title: Synthesis and evaluation of nicotine alpha4beta2 receptor radioligand, 5-(3'-18F-fluoropropyl)-3-(2-(S)-pyrrolidinylmethoxy)pyridine, in rodents and PET in nonhuman primate.
abstract: Nicotine alpha(4)beta(2) receptor subtypes are implicated in the study of Alzheimer's disease, schizophrenia, substance abuse, lung cancer, and other disorders. We report the development and evaluation of a putative antagonist, 5-(3'-fluoropropyl)-3-(2-(S)-pyrrolidinylmethoxy)pyridine (nifrolidine) as a PET agent for nicotine alpha(4)beta(2) receptors.
  
  
  METHODS
  In vitro binding affinity of nifrolidine was measured in rat brain slices labeled with (125)I-iodoepibatidine or (125)I-bungaratoxin. Selectivity of binding was measured in the presence of cytisine. (18)F radiolabeling was performed by reacting the tosylate precursor with (18)F-fluoride followed by deprotection. In vitro autoradiographic studies in rat brain slices with 5-(3'-(18)F-fluoropropyl)-3-(2-(S)-pyrrolidinylmethoxy)pyridine ((18)F-nifrolidine) were read on a phosphor imager. Rats were injected with (18)F-nifrolidine (3.7 MBq each), and brain regions were counted at various times (2-120 min). Blocking studies were performed by subcutaneous injection of nicotine (10 mg/kg). A PET study of (18)F-nifrolidine (approximately 148 MBq) was performed on an anesthetized rhesus monkey using a high-resolution scanner.
  
  
  RESULTS
  In vitro binding affinity of nifrolidine exhibited an inhibition constant of 2.89 nmol/L for the alpha(4)beta(2) sites. Radiosynthesis and high-performance liquid chromatography purifications yielded the product in approximately 20%-40% decay-corrected radiochemical yield to provide (18)F-nifrolidine specific activities of approximately 111-185 GBq/mumol. In vitro autoradiography in rat brain slices revealed selective binding of (18)F-nifrolidine to the anteroventral thalamic nucleus, ventral posteriomedial thalamus, dorsolateral geniculate, and, to a lesser extent, cortex and striata, which are known to contain alpha(4)beta(2) sites. This specific binding was completely abolished by 300 mumol/L nicotine. Ex vivo rat brain distribution studies indicated selective binding in the thalamus with a maximal thalamus-to-cerebellum ratio of approximately 3. The PET study revealed selective maximal uptake (0.01% injected dose/mL) in regions of the thalamus (anteroventral and anteromedial thalamus, ventrolateral thalamus) and extrathalamic regions such as cingulate gyrus, lateral geniculate, temporal cortex, and frontal cortex.
  
  
  CONCLUSIONS
  Binding of (18)F-nifrolidine to alpha(4)beta(2) receptor-rich regions in rats and monkeys indicates promise as a PET agent. Additionally, the thalamus-to-cerebellum ratio approached a plateau of 1.7 in 120 min, indicating relatively faster kinetics compared with previously reported imaging agents.
authors.affiliation: Department of Psychiatry and Human Behavior. Brain Imaging Center, University of California, Irvine, CA 92697, USA.
authors.forename: Sankha
authors.initials: S
authors.lastname: Chattopadhyay
authors.forename: Baogang
authors.initials: B
authors.lastname: Xue
authors.forename: Daphne
authors.initials: D
authors.lastname: Collins
authors.forename: Rama
authors.initials: R
authors.lastname: Pichika
authors.forename: Rudy
authors.initials: R
authors.lastname: Bagnera
authors.forename: Frances M
authors.initials: FM
authors.lastname: Leslie
authors.forename: Bradley T
authors.initials: BT
authors.lastname: Christian
authors.forename: Bingzhi
authors.initials: B
authors.lastname: Shi
authors.forename: Tanjore K
authors.initials: TK
authors.lastname: Narayanan
authors.forename: Steven G
authors.initials: SG
authors.lastname: Potkin
authors.forename: Jogeshwar
authors.initials: J
authors.lastname: Mukherjee
mesh_terms: D000818:Animals; D001921:Brain / Q000000981:diagnostic imaging* / Q000378:metabolism*; D019086:Bridged Bicyclo Compounds, Heterocyclic / Q000737:chemistry / Q000493:pharmacokinetics*; D007553:Isotope Labeling / Q000379:methods; D008253:Macaca mulatta; D008297:Male; D008657:Metabolic Clearance Rate; D049268:Positron-Emission Tomography; D011485:Protein Binding; D011725:Pyridines / Q000737:chemistry / Q000493:pharmacokinetics*; D011759:Pyrrolidines / Q000737:chemistry / Q000493:pharmacokinetics*; D019275:Radiopharmaceuticals / Q000138:chemical synthesis / Q000493:pharmacokinetics; D051381:Rats; D017207:Rats, Sprague-Dawley; D011978:Receptors, Nicotinic / Q000378:metabolism*; D014018:Tissue Distribution
chemical_list: C516423:2-(6-iodo-3-pyridinyl)-7-azabicyclo(2.2.1)heptane; C498053:5-(3'-fluoropropyl)-3-(2-pyrrolidinylmethoxy)pyridine; D019086:Bridged Bicyclo Compounds, Heterocyclic; D011725:Pyridines; D011759:Pyrrolidines; D019275:Radiopharmaceuticals; D011978:Receptors, Nicotinic; C403531:nicotinic receptor alpha4beta2