Synthesis and characterization of binding of 5-[⁷⁶Br]bromo-3-[[2(S)- azetidinyl]methoxy]pyridine, a novel nicotinic acetylcholine receptor ligand, in rat brain

5-[⁷⁶Br]Bromo-3-[[2(S)-azetidinyl]methoxy]pyridine ([⁷⁶Br]BAP), a novel nicotinic acetylcholine receptor ligand, was synthesized using [⁷⁶Br]bromide in an oxidative bromodestannylation of the corresponding trimethylstannyl compound. The radiochemical yield was 25%, and the specific radioactivity was on the order of 1 Ci/μmol. The binding properties of [⁷⁶Br]BAP were characterized in vitro and in vivo in rat brain, and positron emission tomography (PET) experiments were performed in two rhesus monkeys. In association experiments on membranes of the cortex and thalamus, >90% of maximal specific [⁷⁶Br]BAP binding was obtained after 60 min. The dissociation half-life of [⁷⁶Br]BAP was 51 ± 6 min in cortical membranes and 56 ± 3 min in thalamic membranes. Saturation experiments with [⁷⁶Br]BAP revealed one population of binding sites with dissociation constant (K(D)) values of 36 ± 9 and 30 ± 9 pM in membranes of cortex and thalamus, respectively. The maximal binding site density (B(max)) values were 90 ± 17 and 207 ± 33 fmol/mg in membranes of cortex and thalamus, respectively. Scatchard plots were nonlinear, and the Hill coefficients were <1, suggesting the presence of a lower-affinity binding site. In vitro autoradiography studies showed that binding of [⁷⁶Br]BAP was high in the thalamus and presubiculum, moderate in the cortex and striatum, and low in the cerebellum and hippocampus. A similar pattern of [⁷⁶Br]BAP accumulation was observed by ex vivo autoradiography. In vivo, binding of [⁷⁶Br]BAP in whole rat brain was blocked by preinjection of (S)(-)-nicotine (0.3 mg/kg) by 27, 52, 68, and 91% at survival times of 10, 25, 40, 120, and 300 min, respectively. In a preliminary PET study in rhesus monkeys, the highest [⁷⁶Br]BAP uptake was found in the thalamus, and radioactivity was displaceable by ~60% with cytisine and by 50% with (S)(-)-nicotine. The data of this study indicate that [⁷⁶Br]BAP is a promising radioligand for the characterization of nicotinic acetylcholine receptors in vivo.