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

Abstract

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.