Ecstasy-induced reduction of the availability of the brain serotonin transporter as revealed by [11C](+)McN5652-PET and the multi-linear reference tissue model: loss of transporters or artifact of tracer kinetic modelling?

In a previous positron emission tomography (PET) study with the serotonin transporter (SERT) ligand [(11)C](+)McN5652, we found protracted reduction of the availability of the brain SERT in users of the drug ecstasy. However, the multi-linear reference tissue method for the quantification of SERT availability used in this study is prone to effects of altered levels of statistical noise that could simulate reduction of SERT. The aim of the present study was to take into account this confound by re-evaluation of the data now using a modelling approach that is rather insensitive to alterations in the level of statistical noise. A total of 116 subjects (30 current, 29 former ecstasy users, 29 drug-naive, 28 polydrug controls) in whom [(11)C](+)McN5652-PET had been performed previously were re-evaluated. The equilibrium specific-to-non-specific partition coefficient V"( 3) was obtained voxel-wise by application of the simplified reference tissue method (SRTM), which provides quite unbiased results up to rather large noise levels. Voxel-based comparisons between the groups were performed using statistical parametric mapping. V"(3) was reduced in the striatum and in the thalamus in current ecstasy users. This was confirmed by volume-of-interest-based analysis. This result suggests that the ecstasy-induced reduction of SERT availability in SERT-rich brain regions reported previously indicates reduced SERT binding potential rather than being an artifact of tracer kinetic modelling. SRTM analysis did not confirm previous findings in neocortical brain areas.     

Radioligands for the study of the 5-HT transporter in vivo

Loss of 5-HT transporter (SERT) sites has been implicated in various neurodegenerative diseases and users of some amphetamine derivatives such as MDMA. Therefore, the development of suitable radioligands for neuroimaging of the SERT in the human brain is important. A large number of drugs have been labeled with 11C, 18F or (123)I over the last ten years in order to achieve such radioligands. Despite these attempts most of the compounds were found unsuitable because of low target-to-nontarget ratios. Some cocaine-derived radioligands allow SERT imaging of the human brain using positron emission tomography (PET) although they have a limited selectivity. Among the various specific 5-HT uptake inhibitors only [(123)I]iodo-nitroquipazine for single photon emission computed tomography (SPECT) and [11C](+)McN5652 for PET appear to meet the criteria of a useful radioligand. There is still a need for the development of new radioligands for SERT imaging. Advances in tracer synthetic methodologies may bring further progress in this field.     

Autoradiographic imaging of the serotonin transporter in the brain of rats and pigs using S-([18F]fluoromethyl)-(+)-McN5652.

The [18F]fluoromethyl analogue of (+)-McN5652 ([18F]FMe-McN) has recently been developed as a radioligand for imaging the neuronal serotonin transporter (SERT) with positron emission tomography (PET). We describe here the autoradiographic evaluation of [18F]FMe-McN in the brain of rats and pigs. Autoradiographic studies of [18F]FMe-McN performed on rat and pig brain in vitro showed a high accumulation of radioactivity in the regions rich in SERT, such as amygdala, hypothalamus, superficial gray layer of the superior colliculus, various nuclei of thalamus and substantia nigra. The binding of [18F]FMe-McN was reduced by citalopram, a highly selective inhibitor for SERT. Similar regional specific binding densities of [18F]FMe-McN were observed in both species. The regional distribution and specific binding of this radiotracer correlates well with the distribution and regional brain binding of [3H]citalopram. Region-to-cerebellum ratios of [18F]FMe-McN in vitro reached a maximum value of 20.6 in the rat and 14.5 in the pig. In addition, ex vivo autoradiography of the rat brain was performed 90 min after i.v. administration of [18F]FMe-McN. The highest regional uptake of [18F]FMe-McN was observed in the hypothalamic area, substantia nigra and amygdaloid area. There is a high correlation between the in vitro and in vivo binding. The region-to-cerebellum ratio in vivo reached a maximum value of 5.1 in the substantia nigra, the highest yet reported for an 18F-labelled SERT tracer in vivo in this region. Furthermore, the distribution volume of [18F]FMe-McN calculated from the PET data in various regions of the porcine brain is highly correlated with the SERT density as determined by in vitro autoradiography with [3H]citalopram. Thus, [18F]FMe-McN has a clear potential as a radiotracer for studies of the SERT distribution in man with PET.     

New PET imaging agent for the serotonin transporter: [(18)F]ACF (2-[(2-amino-4-chloro-5-fluorophenyl)thio]-N,N-dimethyl-benzenmethanamine)

A new F-18-labeled phenylthiophenyl derivative specific for imaging of serotonin transporters (SERT) in the brain by positron emission tomography (PET) is described. Fluorinated phenylthiophenyl derivative, ACF, 2-[(2-amino-4-chloro-5-fluorophenyl)thio]-N,N-dimethyl-benzenmethanamine, was prepared by first coupling 2,5-dichloro-4-nitroaniline with 2-mercapto-N,N-dimethylbenzamide. The amino group of the coupled adduct was converted to a fluoro group through a Schiemann reaction. Subsequently, a one pot reduction of both nitro and amide groups by BH(3)-tetrahydrofuran yielded the nonradioactive ACF (yield 25%). In vitro binding assays using cell membrane homogenates of LLC cells expressing SERT, dopamine transporters (DAT), or norepinephrine transporters (NET) showed excellent binding affinity and selectivity for SERT (K(i) = 0.05, 3020, and 650 nM for SERT, DAT, and NET, respectively). For preparation of the [(18)F]ACF, the NH(2) group of the initially coupled adduct was converted to the trimethylammonium salt, which was replaced by [(18)F]fluoride in the presence of Kryptofix 222 and potassium carbonate. The final product, [(18)F]ACF, was obtained after a borane and stannous chloride reduction reaction. The combined two step reaction gave a radiochemical yield of 10-15% (EOB) and a radiochemical purity of >99%. Synthesis of the novel PET tracer, [(18)F]ACF, as a probe for binding to SERT in the brain was successfully achieved. The new tracer [(18)F]ACF showed excellent brain penetration and selective localization after an iv injection in rats (brain uptake at 2, 30, 60, 120, and 240 min was 3.27, 1.28, 0.69, 0.21, and 0.06% dose/organ, respectively). The hypothalamus/cerebellum ratio at 60 min post iv injection was 3.55. This specific localization in the hypothalamus was blocked by pretreatment of (+)McN5652. This novel ligand is a potential PET tracer for in vivo evaluation of SERT in the brain.     

Quantitative PET studies of the serotonin transporter in MDMA users and controls using [11C]McN5652 and [11C]DASB.

(+/-)3,4-Methylenedioxymethamphetamine (MDMA, 'Ecstasy') is a widely used illicit drug that produces toxic effects on brain serotonin axons and axon terminals in animals. The results of clinical studies addressing MDMA's serotonin neurotoxic potential in humans have been inconclusive. In the present study, 23 abstinent MDMA users and 19 non-MDMA controls underwent quantitative positron emission tomography (PET) studies using [11C]McN5652 and [11C]DASB, first- and second-generation serotonin transporter (SERT) ligands previously validated in baboons for detecting MDMA-induced brain serotonin neurotoxicity. Global and regional distribution volumes (DVs) and two additional SERT-binding parameters (DV(spec) and DVR) were compared in the two subject populations using parametric statistical analyses. Data from PET studies revealed excellent correlations between the various binding parameters of [11C]McN5652 and [11C]DASB, both in individual brain regions and individual subjects. Global SERT reductions were found in MDMA users with both PET ligands, using all three of the above-mentioned SERT-binding parameters. Preplanned comparisons in 15 regions of interest demonstrated reductions in selected cortical and subcortical structures. Exploratory correlational analyses suggested that SERT measures recover with time, and that loss of the SERT is directly associated with MDMA use intensity. These quantitative PET data, obtained using validated first- and second-generation SERT PET ligands, provide strong evidence of reduced SERT density in some recreational MDMA users.     

5-substituted derivatives of 6-halogeno-3-((2-(S)-azetidinyl)methoxy)pyridine and 6-halogeno-3-((2-(S)-pyrrolidinyl)methoxy)pyridine with low picomolar affinity for alpha4beta2 nicotinic acetylcholine receptor and wide range of lipophilicity: potential probes for imaging with positron emission tomography

Potential positron emission tomography (PET) ligands with low picomolar affinity at the nicotinic acetylcholine receptor (nAChR) and with lipophilicity (log D) ranging from -1.6 to +1.5 have been synthesized. Most members of the series, which are derivatives of 5-substituted-6-halogeno-A-85380, exhibited a higher binding affinity at alpha4beta2-nAChRs than epibatidine. An analysis, by molecular modeling, revealed an important role of the orientation of the additional heterocyclic ring on the binding affinity of the ligands with nAChRs. The existing nicotinic pharmacophore models do not accommodate this finding. Two compounds of the series, 6-[(18)F]fluoro-5-(pyridin-3-yl)-A-85380 ([(18)F]31) and 6-chloro-3-((2-(S)-azetidinyl)methoxy)-5-(2-[(18)F]fluoropyridin-5-yl)pyridine) ([(18)F]35), were radiolabeled with (18)F. Comparison of PET data for [(18)F]31 and 2-[(18)F]FA shows the influence of lipophilicity on the binding potential. Our recent PET studies with [(18)F]35 demonstrated that its binding potential values in Rhesus monkey brain were ca. 2.5 times those of 2-[(18)F]FA. Therefore, [(18)F]35 and several other members of the series, when radiolabeled, will be suitable for quantitative imaging of extrathalamic nAChRs.     

Synthesis and in vivo evaluation of halogenated N,N-dimethyl-2-(2'-amino-4'-hydroxymethylphenylthio)benzylamine derivatives as PET serotonin transporter ligands

N, N-dimethyl-2-(2'-amino-4'-hydroxymethylphenylthio)benzylamine (38), substituted on ring A, was reported to display high binding affinity and selectivity to the human brain serotonin transporter (SERT). In an attempt to explore the potential of compounds substituted on ring B of the phenylthiophenyl core structure, three derivatives of 38 were synthesized: N, N-dimethyl-2-(2'-amino-4'-hydroxymethyl-phenylthio)-5-fluorobenzylamine (35), N, N-dimethyl-2-(2'-amino-4'-hydroxymethyl-phenylthio)-5-bromobenzylamine (36), and N, N-dimethyl-2-(2'-amino-4'-hydroxymethyl-phenylthio)-5-iodobenzylamine (37). The in vitro binding studies in cells transfected with human SERT, norepinephrine transporter (NET), and dopamine transporter (DAT) showed that 35, 36, and 37 exhibited high SERT affinity with K is (SERT) = 1.26, 0.29, and 0.31 nM (vs [(3)H]citalopram), respectively. [(11)C]-(35), [(11)C]-(36), and [(11)C]-( 37) were prepared by methylation of their monomethyl precursors 16, 17, and 18, with [(11)C]iodomethane in 28, 11, and 14% radiochemical yields, respectively. The microPET images of [(11)C]-(35), [(11)C]-(36), and [(11)C]-(37) showed high uptake in the monkey brain regions rich in SERT with peak midbrain to cerebellum ratios of 3.41, 3.24, and 3.00 at 85 min post-injection, respectively. In vivo bindings of [(11)C]-(35), [(11)C]-(36), and [(11)C]-(37) were shown to be specific to the SERT as displacement with citalopram (a potent SERT ligand) reduced radioactivity in SERT-rich regions to the cerebellum level. These results suggest that [(11)C]-(35), [(11)C]-(36), and [(11)C]-(37) could be potential agents for mapping human SERT by PET and radiolabeling 37 with iodine-123, which could afford the first SPECT SERT imaging agent exhibiting fast kinetics.     

Development of a new radioligand, N-(5-fluoro-2-phenoxyphenyl)-N-(2-[18F]fluoroethyl-5-methoxybenzyl)acetamide, for pet imaging of peripheral benzodiazepine receptor in primate brain

To develop a positron emission tomography (PET) ligand for imaging the 'peripheral benzodiazepine receptor' (PBR) in brain and elucidating the relationship between PBR and brain diseases, four analogues (4-7) of N-(2,5-dimethoxybenzyl)-N-(5-fluoro-2-phenoxyphenyl)acetamide (2) were synthesized and evaluated as ligands for PBR. Of these compounds, fluoromethyl (4) and fluoroethyl (5) analogues had similar or higher affinities for PBR than the parent compound 2 (K(i) = 0.16 nM for PBR in rat brain sections). Iodomethyl analogue 6 displayed a moderate affinity, whereas tosyloxyethyl analogue 7 had weak affinity. Radiolabeling was performed for the fluoroalkyl analogues 4 and 5 using fluorine-18 ((18)F, beta(+); 96.7%, T(1/2) = 109.8 min). Ligands [(18)F]4 and [(18)F]5 were respectively synthesized by the alkylation of desmethyl precursor 3 with [(18)F]fluoromethyl iodide ([(18)F]8) and 2-[(18)F]fluoroethyl bromide ([(18)F]9). The distribution patterns of [(18)F]4 and [(18)F]5 in mice were consistent with the known distribution of PBR. However, compared with [(18)F]5, [(18)F]4 displayed a high uptake in the bone of mice. The PET image of [(18)F]4 for monkey brain also showed significant radioactivity in the bone, suggesting that this ligand was unstable for in vivo defluorination and was not a useful PET ligand. Ligand [(18)F]5 displayed a high uptake in monkey brain especially in the occipital cortex, a region with richer PBR than the other regions in the brain. The radioactivity level of [(18)F]5 in monkey brain was 1.5 times higher than that of [(11)C]2, and 6 times higher than that of (R)-(1-(2-chlorophenyl)-N-[(11)C]methyl,N-(1-methylpropyl)isoquinoline ([(11)C]1). Moreover, the in vivo binding of [(18)F]5 was significantly inhibited by PBR-selective 2 or 1, indicating that the binding of [(18)F]5 in the monkey brain was mainly due to PBR. Metabolite analysis revealed that [(18)F]4 was rapidly metabolized by defluorination to [(18)F]F(-) in the plasma and brain of mice, whereas [(18)F]5 was metabolized by debenzylation to a polar product [(18)F]13 only in the plasma. No radioactive metabolite of [(18)F]5 was detected in the mouse brain. The biological data indicate that [(18)F]5 is a useful PET ligand for PBR and is currently used for imaging PBR in human brain.     

Synthesis and evaluation of novel radioligands for positron emission tomography imaging of metabotropic glutamate receptor subtype 1 (mGluR1) in rodent brain

We designed three novel positron emission tomography ligands, N-(4-(6-(isopropylamino)pyrimidin-4-yl)-1,3-thiazol-2-yl)-4-[(11)C]methoxy-N-methylbenzamide ([(11)C]6), 4-[(18)F]fluoroethoxy-N-[4-[6-(isopropylamino)pyrimidin-4-yl]-1,3-thiazol-2-yl]-N-methylbenzamide ([(18)F]7), and 4-[(18)F]fluoropropoxy-N-[4-[6-(isopropylamino)pyrimidin-4-yl]-1,3-thiazol-2-yl]-N-methylbenzamide ([(18)F]8), for imaging metabotropic glutamate receptor type 1 (mGluR1) in rodent brain. Unlabeled compound 6 was synthesized by benzoylation of 4-pyrimidinyl-2-methylaminothiazole 10, followed by reaction with isopropylamine. Removal of the methyl group in 6 gave phenol precursor 12 for radiosynthesis. Two fluoroalkoxy analogues 7 and 8 were prepared by reacting 12 with tosylates 13 and 14. Radioligands [(11)C]6, [(18)F]7, and [(18)F]8 were synthesized by O-[(11)C]methylation or [(18)F]fluoroalkylation of 12. Compound 6 showed high in vitro binding affinity for mGluR1, whereas 7 and 8 had weak affinity. Autoradiography using rat brain sections showed that [(11)C]6 binding is aligned with the reported distribution of mGluR1 with high specific binding in the cerebellum and thalamus. PET study with [(11)C]6 in rats showed high brain uptake and a similar distribution pattern to that in autoradiography, indicating the usefulness of [(11)C]6 for imaging brain mGluR1.     

2-11C]isopropyl-, [1-11C]ethyl-, and [11C]methyl-labeled phenoxyphenyl acetamide derivatives as positron emission tomography ligands for the peripheral benzodiazepine receptor: radiosynthesis, uptake, and in vivo binding in brain

The peripheral benzodiazepine receptor (PBR) is widely expressed in peripheral tissues, blood cells, and in glia cells in the brain. We have previously developed two positron emission tomography (PET) ligands, N-(2-[(11)C],5-dimethoxybenzyl)-N-(5-fluoro-2-phenoxyphenyl)acetamide ([(11)C]2) and its [(18)F]fluoroethyl analogue ([(18)F]6), for the current investigation of PBR in the human brain. The aim of this study was to label the potent PBR agonist N-(4-chloro-2-phenoxyphenyl)-N-(isopropoxybenzyl)acetamide (3) and its ethyl (7) and methyl (8) homologues with (11)C and to evaluate them as PET ligands for PBR with mice, rats, and monkeys. Ligands [(11)C]3, [(11)C]7, and [(11)C]8 were synthesized by alkylation of phenol precursor 9 with 2-[2-(11)C]iodopropane ([(11)C]10), [1-(11)C]iodoethane ([(11)C]11), and [(11)C]iodomethane ([(11)C]12), respectively. The alkylating agent [(11)C]10 or [(11)C]11 was prepared by reacting CH(3)MgBr with [(11)C]CO(2), followed by reduction with LiAlH(4) and iodination with HI. In vitro quantitative autoradiography determined that 3, 7, and 8 had potent binding affinities (K(i) = 0.07-0.19 nM) for PBR in the rat brain. These [(11)C]ligands could pass across the blood-brain barrier and enter the rat brain (0.17-0.32% of injected dose per gram wet tissue). Ex vivo autoradiography showed that the [(11)C]ligands preferably distributed in the olfactory bulb and cerebellum, two regions with richer PBR density in the rat brain. The co-injection of PBR-selective 2 reduced the [(11)C]ligand binding in the two regions, suggesting that binding in the rat brain was specific to PBR. PET study determined that the [(11)C]ligands preferably accumulate in the occipital cortex of the monkey brain, a region with a high density of PBR in the primate brain. Moreover, in vivo binding of the methyl homologue [(11)C]8 in the monkey brain could be inhibited by PBR-selective 2 or 1, indicating that some of the [(11)C]8 binding was due to PBR. Metabolite analysis demonstrated that these [(11)C]ligands were metabolized by debenzylation to polar products mainly in the plasma.     

Serotonin transporter binding after recovery from eating disorders

RATIONALE: Several lines of evidence suggest that altered serotonin (5-HT) function persists after recovery from anorexia nervosa (AN) and bulimia nervosa (BN). OBJECTIVES: We compared 11 subjects who recovered (>1 year normal weight, regular menstrual cycles, no binging or purging) from restricting-type AN (REC RAN), 7 who recovered from bulimia-type AN (REC BAN), 9 who recovered from BN (REC BN), and 10 healthy control women (CW). MATERIALS AND METHODS: Positron emission tomography (PET) imaging with [11C]McN5652 was used to assess the 5-HT transporter (5-HTT). For [11C]McN5652, distribution volume (DV) values were determined using a two-compartment, three-parameter tracer kinetic model, and specific binding was assessed using the binding potential (BP, BP=DVregion of interest/DVcerebellum-1). RESULTS: After correction for multiple comparisons, the four groups showed significant (p<0.05) differences for [11C]McN5652 BP values for the dorsal raphe and antero-ventral striatum (AVS). Post-hoc analysis revealed that REC RAN had significantly increased [11C]McN5652 BP compared to REC BAN in these regions. CONCLUSIONS: Divergent 5-HTT activity in subtypes of eating disorder subjects may provide important insights as to why these groups have differences in affective regulation and impulse control.     

Occupancy of brain serotonin transporters during treatment with paroxetine in patients with social phobia: a positron emission tomography study with 11C McN 5652

Abstract Rationale. Although selective serotonin reuptake inhibitors (SSRIs) are widely used in the treatment of anxiety and depressive disorders, the occupancy of the serotonin reuptake transporter (SERT) achieved in humans at typical clinical doses by these agents remains poorly characterized. Objective. The purpose of this study was to determine the occupancy of the SERT achieved in vivo by the SSRI paroxetine in social phobia patients at typical antianxiety doses. Methods. Measures of SERT availability were obtained with positron emission tomography and the SERT radiotracer [¹¹C](+)-McN 5652 in five patients with social phobia before and during treatment with paroxetine at usual therapeutic doses (20–40 mg per day). Results. Occupancy of the SERT by paroxetine was high in all subjects and in all regions measured after 3–6 months of continuous treatment. Conclusions. The results of this study in an anxiety disorder sample are consistent with previously reported results in a depressed sample and suggest that paroxetine at therapeutic doses achieves very high occupancy levels of the SERT. Electronic Publication     

Discovery of (-)-7-methyl-2-exo-[3'-(6-[18F]fluoropyridin-2-yl)-5'-pyridinyl]-7-azabicyclo[2.2.1]heptane, a radiolabeled antagonist for cerebral nicotinic acetylcholine receptor (alpha4beta2-nAChR) with optimal positron emission tomography imaging properties

Several isomers of 7-methyl-2-exo-([(18)F]fluoropyridinyl-5'-pyridinyl)-7-azabicyclo[2.2.1]heptane have been developed as radioligands with optimized brain kinetics for PET imaging of nAChR. The binding assay demonstrated that all isomers are beta-nAChR selective ligands with Ki = 0.02-0.3 nM. The experimental lipophilicity values of all isomers were in the optimal range for the cerebral radioligands (log D7.4= 0.67-0.99). The isomers with higher binding affinity manifested slow baboon brain kinetics, whereas the isomer with the lowest binding affinity (Ki = 0.3 nM) ((-)-7-methyl-2- exo-[3'-(6-[(18)F]fluoropyridin-2-yl)-5'-pyridinyl]-7-azabicyclo[2.2.1]heptane, [(18)F](-)-6c) and greatest lipophilicity (log D 7.4 = 0.99) exhibited optimal brain kinetics. [(18)F](-)-6c manifests a unique combination of the optimally rapid brain kinetics, high BP and brain uptake, and favorable metabolic profile. Pharmacological studies showed that (-)-6c is an alpha4beta2-nAChR antagonist with low side effects in mice. This combination of imaging properties suggests that [(18)F]-(-)- 6c is a potentially superior replacement for 2-[(18)F]fluoro-A-85380 and 6-[(18)F]fluoro-A-85380, the only available nAChR PET radioligands for humans.     

Pharmacodynamics of radiolabelled anticancer drugs for positron emission tomography

Positron Emission Tomography (PET) offers an exciting opportunity to monitor key pathways involved in malignant transformation due to the ability to radiolabel and image the behaviour of biological probes. In this review, we will describe how PET can use various radiolabelled compounds to monitor various targets including ligand-receptor interactions using 16alpha-[(18)F]fluoro-17beta-oestradiol (FES) pathways involved in metabolism with [(18)F]fluorodeoxy-glucose ([(18)F]FDG), (11)C-methyl-choline for signal transduction, cell cycle and proliferation with 2-[(11)C]thymidine, cell death using [(124)I]annexin V, [(124)C]colchicine for drug resistance and angiogenesis using [(124)I]anti-VEGF.     

Radiosynthesis and biological evaluation of L- and D-S-(3-[18F]fluoropropyl)homocysteine for tumor imaging using positron emission tomography

Interest in radiolabeled amino acids for metabolic imaging of cancer and limitations with [(11)C]methionine has prompted the development of a new (18)F-labeled methionine derivative S-(3-[(18)F]fluoropropyl)homocysteine ([(18)F]FPHCys). The L and D enantiomers of [(18)F]FPHCys were prepared from their respective protected S-(3-tosyloxypropyl)homocysteine precursors 1 by [(18)F]fluoride substitution using K(2.2.2) and potassium oxalate, followed by acid hydrolysis on a Tracerlab FX(FN) synthesis module. [(18)F]-L-FPHCys and [(18)F]-D-FPHCys were isolated in 20 ± 5% radiochemical yield and >98% radiochemical and enantiomeric purity in 65 min. Competitive uptake studies in A375 and HT29 tumor cells suggest that L- and D-[(18)F]FPHCys are taken up by the L-transporter system. [(18)F]-L-FPHCys and [(18)F]-D-FPHCys displayed good stability In Vivo without incorporation into protein at least 2 h postinjection. Biodistribution studies demonstrate good uptake in A375 tumor-bearing rodents with tumor to blood ratios of 3.5 and 5.0 for [(18)F]-L-FPHCys and [(18)F]-D-FPHCys, respectively, at 2 h postinjection.     

Novel radiotracers for imaging the serotonin transporter by positron emission tomography: synthesis, radiosynthesis, and in vitro and ex vivo evaluation of (11)C-labeled 2-(phenylthio)araalkylamines

A series of four 2-(phenylthio)araalkylamines have been radiolabeled with (11)C and evaluated as potential radiotracers for imaging the serotonin transporter (SERT) by positron emission tomography (PET). All four candidates display high affinity for SERT and low affinity for the dopamine or norepinephrine transporters using in vitro binding assays. Biodistribution studies in rats demonstrated that tail-vein injection of the (11)C-labeled radiotracers resulted in high brain uptake of radioactivity with a preferential distribution in brain regions known to be rich in SERT such as hypothalamus and thalamus. The most promising candidate, 16, had hypothalamus-to-cerebellum ratios of 9:1, 1 h postinjection, an indication of high specific to nonspecific binding. Ex vivo pharmacological studies demonstrated that uptake in SERT-rich brain regions was both saturable and selective for SERT. Two of the tested radiotracers, 15 and 16, have highly favorable properties for imaging SERT and will be used in pilot human PET imaging studies.     

Synthesis, biodistribution, and primate imaging of fluorine-18 labeled 2beta-carbo-1'-fluoro-2-propoxy-3beta-(4-chlorophenyl)tr opanes. Ligands for the imaging of dopamine transporters by positron emission tomography

2beta-(R)-Carbo-1-fluoro-2-propoxy-3beta-(4-chlorophenyl) tro pane ((R)-FIPCT, R-6) and 2beta-(S)-carbo-1-fluoro-2-propoxy-3beta-(4-chlorophenyl) tro pane ((S)-FIPCT, S-6) were prepared and evaluated in vitro and in vivo for dopamine transporter (DAT) selectivity and specificity. High specific activity [(18)F](R)-FIPCT and [(18)F](S)-FIPCT were synthesized in 5% radiochemical yield (decay-corrected to end of bombardment (EOB)) by preparation of the precursors 2beta-carbo-R-1-mesyloxy-2-propoxy-3beta-(4-chlorop hen yl)tropane (R-12) and 2beta-carbo-S-1-mesyloxy-2-propoxy-3beta-(4-chlorop hen yl)tropane (S-12) followed by treatment with no carrier-added potassium[(18)F]fluoride and kyrptofix K222 in acetonitrile. Competition binding in cells stably expressing the transfected human DAT and serotonin transporter (SERT) labeled by [(3)H]WIN 35428 and [(3)H]citalopram, respectively, demonstrated the following order of DAT affinity (K(i) in nM): GBR 12909 (0.36) > CIT (0.48) > (S)-FIPCT (0.67) > (R)-FIPCT (3.2). The affinity of (S)-FIPCT and (R)-FIPCT for SERT was 127- and 20-fold lower, respectively, than for DAT. In vivo biodistribution studies were performed in male rats and demonstrated that the brain uptake of [(18)F](R)-FIPCT and [(18)F](S)-FIPCT were selective and specific for DAT rich regions (caudate and putamen). PET brain imaging studies in monkeys demonstrated high [(18)F](R)-FIPCT and [(18)F](S)-FIPCT uptake in the caudate and putamen which resulted in caudate-to-cerebellum and putamen-to-cerebellum ratios of 2.5-3.5 at 115 min. [(18)F](R)-FIPCT uptake in the caudate/putamen achieved transient equilibrium at 75 min. In an imaging experiment with [(18)F](S)-FIPCT in a rhesus monkey with its left hemisphere lesioned with MPTP, radioactivity was reduced to background in the caudate and putamen of the lesioned hemisphere. The high specific activity one-step radiolabeling preparation and high specificity and selectivity of [(18)F](R)-FIPCT and [(18)F](S)-FIPCT for DAT indicate [(18)F](R)-FIPCT and [(18)F](S)-FIPCT are potential radioligands for mapping brain DAT in humans using PET.     

Study on the highly efficient synthesis and pharmaceutical evaluation of PET radiopharmaceuticals for the clinical application

Positron Emission Tomography (PET) is an advanced non-invasive technology used in the field of nuclear medicine for clinical diagnosis using radiotracers labeled with short-lived positron emitting radionuclides such as (11)C (half-life: 20.4 min), (13)N, (15)O and (18)F. The present study describes an efficient rapid synthesis method for [(11)C]Phosgene ([(11)C]COCl(2)) which is an important potential precursor for preparation of PET radiopharmaceuticals. Catalytic oxidation of [(11)C]CCl(4) using Fe(2)O(3) powder mixed with Fe granules as an oxidizing agent was newly accomplished with a development of fully automated synthetic apparatus. Utilization of produced [(11)C]COCl(2) provided a substantial synthesis of [2-(11)C]thymine as a key intermediate for preparation of [2-(11)C]thymidine, a PET tracer to evaluate cellular proliferation. Direct ring closure reaction of the alkali metal salt of beta-(N-benzoyl-amino)methacrylamide with [(11)C]COCl(2) readily proceeded under mild conditions to afford [2-(11)C]thymine in fair yield reproducibly. By way of further application, a useful PET ligand for beta-adrenoreceptors, S-(-)-[(11)C]CGP-12177 (CGP) was synthesized in markedly high yield with high specific activity and radiochemical purity. CGP for intravenous injection was prepared in 25 min after EOB with a yield of 1.5+/-0.2 GBq. These results of quality control tests demonstrated that CGP preparation is suitable for routine clinical use. Thus, CGP-PET study has been newly added to clinical PET for cardiac functional investigation in Hokkaido University Hospital.     

The discovery of epidepride and its analogs as high-affinity radioligands for imaging extrastriatal dopamine D(2) receptors in human brain

[(123)I]Epidepride, [(18)F]fallypride, and [(76)Br]isoremoxipride (FLB-457) and their corresponding [(11)C]labeled derivatives belong to a class of high-affinity radioligands for SPECT or PET imaging of dopamine D(2) receptors in the human brain. In contrast to previously used imaging agents, these ligands are capable of identifying extrastriatal dopamine D(2) receptors. The design of these substituted benzamides derive its origin from the atypical antipsychotic agent, remoxipride. Starting in the late 1970's, halogenated analogs of (S)-sulpiride were evaluated in binding assays and behavioral studies, leading to the discovery of remoxipride. Remoxipride was 10 times weaker than sulpiride in vitro but 50 times more potent in vivo. Search for a putative active metabolite of remoxipride led to the discovery of raclopride and eticlopride, the former becoming a useful radioligand as tritium or carbon-11 labeled form for receptor binding and PET studies, respectively. In the US, the mono-iodine analog of raclopride, [(123)I]iodobenzamide (IBZM), was found to have moderate putamen-to-cerebellum ratio in rat and human brain. Continued search for metabolites of remoxipride led to the discovery of its 3,6-dihydroxy derivative, NCQ-344, with an extremely potent in vivo activity in the rat. SAR studies of the metabolites of remoxipride led to the discovery of the 3-methoxy isomer, isoremoxipride (FLB-457) and its corresponding 6-hydroxy analog, FLB-463, both having affinities for the dopamine D(2) receptor in the 20-30 pM range. Later, the 5-[(123)I]iodo analog of FLB-463, [(123)I]ioxipride ([(123)I]NCQ-298), became a potential SPECT imaging agent. In the mean time, the deshydroxy analog of IBZM, [(125)I]iodopride, showed binding potential in the rat similar to [(125)I]IBZM. Epidepride was designed by combining the structure of isoremoxipride with that of iodopride. In 1988, epidepride was independently prepared and radiolabeled in three separate laboratories in Stockholm, Berkeley, and Nashville. Evaluation of seven [(125)I]iodine substituted analogs of raclopride, including IBZM, revealed the unusual high striatum-to-cerebellum ratio of 234 of [(125)I]epidepride in the rat. Subsequent SPECT images with [(123)I]epidepride demonstrated its ability to identify extrastriatal dopamine D(2) receptors in the human brain. Exploration of the structure of epidepride confirmed its exceptional properties, to be exceeded only by its N-allyl homolog, [(125)I]nalepride. The design by others of a series of potent 5-(3-[(18)F]fluoropropyl) substituted analogs of epidepride for PET imaging, lead to the discovery of [(18)F]fallypride. By elucidating the role of lipophilicity in the substituted benzamides, the excellent imaging characteristics of [(11)C]/[(123)I]epidepride, [(11)C]/[(76)Br]isoremoxipride and [(18)F]fallypride, could not only be explained but predicted with remarkable accuracy. By using the inverse product of the receptor affinity (K(D)), and the apparent partition constant of the radioligand (P((7.4))), estimates of maximal binding potential of any radioligand for imaging of any neurotransmitter receptor or transporter site seem possible.     

In vivo positron emission tomography (PET) imaging with an alphavbeta6 specific peptide radiolabeled using 18F-"click" chemistry: evaluation and comparison with the corresponding 4-[18F]fluorobenzoyl- and 2-[18F]fluoropropionyl-peptides

Numerous radiolabeled peptides have been utilized for in vivo imaging of a variety of cell surface receptors. For applications in PET using [(18)F]fluorine, peptides are radiolabeled via a prosthetic group approach. We previously developed solution-phase (18)F-"click" radiolabeling and solid-phase radiolabeling using 4-[(18)F]fluorobenzoic and 2-[(18)F]fluoropropionic acids. Here we compare the three different radiolabeling approaches and report the effects on PET imaging and pharmacokinetics. The prosthetic groups did have an effect; metabolites with significantly different polarities were observed.