Synthesis and biodistribution of radiolabeled alpha 7 nicotinic acetylcholine receptor ligands.
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.


CONCLUSIONS
With further structural optimization, selective imaging of alpha(7)-nAChR may be possible.
Pomper
Martin G
MG
Department of Radiology, Johns Hopkins University, Baltimore, Maryland 21287-2182, USA. mpomper@jhmi.edu
Phillips
Eifion
E
Fan
Hong
H
McCarthy
Dennis J
DJ
Keith
Richard A
RA
Gordon
John C
JC
Scheffel
Ursula
U
Dannals
Robert F
RF
Musachio
John L
JL
D000818:Animals; D001921:Brain / Q000000981:diagnostic imaging* / Q000378:metabolism*; D002250:Carbon Radioisotopes / Q000737:chemistry / Q000493:pharmacokinetics*; D007457:Iodine Radioisotopes / Q000737:chemistry / Q000493:pharmacokinetics*; D007700:Kinetics; D008024:Ligands; D008657:Metabolic Clearance Rate; D051379:Mice; D009928:Organ Specificity; D011877:Radionuclide Imaging; D019275:Radiopharmaceuticals / Q000138:chemical synthesis / Q000493:pharmacokinetics; D011978:Receptors, Nicotinic / Q000378:metabolism*; D014018:Tissue Distribution; D064569:alpha7 Nicotinic Acetylcholine Receptor
D002250:Carbon Radioisotopes; C579166:Chrna7 protein, mouse; D007457:Iodine Radioisotopes; D008024:Ligands; D019275:Radiopharmaceuticals; D011978:Receptors, Nicotinic; D064569:alpha7 Nicotinic Acetylcholine Receptor