1
|
Chauveau F, Winkeler A, Chalon S, Boutin H, Becker G. PET imaging of neuroinflammation: any credible alternatives to TSPO yet? Mol Psychiatry 2024:10.1038/s41380-024-02656-9. [PMID: 38997465 DOI: 10.1038/s41380-024-02656-9] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Grants] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 12/19/2023] [Revised: 06/26/2024] [Accepted: 07/01/2024] [Indexed: 07/14/2024]
Abstract
Over the last decades, the role of neuroinflammation in neuropsychiatric conditions has attracted an exponentially growing interest. A key driver for this trend was the ability to image brain inflammation in vivo using PET radioligands targeting the Translocator Protein 18 kDa (TSPO), which is known to be expressed in activated microglia and astrocytes upon inflammatory events as well as constitutively in endothelial cells. TSPO is a mitochondrial protein that is expressed mostly by microglial cells upon activation but is also expressed by astrocytes in some conditions and constitutively by endothelial cells. Therefore, our current understanding of neuroinflammation dynamics is hampered by the lack of alternative targets available for PET imaging. We performed a systematic search and review on radiotracers developed for neuroinflammation PET imaging apart from TSPO. The following targets of interest were identified through literature screening (including previous narrative reviews): P2Y12R, P2X7R, CSF1R, COX (microglial targets), MAO-B, I2BS (astrocytic targets), CB2R & S1PRs (not specific of a single cell type). We determined the level of development and provided a scoping review for each target. Strikingly, astrocytic biomarker MAO-B has progressed in clinical investigations the furthest, while few radiotracers (notably targeting S1P1Rs, CSF1R) are being implemented in clinical investigations. Other targets such as CB2R and P2X7R have proven disappointing in clinical studies (e.g. poor signal, lack of changes in disease conditions, etc.). While astrocytic targets are promising, development of new biomarkers and tracers specific for microglial activation has proven challenging.
Collapse
Affiliation(s)
- Fabien Chauveau
- Université Claude Bernard Lyon 1, Centre de Recherche en Neurosciences de Lyon, Inserm U1028, CNRS UMR5292, BIORAN, Groupement Hospitalier Est - CERMEP, 59 boulevard Pinel, 69677, Bron, Cedex, France
| | - Alexandra Winkeler
- Université Paris-Saclay, Inserm, CNRS, CEA, BioMaps, Service Hospitalier Frédéric Joliot, 4 place du général Leclerc, 91401, Orsay, France
| | - Sylvie Chalon
- UMR 1253 iBrain, Université de Tours - INSERM, Bâtiment Planiol, UFR de Médecine, 10 Boulevard Tonnellé, 37032, Tours, Cedex 01, France
| | - Hervé Boutin
- UMR 1253 iBrain, Université de Tours - INSERM, Bâtiment Planiol, UFR de Médecine, 10 Boulevard Tonnellé, 37032, Tours, Cedex 01, France.
| | - Guillaume Becker
- Université Claude Bernard Lyon 1, Centre de Recherche en Neurosciences de Lyon, Inserm U1028, CNRS UMR5292, BIORAN, Groupement Hospitalier Est - CERMEP, 59 boulevard Pinel, 69677, Bron, Cedex, France
- Agence Nationale de Sécurité Sanitaire de l'Alimentation, de l'Environnement et du Travail, 14 rue Pierre et Marie Curie, 94701, Maisons-Alfort, Cedex, France
| |
Collapse
|
2
|
Qiu L, Jiang H, Cho K, Yu Y, Jones LA, Huang T, Perlmutter JS, Gropler RJ, Brier MR, Patti GJ, Benzinger TLS, Tu Z. Metabolite Study and Structural Authentication for the First-in-Human Use Sphingosine-1-phosphate Receptor 1 Radiotracer. ACS Chem Neurosci 2024; 15:1882-1892. [PMID: 38634759 PMCID: PMC11103254 DOI: 10.1021/acschemneuro.4c00077] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 04/19/2024] Open
Abstract
The sphingosine-1-phosphate receptor 1 (S1PR1) radiotracer [11C]CS1P1 has shown promise in proof-of-concept PET imaging of neuroinflammation in multiple sclerosis (MS). Our HPLC radiometabolite analysis of human plasma samples collected during PET scans with [11C]CS1P1 detected a radiometabolite peak that is more lipophilic than [11C]CS1P1. Radiolabeled metabolites that cross the blood-brain barrier complicate quantitative modeling of neuroimaging tracers; thus, characterizing such radiometabolites is important. Here, we report our detailed investigation of the metabolite profile of [11C]CS1P1 in rats, nonhuman primates, and humans. CS1P1 is a fluorine-containing ligand that we labeled with C-11 or F-18 for preclinical studies; the brain uptake was similar for both radiotracers. The same lipophilic radiometabolite found in human studies also was observed in plasma samples of rats and NHPs for CS1P1 labeled with either C-11 or F-18. We characterized the metabolite in detail using rats after injection of the nonradioactive CS1P1. To authenticate the molecular structure of this radiometabolite, we injected rats with 8 mg/kg of CS1P1 to collect plasma for solvent extraction and HPLC injection, followed by LC/MS analysis of the same metabolite. The LC/MS data indicated in vivo mono-oxidation of CS1P1 produces the metabolite. Subsequently, we synthesized three different mono-oxidized derivatives of CS1P1 for further investigation. Comparing the retention times of the mono-oxidized derivatives with the metabolite observed in rats injected with CS1P1 identified the metabolite as N-oxide 1, also named TZ82121. The MS fragmentation pattern of N-oxide 1 also matched that of the major metabolite in rat plasma. To confirm that metabolite TZ82121 does not enter the brain, we radiosynthesized [18F]TZ82121 by the oxidation of [18F]FS1P1. Radio-HPLC analysis confirmed that [18F]TZ82121 matched the radiometabolite observed in rat plasma post injection of [18F]FS1P1. Furthermore, the acute biodistribution study in SD rats and PET brain imaging in a nonhuman primate showed that [18F]TZ82121 does not enter the rat or nonhuman primate brain. Consequently, we concluded that the major lipophilic radiometabolite N-oxide [11C]TZ82121, detected in human plasma post injection of [11C]CS1P1, does not enter the brain to confound quantitative PET data analysis. [11C]CS1P1 is a promising S1PR1 radiotracer for detecting S1PR1 expression in the CNS.
Collapse
Affiliation(s)
- Lin Qiu
- Department of Radiology, Washington University School of Medicine, Saint Louis, Missouri 63110, United States
| | - Hao Jiang
- Department of Radiology, Washington University School of Medicine, Saint Louis, Missouri 63110, United States
| | - Kevin Cho
- Center for Mass Spectrometry and Metabolic Tracing, Department of Chemistry, Department of Medicine, Washington University, Saint Louis, Missouri 63130, United States
| | - Yanbo Yu
- Department of Radiology, Washington University School of Medicine, Saint Louis, Missouri 63110, United States
| | - Lynne A Jones
- Department of Radiology, Washington University School of Medicine, Saint Louis, Missouri 63110, United States
| | - Tianyu Huang
- Department of Radiology, Washington University School of Medicine, Saint Louis, Missouri 63110, United States
| | - Joel S Perlmutter
- Department of Radiology, Washington University School of Medicine, Saint Louis, Missouri 63110, United States
- Departments of Neurology, Neuroscience, Physical Therapy and Occupational Therapy, Washington University School of Medicine, Saint Louis, Missouri 63110, United States
| | - Robert J Gropler
- Department of Radiology, Washington University School of Medicine, Saint Louis, Missouri 63110, United States
| | - Matthew R Brier
- Departments of Neurology, Neuroscience, Physical Therapy and Occupational Therapy, Washington University School of Medicine, Saint Louis, Missouri 63110, United States
| | - Gary J Patti
- Center for Mass Spectrometry and Metabolic Tracing, Department of Chemistry, Department of Medicine, Washington University, Saint Louis, Missouri 63130, United States
| | - Tammie L S Benzinger
- Department of Radiology, Washington University School of Medicine, Saint Louis, Missouri 63110, United States
| | - Zhude Tu
- Department of Radiology, Washington University School of Medicine, Saint Louis, Missouri 63110, United States
| |
Collapse
|
3
|
Prasad VP, Wagner S, Keul P, Hermann S, Levkau B, Schäfers M, Haufe G. Synthesis, radiosynthesis and biochemical evaluation of fluorinated analogues of sphingosine-1-phosphate receptor 3 specific antagonists using PET. Bioorg Med Chem 2024; 104:117697. [PMID: 38599005 DOI: 10.1016/j.bmc.2024.117697] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/16/2023] [Revised: 02/21/2024] [Accepted: 03/22/2024] [Indexed: 04/12/2024]
Abstract
Sphingosine-1-phosphate and its receptors (S1PRs) are involved in several diseases such as auto immunity, inflammation and cardiovascular disorders. The S1P analogue fingolimod (Gilenya®) is currently in use for the treatment of relapsing multiple sclerosis. S1PRs are also promising targets for clinical molecular imaging in vivo. The organ distribution of individual S1PRs can be potentially achieved by using S1PR subtype-specific (radiolabeled) chemical probes. Here, we report our efforts on synthesis and in vivo potency determination of new ligands for the S1P receptor 3 (S1P3) based on the S1P3 antagonist TY-52156 and in validation of a potential imaging tracer in vivo using Positron Emission Tomography (PET) after 18F-labelling. A p-fluorophenyl derivative exhibited excellent S1P3 antagonist activity in vitro, good serum stability, and medium lipophilicity. In vivo biodistribution experiments using 18F-PET exhibited significant uptake in the myocardium suggesting potential applications in cardiac imaging.
Collapse
Affiliation(s)
- Vysakh Puspha Prasad
- Organic Chemistry Institute, University of Münster, Corrensstraße 40, 48149 Münster, Germany; NRW Graduate School of Chemistry, University of Münster, Wilhelm-Klemm-Straße 10, 48149 Münster, Germany
| | - Stefan Wagner
- Department of Nuclear Medicine, University Hospital Münster, Albert-Schweitzer-Campus 1, 48149 Münster, Germany
| | - Petra Keul
- Institute of Molecular Medicine III, University Hospital Düsseldorf, Heinrich Heine University Düsseldorf, Universitätsstraße 1, 40225 Düsseldorf, Germany
| | - Sven Hermann
- European Institute for Molecular Imaging (EIMI), University of Münster, Multiscale Imaging Centre, Röntgenstraße 16, 48149 Münster, Germany
| | - Bodo Levkau
- Institute of Molecular Medicine III, University Hospital Düsseldorf, Heinrich Heine University Düsseldorf, Universitätsstraße 1, 40225 Düsseldorf, Germany
| | - Michael Schäfers
- Department of Nuclear Medicine, University Hospital Münster, Albert-Schweitzer-Campus 1, 48149 Münster, Germany; European Institute for Molecular Imaging (EIMI), University of Münster, Multiscale Imaging Centre, Röntgenstraße 16, 48149 Münster, Germany; Cells-in-Motion Interfaculty Centre, University of Münster, Röntgenstraße 16, 48149 Münster, Germany
| | - Günter Haufe
- Organic Chemistry Institute, University of Münster, Corrensstraße 40, 48149 Münster, Germany; NRW Graduate School of Chemistry, University of Münster, Wilhelm-Klemm-Straße 10, 48149 Münster, Germany; European Institute for Molecular Imaging (EIMI), University of Münster, Multiscale Imaging Centre, Röntgenstraße 16, 48149 Münster, Germany; Cells-in-Motion Interfaculty Centre, University of Münster, Röntgenstraße 16, 48149 Münster, Germany.
| |
Collapse
|
4
|
Brier MR, Hamdi M, Rajamanikam J, Zhao H, Mansor S, Jones LA, Rahmani F, Jindal S, Koudelis D, Perlmutter JS, Wong DF, Nickels M, Ippolito JE, Gropler RJ, Schindler TH, Laforest R, Tu Z, Benzinger TLS. Phase 1 Evaluation of 11C-CS1P1 to Assess Safety and Dosimetry in Human Participants. J Nucl Med 2022; 63:1775-1782. [PMID: 35332093 PMCID: PMC9635683 DOI: 10.2967/jnumed.121.263189] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/03/2021] [Revised: 03/02/2022] [Indexed: 01/07/2023] Open
Abstract
This study evaluated the safety, dosimetry, and characteristics of 3-((2-fluoro-4-(5-(2'-methyl-2-(trifluoromethyl)-[1,1'-biphenyl]-4-yl)-1,2,4-oxadiazol-3-yl)benzyl)(methyl-11C)amino)propanoic acid (11C-CS1P1), a radiotracer targeting sphingosine-1-phosphate receptor (S1PR) 1 (S1PR1). S1PR1 is of clinical interest because of its role in multiple sclerosis (and other conditions), with an expanding class of S1PR modulators approved for relapsing multiple sclerosis. 11C-CS1P1 binds S1PR1 with high specificity and has shown promise in animal models of inflammatory diseases. Methods: 11C-CS1P1 was injected into 5 male and 6 female healthy participants. Ten participants were imaged with PET using a multipass whole-body continuous-bed-motion acquisition, and one had dedicated head and neck PET and MRI. Participants were continuously monitored for safety events. Organ time-activity curve data were collected, integrated, and normalized to the injected activity. Organ radiation doses and effective dose were computed using the adult male and female models in OLINDA, version 2.2. SUV images were evaluated for qualitative biodistribution. Results: No adverse events were observed after the dose, including no bradycardia. The liver was the critical organ from dosimetry analysis (mean ± SD: female, 23.12 ± 5.19 μSv/MBq; male, 21.06 ± 1.63 μSv/MBq). The whole-body effective dose (as defined by International Commission on Radiological Protection publication 103) was 4.18 ± 0.30 μSv/MBq in women and 3.54 ± 0.14 μSv/MBq in men. Using a maximum delivered dose of 740 MBq (20 mCi), the effective dose for women would be 3.1 mSv (0.31 rem), with a liver dose of 17.1 mSv (1.7 rem); the effective dose for men would be 2.6 mSv (0.26 rem), with a liver dose of 15.6 mSv (1.56 rem). Brain uptake was seen predominantly in gray matter and correlated with regional S1PR1 RNA expression (r = 0.84). Conclusion: These results support the safety of 11C-CS1P1 for evaluation of inflammation in human clinical populations. Dosimetry permits repeated measures in the same participants. Brain uptake correlates well with known target topography.
Collapse
Affiliation(s)
- Matthew R Brier
- Department of Neurology, Washington University, St. Louis, Missouri
| | - Mahdjoub Hamdi
- Mallinckrodt Institute of Radiology, Washington University, St. Louis, Missouri
| | | | - Haiyang Zhao
- Mallinckrodt Institute of Radiology, Washington University, St. Louis, Missouri
| | - Syahir Mansor
- Mallinckrodt Institute of Radiology, Washington University, St. Louis, Missouri
| | - Lynne A Jones
- Mallinckrodt Institute of Radiology, Washington University, St. Louis, Missouri
| | - Farzaneh Rahmani
- Mallinckrodt Institute of Radiology, Washington University, St. Louis, Missouri
| | - Saurabh Jindal
- Mallinckrodt Institute of Radiology, Washington University, St. Louis, Missouri
| | - Deborah Koudelis
- Mallinckrodt Institute of Radiology, Washington University, St. Louis, Missouri
| | - Joel S Perlmutter
- Department of Neurology, Washington University, St. Louis, Missouri
- Mallinckrodt Institute of Radiology, Washington University, St. Louis, Missouri
- Department of Neuroscience, Washington University, St. Louis, Missouri
- Department of Physical and Occupational Therapy, Washington University, St. Louis, Missouri; and
| | - Dean F Wong
- Department of Neurology, Washington University, St. Louis, Missouri
- Mallinckrodt Institute of Radiology, Washington University, St. Louis, Missouri
- Department of Neuroscience, Washington University, St. Louis, Missouri
- Department of Psychiatry, Washington University, St. Louis, Missouri
| | - Michael Nickels
- Mallinckrodt Institute of Radiology, Washington University, St. Louis, Missouri
| | - Joseph E Ippolito
- Mallinckrodt Institute of Radiology, Washington University, St. Louis, Missouri
| | - Robert J Gropler
- Mallinckrodt Institute of Radiology, Washington University, St. Louis, Missouri
| | - Thomas H Schindler
- Mallinckrodt Institute of Radiology, Washington University, St. Louis, Missouri
| | - Richard Laforest
- Mallinckrodt Institute of Radiology, Washington University, St. Louis, Missouri
| | - Zhude Tu
- Mallinckrodt Institute of Radiology, Washington University, St. Louis, Missouri;
| | - Tammie L S Benzinger
- Mallinckrodt Institute of Radiology, Washington University, St. Louis, Missouri;
| |
Collapse
|
5
|
Chand GB, Jiang H, Miller JP, Rhodes CH, Tu Z, Wong DF. Differential Sphingosine-1-Phosphate Receptor-1 Protein Expression in the Dorsolateral Prefrontal Cortex Between Schizophrenia Type 1 and Type 2. Front Psychiatry 2022; 13:827981. [PMID: 35350429 PMCID: PMC8957823 DOI: 10.3389/fpsyt.2022.827981] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 12/02/2021] [Accepted: 02/03/2022] [Indexed: 11/25/2022] Open
Abstract
Understanding the etiology and treatment approaches in schizophrenia is challenged in part by the heterogeneity of this disorder. One encouraging progress is the growing evidence that there are subtypes of schizophrenia. Recent in vitro findings of messenger ribonucleic acid (mRNA) gene expression on postmortem dorsolateral prefrontal cortex (DLPFC) showed that schizophrenia has two subtypes, those with a relatively normal DLPFC transcriptome (Type 1) and those with differentially expressed genes (Type 2). Sphingosine-1-phosphate receptor-1 (S1PR1) is one of the genes that was highly upregulated in Type 2 compared to Type 1 and controls. The impact of that finding is limited because it only can be confirmed through analysis of autopsy tissue, and the clinical characteristics such as symptoms severity or illness duration except for cause of death was not available from that Medical Examiner based autopsy study. However, S1PR1 has great potential because it is a target gene that can be accessed via positron emission tomography (PET) in vivo using specific radioligands (starting with [11C]CS1P1) successfully developed at our center in human brain imaging. As a preliminary study to validate this PET target in schizophrenia, S1PR1 protein expression was assessed by receptor autoradiography (ARG) using [3H]CS1P1 and immunohistochemistry (IHC) in the DLPFC from patients with schizophrenia classified as Type 1 or Type 2 based on their DLPFC transcriptomes and from controls. Our analyses demonstrate that ARG S1PR1 protein expression is significantly higher in Type 2 compared to Type 1 (p < 0.05) and controls (p < 0.05), which was consistent with previous mRNA S1PR1. These findings support the possibility that PET S1PR1 can be used as a future imaging biomarker to distinguish these subgroups of schizophrenic patients during life with obvious implications for both patient management and the design of clinical trials to validate novel pharmacologic therapies.
Collapse
Affiliation(s)
- Ganesh B. Chand
- Mallinckrodt Institute of Radiology, Washington University School of Medicine, St. Louis, MO, United States
| | - Hao Jiang
- Mallinckrodt Institute of Radiology, Washington University School of Medicine, St. Louis, MO, United States
| | - J. Philip Miller
- Division of Biostatistics, Washington University School of Medicine, St. Louis, MO, United States
| | | | - Zhude Tu
- Mallinckrodt Institute of Radiology, Washington University School of Medicine, St. Louis, MO, United States
| | - Dean Foster Wong
- Mallinckrodt Institute of Radiology, Washington University School of Medicine, St. Louis, MO, United States
- Department of Psychiatry, Neuroscience, and Neurology, Washington University School of Medicine, St. Louis, MO, United States
| |
Collapse
|
6
|
Chen Z, Haider A, Chen J, Xiao Z, Gobbi L, Honer M, Grether U, Arnold SE, Josephson L, Liang SH. The Repertoire of Small-Molecule PET Probes for Neuroinflammation Imaging: Challenges and Opportunities beyond TSPO. J Med Chem 2021; 64:17656-17689. [PMID: 34905377 PMCID: PMC9094091 DOI: 10.1021/acs.jmedchem.1c01571] [Citation(s) in RCA: 24] [Impact Index Per Article: 8.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/21/2023]
Abstract
Neuroinflammation is an adaptive response of the central nervous system to diverse potentially injurious stimuli, which is closely associated with neurodegeneration and typically characterized by activation of microglia and astrocytes. As a noninvasive and translational molecular imaging tool, positron emission tomography (PET) could provide a better understanding of neuroinflammation and its role in neurodegenerative diseases. Ligands to translator protein (TSPO), a putative marker of neuroinflammation, have been the most commonly studied in this context, but they suffer from serious limitations. Herein we present a repertoire of different structural chemotypes and novel PET ligand design for classical and emerging neuroinflammatory targets beyond TSPO. We believe that this Perspective will support multidisciplinary collaborations in academic and industrial institutions working on neuroinflammation and facilitate the progress of neuroinflammation PET probe development for clinical use.
Collapse
Affiliation(s)
- Zhen Chen
- Division of Nuclear Medicine and Molecular Imaging, Massachusetts General Hospital & Department of Radiology, Harvard Medical School, Boston, MA, 02114, United States
| | - Ahmed Haider
- Division of Nuclear Medicine and Molecular Imaging, Massachusetts General Hospital & Department of Radiology, Harvard Medical School, Boston, MA, 02114, United States
| | - Jiahui Chen
- Division of Nuclear Medicine and Molecular Imaging, Massachusetts General Hospital & Department of Radiology, Harvard Medical School, Boston, MA, 02114, United States
| | - Zhiwei Xiao
- Division of Nuclear Medicine and Molecular Imaging, Massachusetts General Hospital & Department of Radiology, Harvard Medical School, Boston, MA, 02114, United States
| | - Luca Gobbi
- Pharma Research and Early Development, F. Hoffmann-La Roche Ltd, CH-4070 Basel, Switzerland
| | - Michael Honer
- Pharma Research and Early Development, F. Hoffmann-La Roche Ltd, CH-4070 Basel, Switzerland
| | - Uwe Grether
- Pharma Research and Early Development, F. Hoffmann-La Roche Ltd, CH-4070 Basel, Switzerland
| | - Steven E. Arnold
- Department of Neurology and the Massachusetts Alzheimer’s Disease Research Center, Massachusetts General Hospital, Harvard Medical School, 114 16th Street, Charlestown, Massachusetts 02129, USA
| | - Lee Josephson
- Division of Nuclear Medicine and Molecular Imaging, Massachusetts General Hospital & Department of Radiology, Harvard Medical School, Boston, MA, 02114, United States
| | - Steven H. Liang
- Division of Nuclear Medicine and Molecular Imaging, Massachusetts General Hospital & Department of Radiology, Harvard Medical School, Boston, MA, 02114, United States
| |
Collapse
|
7
|
Positron Emission Tomography in the Inflamed Cerebellum: Addressing Novel Targets among G Protein-Coupled Receptors and Immune Receptors. Pharmaceutics 2020; 12:pharmaceutics12100925. [PMID: 32998351 PMCID: PMC7601272 DOI: 10.3390/pharmaceutics12100925] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/17/2020] [Revised: 09/17/2020] [Accepted: 09/25/2020] [Indexed: 01/12/2023] Open
Abstract
Inflammatory processes preceding clinical manifestation of brain diseases are moving increasingly into the focus of positron emission tomographic (PET) investigations. A key role in inflammation and as a target of PET imaging efforts is attributed to microglia. Cerebellar microglia, with a predominant ameboid and activated subtype, is of special interest also regarding improved and changing knowledge on functional involvement of the cerebellum in mental activities in addition to its regulatory role in motor function. The present contribution considers small molecule ligands as potential PET tools for the visualization of several receptors recognized to be overexpressed in microglia and which can potentially serve as indicators of inflammatory processes in the cerebellum. The sphingosine 1 phosphate receptor 1 (S1P1), neuropeptide Y receptor 2 (NPY2) and purinoceptor Y12 (P2Y12) cannabinoid receptors and the chemokine receptor CX3CR1 as G-protein-coupled receptors and the ionotropic purinoceptor P2X7 provide structures with rather classical binding behavior, while the immune receptor for advanced glycation end products (RAGE) and the triggering receptor expressed on myeloid cells 2 (TREM2) might depend for instance on further accessory proteins. Improvement in differentiation between microglial functional subtypes in comparison to the presently used 18 kDa translocator protein ligands as well as of the knowledge on the role of polymorphisms are special challenges in such developments.
Collapse
|