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

5-[Br-76]Bromo-3-[[2(S)-azetidinyl]methoxy]-pyridine ([Br-76]BAP), a novel nicotinic acetylcholine receptor ligand, was synthesized using [Br-76]bromi de in an oxidative bromodestannylation of the corresponding trimethylstanny l compound. The radiochemical yield was 25%, and the specific radioactivity was on the order of 1 Ci/mu mol. The binding properties of [Br-76]BAP were characterized in vitro and in vivo in rat brain, and positron emission tom ography (PET) experiments were performed in two rhesus monkeys. In associat ion experiments on membranes of the cortex and thalamus, >90% of maximal sp ecific [Br-76]BAP binding was obtained after 60 min. The dissociation half- life of [Br-76]BAP was 51 +/- 6 min in cortical membranes and 56 +/- 3 min in thalamic membranes. Saturation experiments with [Br-76]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 t hat binding of [Br-76]BAP was high in the thalamus and presubiculum, modera te in the cortex and striatum, and low in the cerebellum and hippocampus. A similar pattern of [Br-76]BAP accumulation was observed by ex vivo autorad iography. In vivo, binding of [Br-76]BAP in whole rat brain was blocked by preinjection of (S)(-)-nicotine (0.3 mg/kg) by 27, 52, 68, and 91% at survi val times of 10, 25, 40, 120, and 300 min, respectively. In a preliminary P ET study in rhesus monkeys, the highest [76Br]BAP uptake was found in the t halamus, and radioactivity was displaceable by similar to 60% with cytisine and by 50% with (S)(-)-nicotine. The data of this study indicate that [Br- 76]BAP is a promising radioligand for the characterization of nicotinic ace tylcholine receptors in vivo.