UC Irvine

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 I-125-iodoepibatidine or I-125-bungaratoxin. Selectivity of binding was measured in the presence of cytisine. F-18 radiolabeling was performed by reacting the tosylate precursor with F-18-fluoride followed by cleprotection. In vitro autoradiographic studies in rat brain slices with 5-(3'-F-18-fluoropropyl)-3-(2-(S)-pyrrolidinylmethoxy)pyridine (F-18-nifrolidine) were read on a phosphor imager. Rats were injected with F-18-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 F-18-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 18F-nifrolidine specific activities of approximately 111-185 GBq/mumol. In vitro autoradiography in rat brain slices revealed selective binding of F-18-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 mus, ventrolateral thalamus) and extrathalamic regions such as cingulate gyrus, lateral geniculate, temporal cortex, and frontal cortex. Conclusion: Binding of F-18-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.