Synthesis and evaluation of nicotine alpha4beta2 receptor radioligand, 5-(3'-18F-fluoropropyl)-3-(2-(S)-pyrrolidinylmethoxy)pyridine, in rodents and PET in nonhuman primate.
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.
Chattopadhyay
Sankha
S
Department of Psychiatry and Human Behavior. Brain Imaging Center, University of California, Irvine, CA 92697, USA.
Xue
Baogang
B
Collins
Daphne
D
Pichika
Rama
R
Bagnera
Rudy
R
Leslie
Frances M
FM
Christian
Bradley T
BT
Shi
Bingzhi
B
Narayanan
Tanjore K
TK
Potkin
Steven G
SG
Mukherjee
Jogeshwar
J
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
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