Synthesis and biodistribution of radiolabeled α₇ nicotinic acetylcholine receptor ligands

Our objective was to develop an array of α₇-selective nicotinic cholinergic receptor (nAChR)-based imaging agents for PET and SPECT. Methods: (2′R)-N-¹¹C-Methyl-N-(phenylmethyl)-spiro[1- azabicyclo[2.2.2]octane-3,2′(3′H)-furo[2,3-b]pyridin] -5′-amine 1 was synthesized by reaction of the corresponding desmethyl precursor with ¹¹C-CO₂ and reduction. N-(R)-1-Aza- bicyclo[2.2.2]oct-3-yl-4-¹¹C-methylsulfanyl-benzamide 2 was synthesized by reduction of the corresponding disutfide precursor and reaction with ¹¹C-iodomethane. N-(R)-1-Aza-bicyclo[2.2.2]oct-3-yl-4- ¹²⁵]-iodo-benzamide 3 was synthesized by halogen exchange of the corresponding bromide. (2′R)-5′-(2-¹²⁵I-iodo-3-furanyl) spiro[1-azabicyclo[2.2.2] octane]-3,2′(3′H)-furo[2,3-b]pyridine 4 was synthesized by the chloramine-T method. Kinetic biodistribution studies were done in male CD-1 mice by tail vein injection of 3.7 MBq (100 μCi) of the ¹¹C-labeled radiotracer or 0.67 MBq (2 μCi) of the ¹²⁵I-labeled radiotracer followed by brain dissection and tissue counting. Receptor blockade was determined by pretreatment of the mice with an excess of either unlabeted precursor or nicotine. Results: We synthesized 4 radiolabeled, moderate- to high-affinity, α₇-nAChR-based ligands. The compounds were a series of quinudidine derivatives with an inhibition constant (K_i) < 6 nmol/L (33 pmol/L for 4) for α₇-nAChR and selectivities of α₇/ α₄β₂ subtypes of ≥14,000. All of the compounds were produced in adequate radiochemical yield and specific radioactivity (>74 GBq/μmol [2,000 Ci/mmol]). No site selectivity or receptor blockade was shown for 1 and 2 (0.91 ± 0.05 and 0.14 ± 0.03 %ID/g [percentage injected dose per gram] in the hippocampus [target tissue], respectively). Compound 3 showed low hippocampal uptake (0.25 ± 0.05 %ID/g) but prolonged retention within that structure. Pretreatment with nicotine decreased its uptake by up to 50% in the hippocampus. Similar reductions were also observed within the cerebellum (nontarget tissue). Compound 4 showed hippocampal uptake of 2.41 ± 0.03 %ID/g and target-to-nontarget uptake ratios of up to 2. Pretreatment of animals with unlabeled 4 resulted in a decrease of hippocampal uptake to 60% of its preblockade value without a corresponding decrease in cerebellar uptake. Conclusion: With further structural optimization, selective imaging of α₇-nAChR may be possible.