title: A PET Tracer for Brain α2C Adrenoceptors, (11)C-ORM-13070: Radiosynthesis and Preclinical Evaluation in Rats and Knockout Mice.
abstract: We report the development of a PET tracer for α2C adrenoceptor imaging and its preliminary preclinical evaluation. α2C adrenoceptors in the human brain may be involved in various neuropsychiatric disorders, such as depression, schizophrenia, and neurodegenerative diseases. PET tracers are needed for imaging of this receptor system in vivo.
  
  
  METHODS
  High-specific-activity (11)C-ORM-13070 (1-[(S)-1-(2,3-dihydrobenzo[1,4]dioxin-2-yl)methyl]-4-(3-(11)C-methoxymethylpyridin-2-yl)-piperazine) was synthesized by (11)C-methylation of O-desmethyl-ORM-13070 with (11)C-methyl triflate, which was prepared from cyclotron-produced (11)C-methane via (11)C-methyl iodide. Rats and mice were investigated in vivo with PET and ex vivo with autoradiography. The specificity of (11)C-ORM-13070 binding to α2 adrenoceptors was demonstrated in rats pretreated with atipamezole, an α2 adrenoceptor antagonist. The α2C adrenoceptor selectivity of the tracer was determined by comparing tracer binding in wild-type and α2A- and α2AC adrenoceptor knockout (KO) mice. (11)C-ORM-13070 and its radioactive metabolites in rat plasma and brain tissue were analyzed with radio-high-performance liquid chromatography and mass spectroscopy. Human radiation dose estimates were extrapolated from rat biodistribution data.
  
  
  RESULTS
  The radiochemical yield, calculated from initial cyclotron-produced (11)C-methane, was 9.6% ± 2.7% (decay-corrected to end of bombardment). The specific activity of the product was 640 ± 390 GBq/μmol (decay-corrected to end of synthesis). The radiochemical purity exceeded 99% in all syntheses. The highest levels of tracer binding were observed in the striatum and olfactory tubercle of rats and control and α2A KO mice-that is, in the brain regions known to contain the highest densities of α2C adrenoceptors. In rats pretreated with atipamezole and in α2AC KO mice, (11)C tracer binding in the striatum and olfactory tubercle was low, similar to that of the frontal cortex and thalamus, regions with low densities of α2C adrenoceptors. Two radioactive metabolites were found in rat plasma, but only one of them was found in the brain; their identity was not revealed. The estimated effective radiation dose was comparable with the average exposure level in PET studies with (11)C tracers.
  
  
  CONCLUSIONS
  An efficient method for the radiosynthesis of (11)C-ORM-13070 was developed. (11)C-ORM-13070 emerged as a potential novel radiotracer for in vivo imaging of brain α2C adrenoceptors.
authors.affiliation: Radiopharmaceutical Chemistry Laboratory, Turku PET Centre, University of Turku, Turku, Finland.
authors.forename: Eveliina
authors.initials: E
authors.lastname: Arponen
authors.affiliation: Radiopharmaceutical Chemistry Laboratory, Turku PET Centre, University of Turku, Turku, Finland.
authors.forename: Semi
authors.initials: S
authors.lastname: Helin
authors.affiliation: MediCity/PET Preclinical Laboratory, Turku PET Centre, University of Turku, Turku, Finland.
authors.forename: Päivi
authors.initials: P
authors.lastname: Marjamäki
authors.affiliation: MediCity/PET Preclinical Laboratory, Turku PET Centre, University of Turku, Turku, Finland.
authors.forename: Tove
authors.initials: T
authors.lastname: Grönroos
authors.affiliation: Orion Pharma, Research and Development, Turku, Finland.
authors.forename: Patrik
authors.initials: P
authors.lastname: Holm
authors.affiliation: Department of Biostatistics, University of Turku, Turku, Finland.
authors.forename: Eliisa
authors.initials: E
authors.lastname: Löyttyniemi
authors.affiliation: Radiopharmaceutical Chemistry Laboratory, Turku PET Centre, University of Turku, Turku, Finland.
authors.forename: Kjell
authors.initials: K
authors.lastname: Någren
authors.affiliation: Department of Pharmacology, Drug Development and Therapeutics, University of Turku, and Unit of Clinical Pharmacology, Turku University Hospital, Turku, Finland; and.
authors.forename: Mika
authors.initials: M
authors.lastname: Scheinin
authors.affiliation: MediCity/PET Preclinical Laboratory, Turku PET Centre, University of Turku, Turku, Finland.
authors.forename: Merja
authors.initials: M
authors.lastname: Haaparanta-Solin
authors.affiliation: Orion Pharma, Research and Development, Turku, Finland.
authors.forename: Jukka
authors.initials: J
authors.lastname: Sallinen
authors.affiliation: Radiopharmaceutical Chemistry Laboratory, Turku PET Centre, University of Turku, Turku, Finland Accelerator Laboratory, Åbo Akademi University, Turku, Finland olof.solin@abo.fi.
authors.forename: Olof
authors.initials: O
authors.lastname: Solin
mesh_terms: D000818:Animals; D001921:Brain / Q000000981:diagnostic imaging* / Q000378:metabolism*; D004146:Dioxanes / Q000138:chemical synthesis* / Q000378:metabolism; D006801:Humans; D008297:Male; D051379:Mice; D018345:Mice, Knockout; D010879:Piperazines / Q000138:chemical synthesis* / Q000378:metabolism; D049268:Positron-Emission Tomography / Q000379:methods*; D011849:Radioactive Tracers; D011854:Radiochemistry; D051381:Rats; D018341:Receptors, Adrenergic, alpha-2 / Q000378:metabolism*; D013379:Substrate Specificity; D014057:Tomography, X-Ray Computed
chemical_list: C000598340:1-(1-(2,3-dihydrobenzo(1,4)dioxin-2-yl)methyl)-4-(3-methoxymethylpyridin-2-yl)piperazine; D004146:Dioxanes; D010879:Piperazines; D011849:Radioactive Tracers; D018341:Receptors, Adrenergic, alpha-2
keywords: 11C-ORM-13070; PET; autoradiography; radiosynthesis; α2C adrenoceptor