title: Radiolabeling of [18F]-fluoroethylnormemantine and initial in vivo evaluation of this innovative PET tracer for imaging the PCP sites of NMDA receptors.
abstract: BACKGROUND
  The N-methyl-D-aspartate receptor (NMDAr) is an ionotropic receptor that mediates excitatory transmission. NMDAr overexcitation is thought to be involved in neurological and neuropsychiatric disorders such as Alzheimer disease and schizophrenia. We synthesized [(18)F]-fluoroethylnormemantine ([(18)F]-FNM), a memantine derivative that binds to phencyclidine (PCP) sites within the NMDA channel pore. These sites are primarily accessible when the channel is in the active and open state.
  
  
  METHODS
  Radiosynthesis was carried out using the Raytest® SynChrom R&D fluorination module. Affinity of this new compound was determined by competition assay. We ran a kinetic study in rats and computed a time-activity curve based on a volume-of-interest analysis, using CARIMAS® software. We performed an ex vivo autoradiography, exposing frozen rat brain sections to a phosphorscreen. Adjacent sections were used to detect NMDAr by immunohistochemistry with an anti-NR1 antibody. As a control of the specificity of our compound for NMDAr, we used a rat anesthetized with ketamine. Correlation analysis was performed with ImageJ software between signal of autoradiography and immunostaining.
  
  
  RESULTS
  Fluorination yield was 10.5% (end of synthesis), with a mean activity of 3145 MBq and a specific activity above 355 GBq/μmol. Affinity assessment allowed us to determine [(19)F]-FNM IC50 at 6.1 10(-6)M. [(18)F]-FMN concentration gradually increased in the brain, stabilizing at 40 minutes post injection. The brain-to-blood ratio was 6, and 0.4% of the injected dose was found in the brain. Combined ex vivo autoradiography and immunohistochemical staining demonstrated colocalization of NMDAr and [(18)F]-FNM (r=0.622, p<0.0001). The highest intensity was found in the cortex and cerebellum, and the lowest in white matter. A low and homogeneous signal corresponding to unspecific binding was observed when PCP sites were blocked with ketamine.
  
  
  CONCLUSIONS
  [(18)F]-FNM appears to be a promising tracer for imaging NMDAr activity for undertaking preclinical studies in perspective of clinical detection of neurological or neuropsychological disorders.
authors.affiliation: Brain Imaging and Neurological Disability UMR 825, INSERM, F-31059 Toulouse, France; Brain imaging and neurological disability UMR 825, University of Toulouse, UPS, CHU Purpan, Place du Dr Baylac, F-31059 Toulouse Cedex 9, France; Radiopharmacy Department, University Hospital, Toulouse, France. Electronic address: anne-sophie.salabert@inserm.fr.
authors.forename: Anne-Sophie
authors.initials: AS
authors.lastname: Salabert
authors.affiliation: Research Center for Brain and Cognition, University of Toulouse UPS, Toulouse, France; CerCo, CNRS, Toulouse, France.
authors.forename: Caroline
authors.initials: C
authors.lastname: Fonta
authors.affiliation: Brain Imaging and Neurological Disability UMR 825, INSERM, F-31059 Toulouse, France; Brain imaging and neurological disability UMR 825, University of Toulouse, UPS, CHU Purpan, Place du Dr Baylac, F-31059 Toulouse Cedex 9, France; Radiopharmacy Department, University Hospital, Toulouse, France.
authors.forename: Charlotte
authors.initials: C
authors.lastname: Fontan
authors.affiliation: Brain Imaging and Neurological Disability UMR 825, INSERM, F-31059 Toulouse, France; Brain imaging and neurological disability UMR 825, University of Toulouse, UPS, CHU Purpan, Place du Dr Baylac, F-31059 Toulouse Cedex 9, France.
authors.forename: Djilali
authors.initials: D
authors.lastname: Adel
authors.affiliation: Radiopharmacy Department, University Hospital, Toulouse, France.
authors.forename: Mathieu
authors.initials: M
authors.lastname: Alonso
authors.affiliation: Animal experimentation platform, UMS 006, Toulouse, France.
authors.forename: Carine
authors.initials: C
authors.lastname: Pestourie
authors.affiliation: Research and Expertise Group, French Association for the Promotion of Medical Research (AFPREMED), Toulouse, France.
authors.forename: Hafid
authors.initials: H
authors.lastname: Belhadj-Tahar
authors.affiliation: Brain Imaging and Neurological Disability UMR 825, INSERM, F-31059 Toulouse, France; Brain imaging and neurological disability UMR 825, University of Toulouse, UPS, CHU Purpan, Place du Dr Baylac, F-31059 Toulouse Cedex 9, France; Radiopharmacy Department, University Hospital, Toulouse, France.
authors.forename: Mathieu
authors.initials: M
authors.lastname: Tafani
authors.affiliation: Brain Imaging and Neurological Disability UMR 825, INSERM, F-31059 Toulouse, France; Brain imaging and neurological disability UMR 825, University of Toulouse, UPS, CHU Purpan, Place du Dr Baylac, F-31059 Toulouse Cedex 9, France; Nuclear Medicine Department, University Hospital, Toulouse, France.
authors.forename: Pierre
authors.initials: P
authors.lastname: Payoux
mesh_terms: D000818:Animals; D001345:Autoradiography; D001921:Brain / Q000000981:diagnostic imaging* / Q000378:metabolism*; D005462:Fluorine Radioisotopes / Q000493:pharmacokinetics; D007553:Isotope Labeling; D007700:Kinetics; D008297:Male; D008559:Memantine / Q000031:analogs & derivatives* / Q000493:pharmacokinetics*; D008657:Metabolic Clearance Rate; D010622:Phencyclidine / Q000378:metabolism*; D049268:Positron-Emission Tomography / Q000379:methods*; D019275:Radiopharmaceuticals / Q000493:pharmacokinetics*; D051381:Rats; D017207:Rats, Sprague-Dawley; D016194:Receptors, N-Methyl-D-Aspartate / Q000378:metabolism*; D014018:Tissue Distribution
chemical_list: D005462:Fluorine Radioisotopes; D019275:Radiopharmaceuticals; D016194:Receptors, N-Methyl-D-Aspartate; C000606130:fluoroethylnormemantine; D010622:Phencyclidine; D008559:Memantine
keywords: Activated NMDA receptor; Excitatory neurotransmission; Innovative PET probe; Memantine; Neuropathology; preclinical