Increased central microglial activation associated with peripheral cytokine levels in premanifest Huntington's disease gene carriers

Previous studies have shown activation of the immune system and altered immune response in Huntington's disease (HD) gene carriers. Here, we hypothesized that peripheral and central immune responses could be concurrent pathophysiological events and represent a global innate immune response to the toxic effects of mutant huntingtin in HD gene carriers. We sought to investigate our hypothesis using [(11)C]PK11195 PET as a translocator protein (TSPO) marker of central microglial activation, together with assessment of peripheral plasma cytokine levels in a cohort of premanifest HD gene carriers who were more than a decade from predicted symptomatic conversion. Data were also compared to those from a group of healthy controls matched for age and gender. We found significantly increased peripheral plasma IL-1β levels in premanifest HD gene carriers compared to the group of normal controls (P=0.018). Premanifest HD gene carriers had increased TSPO levels in cortical, basal ganglia and thalamic brain regions (P<0.001). Increased microglial activation in somatosensory cortex correlated with higher plasma levels of IL-1β (rs=0.87, P=0.013), IL-6 (rs=0.85, P=0.013), IL-8 (rs=0.68, P=0.045) and TNF-α (rs=0.79; P=0.013). Our findings provide first in vivo evidence for an association between peripheral and central immune responses in premanifest HD gene carriers, and provide further supporting evidence for the role of immune dysfunction in the pathogenesis of HD.

Imaging Striatal Microglial Activation in Patients with Parkinson's Disease

This study investigated whether the second-generation translocator protein 18kDa (TSPO) radioligand, [¹⁸F]-FEPPA, could be used in neurodegenerative parkinsonian disorders as a biomarker for detecting neuroinflammation in the striatum. Neuroinflammation has been implicated as a potential mechanism for the progression of Parkinson’s disease (PD). Positron Emission Tomography (PET) radioligand targeting for TSPO allows for the quantification of neuroinflammation in vivo. Based on genotype of the rs6791 polymorphism in the TSPO gene, 16 mixed-affinity binders (MABs) (8 PD and age-matched 8 healthy controls (HCs)), 16 high-affinity binders (HABs) (8 PD and age-matched 8 HCs) and 4 low-affinity binders (LABs) (3 PD and 1 HCs) were identified. Total distribution volume (V_T) values in the striatum were derived from a two-tissue compartment model with arterial plasma as an input function. There was a significant main effect of genotype on [¹⁸F]-FEPPA V_T values in the caudate nucleus (p = 0.001) and putamen (p < 0.001), but no main effect of disease or disease x genotype interaction in either ROI. In the HAB group, the percentage difference between PD and HC was 16% in both caudate nucleus and putamen; in the MAB group, it was -8% and 3%, respectively. While this PET study showed no evidence of increased striatal TSPO expression in PD patients, the current findings provide some insights on the possible interactions between rs6791 polymorphism and neuroinflammation in PD.

Cannabinoid CB2 Receptors in a Mouse Model of Aβ Amyloidosis: Immunohistochemical Analysis and Suitability as a PET Biomarker of Neuroinflammation

In Alzheimer's disease (AD), one of the early responses to Aβ amyloidosis is recruitment of microglia to areas of new plaque. Microglial receptors such as cannabinoid receptor 2 (CB2) might be a suitable target for development of PET radiotracers that could serve as imaging biomarkers of Aβ-induced neuroinflammation. Mouse models of amyloidosis (J20APPswe/ind and APPswe/PS1ΔE9) were used to investigate the cellular distribution of CB2 receptors. Specificity of CB2 antibody (H60) was confirmed using J20APPswe/ind mice lacking CB2 receptors. APPswe/PS1ΔE9 mice were used in small animal PET with a CB2-targeting radiotracer, [11C]A836339. These studies revealed increased binding of [11C]A836339 in amyloid-bearing mice. Specificity of the PET signal was confirmed in a blockade study with a specific CB2 antagonist, AM630. Confocal microscopy revealed that CB2-receptor immunoreactivity was associated with astroglial (GFAP) and, predominantly, microglial (CD68) markers. CB2 receptors were observed, in particular, in microglial processes forming engulfment synapses with Aβ plaques. In contrast to glial cells, neuron (NeuN)-derived CB2 signal was equal between amyloid-bearing and control mice. The pattern of neuronal CB2 staining in amyloid-bearing mice was similar to that in human cases of AD. The data collected in this study indicate that Aβ amyloidosis without concomitant tau pathology is sufficient to activate CB2 receptors that are suitable as an imaging biomarker of neuroinflammation. The main source of enhanced CB2 PET binding in amyloid-bearing mice is increased CB2 immunoreactivity in activated microglia. The presence of CB2 immunoreactivity in neurons does not likely contribute to the enhanced CB2 PET signal in amyloid-bearing mice due to a lack of significant neuronal loss in this model. However, significant loss of neurons as seen at late stages of AD might decrease the CB2 PET signal due to loss of neuronally-derived CB2. Thus this study in mouse models of AD indicates that a CB2-specific radiotracer can be used as a biomarker of neuroinflammation in the early preclinical stages of AD, when no significant neuronal loss has yet developed.

Neuroinflammation in healthy aging: a PET study using a novel Translocator Protein 18kDa (TSPO) radioligand, [(18)F]-FEPPA

One of the cellular markers of neuroinflammation is increased microglia activation, characterized by overexpression of mitochondrial 18kDa Translocator Protein (TSPO). TSPO expression can be quantified in-vivo using the positron emission tomography (PET) radioligand [(18)F]-FEPPA. This study examined microglial activation as measured with [(18)F]-FEPPA PET across the adult lifespan in a group of healthy volunteers. We performed genotyping for the rs6971 TS.PO gene polymorphism to control for the known variability in binding affinity. Thirty-three healthy volunteers (age range: 19-82years; 22 high affinity binders (HAB), 11 mixed affinity binders (MAB)) underwent [(18)F]-FEPPA PET scans, acquired on the High Resolution Research Tomograph (HRRT) and analyzed using a 2-tissue compartment model. Regression analyses were performed to examine the effect of age adjusting for genetic status on [(18)F]-FEPPA total distribution volumes (VT) in the hippocampus, temporal, and prefrontal cortex. We found no significant effect of age on [(18)F]-FEPPA VT (F (1,30)=0.918; p=0.346), and a significant effect of genetic polymorphism (F (1,30)=8.767; p=0.006). This is the first in-vivo study to evaluate age-related changes in TSPO binding, using the new generation TSPO radioligands. Increased neuroinflammation, as measured with [(18)F]-FEPPA PET was not associated with normal aging, suggesting that healthy elderly individuals may serve as useful benchmark against patients with neurodegenerative disorders where neuroinflammation may be present.

Increased PK11195 PET binding in the cortex of patients with MS correlates with disability

OBJECTIVE

Activated microglia are thought to play a major role in cortical gray matter (GM) demyelination in multiple sclerosis (MS). Our objective was to evaluate microglial activation in cortical GM of patients with MS in vivo and to explore its relationship to measures of disability.

METHODS

Using PET and optimized modeling and segmentation procedures, we investigated cortical (11)C-PK11195 (PK11195) binding in patients with relapsing-remitting MS (RRMS), patients with secondary progressive MS (SPMS), and healthy controls. Disability was assessed with the Expanded Disability Status Scale (EDSS) and Multiple Sclerosis Impact Scale (MSIS-29).

RESULTS

Patients with MS showed increased cortical GM PK11195 binding relative to controls, which was multifocal and highest in the postcentral, middle frontal, anterior orbital, fusiform, and parahippocampal gyri. Patients with SPMS also showed additional increases in precentral, superior parietal, lingual and anterior superior, medial and inferior temporal gyri. Total cortical GM PK11195 binding correlated with EDSS scores, with a stronger correlation for the subgroup of patients with SPMS. In patients with SPMS, PK11195 binding also correlated with MSIS-29 scores. No correlation with disability measures was seen for PK11195 binding in white matter. Higher EDSS scores correlated with higher levels of GM PK11195 binding in the postcentral gyrus for patients with RRMS and in precentral gyrus for those with SPMS.

CONCLUSIONS

Microglial activation in cortical GM of patients with MS can be assessed in vivo. The distribution is not uniform and shows a relationship to clinical disability. We speculate that the increased PK11195 binding corresponds to enhanced microglial activation described in postmortem SPMS cortical GM.

Detection and quantification of remote microglial activation in rodent models of focal ischaemia using the TSPO radioligand CLINDE

PURPOSE

Neuroinflammation is involved in stroke pathophysiology and might be imaged using radioligands targeting the 18 kDa translocator protein (TSPO).

METHODS

We studied microglial reaction in brain areas remote from the primary lesion site in two rodent models of focal cerebral ischaemia (permanent or transient) using [125I]-CLINDE, a promising TSPO single photon emission computed tomography radioligand.

RESULTS

In a mouse model of permanent middle cerebral artery occlusion (MCAO), ex vivo autoradiographic studies demonstrated, besides in the ischaemic territory, accumulation of [125I]-CLINDE in the ipsilateral thalamus with a binding that progressed up to 3 weeks after MCAO. [125I]-CLINDE binding markedly decreased in animals pre-injected with either unlabelled CLINDE or PK11195, while no change was observed with flumazenil pre-treatment, demonstrating TSPO specificity. In rats subjected to transient MCAO, [125I]-CLINDE binding in the ipsilateral thalamus and substantia nigra pars reticulata (SNr) was significantly higher than that in contralateral tissue. Moreover, [125I]-CLINDE binding in the thalamus and SNr was quantitatively correlated to the ischaemic volume assessed by MRI in the cortex and striatum, respectively.

CONCLUSION

Clinical consequences of secondary neuronal degeneration in stroke might be better treated thanks to the discrimination of neuronal processes using in vivo molecular imaging and potent TSPO radioligands like CLINDE to guide therapeutic interventions.

PK11195 labels activated microglia in Alzheimer's disease and in vivo in a mouse model using PET

Activated microglia may promote neurodegeneration in Alzheimer's disease (AD) and may also help in amyloid clearance in immunization therapies. In vivo imaging of activated microglia using positron emission tomography (PET) could assist in defining the role of activated microglia during AD progression and therapeutics. We hypothesized that PK11195, a ligand that binds activated microglia, could label these cells in postmortem AD tissues and in vivo in an animal model of AD using PET. [(3)H](R)-PK11195 binding was significantly higher in AD frontal cortex compared to controls and correlated mainly with the abundance of immunohistochemically labeled activated microglia. With age, the brains of APP/PS1 transgenic mice showed progressive increase in [(3)H](R)-PK11195 binding and [(11)C](R)-PK11195 retention in vivo assessed using microPET, which correlated with the histopathological abundance of activated microglia. These results suggest that PK11195 binding in AD postmortem tissue and transgenic mice in vivo correlates with the extent of microglial activation and may help define the role of activated microglia in the pathogenesis and treatment of AD.

Evaluation of methods for generating parametric (R-[11C]PK11195 binding images

Activated microglia can be visualised using (R)-[(11)C]PK11195 (1-[2-chlorophenyl]-N-methyl-N-[1-methyl-propyl]-3-isoquinoline carboxamide) and positron emission tomography (PET). In previous studies, various methods have been used to quantify (R)-[(11)C]PK11195 binding. The purpose of this study was to determine which parametric method would be best suited for quantifying (R)-[(11)C]PK11195 binding at the voxel level. Dynamic (R)-[(11)C]PK11195 scans with arterial blood sampling were performed in 20 healthy and 9 Alzheimer's disease subjects. Parametric images of both volume of distribution (V(d)) and binding potential (BP) were obtained using Logan graphical analysis with plasma input. In addition, BP images were generated using two versions of the basis function implementation of the simplified reference tissue model, two versions of Ichise linearisations, and Logan graphical analysis with reference tissue input. Results of the parametric methods were compared with results of full compartmental analysis using nonlinear regression. Simulations were performed to assess accuracy and precision of each method. It was concluded that Logan graphical analysis with arterial input function is an accurate method for generating parametric images of V(d). Basis function methods, one of the Ichise linearisations and Logan graphical analysis with reference tissue input provided reasonably accurate and precise estimates of BP. In pathological conditions with reduced flow rates or large variations in blood volume, the basis function method is preferred because it produces less bias and is more precise.

Positron emission tomography imaging of neuroinflammation

In the diseased brain, upon activation microglia express binding sites for synthetic ligands designed to recognize the 18-kDa translocator protein TP-18, which is part of the so-called peripheral benzodiazepine receptor complex. PK11195 [1-(2-chlorophenyl)-N-methyl-N-(1-methylpropyl)-3-isoquinoline carboxamide], the prototype synthetic ligand, has been widely used for the functional characterization of TP-18. Its cellular source in activated microglia has been established using high-resolution, single-cell autoradiography with the R-enantiomer [3H](R)-PK11195. Radiolabeled [11C](R)-PK11195 has been used to image active brain disease with positron emission tomography. Consistent with experimental and postmortem observations of a characteristically distributed pattern of microglia activation in areas of focal pathology, as well as in anterograde and retrograde projection areas, the in vivo regional [11C](R)-PK11195 signal is found in active focal lesions and over time also along the affected neural tracts and their respective cortical and subcortical projection areas. Thus, a profile of active disease emerges that matches some of the typical distribution patterns known from structural neuroimaging techniques, but additionally shows involvement of brain regions linked through neural pathways. In the context of cell-based in vivo neuropathology, the image data are thus best interpreted in the context of the emerging cellular understanding of brain disease or damage, rather than the definitions of clinical diagnosis. One important observation, borne out by experiment, is the long latency with which activated microglia or increased PK11195 retention appear to gradually emerge and remain in distal areas secondarily affected by disease, supporting speculations that the presence of activated microglia is an important corollary of brain plasticity.

Microglial Activation in Perinatal Rabbit Brain Induced by Intrauterine Inflammation: Detection with 11C-(R)-PK11195 and Small-Animal PET

Intrauterine infection can lead to a fetal inflammatory response syndrome that has been implicated as one of the causes of perinatal brain injury leading to periventricular leukomalacia (PVL) and cerebral palsy. The presence of activated microglial cells has been noted in autopsy specimens of patients with PVL and in models of neonatal hypoxia and ischemia. Activated microglial cells can cause oligodendrocyte damage and white matter injury by release of inflammatory cytokines and production of excitotoxic metabolites. We hypothesized that exposure to endotoxin in utero leads to microglial activation in the fetal brain that can be monitored in vivo by (11)C-(R)-PK11195 (1-[2-chlorophenyl]-N-methyl-N-[1-methylpropyl]-3-isoquinoline carboxamide)--a positron-emitting ligand that binds peripheral benzodiazepine receptor sites in activated microglia--using small-animal PET.

METHODS

Pregnant New Zealand White rabbits underwent laparotomy and were injected with 20 and 30 microg/kg of Escherichia coli lipopolysaccharide along the length of the uterus on day 28 of gestation. The pups were born spontaneously at term (31 d) and were scanned using small-animal PET after intravenous administration of (11)C-(R)-PK11195 and by MRI on postnatal day 1. The standard uptake values (SUVs) of the tracer were calculated for the whole brain at 10-min intervals for 60 min after tracer injection. The pups were euthanized after the scan, and brains were fixed, sectioned, and stained for microglial cells using biotinylated tomato lectin.

RESULTS

There was increased brain retention of (11)C-(R)-PK11195--as determined by a significant difference in the slope of the SUV over time--in the endotoxin-treated pups when compared with that of age-matched controls. Immunohistochemical staining showed dose-dependent changes in activated microglia (increased number and morphologic changes) in the periventricular region and hippocampus of the brain of newborn rabbit pups exposed to endotoxin in utero.

CONCLUSION

Intrauterine inflammation leads to activation of microglial cells that may be responsible for the development of brain injury and white matter damage in the perinatal period. PET with the tracer (11)C-(R)-PK11195 can be used as a noninvasive, sensitive tool for determining the presence and progress of neuroinflammation due to perinatal insults in newborns.

A systematic comparison of kinetic modelling methods generating parametric maps for [(11)C]-(R)-PK11195

[(11)C]-(R)-PK11195 is presently the most widely used radiotracer for the monitoring of microglia activity in the central nervous system (CNS). Microglia, the resident immune cells of the brain, play a critical role in acute and chronic diseases of the central nervous system and in host defence against neoplasia. The purpose of this investigation was to evaluate the reliability and sensitivity of five kinetic modelling methods for the formation of parametric maps from dynamic [(11)C]-(R)-PK11195 studies. The methods we tested were the simplified reference tissue model (SRTM), basis pursuit, a simple target-to-reference ratio, the Logan plot and a wavelet based Logan plot. For the reliability assessment, the test-retest data consisted of four Alzheimer's patients that were scanned twice at approximately a six-week interval. For the sensitivity assessment, comparison of [(11)C]-(R)-PK11195 binding in Huntington's disease (HD) patients and normal subjects was performed using a group contrast to localize significant increases in mean pixel volume of distribution (VD) in HD. In all instances, a reference region kinetic extracted by a supervised clustering technique was used as input function. Reliability was assessed by use of the intra-class correlation coefficient (ICC) across a wide set of anatomical regions and it was found that the wavelet-based Logan plot, basis pursuit and SRTM gave the highest ICC values on average. The same methods produced the highest z-scores resulting from increases in mean striatal VD in HD patients compared with controls. The reference-to-target ratio and the Logan graphical approach were significantly less reliable and less sensitive.

The peripheral benzodiazepine receptor (Translocator protein 18kDa) in microglia: from pathology to imaging

Microglia constitute the primary resident immune surveillance cell in the brain and are thought to play a significant role in the pathogenesis of several neurodegenerative disorders, such as Alzheimer's disease, multiple sclerosis, Parkinson's disease and HIV-associated dementia. Measuring microglial activation in vivo in patients suffering from these diseases may help chart progression of neuroinflammation as well as assess efficacy of therapies designed to modulate neuroinflammation. Recent studies suggest that activated microglia in the CNS may be detected in vivo using positron emission tomography (PET) utilizing pharmacological ligands of the mitochondrial peripheral benzodiazepine receptor (PBR (recently renamed as Translocator protein (18kDa)). Beginning with the molecular characterization of PBR and regulation in activated microglia, we examine the rationale behind using PBR ligands to image microglia with PET. Current evidence suggests these findings might be applied to the development of clinical assessments of microglial activation in neurological disorders.

Microglial activation correlates with severity in Huntington disease: a clinical and PET study

BACKGROUND

Huntington disease (HD) is characterized by the progressive death of medium spiny dopamine receptor bearing striatal GABAergic neurons. In addition, microglial activation in the areas of neuronal loss has recently been described in postmortem studies. Activated microglia are known to release neurotoxic cytokines, and these may contribute to the pathologic process.

METHODS

To evaluate in vivo the involvement of microglia activation in HD, the authors studied patients at different stages of the disease using [(11)C](R)-PK11195 PET, a marker of microglia activation, and [(11)C]raclopride PET, a marker of dopamine D2 receptor binding and hence striatal GABAergic cell function.

RESULTS

In HD patients, a significant increase in striatal [(11)C](R)-PK11195 binding was observed, which significantly correlated with disease severity as reflected by the striatal reduction in [(11)C]raclopride binding, the Unified Huntington's Disease Rating Scale score, and the patients' CAG index. Also detected were significant increases in microglia activation in cortical regions including prefrontal cortex and anterior cingulate.

CONCLUSIONS

These [(11)C](R)-PK11195 PET findings show that the level of microglial activation correlates with Huntington disease (HD) severity. They lend support to the view that microglia contribute to the ongoing neuronal degeneration in HD and indicate that [(11)C](R)-PK11195 PET provides a valuable marker when monitoring the efficacy of putative neuroprotecting agents in this relentlessly progressive genetic disorder.

In vivo imaging of microglial activation with [11C](R)-PK11195 PET in progressive supranuclear palsy

Progressive supranuclear palsy (PSP) is a neurodegenerative disease presenting with voluntary gaze difficulties, early falls, and Parkinsonism. Neuronal loss, associated with intracellular neurofibrillary tangles and activated microglia, is found targeting the basal ganglia, brainstem nuclei, and frontal cortex. [11C](R)-PK11195 PET is a marker of peripheral benzodiazepine binding sites (PBBS) expressed by activated microglia. We have used [11C](R)-PK11195 PET to demonstrate in vivo the degree and distribution of the glial response to the degenerative process in four patients with PSP. Compared to normal age-matched controls, the PSP patient group showed significantly increased mean [11C](R)-PK11195 binding in the basal ganglia, midbrain, the frontal lobe, and the cerebellum. Two of the patients were rescanned after 6 to 10 months and during that time the level of microglial activation remained stable. [11C](R)-PK11195 PET reveals a pattern of increased microglial activation in PSP patients involving cortical and subcortical regions that corresponds well with the known distribution of neuropathological changes. [11C](R)-PK11195 PET, therefore, may help in characterizing in vivo the underlying disease activity in PSP.

In vivo imaging of microglial activation with [11C](R)-PK11195 PET in corticobasal degeneration

Corticobasal degeneration (CBD) is a neurodegenerative parkinsonian disorder of unknown cause that shows considerable clinical heterogeneity. In CBD, activated microglia have been shown to be associated closely with the extensive tau pathology found in the affected basal ganglia, brainstem nuclei, and cortical regions. We report on the use of [(11)C](R)-(1-[2-chlorophenyl]-N-methyl-N-[1-methylpropyl]-3-isoquinoline carboxamide) (PK11195) positron emission tomography (PET), a marker of peripheral benzodiazepine binding sites (PBBS) that are expressed by activated microglia, to demonstrate in vivo the degree and distribution of glial response to the degenerative process in 4 patients with CBD. Compared with normal age-matched controls, the CBD patient group showed significantly increased mean [(11)C](R)-PK11195 binding in the caudate nucleus, putamen, substantia nigra, pons, pre- and postcentral gyrus, and the frontal lobe. [11C](R)-PK11195 PET reveals a pattern of increased microglial activation in CBD patients involving cortical regions and the basal ganglia that corresponds well with the known distribution of neuropathological changes, which may therefore help to characterize in vivo the underlying disease activity in CBD.

Imaging of activated microglia with PET and [11C]PK 11195 in corticobasal degeneration

Positron emission tomography (PET) using [(11)C]PK 11195, a ligand for peripheral benzodiazepine receptor binding sites, offers the opportunity to image activated microglia in vivo. This tool may therefore be used to display the occurrence of microglial activation in the course of neurodegeneration. A patient with the clinical diagnosis of corticobasal degeneration (CBD) and left-sided symptoms was studied using fluorodeoxyglucose (FDG) and [(11)C]PK 11195 PET. We found a marked right hemispheric hypometabolism and asymmetric microglial activation in corresponding areas of the basal ganglia and right temporal and parietal cortex. [(11)C]PK 11195 PET suggests involvement of microglial activation in the pathogenesis of CBD.

[11C](R)-PK11195 PET imaging of microglial activation in multiple system atrophy

Microglia, the brain's intrinsic macrophages, bind (R)-PK11195 when activated by neuronal injury. The authors used [11C](R)-PK11195 PET to localize in vivo microglial activation in patients with multiple system atrophy (MSA). Increased [11C](R)-PK11195 binding was primarily found in the dorsolateral prefrontal cortex, putamen, pallidum, pons, and substantia nigra, reflecting the known distribution of neuropathologic changes in MSA. Providing an indicator of disease activity, [11C](R)-PK11195 PET can thus be used to characterize the in vivo neuropathology of MSA.

Assessment of neuroinflammation and microglial activation in Alzheimer's disease with radiolabelled PK11195 and single photon emission computed tomography. A pilot study

OBJECTIVES

Inflammation contributes to degeneration in Alzheimer's disease (AD), not simply as a secondary phenomenon, but primarily as a significant source of pathology. [(123)I]iodo-PK11195 is a single photon emission computed tomography (SPECT) ligand for the peripheral benzodiazepine receptor, the latter being expressed on microglia (brain resident macrophages) and upregulated under inflammatory circumstances. The objectives were to assess AD inflammation by detecting [(123)I]iodo-PK11195 uptake changes and investigate how uptake values relate with perfusion SPECT and neuropsychological findings.

METHODS

Ten AD and 9 control subjects were included. [(123)I]iodo-PK11195 SPECT images were realigned into stereotactic space where binding indices, normalized on cerebellar uptake, were calculated.

RESULTS

The mean [(123)I]iodo-PK11195 uptake was increased in AD patients compared with controls in nearly all neocortical regions; however, statistical significance was only reached in the frontal and right mesotemporal regions. Significant correlations were found between regional increased [(123)I]iodo-PK11195 uptake and cognitive deficits.

CONCLUSIONS

[(123)I]iodo-PK11195 is a cellular disease activity marker and allows in vivo assessment of microglial inflammation in AD.

PET visualization of microglia in multiple sclerosis patients using [11C]PK11195

Activated microglia are involved in the immune response of multiple sclerosis (MS). The peripheral benzodiazepine receptor (PBR) is expressed on microglia and up-regulated after neuronal injury. [11C]PK11195 is a positron emission tomography (PET) radioligand for the PBR. The objective of the present study was to investigate [11C]PK11195 imaging in MS patients and its additional value over magnetic resonance imaging (MRI) concerning the immuno-pathophysiological process. Seven healthy and 22 MS subjects were included. Semiquantitative [11C]PK11195 uptake values were assessed with normalization on cortical grey matter. Uptake in Gadolinium-lesions was significantly increased compared with normal white matter. Uptake in T2-lesions was generally decreased, suggesting a PBR down-regulation. However, uptake values increased whenever a clinical or MR-relapse was present, suggestive for a dynamic process with a transient PBR up-regulation. During disease progression, an increase of normal-appearing white matter (NAWM) uptake was found, propagating NAWM as the possible real burden of disease. In conclusion, [11C]PK11195 and PET are able to demonstrate inflammatory processes with microglial involvement in MS.

Scintigraphic visualization of inflammation in neurodegenerative disorders

In the past few decades, our understanding of the central nervous system has evolved from one of an immune-privileged site, to one where inflammation is pathognomonic for some of the most prevalent and tragic neurodegenerative diseases. Current research indicates that diseases as diverse as multiple sclerosis, stroke and Alzheimer's disease exhibit inflammatory processes that contribute to cellular dysfunction or loss. Inflammation, whether in the brain or periphery, is almost always a secondary response to a primary pathogen. In head trauma, for example, the blow to the head is the primary event. What typically concerns the neurologist and neurosurgeon more, however, is the secondary inflammatory response that will ensue and likely cause more neuron loss than the initial injury. This paper reviews the basic neuroinflammatory mechanisms, the potential neurotoxic mediators during activation of microglia, the brain resident macrophages, and their role in neurodegeneration. Alzheimer's disease is taken as a prototype for exploring these mechanisms, as it expresses more than 40 inflammatory mediators, it is the most extensively studied disorder in terms of immune-related pathogenesis, and because of its importance as the most prevalent type of dementia. Tools for the visualization of these neuroinflammatory processes, both structural and mainly functional, are critically reviewed and discussed.

[11C](R)-PK11195 positron emission tomography imaging of activated microglia in vivo in Rasmussen's encephalitis

This study was designed to explore the feasibility of PET using [11C](R)-PK11195 as an in vivo marker of activated microglia/brain macrophages for the assessment of neuroinflammation in Rasmussen's encephalitis (RE). [11C](R)-PK11195 PET was carried out in four normal subjects, two patients with histologically confirmed RE, and three patients with clinically stable hippocampal sclerosis and low seizure frequency. Binding potential maps showing specific binding of [11C](R)-PK11195 were generated for each subject. Regional binding potential values were calculated for anatomically defined regions of interest after coregistration to and spatial transformation into the subjects' own MRI. In one patient with RE who underwent hemispherectomy, the resected, paraffin-embedded brain tissue was stained with an antibody (CR3/43) that labels activated human microglia. Whereas specific binding of [11C](R)-PK11195 in clinically stable hippocampal sclerosis was similar to that in normal brain, patients with RE showed a focal and diffuse increase in binding throughout the affected hemisphere. In RE, [11C](R)-PK11195 PET can reveal in vivo the characteristic, unilateral pattern known from postmortem neuropathologic study. PET imaging of activated microglia/brain macrophages offers a tool for investigation of a range of brain diseases where neuroinflammation is a component and in which conventional MRI does not unequivocally indicate an inflammatory tissue reaction. [11C](R)-PK11195 PET may help in the choice of appropriate biopsy sites and, further, may allow assessment of the efficacy of antiinflammatory disease-modifying treatment.

PET of peripheral benzodiazepine binding sites in the microgliosis of Alzheimer's disease

Animal and human autoradiographic studies have shown increased in vitro binding of the peripheral benzodiazepine binding site antagonist PK 11195 in areas of microgliosis, including the temporal association cortex of patients with Alzheimer's disease. To further elucidate the role of cellular inflammation and microgliosis in Alzheimer's disease during life, we used PET and [11C]PK 11195, a peripheral benzodiazepine receptor ligand known to bind avidly to microglia.

METHODS

Eight patients with a diagnosis of probable Alzheimer's disease underwent PET of the brain using [11C]PK 11195 and, for comparison, with [18F]FDG to determine cerebral glucose metabolism. Uptake of [11C]PK 11195 in various brain regions was expressed relative to that in the cerebellum and compared to values determined in one normal elderly subject and in clinically and anatomically unaffected hemispheres of seven patients with small unilateral gliomas.

RESULTS

No increases in peripheral benzodiazepine binding were identified in patients with probable Alzheimer's disease, and binding was lowest in regions that were most hypometabolic.

CONCLUSION

The peripheral benzodiazepine binding sites associated with microgliosis and cellular inflammation in Alzheimer's disease at postmortem are undetectable by PET using [11C]PK 11195 in patients with mild-to-moderate dementia.