this study, we further radiosynthesized and evaluated another novel TSPO probe, 2-(7-butyl-2-(4-(2-[¹⁸F]fluoroethoxy)phenyl)-5-methylpyrazolo[1,5-a]pyrimidin-3-yl)-N,N-diethylacetamide ([¹⁸F]VUIIS1018A), which features a 700-fold higher binding affinity for TSPO than that of [¹⁸F]DPA-714. We evaluated the performance of [¹⁸F]VUIIS1018A using dynamic in vivo PET imaging, radiometabolite analysis, in vitro autoradiography assays, biodistribution analysis, and blocking assays. In vivo study using this probe demonstrated high signal-to-noise ratio, binding potential (BP_ND), and binding specificity in preclinical neuroinflammation studies. Taken together, these findings indicate that [¹⁸F]VUIIS1018A may serve as a novel TSPO PET probe for neuroinflammation imaging.