n-(2-methoxybenzyl)-n-(4-phenoxypyridin-3-yl)acetamide and Multiple-Sclerosis

The 18 kDa translocator protein (TSPO) is increasingly used to study brain and spinal cord inflammation in degenerative diseases of the CNS such as multiple sclerosis. The enhanced TSPO PET signal that arises during disease is widely considered to reflect activated pathogenic microglia, although quantitative neuropathological data to support this interpretation have not been available. With the increasing interest in the role of chronic microglial activation in multiple sclerosis, characterising the cellular neuropathology associated with TSPO expression is of clear importance for understanding the cellular and pathological processes on which TSPO PET imaging is reporting. Here we have studied the cellular expression of TSPO and specific binding of two TSPO targeting radioligands (3H-PK11195 and 3H-PBR28) in tissue sections from 42 multiple sclerosis cases and 12 age-matched controls. Markers of homeostatic and reactive microglia, astrocytes, and lymphocytes were used to investigate the phenotypes of cells expressing TSPO. There was an approximate 20-fold increase in cells double positive for TSPO and HLA-DR in active lesions and in the rim of chronic active lesion, relative to normal appearing white matter. TSPO was uniformly expressed across myeloid cells irrespective of their phenotype, rather than being preferentially associated with pro-inflammatory microglia or macrophages. TSPO+ astrocytes were increased up to 7-fold compared to normal-appearing white matter across all lesion subtypes and accounted for 25% of the TSPO+ cells in these lesions. To relate TSPO protein expression to ligand binding, specific binding of the TSPO ligands 3H-PK11195 and 3H-PBR28 was determined in the same lesions. TSPO radioligand binding was increased up to seven times for 3H-PBR28 and up to two times for 3H-PK11195 in active lesions and the centre of chronic active lesions and a strong correlation was found between the radioligand binding signal for both tracers and the number of TSPO+ cells across all of the tissues examined. In summary, in multiple sclerosis, TSPO expression arises from microglia of different phenotypes, rather than being restricted to microglia which express classical pro-inflammatory markers. While the majority of cells expressing TSPO in active lesions or chronic active rims are microglia/macrophages, our findings also emphasize the significant contribution of activated astrocytes, as well as smaller contributions from endothelial cells. These obser

Topics: Acetamides; Aged; Aged, 80 and over; Astrocytes; Autopsy; Female; Genotype; Homeostasis; Humans; Isoquinolines; Lymphocytes; Male; Microglia; Middle Aged; Multiple Sclerosis; Positron-Emission Tomography; Pyridines; Radiopharmaceuticals; Receptors, GABA

Translocator protein (18 kDa) (TSPO) is a marker of inflammation in the brain. Positron emission tomography (PET) scans with ligands for this receptor show increased expression of TSPO in many neuropathologic conditions. However, expression of TSPO in the periphery and its possible correlation to central nervous system (CNS) inflammation has been largely unstudied. In this paper PBR28, a recently synthesized ligand for TSPO that is shown to have 80-fold higher specific binding than its predecessor PK11195, is used to quantify peripheral TSPO. Data presented in this study show that monocytes account for the majority of TSPO measured in peripheral blood mononuclear cells (PBMC), and that TSPO expression is stable over time in healthy individuals. Previous studies show that areas of increased PBR28 binding in the brains of multiple sclerosis (MS) patients correlate with active demylinating lesions found during magnetic resonance imaging (MRI). To measure peripheral TSPO expression in an inflammatory disease of the CNS, PBR28 is used in an in vitro radioligand binding assay to measure the amount of TSPO in the PBMC of MS and healthy donor cohorts. Surprisingly, MS patients are found to have a significantly lower amount of peripheral TSPO than healthy donors. We suggest that TSPO protein expression is a potential peripheral biomarker of MS, more research is needed to determine if peripheral TSPO expression may also be altered in other neuroinflammatory conditions.

Topics: Acetamides; Adult; Biomarkers; Brain; Cell Separation; Female; Humans; Isoquinolines; Leukocytes, Mononuclear; Lipopolysaccharide Receptors; Magnetic Resonance Imaging; Male; Membrane Potential, Mitochondrial; Middle Aged; Mitochondria; Multiple Sclerosis; Positron-Emission Tomography; Protein Binding; Pyridines; Radioligand Assay; Radiopharmaceuticals; Real-Time Polymerase Chain Reaction; Receptors, GABA

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.

Topics: Acetamides; Adult; Aged; Biomarkers; Carbon Radioisotopes; Female; Humans; Magnetic Resonance Imaging; Male; Middle Aged; Multiple Sclerosis; Neuroglia; Positron-Emission Tomography; Pyridines; Radiopharmaceuticals; Receptors, GABA