The development of an automated and GMP compliant FASTlab™ Synthesis of [(18) F]GE-180; a radiotracer for imaging translocator protein (TSPO)

Enhancing predictability of IDH mutation status in glioma patients at initial diagnosis: a comparative analysis of radiomics from MRI, [18F]FET PET, and TSPO PET

PURPOSE

According to the World Health Organization classification for tumors of the central nervous system, mutation status of the isocitrate dehydrogenase (IDH) genes has become a major diagnostic discriminator for gliomas. Therefore, imaging-based prediction of IDH mutation status is of high interest for individual patient management. We compared and evaluated the diagnostic value of radiomics derived from dual positron emission tomography (PET) and magnetic resonance imaging (MRI) data to predict the IDH mutation status non-invasively.

METHODS

Eighty-seven glioma patients at initial diagnosis who underwent PET targeting the translocator protein (TSPO) using [18F]GE-180, dynamic amino acid PET using [18F]FET, and T1-/T2-weighted MRI scans were examined. In addition to calculating tumor-to-background ratio (TBR) images for all modalities, parametric images quantifying dynamic [18F]FET PET information were generated. Radiomic features were extracted from TBR and parametric images. The area under the receiver operating characteristic curve (AUC) was employed to assess the performance of logistic regression (LR) classifiers. To report robust estimates, nested cross-validation with five folds and 50 repeats was applied.

RESULTS

TBRGE-180 features extracted from TSPO-positive volumes had the highest predictive power among TBR images (AUC 0.88, with age as co-factor 0.94). Dynamic [18F]FET PET reached a similarly high performance (0.94, with age 0.96). The highest LR coefficients in multimodal analyses included TBRGE-180 features, parameters from kinetic and early static [18F]FET PET images, age, and the features from TBRT2 images such as the kurtosis (0.97).

CONCLUSION

The findings suggest that incorporating TBRGE-180 features along with kinetic information from dynamic [18F]FET PET, kurtosis from TBRT2, and age can yield very high predictability of IDH mutation status, thus potentially improving early patient management.

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.

Targeting the Translocator Protein (18 kDa) in Cardiac Diseases: State of the Art and Future Opportunities

Mitochondria dysfunctions are typical hallmarks of cardiac disorders (CDs). The multiple tasks of this energy-producing organelle are well documented, but its pathophysiologic involvement in several manifestations of heart diseases, such as altered electromechanical coupling, excitability, and arrhythmias, is still under investigation. The human 18 kDa translocator protein (TSPO) is a protein located on the outer mitochondrial membrane whose expression is altered in different pathological conditions, including CDs, making it an attractive therapeutic and diagnostic target. Currently, only a few TSPO ligands are employed in CDs and cardiac imaging. In this Perspective, we report an overview of the emerging role of TSPO at the heart level, focusing on the recent literature concerning the development of TSPO ligands used for fighting and imaging heart-related disease conditions. Accordingly, targeting TSPO might represent a successful strategy to achieve novel therapeutic and diagnostic strategies to unravel the fundamental mechanisms and to provide solutions to still unanswered questions in CDs.

Astrocytes: Lessons Learned from the Cuprizone Model

A diverse array of neurological and psychiatric disorders, including multiple sclerosis, Alzheimer's disease, and schizophrenia, exhibit distinct myelin abnormalities at both the molecular and histological levels. These aberrations are closely linked to dysfunction of oligodendrocytes and alterations in myelin structure, which may be pivotal factors contributing to the disconnection of brain regions and the resulting characteristic clinical impairments observed in these conditions. Astrocytes, which significantly outnumber neurons in the central nervous system by a five-to-one ratio, play indispensable roles in the development, maintenance, and overall well-being of neurons and oligodendrocytes. Consequently, they emerge as potential key players in the onset and progression of a myriad of neurological and psychiatric disorders. Furthermore, targeting astrocytes represents a promising avenue for therapeutic intervention in such disorders. To gain deeper insights into the functions of astrocytes in the context of myelin-related disorders, it is imperative to employ appropriate in vivo models that faithfully recapitulate specific aspects of complex human diseases in a reliable and reproducible manner. One such model is the cuprizone model, wherein metabolic dysfunction in oligodendrocytes initiates an early response involving microglia and astrocyte activation, culminating in multifocal demyelination. Remarkably, following the cessation of cuprizone intoxication, a spontaneous process of endogenous remyelination occurs. In this review article, we provide a historical overview of studies investigating the responses and putative functions of astrocytes in the cuprizone model. Following that, we list previously published works that illuminate various aspects of the biology and function of astrocytes in this multiple sclerosis model. Some of the studies are discussed in more detail in the context of astrocyte biology and pathology. Our objective is twofold: to provide an invaluable overview of this burgeoning field, and, more importantly, to inspire fellow researchers to embark on experimental investigations to elucidate the multifaceted functions of this pivotal glial cell subpopulation.

A feasibility study for quantitative assessment of cerebrovascular malformations using flutriciclamide ([18F]GE-180) PET/MRI

Aim

Neuroinflammation plays a key role in both the pathogenesis and the progression of cerebral cavernous malformations (CCM). Flutriciclamide ([18F]GE-180) is a translocator protein (TSPO) targeting positron emission tomography (PET) tracer, developed for imaging neuroinflammation. The objectives of this study were to describe characteristics of flutriciclamide uptake in different brain tissue regions in CCM patients compared to controls, and to evaluate flutriciclamide uptake and iron deposition within CCM lesions.

Materials and methods

Five patients with CCM and six controls underwent a 60 or 90 min continuous PET/MRI scan following 315 ± 68.9 MBq flutriciclamide administration. Standardized uptake value (SUV) and standardized uptake value ratio (SUVr) were obtained using the striatum as a pseudo-reference. Quantitative susceptibility maps (QSM) were used to define the location of the vascular malformation and calculate the amount of iron deposition in each lesion.

Results

Increased flutriciclamide uptake was observed in all CCM lesions. The temporal pole demonstrated the highest radiotracer uptake; the paracentral lobule, cuneus and hippocampus exhibited moderate uptake; while the striatum had the lowest uptake, with average SUVs of 0.66, 0.55, 0.63, 0.55, and 0.33 for patient with CCM and 0.57, 0.50, 0.48, 0.42, and 0.32 for controls, respectively. Regional SUVr showed similar trends. The average SUV and QSM values in CCM lesions were 0.58 ± 0.23 g/ml and 0.30 ± 0.10 ppm. SUVs and QSM were positively correlated in CCM lesions (r = 0.53, p = 0.03).

Conclusion

The distribution of flutriciclamide ([18F]GE-180) in the human brain and CCM lesions demonstrated the potential of this TSPO PET tracer as a marker of neuroinflammation that may be relevant for characterizing CCM disease progression along with QSM.

Feasibility of radiomic feature harmonization for pooling of [18F]FET or [18F]GE-180 PET images of gliomas

INTRODUCTION

Large datasets are required to ensure reliable non-invasive glioma assessment with radiomics-based machine learning methods. This can often only be achieved by pooling images from different centers. Moreover, trained models should perform with high accuracy when applied to data from different centers. In this study, the impact of reconstruction settings and segmentation methods on radiomic features derived from amino acid and TSPO PET images of glioma patients was examined. Additionally, the ability to model and thus reduce feature differences was investigated.

METHODS

[¹⁸F]FET and [¹⁸F]GE-180 PET data were acquired from 19 glioma patients. For each acquisition, 10 reconstruction settings and 9 segmentation methods were included to emulate multicentric data. Statistical robustness measures were calculated before and after ComBat harmonization. Differences between features due to setting variations were assessed using Friedman test, coefficient of variation (CV) and inter-rater reliability measures, including intraclass and Spearman's rank correlation coefficients and Fleiss' Kappa.

RESULTS

According to Friedman analyses, most features (>60%) showed significant differences. Yet, CV and inter-rater reliability measures indicated higher robustness. ComBat resulted in almost complete harmonization (>87%) according to Friedman test and little to no improvement according to CV and inter-rater reliability measures. [¹⁸F]GE-180 features were more sensitive to reconstruction settings than [¹⁸F]FET features.

CONCLUSIONS

According to Friedman test, feature distributions could be successfully aligned using ComBat. However, depending on settings, changes in patient ranks were observed for some features and could not be eliminated by harmonization. Thus, for clinical utilization it is recommended to exclude affected features.

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.

[18F]BIBD-239: 18F-Labeled ER176, a Positron Emission Tomography Tracer Specific for the Translocator Protein

[¹¹C]ER176 has adequate sensitivity to image the human brain translocator protein (TSPO) in all three genotypes by positron emission tomography (PET). However, its clinical application is limited by the short half-life of ¹¹C (20.38 min). To overcome the deficiency of [¹¹C]ER176 and keep the pharmacophore features of ER176 to the maximum extent, we designed four fluorine-labeled ER176 derivatives using the deuterium method. In vitro competition binding confirmed that the designed compounds had high affinity for TSPO. Biodistribution experiments showed that tissues with high expression of TSPO had high uptake of these compounds, as well as that the compound showed high brain penetration and mild defluorination in vivo. Therefore, [¹⁸F]BIBD-239 with simple synthesis conditions was selected for further biological evaluation. Theoretical simulations showed that BIBD-239 and ER176 have similar binding modes and sites to Ala147-TSPO and Thr147-TSPO, which indicated that the tracers may have consistent sensitivity to the three affinity genotypes. In vitro autoradiography and in vivo PET studies of the ischemic rat brain showed dramatically higher uptake of [¹⁸F]BIBD-239 on the lesion site compared to the contralateral side with good brain kinetics. Additionally, [¹⁸F]BIBD-239 provided clear tumor PET images in a GL261 glioma model. Importantly, PET imaging and liquid chromatography-high-resolution mass spectrometry (LC-HRMS) results showed that in vivo defluorination and other metabolites of [¹⁸F]BIBD-239 did not interfere with brain imaging. Conclusively, [¹⁸F]BIBD-239, similar to ER176 with low polymorphism sensitivity, has simple labeling conditions, high labeling yield, high affinity, and high specificity for TSPO, and it is planned for further evaluation in higher species.

Longitudinal [18F]GE-180 PET Imaging Facilitates In Vivo Monitoring of TSPO Expression in the GL261 Glioblastoma Mouse Model

The 18 kDa translocator protein (TSPO) is increasingly recognized as an interesting target for the imaging of glioblastoma (GBM). Here, we investigated TSPO PET imaging and autoradiography in the frequently used GL261 glioblastoma mouse model and aimed to generate insights into the temporal evolution of TSPO radioligand uptake in glioblastoma in a preclinical setting. We performed a longitudinal [18F]GE-180 PET imaging study from day 4 to 14 post inoculation in the orthotopic syngeneic GL261 GBM mouse model (n = 21 GBM mice, n = 3 sham mice). Contrast-enhanced computed tomography (CT) was performed at the day of the final PET scan (±1 day). [18F]GE-180 autoradiography was performed on day 7, 11 and 14 (ex vivo: n = 13 GBM mice, n = 1 sham mouse; in vitro: n = 21 GBM mice; n = 2 sham mice). Brain sections were also used for hematoxylin and eosin (H&E) staining and TSPO immunohistochemistry. [18F]GE-180 uptake in PET was elevated at the site of inoculation in GBM mice as compared to sham mice at day 11 and later (at day 14, TBRmax +27% compared to sham mice, p = 0.001). In GBM mice, [18F]GE-180 uptake continuously increased over time, e.g., at day 11, mean TBRmax +16% compared to day 4, p = 0.011. [18F]GE-180 uptake as depicted by PET was in all mice co-localized with contrast-enhancement in CT and tissue-based findings. [18F]GE-180 ex vivo and in vitro autoradiography showed highly congruent tracer distribution (r = 0.99, n = 13, p < 0.001). In conclusion, [18F]GE-180 PET imaging facilitates non-invasive in vivo monitoring of TSPO expression in the GL261 GBM mouse model. [18F]GE-180 in vitro autoradiography is a convenient surrogate for ex vivo autoradiography, allowing for straightforward identification of suitable models and scan time-points on previously generated tissue sections.

Dimethyl fumarate decreases short-term but not long-term inflammation in a focal EAE model of neuroinflammation

Background

Dimethyl fumarate (DMF) is an oral immunomodulatory drug used in the treatment of autoimmune diseases. Here, we sought to study whether the effect of DMF can be detected using positron emission tomography (PET) targeting the 18-kDa translocator protein (TSPO) in the focal delayed-type hypersensitivity rat model of multiple sclerosis (fDTH-EAE). The rats were treated orally twice daily from lesion activation (day 0) with either vehicle (tap water with 0.08% Methocel, 200 µL; control group n = 4 (3 after week four)) or 15 mg/kg DMF (n = 4) in 0.08% aqueous Methocel (200 µL) for 8 weeks. The animals were imaged by PET using the TSPO tracer [¹⁸F]GE-180 in weeks 0, 1, 2, 4, 8, and 18 following lesion activation, and the non-displaceable binding potential (BP_ND) was calculated. Immunohistochemical staining for Iba1, CD4, and CD8 was performed in week 18, and in separate cohorts of animals, following 2 or 4 weeks of treatment.

Results

Using the fDTH-EAE model, DMF reduced the [¹⁸F]GE-180 BP_ND in the DMF-treated animals compared to control animals after 1 week of treatment (two-tailed unpaired t test, p = 0.031), but not in weeks 2, 4, 8, or 18 when imaged in vivo by PET. Immunostaining for Iba1 showed that DMF had no effect on the perilesional volume or the core lesion volume after 2 or 4 weeks of treatment, or at 18 weeks. However, the optical density (OD) measurements of CD4⁺ staining showed reduced OD in the lesions of the treated rats.

Conclusions

DMF reduced the microglial activation in the fDTH-EAE model after 1 week of treatment, as detected by PET imaging of the TSPO ligand [¹⁸F]GE-180. However, over an extended time course, reduced microglial activation was not observed using [¹⁸F]GE-180 or by immunohistochemistry for Iba1⁺ microglia/macrophages. Additionally, DMF did affect the infiltration of CD4⁺ and CD8⁺ T-lymphocytes at the fDTH-EAE lesion.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13550-022-00878-y.

In a focal rat DTH-EAE model of neuroinflammation, dimethyl fumarate decreases the uptake of TSPO PET tracer [18F]GE-180 in the short term.

Long-term [¹⁸F]GE-180 follow-up did not indicate a treatment effect.

Decreased neuroinflammation, CD4⁺ T cell infiltration, and CD8⁺ T cell infiltration were detected using immunohistochemistry.

Impact of Partial Volume Correction on [18F]GE-180 PET Quantification in Subcortical Brain Regions of Patients with Corticobasal Syndrome

Corticobasal syndrome (CBS) is a rare neurodegenerative condition characterized by four-repeat tau aggregation in the cortical and subcortical brain regions and accompanied by severe atrophy. The aim of this study was to evaluate partial volume effect correction (PVEC) in patients with CBS compared to a control cohort imaged with the 18-kDa translocator protein (TSPO) positron emission tomography (PET) tracer [¹⁸F]GE-180. Eighteen patients with CBS and 12 age- and sex-matched healthy controls underwent [¹⁸F]GE-180 PET. The cortical and subcortical regions were delineated by deep nuclei parcellation (DNP) of a 3D-T1 MRI. Region-specific subcortical volumes and standardized uptake values and ratios (SUV and SUVr) were extracted before and after region-based voxel-wise PVEC. Regional volumes were compared between patients with CBS and controls. The % group differences and effect sizes (CBS vs. controls) of uncorrected and PVE-corrected SUVr data were compared. Single-region positivity in patients with CBS was assessed by a >2 SD threshold vs. controls and compared between uncorrected and PVE-corrected data. Smaller regional volumes were detected in patients with CBS compared to controls in the right ventral striatum (p = 0.041), the left putamen (p = 0.005), the right putamen (p = 0.038) and the left pallidum (p = 0.015). After applying PVEC, the % group differences were distinctly higher, but the effect sizes of TSPO uptake were only slightly stronger due to the higher variance after PVEC. The single-region positivity of TSPO PET increased in patients with CBS after PVEC (100 vs. 83 regions). PVEC in the cortical and subcortical regions is valuable for TSPO imaging of patients with CBS, leading to the improved detection of elevated [¹⁸F]GE-180 uptake, although the effect sizes in the comparison against the controls did not improve strongly.

Differential Spatial Distribution of TSPO or Amino Acid PET Signal and MRI Contrast Enhancement in Gliomas

Simple Summary

Radiotracers targeting the translocator protein (TSPO) have recently gained substantial interest, since TSPO is overexpressed in malignant gliomas, where it correlates inversely with patient’s survival. The high-affinity TSPO PET ligand [¹⁸F]GE180 was found to depict tumor areas with a remarkably high contrast and has been shown to provide non-invasive information on histological tumor grades. Yet, its significance was questioned with the argument, that the high contrast may solely arise from nonspecific accumulation in tissue supplied by leaky vessels. This study aimed to address this question by providing a detailed evaluation of spatial associations between TSPO and amino acid PET with relative contrast enhancement in T1-weighted MRI. The results show that [¹⁸F]GE180 contrast does not reflect a disrupted blood–brain barrier (BBB) only and that multi-modal imaging generates complementary information, which may better depict spatial heterogeneity of tumor biology and may be used to individualize the therapy for each patient.

Abstract

In this study, dual PET and contrast enhanced MRI were combined to investigate their correlation per voxel in patients at initial diagnosis with suspected glioblastoma. Correlation with contrast enhancement (CE) as an indicator of BBB leakage was further used to evaluate whether PET signal is likely caused by BBB disruption alone, or rather attributable to specific binding after BBB passage. PET images with [¹⁸F]GE180 and the amino acid [¹⁸F]FET were acquired and normalized to healthy background (tumor-to-background ratio, TBR). Contrast enhanced images were normalized voxel by voxel with the pre-contrast T1-weighted MRI to generate relative CE values (rCE). Voxel-wise analysis revealed a high PET signal even within the sub-volumes without detectable CE. No to moderate correlation of rCE with TBR voxel-values and a small overlap as well as a larger distance of the hotspots delineated in rCE and TBR-PET images were detected. In contrast, voxel-wise correlation between both PET modalities was strong for most patients and hotspots showed a moderate overlap and distance. The high PET signal in tumor sub-volumes without CE observed in voxel-wise analysis as well as the discordant hotspots emphasize the specificity of the PET signals and the relevance of combined differential information from dual PET and MRI images.

Long-Term Sex- and Genotype-Specific Effects of 56Fe Irradiation on Wild-Type and APPswe/PS1dE9 Transgenic Mice

Space radiation presents a substantial threat to travel beyond Earth. Relatively low doses of high-energy particle radiation cause physiological and behavioral impairments in rodents and may pose risks to human spaceflight. There is evidence that ⁵⁶Fe irradiation, a significant component of space radiation, may be more harmful to males than to females and worsen Alzheimer's disease pathology in genetically vulnerable models. Yet, research on the long-term, sex- and genotype-specific effects of ⁵⁶Fe irradiation is lacking. Here, we irradiated 4-month-old male and female, wild-type and Alzheimer's-like APP/PS1 mice with 0, 0.10, or 0.50 Gy of ⁵⁶Fe ions (1GeV/u). Mice underwent microPET scans before and 7.5 months after irradiation, a battery of behavioral tests at 11 months of age and were sacrificed for pathological and biochemical analyses at 12 months of age. ⁵⁶Fe irradiation worsened amyloid-beta (Aβ) pathology, gliosis, neuroinflammation and spatial memory, but improved motor coordination, in male transgenic mice and worsened fear memory in wild-type males. Although sham-irradiated female APP/PS1 mice had more cerebral Aβ and gliosis than sham-irradiated male transgenics, female mice of both genotypes were relatively spared from radiation effects 8 months later. These results provide evidence for sex-specific, long-term CNS effects of space radiation.

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.

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.

Robust and Facile Automated Radiosynthesis of [18F]FSPG on the GE FASTlab

Purpose

(S)-4-(3-¹⁸F-Fluoropropyl)-ʟ-Glutamic Acid ([¹⁸F]FSPG) is a radiolabeled non-natural amino acid that is used for positron emission tomography (PET) imaging of the glutamate/cystine antiporter, system x_C^-, whose expression is upregulated in many cancer types. To increase the clinical adoption of this radiotracer, reliable and facile automated procedures for [¹⁸F]FSPG production are required. Here, we report a cassette-based method to produce [¹⁸F]FSPG at high radioactivity concentrations from low amounts of starting activity.

Procedures

An automated synthesis and purification of [¹⁸F]FSPG was developed using the GE FASTlab. Optimization of the reaction conditions and automated manipulations were performed by measuring the isolated radiochemical yield of [¹⁸F]FSPG and by assessing radiochemical purity using radio-HPLC. Purification of [¹⁸F]FSPG was conducted by trapping and washing of the radiotracer on Oasis MCX SPE cartridges, followed by a reverse elution of [¹⁸F]FSPG in phosphate-buffered saline. Subsequently, the [¹⁸F]FSPG obtained from the optimized process was used to image an animal model of non-small cell lung cancer.

Results

The optimized protocol produced [¹⁸F]FSPG in 38.4 ± 2.6 % radiochemical yield and >96 % radiochemical purity with a molar activity of 11.1 ± 7.7 GBq/μmol. Small alterations, including the implementation of a reverse elution and an altered Hypercarb cartridge, led to significant improvements in radiotracer concentration from <10 MBq/ml to >100 MBq/ml. The improved radiotracer concentration allowed for the imaging of up to 20 mice, starting with just 1.5 GBq of [¹⁸F]Fluoride.

Conclusions

We have developed a robust and facile method for [¹⁸F]FSPG radiosynthesis in high radiotracer concentration, radiochemical yield, and radiochemical purity. This cassette-based method enabled the production of [¹⁸F]FSPG at radioactive concentrations sufficient to facilitate large-scale preclinical experiments with a single prep of starting activity. The use of a cassette-based radiosynthesis on an automated synthesis module routinely used for clinical production makes the method amenable to rapid and widespread clinical translation.

Supplementary Information

The online version contains supplementary material available at 10.1007/s11307-021-01609-w.

Evading the Immune System: Immune Modulation and Immune Matching in Cell Replacement Therapies for Parkinson's Disease

Stem cell-based therapies for Parkinson’s disease are now being applied clinically. Notably, studies have shown that controlling the graft-induced immune response improves the results. In this mini-review, we concisely summarize current approaches used for this control. We focus on four modes of stem cell-based therapies: autologous transplantation, allogeneic transplantation with human leukocyte antigen-matching and allogeneic transplantation without, and finally the application of “universal” pluripotent stem cells. We also discuss immuno-suppressive treatments and the monitoring of immune reactions in the brain.

Fully Automated Synthesis of Novel TSPO PET Imaging Ligand [18F]Fluoroethyltemazepam

INTRODUCTION

Benzodiazepines, including temazepam are described as TSPO antagonists. In fact, TSPO was initially described as a peripheral benzodiazepine receptor (PBR) with a secondary binding site for diazepam. TSPO is a potential imaging target of neuroinflammation because there is an amplification of the expression of this receptor.

OBJECTIVES

Herein, we developed a novel fluorinated benzodiazepine ligand, [18F]Fluoroethyltemazepam ([18F]F-FETEM), for positron emission tomography (PET) imaging of translocator protein (18 kDa).

METHODS

[18F]F-FETEM was radiolabelled with an automated synthesizer via a one-pot procedure. We conducted a [18F]F-aliphatic nucleophilic substitution of a tosylated precursor followed by purification on C18 and Alumina N SPE cartridges. Quality control tests was also carried out.

RESULTS

We obtained 2.0-3.0% decay-uncorrected radiochemical activity yield (3.7% decay-corrected) within the whole synthesis time about 33 min. The radiochemical purity of [18F]F-FETEM was over 90% by TLC analysis.

CONCLUSIONS

This automated procedure may be used as basis for future production of [18F]F-FETEM for preclinical PET imaging studies.

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.

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.

Development of an automated, GMP compliant FASTlab™ radiosynthesis of [18 F]GE-179 for the clinical study of activated NMDA receptors

N-(2-chloro-5-(S-2-[18 F]fluoroethyl)thiophenyl)-N'-(3-thiomethylphenyl)-N'-methylguanidine, ([18 F]GE-179), has been identified as a promising positron emission tomography (PET) ligand for the intra-channel phencyclidine (PCP) binding site of the N-methyl-D-aspartate (NMDA) receptor. The radiosynthesis of [18 F]GE-179 has only been performed at low radioactivity levels. However, the manufacture of a GMP compliant product at high radioactivity levels was required for clinical studies. We describe the development of a process using the GE FASTlab™ radiosynthesis platform coupled with HPLC purification. The radiosynthesis is a two-step process, involving the nucleophilic fluorination of ethylene ditosylate, 11, followed by alkylation to the deprotonated thiol precursor, N-(2-chloro-5-thiophenol)-N'-(3-thiomethylphenyl)-N'-methyl guanidine, 8. The crude product was purified by semi-preparative HPLC to give the formulated product in an activity yield (AY) of 7 ± 2% (n = 15) with a total synthesis time of 120 minutes. The radioactive concentration (RAC) and radiochemical purity (RCP) were 328 ± 77 MBq/mL and 96.5 ± 1% respectively and the total chemical content was 2 ± 1 μg. The final formulation volume was 14 mL. The previously described radiosynthesis of [18 F]GE-179 was successfully modified to deliver an process on the FASTlab™ that allows the manufacture of a GMP quality product from high starting radioactivitity (up to 80 GBq) and delivers a product suitable for clinical use.

TSPO PET, tumour grading and molecular genetics in histologically verified glioma: a correlative 18F-GE-180 PET study

BACKGROUND

The 18-kDa translocator protein (TSPO) is overexpressed in brain tumours and represents an interesting target for glioma imaging. ¹⁸F-GE-180, a novel TSPO ligand, has shown improved binding affinity and a high target-to-background contrast in patients with glioblastoma. However, the association of uptake characteristics on TSPO PET using ¹⁸F-GE-180 with the histological WHO grade and molecular genetic features so far remains unknown and was evaluated in the current study.

METHODS

Fifty-eight patients with histologically validated glioma at initial diagnosis or recurrence were included. All patients underwent ¹⁸F-GE-180 PET, and the maximal and mean tumour-to-background ratios (TBR_max, TBR_mean) as well as the PET volume were assessed. On MRI, presence/absence of contrast enhancement was evaluated. Imaging characteristics were correlated with neuropathological parameters (i.e. WHO grade, isocitrate dehydrogenase (IDH) mutation, O-6-methylguanine-DNA methyltransferase (MGMT) promoter methylation and telomerase reverse transcriptase (TERT) promoter mutation).

RESULTS

Six of 58 patients presented with WHO grade II, 16/58 grade III and 36/58 grade IV gliomas. An (IDH) mutation was found in 19/58 cases, and 39/58 were classified as IDH-wild type. High ¹⁸F-GE-180-uptake was observed in all but 4 cases (being WHO grade II glioma, IDH-mutant). A high association of ¹⁸F-GE-180-uptake and WHO grades was seen: WHO grade IV gliomas showed the highest uptake intensity compared with grades III and II gliomas (median TBR_max 5.15 (2.59-8.95) vs. 3.63 (1.85-7.64) vs. 1.63 (1.50-3.43), p < 0.001); this association with WHO grades persisted within the IDH-wild-type and IDH-mutant subgroup analyses (p < 0.05). Uptake intensity was also associated with the IDH mutational status with a trend towards higher ¹⁸F-GE-180-uptake in IDH-wild-type gliomas in the overall group (median TBR_max 4.67 (1.56-8.95) vs. 3.60 (1.50-7.64), p = 0.083); however, within each WHO grade, no differences were found (e.g. median TBR_max in WHO grade III glioma 4.05 (1.85-5.39) vs. 3.36 (2.32-7.64), p = 1.000). No association was found between uptake intensity and MGMT or TERT (p > 0.05 each).

CONCLUSION

Uptake characteristics on ¹⁸F-GE-180 PET are highly associated with the histological WHO grades, with the highest ¹⁸F-GE-180 uptake in WHO grade IV glioblastomas and a PET-positive rate of 100% among the investigated high-grade gliomas. Conversely, all TSPO-negative cases were WHO grade II gliomas. The observed association of ¹⁸F-GE-180 uptake and the IDH mutational status seems to be related to the high inter-correlation of the IDH mutational status and the WHO grades.

Space-like 56Fe irradiation manifests mild, early sex-specific behavioral and neuropathological changes in wildtype and Alzheimer's-like transgenic mice

Space travel will expose people to high-energy, heavy particle radiation, and the cognitive deficits induced by this exposure are not well understood. To investigate the short-term effects of space radiation, we irradiated 4-month-old Alzheimer’s disease (AD)-like transgenic (Tg) mice and wildtype (WT) littermates with a single, whole-body dose of 10 or 50 cGy ⁵⁶Fe ions (1 GeV/u) at Brookhaven National Laboratory. At ~1.5 months post irradiation, behavioural testing showed sex-, genotype-, and dose-dependent changes in locomotor activity, contextual fear conditioning, grip strength, and motor learning, mainly in Tg but not WT mice. There was little change in general health, depression, or anxiety. Two months post irradiation, microPET imaging of the stable binding of a translocator protein ligand suggested no radiation-specific change in neuroinflammation, although initial uptake was reduced in female mice independently of cerebral blood flow. Biochemical and immunohistochemical analyses revealed that radiation reduced cerebral amyloid-β levels and microglia activation in female Tg mice, modestly increased microhemorrhages in 50 cGy irradiated male WT mice, and did not affect synaptic marker levels compared to sham controls. Taken together, we show specific short-term changes in neuropathology and behaviour induced by ⁵⁶Fe irradiation, possibly having implications for long-term space travel.

Cessation of anti-VLA-4 therapy in a focal rat model of multiple sclerosis causes an increase in neuroinflammation

Background

Positron emission tomography (PET) can be used for in vivo evaluation of the pathology associated with multiple sclerosis. We investigated the use of longitudinal PET imaging and the 18-kDa translocator protein (TSPO) binding radioligand [¹⁸F]GE-180 to detect changes in a chronic multiple sclerosis-like focal delayed-type hypersensitivity experimental autoimmune encephalomyelitis (fDTH-EAE) rat model during and after anti-VLA-4 monoclonal antibody (mAb) treatment. Thirty days after lesion activation, fDTH-EAE rats were treated with the anti-VLA-4 mAb (n = 4) or a control mAb (n = 4; 5 mg/kg, every third day, subcutaneously) for 31 days. Animals were imaged with [¹⁸F]GE-180 on days 30, 44, 65, 86 and 142. Another group of animals (n = 4) was used for visualisation the microglia with Iba-1 at day 44 after a 2-week treatment period.

Results

After a 2-week treatment period on day 44, there was a declining trend (p = 0.067) in [¹⁸F]GE-180-binding in the anti-VLA-4 mAb-treated animals versus controls. However, cessation of treatment for 4 days after a 31-day treatment period increased [¹⁸F]GE-180 binding in animals treated with anti-VLA-4 mAb compared to the control group (p = 0.0003). There was no difference between the groups in TSPO binding by day 142.

Conclusions

These results demonstrated that cessation of anti-VLA-4 mAb treatment for 4 days caused a transient rebound increase in neuroinflammation. This highlights the usefulness of serial TSPO imaging in the fDTH-EAE model to better understand the rebound phenomenon.

[Cell therapy for Parkinson's disease with induced pluripotent stem cells]

Cell therapy for Parkinson's disease has a history of being applied clinically with aborted embryos as donor source. Efficacy of the therapy under the appropriate condition has been reported. Based on this experience and the advancement of stem cell technology, clinical trials of cell therapy with embryonic stem cells (ESCs) or induced pluripotent stem cells (iPSCs) are going to start soon in several countries. In Japan a physician-initiated clinical trial of iPSC-based therapy for Parkinson's disease has launched since 2018. This trial adopts allogeneic transplantation with a cell line from iPSC stock. This article discusses patient selection, procedure, and risk of the therapy. It also introduces the world's current situation of the cell therapy for Parkinson's disease.

Expression of Translocator Protein and [18F]-GE180 Ligand Uptake in Multiple Sclerosis Animal Models

Positron emission tomography (PET) ligands targeting the translocator protein (TSPO) represent promising tools to visualize neuroinflammation in multiple sclerosis (MS). Although it is known that TSPO is expressed in the outer mitochondria membrane, its cellular localization in the central nervous system under physiological and pathological conditions is not entirely clear. The purpose of this study was to assess the feasibility of utilizing PET imaging with the TSPO tracer, [18F]-GE180, to detect histopathological changes during experimental demyelination, and to determine which cell types express TSPO. C57BL/6 mice were fed with cuprizone for up to 5 weeks to induce demyelination. Groups of mice were investigated by [18F]-GE180 PET imaging at week 5. Recruitment of peripheral immune cells was triggered by combining cuprizone intoxication with MOG_35–55 immunization (i.e., Cup/EAE). Immunofluorescence double-labelling and transgene mice were used to determine which cell types express TSPO. [18F]-GE180-PET reliably detected the cuprizone-induced pathology in various white and grey matter regions, including the corpus callosum, cortex, hippocampus, thalamus and caudoputamen. Cuprizone-induced demyelination was paralleled by an increase in TSPO expression, glia activation and axonal injury. Most of the microglia and around one-third of the astrocytes expressed TSPO. TSPO expression induction was more severe in the white matter corpus callosum compared to the grey matter cortex. Although mitochondria accumulate at sites of focal axonal injury, these mitochondria do not express TSPO. In Cup/EAE mice, both microglia and recruited monocytes contribute to the TSPO expressing cell populations. These findings support the notion that TSPO is a valuable marker for the in vivo visualization and quantification of neuropathological changes in the MS brain. The pathological substrate of an increase in TSPO-ligand binding might be diverse including microglia activation, peripheral monocyte recruitment, or astrocytosis, but not axonal injury.

Microglial response to increasing amyloid load saturates with aging: a longitudinal dual tracer in vivo μPET-study

BACKGROUND

Causal associations between microglia activation and β-amyloid (Aβ) accumulation during the progression of Alzheimer's disease (AD) remain a matter of controversy. Therefore, we used longitudinal dual tracer in vivo small animal positron emission tomography (μPET) imaging to resolve the progression of the association between Aβ deposition and microglial responses during aging of an Aβ mouse model.

METHODS

APP-SL70 mice (N = 17; baseline age 3.2-8.5 months) and age-matched C57Bl/6 controls (wildtype (wt)) were investigated longitudinally for 6 months using Aβ (18F-florbetaben) and 18 kDa translocator protein (TSPO) μPET (18F-GE180). Changes in cortical binding were transformed to Z-scores relative to wt mice, and microglial activation relative to amyloidosis was defined as the Z-score difference (TSPO-Aβ). Using 3D immunohistochemistry for activated microglia (Iba-1) and histology for fibrillary Aβ (methoxy-X04), we measure microglial brain fraction relative to plaque size and the distance from plaque margins.

RESULTS

Aβ-PET binding increased exponentially as a function of age in APP-SL70 mice, whereas TSPO binding had an inverse U-shape growth function. Longitudinal Z-score differences declined with aging, suggesting that microglial response declined relative to increasing amyloidosis in aging APP-SL70 mice. Microglial brain volume fraction was inversely related to adjacent plaque size, while the proximity to Aβ plaques increased with age.

CONCLUSIONS

Microglial activity decreases relative to ongoing amyloidosis with aging in APP-SL70 mice. The plaque-associated microglial brain fraction saturated and correlated negatively with increasing plaque size with aging.

Positron Emission Tomography Imaging of Macrophages in Atherosclerosis with 18F-GE-180, a Radiotracer for Translocator Protein (TSPO)

Intraplaque inflammation plays an important role in the progression of atherosclerosis. The 18 kDa translocator protein (TSPO) expression is upregulated in activated macrophages, representing a potential target to identify inflamed atherosclerotic plaques. We preclinically evaluated ¹⁸F-GE-180, a novel third-generation TSPO radioligand, in a mouse model of atherosclerosis. Methods. Nine hypercholesterolemic mice deficient in low density lipoprotein receptor and apolipoprotein B48 (LDLR^−/−ApoB^100/100) and six healthy C57BL/6N mice were injected with 10 MBq of ¹⁸F-GE-180. Specificity of binding was demonstrated in three LDLR^−/−ApoB^100/100 mice by injection of nonradioactive reference compound of ¹⁸F-GE-180 before ¹⁸F-GE-180. Dynamic 30-minute PET was performed followed by contrast-enhanced CT, and the mice were sacrificed at 60 minutes after injection. Tissue samples were obtained for ex vivo biodistribution measurements, and aortas were cut into serial cryosections for digital autoradiography. The presence of macrophages and TSPO was studied by immunohistochemistry. The ¹⁸F-GE-180 retention in plaque areas with different macrophage densities and lesion-free vessel wall were compared. Results. The LDLR^−/−ApoB^100/100 mice showed large, inflamed plaques in the aorta. Autoradiography revealed significantly higher ¹⁸F-GE-180 retention in macrophage-rich plaque areas than in noninflamed areas (count densities 150 ± 45 PSL/mm² versus 51 ± 12 PSL/mm², p < 0.001). Prominent retention in the vessel wall without plaque was also observed (220 ± 41 PSL/mm²). Blocking with nonradioactive GE-180 diminished the difference in count densities between macrophage-rich and noninflamed areas in atherosclerotic plaques and lowered the count density in vessel wall without plaque. Conclusion. ¹⁸F-GE-180 shows specific uptake in macrophage-rich areas of atherosclerotic plaques in mice. However, retention in atherosclerotic lesions does not exceed that in lesion-free vessel wall. The third-generation TSPO radioligand ¹⁸F-GE-180 did not show improved characteristics for imaging atherosclerotic plaque inflammation compared to previously studied TSPO-targeting tracers.

Data on specificity of [18F]GE180 uptake for TSPO expression in rodent brain and myocardium

Data in this article show radioligand uptake (to gamma counter and positron-emission-tomography) as well as polymerase chain reaction analyses of 18 kDa translocator protein (TSPO) quantification. We confirmed specificity of [¹⁸F]GE180 binding of rodent brain and myocardium by blocking experiments with prior application of non-radioactive GE180, using dynamic in vivo positron-emission-tomography and ex vivo gamma counter measurements. Expression of TSPO was compared between rodent brain and myocardium by quantitative polymerase chain reaction.

TSPO PET with [18F]GE-180 sensitively detects focal neuroinflammation in patients with relapsing-remitting multiple sclerosis

PURPOSE

Expression of the translocator protein (TSPO) is upregulated in activated macrophages/microglia and is considered to be a marker of neuroinflammation. We investigated the novel TSPO ligand [¹⁸F]GE-180 in patients with relapsing-remitting multiple sclerosis (RRMS) to determine the feasibility of [¹⁸F]GE-180 PET imaging in RRMS patients and to assess its ability to detect active inflammatory lesions in comparison with the current gold standard, contrast-enhanced magnetic resonance imaging (MRI).

METHODS

Nineteen RRMS patients were prospectively included in this study. All patients underwent TSPO genotyping and were classified as high-affinity, medium-affinity or low-affinity binders (HAB/MAB/LAB). PET scans were performed after administration of 189 ± 12 MBq [¹⁸F]GE-180, and 60-90 min summation images were used for visual analysis and assessment of standardized uptake values (SUV). The frontal nonaffected cortex served as a pseudoreference region (PRR) for evaluation of SUV ratios (SUVR). PET data were correlated with MRI signal abnormalities, i.e. T2 hyperintensity or contrast enhancement (CE). When available, previous MRI data were used to follow the temporal evolution of individual lesions.

RESULTS

Focal lesions were identified as hot spots by visual inspection. Such lesions were detected in 17 of the 19 patients and overall 89 [¹⁸F]GE-180-positive lesions were found. TSPO genotyping revealed 11 patients with HAB status, 5 with MAB status and 3 with LAB status. There were no associations between underlying binding status (HAB, MAB and LAB) and the signal intensity in either lesions (SUVR 1.87 ± 0.43, 1.95 ± 0.48 and 1.86 ± 0.80, respectively; p = 0.280) or the PRR (SUV 0.36 ± 0.03, 0.40 ± 0.06 and 0.37 ± 0.03, respectively; p = 0.990). Of the 89 [¹⁸F]GE-180-positive lesions, 70 showed CE on MRI, while the remainder presented as T2 lesions without CE. SUVR were significantly higher in lesions with CE than in those without (2.00 ± 0.53 vs. 1.60 ± 0.15; p = 0.001). Notably, of 19 [¹⁸F]GE-180-positive lesions without CE, 8 previously showed CE, indicating that [¹⁸F]GE-180 imaging may be able to detect lesional activity that is sustained beyond the blood-brain barrier breakdown.

CONCLUSION

[¹⁸F]GE-180 PET can detect areas of focal macrophage/microglia activation in patients with RRMS in lesions with and without CE on MRI. Therefore, [¹⁸F]GE-180 PET imaging is a sensitive and quantitative approach to the detection of active MS lesions. It may provide information beyond contrast-enhanced MRI and is readily applicable to all patients. [¹⁸F]GE-180 PET imaging is therefore a promising new tool for the assessment of focal inflammatory activity in MS.

In Vivo Imaging of Glial Activation after Unilateral Labyrinthectomy in the Rat: A [18F]GE180-PET Study

The functional relevance of reactive gliosis for recovery from acute unilateral vestibulopathy is unknown. In the present study, glial activation was visualized in vivo by [¹⁸F]GE180-PET in a rat model of unilateral labyrinthectomy (UL) and compared to behavioral vestibular compensation (VC) overtime. 14 Sprague-Dawley rats underwent a UL by transtympanic injection of bupivacaine/arsenilate, 14 rats a SHAM UL (injection of normal saline). Glial activation was depicted with [¹⁸F]GE180-PET and ex vivo autoradiography at baseline and 7, 15, 30 days after UL/SHAM UL. Postural asymmetry and nystagmus were registered at 1, 2, 3, 7, 15, 30 days after UL/SHAM UL. Signs of vestibular imbalance were found only after UL, which significantly decreased until days 15 and 30. In parallel, [¹⁸F]GE180-PET and ex vivo autoradiography depicted glial activation in the ipsilesional vestibular nerve and nucleus on days 7 and 15 after UL. Correlation analysis revealed a strong negative association of [¹⁸F]GE180 uptake in the ipsilesional vestibular nucleus on day 7 with the rate of postural recovery (R = −0.90, p < 0.001), suggesting that glial activation accelerates VC. In conclusion, glial activation takes place in the ipsilesional vestibular nerve and nucleus within the first 30 days after UL in the rat and can be visualized in vivo by [¹⁸F]GE180-PET.

TSPO imaging using the novel PET ligand [18F]GE-180: quantification approaches in patients with multiple sclerosis

Background

PET ligands targeting the translocator protein (TSPO) represent promising tools to visualise neuroinflammation. Here, we analysed parameters obtained in dynamic and static PET images using the novel TSPO ligand [¹⁸F]GE-180 in patients with relapsing remitting multiple sclerosis (RRMS) and an approach for semi-quantitative assessment of this disease in clinical routine.

Seventeen dynamic [¹⁸F]GE-180 PET scans of RRMS patients were evaluated (90 min). A pseudo-reference region (PRR) was defined after identification of the least disease-affected brain area by voxel-based comparison with six healthy controls (HC) and upon exclusion of voxels suspected of being affected in static 60–90 min p.i. images. Standardised uptake value ratios (SUVR) obtained from static images normalised to PRR were correlated to the distribution volume ratios (DVR) derived from dynamic data with Logan reference tissue model.

Results

Group comparison with HC revealed white matter and thalamus as most affected regions. Fewest differences were found in grey matter, and normalisation to frontal cortex (FC) yielded the greatest reduction in variability of healthy grey and white matter. Hence, FC corrected for affected voxels was chosen as PRR, leading to time-activity curves of FC which were congruent to HC data (SUV_60–90 0.37, U test P = 0.42). SUVR showed a very strong correlation with DVR (Pearson ρ > 0.9). Focal MS lesions exhibited a high SUVR (range, 1.3–3.2).

Conclusions

This comparison with parameters from dynamic data suggests that SUVR normalised to corrected frontal cortex as PRR is suitable for the quantification of [¹⁸F]GE-180 uptake in lesions and different brain regions of RRMS patients. This efficient diagnostic protocol based on static [¹⁸F]GE-180 PET scans acquired 60–90 min p.i. allows the semi-quantitative assessment of neuroinflammation in RRMS patients in clinical routine.

Comparative Evaluation of Three TSPO PET Radiotracers in a LPS-Induced Model of Mild Neuroinflammation in Rats

Purpose

Over the past 20 years, neuroinflammation (NI) has increasingly been recognised as having an important role in  many neurodegenerative diseases, including Alzheimer’s disease. As such, being able to image NI non-invasively in patients is critical to monitor pathological processes and potential therapies targeting neuroinflammation. The translocator protein (TSPO) has proven a reliable NI biomarker for positron emission tomography (PET) imaging. However, if TSPO imaging in acute conditions such as stroke provides strong and reliable signals, TSPO imaging in neurodegenerative diseases has proven more challenging. Here, we report results comparing the recently developed TSPO tracers [¹⁸F]GE-180 and [¹⁸F]DPA-714 with (R)-[¹¹C]PK11195 in a rodent model of subtle focal inflammation.

Procedures

Adult male Wistar rats were stereotactically injected with 1 μg lipopolysaccharide in the right striatum. Three days later, animals underwent a 60-min PET scan with (R)-[¹¹C]PK11195 and [¹⁸F]GE-180 (n = 6) or [¹⁸F]DPA-714 (n = 6). Ten animals were scanned with either [¹⁸F]GE-180 (n = 5) or [¹⁸F]DPA-714 (n = 5) only. Kinetic analysis of PET data was performed using the simplified reference tissue model (SRTM) with a contralateral reference region or a novel data-driven input to estimate binding potential BP_ND. Autoradiography and immunohistochemistry were performed to confirm in vivo results.

Results

At 40–60 min post-injection, [¹⁸F]GE-180 dual-scanned animals showed a significantly increased core/contralateral uptake ratio vs. the same animals scanned with (R)-[¹¹C]PK11195 (3.41 ± 1.09 vs. 2.43 ± 0.39, p = 0.03); [¹⁸]DPA-714 did not (2.80 ± 0.69 vs. 2.26 ± 0.41). Kinetic modelling with a contralateral reference region identified significantly higher binding potential (BP_ND) in the core of the LPS injection site with [¹⁸F]GE-180 but not with [¹⁸F]DPA-714 vs. (R)-[¹¹C]PK11195. A cerebellar reference region and novel data-driven input to the SRTM were unable to distinguish differences in tracer BP_ND.

Conclusions

Second-generation TSPO-PET tracers are able to accurately detect mild-level NI. In this model, [¹⁸F]GE-180 shows a higher core/contralateral ratio and BP_ND when compared to (R)-[¹¹C]PK11195, while [¹⁸F]DPA-714 did not.

Electronic supplementary material

The online version of this article (doi:10.1007/s11307-016-0984-3) contains supplementary material, which is available to authorized users.

Time Courses of Cortical Glucose Metabolism and Microglial Activity Across the Life Span of Wild-Type Mice: A PET Study

Contrary to findings in the human brain, ¹⁸F-FDG PET shows cerebral hypermetabolism of aged wild-type (WT) mice relative to younger animals, supposedly due to microglial activation. Therefore, we used dual-tracer small-animal PET to examine directly the link between neuroinflammation and hypermetabolism in aged mice. Methods: WT mice (5-20 mo) were investigated in a cross-sectional design using ¹⁸F-FDG (n = 43) and translocator protein (TSPO) (¹⁸F-GE180; n = 58) small-animal PET, with volume-of-interest and voxelwise analyses. Biochemical analysis of plasma cytokine levels and immunohistochemical confirmation of microglial activity were also performed. Results: Age-dependent cortical hypermetabolism in WT mice relative to young animals aged 5 mo peaked at 14.5 mo (+16%, P < 0.001) and declined to baseline at 20 mo. Similarly, cortical TSPO binding increased to a maximum at 14.5 mo (+15%, P < 0.001) and remained high to 20 mo, resulting in an overall correlation between ¹⁸F-FDG uptake and TSPO binding (R = 0.69, P < 0.005). Biochemical and immunohistochemical analyses confirmed the TSPO small-animal PET findings. Conclusion: Age-dependent neuroinflammation is associated with the controversial observation of cerebral hypermetabolism in aging WT mice.

The FTD-like syndrome causing TREM2 T66M mutation impairs microglia function, brain perfusion, and glucose metabolism

Genetic variants in the triggering receptor expressed on myeloid cells 2 (TREM2) increase the risk for several neurodegenerative diseases including Alzheimer's disease and frontotemporal dementia (FTD). Homozygous TREM2 missense mutations, such as p.T66M, lead to the FTD-like syndrome, but how they cause pathology is unknown. Using CRISPR/Cas9 genome editing, we generated a knock-in mouse model for the disease-associated Trem2 p.T66M mutation. Consistent with a loss-of-function mutation, we observe an intracellular accumulation of immature mutant Trem2 and reduced generation of soluble Trem2 similar to patients with the homozygous p.T66M mutation. Trem2 p.T66M knock-in mice show delayed resolution of inflammation upon in vivo lipopolysaccharide stimulation and cultured macrophages display significantly reduced phagocytic activity. Immunohistochemistry together with in vivo TSPO small animal positron emission tomography (μPET) demonstrates an age-dependent reduction in microglial activity. Surprisingly, perfusion magnetic resonance imaging and FDG-μPET imaging reveal a significant reduction in cerebral blood flow and brain glucose metabolism. Thus, we demonstrate that a TREM2 loss-of-function mutation causes brain-wide metabolic alterations pointing toward a possible function of microglia in regulating brain glucose metabolism.

Identification of brain regions predicting epileptogenesis by serial [18F]GE-180 positron emission tomography imaging of neuroinflammation in a rat model of temporal lobe epilepsy

Excessive activation of inflammatory signaling pathways seems to be a hallmark of epileptogenesis. Positron emission tomography (PET) allows in vivo detection of brain inflammation with spatial information and opportunities for longitudinal follow-up scanning protocols. Here, we assessed whether molecular imaging of the 18 kDa translocator protein (TSPO) can serve as a biomarker for the development of epilepsy. Therefore, brain uptake of [¹⁸F]GE-180, a highly selective radioligand of TSPO, was investigated in a longitudinal PET study in a chronic rat model of temporal lobe epilepsy. Analyses revealed that the influence of the epileptogenic insult on [¹⁸F]GE-180 brain uptake was most pronounced in the earlier phase of epileptogenesis. Differences were evident in various brain regions during earlier phases of epileptogenesis with [¹⁸F]GE-180 standardized uptake value enhanced by 2.1 to 2.7fold. In contrast, brain regions exhibiting differences seemed to be more restricted with less pronounced increases of tracer uptake by 1.8-2.5fold four weeks following status epilepticus and by 1.5-1.8fold in the chronic phase. Based on correlation analysis, we were able to identify regions with a predictive value showing a correlation with seizure development. These regions include the amygdala as well as a cluster of brain areas. This cluster comprises parts of different brain regions, e.g. the hippocampus, parietal cortex, thalamus, and somatosensory cortex. In conclusion, the data provide evidence that [¹⁸F]GE-180 PET brain imaging can serve as a biomarker of epileptogenesis. The identification of brain regions with predictive value might facilitate the development of preventive concepts as well as the early assessment of the interventional success. Future studies are necessary to further confirm the predictivity of the approach.

Increase of TREM2 during Aging of an Alzheimer's Disease Mouse Model Is Paralleled by Microglial Activation and Amyloidosis

Heterozygous missense mutations in the triggering receptor expressed on myeloid cells 2 (TREM2) have been reported to significantly increase the risk of developing Alzheimer’s disease (AD). Since TREM2 is specifically expressed by microglia in the brain, we hypothesized that soluble TREM2 (sTREM2) levels may increase together with in vivo biomarkers of microglial activity and amyloidosis in an AD mouse model as assessed by small animal positron-emission-tomography (μPET). In this cross-sectional study, we examined a strong amyloid mouse model (PS2APP) of four age groups by μPET with [¹⁸F]-GE180 (glial activation) and [¹⁸F]-florbetaben (amyloidosis), followed by measurement of sTREM2 levels and amyloid levels in the brain. Pathology affected brain regions were compared between tracers (dice similarity coefficients) and pseudo-longitudinally. μPET results of both tracers were correlated with terminal TREM2 levels. The brain sTREM2 levels strongly increased with age of PS2APP mice (5 vs. 16 months: +211%, p < 0.001), and correlated highly with μPET signals of microglial activity (R = 0.89, p < 0.001) and amyloidosis (R = 0.92, p < 0.001). Dual μPET enabled regional mapping of glial activation and amyloidosis in the mouse brain, which progressed concertedly leading to a high overlap in aged PS2APP mice (dice similarity 67%). Together, these results substantiate the use of in vivo μPET measurements in conjunction with post mortem sTREM2 in future anti-inflammatory treatment trials. Taking human data into account sTREM2 may increase during active amyloid deposition.

Kinetic analysis of the translocator protein positron emission tomography ligand [18F]GE-180 in the human brain

Purpose

PET can image neuroinflammation by targeting the translocator protein (TSPO), which is upregulated in activated microglia. The high nonspecific binding of the first-generation TSPO radioligand [¹¹C]PK-11195 limits accurate quantification. [¹⁸F]GE-180, a novel TSPO ligand, displays superior binding to [¹¹C]PK-11195 in vitro. Our objectives were to: (1) evaluate tracer characteristics of [¹⁸F]GE-180 in the brains of healthy human subjects; and (2) investigate whether the TSPO Ala147Thr polymorphism influences outcome measures.

Methods

Ten volunteers (five high-affinity binders, HABs, and five mixed-affinity binders, MABs) underwent a dynamic PET scan with arterial sampling after injection of [¹⁸F]GE-180. Kinetic modelling of time–activity curves with one-tissue and two-tissue compartment models and Logan graphical analysis was applied to the data. The primary outcome measure was the total volume of distribution (V_T) across various regions of interest (ROIs). Secondary outcome measures were the standardized uptake values (SUV), the distribution volume and SUV ratios estimated using a pseudoreference region.

Results

The two-tissue compartment model was the best model. The average regional delivery rate constant (K₁) was 0.01 mL cm⁻³ min⁻¹ indicating low extraction across the blood–brain barrier (1 %). The estimated median V_T across all ROIs was also low, ranging from 0.16 mL cm⁻³ in the striatum to 0.38 mL cm⁻³ in the thalamus. There were no significant differences in V_T between HABs and MABs across all ROIs.

Conclusion

A reversible two-tissue compartment model fitted the data well and determined that the tracer has a low first-pass extraction (approximately 1 %) and low V_T estimates in healthy individuals. There was no observable dependency on the rs6971 polymorphism as compared to other second-generation TSPO PET tracers. Investigation of [¹⁸F]GE-180 in populations with neuroinflammatory disease is needed to determine its suitability for quantitative assessment of TSPO expression.

Electronic supplementary material

The online version of this article (doi:10.1007/s00259-016-3444-z) contains supplementary material, which is available to authorized users.

In vivo PET imaging of the neuroinflammatory response in rat spinal cord injury using the TSPO tracer [(18)F]GE-180 and effect of docosahexaenoic acid

Purpose

Traumatic spinal cord injury (SCI) is a devastating condition which affects millions of people worldwide causing major disability and substantial socioeconomic burden. There are currently no effective treatments. Modulating the neuroinflammatory (NI) response after SCI has evolved as a major therapeutic strategy. PET can be used to detect the upregulation of the 18-kDa translocator protein (TSPO), a hallmark of activated microglia in the CNS. We investigated whether PET imaging using the novel TSPO tracer [¹⁸F]GE-180 can be used as a clinically relevant biomarker for NI in a contusion SCI rat model, and we present data on the modulation of NI by the lipid docosahexaenoic acid (DHA).

Methods

A total of 22 adult male Wistar rats were subjected to controlled spinal cord contusion at the T10 spinal cord level. Six non-injured and ten T10 laminectomy only (LAM) animals were used as controls. A subset of six SCI animals were treated with a single intravenous dose of 250 nmol/kg DHA (SCI-DHA group) 30 min after injury; a saline-injected group of six animals was used as an injection control. PET and CT imaging was carried out 7 days after injury using the [¹⁸F]GE-180 radiotracer. After imaging, the animals were killed and the spinal cord dissected out for biodistribution and autoradiography studies. In vivo data were correlated with ex vivo immunohistochemistry for TSPO.

Results

In vivo dynamic PET imaging revealed an increase in tracer uptake in the spinal cord of the SCI animals compared with the non-injured and LAM animals from 35 min after injection (P < 0.0001; SCI vs. LAM vs. non-injured). Biodistribution and autoradiography studies confirmed the high affinity and specific [¹⁸F]GE-180 binding in the injured spinal cord compared with the binding in the control groups. Furthermore, they also showed decreased tracer uptake in the T10 SCI area in relation to the non-injured remainder of the spinal cord in the SCI-DHA group compared with the SCI-saline group (P < 0.05), supporting a NI modulatory effect of DHA. Immunohistochemistry showed a high level of TSPO expression (38 %) at the T10 injury site in SCI animals compared with that in the non-injured animals (6 %).

Conclusion

[¹⁸F]GE-180 PET imaging can reveal areas of increased TSPO expression that can be visualized and quantified in vivo after SCI, offering a minimally invasive approach to the monitoring of NI in SCI models and providing a translatable clinical readout for the testing of new therapies.

Electronic supplementary material

The online version of this article (doi:10.1007/s00259-016-3391-8) contains supplementary material, which is available to authorized users.

In Vivo Detection of Age- and Disease-Related Increases in Neuroinflammation by 18F-GE180 TSPO MicroPET Imaging in Wild-Type and Alzheimer's Transgenic Mice

Alzheimer's disease (AD) is the most common cause of dementia. Neuroinflammation appears to play an important role in AD pathogenesis. Ligands of the 18 kDa translocator protein (TSPO), a marker for activated microglia, have been used as positron emission tomography (PET) tracers to reflect neuroinflammation in humans and mouse models. Here, we used the novel TSPO-targeted PET tracer (18)F-GE180 (flutriciclamide) to investigate differences in neuroinflammation between young and old WT and APP/PS1dE9 transgenic (Tg) mice. In vivo PET scans revealed an overt age-dependent elevation in whole-brain uptake of (18)F-GE180 in both WT and Tg mice, and a significant increase in whole-brain uptake of (18)F-GE180 (peak-uptake and retention) in old Tg mice compared with young Tg mice and all WT mice. Similarly, the (18)F-GE180 binding potential in hippocampus was highest to lowest in old Tg > old WT > young Tg > young WT mice using MRI coregistration. Ex vivo PET and autoradiography analysis further confirmed our in vivo PET results: enhanced uptake and specific binding (SUV75%) of (18)F-GE180 in hippocampus and cortex was highest in old Tg mice followed by old WT, young Tg, and finally young WT mice. (18)F-GE180 specificity was confirmed by an in vivo cold tracer competition study. We also examined (18)F-GE180 metabolites in 4-month-old WT mice and found that, although total radioactivity declined over 2 h, of the remaining radioactivity, ∼90% was due to parent (18)F-GE180. In conclusion, (18)F-GE180 PET scans may be useful for longitudinal monitoring of neuroinflammation during AD progression and treatment.

SIGNIFICANCE STATEMENT

Microglial activation, a player in Alzheimer's disease (AD) pathogenesis, is thought to reflect neuroinflammation. Using in vivo microPET imaging with a novel TSPO radioligand, (18)F-GE180, we detected significantly enhanced neuroinflammation during normal aging in WT mice and in response to AD-associated pathology in APP/PS1dE9 Tg mice, an AD mouse model. Increased uptake and specific binding of (18)F-GE180 in whole brain and hippocampus were confirmed by ex vivo PET and autoradiography. The binding specificity and stability of (18)F-GE180 was further confirmed by a cold tracer competition study and a metabolite study, respectively. Therefore, (18)F-GE180 PET imaging may be useful for longitudinal monitoring of neuroinflammation during AD progression and treatment and may also be useful for other neurodegenerative diseases.

Glial Activation and Glucose Metabolism in a Transgenic Amyloid Mouse Model: A Triple-Tracer PET Study

Amyloid imaging by small-animal PET in models of Alzheimer disease (AD) offers the possibility to track amyloidogenesis and brain energy metabolism. Because microglial activation is thought to contribute to AD pathology, we undertook a triple-tracer small-animal PET study to assess microglial activation and glucose metabolism in association with amyloid plaque load in a transgenic AD mouse model.

METHODS

Groups of PS2APP and C57BL/6 wild-type mice of various ages were examined by small-animal PET. We acquired 90-min dynamic emission data with (18)F-GE180 for imaging activated microglia (18-kD translocator protein ligand [TSPO]) and static 30- to 60-min recordings with (18)F-FDG for energy metabolism and (18)F-florbetaben for amyloidosis. Optimal fusion of PET data was obtained through automatic nonlinear spatial normalization, and SUVRs were calculated. For the novel TSPO tracer (18)F-GE180, we then calculated distribution volume ratios after establishing a suitable reference region. Immunohistochemical analyses with TSPO antisera, methoxy-X04 staining for fibrillary β-amyloid, and ex vivo autoradiography served as terminal gold standard assessments.

RESULTS

SUVR at 60-90 min after injection gave robust quantitation of (18)F-GE180, which correlated well with distribution volume ratios calculated from the entire recording and using a white matter reference region. Relative to age-matched wild-type, (18)F-GE180 SUVR was slightly elevated in PS2APP mice at 5 mo (+9%; P < 0.01) and distinctly increased at 16 mo (+25%; P < 0.001). Over this age range, there was a high positive correlation between small-animal PET findings of microglial activation with amyloid load (R = 0.85; P < 0.001) and likewise with metabolism (R = 0.61; P < 0.005). Immunohistochemical and autoradiographic findings confirmed the in vivo small-animal PET data.

CONCLUSION

In this first triple-tracer small-animal PET in a well-established AD mouse model, we found evidence for age-dependent microglial activation. This activation, correlating positively with the amyloid load, implies a relationship between amyloidosis and inflammation in the PS2APP AD mouse model.

Clinical translation of molecular imaging agents used in PET studies of cancer

Over recent years, there has been a rapid expansion in our knowledge of the factors that regulate tumor growth; this has resulted in the identification of new therapeutic targets and improvements in the long-term survival of cancer patients. New noninvasive biomarkers of drug targets and pathway modulation in vivo are needed to guide therapy selection and detect drug resistance early so that alternative, more effective treatments can be offered. The translation of new therapeutics into the clinic is disappointingly slow, expensive, and subject to high rates of attrition often occurring at late stages (phase 3) of development. In an attempt to mitigate these delays and failures, there has been resurgence in the development of new molecular imaging probes for studies with positron emission tomography (PET) to characterize tumor biology. In the assessment of therapeutic effects, PET allows imaging of entire tumor burden in a noninvasive repeatable manner. This chapter focuses on the clinical translation of PET imaging agents from bench to bedside. New probes are being used to study a diverse range of processes such as angiogenesis, apoptosis, fatty acid metabolism, and growth factor receptor expression. In the future, validation of these novel imaging probes could allow more innovative therapies to be adapted earlier in the clinic leading to improved patient outcomes.