title: Brain penetrant small molecule (18)F-GnRH receptor (GnRH-R) antagonists: Synthesis and preliminary positron emission tomography imaging in rats.
abstract: The gonadotropin releasing hormone receptor (GnRH-R) has a well-described neuroendocrine function in the anterior pituitary. However, little is known about its function in the central nervous system (CNS), where it is most abundantly expressed in hippocampus and amygdala. Since peptide ligands based upon the endogenous decapetide GnRH do not pass the blood-brain-barrier, we are seeking a high-affinity small molecule GnRH-R ligand suitable for brain imaging by positron emission tomography. We have previously reported the radiosynthesis and in vitro evaluation of two novel [(18)F]fluorinated GnRH-R ligands belonging to the furamide class of antagonists, with molecular weight less than 500 Da. We now extend this work using palladium coupling for the synthesis of four novel radioligands, with putatively reduced polar surface area and hydrophilicity relative to the two previously described compounds, and report the uptake of these (18)F-labeled compounds in brain of living rats.
  
  
  
  We synthesized reference standards of the small molecule GnRH-R antagonists as well as mesylate precursors for (18)F-labeling. The antagonists were tested for binding affinity for both human and rat GnRH-R. Serum and blood stability in vitro and in vivo were studied. Biodistribution and PET imaging studies were performed in male rats in order to assess brain penetration in vivo.
  
  
  
  A palladium coupling methodology served for the synthesis of four novel fluorinated furamide GnRH receptor antagonists with reduced heteroatomic count. Radioligand binding assays in vitro revealed subnanomolar affinity of the new fluorinated compounds for both human and rat GnRH-R. The (18)F-GnRH antagonists were synthesized from the corresponding mesylate precursors in 5-15% overall radiochemical yield. The radiolabeled compounds demonstrated good in vivo stability. PET imaging with the (18)F-radiotracers in naive rats showed good permeability into brain and rapid washout, but absence of discernible specific binding in vivo.
  
  
  
  The novel small molecule (18)F-fluorinated GnRH-R antagonist compounds show high receptor affinity in vitro, and may prove useful for quantitative autoradiographic studies in vitro. The compounds were permeable to the blood-brain barrier, but nonetheless failed to reveal significant specific binding in brain of living rats. Nonetheless, our approach may serve as a foundation for designing PET ligands suitable to image the GnRH-R distribution in brain.
authors.affiliation: Norsk medisinsk syklotronsenter AS, Postboks 4950 Nydalen, 0424 Oslo, Norway; School of Pharmacy, University of Oslo, P.O.Box 1068, Blindern, 0316 Oslo, Norway. Electronic address: dag.erlend.olberg@petsenteret.no.
authors.forename: Dag E
authors.initials: DE
authors.lastname: Olberg
authors.affiliation: Department of Biomedical Engineering, University of California Davis, Davis, CA, USA.
authors.forename: Nadine
authors.initials: N
authors.lastname: Bauer
authors.affiliation: Department of Pharmacology, Faculty of Medicine, University of Oslo and Oslo, University Hospital, Oslo, Norway; K.G. Jebsen Cardiac Research Centre and Center for Heart Failure Research, Faculty of Medicine, University of Oslo, Oslo, Norway.
authors.forename: Kjetil W
authors.initials: KW
authors.lastname: Andressen
authors.affiliation: The Intervention Centre, Oslo University Hospital, PO 4950, Nydalen, 0424 Oslo, Norway.
authors.forename: Trine
authors.initials: T
authors.lastname: Hjørnevik
authors.affiliation: Department of Neuropsychiatry and Psychosomatic Medicine, Oslo University Hospital, Oslo, Norway; School of Psychology and Conselling, Queensland University of Technology and QIMR Berghofer, Brisbane, Australia.
authors.forename: Paul
authors.initials: P
authors.lastname: Cumming
authors.affiliation: Department of Pharmacology, Faculty of Medicine, University of Oslo and Oslo, University Hospital, Oslo, Norway; K.G. Jebsen Cardiac Research Centre and Center for Heart Failure Research, Faculty of Medicine, University of Oslo, Oslo, Norway.
authors.forename: Finn O
authors.initials: FO
authors.lastname: Levy
authors.affiliation: School of Pharmacy, University of Oslo, P.O.Box 1068, Blindern, 0316 Oslo, Norway.
authors.forename: Jo
authors.initials: J
authors.lastname: Klaveness
authors.affiliation: Department of Neuropsychiatry and Psychosomatic Medicine, Oslo University Hospital, Oslo, Norway.
authors.forename: Ira
authors.initials: I
authors.lastname: Haraldsen
authors.affiliation: Department of Biomedical Engineering, University of California Davis, Davis, CA, USA; Division of Hematology/Oncology, Department of Internal Medicine, University of California Davis, Sacramento, CA; Center for Molecular and Genomic Imaging, University of California Davis, Davis, CA; Radiochemistry Research and Training Facility, University of California Davis, Sacramento, CA.
authors.forename: Julie L
authors.initials: JL
authors.lastname: Sutcliffe
mesh_terms: D000818:Animals; D001921:Brain / Q000000981:diagnostic imaging* / Q000378:metabolism*; D060326:Chemistry Techniques, Synthetic; D005462:Fluorine Radioisotopes; D005663:Furans / Q000138:chemical synthesis* / Q000378:metabolism* / Q000493:pharmacokinetics / Q000494:pharmacology; D008297:Male; D010539:Permeability; D049268:Positron-Emission Tomography; D011854:Radiochemistry; D051381:Rats; D017207:Rats, Sprague-Dawley; D011966:Receptors, LHRH / Q000037:antagonists & inhibitors*; D013379:Substrate Specificity; D014018:Tissue Distribution
chemical_list: D005462:Fluorine Radioisotopes; D005663:Furans; D011966:Receptors, LHRH; C004932:diloxanide furoate
keywords: Brain imaging; Buchwald–Hartwig; GnRH receptor; GnRH-R furamide antagonists; Positron emission tomography