title: [¹¹C]NS9531, [¹¹C]NS9762 and [¹¹C]NS6417, specific SERT tracers: pre-clinical evaluation in pigs and optimization of synthesis conditions using [¹¹C]methyl triflate.
abstract: BACKGROUND
  NS9531, NS9762 and NS6417 are nitroquinolinyl-diazabicyclo-alkane derivatives that have been developed as inhibitors of serotonin reuptake transporters (SERT) by NeuroSearch A/S.
  
  
  METHODS
  IC50 was measured on the up-take of serotonin, dopamine and noradrenaline in synaptosomes prepared from selected rat brain regions. For the pre-clinical evaluation in pigs, [(11)C]NS9531, [(11)C]NS9762 and [(11)C]NS6417 were prepared by N-methylation using [(11)C]methyl iodide. These syntheses were later on optimized regarding: 1) choice of labelled precursor; 2) HPLC purification conditions; and 3) formulation using SPE columns. The synthesis protocols were then fully automated on a GE FXc Pro. Preclinical evaluation was performed by PET studies in landrace pigs before and after treatment with citalopram.
  
  
  RESULTS
  IC50 measurements showed that all three compounds have low nanomolar affinity for SERT, and micromolar affinity for DAT and NET. The radiochemical yield (r.y.) of all three ligands from [(11)C]methyl iodide was higher than 30%. From [(11)C]methyl triflate, the r.y. of [(11)C]NS9531 and [(11)C]NS9762 were higher than 80% whereas the r.y. of [(11)C]NS6417 was 65%. Residual precursor amounts in final products could be significantly reduced by the use of [(11)C]methyl triflate, <0.2 μg compared with <10 μg, calculated for a 300 MBq injection at 20 minutes EOS. The optimized conditions gave 2.5-4.5 GBq of products with a specific radioactivity of 20-70 MBq/nmol, residual acetonitrile 15-30 ppm, and pH 6.5-7.1. All three compounds showed a rapid and comparable high pig brain uptake of about 3%, producing PET images of good contrast, and uptake was reduced after pre-administration with citalopram.
  
  
  CONCLUSIONS
  The three (11)C labelled PET tracers could be prepared in medium to high yield and high purity. IC50 measurements showed that the three NS compounds were highly selective, high affinity SERT inhibitors. PET studies in pig showed high brain uptake that could be blocked by citalopram pre-treatment.
authors.affiliation: PET & Cyclotron Unit, Department of Nuclear Medicine, Odense University Hospital, Sdr. Boulevard 29, DK-5000, Odense, Denmark.
authors.forename: Johan Hygum
authors.initials: JH
authors.lastname: Dam
authors.affiliation: Department of Nuclear Medicine & PET Centre, Aarhus University Hospital, DK-8000, Aarhus C, Denmark.
authors.forename: Dirk
authors.initials: D
authors.lastname: Bender
authors.affiliation: DanPET AB, Malmö, Sweden.
authors.forename: Dan
authors.initials: D
authors.lastname: Peters
authors.affiliation: PET & Cyclotron Unit, Department of Nuclear Medicine, Odense University Hospital, Sdr. Boulevard 29, DK-5000, Odense, Denmark. Electronic address: kjell.naagren@rsyd.dk.
authors.forename: Kjell
authors.initials: K
authors.lastname: Någren
mesh_terms: D000818:Animals; D002250:Carbon Radioisotopes; D060326:Chemistry Techniques, Synthetic; D002626:Chemistry, Pharmaceutical; D005260:Female; D006847:Hydrocarbons, Iodinated / Q000737:chemistry; D007553:Isotope Labeling; D008698:Mesylates / Q000737:chemistry*; D009600:Nitroquinolines / Q000138:chemical synthesis* / Q000737:chemistry*; D049268:Positron-Emission Tomography; D011786:Quality Control; D011849:Radioactive Tracers; D019275:Radiopharmaceuticals / Q000138:chemical synthesis* / Q000737:chemistry*; D051381:Rats; D050486:Serotonin Plasma Membrane Transport Proteins / Q000378:metabolism*; D013552:Swine
chemical_list: D002250:Carbon Radioisotopes; D006847:Hydrocarbons, Iodinated; D008698:Mesylates; D009600:Nitroquinolines; D011849:Radioactive Tracers; D019275:Radiopharmaceuticals; D050486:Serotonin Plasma Membrane Transport Proteins; C078465:methyl triflate; C014055:methyl iodide
keywords: Preclinical evaluation; SERT; SPE formulation; [(11)C]NS6417; [(11)C]NS9531; [(11)C]NS9762