Microglial imaging with positron emission tomography and atrophy measurements with magnetic resonance imaging in multiple sclerosis: a correlative study

In Vivo Assessment of Brain White Matter Inflammation in Multiple Sclerosis with (18)F-PBR111 PET

PET radioligand binding to the 18-kD translocator protein (TSPO) in the brains of patients with multiple sclerosis (MS) primarily reflects activated microglia and macrophages. We previously developed genetic stratification for accurate quantitative estimation of TSPO using second-generation PET radioligands. In this study, we used (18)F-PBR111 PET and MR imaging to measure relative binding in the lesional, perilesional, and surrounding normal-appearing white matter of MS patients, as an index of the innate immune response.

METHODS

(18)F-PBR111 binding was quantified in 11 MS patients and 11 age-matched healthy volunteers, stratified according to the rs6971 TSPO gene polymorphism. Fluid-attenuated inversion recovery and magnetization transfer ratio (MTR) MR imaging were used to segment the white matter in MS patients as lesions, perilesional volumes, nonlesional white matter with reduced MTR, and nonlesional white matter with normal MTR.

RESULTS

(18)F-PBR111 binding was higher in the white matter lesions and perilesional volumes of MS patients than in white matter of healthy controls (P < 0.05). Although there was substantial heterogeneity in binding between different lesions, a within-subject analysis showed higher (18)F-PBR111 binding in MS lesions (P < 0.05) and in perilesional (P < 0.05) and nonlesional white matter with reduced MTR (P < 0.005) than in nonlesional white matter with a normal MTR. A positive correlation was observed between the mean (18)F-PBR111 volume of distribution increase in lesions relative to nonlesional white matter with a normal MTR and the MS severity score (Spearman ρ = 0.62, P < 0.05).

CONCLUSION

This study demonstrates that quantitative TSPO PET with a second-generation radioligand can be used to characterize innate immune responses in MS in vivo and provides further evidence supporting an association between the white matter TSPO PET signal in lesions and disease severity. Our approach is practical for extension to studies of the role of the innate immune response in MS for differentiation of antiinflammatory effects of new medicines and their longer term impact on clinical outcome.

PET imaging in multiple sclerosis

Positron emission tomography (PET) is a non-invasive technique for quantitative imaging of biochemical and physiological processes in animals and humans. PET uses probes labeled with a radioactive isotope, called PET tracers, which can bind to or be converted by a specific biological target and thus can be applied to detect and monitor different aspects of diseases. The number of applications of PET imaging in multiple sclerosis is still limited. Clinical studies using PET are basically focused on monitoring changes in glucose metabolism and the presence of activated microglia/macrophages in sclerotic lesions. In preclinical studies, PET imaging of targets for other processes, like demyelination and remyelination, has been investigated and may soon be translated to clinical applications. Moreover, more PET tracers that could be relevant for MS are available now, but have not been studied in this context yet. In this review, we summarize the PET imaging studies performed in multiple sclerosis up to now. In addition, we will identify potential applications of PET imaging of processes or targets that are of interest to MS research, but have yet remained largely unexplored.

In Vivo Detection of Diffuse Inflammation in Secondary Progressive Multiple Sclerosis Using PET Imaging and the Radioligand ¹¹C-PK11195

Patients with secondary progressive multiple sclerosis (SPMS) are lacking efficient medication to slow down the progression of their disease. PET imaging holds promise as a method to study, at the molecular level and in vivo, the central nervous system pathology of SPMS. PET might thus help to elucidate potential therapeutic targets and be useful as an imaging biomarker in future treatment trials of progressive multiple sclerosis. The objective of this study was to evaluate whether translocator protein (TSPO) imaging could be used to visualize the diffuse inflammation located in the periplaque area and in the normal-appearing white matter (NAWM) in the brains of patients with SPMS.

METHODS

This was an imaging study using MR imaging and PET with (11)C-PK11195 binding to TSPO, which is expressed in activated, but not in resting, microglia. Ten SPMS patients with a mean expanded disability status scale score of 6.3 (SD, 1.5) and eight age-matched healthy controls were studied. The imaging was performed using High-Resolution Research Tomograph PET and 1.5-T MR imaging scanners. Microglial activation was evaluated as the distribution volume ratio (DVR) of (11)C-PK11195 from dynamic PET images. DVR estimations were performed with special interest in NAWM and gray matter using region-of-interest and parametric image-based approaches.

RESULTS

The DVR of (11)C-PK11195 was significantly increased in the periventricular and total NAWM (P = 0.016 and P < 0.001, respectively) and in the thalamic ROIs (P = 0.027) of SPMS patients, compared with the control group. Similarly, parametric image analysis showed widespread increases of (11)C-PK11195 in the white matter of SPMS patients, compared with healthy controls. Increased perilesional TSPO uptake was present in 57% of the chronic T1 lesions in MR imaging.

CONCLUSION

The finding of increased (11)C-PK11195 binding in the NAWM of SPMS patients is in line with the neuropathologic demonstration that activated microglial cells are the source of diffuse NAWM inflammation. Evaluating microglial activation with TSPO-binding PET ligands provides a unique tool to assess diffuse brain inflammation and perilesional activity in progressive multiple sclerosis in vivo.

Translocator protein 18 kDa negatively regulates inflammation in microglia

Translocator protein 18 kDa (TSPO) is a mitochondrial outer membrane protein. Although TSPO expression is up-regulated during neuroinflammation, the role of TSPO and its signaling mechanisms in regulation of neuroinflammation remains to be elucidated at the molecular level. Here we demonstrate that TSPO is a negative regulator of neuroinflammation in microglia. Over-expression of TSPO decreased production of pro-inflammatory cytokines upon lipopolysaccharide treatment while TSPO knock-down had the opposite effect. Anti-inflammatory activity of TSPO is also supported by increased expression of alternatively activated M2 stage-related genes. These data suggest that up-regulation of TSPO level during neuroinflammation may be an adaptive response mechanism. We also provide the evidence that the repressive activity of TSPO is at least partially mediated by the attenuation of NF-κB activation. Neurodegenerative diseases are characterized by loss of specific subsets of neurons at the particular anatomical regions of the central nervous system. Cause of neuronal death is still largely unknown, but it is becoming clear that neuroinflammation plays a significant role in the pathophysiology of neurodegenerative diseases. Understanding the mechanisms underlying the inhibitory effects of TSPO on neuroinflammation can contribute to the therapeutic design for neurodegenerative diseases.

PET imaging of glucose metabolism, neuroinflammation and demyelination in the lysolecithin rat model for multiple sclerosis

Background:

Injection of lysolecithin in the central nervous system results in demyelination accompanied by local activation of microglia and recruitment of monocytes. Positron-emission tomography (PET) imaging, using specific tracers, may be an adequate technique to monitor these events in vivo and therefore may become a tool for monitoring disease progression in multiple sclerosis (MS) patients.

Objectives:

The objective of this paper is to evaluate the potential of PET imaging in monitoring local lesions, using [11C]MeDAS, [11C]PK11195 and [18F]FDG as PET tracers for myelin density, microglia activation and glucose metabolism, respectively.

Methods:

Sprague-Dawley rats were stereotactically injected with either 1% lysolecithin or saline in the corpus callosum and striatum of the right brain hemisphere. PET imaging was performed three days, one week and four weeks after injection. Animals were terminated after PET imaging and the brains were explanted for (immuno)histochemical analysis.

Results:

PET imaging was able to detect local demyelination induced by lysolecithin in the corpus callosum and striatum with [11C]MeDAS and concomitant microglia activation and monocyte recruitment with [11C]PK11195. [18F]FDG imaging demonstrated that glucose metabolism was maintained in the demyelinated lesions.

Conclusion:

PET imaging with multiple tracers allows simultaneous in vivo monitoring of myelin density, neuroinflammation and brain metabolism in small MS-like lesions, indicating its potential to monitor disease progression in MS patients.

Imaging Neuroinflammation - from Bench to Bedside

Neuroinflammation plays a central role in a variety of neurological diseases, including stroke, multiple sclerosis, Alzheimer’s disease, and malignant CNS neoplasms, among many other. Different cell types and molecular mediators participate in a cascade of events in the brain that is ultimately aimed at control, regeneration and repair, but leads to damage of brain tissue under pathological conditions. Non-invasive molecular imaging of key players in the inflammation cascade holds promise for identification and quantification of the disease process before it is too late for effective therapeutic intervention. In this review, we focus on molecular imaging techniques that target inflammatory cells and molecules that are of interest in neuroinflammation, especially those with high translational potential. Over the past decade, a plethora of molecular imaging agents have been developed and tested in animal models of (neuro)inflammation, and a few have been translated from bench to bedside. The most promising imaging techniques to visualize neuroinflammation include MRI, positron emission tomography (PET), single photon emission computed tomography (SPECT), and optical imaging methods. These techniques enable us to image adhesion molecules to visualize endothelial cell activation, assess leukocyte functions such as oxidative stress, granule release, and phagocytosis, and label a variety of inflammatory cells for cell tracking experiments. In addition, several cell types and their activation can be specifically targeted in vivo, and consequences of neuroinflammation such as neuronal death and demyelination can be quantified. As we continue to make progress in utilizing molecular imaging technology to study and understand neuroinflammation, increasing efforts and investment should be made to bring more of these novel imaging agents from the “bench to bedside.”

Neuroimaging biomarkers of neurodegenerative diseases and dementia

Neurodegenerative disorders leading to dementia are common diseases that affect many older and some young adults. Neuroimaging methods are important tools for assessing and monitoring pathological brain changes associated with progressive neurodegenerative conditions. In this review, the authors describe key findings from neuroimaging studies (magnetic resonance imaging and radionucleotide imaging) in neurodegenerative disorders, including Alzheimer's disease (AD) and prodromal stages, familial and atypical AD syndromes, frontotemporal dementia, amyotrophic lateral sclerosis with and without dementia, Parkinson's disease with and without dementia, dementia with Lewy bodies, Huntington's disease, multiple sclerosis, HIV-associated neurocognitive disorder, and prion protein associated diseases (i.e., Creutzfeldt-Jakob disease). The authors focus on neuroimaging findings of in vivo pathology in these disorders, as well as the potential for neuroimaging to provide useful information for differential diagnosis of neurodegenerative disorders.

A TSPO ligand is protective in a mouse model of multiple sclerosis

Local production of neurosteroids such as progesterone and allopregnanolone confers neuroprotection in central nervous system (CNS) inflammatory diseases. The mitochondrial translocator protein (TSPO) performs a rate-limiting step in the conversion of cholesterol to pregnenolone and its steroid derivatives. Previous studies have shown that TSPO is upregulated in microglia and astroglia during neural inflammation, and radiolabelled TSPO ligands such as PK11195 have been used to image and localize injury in the CNS. Recent studies have shown that modulating TSPO activity with pharmacological ligands such as etifoxine can initiate the production of neurosteroids locally in the injured CNS. In this study, we examined the effects of etifoxine, a clinically available anxiolytic drug, in the development and progression of mouse experimental autoimmune encephalomyelitis (EAE), an experimental model for multiple sclerosis (MS). Our results showed that etifoxine attenuated EAE severity when administered before the development of clinical signs and also improved symptomatic recovery when administered at the peak of the disease. In both cases, recovery was correlated with diminished inflammatory pathology in the lumbar spinal cord. Modulation of TSPO activity by etifoxine led to less peripheral immune cell infiltration of the spinal cord, and increased oligodendroglial regeneration after inflammatory demyelination in EAE. Our results suggest that a TSPO ligand, e.g. etifoxine, could be a potential new therapeutic option for MS with benefits that could be comparable to the administration of systemic steroids but potentially avoiding the detrimental side effects of long-term direct use of steroids.

Positron emission tomography imaging in neurological disorders

Positron emission tomography (PET) is a powerful tool for in vivo imaging investigations of human brain function. It provides non-invasive quantification of brain metabolism, receptor binding of various neurotransmitter systems, and alterations in regional blood flow. The use of PET in a clinical setting is still limited due to the high costs of cyclotrons and radiochemical laboratories. However, once these limitations can be bypassed, PET could aid clinical practice by providing a useful imaging technique for the diagnosis, the planning of treatment, and the prediction outcome in various neurological diseases.This review aims to explain the PET imaging technique and its applications in neurological disorders such as Parkinson’s disease, Huntington’s disease, multiple sclerosis, and dementias.

Neuroinflammatory imaging biomarkers: relevance to multiple sclerosis and its therapy

Magnetic resonance imaging is an established tool in the management of multiple sclerosis (MS). Loss of blood brain barrier integrity assessed by gadolinium (Gd) enhancement is the current standard marker of MS activity. To explore the complex cascade of the inflammatory events, other magnetic resonance imaging, but also positron emission tomographic markers reviewed in this article are being developed to address active neuroinflammation with increased sensitivity and specificity. Alternative magnetic resonance contrast agents, positron emission tomographic tracers and imaging techniques could be more sensitive than Gd to early blood brain barrier alteration, and they could assess the inflammatory cell recruitment and/or the associated edema accumulation. These markers of active neuroinflammation, although some of them are limited to experimental studies, could find great relevance to complete Gd information and thereby increase our understanding of acute lesion pathophysiology and its noninvasive follow-up, especially to monitor treatment efficacy. Furthermore, such accurate markers of inflammation combined with those of neurodegeneration hold promise to provide a more complete picture of MS, which will be of great benefit for future therapeutic strategies.

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.

Noninvasive molecular imaging of neuroinflammation

Inflammation is a highly dynamic and complex adaptive process to preserve and restore tissue homeostasis. Originally viewed as an immune-privileged organ, the central nervous system (CNS) is now recognized to have a constant interplay with the innate and the adaptive immune systems, where resident microglia and infiltrating immune cells from the periphery have important roles. Common diseases of the CNS, such as stroke, multiple sclerosis (MS), and neurodegeneration, elicit a neuroinflammatory response with the goal to limit the extent of the disease and to support repair and regeneration. However, various disease mechanisms lead to neuroinflammation (NI) contributing to the disease process itself. Molecular imaging is the method of choice to try to decipher key aspects of the dynamic interplay of various inducers, sensors, transducers, and effectors of the orchestrated inflammatory response in vivo in animal models and patients. Here, we review the basic principles of NI with emphasis on microglia and common neurologic disease mechanisms, the molecular targets which are being used and explored for imaging, and molecular imaging of NI in frequent neurologic diseases, such as stroke, MS, neurodegeneration, epilepsy, encephalitis, and gliomas.

Imaging of microglia in patients with neurodegenerative disorders

Microglia constitute the main immune defense in the central nervous system. In response to neuronal injury, microglia become activated, acquire phagocytic properties, and release a wide range of pro-inflammatory mediators that are essential for the annihilation of the neuronal insult. Although the role of microglial activation in acute neuronal damage is well defined, the pathophysiological processes underlying destructive or protective role to neurons following chronic exposure to microglial activation is still a subject of debate. It is likely that chronic exposure induces detrimental effects by promoting neuronal death through the release of neurotoxic factors. Positron emission tomography (PET) imaging with the use of translocator protein (TSPO) radioligands provides an in vivo tool for tracking the progression and severity of neuroinflammation in neurodegenerative disease. TSPO expression is correlated to the extent of microglial activation and the measurement of TSPO uptake in vivo with PET is a useful indicator of active disease. Although understanding of the interaction between radioligands and TSPO is not completely clear, there is a wide interest in application of TSPO imaging in neurodegenerative disease. In this article, we aim to review the applications of in vivo microglia imaging in neurodegenerative disorders such as Parkinson's disease, Huntington's disease, Dementias, and Multiple Sclerosis.

Clinical correlates of grey matter pathology in multiple sclerosis

Traditionally, multiple sclerosis has been viewed as a disease predominantly affecting white matter. However, this view has lately been subject to numerous changes, as new evidence of anatomical and histological changes as well as of molecular targets within the grey matter has arisen. This advance was driven mainly by novel imaging techniques, however, these have not yet been implemented in routine clinical practice. The changes in the grey matter are related to physical and cognitive disability seen in individuals with multiple sclerosis. Furthermore, damage to several grey matter structures can be associated with impairment of specific functions. Therefore, we conclude that grey matter damage - global and regional - has the potential to become a marker of disease activity, complementary to the currently used magnetic resonance markers (global brain atrophy and T2 hyperintense lesions). Furthermore, it may improve the prediction of the future disease course and response to therapy in individual patients and may also become a reliable additional surrogate marker of treatment effect.

Decreased microglial activation in MS patients treated with glatiramer acetate

Activated microglia are thought to be an important contributor to tissue damage in multiple sclerosis (MS). The level of microglial activation can be measured non-invasively using [(11)C]-R-PK11195, a radiopharmaceutical for positron emission tomography (PET). Prior studies have identified abnormalities in the level of [(11)C]-R-PK11195 uptake in patients with MS, but treatment effects have not been evaluated. Nine previously untreated relapsing-remitting MS patients underwent PET and magnetic resonance imaging of the brain at baseline and after 1 year of treatment with glatiramer acetate. Parametric maps of [(11)C]-R-PK11195 uptake were obtained for baseline and post-treatment PET scans, and the change in [(11)C]-R-PK11195 uptake pre- to post-treatment was evaluated across the whole brain. Region-of-interest analysis was also applied to selected subregions. Whole brain [(11)C]-R-PK11195 binding potential per unit volume decreased 3.17% (95% CI: -0.74, -5.53%) between baseline and 1 year (p = 0.018). A significant decrease was noted in cortical gray matter and cerebral white matter, and a trend towards decreased uptake was seen in the putamen and thalamus. The results are consistent with a reduction in inflammation due to treatment with glatiramer acetate, though a larger controlled study would be required to prove that association. Future research will focus on whether the level of baseline microglial activation predicts future tissue damage in MS and whether [(11)C]-R-PK11195 uptake in cortical gray matter correlates with cortical lesion load.

Current paradigm of the 18-kDa translocator protein (TSPO) as a molecular target for PET imaging in neuroinflammation and neurodegenerative diseases

Neuroinflammation is a process characterised by drastic changes in microglial morphology and by marked upregulation of the 18-kDa translocator protein (TSPO) on the mitochondria. The continual increase in incidence of neuroinflammation and neurodegenerative diseases poses a major health issue in many countries, requiring more innovative diagnostic and monitoring tools. TSPO expression may constitute a biomarker for brain inflammation that could be monitored by using TSPO tracers as neuroimaging agents. From medical imaging perspectives, this review focuses on the current concepts related to the TSPO, and discusses briefly on the status of its PET imaging related to neuroinflammation and neurodegenerative diseases in humans.

Translocator protein PET imaging for glial activation in multiple sclerosis

Glial activation in the setting of central nervous system inflammation is a key feature of the multiple sclerosis (MS) pathology. Monitoring glial activation in subjects with MS, therefore, has the potential to be informative with respect to disease activity. The translocator protein 18 kDa (TSPO) is a promising biomarker of glial activation that can be imaged by positron emission tomography (PET). To characterize the in vivo TSPO expression in MS, we analyzed brain PET scans in subjects with MS and healthy volunteers in an observational study using [(11)C]PBR28, a newly developed translocator protein-specific radioligand. The [(11)C]PBR28 PET showed altered compartmental distribution of TSPO in the MS brain compared to healthy volunteers (p = 0.019). Focal increases in [(11)C]PBR28 binding corresponded to areas of active inflammation as evidenced by significantly greater binding in regions of gadolinium contrast enhancement compared to contralateral normal-appearing white matter (p = 0.0039). Furthermore, increase in [(11)C]PBR28 binding preceded the appearance of contrast enhancement on magnetic resonance imaging in some lesions, suggesting a role for early glial activation in MS lesion formation. Global [(11)C]PBR28 binding showed correlation with disease duration (p = 0.041), but not with measures of clinical disability. These results further define TSPO as an informative marker of glial activation in MS.

11C-(R)-PK11195 PET imaging of microglial activation and response to minocycline in zymosan-treated rats

We sought to advance methodology for studying microglial activation and putative therapeutic downregulation in response to minocycline by means of noninvasive in vivo imaging. A reproducible focal white matter lesion was used to reliably compare treatment conditions.

METHODS

The corpus callosum of female Sprague Dawley rats was injected with zymosan to promote microglial activation as confirmed by hematoxylin and eosin staining, (3)H-PK11195 autoradiography, and CD11b immunohistochemistry. A subset of subjects was treated systemically with minocycline to potentially alter microglial activation. Seven days after zymosan injection, subjects were imaged with PET using the radiotracer (11)C-(R)-PK11195. In vivo binding was evaluated using the distribution volume ratio (DVR) with respect to a reference region.

RESULTS

At the lesion site, the observed (11)C-(R)-PK11195 DVR for each treatment was as follows: mean saline DVR ± SD, 1.17 ± 0.05 (n = 5); zymosan-only DVR, 1.96 ± 0.33 (n = 10); and zymosan with minocycline DVR, 1.58 ± 0.12 (n = 9). Therefore, compared with controls, zymosan increased binding (P = 0.0001, 2-tailed t test) and minocycline treatment reduced zymosan-induced binding by 46% (P = 0.004, 2-tailed t test).

CONCLUSION

Zymosan-induced microglial activation and its response to minocycline can be quantitatively imaged in the rat brain using (11)C-(R)-PK11195 PET. The ability to detect a treatment effect in a focal white-matter lesion may be of use in studying therapies for multiple sclerosis (MS).

Translocator protein (18 kDa) (TSPO) as a therapeutic target for neurological and psychiatric disorders

The translocator protein (18 kDa) (TSPO) is localized primarily in the outer mitochondrial membrane of steroid-synthesizing cells, including those in the central and peripheral nervous system. One of its main functions is the transport of the substrate cholesterol into mitochondria, a prerequisite for steroid synthesis. TSPO expression may constitute a biomarker of brain inflammation and reactive gliosis that could be monitored by using TSPO ligands as neuroimaging agents. Moreover, initial clinical trials have indicated that TSPO ligands might be valuable in the treatment of neurological and psychiatric disorders. This Review focuses on the biology and pathophysiology of TSPO and the potential of currently available TSPO ligands for the diagnosis and treatment of neurological and psychiatric disorders.

Towards molecular imaging of multiple sclerosis

Magnetic resonance imaging (MRI) has had a profound impact on both research and clinical management of multiple sclerosis (MS), but signal changes reflect underlying neuropathology only indirectly and often non-specifically. Positron emission tomography (PET) offers the potential to complement MRI with quantitative measures of molecularly specific markers of cellular and metabolic processes. PET radiotracers already available promise new insights into the dynamics of the innate immune response, neuronal function, neurodegeneration and remyelination. Because PET is an exquisitely sensitive technique (able to image even picomolar concentrations), only microdoses of radioligand (<10 µg) are needed for imaging. This facilitates rapid implementation of novel radioligands because extensive toxicology data is not required. In the future, molecular imaging could assist clinical decision-making with patient stratification for optimization of treatment selection.

Radioisotopic imaging of neuroinflammation

Inflammatory responses are closely associated with many neurologic disorders and influence their outcome. In vivo imaging can document events accompanying neuroinflammation, such as changes in blood flow, vascular permeability, tightness of the blood-to-brain barrier, local metabolic activity, and expression of specific molecular targets. Here, we briefly review current methods for imaging neuroinflammation, with special emphasis on nuclear imaging techniques.

Reference tissue models and blood-brain barrier disruption: lessons from (R)-[11C]PK11195 in traumatic brain injury

(R)-[(11)C]PK11195 is a tracer for activated microglia. The purpose of this study was to assess the validity of the simplified reference tissue model for analyzing (R)-[(11)C]PK11195 studies in traumatic brain injury (TBI), where blood-brain barrier disruptions are likely.

METHODS

Dynamic (R)-[(11)C]PK11195 scans were acquired at 3 time points after TBI. Plasma input-derived binding potential (BP(ND)(PI)), volume of distribution (V(T)) and K(1)/k(2), and simplified reference tissue model-derived binding potential (BP(ND)(SRTM)) were obtained. Simulations were performed to assess the effect of varying K(1)/k(2).

RESULTS

Early after TBI, an increase in V(T), but not in BP(ND)(PI), was found. Early K(1)/k(2) correlated with V(T) and BP(ND)(SRTM) but not with BP(ND)(PI). One and 6 mo after TBI, BP(ND)(SRTM) correlated with BP(ND)(PI).

CONCLUSION

Early after TBI, (R)-[(11)C]PK11195 studies should be analyzed using plasma input models.

Imaging microglial activation during neuroinflammation and Alzheimer's disease

Microglial activation is an important pathogenic component of neurodegenerative disease processes. This state of increased inflammation is associated not only with neurotoxic consequences but also neuroprotective effects, e.g., phagocytosis and clearance of amyloid in Alzheimer's disease. In addition, activation of microglia appears to be one of the major mechanisms of amyloid clearance following active or passive immunotherapy. Imaging techniques may provide a minimally invasive tool to elucidate the complexities and dynamics of microglial function and dysfunction in aging and neurodegenerative diseases. Imaging microglia in vivo in live subjects by confocal or two/multiphoton microscopy offers the advantage of studying these cells over time in their native environment. Imaging microglia in human subjects by positron emission tomography scanning with translocator protein-18 kDa ligands can offer a measure of the inflammatory process and a means of detecting progression of disease and efficacy of therapeutics over time.

Use of ultrasmall superparamagnetic particles of iron oxide (USPIO)-enhanced MRI to demonstrate diffuse inflammation in the normal-appearing white matter (NAWM) of multiple sclerosis (MS) patients: an exploratory study

PURPOSE

To explore ultrasmall superparamagnetic particles of iron oxide (USPIO) as a marker for diffuse inflammation in multiple sclerosis (MS) normal-appearing white matter (NAWM), using quantitative MRI. Disease activity in the NAWM of MS patients partly explains why MRI lesion burden correlates only moderately with disability. USPIO have been shown to visualize the cellular component of inflammation in focal MS lesions. In this study, we aimed to explore USPIO as a marker for the more diffuse inflammation in MS NAWM, using quantitative MRI.

MATERIALS AND METHODS

In this prospective MRI study, 16 MS patients (eight relapsing-remitting MS [RRMS] and eight primary-progressive MS [PPMS] cases) and five healthy control (HC) subjects were included. Using a flip-angle (FA) array, B1-corrected T1 maps were generated before and 24 hours after USPIO (SHU555C) injection. White-matter (WM) T1 histogram and region-of-interest (ROI) characteristics were compared between both time points using Wilcoxon signed-rank test.

RESULTS

Both NAWM ROI and histogram analyses showed T1 shortening after USPIO injection in MS patients (P < 0.01), but not in HCs (P = 0.68).

CONCLUSION

This exploratory study suggests that USPIO-enhanced MRI may be a new potential marker for subtle inflammatory activity in MS NAWM. Further studies should focus on relating diffuse inflammation to clinical disease activity and treatment efficacy.

[11C]-DPA-713 and [18F]-DPA-714 as new PET tracers for TSPO: a comparison with [11C]-(R)-PK11195 in a rat model of herpes encephalitis

Background

Activation of microglia cells plays an important role in neurological diseases. Positron emission tomography (PET) with [¹¹C]-(R)-PK11195 has already been used to visualize activated microglia cells in neurological diseases. However, [¹¹C]-(R)-PK11195 may not possess the required sensitivity to visualize mild neuroinflammation. In this study, we evaluated the PET tracers [¹¹C]-DPA-713 and [¹⁸F]-DPA-714 as agents for imaging of activated microglia in a rat model of herpes encephalitis.

Materials and Methods

Rats were intranasally inoculated with HSV-1. On day 6 or 7 after inoculation, small animal PET studies were performed to compare [¹¹C]-(R)-PK11195, [¹¹C]-DPA-713, and [¹⁸F]-DPA-714.

Results

Uptake of [¹¹C]-DPA-713 in infected brain areas was comparable to that of [¹¹C]-(R)-PK11195, but [¹¹C]-DPA-713 showed lower non-specific binding. Non-specific uptake of [¹⁸F]-DPA-714 was lower than that of [¹¹C]-(R)-PK11195. In the infected brain, total [¹⁸F]-DPA-714 uptake was lower than that of [¹¹C]-(R)-PK11195, with comparable specific uptake.

Conclusions

[¹¹C]-DPA-713 may be more suitable for visualizing mild inflammation than [¹¹C]-(R)-PK11195. In addition, the fact that [¹⁸F]-DPA-714 is an agonist PET tracer opens new possibilities to evaluate different aspects of neuroinflammation. Therefore, both tracers warrant further investigation in animal models and in a clinical setting.

The positron emission tomography ligand DAA1106 binds with high affinity to activated microglia in human neurological disorders

Chronic microglial activation is an important component of many neurological disorders, and imaging activated microglia in vivo will enable the detection and improved treatment of neuroinflammation. 1-(2-chlorphenyl)-N-methyl-N-(1-methylpropyl)-3-isoquinoline-carbox-amide (PK11195), a peripheral benzodiazepine receptor ligand, has been used to image neuroinflammation, but the extent to which PK11195 binding distinguishes activated microglia and reactive astrocytes is unclear. Moreover, PK11195 may lack sufficient sensitivity for detecting mild neuroinflammation. We hypothesized that N-(2,5-dimethoxybenzyl)-N-(4-fluoro-2-phenoxyphenyl) acetamide (DAA1106), a new ligand that binds specifically to peripheral benzodiazepine receptor, binds to activated microglia in human neurological diseases with higher affinity than does PK11195. We therefore compared the pharmacological binding properties of [3H](R)-PK11195 and [3H]DAA1106 in postmortem tissues from patients with cerebral infarcts, amyotrophic lateral sclerosis, Alzheimer disease, frontotemporal dementia, and multiple sclerosis (n=10 each). In all diseases, [3H]DAA1106 showed a higher binding affinity as reflected by lower dissociation constant (KD) values than that of [3H](R)-PK11195. Moreover, specific binding of both ligands correlated with the presence of activated microglia identified by immunohistochemistry in situ. We conclude that 1) ligands that bind peripheral benzodiazepine receptor mainly label activated microglia in human neurological disorders and that 2) DAA1106 may possess binding characteristics superior to those of PK11195, which may be beneficial for in vivo positron emission tomography imaging.

Translocator protein 18 kDa (TSPO): molecular sensor of brain injury and repair

For over 15 years, the peripheral benzodiazepine receptor (PBR), recently named translocator protein 18 kDa (TSPO) has been studied as a biomarker of reactive gliosis and inflammation associated with a variety of neuropathological conditions. Early studies documented that in the brain parenchyma, TSPO is exclusively localized in glial cells. Under normal physiological conditions, TSPO levels are low in the brain neuropil but they markedly increase at sites of brain injury and inflammation making it uniquely suited for assessing active gliosis. This research has generated significant efforts from multiple research groups throughout the world to apply TSPO as a marker of "active" brain pathology using in vivo imaging modalities such as Positron Emission Tomography (PET) in experimental animals and humans. Further, in the last few years, there has been an increased interest in understanding the molecular and cellular function(s) of TSPO in glial cells. The latest evidence suggests that TSPO may not only serve as a biomarker of active brain disease but also the use of TSPO-specific ligands may have therapeutic implications in brain injury and repair. This review presents an overview of the history and function of TSPO focusing on studies related to its use as a sensor of active brain disease in experimental animals and in human studies.

The high affinity peripheral benzodiazepine receptor ligand DAA1106 binds specifically to microglia in a rat model of traumatic brain injury: implications for PET imaging

Traumatic brain injury (TBI) is a significant cause of mortality, morbidity, and disability. Microglial activation is commonly observed in response to neuronal injury which, when prolonged, is thought to be detrimental to neuronal survival. Activated microglia can be labeled using PK11195, a ligand that binds the peripheral benzodiazepine receptor (PBR), receptors which are increased in activated microglia and sparse in the resting brain. We compared the binding properties of two PBR ligands PK11195 and DAA1106 in rats using the controlled cortical impact (CCI) model of experimental TBI. While both ligands showed relative increases with specific binding in the cortex ipsilateral to injury compared to the contralateral side, [(3)H]DAA1106 showed higher binding affinity compared with [(3)H](R)-PK11195. Combined immunohistochemistry and autoradiography in brain tissues near the injury site showed that [(3)H]DAA1106 binding co-registered with activated microglia more than astrocytes. Further, increased [(3)H]DAA1106-specific binding positively correlated with the degree of microglial activation, and to a lesser degree with reactive astrocytosis. Finally, in vivo administration of each ligand in rats with TBI showed greater retention of [(11)C]DAA1106 compared to [(11)C](R)-PK11195 at the site of the contusion as assessed by ex vivo autoradiography. These results in a rat model of TBI indicate that [(11)C]DAA1106 binds with higher affinity to microglia when compared with PK11195, suggesting that [(11)C]DAA1106 may represent a better ligand than [(11)C](R)-PK11195 for in vivo PET imaging of activated microglia in TBI.

A comparison of the high-affinity peripheral benzodiazepine receptor ligands DAA1106 and (R)-PK11195 in rat models of neuroinflammation: implications for PET imaging of microglial activation

Activated microglia are an important feature of many neurological diseases and can be imaged in vivo using 1-(2-chlorophenyl)-N-methyl-N-(1-methylpropyl)-3-isoquinolinecarboxamide (PK11195), a ligand that binds the peripheral benzodiazepine receptor (PBR). N-(2,5-dimethoxybenzyl)-N-(5-fluoro-2-phenoxyphenyl) acetamide (DAA1106) is a new PBR-specific ligand that has been reported to bind to PBR with higher affinity compared with PK11195. We hypothesized that this high-affinity binding of DAA1106 to PBR will enable better delineation of microglia in vivo using positron emission tomography. [(3)H]DAA1106 showed higher binding affinity compared with [(3)H](R)-PK11195 in brain tissue derived from normal rats and the rats injected intrastriatally with 6-hydroxydopamine or lipopolysaccharide at the site of the lesion. Immunohistochemistry combined with autoradiography in brain tissues as well as correlation analyses showed that increased [(3)H]DAA1106 binding corresponded mainly to activated microglia. Finally, ex vivo autoradiography and positron emission tomography imaging in vivo showed greater retention of [(11)C]DAA1106 compared with [(11)C](R)-PK11195 in animals injected with either lipopolysaccaride or 6-hydroxydopamine at the site of lesion. These results indicate that DAA1106 binds with higher affinity to microglia in rat models of neuroinflammation when compared with PK11195, suggesting that [(11)C]DAA1106 may represent a significant improvement over [(11)C](R)-PK11195 for in vivo imaging of activated microglia in human neuroinflammatory disorders.

Positron emission tomography with computed tomography imaging of neuroinflammation in experimental autoimmune encephalomyelitis

2-[(18)F]Fluoro-2-deoxy-d-glucose positron emission tomography ([(18)F]FDG PET) detection of the up-regulated glycolysis associated with malignant transformation is a noninvasive imaging technique used extensively in cancer diagnosis. Although striking similarities exist in glucose transport and metabolism between tumor cells and activated immune cells, the potential use of [(18)F]FDG PET for the diagnosis and evaluation of autoimmune disorders has not been systematically investigated. Here we ask whether [(18)F]FDG PET in conjunction with computed tomography (CT) could be used to monitor a complex autoimmune disorder such as murine experimental autoimmune encephalomyelitis (EAE) and whether this approach is sensitive enough to evaluate therapeutic interventions. We found that (i) coregistration of metabolic (i.e., microPET) and high-resolution anatomical (i.e., CT) images allows serial quantification of glycolysis with [(18)F]FDG in various spinal column segments; (ii) [(18)F]FDG PET/CT can detect the increased glycolysis associated with paralysis-causing inflammatory infiltrates in the spinal cord; and (iii) the [(18)F]FDG measure of glycolysis in the spinal cord is sensitive to systemic immunosuppressive therapy. These results highlight the potential use of serial [(18)F]FDG PET/CT imaging to monitor neuroinflammation in EAE and suggest that similar approaches could be applied to the diagnosis and evaluation of other autoimmune and inflammatory disorders in animal models and in humans.

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.

Up-regulation of PK11195 binding in areas of axonal degeneration coincides with early microglial activation in mouse brain

Increased binding of the peripheral benzodiazepine binding site (PBBS) ligand [(3)H]PK11195 in the central nervous system of patients suffering from acute and chronic neuropathology has been associated with reactive microgliosis. However, it remains uncertain which stages of microglial activation occur in conjunction with the increased [(3)H]PK11195 binding. We used quantitative autoradiography for [(3)H]PK11195 and quantitative polymerase chain reaction for PBBS mRNA and markers of early and late microglial activation to investigate the time-course of cellular responses in the hippocampus of mice with degeneration of the entorhinal-hippocampal perforant path. The axonal lesion evoked an increase in the B(max) for [(3)H]PK11195 in hippocampus which peaked at 2 days post-lesion, remained elevated at day 5 and began to decline at 10 days post-lesion. These changes occurred in the absence of significant changes in affinity in vitro. Quantitative polymerase chain reaction analysis of isolated hippocampi using exon-specific primers indicated the presence of several splice variants of PBBS mRNA, which appeared to be affected differentially by the lesion. The changes in PBBS mRNA and CD11b mRNA levels correlated with the B(max) for [(3)H]PK11195 during 10 days post-lesion, suggesting that microglial activation couples with increases in mRNA levels for these markers. In addition, the onset of changes in PBBS mRNA levels coincided with the significantly elevated tumor necrosis factor mRNA levels present during early microglial activation at 2 days post-lesion. We conclude that up-regulation of [(3)H]PK11195 binding and PBBS mRNA levels coincided with early microglial activation, characterized by concomitantly increased microglial tumor necrosis factor mRNA levels, and persisted throughout the period with reactive microgliosis.

Distribution of PK11195 binding sites in porcine brain studied by autoradiography in vitro and by positron emission tomography

The cerebral distribution of peripheral-type benzodiazepine binding sites (PBBS) in human brain has been investigated by positron emission tomography (PET) with the specific radioligand [11C]PK11195 in diverse neuropathological conditions. However, little is known about the pattern of PK11195 binding sites in healthy brain. Therefore, we used quantitative autoradiography to measure the saturation binding parameters for [3H]PK11195 in cryostat sections from young Landrace pigs. Specific binding was lowest in the cerebellar white matter (85 fmol mg(-1)) and highest in the caudate nucleus (370 fmol mg(-1)), superior colliculus (400 fmol mg(-1)), and anterior thalamic nucleus (588 fmol mg(-1)). The apparent affinity was in the range of 2-6 nM in vitro, predicting high specific binding in PET studies of living brain. However, the distribution volume (V(d), ml g(-1)) of high specific activity [11C]PK11195 was nearly homogeneous (3 ml g(-1)) throughout brain of healthy Landrace pigs, and was nearly identical in studies with lower specific activity, suggesting that factors in vivo disfavor the detection of PBBS in Landrace pigs with this radioligand. In young, adult Göttingen minipig brain, the magnitude of V(d) for [11C]PK11195 was in the range 5-10 ml g(-1), and had a heterogeneous distribution resembling the in vitro findings in Landrace pigs. There was a trend toward globally increased V(d) in a group of minipigs with acute MPTP-induced parkinsonism, but no increase in V(d) was evident in the same pigs rescanned at 2 weeks after grafting of fetal mesencephalon to the partially denervated striatum. Thus, [11C]PK11195 binding was not highly sensitive to constituitively expressed PBBS in brain of young Landrace pigs, and did not clearly demonstrate the expected microglial activation in the MPTP/xenograft model of minipigs.

Cognitive dysfunction and emotional–behavioural changes in MS: The potential of positron emission tomography

Cognitive dysfunction and emotional-behavioral changes are symptoms with increasing clinical relevance during progression of the disease. They cannot be explained by demyelination of white matter alone but clearly indicate cortical dysfunction. Positron emission tomography (PET) provides methods to assess cortical dysfunction quantitatively by measuring cerebral glucose metabolism using the tracer (18)F-2-fluoro-2-deoxy-d-glucose (FDG). The technique has been employed to study fatigue and disease progression. Microglial activation was studied by 11C-PK-11195 PET. It was found not only in active plaques but also in degenerating fibre tracks. Other tracers offer a broad spectrum of measuring local physiological functions and pathophysiological processes, but some of them are still limited to experimental animal research.

Cell-cell communication by endocannabinoids during immune surveillance of the central nervous system

The immune system is designed to defend the organism from hazardous infection. The way by which cells of the immune system perform this function can be dangerous for the survival and function of the neuronal network in the brain. An attack of immune cells inside the brain includes the potential for severe neuronal damage or cell death and therefore impairment of CNS function. To avoid such undesirable action of the immune system, the CNS harbours an impressive arsenal of cellular and molecular mechanisms enabling strict control of immune reactions--the so-called "immune privilege". Under inflammatory and pathological conditions, loss of control of the CNS immune system results in the activation of neuronal damage cascades frequently associated with neurological disease. On the other hand, processes of neuroprotection and neurorepair after neuronal damage depend on a steady and tightly controlled immune surveillance. Accordingly, the immune system serves a highly specialized function in the CNS including negative feedback mechanisms that control immune reactions. Recent studies have revealed that endocannabinoids participate in one of the most important ones of the brain's negative feedback system. The CNS endocannabinoid system consists of cannabinoid receptors, their endogenous ligands and enzymes for the synthesis and degradation of endocannabinoids. It participates crucially in neuronal cell-cell-communication and signal transduction, e.g., by modulating synaptic input and protecting neurons from excitotoxic damage. Over the last decade, it has also become evident that endocannabinoids play an important role in the communication between immune cells, and in the interaction between nerve and immune system during CNS damage. Thus, therapeutic intervention in the CNS endocannabinoid system may help to restore the well-controlled and finely tuned balance of immune reactions in pathological conditions.

Peripheral benzodiazepine receptors in the brain of cirrhosis patients with manifest hepatic encephalopathy

PURPOSE

It has been suggested that ammonia-induced enhancement of peripheral benzodiazepine receptors (PBRs) in the brain is involved in the development of hepatic encephalopathy (HE). This hypothesis is based on animal experiments and studies of post-mortem human brains using radiolabelled PK11195, a specific ligand for PBR, but to our knowledge has not been tested in living patients. The aim of the present study was to test this hypothesis by measuring the number of cerebral PBRs in specific brain regions in cirrhotic patients with an acute episode of clinically manifest HE and healthy subjects using dynamic (11)C-PK11195 brain PET.

METHODS

Eight cirrhotic patients with an acute episode of clinically manifest HE (mean arterial ammonia 81 micromol/l) and five healthy subjects (22 micromol/l) underwent dynamic (11)C-PK11195 and (15)O-H(2)O PET, co-registered with MR images. Brain regions (putamen, cerebellum, cortex and thalamus) were delineated on co-registered (15)O-H(2) (15)O and MR images and copied to the dynamic (15)O-H(2)O and (11)C-PK11195 images. Regional cerebral blood flow (CBF) ((15)O-H(2)O scan) and the volume of distribution of PK11195 ((11)C-PK11195 scan) were calculated by kinetic analysis.

RESULTS

There were regional differences in the CBF, with lowest values in the cortex and highest values in the putamen in both groups of subjects (p<0.05), but no significant differences between the groups. There were no significant differences in the volume of distribution of PK11195 (V (d)) between regions or between the two groups of subjects. Mean values of V (d) ranged from 1.0 to 1.1 in both groups of subjects.

CONCLUSION

The results do not confirm the hypothesis of an increased number of PBRs in patients with HE.

Clinical aspects of inflammation in Alzheimer's disease

In Alzheimer's disease (AD) there is increasing evidence that neurotoxicity is mediated by CNS inflammatory processes. These processes involve activation of microglia by amyloid-beta leading to release of pro-inflammatory cytokines including IL-1beta, IL-6, and TNF-alpha among others. Neurotoxic processes mediated by these cytokines may include direct neuronal death by enhancement of apoptosis, decreased synaptic function as evidence by inhibition of long-term potentiation, and inhibition of hippocampal neurogenesis. Central nervous system (CNS) inflammation may predate the development of senile plaques and neurofibrillary tangles in AD and may prove to be a more sensitive marker of prodromal AD. New developments in measuring CNS inflammation include measuring cytokine release by peripheral blood mononuclear cells and the development of PET markers of microglial activation. There is epidemiological evidence that circulating serum IL-6 is associated with poorer cognition. While epidemiological studies suggest a protective effect of NSAIDs against development of AD, controlled trials of NSAIDs to date have not shown any protective effect of drug. New anti-inflammatory agents for treating or preventing AD may include novel NSAIDs and opioid antagonists. These developments provide an alternative or potential adjunct to anti-amyloid therapies for AD.