Synthesis and characterization of iodine-123 labeled 2beta-carbomethoxy-3beta-(4'-((Z)-2-iodoethenyl)phenyl)nortropane. A ligand for in vivo imaging of serotonin transporters by single-photon-emission tomography

2beta-Carbomethoxy-3beta-(4'-((Z)-2-iodoethenyl)phenyl)nortropane (ZIENT) (6) and 2beta-carbomethoxy-3beta-(4'-((E)-2-iodoethenyl)phenyl)nortropane (EIENT) (10) were prepared and evaluated in vitro and in vivo for serotonin transporter (SERT) selectivity and specificity. High specific activity [(123)I]ZIENT and [(123)I]EIENT were synthesized in 45% (n = 5) and 42% (n = 4) radiochemical yield (decay-corrected to end of bombardment (EOB)), respectively, by preparation of the precursor carbomethoxy-3beta-(4'-((Z)-2-trimethylstannylethenyl)phenyl)nortropane (7) and 2beta-carbomethoxy-3beta-(4'-((E)-2-tributylstannylethenyl)phenyl)nortropane (9), respectively, followed by treatment with no carrier-added sodium [(123)I]iodide and hydrogen peroxide in ethanolic HCl. Competition binding in cells stably expressing the transfected human SERT, dopamine transporter (DAT), and norepinephrine transporter (NET) using [(3)H]citalopram, [(3)H]WIN 35,428, and [(3)H]nisoxetine, respectively, demonstrated the following order of SERT affinity (K(i) in nM): ZIENT (0.05) > nor-CIT (0.12) > EIENT (1.15) > fluvoxamine (1.46). The affinity of ZIENT and EIENT for DAT was 69 and 1.6-fold lower, respectively, than for SERT. In vivo biodistribution and blocking studies were performed in male rats and demonstrated that the brain uptake of [(123)I]ZIENT was selective and specific for SERT-rich regions (hypothalamus, striatum, pons, and prefrontal cortex). SPECT brain imaging studies in monkeys demonstrated high [(123)I]ZIENT uptake in the diencephalon, which resulted in diencephalon-to-cerebellum ratios of 2.12 at 190 min. [(123)I]ZIENT uptake in the diencephalon achieved transient equilibrium at 157 min. In a displacement experiment of [(123)I]ZIENT in a cynomolgus monkey, radioactivity was reduced by 39% in the diencephalon at 101 min following injection of citalopram. The high specific activity one-step radiolabeling preparation and high selectivity of [(123)I]ZIENT for SERT support its candidacy as a radioligand for mapping brain SERT sites.     

Synthesis, radiosynthesis, and biological evaluation of carbon-11 and iodine-123 labeled 2beta-carbomethoxy-3beta-[4'-((Z)-2-haloethenyl)phenyl]tropanes: candidate radioligands for in vivo imaging of the serotonin transporter

2beta-Carbomethoxy-3beta-[4'-((Z)-2-iodoethenyl)phenyl]tropane (ZIET) and 2beta-carbomethoxy-3beta-[4'-((Z)-2-bromoethenyl)phenyl]tropane (ZBrET) were synthesized as well as their nortropane congeners ZIENT and ZBrENT. Binding affinities of these compounds were determined in cells transfected to express human SERT, DAT, and NET using [3H]citalopram, [125I]RTI-55, and [3H]nisoxetine, respectively. Both ZIET and ZBrET displayed high affinity for the SERT (Ki = 0.11 and 0.08 nM, respectively).The affinities of ZIET and ZBrET for the DAT were 200 and 38-fold lower, respectively, than for the SERT. [11C]ZIET and [11C]ZBrET were prepared by alkylation of their corresponding nortropanes with [11C]methyl iodide in approximately 30% radiochemical yield (decay-corrected to end of bombardment, EOB). High specific activity [123I]ZIET was synthesized in 33% radiochemical yield (decay-corrected) by treating the 2beta-carbomethoxy-3beta-[4'-((Z)-2-trimethylstannylethenyl)phenyl]tropane (3) with no carrier-added sodium [123I]iodide and hydrogen peroxide in ethanolic HCl. Biodistribution studies in rats indicated that [123I]ZIET enters the brain readily and accumulates in SERT-rich regions. Blocking studies performed in rats demonstrated that [123I]ZIET was selective and specific for SERT-rich regions (e.g. thalamus, brainstem, and striatum). MicroPET brain imaging studies in monkeys demonstrated that [11C]ZIET and [11C]ZBrET uptakes were selectivity localized in the putamen, midbrain, caudate, thalamus, pons, and medulla. Radioactivity in the regions of high SERT density of monkey brain was displaceable with citalopram except in the putamen and caudate. Radioactivity uptake in these DAT-rich regions was significantly displaceable either by preadministration of citalopram followed by injection of RTI-113 (or vice-versa) or by administration of a mixture of DAT and SERT ligands. In conclusion, the high yield, high specific activity, one-step radiolabeling method, high selectivity and favorable kinetics, and the good results obtained with [123I]ZIET in rats support the candidacy of [11C]ZIET for in vivo visualization and quantification of brain SERT.     

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.     

Synthesis, radiosynthesis, and biological evaluation of carbon-11 labeled 2beta-carbomethoxy-3beta-(3'-((Z)-2-haloethenyl)phenyl)nortropanes: candidate radioligands for in vivo imaging of the serotonin transporter with positron emission tomography

2beta-carbomethoxy-3beta-(3'-((Z)-2-iodoethenyl)phenyl)nortropane (mZIENT, 1) and 2beta-carbomethoxy-3beta-(3'-((Z)-2-bromoethenyl)phenyl)nortropane (mZBrENT, 2) were synthesized and evaluated for binding to the human serotonin, dopamine, and norepinephrine transporters (SERT, DAT, and NET, respectively) using transfected cells. Both 1 and 2 have a high affinity for the SERT (Ki=0.2 nM) and are approximately 160 times more selective for the SERT than the DAT. Compound 2 has a significantly higher affinity for the NET than 1, and this may be a result of the different size and electronegativity of the halogen atoms. MicroPET imaging in nonhuman primates with [11C]1 and [11C]2 demonstrated that both tracers behave similarly in vivo with high uptake being observed in the SERT-rich brain regions and peak uptake being achieved in about 55 min postinjection. Chase studies with citalopram and methylphenidate demonstrated that this uptake is the result of preferential binding to the SERT.     

Synthesis and in vivo evaluation of fluorine-18 and iodine-123 labeled 2beta-carbo(2-fluoroethoxy)-3beta-(4'-((Z)-2-iodoethenyl)phenyl)nortropane as a candidate serotonin transporter imaging agent

2Beta-carbo(2-fluoroethoxy)-3beta-(4'-((Z)-2-iodoethenyl)phenyl)nortropane (betaFEpZIENT, 1) was synthesized as a serotonin transporter (SERT) imaging agent for both positron emission tomography (PET) and single photon emission computerized tomography (SPECT). The binding affinity of 1 to human monoamine transporters showed a high affinity for the SERT (Ki = 0.08 nM) with respect to the dopamine transporter (DAT) (Ki = 13 nM) and the norepinephrine transporter (NET) (Ki = 28 nM). In vivo biodistribution and blocking studies performed in male rats demonstrated that [123I]1 was selective and specific for SERT. In vivo microPET brain imaging studies in an anesthetized monkey with [18F]1 showed high uptake in the diencephalon and brainstem with peak uptake achieved at 120 min. A chase study with (R,S)-citalopram.HBr displaced [18F]1 radioactivity from all SERT-rich brain regions. A chase study with the DAT ligand 2beta-carbophenoxy-3beta-(4-chlorophenyl)tropane (9, RTI-113) failed to displace [18F]1, indicating that [18F]1 is specific to the SERT. The in vivo evaluation of [18F]1 indicates that this radiotracer is a good candidate for mapping and quantifying CNS SERT.     

Synthesis, radiosynthesis, and biological evaluation of carbon-11 and fluorine-18 (N-fluoroalkyl) labeled 2beta-carbomethoxy-3beta-(4'-(3-furyl)phenyl)tropanes and -nortropanes: candidate radioligands for in vivo imaging of the serotonin transporter with positron emission tomography

2beta-Carbomethoxy-3beta-(4'-(3-furyl)phenyl)nortropane (1) was synthesized along with the N-methyl (2), N-fluoroethyl (3), N-fluoropropyl (4), and N-fluorobutyl (5) derivatives. The binding affinity for each compound to the human serotonin, dopamine, and norepinephrine transporters was determined using transfected HEK-293 cells. Radiolabeling and microPET brain imaging studies were performed with [(11)C]1, [(11)C]2, and [(18)F]3 to determine their utility as in vivo imaging agents.     

In vivo measurement of the serotonin transporter with (S)-([18F]fluoromethyl)-(+)-McN5652.

The radiolabeled serotonin transporter (SERT) ligand [(11)C](+)-McN5652 has recently been used in clinical positron emission tomography (PET) studies for SERT imaging. However, this radioligand offers disadvantages in routine clinical settings because of its short radioisotope half-life (eg PET facilities within hospitals without a cyclotron need to acquire such radioligands from distant cyclotron units for clinical use). S-([(18)F]fluoromethyl)-(+)-McN5652 ([(18)F](+)-FMe-McN5652) is an analogue which has been synthesized newly, and has a significantly longer radioisotope half-life. In the porcine brain, it demonstrates the same characteristic distribution pattern of serotonin-uptake sites like the (11)C-labeled congener with the highest binding in the midbrain and thalamus and the lowest in the cerebellum and occipital cortex. It shows a 30% higher blood-brain transfer and a slower peripheral metabolism than [(11)C](+)-McN5652. Rather uniform brain binding was observed after injection of the pharmacologically inactive radiolabeled enantiomer, or after pretreatment with the highly selective SERT inhibitor citalopram. The norepinephrine uptake inhibitor maprotiline did not show any inhibitory effect. Using a one-tissue compartment model (K(1), k"(2)) or a two-tissue compartment model (K(1) to k(4)) with or without constraints for calculation, the regional binding parameters of [(11)C](+)-McN5652 and [(18)F](+)-FMe-McN5652 are highly correlated among each other and with the SERT density, as determined by in vitro binding of [(3)H]citalopram. Using constraints to correct for the free fraction and nonspecific binding of the radiotracers, a considerable increase of the midbrain-occipital cortex ratios with higher values for [(18)F](+)-FMe-McN5652 compared to [(11)C](+)McN5652 was revealed. It is concluded that [(18)F](+)-FMe-McN5652 has better features than [(11)C](+)McN5652 for SERT imaging with PET.     

2-(2-(dimethylaminomethyl)phenoxy)-5-iodophenylamine: an improved serotonin transporter imaging agent

Imaging serotonin transporters (SERT) is an emerging research tool potentially useful to cast light on the mechanisms of drug action as well as to monitor the treatment of depressed patients. We have prepared two new derivatives of 3, 2-(2-(dimethylaminomethyl)phenoxy)-5-iodophenylamine (4) and 2-(2-(dimethylaminomethyl)benzyl)-5-iodophenylamine (5) (K(i) for SERT = 0.37 and 48.6 nM, respectively). Both [(125)I]4 and [(125)I]5 displayed excellent brain uptakes in rats, and they showed a highest uptake in hypothalamus (between 60 and 240 min), a region populated with the highest density of SERT. The specific uptake of [(125)I]4 in the hypothalamus resulted in a target to nontarget ratio ([hypothalamus-cerebellum]/cerebellum) of 4.3 at 2 h. Autoradiography of rat brain sections (ex vivo at 2 h) of [(125)I]4 showed an excellent regional distribution pattern consistent with known SERT localization. These data suggest that [(123)I]4 may be useful for imaging SERT binding sites in the brain by single photon emission computed tomography (SPECT).     

Synthesis, in vitro characterization, and radiolabeling of N,N-dimethyl-2-(2'-amino-4'-substituted-phenylthio)benzylamines: potential candidates as selective serotonin transporter radioligands

A series of N,N-dimethylated and N-monomethylated analogues of N,N-dimethyl-2-(2'-amino-4'-iodophenylthio)benzylamine substituted at the 4'-phenyl position have been prepared and evaluated in vitro for serotonin transporter (SERT) selectivity. Several derivatives were prepared where the 4'-position was either unsubstituted 13 and 33a or substituted with methyl 14a and 33b, ethenyl 14b and 34, ethyl 16 and 35, hydroxymethyl 20 and 41, hydroxyethyl 22, fluoroethyl 23, hydroxypropyl 27, and fluoropropyl 28. Competition binding in cells stably expressing the transfected human SERT, dopamine transporter (DAT), and norepinephrine transporter (NET) using [(3)H]citalopram, [(3)H]WIN 35,428 or [(125)I]RTI-55, and [(3)H]nisoxetine, respectively, demonstrated the following order of SERT affinity (K(i) (nM)): 14a (0.25) > 16 (0.49) > 20 (0.57) > 14b (1.12) > 13 (1.59) > 33b (1.94) = 35 (2.04) > 23 (8.50) = 28 (8.55) > 41 (15.11) > 22 (51) > 33a (83.43) > 27 (92). The K(i) values revealed that most of these derivatives displayed a high affinity for the SERT and a high selectivity over the DAT and NET. Moreover, substitution at the 4'-position of the dimethylated and monomethylated benzylamines differently influenced SERT binding: (i) the dimethylated benzylamines exhibited higher SERT affinity than the monomethylated ones, (ii) alkyl, alkenyl, or hydroxymethyl functions at the 4'-position afford compounds with high SERT affinity, and (iii) omega-hydroxy and fluoro-substituted ethyl and propyl groups at the 4'-position decrease the SERT affinity. From this series, the dimethylated derivatives 13, 14a, 14b, 16, and 20 were radiolabeled with carbon-11 and their log P(7.4) was calculated as a measure of their potential brain penetrance as positron emission tomography SERT imaging agents.     

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.     

Discovery of novel trisubstituted asymmetric derivatives of (2S,4R,5R)-2-benzhydryl-5-benzylaminotetrahydropyran-4-ol, exhibiting high affinity for serotonin and norepinephrine transporters in a stereospecific manner

In our structure-activity relationship study on 3,6-disubstituted pyran derivatives, we have carried out asymmetric synthesis and biological characterization of trisubstituted (2S,4R,5R)-2-benzhydryl-5-benzylaminotetrahydropyran-4-ol and (3S,4R,6S)-6-benzhydryl-4-benzylaminotetrahydropyran-3-ol derivatives and their enantiomers. All synthesized derivatives were tested for their affinities for the dopamine transporter (DAT), serotonin transporter (SERT), and norepinephrine transporter (NET) in the brain by measuring their potency in inhibiting the uptake of [(3)H]DA, [(3)H]-5-HT, and [(3)H]NE, respectively. Compounds were also tested for their binding affinity at the DAT by their inhibition of [(3)H]WIN 35,428. Biological results indicated that regioselectivity and stereoselectivity played important roles in determining activity for monoamine transporters as only (-)-isomers of 2-benzhydryl-5-benzylaminotetrahydropyran-4-ol derivatives exhibited appreciable potency for the monoamine transporters, in particular for the SERT and NET. Among the active analogues, (-)-9d exhibited potent and selective affinity at the NET (K(i), [(3)H]NE = 4.92 nM; DAT/NET = 91 and SERT/NET = 140). One of the derivatives with p-methoxybenzyl substitution, (-)-9a, was potent at both SERT and NET (K(i), [(3)H]-5-HT = 25.9 and [(3)H]NE = 15.8 nM, respectively). In the active analogue series ((-)-9a-(-)-9e), a cis-relationship between the biphenyl and the amino moiety was maintained for the SERT and NET interactions, as was observed with our earlier 3,6-disubstituted pyran compounds for the DAT interaction. To the best of our knowledge, this current series of compounds represents a novel class of pyran derivatives as blockers for monoamine transporters.     

Differential effects of cocaine and MDMA self-administration on cortical serotonin transporter availability in monkeys

Cocaine self-administration alters brain dopaminergic and serotonergic function primarily in mesolimbic and prefrontal brain regions whereas 3,4-methylenedioxymethamphetamine (MDMA) self-administration predominately alters brain serotonergic function in a more widespread distribution across cortical regions. We previously reported that, compared to drug-na?ve rhesus monkeys, self-administration of cocaine but not MDMA was associated with increased serotonin transporter (SERT) availability in two mesolimbic regions, the caudate nucleus and putamen, as measured by positron emission tomography (PET) using the SERT-specific ligand [(11)C]-3-amino-4(2-dimethylamino-methyl-phenylsulfanyl)-benzonitrile ([(11)C]DASB). The goal of the present study was to extend this comparison between cocaine and MDMA self-administration to SERT availability in cortical regions, which have been shown previously to be affected in human drug abusers and are associated with executive function. PET studies using [(11)C]DASB were conducted in adult male rhesus monkeys with a history of cocaine (mean intake?=?742.6?mg/kg) or MDMA (mean intake?=?121.0?mg/kg) self-administration, and drug-na?ve controls (n?=?4/group). Regions of interest were drawn for several cortical (prefrontal, temporal, parietal, occipital and midcingulate) and subcortical (thalamus, amygdala and hippocampus) areas. Cortical SERT availability was significantly higher in monkeys with a cocaine self-administration history compared to controls whereas MDMA self-administration resulted in lower levels of SERT availability. These data extend our previous findings indicating that cocaine and MDMA self-administration differentially alter SERT availability in subcortical and cortical regions, which may have implications for development of treatment drugs.     

Design and synthesis of promiscuous high-affinity monoamine transporter ligands: unraveling transporter selectivity

A series of 4-[2-[bis(4-fluorophenyl)methoxy]ethyl]-piperidines and 4-[2-[(bisphenyl)methoxy]ethyl]-piperidines with different types of substituents in the phenylpropyl side-chain were synthesized and examined for their ability to bind to the dopamine transporter (DAT), the serotonin transporter (SERT), and the norepinephrine transporter (NET). All of the compounds showed high binding affinities for the DAT in the low to subnanomolar range. Their ability to bind to the SERT and the NET, while maintaining their high affinity for the DAT, could be altered by substitution in positions C2 and C3 of the phenylpropyl side-chain. This approach gave rise to a new set of compounds with selectivity for the DAT, the DAT and the SERT, or the DAT and the NET. Six compounds (7, 9, 11, 12, 14, and 20) with relatively low SERT/DAT ratios were selected for additional study in biogenic amine uptake inhibition assays based on the biogenic amine transporter binding results. Some of the new ligands can serve as pharmacological tools to block DAT or DAT and another transporter simultaneously.     

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.     

Effects of cocaine and MDMA self-administration on serotonin transporter availability in monkeys.

Although serotonin (5-HT) can interact with dopamine (DA) systems to modulate the subjective and reinforcing effects of psychostimulants such as cocaine and 3,4-methyldioxymethamphetamine (MDMA, ecstasy), the long-term effects of exposure to psychostimulants on brain 5-HT systems are not well characterized. The present study assessed 5-HT transporter (SERT) availability using positron emission tomography (PET) in rhesus monkeys with the SERT-specific radioligand [(11)C]3-amino-4-(2-dimethylaminomethyl-phenylsulfanyl)-benzonitrile (DASB). SERT availability was assessed in regions of interest including the caudate nucleus, putamen, anterior cingulate cortex, and cerebellum. [(11)C]DASB distribution volume ratios (DVRs) were calculated using the cerebellum as the reference region. DVRs were calculated in control monkeys and in cocaine or MDMA self-administering monkeys approximately 24 h after the last self-administration (SA) session. SERT availability did not differ between monkeys with a history of MDMA SA and control monkeys in any region examined. In contrast, monkeys with a history of cocaine SA showed significantly higher levels of SERT availability in the caudate nucleus and putamen compared to control subjects. These results suggest that chronic SA of cocaine, but not MDMA, leads to alterations in serotonergic function in brain areas relevant to drug abuse. The higher level of SERT availability in cocaine-experienced monkeys may lead to a reduced inhibitory tone of 5-HT on the DA system, which may explain, in part, differences in the abuse liability between cocaine and MDMA.     

Deuterium‐substituted 2‐(2′‐((dimethylamino)methyl)‐4′‐[18F](fluoropropoxy)phenylthio)benzenamine as a serotonin transporter imaging agent

Positron emission tomography imaging of serotonin transporter (SERT) is useful for studying brain diseases with altered serotonergic function. A deuterated imaging agent, ([¹⁸F]2‐((2‐((bis(methyl‐d₃)amino)methyl)‐4‐(3‐fluoropropoxy‐1,1,2,2,3,3‐d₆)phenyl)thio)aniline, [¹⁸F]D12FPBM, [¹⁸F] 1 ), was prepared as a new chemical entity. The deuterated agent, 1 , showed excellent binding affinity to SERT; Ki was 0.086 nM, comparable with the undeuterated FPBM. In vivo biodistribution studies in rats with [¹⁸F] 1 showed good brain uptake (1.09% dose/g at 2 min post injection) and high specific uptake into the hypothalamus (HY) as compared with cerebellum (CB) (HY/CB = 7.55 at 120 min), suggesting a specific localization to SERT binding sites. Regional brain distribution in rats provided clear indication that [¹⁸F] 1 concentrated in the hypothalamus, hippocampus, and striatum, areas with a high SERT density. Results indicate that very little D to H substitution effect was found; [¹⁸F]FPBM and [¹⁸F] 1 showed very similar SERT binding. [¹⁸F] 1 might be an excellent candidate for SERT imaging.     

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.     

Further structural exploration of trisubstituted asymmetric pyran derivatives (2S,4R,5R)-2-benzhydryl-5-benzylamino-tetrahydropyran-4-ol and their corresponding disubstituted (3S,6S) pyran derivatives: a proposed pharmacophore model for high-affinity interaction with the dopamine, serotonin, and norepinephrine transporters

In our previous report, we described a novel series of asymmetric pyran derivatives (2S,4R,5R)-2-benzhydryl-5-benzylamino-tetrahydropyran-4-ol and their enantiomers as blockers of monoamine transporters in the brain. In this report, we describe the further exploration of this series of molecules by incorporating functional groups in the molecular template, which should promote the formation of H bonds with the transporters. In addition, a new synthetic scheme for the asymmetric synthesis of disubstituted cis-(6-benzhydryl-tetrahydro-pyran-3-yl)-benzylamine analogues and their biological characterization is reported. All synthesized derivatives were tested for their affinities for the dopamine transporter (DAT), serotonin transporter (SERT), and norepinephrine transporter (NET) in the brain by measuring their potency in inhibiting the uptake of [(3)H]DA, [(3)H]5-HT, and [(3)H]NE, respectively. The compounds were also tested for their binding potency at the DAT by their ability to inhibit binding of [(3)H]WIN 35, 428. The results indicated that the presence of functional groups, such as -OH, -NH(2), and the bioisosteric 5-substituted indole moiety in both di and trisubstituted compounds, significantly increased their potencies for the SERT and NET, especially for the NET. Among the trisubstituted compounds, (-)-4b exhibited the highest potency for the NET and the SERT (K(i) of 2.13 and 15.3 nM, respectively) and was a serotonin norepinephrine reuptake inhibitor (SNRI). Compound (-)-4a exhibited the highest selectivity for the NET. Among the disubstituted compounds, a number of compounds, such as (-)-9a, (+)-9b, (-)-9b, and (+)-9d, exhibited significant low-nanomolar potencies for the SERT and the NET. Interestingly, compound (-)-9d exhibited appreciable potencies at all three transporters. On the basis of our present and past findings, we propose a qualitative model for the interaction of these compounds with monoamine transporters, which will be refined further in the future.     

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.     

Design and synthesis of a novel photoaffinity ligand for the dopamine and serotonin transporters based on 2beta-carbomethoxy-3beta-biphenyltropane

Tropane-based photoaffinity ligands covalently bind to discrete points of attachment on the dopamine transporter (DAT). To further explore structure-activity relations, a ligand in which the photoactivated group was extended from the 3-position of the tropane ring was synthesized from cocaine via a Stille or Suzuki coupling strategy. 3-(4'-Azido-3'-iodo-biphenyl-4-yl)-8-methyl-8-aza-bicyclo[3.2.1]octane-2-carboxylic acid methyl ester (11; K(i) = 15.1 +/- 2.2 nM) demonstrated high binding affinity for the DAT. Moreover, this compound showed moderate binding affinity for the serotonin transporter (SERT, K(i) = 109 +/- 14 nM), suggesting the potential utility of [(125)I]11 in both DAT and SERT protein structure studies.