Two binding sites for [3H]PBR28 in human brain: implications for TSPO PET imaging of neuroinflammation

Prospects and challenges of imaging neuroinflammation beyond TSPO in Alzheimer's disease

Neuroinflammation, as defined by the activation of microglia and astrocytes, has emerged in the last years as a key element of the pathogenesis of neurodegenerative diseases based on genetic findings and preclinical and human studies. This has raised the need for new methodologies to assess and follow glial activation in patients, prompting the development of PET ligands for molecular imaging of glial cells and novel structural MRI and DTI tools leading to a multimodal approach. The present review describes the recent advancements in microglia and astrocyte biology in the context of health, ageing, and Alzheimer's disease, the most common dementia worldwide. The review further delves in molecular imaging discussing the challenges associated with past and present targets, including conflicting findings, and finally, presenting novel methodologies currently explored to improve our in vivo knowledge of the neuroinflammatory patterns in Alzheimer's disease. With glial cell activation as a potential therapeutic target in neurodegenerative diseases, the translational research between cell biologists, chemists, physicists, radiologists, and neurologists should be strengthened.

Translocator protein in late stage Alzheimer's disease and Dementia with Lewy bodies brains

OBJECTIVE

Increased translocator protein (TSPO), previously known as the peripheral benzodiazepine receptor (PBR), in glial cells of the brain has been used as a neuroinflammation marker in the early and middle stages of neurodegenerative diseases, such as Alzheimer's disease (AD) and Dementia with Lewy Bodies (DLB). In this study, we investigated the changes in TSPO density with respect to late stage AD and DLB.

METHODS

TSPO density was measured in multiple regions of postmortem human brains in 20 different cases: seven late stage AD cases (Braak amyloid average: C; Braak tangle average: VI; Aged 74-88, mean: 83 ± 5 years), five DLB cases (Braak amyloid average: C; Braak tangle average: V; Aged 79-91, mean: 84 ± 4 years), and eight age-matched normal control cases (3 males, 5 females: aged 77-92 years; mean: 87 ± 6 years). Measurements were taken by quantitative autoradiography using [3 H]PK11195 and [3 H]PBR28.

RESULTS

No significant changes were found in TSPO density of the frontal cortex, striatum, thalamus, or red nucleus of the AD and DLB brains. A significant reduction in TSPO density was found in the substantia nigra (SN) of the AD and DLB brains compared to that of age-matched healthy controls.

INTERPRETATION

This distinct pattern of TSPO density change in late stage AD and DLB cases may imply the occurrence of microglia dystrophy in late stage neurodegeneration. Furthermore, TSPO may not only be a microglia activation marker in early stage AD and DLB, but TSPO may also be used to monitor microglia dysfunction in the late stage of these diseases.

Detecting neuroinflammation in the brain following chronic alcohol exposure in rats: A comparison between in vivo and in vitro TSPO radioligand binding

Excessive alcohol consumption is associated with neuroinflammation, which likely contributes to alcohol-related pathology. However, positron emission tomography (PET) studies using radioligands for the 18-kDa translocator protein (TSPO), which is considered a biomarker of neuroinflammation, reported decreased binding in alcohol use disorder (AUD) participants compared to controls. In contrast, autoradiographic findings in alcohol exposed rats reported increases in TSPO radioligand binding. To assess if these discrepancies reflected differences between in vitro and in vivo methodologies, we compared in vitro autoradiography (using [3 H]PBR28 and [3 H]PK11195) with in vivo PET (using [11 C]PBR28) in male, Wistar rats exposed to chronic alcohol-vapor (dependent n = 10) and in rats exposed to air-vapor (nondependent n = 10). PET scans were obtained with [11 C]PBR28, after which rats were euthanized and the brains were harvested for autoradiography with [3 H]PBR28 and [3 H]PK11195 (n = 7 dependent and n = 7 nondependent), and binding quantified in hippocampus, thalamus, and parietal cortex. Autoradiography revealed significantly higher binding in alcohol-dependent rats for both radioligands in thalamus and hippocampus (trend level for [3 H]PBR28) compared to nondependent rats, and these group differences were stronger for [3 H]PK11195 than [3 H]PBR28. In contrast, PET measures obtained in the same rats showed no group difference in [11 C]PBR28 binding. Our in vitro data are consistent with neuroinflammation associated with chronic alcohol exposure. Failure to observe similar increases in [11 C]PBR28 binding in vivo suggests the possibility that a mechanism mediated by chronic alcohol exposure interferes with [11 C]PBR28 binding to TSPO in vivo. These data question the sensitivity of PBR28 PET as a methodology to assess neuroinflammation in AUD.

Recent Developments in TSPO PET Imaging as A Biomarker of Neuroinflammation in Neurodegenerative Disorders

Neuroinflammation is an inflammatory response in the brain and spinal cord, which can involve the activation of microglia and astrocytes. It is a common feature of many central nervous system disorders, including a range of neurodegenerative disorders. An overlap between activated microglia, pro-inflammatory cytokines and translocator protein (TSPO) ligand binding was shown in early animal studies of neurodegeneration. These findings have been translated in clinical studies, where increases in TSPO positron emission tomography (PET) signal occur in disease-relevant areas across a broad spectrum of neurodegenerative diseases. While this supports the use of TSPO PET as a biomarker to monitor response in clinical trials of novel neurodegenerative therapeutics, the clinical utility of current TSPO PET radioligands has been hampered by the lack of high affinity binding to a prevalent form of polymorphic TSPO (A147T) compared to wild type TSPO. This review details recent developments in exploration of ligand-sensitivity to A147T TSPO that have yielded ligands with improved clinical utility. In addition to developing a non-discriminating TSPO ligand, the final frontier of TSPO biomarker research requires developing an understanding of the cellular and functional interpretation of the TSPO PET signal. Recent insights resulting from single cell analysis of microglial phenotypes are reviewed.

A new perspective for advanced positron emission tomography-based molecular imaging in neurodegenerative proteinopathies

Recent studies in neurodegenerative conditions have increasingly highlighted that the same neuropathology can trigger different clinical phenotypes or, vice-versa, that similar phenotypes can be triggered by different neuropathologies. This evidence has called for the adoption of a pathology spectrum-based approach to study neurodegenerative proteinopathies. These conditions share brain deposition of abnormal protein aggregates, leading to aberrant biochemical, metabolic, functional, and structural changes. Positron emission tomography (PET) is a well-recognized and unique tool for the in vivo assessment of brain neuropathology, and novel PET techniques are emerging for the study of specific protein species. Today, key applications of PET range from early research and clinical diagnostic tools to their use in clinical trials for both participants screening and outcome evaluation. This position article critically reviews the role of distinct PET molecular tracers for different neurodegenerative proteinopathies, highlighting their strengths, weaknesses, and opportunities, with special emphasis on methodological challenges and future applications.

Neuroinflammation and amyloid-beta 40 are associated with reduced serotonin transporter (SERT) activity in a transgenic model of familial Alzheimer's disease

Background

Discrepant and often contradictory results have accumulated regarding the antidepressant and pro-cognitive effects of serotonin transporter (SERT) antagonists in Alzheimer’s disease.

Methods

To address the discrepancy, we measured the activity and density of SERT in the neocortex of 3–24-month-old APP_swe/PS1_dE9 and wild-type littermate mice, by using [³H]DASB autoradiography and the [³H]5-HT uptake assay. Levels of soluble amyloid-β (Aβ), and pro-inflammatory cytokines that can regulate SERT function, such as interleukin-1β (IL-1β), interleukin-6 (IL-6), and tumor necrosis factor (TNF), were measured in parallel. Neuroinflammation in aging APP_swe/PS1_dE9 mice was further evaluated by [³H]PK11195 autoradiography.

Results

Decreased SERT density was observed in the parietal and frontal cortex of 18–24-month-old APP_swe/PS1_dE9 mice, compared to age-matched, wild-type animals. The maximal velocity uptake rate (V_max) of [³H]5-HT was reduced in neocortical preparations from 20-month-old transgenic vs. wild-type mice. The reduction was observed when the proportion of soluble Aβ₄₀ in the Aβ_40/42 ratio increased in the aged transgenic brain. At concentrations compatible with those measured in 20-month-old APP_swe/PS1_dE9 mice, synthetic human Aβ₄₀, but not Aβ₄₂, reduced the baseline V_max of [³H]5-HT by ~ 20%. Neuroinflammation in APP_swe/PS1_dE9 vs. wild-type mice was evidenced by elevated [³H]PK11195 binding levels and increased concentration of IL-1β protein, which preceded the reductions in neocortical SERT density and activity. Age-induced increases in the levels of IL-1β, IL-6, and TNF were observed in both transgenic and wild-type animals.

Conclusions

The progression of cerebral amyloidosis is associated with neuroinflammation and decreased presynaptic markers of serotonergic integrity and activity. The Aβ₄₀-induced reduction in the uptake kinetics of [³H]5-HT suggests that the activity of SERT, and potentially the effects of SERT antagonism, depend on the levels of interstitial Aβ₄₀.

Prostate cancer and inflammation: A new molecular imaging challenge in the era of personalized medicine

The relationship between cancer and inflammation is one of the most important fields for both clinical and translational research. Despite numerous studies reported interesting and solid data about the prognostic value of the presence of inflammatory infiltrate in cancers, the biological role of inflammation in prostate cancer development is not yet fully clarified. The characterization of molecular pathways that connect altered inflammatory response and prostate cancer progression can provide the scientific rationale for the identification of new prognostic and predictive biomarkers. Specifically, the detection of infiltrating immune cells or related-cytokines by histology and/or by molecular imaging techniques could profoundly change the management of prostate cancer patients. In this context, the anatomic pathology and imaging diagnostic teamwork can provide a valuable support for the validation of new targets for diagnosis and therapy of prostate cancer lesions associated to the inflammatory infiltrate. The aim of this review is to summarize the current literature about the role of molecular imaging technique and anatomic pathology in the study of the mutual interaction occurring between prostate cancer and inflammation. Specifically, we reported the more recent advances in molecular imaging and histological methods for the early detection of prostate lesions associated to the inflammatory infiltrate.

Neuroinflammation and Cytokines in Myalgic Encephalomyelitis/Chronic Fatigue Syndrome (ME/CFS): A Critical Review of Research Methods

Myalgic encephalomyelitis/chronic fatigue syndrome (ME/CFS) is the label given to a syndrome that can include long-term flu-like symptoms, profound fatigue, trouble concentrating, and autonomic problems, all of which worsen after exertion. It is unclear how many individuals with this diagnosis are suffering from the same condition or have the same underlying pathophysiology, and the discovery of biomarkers would be clarifying. The name "myalgic encephalomyelitis" essentially means "muscle pain related to central nervous system inflammation" and many efforts to find diagnostic biomarkers have focused on one or more aspects of neuroinflammation, from periphery to brain. As the field uncovers the relationship between the symptoms of this condition and neuroinflammation, attention must be paid to the biological mechanisms of neuroinflammation and issues with its potential measurement. The current review focuses on three methods used to study putative neuroinflammation in ME/CFS: (1) positron emission tomography (PET) neuroimaging using translocator protein (TSPO) binding radioligand (2) magnetic resonance spectroscopy (MRS) neuroimaging and (3) assays of cytokines circulating in blood and cerebrospinal fluid. PET scanning using TSPO-binding radioligand is a promising option for studies of neuroinflammation. However, methodological difficulties that exist both in this particular technique and across the ME/CFS neuroimaging literature must be addressed for any results to be interpretable. We argue that the vast majority of ME/CFS neuroimaging has failed to use optimal techniques for studying brainstem, despite its probable centrality to any neuroinflammatory causes or autonomic effects. MRS is discussed as a less informative but more widely available, less invasive, and less expensive option for imaging neuroinflammation, and existing studies using MRS neuroimaging are reviewed. Studies seeking to find a peripheral circulating cytokine "profile" for ME/CFS are reviewed, with attention paid to the biological and methodological reasons for lack of replication among these studies. We argue that both the biological mechanisms of cytokines and the innumerable sources of potential variance in their measurement make it unlikely that a consistent and replicable diagnostic cytokine profile will ever be discovered.

Brain glial activation in fibromyalgia - A multi-site positron emission tomography investigation

Fibromyalgia (FM) is a poorly understood chronic condition characterized by widespread musculoskeletal pain, fatigue, and cognitive difficulties. While mounting evidence suggests a role for neuroinflammation, no study has directly provided evidence of brain glial activation in FM. In this study, we conducted a Positron Emission Tomography (PET) study using [¹¹C]PBR28, which binds to the translocator protein (TSPO), a protein upregulated in activated microglia and astrocytes. To enhance statistical power and generalizability, we combined datasets collected independently at two separate institutions (Massachusetts General Hospital [MGH] and Karolinska Institutet [KI]). In an attempt to disentangle the contributions of different glial cell types to FM, a smaller sample was scanned at KI with [¹¹C]-_L-deprenyl-D₂ PET, thought to primarily reflect astrocytic (but not microglial) signal. Thirty-one FM patients and 27 healthy controls (HC) were examined using [¹¹C]PBR28 PET. 11 FM patients and 11 HC were scanned using [¹¹C]-_L-deprenyl-D₂ PET. Standardized uptake values normalized by occipital cortex signal (SUVR) and distribution volume (V_T) were computed from the [¹¹C]PBR28 data. [¹¹C]-_L-deprenyl-D₂ was quantified using λ k₃. PET imaging metrics were compared across groups, and when differing across groups, against clinical variables. Compared to HC, FM patients demonstrated widespread cortical elevations, and no decreases, in [¹¹C]PBR28 V_T and SUVR, most pronounced in the medial and lateral walls of the frontal and parietal lobes. No regions showed significant group differences in [¹¹C]-_L-deprenyl-D₂ signal, including those demonstrating elevated [¹¹C]PBR28 signal in patients (p's ≥ 0.53, uncorrected). The elevations in [¹¹C]PBR28 V_T and SUVR were correlated both spatially (i.e., were observed in overlapping regions) and, in several areas, also in terms of magnitude. In exploratory, uncorrected analyses, higher subjective ratings of fatigue in FM patients were associated with higher [¹¹C]PBR28 SUVR in the anterior and posterior middle cingulate cortices (p's < 0.03). SUVR was not significantly associated with any other clinical variable. Our work provides the first in vivo evidence supporting a role for glial activation in FM pathophysiology. Given that the elevations in [¹¹C]PBR28 signal were not also accompanied by increased [¹¹C]-_L-deprenyl-D₂ signal, our data suggests that microglia, but not astrocytes, may be driving the TSPO elevation in these regions. Although [¹¹C]-_L-deprenyl-D₂ signal was not found to be increased in FM patients, larger studies are needed to further assess the role of possible astrocytic contributions in FM. Overall, our data support glial modulation as a potential therapeutic strategy for FM.

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).

Novel potential pyrazolopyrimidine based translocator protein ligands for the evaluation of neuroinflammation with PET

Translocator protein (TSPO) is an interesting biological target because TSPO overexpression is associated with microglial activation caused by neuronal damage or neuroinflammation, and these activated microglia are involved in several central nervous system diseases. Herein, novel fluorinated ligands (14a-c and 16a-c) based on a 2-phenylpyrazolo[1,5-a]pyrimidin-3-yl acetamide scaffold were synthesized, and in vitro characterization of each of the novel ligands was performed to elucidate structure activity relationships. All of the newly synthesized ligands displayed nano-molar affinity for TSPO. Particularly, an in vitro affinity study suggests that 2-(5,7-diethyl-2-(4-(3-fluoro-2-methylpropoxy)phenyl)pyrazolo[1,5-a]pyrimidin-3-yl)-N,N-diethylacetamide (14a), which exhibited high nano-molar affinity for TSPO and proper lipophilicity, was suitable for in vivo brain studies. Thus, radiosynthesis from tosylate precursor 13a using fluorine-18 was performed, and [¹⁸F]14a was obtained in a 31% radiochemical yield (decay-corrected). Dynamic positron emission tomography (PET) imaging studies were performed in a lipopolysaccharide (LPS)-induced neuroinflammation rat model using [¹⁸F]14a to identify the location of inflammation in the brain with a high target-to-background signal ratio. In addition, we validated that the locations of inflammatory lesions found by PET imaging were consistent with the locations observed by histological examination of dissected brains using antibodies. These results suggest that [¹⁸F]14a is a novel promising PET imaging agent for diagnosing neuroinflammation, and it may also prove to be applicable for diagnosing other diseases, including cancers associated with altered TSPO expression, using PET techniques.

Advances in PET Methodology

Recent advances in disease understanding, instrumentation technology, and computationally demanding image analysis approaches are opening new frontiers in the investigation of movement disorders and brain disease in general. A key aspect is the recognition of the need to determine molecular correlates to early functional and metabolic connectivity alterations, which are increasingly recognized as useful signatures of specific clinical disease phenotypes. Such multi-modal approaches are highly likely to provide new information on pathogenic mechanisms and to help the identification of novel therapeutic targets. This chapter describes recent methodological developments in PET starting with a very brief overview of radiotracers relevant to movement disorders while emphasizing the development of instrumentation, algorithms and imaging analysis methods relevant to multi-modal investigation of movement disorders.

Does inflammation precede tau aggregation in early Alzheimer's disease? A PET study

OBJECTIVE

Our aim was to assess with positron emission tomography (PET) the temporal and spatial inter-relationships between levels of cortical microglial activation and the aggregated amyloid-β and tau load in mild cognitive impairment (MCI) and early Alzheimer's disease (AD).

METHODS

Six clinically probable AD and 20 MCI subjects had inflammation (¹¹C-(R)-PK11195), amyloid (¹¹C-PiB) and tau (¹⁸F-flortaucipir) PET, magnetic resonance imaging (MRI) and a neuropsychological assessment. Parametric images of tracer binding were interrogated at a voxel level and by region of interest analyses.

RESULTS

55% of MCI and 83% of AD subjects had a high amyloid-β load. We have previously reported that clusters of correlated amyloid and inflammation levels are present in cortex. Here we found no correlation between levels of inflammation (¹¹C-(R)-PK11195 BP_ND) and tau (¹⁸F-flortaucipir SUVR) or MMSE scores in high amyloid-β cases.

INTERPRETATION

While correlated levels of amyloid-β and inflammation can be seen in MCI, we did not detect an association between levels of cortical tau tangles and inflammation in our series of high amyloid-β cases. High levels of inflammation could be seen in amyloid-β positive MCI cases where ¹⁸F-flortaucipir signals were low suggesting microglial activation precedes tau tangle formation. Inflammation levels were higher in high amyloid-β MCI than in early AD cases, compatible with it initially playing a protective role.

Integrated magnetic resonance imaging and [11 C]-PBR28 positron emission tomographic imaging in amyotrophic lateral sclerosis

OBJECTIVE

To characterize [11 C]-PBR28 brain uptake using positron emission tomography (PET) in people with amyotrophic lateral sclerosis (ALS) and primary lateral sclerosis (PLS). We have previously shown increased [11 C]-PBR28 uptake in the precentral gyrus in a small group of ALS patients. Herein, we confirm our initial finding, study the longitudinal changes, and characterize the gray versus white matter distribution of [11 C]-PBR28 uptake in a larger cohort of patients with ALS and PLS.

METHODS

Eighty-five participants including 53 with ALS, 11 with PLS, and 21 healthy controls underwent integrated [11 C]-PBR28 PET-magnetic resonance brain imaging. Patients were clinically assessed using the Upper Motor Neuron Burden (UMNB) and the Amyotrophic Lateral Sclerosis Functional Rating Scale-Revised (ALSFRS-R). [11 C]-PBR28 uptake was quantified as standardized uptake value ratio (SUVR) and compared between groups. Cortical thickness and fractional anisotropy were compared between groups and correlated with SUVR and the clinical data. [11 C]-PBR28 uptake and ALSFRS-R were compared longitudinally over 6 months in 10 ALS individuals.

RESULTS

Whole brain voxelwise, surface-based, and region of interest analyses revealed increased [11 C]-PBR28 uptake in the precentral and paracentral gyri in ALS, and in the subcortical white matter for the same regions in PLS, compared to controls. The increase in [11 C]-PBR28 uptake colocalized and correlated with cortical thinning, reduced fractional anisotropy, and increased mean diffusivity, and correlated with higher UMNB score. No significant changes were detected in [11 C]-PBR28 uptake over 6 months despite clinical progression.

INTERPRETATION

Glial activation measured by in vivo [11 C]-PBR28 PET is increased in pathologically relevant regions in people with ALS and correlates with clinical measures. Ann Neurol 2018;83:1186-1197.

In vivo imaging biomarkers of neuroinflammation in the development and assessment of stroke therapies - towards clinical translation

Modulation of the inflammatory microenvironment after stroke opens a new avenue for the development of novel neurorestorative therapies in stroke. Understanding the spatio-temporal profile of (neuro-)inflammatory imaging biomarkers in detail thereby represents a crucial factor in the development and application of immunomodulatory therapies. The early integration of quantitative molecular imaging biomarkers in stroke drug development may provide key information about (i) early diagnosis and follow-up, (ii) spatio-temporal drug-target engagement (pharmacodynamic biomarker), (iii) differentiation of responders and non-responders in the patient cohort (inclusion/exclusion criteria; predictive biomarkers), and (iv) the mechanism of action. The use of targeted imaging biomarkers for may thus allow clinicians to decipher the profile of patient-specific inflammatory activity and the development of patient-tailored strategies for immunomodulatory and neuro-restorative therapies in stroke. Here, we highlight the recent developments in preclinical and clinical molecular imaging biomarkers of neuroinflammation (endothelial markers, microglia, MMPs, cell labeling, future developments) in stroke and outline how imaging biomarkers can be used in overcoming current translational roadblocks and attrition in order to advance new immunomodulatory compounds within the clinical pipeline.

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.

11C-DPA-713 has much greater specific binding to translocator protein 18 kDa (TSPO) in human brain than 11C-( R)-PK11195

Positron emission tomography (PET) radioligands for translocator protein 18 kDa (TSPO) are widely used to measure neuroinflammation, but controversy exists whether second-generation radioligands are superior to the prototypical agent ¹¹C-( R)-PK11195 in human imaging. This study sought to quantitatively measure the "signal to background" ratio (assessed as binding potential ( BP_ND)) of ¹¹C-( R)-PK11195 compared to one of the most promising second-generation radioligands, ¹¹C-DPA-713. Healthy subjects had dynamic PET scans and arterial blood measurements of radioligand after injection of either ¹¹C-( R)-PK11195 (16 subjects) or ¹¹C-DPA-713 (22 subjects). To measure the amount of specific binding, a subset of these subjects was scanned after administration of the TSPO blocking drug XBD173 (30-90 mg PO). ¹¹C-DPA-713 showed a significant sensitivity to genotype in brain, whereas ¹¹C-( R)-PK11195 did not. Lassen occupancy plot analysis revealed that the specific binding of ¹¹C-DPA-713 was much greater than that of ¹¹C-( R)-PK11195. The BP_ND in high-affinity binders was about 10-fold higher for ¹¹C-DPA-713 (7.3) than for ¹¹C-( R)-PK11195 (0.75). Although the high specific binding of ¹¹C-DPA-713 suggests it is an ideal ligand to measure TSPO, we also found that its distribution volume increased over time, consistent with the accumulation of radiometabolites in brain.

Sifting through the surfeit of neuroinflammation tracers

The first phase of molecular brain imaging of microglial activation in neuroinflammatory conditions began some 20 years ago with the introduction of [11C]-( R)-PK11195, the prototype isoquinoline ligand for translocator protein (18 kDa) (TSPO). Investigations by positron emission tomography (PET) revealed microgliosis in numerous brain diseases, despite the rather low specific binding signal imparted by [11C]-( R)-PK11195. There has since been enormous expansion of the repertoire of TSPO tracers, many with higher specific binding, albeit complicated by allelic dependence of the affinity. However, the specificity of TSPO PET for revealing microglial activation not been fully established, and it has been difficult to judge the relative merits of the competing tracers and analysis methods with respect to their sensitivity for detecting microglial activation. We therefore present a systematic comparison of 13 TSPO PET and single photon computed tomography (SPECT) tracers belonging to five structural classes, each of which has been investigated by compartmental analysis in healthy human brain relative to a metabolite-corrected arterial input. We emphasize the need to establish the non-displaceable binding component for each ligand and conclude with five recommendations for a standard approach to define the cellular distribution of TSPO signals, and to characterize the properties of candidate TSPO tracers.

Translocator Protein as an Imaging Marker of Macrophage and Stromal Activation in Rheumatoid Arthritis Pannus

PET radioligands targeted to translocator protein (TSPO) offer a highly sensitive and specific means of imaging joint inflammation in rheumatoid arthritis (RA). Through high expression of TSPO on activated macrophages, TSPO PET has been widely reported in several studies of RA as a means of imaging synovial macrophages in vivo. However, this premise does not take into account the ubiquitous expression of TSPO. This study aimed to investigate TSPO expression in major cellular constituents of RA pannus-monocytes, macrophages, fibroblastlike synoviocytes (FLS cells), and CD4-positive (CD4+) T lymphocytes (T cells)-to more accurately interpret TSPO PET signal from RA synovium. Methods: Three RA patients and 3 healthy volunteers underwent PET of both knees using the TSPO radioligand ¹¹C-PBR28. Through ³H-PBR28 autoradiography and immunostaining of synovial tissue in 6 RA patients and 6 healthy volunteers, cellular expression of TSPO in synovial tissue was evaluated. TSPO messenger RNA expression and ³H-PBR28 radioligand binding was assessed using in vitro monocytes, macrophages, FLS cells, and CD4+ T cells. Results:¹¹C-PBR28 PET signal was significantly higher in RA joints than in healthy joints (average SUV, 0.82 ± 0.12 vs. 0.03 ± 0.004; P < 0.01). Further, ³H-PBR28-specific binding in synovial tissue was approximately 10-fold higher in RA patients than in healthy controls. Immunofluorescence revealed TSPO expression on macrophages, FLS cells, and CD4+ T cells. The in vitro study demonstrated the highest TSPO messenger RNA expression and ³H-PBR28-specific binding in activated FLS cells, nonactivated M0 macrophages, and activated M2 reparative macrophages, with the least TSPO expression being in activated and nonactivated CD4+ T cells. Conclusion: To our knowledge, this study was the first evaluation of cellular TSPO expression in synovium, with the highest TSPO expression and PBR28 binding being found on activated synovial FLS cells and M2 macrophages. TSPO-targeted PET may therefore have a unique sensitivity in detecting FLS cells and macrophage-predominant inflammation in RA, with potential utility for assessing treatment response in trials using novel FLS-cell-targeted therapies.

Update on Molecular Imaging in Parkinson's Disease

Advances in radionuclide tracers have allowed for more accurate imaging that reflects the actions of numerous neurotransmitters, energy metabolism utilization, inflammation, and pathological protein accumulation. All of these achievements in molecular brain imaging have broadened our understanding of brain function in Parkinson's disease (PD). The implementation of molecular imaging has supported more accurate PD diagnosis as well as assessment of therapeutic outcome and disease progression. Moreover, molecular imaging is well suited for the detection of preclinical or prodromal PD cases. Despite these advances, future frontiers of research in this area will focus on using multi-modalities combining positron emission tomography and magnetic resonance imaging along with causal modeling with complex algorithms.

TSPO mutations in rats and a human polymorphism impair the rate of steroid synthesis

The 18 kDa translocator protein (TSPO) is a ubiquitous conserved outer mitochondrial membrane protein implicated in numerous cell and tissue functions, including steroid hormone biosynthesis, respiration, cell proliferation, and apoptosis. TSPO binds with high affinity to cholesterol and numerous compounds, is expressed at high levels in steroid-synthesizing tissues, and mediates cholesterol import into mitochondria, which is the rate-limiting step in steroid formation. In humans, the rs6971 polymorphism on the TSPO gene leads to an amino acid substitution in the fifth transmembrane loop of the protein, which is where the cholesterol-binding domain of TSPO is located, and this polymorphism has been associated with anxiety-related disorders. However, recent knockout mouse models have provided inconsistent conclusions of whether TSPO is directly involved in steroid synthesis. In this report, we show that TSPO deletion mutations in rat and its corresponding rs6971 polymorphism in humans alter adrenocorticotropic hormone-induced plasma corticosteroid concentrations. Rat tissues examined show increased cholesteryl ester accumulation, and neurosteroid formation was undetectable in homozygous rats. These results also support a role for TSPO ligands in diseases with steroid-dependent stress and anxiety elements.

Imaging Translocator Protein as a Biomarker of Neuroinflammation in Dementia

Neuroinflammation has long been considered a potential contributor to neurodegenerative disorders that result in dementia. Accumulation of abnormal protein aggregates in Alzheimer's disease, frontotemporal dementia, and dementia with Lewy bodies is associated with the activation of microglia and astrocytes into proinflammatory states, and chronic low-level activation of glial cells likely contributes to the pathological changes observed in these and other neurodegenerative diseases. The 18kDa translocator protein (TSPO) is a key biomarker for measuring inflammation in the brain via positron emission tomography (PET). Increased TSPO density has been observed in brain tissue from patients with neurodegenerative diseases and colocalizes to activated microglia and reactive astrocytes. Several radioligands have been developed to measure TSPO density in vivo with PET, and these have been used in clinical studies of different dementia syndromes. However, TSPO radioligands have limitations, including low specific-to-nonspecific signal and differential affinity to a polymorphism on the TSPO gene, which must be taken into consideration in designing and interpreting human PET studies. Nonetheless, most PET studies have shown that increased TSPO binding is associated with various dementias, suggesting that TSPO has potential as a biomarker to further explore the role of neuroinflammation in dementia pathogenesis and may prove useful in monitoring disease progression.

The emerging role of PET imaging in dementia

A compelling need in the field of neurodegenerative diseases is the development and validation of biomarkers for early identification and differential diagnosis. The availability of positron emission tomography (PET) neuroimaging tools for the assessment of molecular biology and neuropathology has opened new venues in the diagnostic design and the conduction of new clinical trials. PET techniques, allowing the in vivo assessment of brain function and pathology changes, are increasingly showing great potential in supporting clinical diagnosis also in the early and even preclinical phases of dementia. This review will summarize the most recent evidence on fluorine-18 fluorodeoxyglucose-, amyloid -, tau -, and neuroinflammation - PET tools, highlighting strengths and limitations and possible new perspectives in research and clinical applications. Appropriate use of PET tools is crucial for a prompt diagnosis and target evaluation of new developed drugs aimed at slowing or preventing dementia.

PET imaging of the neurovascular interface in cerebrovascular disease

Cerebrovascular disease encompasses a range of pathologies that affect different components of the cerebral vasculature and brain parenchyma. Large artery atherosclerosis, acute cerebral ischaemia, and intracerebral small vessel disease all demonstrate altered metabolic processes that are key to their pathogenesis. Although structural imaging techniques such as MRI are the mainstay of clinical care and research in cerebrovascular disease, they have limited ability to detect these pathophysiological processes in vivo. By contrast, PET can detect and quantify metabolic processes that are relevant to each facet of cerebrovascular disease. Information obtained from PET studies has helped to shape the understanding of key concepts in cerebrovascular medicine, including vulnerable atherosclerotic plaque, salvageable ischaemic penumbra, neuroinflammation and selective neuronal loss after ischaemic insult. PET has also helped to elucidate the relationships between chronic hypoxia, neuroinflammation, and amyloid-β deposition in cerebral small vessel disease. This Review describes how PET-based imaging of metabolic processes at the neurovascular interface has contributed to our understanding of cerebrovascular disease.

The macrophage marker translocator protein (TSPO) is down-regulated on pro-inflammatory 'M1' human macrophages

The translocator protein (TSPO) is a mitochondrial membrane protein, of as yet uncertain function. Its purported high expression on activated macrophages, has lent utility to TSPO targeted molecular imaging in the form of positron emission tomography (PET), as a means to detect and quantify inflammation in vivo. However, existing literature regarding TSPO expression on human activated macrophages is lacking, mostly deriving from brain tissue studies, including studies of brain malignancy, and inflammatory diseases such as multiple sclerosis. Here, we utilized three human sources of monocyte derived macrophages (MDM), from THP-1 monocytes, healthy peripheral blood monocytes and synovial fluid monocytes from patients with rheumatoid arthritis, to undertake a detailed investigation of TSPO expression in activated macrophages. In this work, we demonstrate a consistent down-regulation of TSPO mRNA and protein in macrophages activated to a pro-inflammatory, or ‘M1’ phenotype. Conversely, stimulation of macrophages to an M2 phenotype with IL-4, dexamethasone or TGF-β1 did not alter TSPO expression, regardless of MDM source. The reasons for this are uncertain, but our study findings add some supporting evidence for recent investigations concluding that TSPO may be involved in negative regulation of inflammatory responses in macrophages.

The Low-Affinity Binding of Second Generation Radiotracers Targeting TSPO is Associated with a Unique Allosteric Binding Site

[¹¹C]-PK11195 (PK11195) has been widely used with positron emission tomography (PET) to assess levels of the translocator protein 18 kDa (TSPO) as a marker of neuroinflammation. Recent ligands, such as [¹¹C]-PBR28 and [¹¹C]-DPA713, have improved signal-to-noise ratio and specificity for TSPO over PK11195. However, these second generation radiotracers exhibit binding differences due to a single polymorphism (rs6971) that leads to three genotypes: C/C, C/T and T/T associated with high, mixed and low binding affinities, respectively. Here we report that [³H]-DPA-713 in the presence of cholesterol or PK11195 has an accelerated dissociation rate from TSPO in platelets isolated from individuals with the T/T genotype. This allosteric interaction was not observed in platelets isolated from individuals with the C/C or C/T genotype. The results provide a molecular rationale for low binding affinity of T/T TSPO and further support the exclusion of these subjects from PET imaging studies using second generation TSPO ligands.

Pro-inflammatory activation of primary microglia and macrophages increases 18 kDa translocator protein expression in rodents but not humans

The 18kDa Translocator Protein (TSPO) is the most commonly used tissue-specific marker of inflammation in positron emission tomography (PET) studies. It is expressed in myeloid cells such as microglia and macrophages, and in rodent myeloid cells expression increases with cellular activation. We assessed the effect of myeloid cell activation on TSPO gene expression in both primary human and rodent microglia and macrophages in vitro, and also measured TSPO radioligand binding with ³H-PBR28 in primary human macrophages. As observed previously, we found that TSPO expression increases (∼9-fold) in rodent-derived macrophages and microglia upon pro-inflammatory stimulation. However, TSPO expression does not increase with classical pro-inflammatory activation in primary human microglia (fold change 0.85 [95% CI 0.58-1.12], p = 0.47). In contrast, pro-inflammatory activation of human monocyte-derived macrophages is associated with a reduction of both TSPO gene expression (fold change 0.60 [95% CI 0.45-0.74], p = 0.02) and TSPO binding site abundance (fold change 0.61 [95% CI 0.49-0.73], p < 0.0001). These findings have important implications for understanding the biology of TSPO in activated macrophages and microglia in humans. They are also clinically relevant for the interpretation of PET studies using TSPO targeting radioligands, as they suggest changes in TSPO expression may reflect microglial and macrophage density rather than activation phenotype.

In vivo PET imaging of neuroinflammation in Alzheimer's disease

Increasing evidence suggests that neuroinflammation contributes to the pathophysiology of many neurodegenerative diseases, especially Alzheimer's disease (AD). Molecular imaging by PET may be a useful tool to assess neuroinflammation in vivo, thus helping to decipher the complex role of inflammatory processes in the pathophysiology of neurodegenerative diseases and providing a potential means of monitoring the effect of new therapeutic approaches. For this objective, the main target of PET studies is the 18 kDa translocator protein (TSPO), as it is overexpressed by activated microglia. In the present review, we describe the most widely used PET tracers targeting the TSPO, the methodological issues in tracer quantification and summarize the results obtained by TSPO PET imaging in AD, as well as in neurodegenerative disorders associated with AD, in psychiatric disorders and ageing. We also briefly describe alternative PET targets and imaging modalities to study neuroinflammation. Lastly, we question the meaning of PET imaging data in the context of a highly complex and multifaceted role of neuroinflammation in neurodegenerative diseases. This overview leads to the conclusion that PET imaging of neuroinflammation is a promising way of deciphering the enigma of the pathophysiology of AD and of monitoring the effect of new therapies.

Quantitative longitudinal imaging of activated microglia as a marker of inflammation in the pilocarpine rat model of epilepsy using [11C]-( R)-PK11195 PET and MRI

Inflammation may play a role in the development of epilepsy after brain insults. [¹¹C]-( R)-PK11195 binds to TSPO, expressed by activated microglia. We quantified [¹¹C]-( R)-PK11195 binding during epileptogenesis after pilocarpine-induced status epilepticus (SE), a model of temporal lobe epilepsy. Nine male rats were studied thrice (D0-1, D0 + 6, D0 + 35, D0 = SE induction). In the same session, 7T T2-weighted images and DTI for mean diffusivity (MD) and fractional anisotropy (FA) maps were acquired, followed by dynamic PET/CT. On D0 + 35, femoral arterial blood was sampled for rat-specific metabolite-corrected arterial plasma input functions (AIFs). In multiple MR-derived ROIs, we assessed four kinetic models (two with AIFs; two using a reference region), standard uptake values (SUVs), and a model with a mean AIF. All models showed large (up to two-fold) and significant TSPO binding increases in regions expected to be affected, and comparatively little change in the brainstem, at D0 + 6. Some individuals showed increases at D0 + 35. AIF models yielded more consistent increases at D0 + 6. FA values were decreased at D0 + 6 and had recovered by D0 + 35. MD was increased at D0 + 6 and more so at D0 + 35. [¹¹C]-( R)-PK11195 PET binding and MR biomarker changes could be detected with only nine rats, highlighting the potential of longitudinal imaging studies.

Imaging Microglial Activation in Untreated First-Episode Psychosis: A PET Study With [18F]FEPPA

OBJECTIVE

Neuroinflammation and abnormal immune responses are increasingly implicated in the pathophysiology of schizophrenia. Previous positron emission tomography (PET) studies targeting the translocator protein 18 kDa (TSPO) have been limited by high nonspecific binding of the first-generation radioligand, low-resolution scanners, small sample sizes, and psychotic patients being on antipsychotics or not being in the first episode of their illness. The present study uses the novel second-generation TSPO PET radioligand [18F]FEPPA to evaluate whether microglial activation is elevated in the dorsolateral prefrontal cortex and hippocampus of untreated patients with first-episode psychosis.

METHOD

Nineteen untreated patients with first-episode psychosis (14 of them antipsychotic naive) and 20 healthy volunteers underwent a high-resolution [18F]FEPPA PET scan and MRI. Dynamic PET data were analyzed using the validated two-tissue compartment model with arterial plasma input function with total volume of distribution (VT) as outcome measure. All analyses were corrected for TSPO rs6971 polymorphism (which is implicated in differential binding affinity).

RESULTS

No significant differences were observed between patients and healthy volunteers in microglial activation, as indexed by [18F]FEPPA VT, in either the dorsolateral prefrontal cortex or the hippocampus. There were no significant correlations between [18F]FEPPA VT and duration of illness, clinical presentation, or neuropsychological measures after adjusting for multiple testing.

CONCLUSIONS

The lack of significant differences in [18F]FEPPA VT between groups suggests that microglial activation is not present in first-episode psychosis.

Microglial activation in Parkinson's disease using [18F]-FEPPA

Background

Neuroinflammatory processes including activated microglia have been reported to play an important role in Parkinson’s disease (PD). Increased expression of translocator protein (TSPO) has been observed after brain injury and inflammation in neurodegenerative diseases. Positron emission tomography (PET) radioligand targeting TSPO allows for the quantification of neuroinflammation in vivo.

Methods

Based on the genotype of the rs6791 polymorphism in the TSPO gene, we included 25 mixed-affinity binders (MABs) (14 PD patients and 11 age-matched healthy controls (HC)) and 27 high-affinity binders (HABs) (16 PD patients and 11 age-matched HC) to assess regional differences in the second-generation radioligand [¹⁸F]-FEPPA between PD patients and HC. FEPPA total distribution volume (V_T) values in cortical as well as subcortical brain regions were derived from a two-tissue compartment model with arterial plasma as an input function.

Results

Our results revealed a significant main effect of genotype on [¹⁸F]-FEPPA V_T in every brain region, but no main effect of disease or disease × genotype interaction in any brain region. The overall percentage difference of the mean FEPPA V_T between HC-MABs and HC-HABs was 32.6% (SD = 2.09) and for PD-MABs and PD-HABs was 43.1% (SD = 1.21).

Conclusions

Future investigations are needed to determine the significance of [¹⁸F]-FEPPA as a biomarker of neuroinflammation as well as the importance of the rs6971 polymorphism and its clinical consequence in PD.

11C-ER176, a Radioligand for 18-kDa Translocator Protein, Has Adequate Sensitivity to Robustly Image All Three Affinity Genotypes in Human Brain

For PET imaging of 18-kDa translocator protein (TSPO), a biomarker of neuroinflammation, most second-generation radioligands are sensitive to the single nucleotide polymorphism rs6971; however, this is probably not the case for the prototypical agent ¹¹C-PK11195 (¹¹C-labeled N-butan-2-yl-1-(2-chlorophenyl)-N-methylisoquinoline-3-carboxamide), which has a relatively lower signal-to-noise ratio. We recently found that ¹¹C-ER176 (¹¹C-(R)-N-sec-butyl-4-(2-chlorophenyl)-N-methylquinazoline-2-carboxamide), a new analog of ¹¹C-(R)-PK11195, showed little sensitivity to rs6971 when tested in vitro and had high specific binding in monkey brain. This study sought, first, to determine whether the sensitivity of ¹¹C-ER176 in humans is similar to the low sensitivity measured in vitro and, second, to measure the nondisplaceable binding potential (BP_ND, or the ratio of specific-to-nondisplaceable uptake) of ¹¹C-ER176 in human brain.

METHODS

Nine healthy volunteers-3 high-affinity binders (HABs), 3 mixed-affinity binders (MABs), and 3 low-affinity binders (LABs)-were studied with whole-body ¹¹C-ER176 PET imaging. SUVs from 60 to 120 min after injection derived from each organ were compared between genotypes. Eight separate healthy volunteers-3 HABs, 3 MABs, and 2 LABs-underwent brain PET imaging. The 3 HABs underwent a repeated brain scan after TSPO blockade with XBD173 (N-benzyl-N-ethyl-2-(7-methyl-8-oxo-2-phenylpurin-9-yl)acetamide) to determine nondisplaceable distribution volume (V_ND) via Lassen occupancy plotting and thereby estimate BP_ND in brain.

RESULTS

Regional SUV averaged from 60 to 120 min after injection in brain and peripheral organs with high TSPO densities such as lung and spleen were greater in HABs than in LABs. On the basis of V_ND determined via the occupancy plot, the whole-brain BP_ND for LABs was estimated to be 1.4 ± 0.8, which was much lower than that for HABs (4.2 ± 1.3) but about the same as that for HABs with ¹¹C-PBR28 ([methyl-¹¹C]N-acetyl-N-(2-methoxybenzyl)-2-phenoxy-5-pyridinamine)) (∼1.2).

CONCLUSION

Obvious in vivo sensitivity to rs6971 was observed in ¹¹C-ER176 that had not been expected from in vitro studies, suggesting that the future development of any improved radioligand for TSPO should consider the possibility that in vitro properties will not be reflected in vivo. We also found that ¹¹C-ER176 has adequately high BP_ND for all rs6971 genotypes. Thus, the new radioligand would likely have greater sensitivity in detecting abnormalities in patients.

Determination and reduction of translocator protein (TSPO) ligand rs6971 discrimination

The 18 kDa translocator protein (TSPO) is a target for development of diagnostic imaging agents for glioblastoma and neuroinflammation.

The 18 kDa translocator protein (TSPO) is a target for development of diagnostic imaging agents for glioblastoma and neuroinflammation. Clinical translation of TSPO imaging agents has been hindered by the presence of a polymorphism, rs6971, which causes a non-conservative substitution of alanine for threonine at amino acid residue 147 (TSPO A147T). Disclosed brain-permeant second-generation TSPO ligands bind TSPO A147T with reduced affinity compared to the wild type protein (TSPO WT). Efforts to develop a TSPO ligand that binds TSPO WT and TSPO A147T with similarly high affinity have been hampered by a lack of knowledge about how ligand structure differentially influences interaction with the two forms of TSPO. To gain insight, we have established human embryonic kidney cell lines stably over-expressing human TSPO WT and TSPO A147T, and tested how modifications of a novel N-alkylated carbazole scaffold influence affinity to both TSPO isoforms. Most of the new analogues developed in this study showed high affinity to TSPO WT and a 5–6-fold lower affinity to TSPO A147T. Addition of electron-withdrawing substituents yielded analogues with highest affinity for TSPO A147T without decreasing affinity for TSPO WT. This knowledge can be used to inform further development of non-discriminating TSPO ligands for use as diagnostic markers for glioblastoma and neuroinflammation irrespective of rs6971.

Evaluation of PET Imaging Performance of the TSPO Radioligand [18F]DPA-714 in Mouse and Rat Models of Cancer and Inflammation

Purpose

Many radioligands have been explored for imaging the 18-kDa translocator protein (TSPO), a diagnostic and therapeutic target for inflammation and cancer. Here, we investigated the TSPO radioligand [¹⁸F]DPA-714 for positron emission tomography (PET) imaging of cancer and inflammation.

Procedures

[¹⁸F]DPA-714 PET imaging was performed in 8 mouse and rat models of breast and brain cancer and 4 mouse and rat models of muscular and bowel inflammation.

Results

[¹⁸F]DPA-714 showed different uptake levels in healthy organs and malignant tissues of mice and rats. Although high and displaceable [¹⁸F]DPA-714 binding is observed ex vivo, TSPO-positive PET imaging of peripheral lesions of cancer and inflammation in mice did not show significant lesion-to-background signal ratios. Slower [¹⁸F]DPA-714 metabolism and muscle clearance in mice compared to rats may explain the elevated background signal in peripheral organs in this species.

Conclusion

Although TSPO is an evolutionary conserved protein, inter- and intra-species differences call for further exploration of the pharmacological parameters of TSPO radioligands.

Electronic supplementary material

The online version of this article (doi:10.1007/s11307-015-0877-x) contains supplementary material, which is available to authorized users.

PET Neurochemical Imaging Modes

PET has deep roots in neuroscience stemming from its first application in brain tumor and brain metabolism imaging. PET emerged over the past few decades and continues to play a prominent role in the study of neurochemistry in the living human brain. Over time, neurochemical imaging with PET has been expanded to address a host of research questions related to, among many others, protein density, drug occupancy, and endogenous neurochemical release. Each of these imaging modes has distinct design and analysis considerations that are critical for enabling quantitative measurements. The number of considerations required for a neurochemical PET study can make it unapproachable. This article aims to orient those interested in neurochemical PET imaging to three of the common imaging modes and to provide some perspective on needs that exist for expansion of neurochemical PET imaging.

In Vivo Imaging of Human Neuroinflammation

Neuroinflammation is implicated in the pathophysiology of a growing number of human disorders, including multiple sclerosis, chronic pain, traumatic brain injury, and amyotrophic lateral sclerosis. As a result, interest in the development of novel methods to investigate neuroinflammatory processes, for the purpose of diagnosis, development of new therapies, and treatment monitoring, has surged over the past 15 years. Neuroimaging offers a wide array of non- or minimally invasive techniques to characterize neuroinflammatory processes. The intent of this Review is to provide brief descriptions of currently available neuroimaging methods to image neuroinflammation in the human central nervous system (CNS) in vivo. Specifically, because of the relatively widespread accessibility of equipment for nuclear imaging (positron emission tomography [PET]; single photon emission computed tomography [SPECT]) and magnetic resonance imaging (MRI), we will focus on strategies utilizing these technologies. We first provide a working definition of "neuroinflammation" and then discuss available neuroimaging methods to study human neuroinflammatory processes. Specifically, we will focus on neuroimaging methods that target (1) the activation of CNS immunocompetent cells (e.g. imaging of glial activation with TSPO tracer [(11)C]PBR28), (2) compromised BBB (e.g. identification of MS lesions with gadolinium-enhanced MRI), (3) CNS-infiltration of circulating immune cells (e.g. tracking monocyte infiltration into brain parenchyma with iron oxide nanoparticles and MRI), and (4) pathological consequences of neuroinflammation (e.g. imaging apoptosis with [(99m)Tc]Annexin V or iron accumulation with T2* relaxometry). This Review provides an overview of state-of-the-art techniques for imaging human neuroinflammation which have potential to impact patient care in the foreseeable future.