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

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-yl α,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 [K(i), nM: ml, (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 radiotracer. 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 further evaluation in the design of selective PET mAChR imaging ligands.