Our objective was to develop an array of alpha(7)-selective nicotinic cholinergic receptor (nAChR)-based imaging agents for PET and SPECT.
Methods: (2'R)-N-(11)C-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 (11)C-CO(2) and reduction. N-(R)-1-Aza-bicyclo[2.2.2]oct-3-yl-4-(11)C-methylsulfanyl-benzamide 2 was synthesized by reduction of the corresponding disulfide precursor and reaction with (11)C-iodomethane. N-(R)-1-Aza-bicyclo[2.2.2]oct-3-yl-4-(125)I-iodo-benzamide 3 was synthesized by halogen exchange of the corresponding bromide. (2'R)-5'-(2-(125)I-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 microCi) of the (11)C-labeled radiotracer or 0.67 MBq (2 microCi) of the (125)I-labeled radiotracer followed by brain dissection and tissue counting. Receptor blockade was determined by pretreatment of the mice with an excess of either unlabeled precursor or nicotine.
Results: We synthesized 4 radiolabeled, moderate- to high-affinity, alpha(7)-nAChR-based ligands. The compounds were a series of quinuclidine derivatives with an inhibition constant (K(i)) < 6 nmol/L (33 pmol/L for 4) for alpha(7)-nAChR and selectivities of alpha(7)/alpha(4)beta(2) subtypes of > or =14,000. All of the compounds were produced in adequate radiochemical yield and specific radioactivity (>74 GBq/micromol [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 alpha(7)-nAChR may be possible.