title: Detection of microglial activation in an acute model of neuroinflammation using PET and radiotracers 11C-(R)-PK11195 and 18F-GE-180.
abstract: 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.
  
  
  CONCLUSIONS
  (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.
authors: Department of Pharmacology, Drug Development and Therapeutics, University of Turku, Turku, Finland.
authors: Alex M
authors: AM
authors: Dickens
authors: Susanne
authors: S
authors: Vainio
authors: Päivi
authors: P
authors: Marjamäki
authors: Jarkko
authors: J
authors: Johansson
authors: Paula
authors: P
authors: Lehtiniemi
authors: Johanna
authors: J
authors: Rokka
authors: Juha
authors: J
authors: Rinne
authors: Olof
authors: O
authors: Solin
authors: Merja
authors: M
authors: Haaparanta-Solin
authors: Paul A
authors: PA
authors: Jones
authors: William
authors: W
authors: Trigg
authors: Daniel C
authors: DC
authors: Anthony
authors: Laura
authors: L
authors: Airas
mesh_terms: D000577:Amides; D000818:Animals; D001253:Astrocytes / Q000000981:diagnostic imaging; D002227:Carbazoles; D004195:Disease Models, Animal; D007249:Inflammation / Q000000981:diagnostic imaging; D007546:Isoquinolines; D008297:Male; D017628:Microglia / Q000000981:diagnostic imaging*; D049268:Positron-Emission Tomography; D011849:Radioactive Tracers; D051381:Rats
chemical_list: C504060:(R)-(11C)1-(2-chlorophenyl)-N-methyl-N-(1-methylpropyl)-3-isoquinolinecarboxamide; D000577:Amides; D002227:Carbazoles; C571664:GE-180; D007546:Isoquinolines; D011849:Radioactive Tracers
keywords: astrocyte; brain; neuroinflammation; positron emission tomography; second-generation TSPO ligand