Synthesis of the enantiomers of [N-methyl-11C]PK 11195 and comparison of their behaviours as radioligands for PK binding sites in rats

The role of neuroinflammation in dementias

The molecular mechanism of neuronal loss and synaptic damage in Alzheimer's disease (AD), Parkinson's disease dementia (PDD), frontotemporal dementia (FTD) and Lewy body dementia (LBD) is poorly understood and could differ among different types of neurodegenerative processes. However, the presence of neuroinflammation is a common feature of dementia. In this setting, reactive microgliosis, oxidative damage and mitochondrial dysfunction are associated with the pathogenesis of all types of neurodegenerative dementia. Moreover, an increased body of evidence suggests that microglia may play a central role in AD progression. In this paper, we review the scientific literature on neuroinflammation related to the most common neurodegenerative dementias (AD, PDD, FTD and LBD) focussing on the possible molecular mechanisms and the available clinical evidence. Furthermore, we discuss the neuroimaging techniques that are currently used for the study of neuroinflammation in human brain.

Radiosynthesis and in vivo evaluation of two imidazopyridineacetamides, [(11)C]CB184 and [ (11)C]CB190, as a PET tracer for 18 kDa translocator protein: direct comparison with [ (11)C](R)-PK11195

OBJECTIVE

We report synthesis of two carbon-11 labeled imidazopyridines TSPO ligands, [(11)C]CB184 and [(11)C]CB190, for PET imaging of inflammatory process along with neurodegeneration, ischemia or brain tumor. Biodistribution of these compounds was compared with that of [(11)C]CB148 and [(11)C](R)-PK11195.

METHODS

Both [(11)C]CB184 and [(11)C]CB190 having (11)C-methoxyl group on an aromatic ring were readily prepared using [(11)C]methyl triflate. Biodistribution and metabolism of the compounds were examined with normal mice. An animal PET study using 6-hydroxydopamine treated rats as a model of neurodegeneration was pursued for proper estimation of feasibility of the radioligands to determine neuroinflammation process.

RESULTS

[(11)C]CB184 and [(11)C]CB190 were obtained via O-methylation of corresponding desmethyl precursor using [(11)C]methyl triflate in radiochemical yield of 73 % (decay-corrected). In vivo validation as a TSPO radioligand was carried out using normal mice and lesioned rats. In mice, [(11)C]CB184 showed more uptake and specific binding than [(11)C]CB190. Metabolism studies showed that 36 % and 25 % of radioactivity in plasma remained unchanged 30 min after intravenous injection of [(11)C]CB184 and [(11)C]CB190, respectively. In the PET study using rats, lesioned side of the brain showed significantly higher uptake than contralateral side after i.v. injection of either [(11)C]CB184 or [(11)C](R)-PK11195. Indirect Logan plot analysis revealed distribution volume ratio (DVR) between the two sides which might indicate lesion-related elevation of TSPO binding. The DVR was 1.15 ± 0.10 for [(11)C](R)-PK11195 and was 1.15 ± 0.09 for [(11)C]CB184.

CONCLUSION

The sensitivity to detect neuroinflammation activity was similar for [(11)C]CB184 and [(11)C](R)-PK11195.

The 18-kDa mitochondrial translocator protein in human gliomas: an 11C-(R)PK11195 PET imaging and neuropathology study

The 18-kDa mitochondrial translocator protein (TSPO) is upregulated in high-grade astrocytomas and can be imaged by PET using the selective radiotracer (11)C-(R)PK11195. We investigated (11)C-(R)PK11195 binding in human gliomas and its relationship with TSPO expression in tumor tissue and glioma-associated microglia/macrophages (GAMs) within the tumors.

METHODS

Twenty-two glioma patients underwent dynamic (11)C-(R)PK11195 PET scans and perfusion MR imaging acquisition. Parametric maps of (11)C-(R)PK11195 binding potential (BPND) were generated. Coregistered MR/PET images were used to guide tumor biopsy. The tumor tissue was quantitatively assessed for TSPO expression and infiltration of GAMs using immunohistochemistry and double immunofluorescence. The imaging and histopathologic parameters were compared among different histotypes and grades and correlated with each other.

RESULTS

BPND of (11)C-(R)PK11195 in high-grade gliomas was significantly higher than in low-grade astrocytomas and low-grade oligodendrogliomas. TSPO in gliomas was expressed predominantly by neoplastic cells, and its expression correlated positively with BPND in the tumors. GAMs only partially contributed to the overall TSPO expression within the tumors, and TSPO expression in GAMs did not correlate with tumor BPND.

CONCLUSION

PET with (11)C-(R)PK11195 in human gliomas predominantly reflects TSPO expression in tumor cells. It therefore has the potential to effectively stratify patients who are suitable for TSPO-targeted treatment.

Roles of microglia in brain development, tissue maintenance and repair

The emerging roles of microglia are currently being investigated in the healthy and diseased brain with a growing interest in their diverse functions. In recent years, it has been demonstrated that microglia are not only immunocentric, but also neurobiological and can impact neural development and the maintenance of neuronal cell function in both healthy and pathological contexts. In the disease context, there is widespread consensus that microglia are dynamic cells with a potential to contribute to both central nervous system damage and repair. Indeed, a number of studies have found that microenvironmental conditions can selectively modify unique microglia phenotypes and functions. One novel mechanism that has garnered interest involves the regulation of microglial function by microRNAs, which has therapeutic implications such as enhancing microglia-mediated suppression of brain injury and promoting repair following inflammatory injury. Furthermore, recently published articles have identified molecular signatures of myeloid cells, suggesting that microglia are a distinct cell population compared to other cells of myeloid lineage that access the central nervous system under pathological conditions. Thus, new opportunities exist to help distinguish microglia in the brain and permit the study of their unique functions in health and disease.

In vivo analysis of neuroinflammation in the late chronic phase after experimental stroke

BACKGROUND AND PURPOSE

In vivo imaging of inflammatory processes is a valuable tool in stroke research. We here investigated the combination of two imaging modalities in the chronic phase after cerebral ischemia: magnetic resonance imaging (MRI) using intravenously applied ultra small supraparamagnetic iron oxide particles (USPIO), and positron emission tomography (PET) with the tracer [(11)C]PK11195.

METHODS

Rats were subjected to permanent middle cerebral artery occlusion (pMCAO) by the macrosphere model and monitored by MRI and PET for 28 or 56 days, followed by immunohistochemical endpoint analysis. To our knowledge, this is the first study providing USPIO-MRI data in the chronic phase up to 8 weeks after stroke.

RESULTS

Phagocytes with internalized USPIOs induced MRI-T2(∗) signal alterations in the brain. Combined analysis with [(11)C]PK11195-PET allowed quantification of phagocytic activity and other neuroinflammatory processes. From 4 weeks after induction of ischemia, inflammation was dominated by phagocytes. Immunohistochemistry revealed colocalization of Iba1+ microglia with [(11)C]PK11195 and ED1/CD68 with USPIOs. USPIO-related iron was distinguished from alternatively deposited iron by assessing MRI before and after USPIO application. Tissue affected by non-phagocytic inflammation during the first week mostly remained in a viably vital but remodeled state after 4 or 8 weeks, while phagocytic activity was associated with severe injury and necrosis accordingly.

CONCLUSIONS

We conclude that the combined approach of USPIO-MRI and [(11)C]PK11195-PET allows to observe post-stroke inflammatory processes in the living animal in an intraindividual and longitudinal fashion, predicting long-term tissue fate. The non-invasive imaging methods do not affect the immune system and have been applied to human subjects before. Translation into clinical applications is therefore feasible.

Increased PK11195-PET binding in normal-appearing white matter in clinically isolated syndrome

The most accurate predictor of the subsequent development of multiple sclerosis in clinically isolated syndrome is the presence of lesions at magnetic resonance imaging. We used in vivo positron emission tomography with (11)C-(R)-PK11195, a biomarker of activated microglia, to investigate the normal-appearing white matter and grey matter of subjects with clinically isolated syndrome to explore its role in the development of multiple sclerosis. Eighteen clinically isolated syndrome and eight healthy control subjects were recruited. Baseline assessment included: history, neurological examination, expanded disability status scale, magnetic resonance imaging and PK11195-positron emission tomography scans. All assessments except the PK11195-positron emission tomography scan were repeated over 2 years. SUPERPK methodology was used to measure the binding potential relative to the non-specific volume, BPND. We show a global increase of normal-appearing white matter PK11195 BPND in clinically isolated syndrome subjects compared with healthy controls (P = 0.014). Clinically isolated syndrome subjects with T2 magnetic resonance imaging lesions had higher PK11195 BPND in normal-appearing white matter (P = 0.009) and their normal-appearing white matter PK11195 BPND correlated with the Expanded Disability Status Scale (P = 0.007; r = 0.672). At 2 years those who developed dissemination in space or multiple sclerosis, had higher PK11195 BPND in normal-appearing white matter at baseline (P = 0.007 and P = 0.048, respectively). Central grey matter PK11195 BPND was increased in subjects with clinically isolated syndrome compared to healthy controls but no difference was found in cortical grey matter PK11195 BPND. Microglial activation in clinically isolated syndrome normal-appearing white matter is diffusely increased compared with healthy control subjects and is further increased in those who have magnetic resonance imaging lesions. Furthermore microglial activation in clinically isolated syndrome normal-appearing white matter is also higher in those subjects who developed multiple sclerosis at 2 years. Our finding, if replicated in a larger study, could be of prognostic value and aid early treatment decisions in clinically isolated syndrome.

The 18 kDa translocator protein, microglia and neuroinflammation

The 18 kDa translocator protein (TSPO), previously known as the peripheral benzodiazepine receptor, is expressed in the injured brain. It has become known as an imaging marker of "neuroinflammation" indicating active disease, and is best interpreted as a nondiagnostic biomarker and disease staging tool that refers to histopathology rather than disease etiology. The therapeutic potential of TSPO as a drug target is mostly based on the understanding that it is an outer mitochondrial membrane protein required for the translocation of cholesterol, which thus regulates the rate of steroid synthesis. This pivotal role together with the evolutionary conservation of TSPO has underpinned the belief that any loss or mutation of TSPO should be associated with significant physiological deficits or be outright incompatible with life. However, against prediction, full Tspo knockout mice are viable and across their lifespan do not show the phenotype expected if cholesterol transport and steroid synthesis were significantly impaired. Thus, the "translocation" function of TSPO remains to be better substantiated. Here, we discuss the literature before and after the introduction of the new nomenclature for TSPO and review some of the newer findings. In light of the controversy surrounding the function of TSPO, we emphasize the continued importance of identifying compounds with confirmed selectivity and suggest that TSPO expression is analyzed within specific disease contexts rather than merely equated with the reified concept of "neuroinflammation."

Synthesis and evaluation of translocator 18 kDa protein (TSPO) positron emission tomography (PET) radioligands with low binding sensitivity to human single nucleotide polymorphism rs6971

The imaging of translocator 18 kDa protein (TSPO) in living human brain with radioligands by positron emission tomography (PET) has become an important means for the study of neuroinflammatory conditions occurring in several neuropsychiatric disorders. The widely used prototypical PET radioligand [¹¹C](R)-PK 11195 ([¹¹C](R)-1; [N-methyl-¹¹C](R)-N-sec-butyl-1-(2-chlorophenyl)-N-methylisoquinoline-3-carboxamide) gives a low PET signal and is difficult to quantify, whereas later generation radioligands have binding sensitivity to a human single nucleotide polymorphism (SNP) rs6971, which imposes limitations on their utility for comparative quantitative PET studies of normal and diseased subjects. Recently, azaisosteres of 1 have been developed with improved drug-like properties, including enhanced TSPO affinity accompanied by moderated lipophilicity. Here we selected three of these new ligands (7–9) for labeling with carbon-11 and for evaluation in monkey as candidate PET radioligands for imaging brain TSPO. Each radioligand was readily prepared by ¹¹C-methylation of an N-desmethyl precursor and was found to give a high proportion of TSPO-specific binding in monkey brain. One of these radioligands, [¹¹C]7, the direct 4-azaisostere of 1, presents many radioligand properties that are superior to those reported for [¹¹C]1, including higher affinity, lower lipophilicity, and stable quantifiable PET signal. Importantly, 7 was also found to show very low sensitivity to the human SNP rs6971 in vitro. Therefore, [¹¹C]7 now warrants evaluation in human subjects with PET to assess its utility for imaging TSPO in human brain, irrespective of subject genotype.

MicroPET imaging and transgenic models: a blueprint for Alzheimer's disease clinical research

Over the past decades, developments in neuroimaging have significantly contributed to the understanding of Alzheimer's disease (AD) pathophysiology. Specifically, positron emission tomography (PET) imaging agents targeting amyloid deposition have provided unprecedented opportunities for refining in vivo diagnosis, monitoring disease propagation, and advancing AD clinical trials. Furthermore, the use of a miniaturized version of PET (microPET) in transgenic (Tg) animals has been a successful strategy for accelerating the development of novel radiopharmaceuticals. However, advanced applications of microPET focusing on the longitudinal propagation of AD pathophysiology or therapeutic strategies remain in their infancy. This review highlights what we have learned from microPET imaging in Tg models displaying amyloid and tau pathology, and anticipates cutting-edge applications with high translational value to clinical research.

Development of N-methyl-(2-arylquinolin-4-yl)oxypropanamides as leads to PET radioligands for translocator protein (18 kDa)

Translocator protein (18 kDa), known as TSPO, is a recognized biomarker of neuroinflammation. Radioligands with PET accurately quantify TSPO in neuroinflammatory conditions. However, the existence of three human TSPO genotypes that show differential affinity to almost all useful TSPO PET radioligands hampers such studies. There is an unmet need for genotype-insensitive, high-affinity, and moderately lipophilic TSPO ligands that may serve as leads for PET radioligand development. To address this need, we varied the known high-affinity TSPO ligand (l)-N,N-diethyl-2-methyl-3-(2-phenylquinolin-4-yl)propanamide in its aryl scaffold, side chain tether, and pendant substituted amido group while retaining an N-methyl group as a site for labeling with carbon-11. From this effort, oxygen-tethered N-methyl-aryloxypropanamides emerged as new high-affinity TSPO ligands with attenuated lipophilicity, including one example with attractive properties for PET radioligand development, namely N-methyl-N-phenyl-2-{[2-(pyridin-2-yl)quinolin-4-yl]oxy}propanamide (22a; rat K_i = 0.10 nM; human TSPO genotypes K_i = 1.4 nM; clogD = 4.18).

(R)-[11C]PK11195 brain uptake as a biomarker of inflammation and antiepileptic drug resistance: evaluation in a rat epilepsy model

Neuroinflammation has been suggested as a key determinant of the intrinsic severity of epilepsy. Glial cell activation and associated inflammatory signaling can influence seizure thresholds as well as the pharmacodynamics and pharmacokinetics of antiepileptic drugs. Based on these data, we hypothesized that molecular imaging of microglia activation might serve as a tool to predict drug refractoriness of epilepsy. Brain uptake of (R)-[11C]PK11195, a ligand of the translocator protein 18 kDa and molecular marker of microglia activation, was studied in a chronic model of temporal lobe epilepsy in rats with selection of phenobarbital responders and non-responders. In rats with drug-sensitive epilepsy, (R)-[11C]PK11195 brain uptake values were comparable to those in non-epileptic controls. Analysis in non-responders revealed enhanced brain uptake of up to 39% in different brain regions. The difference might be related to the fact that non-responders exhibited higher baseline seizure frequencies than responders indicating a more pronounced intrinsic disease severity. In hippocampal sections, ED1 immunostaining argued against a general difference in microglia activation between both groups. Our data suggest that TSPO PET imaging might serve as a biomarker for drug resistance in temporal lobe epilepsy. However, it needs to be considered that our findings indicate that the TSPO PET data might merely reflect seizure frequency. Future experimental and clinical studies should further evaluate the validity of TSPO PET data to predict the response to phenobarbital and other antiepileptic drugs in longitudinal studies with scanning before drug exposure and with a focus on the early phase following an epileptogenic brain insult.

Structure of the mitochondrial translocator protein in complex with a diagnostic ligand

The 18-kilodalton translocator protein TSPO is found in mitochondrial membranes and mediates the import of cholesterol and porphyrins into mitochondria. In line with the role of TSPO in mitochondrial function, TSPO ligands are used for a variety of diagnostic and therapeutic applications in animals and humans. We present the three-dimensional high-resolution structure of mammalian TSPO reconstituted in detergent micelles in complex with its high-affinity ligand PK11195. The TSPO-PK11195 structure is described by a tight bundle of five transmembrane α helices that form a hydrophobic pocket accepting PK11195. Ligand-induced stabilization of the structure of TSPO suggests a molecular mechanism for the stimulation of cholesterol transport into mitochondria.

Characterization of a novel acetamidobenzoxazolone-based PET ligand for translocator protein (18 kDa) imaging of neuroinflammation in the brain

We developed the novel positron emission tomography (PET) ligand 2-[5-(4-[(11)C]methoxyphenyl)-2-oxo-1,3-benzoxazol-3(2H)-yl]-N-methyl-N-phenylacetamide ([(11)C]MBMP) for translocator protein (18 kDa, TSPO) imaging and evaluated its efficacy in ischemic rat brains. [(11)C]MBMP was synthesized by reacting desmethyl precursor (1) with [(11)C]CH3 I in radiochemical purity of ≥ 98% and specific activity of 85 ± 30 GBq/μmol (n = 18) at the end of synthesis. Biodistribution study on mice showed high accumulation of radioactivity in the TSPO-rich organs, e.g., the lungs, heart, kidneys, and adrenal glands. The metabolite analysis in mice brain homogenate showed 80.1 ± 2.7% intact [(11)C]MBMP at 60 min after injection. To determine the specific binding of [(11)C]MBMP with TSPO in the brain, in vitro autoradiography and PET studies were performed in an ischemic rat model. In vitro autoradiography indicated significantly increased binding on the ipsilateral side compared with that on the contralateral side of ischemic rat brains. This result was supported firmly by the contrast of radioactivity between the ipsilateral and contralateral sides in PET images. Displacement experiments with unlabelled MBMP or PK11195 minimized the difference in uptake between the two sides. In summary, [(11)C]MBMP is a potential PET imaging agent for TSPO and, consequently, for the up-regulation of microglia during neuroinflammation.

Detection of microglial activation in an acute model of neuroinflammation using PET and radiotracers 11C-(R)-PK11195 and 18F-GE-180

It remains unclear how different translocator protein (TSPO) ligands reflect the spatial extent of astrocyte or microglial activation in various neuroinflammatory conditions. Here, we use a reproducible lipopolysaccharide (LPS)-induced model of acute central nervous system inflammation to compare the binding performance of a new TSPO ligand (18)F-GE-180 with (11)C-(R)-PK11195. Using immunohistochemistry, we also explore the ability of the TSPO ligands to detect activated microglial cells and astrocytes.

METHODS

Lewis rats (n = 30) were microinjected with LPS (1 or 10 μg) or saline (1 μL) into the left striatum. The animals were imaged in vivo at 16 h after the injection using PET radiotracers (18)F-GE-180 or (11)C-(R)-PK11195 (n = 3 in each group) and were killed afterward for autoradiography of the brain. Immunohistochemical assessment of OX-42 and glial fibrillary acidic protein (GFAP) was performed to identify activated microglial cells and reactive astrocytes.

RESULTS

In vivo PET imaging revealed an increase in the ipsilateral TSPO binding, compared with binding in the contralateral hemisphere, after the microinjection of 10 μg of LPS. No increase was observed with vehicle. By autoradiography, the TSPO radiotracer binding potential in the injected hemisphere was increased after striatal injection of 1 or 10 μg of LPS. However, the significant increase was observed only when using (18)F-GE-180. The area of CD11b-expressing microglial cells extended beyond that of enhanced GFAP staining and mapped more closely to the extent of (18)F-GE-180 binding than to (11)C-(R)-PK11195 binding. The signal from either PET ligand was significantly increased in regions of increased GFAP immunoreactivity and OX-42 colocalization, meaning that the presence of both activated microglia and astrocytes in a given area leads to increased binding of the TSPO radiotracers.

CONCLUSION

(18)F-GE-180 is able to reveal sites of activated microglia in both gray and white matter. However, the signal is increased by the presence of activated astrocytes. Therefore, (18)F-GE-180 is a promising new fluorinated longer-half-life tracer that reveals the presence of activated microglia in a manner that is superior to (11)C-(R)-PK11195 due to the higher binding potential observed for this ligand.

Microglia activation in multiple sclerosis black holes predicts outcome in progressive patients: an in vivo [(11)C](R)-PK11195-PET pilot study

The pathophysiological correlates and the contribution to persisting disability of hypointense T1-weighted MRI lesions, black holes (BH), in multiple sclerosis (MS) are still unclear. In order to study the in vivo functional correlates of this MRI finding, we used 11C-PK11195 PET (PK-PET) to investigate changes in microglial activity. Ten relapsing and 9 progressive MS subjects had a PK-PET scan and a MRI scan alongside a full clinical assessment, including the expanded disability status scale (EDSS) for evaluation of disability. We studied the PK binding potential of the specifically bound radioligand relative to the non-displaceable radioligand in tissue (BPND) in T1 BHs. Out of a total of 1242 BHs identified, 947 were PK enhancing. The PKBPND was correlated with the EDSS (r=0.818; p<0.05) only in the progressive group. In the relapsing patients there was an inverse correlation between PKBPND and BH total lesion volume in whole brain (r=-0.781; p<0.05). When progressive patients were grouped according to the disability outcome at 2years from the PK-PET scan, the total PKBPND in BHs was found to be a significant outcome predictor of disability (p<0.01). Our findings show that relapsing and progressive patients have heterogeneous patterns of PKBPND in T1 BHs and indicate that BHs are not just "holes" representing loss of axons and myelin, but display inflammatory activity in the form of activated microglia. The significant association between PKBPND, neurological impairment and outcome in progressive subjects supports a role for activated microglia in disability progression.

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.”

Direct comparison of autologous and allogeneic transplantation of iPSC-derived neural cells in the brain of a non-human primate

Induced pluripotent stem cells (iPSCs) provide the potential for autologous transplantation using cells derived from a patient's own cells. However, the immunogenicity of iPSCs or their derivatives has been a matter of controversy, and up to now there has been no direct comparison of autologous and allogeneic transplantation in the brains of humans or nonhuman primates. Here, using nonhuman primates, we found that the autologous transplantation of iPSC-derived neurons elicited only a minimal immune response in the brain. In contrast, the allografts caused an acquired immune response with the activation of microglia (IBA-1(+)/MHC class II(+)) and the infiltration of leukocytes (CD45(+)/CD3(+)). Consequently, a higher number of dopaminergic neurons survived in the autografts. Our results suggest that the autologous transplantation of iPSC-derived neural cells is advantageous for minimizing the immune response in the brain compared with allogeneic grafts.

[¹¹C]-(R)PK11195 tracer kinetics in the brain of glioma patients and a comparison of two referencing approaches

PURPOSE

Translocator protein (TSPO) is a biomarker of neuroinflammation that can be imaged by PET using [¹¹C]-(R)PK11195. We sought to characterize the [¹¹C]-(R)PK11195 kinetics in gliomas of different histotypes and grades, and to compare two reference tissue input functions (supervised cluster analysis versus cerebellar grey matter) for the estimation of [¹¹C]-(R)PK11195 binding in gliomas and surrounding brain structures.

METHODS

Twenty-three glioma patients and ten age-matched controls underwent structural MRI and dynamic [¹¹C]-(R)PK11195 PET scans. Tissue time-activity curves (TACs) were extracted from tumour regions as well as grey matter (GM) and white matter (WM) of the brains. Parametric maps of binding potential (BPND) were generated with the simplified reference tissue model using the two input functions, and were compared with each other. TSPO expression was assessed in tumour tissue sections by immunohistochemistry.

RESULTS

Three types of regional kinetics were observed in individual tumour TACs: GM-like kinetics (n=6, clearance of the tracer similar to that in cerebellar GM), WM-like kinetics (n=8, clearance of the tracer similar to that in cerebral WM) and a form of mixed kinetics (n=9, intermediate rate of clearance). Such kinetic patterns differed between low-grade astrocytomas (WM-like kinetics) and oligodendrogliomas (GM-like and mixed kinetics), but were independent of tumour grade. There was good agreement between parametric maps of BPND derived from the two input functions in all controls and 10 of 23 glioma patients. In 13 of the 23 patients, BPND values derived from the supervised cluster input were systematically smaller than those using the cerebellar input. Immunohistochemistry confirmed that TSPO expression increased with tumour grade.

CONCLUSION

The three types of [¹¹C]-(R)PK11195 kinetics in gliomas are determined in part by tracer delivery, and indicated that kinetic analysis is a valuable tool in the study of gliomas with the potential for in vivo discrimination between low-grade astrocytomas and oligodendrogliomas. Supervised cluster and cerebellar input functions produced consistent BPND estimates in approximately half of the gliomas investigated, but had a systematic difference in the remainder. The cerebellar input is preferred based on theoretical and practical considerations.

Microglia, amyloid, and glucose metabolism in Parkinson's disease with and without dementia

[(11)C](R)PK11195-PET measures upregulation of translocator protein, which is associated with microglial activation, [(11)C]PIB-PET is a marker of amyloid, while [(18)F]FDG-PET measures cerebral glucose metabolism (rCMRGlc). We hypothesize that microglial activation is an early event in the Parkinson's disease (PD) spectrum and is independent of the amyloid pathology. The aim of this study is to evaluate in vivo the relationship between microglial activation, amyloid deposition, and glucose metabolism in Parkinson's disease dementia (PDD) and PD subjects without dementia. Here, we evaluated 11 PDD subjects, 8 PD subjects without dementia, and 24 control subjects. Subjects underwent T1 and T2 MRI, [(11)C](R)PK11195, [(18)F]FDG, and [(11)C]PIB PET scans. Parametric maps of [(11)C](R)PK11195 binding potential, rCMRGlc, and [(11)C]PIB uptake were interrogated using region of interest and SPM (statistical parametric mapping) analysis. The PDD patients showed a significant increase of microglial activation in anterior and posterior cingulate, striatum, frontal, temporal, parietal, and occipital cortical regions compared with the controls. The PD subjects also showed a statistically significant increase in microglial activation in temporal, parietal, and occipital regions. [(11)C]PIB uptake was marginally increased in PDD and PD. There was a significant reduction in glucose metabolism in PDD and PD. We have also demonstrated pixel-by-pixel correlation between mini-mental state examination (MMSE) score and microglial activation, and MMSE score and rCMRGlc. In conclusion, we have demonstrated that cortical microglial activation and reduced glucose metabolism can be detected early on in this disease spectrum. Significant microglial activation may be a factor in driving the disease process in PDD. Given this, agents that affect microglial activation could have an influence on disease progression.

[11C]PK11195 PET imaging of spinal glial activation after nerve injury in rats

The role of glial activation has been implicated in the development and persistence of neuropathic pain after nerve injury by recent studies. PK11195 binding to the translocator protein 18kDa (TSPO) has been shown to be enhanced in activated microglia. This study was designed to assess PK11195 imaging in spinal microglia during activation after nerve injury. The development of neuropathic pain was induced by partial sciatic nerve ligation (PSL). PSL rats on days 7 and 14 after nerve injury were subjected to imaging with a small-animal positron emission tomography/computed tomography (PET/CT) scanner using [(11)C]PK11195 to detect spinal microglial activation by means of noninvasive in vivo imaging. Spinal [(3)H]PK11195 autoradiography was performed to confirm the results of [(11)C]PK11195 PET in PSL rats. Quantitative RT-PCR of CD11b and GFAP mRNA, and the immunohistochemistry of Iba1 and GFAP were investigated to detect activated microglia and astrocytes. Mechanical allodynia was observed in the ipsilateral paw of PSL rats from day 3 after nerve injury and stably persisted from days 7 to 14. PET/CT fusion images clearly showed large amounts of accumulation of [(11)C]PK11195 in the lumbar spinal cord on days 7 and 14 after nerve injury. [(11)C]PK11195 enhanced images were restricted to the L3-L6 area of the spinal cord. The standardized uptake value (SUV) of [(11)C]PK11195 was significantly increased in the lumbar spinal cord compared to that of the thoracic region. Increased specific binding of [(11)C]PK11195 to TSPO in the spinal cord of PSL rats was confirmed by competition studies using unlabeled (R, S)-PK11195. Increased [(3)H]PK11195 binding was also observed in the ipsilateral dorsal horn of the L3-L6 spinal cord on days 7 and 14 after nerve injury. CD11b mRNA and Iba1 immunoreactive cells increased significantly on days 7 and 14 after nerve injury by PSL. However, changes in GFAP mRNA and immunoreactivity were slight in the ipsilateral side of PSL rats. In the present study, we showed that glial activation could be quantitatively imaged in the spinal cord of neuropathic pain rats using [(11)C]PK11195 PET, suggesting that high resolution PET using TSPO-specific radioligands might be useful for imaging to assess the role of glial activation, including neuroinflammatory processes, in neuropathic pain patients.

Optimization of [11C]DASB-synthesis: vessel-based and flow-through microreactor methods

The intention for the present study was to implement a microfluidic set-up for N-(11)C-methylations in a flow-through microreactor device with [(11)C]DASB as model-compound and [(11)C]CH(3)I and [(11)C]CH(3)OTf, respectively, as (11)C-methylation agents. Due to an observed "aging" effect of the (11)C-methylation agents' solution, this goal was not achieved. Nevertheless, based on these observations, the time consumption for the vessel-based routine production of [(11)C]DASB was reduced (34±1 min) and RCY was increased to 45.1±4.6% (EOB; 5.2±0.95 GBq EOS).

Microglial activation in Alzheimer's disease: an (R)-[¹¹C]PK11195 positron emission tomography study

Inflammatory mechanisms, like microglial activation, could be involved in the pathogenesis of Alzheimer's disease (AD). (R)-[(11)C]PK11195 (1-(2-chlorophenyl)-N-methyl-N-1(1-methylpropyl)-3-isoquinolinecarboxamide), a positron emission tomography (PET) ligand, can be used to quantify microglial activation in vivo. The purpose of this study was to assess whether increased (R)-[(11)C]PK11195 binding is present in AD and mild cognitive impairment (MCI), currently also known as "prodromal AD."

METHODS

Nineteen patients with probable AD, 10 patients with prodromal AD (MCI), and 21 healthy control subjects were analyzed. Parametric images of binding potential (BP(ND)) of (R)-[(11)C]PK11195 scans were generated using receptor parametric mapping (RPM) with supervised cluster analysis. Differences between subject groups were tested using mixed model analysis, and associations between BP(ND) and cognition were evaluated using Pearson correlation coefficients.

RESULTS

Voxel-wise statistical parametric mapping (SPM) analysis showed small clusters of significantly increased (R)-[(11)C]PK11195 BP(ND) in occipital lobe in AD dementia patients compared with healthy control subjects. Regions of interest (ROI)-based analyses showed no differences, with large overlap between groups. There were no differences in (R)-[(11)C]PK11195 BP(ND) between clinically stable prodromal AD patients and those who progressed to dementia, and BP(ND) did not correlate with cognitive function.

CONCLUSION

Microglial activation is a subtle phenomenon occurring in AD.

Acute HCV/HIV coinfection is associated with cognitive dysfunction and cerebral metabolite disturbance, but not increased microglial cell activation

BACKGROUND

Microglial cell activation and cerebral function impairment are described in both chronic hepatitis C viral (HCV) and Human-Immune-Deficiency viral (HIV) infections. The aim of this study was to investigate the effect of acute HCV infection upon cerebral function and microglial cell activation in HIV-infected individuals.

METHODS

A case-control study was conducted. Subjects with acute HCV and chronic HIV coinfection (aHCV) were compared to matched controls with chronic HIV monoinfection (HIVmono). aHCV was defined as a new positive plasma HCV RNA within 12 months of a negative RNA test. Subjects underwent neuro-cognitive testing (NCT), cerebral proton magnetic resonance spectroscopy ((1)H-MRS) and positron emission tomography (PET) using a (11)C-radiolabeled ligand (PK11195), which is highly specific for translocator protein 18 kDA receptors on activated microglial cells. Differences between cases and controls were assessed using linear regression modelling.

RESULTS

Twenty-four aHCV cases completed NCT and (1)H-MRS, 8 underwent PET. Of 57 HIVmono controls completing NCT, 12 underwent (1)H-MRS and 8 PET. Subjects with aHCV demonstrated on NCT, significantly poorer executive function (mean (SD) error rate 26.50(17.87) versus 19.09(8.12), p = 0.001) and on (1)H-MRS increased myo-inositol/creatine ratios (mI/Cr, a marker of cerebral inflammation) in the basal ganglia (ratio of 0.71(0.22) versus 0.55(0.23), p = 0.03), compared to subjects with HIVmono. On PET imaging, no difference in (11)C-PK11195 binding potential (BP) was observed between study groups (p>0.10 all cerebral locations), however lower BPs were associated with combination antiretroviral therapy (cART) use in the parietal (p = 0.01) and frontal (p = 0.03) cerebral locations.

DISCUSSION

Poorer cognitive performance and disturbance of cerebral metabolites are observed in subjects with aHC,V compared to subjects with HIVmono. Higher (11)C-PK11195 BP was not observed in subjects with aHCV, but was observed in subjects not on cART.

Synthesis and biological evaluation of 4-phenylquinazoline-2-carboxamides designed as a novel class of potent ligands of the translocator protein

A series of novel 4-phenylquinazoline-2-carboxamides (1-58) were designed as aza-isosters of PK11195, the well-known 18 kDa translocator protein (TSPO) reference ligand, and synthesized by means of a very simple and efficient procedure. A number of these derivatives bind to the TSPO with K(i) values in the nanomolar/subnanomolar range, show selectivity toward the central benzodiazepine receptor (BzR) and exhibit structure-affinity relationships consistent with a previously published pharmacophore/topological model of ligand-TSPO interaction.

Applications of positron emission tomography in animal models of neurological and neuropsychiatric disorders

Positron emission tomography (PET) provides dynamic images of the biodistribution of radioactive tracers in the brain. Through application of the principles of compartmental analysis, tracer uptake can be quantified in terms of specific physiological processes such as cerebral blood flow, cerebral metabolic rate, and the availability of receptors in brain. Whereas early PET studies in animal models of brain diseases were hampered by the limited spatial resolution of PET instruments, dedicated small-animal instruments now provide molecular images of rodent brain with resolution approaching 1mm, the theoretic limit of the method. Major applications of PET for brain research have consisted of studies of animal models of neurological disorders, notably Parkinson's disease (PD), Alzheimer's disease (AD), and Huntington's disease (HD), stroke, epilepsy and traumatic brain injury; these studies have particularly benefited from selective neurochemical lesion models (PD), and also transgenic rodent models (AD, HD). Due to their complex and uncertain pathophysiologies, corresponding models of neuropsychiatric disorders have proven more difficult to establish. Historically, there has been an emphasis on PET studies of dopamine transmission, as assessed with a range of tracers targeting dopamine synthesis, plasma membrane transporters, and receptor binding sites. However, notable recent breakthroughs in molecular imaging include the development of greatly improved tracers for subtypes of serotonin, cannabinoid, and metabotropic glutamate receptors, as well as noradrenaline transporters, amyloid-β and neuroinflammatory changes. This article reviews the considerable recent progress in preclinical PET and discusses applications relevant to a number of neurological and neuropsychiatric disorders in humans.

Cu(I)-catalyzed (11)C carboxylation of boronic acid esters: a rapid and convenient entry to (11)C-labeled carboxylic acids, esters, and amides

Rapid and direct: the carboxylation of boronic acid esters with (11)CO(2) provides [(11)C]carboxylic acids as a convenient entry into [(11)C]esters and [(11)C]amides. This conversion of boronates is tolerant to diverse functional groups (e.g., halo, nitro, or carbonyl).

An alteration in the lateral geniculate nucleus of experimental glaucoma monkeys: in vivo positron emission tomography imaging of glial activation

We examined lateral geniculate nucleus (LGN) degeneration as an indicator for possible diagnosis of glaucoma in experimental glaucoma monkeys using positron emission tomography (PET). Chronic intraocular pressure (IOP) elevation was induced by laser trabeculoplasty in the left eyes of 5 cynomolgus monkeys. Glial cell activation was detected by PET imaging with [¹¹C]PK11195, a PET ligand for peripheral-type benzodiazepine receptor (PBR), before and at 4 weeks after laser treatment (moderate glaucoma stage). At mild, moderate, and advanced experimental glaucoma stages (classified by histological changes based on the extent of axonal loss), brains were stained with cresyl violet, or antibodies against PBR, Iba-1 (a microglial marker), and GFAP (an activated astrocyte marker). In laser-treated eyes, IOP was persistently elevated throughout all observation periods. PET imaging showed increased [¹¹C]PK11195 binding potential in the bilateral LGN at 4 weeks after laser treatment; the increase in the ipsilateral LGN was statistically significant (P<0.05, n = 4). Immunostaining showed bilateral activations of microglia and astrocytes in LGN layers receiving input from the laser-treated eye. PBR-positive cells were observed in LGN layers receiving input from laser-treated eye at all experimental glaucoma stages including the mild glaucoma stage and their localization coincided with Iba-1 positive microglia and GFAP-positive astrocytes. These data suggest that glial activation occurs in the LGN at a mild glaucoma stage, and that the LGN degeneration could be detected by a PET imaging with [¹¹C]PK11195 during the moderate experimental glaucoma stage after unilateral ocular hypertension. Therefore, activated glial markers such as PBR in the LGN may be useful in noninvasive molecular imaging for diagnosis of glaucoma.

Identifying improved TSPO PET imaging probes through biomathematics: the impact of multiple TSPO binding sites in vivo

To date, ¹¹C-(R)-PK11195 has been the most widely used TSPO PET imaging probe, although it suffers from high non-specific binding and low signal to noise. A significant number of 2nd generation TSPO radioligands have been developed with higher affinity and/or lower non-specific binding, however there is substantial inter-subject variation in their affinity for the TSPO. TSPO from human tissue samples binds 2nd generation TSPO radioligands with either high affinity (high affinity binders, HABs), or low affinity (LABs) or expresses both HAB and LAB binding sites (mixed affinity binders, MABs). The expression of these different TSPO binding sites in human is encoded by the rs6971 polymorphism in the TSPO gene. Here, we use a predictive biomathematical model to estimate the in vivo performances of three of these 2nd generation radioligands (¹⁸F-PBR111, ¹¹C-PBR28, ¹¹C-DPA713) and ¹¹C-(R)-PK11195 in humans. The biomathematical model only relies on in silico, in vitro and genetic data (polymorphism frequencies in different ethnic groups) to predict the radioactivity time course in vivo. In particular, we provide estimates of the performances of these ligands in within-subject (e.g. longitudinal studies) and between-subject (e.g. disease characterisation) PET studies, with and without knowledge of the TSPO binding class. This enables an assessment of the different radioligands prior to radiolabelling or acquisition of any in vivo data. The within-subject performance was characterised in terms of the reproducibility of the in vivo binding potential (%COV[BP(ND)]) for each separate TSPO binding class in normal and diseased states (50% to 400% increase in TSPO density), whilst the between-subject performance was characterised in terms of the number of subjects required to distinguish between different populations. The results indicated that the within-subject variability for ¹⁸F-PBR111, ¹¹C-PBR28 and ¹¹C-DPA713 (0.9% to 2.2%) was significantly lower than ¹¹C-(R)-PK11195 (16% to 36%) for HABs and MABs in both normal and diseased states. For between-subject studies, sample sizes required to detect 50% differences in TSPO density with the 2nd generation tracers are approximately half that required with ¹¹C-(R)-PK11195 when binding class information is known a priori. As binding class can be identified using a simple genetic test or from peripheral blood assays, the combination of binding class information with 2nd generation TSPO imaging data should provide superior tools to investigate inflammatory processes in humans in vivo.

Initial evaluation in healthy humans of [18F]DPA-714, a potential PET biomarker for neuroinflammation

INTRODUCTION

The translocator protein 18 kDa (TSPO), although minimally expressed in healthy brain, is up-regulated in pathological conditions, coinciding with microglial activation. It is thereby a suitable in vivo biomarker of neuroinflammation for detection, evaluation and therapeutic monitoring of brain diseases. We aimed to estimate the radiation dosimetry of the positron emission tomography (PET) TSPO radioligand [(18)F]DPA-714, and we evaluated in healthy volunteers its whole-body uptake and cerebral kinetics.

METHODS

Biodistribution data from mice were used for the prediction of radiation dosimetry. In human studies, a 90-min dynamic PET scan was performed in seven healthy volunteers after injection of [(18)F]DPA-714 (245±45 MBq). Arterial and venous samples were collected from two subjects, and two additional subjects were submitted to whole-body acquisition. Regions of interest were defined over cerebral structures to obtain mean time-activity curves and to estimate the distribution volume ratios by Logan graphical analysis, and over peripheral organs to obtain standard uptake values.

RESULTS

The effective dose estimated from biodistribution in mice was 17.2 μSv/MBq. Modeling of regional brain and plasma data showed good in vivo stability of [(18)F]DPA-714 in humans, with only 20% of blood metabolites 20 min postinjection (p.i.). Maximum cerebral uptake was observed 5 min p.i., followed by two decreasing phases: a rapid washout (5-30 min) followed by a slower phase for the remainder of PET acquisition. Whole-body images demonstrate high activity in the gallbladder, heart, spleen and kidneys.

CONCLUSIONS

This initial study in humans shows that [(18)F]DPA-714 is a promising PET radioligand with excellent in vivo stability and biodistribution, and acceptable effective dose estimation. Therefore, [(18)F]DPA-714 could provide a sensitive measure of neuroinflammatory changes in subsequent clinical investigations.

Widespread and prolonged increase in (R)-(11)C-PK11195 binding after traumatic brain injury

Our objective was to measure (R)-(11)C-PK11195 binding as an indirect marker of neuronal damage after traumatic brain injury (TBI).

METHODS

Dynamic (R)-(11)C-PK11195 PET scans were acquired for 8 patients 6 mo after TBI and for 7 age-matched healthy controls. (R)-(11)C-PK11195 binding was assessed using the simplified reference tissue model. Because of widespread traumatic changes in TBI, an anatomic reference region could not be defined. Therefore, supervised cluster analysis was used to generate an appropriate reference tissue input.

RESULTS

Increased whole-brain binding of (R)-(11)C-PK11195 was observed in TBI patients. Regional analysis indicated that increased (R)-(11)C-PK11195 binding was widespread over the brain.

CONCLUSION

Six months after TBI, there was a prolonged and widespread increase in (R)-(11)C-PK11195 binding, which is indicative of diffuse neuronal damage.

In vivo expression of cyclooxygenase-1 in activated microglia and macrophages during neuroinflammation visualized by PET with 11C-ketoprofen methyl ester

Cyclooxygenase (COX)-1 and -2 are prostanoid-synthesizing enzymes that play important roles in the regulation of neuroinflammation and in the development of neurodegenerative disorders. However, the specific functions of these isoforms are still unclear. We recently developed (11)C-labeled ketoprofen methyl ester as a PET probe that targets the COXs for imaging neuroinflammation, though its responsible isoform is yet to be determined. In the present study, we performed ex vivo and in vivo imaging studies with (11)C-ketoprofen methyl ester and determined the contributions of the COX isoforms during the neuroinflammatory process.

METHODS

To identify the COX isoform responsible for (11)C-ketoprofen methyl ester in the brain, we examined the ex vivo autoradiography of (11)C-ketoprofen methyl ester using COX-deficient mice. Time-dependent changes in accumulation of (11)C-ketoprofen methyl ester during the neuroinflammatory process were evaluated by PET in rats with hemispheric neuroinflammation induced by intrastriatal injection of lipopolysaccharide or quinolinic acid. In both rat models, cell-type specificity of COX isoform expression during neuroinflammation was identified immunohistochemically.

RESULTS

Ex vivo autoradiographic analysis of COX-deficient mice revealed a significant reduction of (11)C-ketoprofen methyl ester accumulation only in COX-1-deficient mice, not COX-2-deficient mice. PET of rats after intrastriatal injection of lipopolysaccharide showed a significant increase in accumulation of (11)C-ketoprofen methyl ester in the inflamed area. This increase was evident at the early phase of 6 h, peaked at day 1, and then returned to basal levels by day 7. In addition, immunohistochemical analysis revealed that the population of activated microglia and macrophages was elevated at the early phase with COX-1 expression but not COX-2. A significant increase in (11)C-ketoprofen methyl ester accumulation was also observed at day 1 after intrastriatal injection of quinolinic acid, with increased COX-1-expressing activated microglia and macrophages.

CONCLUSION

We have identified (11)C-ketoprofen methyl ester as a COX-1-selective PET probe, and using this, we have also demonstrated a time-dependent expression of COX-1 in activated microglia and macrophages during the neuroinflammatory process in the living brain. Thus, COX-1 may play a crucial role in the pathology of neuroinflammation and might be a critical target for the diagnosis and therapy of neurodegenerative disorders.

Increased cerebral (R)-[(11)C]PK11195 uptake and glutamate release in a rat model of traumatic brain injury: a longitudinal pilot study

BACKGROUND

The aim of the present study was to investigate microglia activation over time following traumatic brain injury (TBI) and to relate these findings to glutamate release.

PROCEDURES

Sequential dynamic (R)-[(11)C]PK11195 PET scans were performed in rats 24 hours before (baseline), and one and ten days after TBI using controlled cortical impact, or a sham procedure. Extracellular fluid (ECF) glutamate concentrations were measured using cerebral microdialysis. Brains were processed for histopathology and (immuno)-histochemistry.

RESULTS

Ten days after TBI, (R)-[(11)C]PK11195 binding was significantly increased in TBI rats compared with both baseline values and sham controls (p < 0.05). ECF glutamate values were increased immediately after TBI (27.6 ± 14.0 μmol·L(-1)) as compared with the sham procedure (6.4 ± 3.6 μmol·L(-1)). Significant differences were found between TBI and sham for ED-1, OX-6, GFAP, Perl's, and Fluoro-Jade B.

CONCLUSIONS

Increased cerebral uptake of (R)-[(11)C]PK11195 ten days after TBI points to prolonged and ongoing activation of microglia. This activation followed a significant acute posttraumatic increase in ECF glutamate levels.

Evaluation of [¹²³I]-CLINDE as a potent SPECT radiotracer to assess the degree of astroglia activation in cuprizone-induced neuroinflammation

PURPOSE

The purpose of this study was to assess the feasibility and sensitivity of the high-affinity translocator protein (TSPO) ligand [(123)I]-CLINDE in imaging TSPO changes in vivo and characterise and compare astroglial and TSPO changes in the cuprizone model of demyelination and remyelination in C57BL/6 mice.

METHODS

C57BL/6 mice were fed with cuprizone for 4 weeks to induce demyelination followed by 2-4 weeks of standard diet (remyelination). Groups of mice were followed by in vivo single photon emission computed tomography (SPECT)/CT imaging using [(123)I]-CLINDE and uptake correlated with biodistribution, autoradiography, immunohistochemistry, immunofluorescence and real-time polymerase chain reaction (RT-PCR).

RESULTS

The uptake of [(123)I]-CLINDE in the brain as measured by SPECT imaging over the course of treatment reflects the extent of the physiological response, with significant increases observed during demyelination followed by a decrease in uptake during remyelination. This was confirmed by autoradiography and biodistribution studies. A positive correlation between TSPO expression and astrogliosis was found and both activated astrocytes and microglial cells expressed TSPO. [(123)I]-CLINDE uptake reflects astrogliosis in brain structures such as corpus callosum, caudate putamen, medium septum and olfactory tubercle as confirmed by both in vitro and in vivo results.

CONCLUSION

The dynamics in the cuprizone-induced astroglial and TSPO changes, observed by SPECT imaging, were confirmed by immunofluorescence, RT-PCR and autoradiography. The highly specific TSPO radioiodinated ligand CLINDE can be used as an in vivo marker for early detection and monitoring of a variety of neuropathological conditions using noninvasive brain imaging techniques.

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.

Evaluation of novel N1-methyl-2-phenylindol-3-ylglyoxylamides as a new chemotype of 18 kDa translocator protein-selective ligand suitable for the development of positron emission tomography radioligands

A novel series of N(1)-methyl-(2-phenylindol-3-yl)glyoxylamides, 19-31, designed in accordance with our previously reported pharmacophore/topological model, showed high affinity for the 18 kDa translocator protein (TSPO) and paved the way for developing a new radiolabeled probe. Thus ligand 31, N,N-di-n-propyl-(N(1)-methyl-2-(4'-nitrophenyl)indol-3-yl)glyoxylamide, featuring the best combination of affinity and lipophilicity, was labeled with carbon-11 for evaluation with positron emission tomography (PET) in monkey. After intravenous injection, [(11)C]31 entered brain to give a high proportion of TSPO-specific binding. These findings augur well for the future application of [(11)C]31 in humans. Consequently, the binding of 31 to human TSPO was tested on samples of brain membranes from deceased subjects who through ethically approved in vitro study had previously been established to be high-affinity binders (HABs), mixed-affinity binders (MABs), or low-affinity binders (LABs) for the known TSPO ligand, PBR28 (2). 31 showed high affinity for HABs, MABs, and LABs. In conclusion, [(11)C]31 represents a promising new chemotype for developing novel TSPO radioligands as biomarkers of neuroinflammation.

Microglial activation in healthy aging

Healthy brain aging is characterized by neuronal loss and decline of cognitive function. Neuronal loss is closely associated with microglial activation and postmortem studies have indeed suggested that activated microglia may be present in the aging brain. Microglial activation can be quantified in vivo using (R)-[(11)C]PK11195 and positron emission tomography. The purpose of this study was to measure specific binding of (R)-[(11)C]PK11195 in healthy subjects over a wide age range. Thirty-five healthy subjects (age range 19-79 years) were included. In all subjects 60-minute dynamic (R)-[(11)C]PK11195 scans were acquired. Specific binding of (R)-[(11)C]PK11195 was calculated using receptor parametric mapping in combination with supervised cluster analysis to extract the reference tissue input function. Increased binding of (R)-[(11)C]PK11195 with aging was found in frontal lobe, anterior and posterior cingulate cortex, medial inferior temporal lobe, insula, hippocampus, entorhinal cortex, thalamus, parietal and occipital lobes, and cerebellum. This indicates that activated microglia appear in several cortical and subcortical areas during healthy aging, suggesting widespread neuronal loss.

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.

Peripheral benzodiazepine receptors in patients with chronic schizophrenia: a PET study with [11C]DAA1106

Inflammatory/immunological process and glial contribution are suggested in the pathophysiology of schizophrenia. We investigated peripheral benzodiazepine receptors in brains of patients with chronic schizophrenia, which were reported to be located on mitochondria of glial cells, using [11C]DAA1106 with positron emission tomography. Fourteen patients and 14 age- and sex-matched normal controls participated in this study. PET data were analysed by two-tissue compartment model with metabolite-corrected plasma input. Clinical symptoms were assessed using the Positive and Negative Syndrome Scale. There was no significant difference between [11C]DAA1106 binding of the cortical regions of normal controls and patients with schizophrenia, whereas the patients showed a positive correlation between cortical [11C]DAA1106 binding and positive symptom scores. There was also a positive correlation between [11C]DAA1106 binding and duration of illness. Although the correlations need to be interpreted very cautiously, involvement of glial reaction process in the pathophysiology of positive symptoms or progressive change of schizophrenia might be suggested.