Resolution, in vitro and in vivo evaluation of fluorine-18-labeled isomers of 1-azabicyclo[2.2.2]oct-3-ylα-(1-fluoropent-5-yl)α-hydroxy-α-phenylacetate (FQNPe) as new PET candidates for the imaging of muscarinic-cholinergic receptor

Abstract

1‐Azabicyclo[2.2.2]oct‐3‐yl α‐(1‐fluoropent‐5‐yl)‐α‐hydroxy‐α‐phenylacetate (FQNPe, 2), an analogue of 1‐azabicyclo[2.2.2]oct‐3‐ylα, α‐(diphenyl)‐α‐hydroxyacetate (QNB), was resolved into its four stereoisomers. In vitro binding assays of the stereoisomers of 2 demonstrated that while the (S,S)‐isomer did not have significant receptor binding, the other stereoisomers of 2 bound with high affinity to the various mAChR subtypes [Ki, nM: m1, (R,R), 0.33; (R,S), 1.4 (S,R), 3.8; m2, (R,R), 0.1; (R,S), 4.2; (S,R), <75% binding; m3, (R,R), 0.34; (R,S), 3.1; (S,R), 7.6]. The (R,R)‐ and (R,S)‐ stereoisomers of 2 were radiolabeled with fluorine‐18 via a two step procedure in radiochemical yields of 12–21% (n=2) and 9% (decayed corrected to beginning of synthesis), respectively. In vivo biodistribution studies demonstrated significant uptake of [18F]‐(R,R)‐2 in cerebral mAChR‐rich regions of rat brains up to 3 h post injection. Low accumulation of fluorine‐18 in the bone indicated that [18F]‐(R,R)‐2 displayed significant in vivo stability. In contrast [18F]‐(R,S)‐2 demonstrated rapid washout from all cerebral regions. Preinjection of (R)‐QNB (3 mg/kg) 1 h prior to the injection of [18F]‐(R,R)‐2 blocked the uptake of activity in cerebral regions by approximately 90% while the preinjection of haloperidol (3 mg/kg) 1 h prior to the injection of [18F]‐(R,R)‐2 demonstrated no statistically significant effect on the binding of the reactor. An ex vivo metabolic study utilizing [18F]‐(R,R)‐2 demonstrated that greater than 96% of the organic soluble radioactivity which localized in the brain and heart at 1 h post‐injection migrated on TLC with the same mobility as the parent. Although [18F]‐(R.R)‐2 did not demonstrate a desired in vitro or in vivo mAChR subtype selectivity, these results suggest that the introduction of a fluoroalkyl group in various benzylic analogues of QNB is an attractive radiolabeling moiety for furthering evaluation in the design of selective PET mAChR imaging ligands.