Comparison of Three 18F-Labeled Butyrophenone Neuroleptic Drugs in the Baboon Using Positron Emission Tomography

The butyrophenone neuroleptics spiroperidol, benperidol, and haloperidol were radiolabeled with fluorine-18 and studied in baboon brain using positron emission transaxial tomography (PETT). Pretreatment of the baboon with a high pharmacological dose of (+)-butaclamol reduced the specifically bound component of radioactivity distribution in the striatum to approximately the radioactivity distribution found in the cerebellum. Comparative studies of brain distribution kinetics over a 4-h period indicated that either [18F]spiroperidol or [18F]benperidol may be suitable for specific labeling of neuroleptic receptors. In an 8-h study with [18F]spiroperidol, striatal radioactivity did not decline, suggesting that spiroperidol either has a very slow dissociation rate or that it binds irreversibly to these receptors in vivo. [18F]Haloperidol may not be suitable for in vivo PETT studies, because of a relatively high component of nonspecific distribution and a faster dissociation from the receptor. Analysis of 18F in plasma after injection of [18F]spiroperidol indicated rapid metabolism to polar and acidic metabolites, with only 40% of the total radioactivity being present as unchanged drug after 30 min. Analysis of the metabolic stability of the radioactively labeled compound in rat striatum indicated that greater than 95% of [18F]spiroperidol remains unchanged after 4 h.     

3-[18F]Acetylcyclofoxy: a useful probe for the visualization of opiate receptors in living animals

A fluoro-analogue of the potent narcotic antagonist, naltrexone, was synthesized and shown to bind with high affinity to opiate receptors in vitro. 3-[18F]acetylcyclofoxy was prepared via a one-step triflate displacement reaction with the positron emitting 18F ion from tetraethylammonium [18F] fluoride. 3-[18F]acetylcyclofoxy accumulation in opiate receptor rich brain regions of both rat and baboon is shown to be completely displaced by the active enantiomer of naloxone [-)-naloxone) while the identical dose of the pharmacologically inert (+)-naloxone has no detectable effect. Moreover, both rat and baboon brain showed the well documented, typical opiate receptor distribution so that basal ganglia and thalamus are clearly visible in the living baboon brain up to 95 min after intravenous injection of 3-[18F] acetylcyclofoxy. We expect that 3-[18F )acetylcyclofoxy will be a useful probe for visualizing opiate receptors in living humans.     

Synthesis,tissue distribution in rats and PET studies in baboon brain of no-carrier-added [18F]RU 52461: in vivo evaluation as a brain glucocorticoid receptor radioligand

11,17β-Dihydroxy-6-methyl-17α -(3-[¹⁸F]fluoro-prop-1 -ynyl)androsta-1,4,6-trien-3-one ([¹⁸F]RU 52461), an ¹⁸F-analog of RU 28362, was synthesized by bromide displacement with [¹⁸F]fluoride in 12–30% overall radiochemical yield (decay-corrected) within 140 min from end of bombardment (EOB). The specific activity was 900–1500 mCi/μmol (33.3–55.5 GBq/μmol) at the end of synthesis (EOS). Biodistribution studies indicated high adrenal and pituitary retention, and uniformly low uptake of [¹⁸F]RU 52461 in all other brain regions of the rat. Except for the pituitary, no specific receptor-mediated uptake of [¹⁸F]RU 52461 could be demonstrated using saturating doses of unlabeled RU 52461 in rat brain. While no change was observed throughout the brain areas in adrenalectomized rats and in animals coinjected with dexamethasone, when compared to controls. PET studies revealed extremely low levels of radioactivity in baboon brain. Therefore, [¹⁸F]RU 52461 does not appear to cross the blood-brain barrier, suggesting that this radiopharmaceutical is not suitable to visualize the brain glucocorticoid binding sites by PET.     

Synthesis and in vivo evaluation of [18F]-4-[5-(4-methylphenyl)-3-(trifluoromethyl)-1H-pyrazol-1-yl]benzenesulfonamide as a PET imaging probe for COX-2 expression

Synthesis of [18F]4-[5-(4-methylphenyl)-3-(trifluoromethyl)-1H-pyrazol-1-yl]benzenesulfonamide ([18F]celecoxib), a selective COX-2 inhibitor, is achieved via a bromide to [18F]F- exchange reaction. Synthesis of the precursor for radiolabeling was achieved from 4'-methylacetophenone in four steps with 22% overall yield. Under non-radioactive conditions, fluorination was achieved using TBAF in DMSO at 135 degrees C in 80% yield. Synthesis of [18F]celecoxib was achieved using [18F]TBAF in DMSO at 135 degrees C in 10+/-2% yield (EOS) with >99% chemical and radiochemical purities. The specific activity was 120+/-40 mCi/micromol (EOB). [18F]celecoxib was found to be stable in ethanol, however, de[18F]fluorination (6.5%) was observed after 4 h in 10% ethanol-saline solution. Rodent PET studies show bone labeling indicating in vivo de[18F]fluorination of [18F]celecoxib. PET studies in baboon indicated a lower rate of de[18F]fluorination than rat and retention of radioactivity in brain regions consistent with the known distribution of COX-2. A radiolabeling method that can generate consistent high specific activity is needed for routine human use.     

Biodistribution of N-alkyl and N-fluoroalkyl derivatives of spiroperidol; Radiopharmaceuticals for PET studies of dopamine receptors

There is great interest in the application of positron labeled ligands to map the dopamine receptor in vivo. A series of fluorine-18-labeled N-alkyl and N-fluoroalkyl spiroperidol (SP) derivatives N-methyl[¹⁸F]SP; N-ethyl[¹⁸F]SP; N-(2-[¹⁸F]fluoroethyl)SP; N-propyl[¹⁸F]SP; N-(3-[¹⁸F]fluoropropyl)SP; N-(3-fluoropropyl) [¹⁸F]SP; N-(2-[¹⁸F]fluoropropyl)SP; N-(2-[¹⁸F]fluorobutyl)SP; N-(2-[¹⁸F]fluoropentyl)SP; and N-(2-[¹⁸F]fluorohexyl) SP were synthesized. The lipophilicity of these ligands (log octanol/water partition coefficient) varies from 2.67 to 5.56 and the initial brain uptake in rats, measured at 2 min, was greatest with the methyl, ethyl, propyl, fluoroethyl, and fluoropropyl derivatives. The highest striatum/cerebellum values 1 h after administration were obtained with the N-methyl, N-propyl, and N-3-fluoropropyl derivatives, while that of N-2-fluoroethyl showed the greatest uptake of total activity in the brain at this time. The uptake of all these ligands in the striatum could be blocked by cold SP showing the striatal uptake to be by the dopamine receptors.     

Click synthesis and biologic evaluation of (R)- and (S)-2-amino-3-[1-(2-[18F]fluoroethyl)-1H-[1,2,3]triazol-4-yl]propanoic acid for brain tumor imaging with positron emission tomography

The (R)- and (S)-enantiomers of 2-amino-3-[1-(2-[18F]fluoroethyl)-1H-[1,2,3]triazol-4-yl]propanoic acid (4) were synthesized and evaluated in the rat 9L gliosarcoma brain tumor model using cell uptake assays, biodistribution studies, and micro-positron emission tomography (microPET). The (R)- and (S)-enantiomers of [18F]4 were radiolabeled separately using the click reaction in 57% and 51% decay-corrected yields, respectively. (S)-[18F]4 was a substrate for cationic amino acid transport and, to a lesser extent, system L transport in vitro. In vivo biodistribution studies demonstrated that (S)-[18F]4 provided higher tumor uptake and higher tumor to brain ratios (15:1 at the 30- and 60-minute time points) compared to the (R)-enantiomer (7:1 at the 30- and 60-minute time points). MicroPET studies with (S)-[18F]4 confirmed that this tracer provides good target to background ratios for both subcutaneous and intracranial 9L gliosarcoma tumors. Based on these results, the 1H-[1,2,3]triazole-substituted amino acid (S)-[18F]4 has promising PET properties for brain tumors and represents a novel class of radiolabeled amino acids for tumor imaging.     

Synthesis and evaluation of (S)-[18F]-fluoroethylcarazolol for in vivo beta-adrenoceptor imaging in the brain

The beta-adrenergic receptor ligand (S)-4-(3-(2'-[18F]-fluoroethylamino)-2-hydroxypropoxy)-carbazol ((S)-[18F]-fluoroethylcarazolol) was prepared by reaction of [18F]-fluoroethylamine with the corresponding (S)-epoxide and was evaluated in rats by studying its pharmacokinetics and its binding profile both in vitro and in vivo. In vitro, (S)-fluoroethylcarazolol binds preferentially to beta-adrenoceptors (pK(i)=9.3 for beta(1) and 9.4 for beta(2)) and has less affinity to 5HT(1A) and 5HT(1D) receptors (pK(i)=6.7 and 5.2). In vivo, standard uptake values (SUVs) up to 0.63+/-0.07 in cortical regions were found after 60 min. Metabolites (90%) appeared within 10 min in plasma, whereas, in brain 70-75% parent compound was found after 60 min. Clearance from plasma occurred within 5 min. Cerebral uptake could be blocked by 'cold' fluoroethylcarazolol in every region, except medulla. Uptake was also blocked by propranolol and pindolol, but not by WAY 100635. ICI 89406 hardly lowered [18F] levels in brain. ICI 118551 reduced uptake of [18F] in cerebellum (mainly beta(2)) by 30%. Specific binding (tissue minus medulla values) in various brain regions corresponded with those observed for [18F]-fluorocarazolol (r(2)=0.95) and with in vitro beta-adrenoceptor densities (r(2)=0.76). Autoradiography using phosphor images of (S)-[18F]-fluoroethylcarazolol in rat brain showed the characteristic binding pattern of beta-antagonists, while propranolol treatment resulted in low and homogenous uptake. Regional tissue minus medulla values corresponded with in vitro beta-adrenoceptor densities (r(2)=0.77). We conclude that (S)-[18F]-fluoroethylcarazolol is a high affinity ligand that binds specifically to cerebral beta-adrenoceptors in vivo and may be of use for beta-adrenoceptor imaging in the brain with PET.     

Synthesis and in vivo evaluation of [18F]N-(2-benzofuranylmethyl)-N'-[4-(2-fluoroethoxy)benzyl]piperazine, a novel σ1 receptor PET imaging agent

N-(2-Benzofuranylmethyl)-N'-[4-(2-fluoroethoxy)benzyl]piperazine (6, σ(1)K(i)=2.6 nM) was radiolabeled with fluorine-18 to provide a potential σ(1) receptor radioligand for use in positron emission tomography (PET). Radiofluorination of the appropriate tosylate precursor furnished [(18)F]6 with a specific activity of 45 GBq/μmol, in an average radiochemical yield of 18% and greater than 98% radiochemical purity. MicroPET imaging in Papio hamadryas baboon brain revealed [(18)F]6 uptake consistent with σ receptor distribution, and specificity for σ receptors was demonstrated in a haloperidol pre-treated animal. [(18)F]6 possesses suitable properties for PET imaging of σ(1) receptors, and further investigation of this σ(1) receptor tracer is warranted.     

Fluorine-18 radiolabelling, biodistribution studies and preliminary PET evaluation of a new memantine derivative for imaging the NMDA receptor

A synthetic method has been established for preparing [18F]1-amino-3-fluoromethyl-5-methyl-adamantane ([18F]AFA). Biodistribution of the radiotracer in mice showed high brain uptake. The peak uptake (3.7% I.D/g organ) for the brain occurred at 30 min after injection. Accumulation of radioactivity in mouse brain was consistent with the known distribution of the NMDA receptors. The binding of [18F]AFA to the phencyclidine (PCP) binding sites of the NMDA receptor complex and the sigma recognition sites in a Rhesus monkey was also examined using positron emission tomography (PET). The regional brain distribution of [18F]AFA was changed by memantine and by (+)-MK-801, indicating competition for the same binding sites. Treatment with haloperidol caused a marked reduction of radioactivity uptake in all the brain regions examined. (-)-Butaclamol, which has pharmacological specificity for sigma sites, did not have any significant effects.     

Mechanistic Positron Emission Tomography Studies of 6-[18F]Fluorodopamine in Living Baboon Heart: Selective Imaging and Control of Radiotracer Metabolism Using the Deuterium Isotope Effect

Abstract: Mechanistic positron emission tomography (PET) studies using the deuterium isotope effect and specific pharmacological intervention were undertaken to examine the behavior of 6-[¹⁸F]fluorodopamine (6-[¹⁸F]FDA; 1 ) and (?)-6-[¹⁸F]fluoronorepinephrine {(?)-6-[¹⁸F]FNE; 2 } in the baboon heart. Two regiospecifically deuterated derivatives of 6-[¹⁸F]FDA [α,α-D₂(3 ) and β,β-D₂ (4 )] were used to assess the contributions of monoamine oxidase (MAO) and dopamine β-hydroxylase, respectively, to the clearance kinetics of 6-[¹⁸F]FDA. Compound 3 showed a reduced rate of clearance, consistent with MAO-catalyzed cleavage of the α C-D bond, whereas compound 4 showed no change, indicating that cleavage of the β C-D bond is not a rate-limiting step. Pretreatment with pargyline, an MAO inhibitor, also decreased the rate of clearance. Desipramine and tomoxetine [norepinephrine (NE) uptake inhibitors], but not GBR-12909 (a dopamine uptake inhibitor), blocked the uptake of both (?)-6-[¹⁸F]FNE and 6-[¹⁸F]FDA, with (?)-6-[¹⁸F]FNE showing a higher degree of blockade. Chiral HPLC demonstrated that 6-[¹⁸F]FDA is stereoselectively converted to (?)-6-[¹⁸F]FNE in vivo in the rat heart. These studies demonstrate that (a) the more rapid clearance of 6-[¹⁸F]FDA relative to (?)-6-[¹⁸F]FNE can be largely accounted for by metabolism by MAO; (b) selective deuterium substitution can be used to protect a radiotracer from metabolism in vivo and to favor a particular pathway; (c) 6-[¹⁸F]FDA and (?)-6-[¹⁸F]FNE share the NE transporter; (d) 6-[¹⁸F]FDA is stereoselectively converted to (?)-6-[¹⁸F]FNE in vivo; and (e) the profile of radioactivity in the heart for 6-[¹⁸F]FDA is complex, probably including labeled metabolites as well as neuronal and nonneuronal uptake.     

Evaluation of [18F]VUF 5000 as a potential PET ligand for brain imaging of the histamine H3 receptor.

[18F]VUF 5000 was evaluated as a potential PET ligand for the histamine H3 receptor. In the rat a high uptake of [18F]VUF 5000 was observed in liver, lung and kidney and a low uptake in the brain. In order to explain these findings we determined the LogD(oct,7.2) of [18F]VUF 5000, studied the biodistribution in the presence of carrier VUF 5000, modified [18F]VUF 5000 chemically and studied the binding of [18F]VUF 5000 to human serum albumin. From the results of these experiments it was concluded that [18F]VUF 5000 is not suitable as a PET ligand for brain imaging of the histamine H3 receptor, since [18F]VUF 5000 hardly penetrates into the brain.     

Imaging the Dopamine Uptake Site with Ex Vivo [18F]GBR 13119 Binding Autoradiography in Rat Brain

We studied the binding of [18F]GBR 13119 (1-[[(4-[18F]fluorophenyl) (phenyl)methoxy]ethyl]-4-(3-phenylpropyl)piperazine) to rat brain with autoradiography after intravenous injection. The rank order of binding was dorsal striatum greater than nucleus accumbens = olfactory tubercle greater than substantia nigra = ventral tegmental area greater than other areas. Binding was blocked by prior injection of dopamine uptake blockers but not by injection of dopamine receptor antagonists or drugs that bind to the dialkylpiperazine site. Unilateral 6-hydroxy-dopamine lesions of dopamine neurons caused a marked decrease in striatal and nigral binding on the side of the lesion. We conclude that intravenous injection of [18F]GBR 13119 provides a useful marker of presynaptic dopamine uptake sites.     

In vivo assessment of 6-deoxy-6-[18F]fluoro-d-galactose as a PET tracer for studying galactose metabolism

The potential of 6-deoxy-6-[¹⁸F]fluoro-d-galactose (6-[¹⁸F]FdGal) as an in vivo tracer for studying galactose metabolism in tumors and liver was investigated. High uptake and rapid clearance of the radioactivity were observed in many organs of mice after i.v. injection of the tracer. d-Galactose loading did not affect liver uptake. Three experimental tumors showed a slightly higher uptake than other tissues, and rat brain tumor was clearly visualized by autoradiography. However, the radioactivity in tumors decreased rapidly. In the liver, a significant amount of the tracer was found in a galactonate form, while this oxidation was a minor metabolic pathway in the tumors. In both tumor and liver tissues, small amounts of the tracer were incorporated into macromolecular glycoconjugate via phosphate and uridylate forms as intermediate precursors. These results indicate that 6-[¹⁸F]FdGal is not suitable for studying galactose metabolism in vivo because of the low affinity of the tracer for the metabolism.     

Characterization of the nicotinic ligand 2-[18F]fluoro-3-[2(S)-2-azetidinylmethoxy]pyridine in vivo

The biodistribution of the nicotinic acetylcholine receptor (nAChR) radioligand 2-[18F]fluoro-3-[2(S)-2-azetidinylmethoxy]pyridine ([18F]fluoro-A-85380, half-life of fluorine-18 = 110 min) in selected rat brain areas was assessed in vivo. The radiotracer showed a good penetration in the brain. The regional distribution of the radioligand was consistent with the density of nAChRs determined from previous studies in vitro. Sixty minutes post-injection, the highest uptake was observed in the thalamus, (1% I.D./g tissue), an intermediate one in the frontal cortex (0.78% I.D./g tissue), and the lowest in the cerebellum (0.5% I.D./g tissue). Pretreatment with several nAChR ligands (nicotine, cytisine, epibatidine, unlabeled fluoro-A-85380) substantially reduced uptake of the radioligand in the three cerebral areas. Pretreatment with the nAChR channel blocker mecamylamine or with the muscarinic receptor antagonist dexetimide had no appreciable effect on the uptake of fluoro-A-85380. These results support the high in vivo selectivity and specificity of fluoro-A-85380. Therefore, [18F]fluoro-A-85380 may be useful for positron emission tomography study of nAChRs in humans.     

3-N-(2-[18F]-fluoroethyl)-spiperone: a novel ligand for cerebral dopamine receptor studies with pet

3-N-(2-[18F]-fluoroethyl)-spiperone [( 18F]-FESP) was synthesized at high specific activity by condensation with 2-[18F]-fluoroethyltosylate (35 TBq/mmol). In vivo binding studies in baboons by positron emission tomography exhibited regio-selective uptake in the striatum which was saturable with the cold ligand and prevented by pretreatment with (+)-butaclamol. The pharmacokinetic behaviour, i.e. the absolute uptake in tissue and the striatum-to-cerebellum ratio, was very similar to that of methylspiperone. Analysis of the radioactivity in mouse brain after administration of [18F]-FESP indicated a high in-vivo stability (greater than 90% after 210 min in the striatum). Comparative distribution studies of other N-fluoroalkylspiperones in mice suggest that FESP and the N-fluoropropyl analogue are the most potent D2 receptor ligands.     

Syntheses and binding affinities of 6-nitroquipazine analogues for serotonin transporter: Part 3. A potential 5-HT transporter imaging agent, 3-(3-[18F]fluoropropyl)-6-nitroquipazine

3-(3-[18F]Fluoropropyl)-6-nitroquipazine ([18F]FPNQ) as a 5-HT transporter imaging agents was designed, synthesized, and evaluated. FPNQ was selected due to its potent in vitro biological activity (K(i)=0.32 nM) in rat brain cortical membranes. The 18F-labeled FPNQ was prepared by reaction of the propyl mesylate as a precursor with tetra-n-butylammonium [18F]fluoride generated under NCA conditions. The precursor mesylate was synthesized from commercially available hydrocarbostyril in nine steps in 21% overall yield. The specific activity of the [18F]FPNQ determined by radioreceptor assay was 27.0 GBq/micromol. Tissue distribution studies in mice showed the highest uptake in the frontal cortex (5.79 %ID/g) at 60 min post-injection.     

Fluorine-substituted corticosteroids: Synthesis and evaluation as potential receptor-based imaging agents for positron emission tomography of the brain

We have prepared eight fluorine-substituted corticosteroids representing ligands selective for Type I and Type II corticosteroid receptor subtypes as potential imaging agents for corticosteroid receptor-containing regions of the brain. Receptor binding affinity assays show that fluorine substitution for hydroxyl or hydrogen in these steroids generally results in some reduction in affinity, with the result that the absolute affinity of these fluorine-substituted ligands for receptor is less than that typical for steroid hormones that show receptor-based, target selective uptake in vivo. Five of these compounds were prepared in fluorine-18 labeled form by a simple sulfonate ester displacement reaction, and their tissue distribution was studied in the adrenalectomized rat. There is no selective accumulation nor selective retention of the Type I selective corticosteroids (¹⁸F-RU 26752, 21-[¹⁸F]fluoroprogesterone, 21-[¹⁸F]fluoro-11β-hydroxyprogesterone) in either the brain, or other target tissues (pituitary, kidney, liver). The Type II selective corticosteroids (¹⁸F-RU 28362, ¹⁸F-triamcinolone acetonide) show uptake into the hippocampus which can be partially blocked by a competing ligand; in target tissues outside the brain, the blocking is more complete. All of the ¹⁸F-labeled compounds show considerable defluorination, evident as high bone activity levels. These results, coupled with earlier findings in the literature, suggest that radiolabeled corticosteroid receptor ligands with both greater metabolic stability and higher receptor binding affinity and selectivity are needed for imaging corticosteroid receptors in the hippocampus.     

Role of acetyl-L-carnitine in the brain: revealed by Bioradiography

To elucidate the role of acetyl-l-carnitine in the brain, we used a novel method, ‘Bioradiography,’ in which the dynamic process could be followed in living slices by use of positron-emitter labeled compounds and imaging plates. We studied the incorporation of 2-[¹⁸F]fluoro-2-deoxy-d-glucose ([¹⁸F]FDG) into rat brain slices incubated in oxygenated Krebs-Ringer solution. Under the glucose-free condition, [¹⁸F]FDG uptake rate decreased with time and plateaued within 350 min in the cerebral cortex and cerebellum, and the addition of 1 or 5 mM acetyl-l-carnitine did not alter the [¹⁸F]FDG uptake rate. When a glutaminase inhibitor, 0.5 mM 6-diazo-5-oxo-l-norleucine (DON), was added under the normal glucose condition, [¹⁸F]FDG uptake rate decreased. Acetyl-l-carnitine (1 mM), which decreased [¹⁸F]FDG uptake rate, reversed this DON-induced decrease in [¹⁸F]FDG uptake rate in the cerebral cortex. These results suggest that acetyl-l-carnitine can be used for the production of releasable glutamate rather than as an energy source in the brain.     

Synthesis and evaluation of 6-[1-(2-[(18)F]fluoro-3-pyridyl)-5-methyl-1H-1,2,3-triazol-4-yl]quinoline for positron emission tomography imaging of the metabotropic glutamate receptor type 1 in brain

The purpose of this study was to synthesize 6-[1-(2-[¹⁸F]fluoro-3-pyridyl)-5-methyl-1H-1,2,3-triazol-4-yl]quinoline ([¹⁸F]FPTQ, [¹⁸F]7a) and to evaluate its potential as a positron emission tomography ligand for imaging metabotropic glutamate receptor type 1 (mGluR1) in the rat brain. Compound [¹⁸F]7a was synthesized by [¹⁸F]fluorination of 6-[1-(2-bromo-3-pyridyl)-5-methyl-1H-1,2,3-triazol-4-yl]quinoline (7b) with potassium [¹⁸F]fluoride. At the end of synthesis, 1280-1830 MBq (n = 8) of [¹⁸F]7a was obtained with >98% radiochemical purity and 118-237 GBq/??mol specific activity using 3300-4000 MBq of [¹⁸F]F⁻. In vitro autoradiography showed that [¹⁸F]7a had high specific binding with mGluR1 in the rat brain. Biodistribution study using a dissection method and small-animal PET showed that [¹⁸F]7a had high uptake in the rat brain. The uptake of radioactivity in the cerebellum was reduced by unlabeled 7a and mGluR1-selective ligand JNJ-16259685 (2), indicating that [¹⁸F]7a had in vivo specific binding with mGluR1. Because of a low amount of radiolabeled metabolite present in the brain, [¹⁸F]7a may have a limiting potential for the in vivo imaging of mGluR1 by PET.     

Synthesis of a fluorine-18 labeled derivative of epibatidine for in vivo nicotinic acetylcholine receptor PET imaging

Epibatidine (exo-2-(2'-chloro-5'-pyridyl)-7-azabicyclo[2.2.1]heptane), a natural compound isolated from the skin of the Ecuadorian poison frog Epipedobates tricolor, is the most potent nicotinic acetylcholine receptor (nAChR) agonist reported to date. In order to visualize and quantify in vivo these receptors in human brain using Positron Emission Tomography (PET), [18F]norchlorofluoroepibatidine (exo-2-(2'-[18F]fluoro-5'-pyridyl)-7-azabicyclo[2.2.1]heptane), a fluorine-18 (t(1/2): 110 min) radiolabeled derivative of epibatidine has been designed. The corresponding 2'-bromo-, 2'-iodo- and 2'-nitro exo-2-(5'-pyridyl)-7-azabicyclo[2.2.1]heptane analogues as labeling precursors, as well as norchlorofluoroepibatidine as a reference compound have been synthesized by reductive, stereoselective, palladium-catalyzed Heck-type coupling between an N-Boc protected azanorbornene and the corresponding halopyridine. [18F]Norchlorofluoroepibatidine has been radiolabeled with fluorine-18 by nucleophilic aromatic substitution from the corresponding Boc-protected halo- and nitro precursors using [18F]FK-K222 complex in DMSO by conventional heating (at 150-180 degrees C for 10 min) or microwave activations (at 100 Watt, for 1 to 2.5 min), followed by TFA-removal of the protective group. Typically, using the microwave activation procedure, 60-80 mCi (2.22-2.96 GBq) of pure [18F]norchlorofluoroepibatidine could be obtained in less than 2 h (110-115 min) from the bromo labeling precursor, with specific radioactivities of 1.5-2.5 Ci/micromol (55.5-92.5 GBq/micromol) calculated for End of Bombardment. The preliminary PET experiments in baboon (Papio papio) with [18F]norchlorofluoroepibatidine show a high uptake and a rapid accumulation of the radiotracer into the brain within 30 min. In the thalamus, a nAChR rich area, uptake of radioactivity reached a maximum at 40 min (10% I.D./100 mL tissue). The ratio of radioactivity thalamus/cerebellum (the latter being a nAChR poor area) was 2 at 40 min and increased with time, up to 4.3 at 160 min. Its specific regiodistribution and its high ratio of specific-to-nonspecific binding confirm the ideal profile of [18F]norchlorofluoroepibatidine as a suitable radioligand for PET imaging of nAChRs in the brain.