Synthesis and Nicotinic Acetylcholine Receptor in Vivo Binding Properties of 2-Fluoro-3-[2(S)-2-azetidinylmethoxy]pyridine: A New Positron Emission Tomography Ligand for Nicotinic Receptors

The lead compound of a new series of 3-pyridyl ethers, the azetidine derivative A-85380 (3-[(S)-2-azetidinylmethoxy]pyridine), is a potent and selective ligand for the human α4β2 nicotinic acetylcholine receptor (nAChR) subtype. In vitro, the fluoro derivative of A-85380 (2-fluoro-3-[(S)-2-azetidinylmethoxy]pyridine or F-A-85380) competitively displaced [3H]cytisine or [3H]epibatidine with K i values of 48 and 46 pM, respectively. F-A-85380 has been labeled with the positron emitter fluorine-18 (t 1/2 (half-life) = 110 min) by no-carrier-added nucleophilic aromatic substitution by K[18F]F-K222 complex with (3-[2(S)-N-(tert-butoxycarbonyl)-2-azetidinylmethoxy]pyridin-2-yl)trimethylammonium trifluoromethanesulfonate as a highly efficient labeling precursor, followed by TFA removal of the Boc protective group. The total synthesis time was 50−53 min from the end of cyclotron fluorine-18 production (EOB). Radiochemical yields, with respect to initial [18F]fluoride ion radioactivity, were 68−72% (decay-corrected) and 49−52% (non-decay-corrected), and the specific radioactivities at EOB were 4−7 Ci/μmol (148−259 GBq/μmol). In vivo characterization of [18F]F-A-85380 showed promising properties for PET imaging of central nAChRs. This compound does not bind in vivo to α7 nicotinic or 5HT3 receptors. Moreover, its cerebral uptake can be modulated by the synaptic concentration of the endogenous ligand acetylcholine. The preliminary PET experiments in baboons with [18F]F-A-85380 show an accumulation of the radiotracer in the brain within 60 min. In the thalamus, a nAChR-rich area, uptake of radioactivity reached a maximum at 60 min (4% I.D./100 mL of tissue). [18F]F-A-85380 appears to be a suitable radioligand for brain imaging nAChRs with PET.