Serotonin Transporter Imaging Agent Research Articles

Serotonin transporter imaging agent as a probe for β-cells of pancreas

ObjectiveDiabetes mellitus (DM) is one of the major diseases in the world. Nuclear medicine imaging may be able to detect functional status of pancreatic β cells in vivo, which might elucidate the pathological mechanisms of diabetes and develop individualized treatment plans. In this study, we evaluated the ability of [125I]ADAM, a serotonin transporter (SERT) imaging agent, as a probe for detecting pancreatic β-cell mass (BCM). MethodsIn vitro cell studies were evaluated in INS-1 cells (rat islet β cell line). Biodistribution studies were performed in male normal Sprague-Dawley rats and alloxan-induced type 1 diabetes mellitus (T1DM) rats. Distribution and expression of SERT protein in pancreas of rats were also measured by immunofluorescence staining and Western blot. ResultsIn vitro cell studies showed that the concentration of [125I]ADAM associated with the INS-1 cells was increased gradually with incubation time, and the SERT specific inhibitor, escitalopram, exhibited the inhibitory effect on this interaction. Biodistribution studies also showed that the uptake of [125I]ADAM in the pancreas of normal rats was decreased in the presence of escitalopram. However, in the T1DM rat model with a significant β cells reduction, the uptake of pancreas was increased when compared with the control. Through immunofluorescence staining and Western blot, it was found that both the endocrine and exocrine cells of the normal pancreas expressed SERT protein, and the level of SERT protein in the exocrine cells was higher than islets. In the diabetic state, the expression of SERT in the exocrine cells was further increased. ConclusionsThe SERT imaging agent, [125I]ADAM, at the present form will not be suitable for imaging β cells, specifically because there were extraordinarily high non-specific signals contributing from the exocrine cells of pancreas. In addition, we noticed that the level of SERT expression was abnormally elevated in the diabetic state, which might provide an unexpected target for studying the pathological mechanisms of diabetes.

Deuterium-substituted 2-(2'-((dimethylamino)methyl)-4'-[18 F](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, ([18 F]2-((2-((bis(methyl-d3 )amino)methyl)-4-(3-fluoropropoxy-1,1,2,2,3,3-d6 )phenyl)thio)aniline, [18 F]D12FPBM, [18 F]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 [18 F]1 showed good brain uptake (1.09% dose/g at 2min post injection) and high specific uptake into the hypothalamus (HY) as compared with cerebellum (CB) (HY/CB=7.55 at 120min), suggesting a specific localization to SERT binding sites. Regional brain distribution in rats provided clear indication that [18 F]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; [18 F]FPBM and [18 F]1 showed very similar SERT binding. [18 F]1 might be an excellent candidate for SERT imaging.

One-step preparation of [18F]FPBM for PET imaging of serotonin transporter (SERT) in the brain

Serotonin transporters (SERT) in the brain play an important role in normal brain function. Selective serotonin reuptake inhibitors such as fluoxetine, sertraline, paroxetine, escitalopram, etc., specifically target SERT binding in the brain. Development of SERT imaging agents may be useful for studying the function of SERT by in vivo imaging. A one-step preparation of [18F]FPBM, 2-(2′-(dimethylamino)methyl)-4′-(3-([18F]fluoropropoxy)phenylthio)benzenamine, for positron emission tomography (PET) imaging of SERT binding in the brain was achieved. An active OTs intermediate, 9, was reacted with [18F]F−/K222 to produce [18F]FPBM in one step and in high radiochemical yield. This labeling reaction was evaluated and optimized under different temperatures, bases, solvents, and varying amounts of precursor 9. The radiolabeling reaction led to the desired [18F]FPBM in one step and the crude product was purified by HPLC purification to give no-carrier-added [18F]FPBM (radiochemical yield, 24–33%, decay corrected; radiochemical purity >99%). PET imaging studies in normal monkeys (n=4) showed fast, pronounced uptakes in the midbrain and thalamus, regions known to be rich in SERT binding sites. A displacement experiment with escitalopram (5mg/kg iv injection at 30min after [18F]FPBM injection) showed a rapid and complete reversal of SERT binding, suggesting that binding by [18F]FPBM was highly specific and reversible. A one-step radiolabeling method coupled with HPLC purification for preparation of [18F]FPBM was developed. Imaging studies suggest that it is feasible to use this method to prepare [18F]FPBM for in vivo PET imaging of SERT binding in the brain.

Fully automated one-pot two-step synthesis of 4-[18F]-ADAM, a potent serotonin transporter imaging agent

IntroductionN,N-dimethyl-2-(2-amino-4-[18F]fluorophenylthio)benzylamine (4-[18F]-ADAM, 2) is a potent serotonin transporter (SERT) imaging agent. In order to fulfill the demand of clinical studies, we have developed a fully automated one-pot two-step synthesis of this potent SERT imaging agent. MethodsThe 4-[18F]-ADAM (2) was synthesized using a commercially available GE TRACERlab FN module. Briefly, the precursor, N,N-dimethyl-2-(2,4-dinitrophenylthio) benzylamine (1) in DMSO was reacted with K[18F]/K2.2.2 in a glassy carbon reaction vessel at 120°C for 7.5min followed by reduction of the intermediate with NaBH4/Cu(OAc)2 in EtOH in the same vessel at 80°C for 20min. The reaction mixture was then purified with high-performance liquid chromatography (HPLC) and solid phase extraction (SPE) to give (2). The quality of (2) synthesized by this method was verified by HPLC and TLC as compared to its authentic sample synthesized by two-pot two-step method. ResultsUsing this automated one-pot two-step method, the radiochemical yield (RCY) of (2) was 2.5±0.8% (n=12, EOS) in a synthesis time of 100±6min from EOB with a specific activity of 4.4±1.9Ci/μmol (n=12, EOS). Radioligand (2) was stable over 4h at room temperature. ConclusionsThis fully automated one-pot two-step synthetic method using a commercially available GE TRACERlab FN module could simplify the synthesis of 4-[18F]-ADAM (2) and fulfill its demand for both animal and human studies, especially for study sites without a cyclotron.

PET Imaging of Serotonin Transporters with 4-[18F]-ADAM in a Parkinsonian Rat Model

This study was undertaken to address the effects of fetal mesencephalic tissue transplantation on the serotonin system in a rat model of Parkinson's disease (PD) while also investigating the usefulness of 4-[18F]-ADAM (a serotonin transporter imaging agent) coupled with micro-PET for imaging serotonin transporters (SERTs). A PD model was induced by unilateral injection of 6-hydroxydopamine (6-OHDA) into the right medial forebrain bundle of the nigrostriatal pathway, while cell transplantation was performed via intrastriatal injection of mesencephalic brain tissue dissected from embryonic (E14) rats. The 4-[18F]-ADAM/micro-PET scanning was performed following both 6-OHDA lesioning and transplantation. Immunohistochemistry (IHC) studies were also performed following the final PET scan, and the results were compared to show a 17-43% decrease in the specific uptake ratio (SUR) and a 23-52% decrease in serotonin transporter immunoreactivity (SERT-ir) within various brain regions on the lesioned side. The number of methamphetamine-induced rotations also decreased significantly at the 4th week postgraft. In addition, striatal SUR and the SERT-ir levels were restored to 77% and 83% 5 weeks postgraft. These results suggest that Parkinson's disease also affects the serotonergic system, while both the dopaminergic and serotonergic systems can be partially restored in a rat model of PD after E14 mesencephalic tissue transplantation. In addition, we have also determined that 4-[18F]-ADAM/micro-PET can be used to detect serotonergic neuron loss, monitor the progress of Parkinson's disease, and oversee the effectiveness of therapy.

Investigating the effects of noise-induced hearing loss on serotonin transporters in rat brain using 4-[18F]-ADAM/small animal PET

The serotonin transporter (SERT) is an important marker of the status of serotonergic neurons. The main function of SERT is to regulate the serotonin concentration in the synapse. Recent studies have shown that SERT is expressed in the central auditory pathway and may play a role in the auditory process. However, little is known about the effects of noise on the cerebral serotonin system. In this study, we explored the status of brain SERT in a rat model of noise-induced hearing loss using 4-[(18)F]-ADAM (a SERT imaging agent) and small animal positron emission tomography (PET) imaging. Male Sprague Dawley rats were exposed to an 8 kHz noise at 118 dB sound pressure level for 3.5h. An auditory brainstem response test and 4-[(18)F]-ADAM/small animal PET were performed at different time points after noise exposure. The specific uptake ratios (SURs) for 4-[(18)F]-ADAM were calculated from the PET imaging data in six brain regions. Immunohistochemistry and surface preparation of the cochleae were performed 30 days after noise exposure. Our data clearly showed that the hearing and cochlear outer hair cells of the rats were lost after noise exposure. In the PET study, the SURs of SERT were markedly reduced by 35%-58% in various brain regions one day after noise exposure. The decrement remained on days 8 and 15 and was approximately 26%-48% on day 29. The distribution and intensity of SERT immunostaining in the brain paralleled the PET imaging data. These results suggest that noise-induced hearing loss involves a reduction in SERT expression in various regions of the rat brain and that changes in SERT are detectable by 4-[(18)F]-ADAM/small animal PET in vivo.

Characterization of 4-[18F]-ADAM as an imaging agent for SERT in non-human primate brain using PET: a dynamic study

Serotonin transporter (SERT) has been associated with many psychiatric diseases. This study investigated the biodistribution of a serotonin transporter imaging agent, N,N-dimethyl-2-(2-amino-4-(18)F-fluorophenylthio)benzylamine (4-[(18)F]-ADAM), in nonhuman primate brain using positron emission tomography (PET). Six and four Macaca cyclopis monkeys were used to determine the transit time (i.e., time necessary to reach biodistribution equilibrium) and the reproducibility of 4-[(18)F]-ADAM biodistribution in the brain, respectively. The sensitivity and specificity of 4-[(18)F]-ADAM binding to SERT were evaluated in one monkey challenged with different doses of fluoxetine and one monkey treated with 3,4-methylendioxymethamphetamine (MDMA). Dynamic PET imaging was performed for 3 h after 4-[(18)F]-ADAM intravenous bolus injection. The specific uptake ratios (SURs) in the midbrain (MB), thalamus (TH), striatum (ST) and frontal cortex (FC) were calculated. The distribution of 4-[(18)F]-ADAM reached equilibrium 120-150 min after injection. The mean SURs were 2.49 ± 0.13 in MB, 1.59 ± 0.17 in TH, 1.35 ± 0.06 in ST and 0.34 ± 0.03 in FC, and the minimum variability was shown 120-150 min after 4-[(18)F]-ADAM injection. Using SURs and intraclass coefficient of correlation, the test/retest variability was under 8% and above 0.8, respectively, in SERT-rich areas. Challenge with fluoxetin (0.75-2 mg) dose-dependently inhibited the SURs in various brain regions. 4-[(18)F]-ADAM binding was markedly reduced in the brain of an MDMA-treated monkey compared to that in brains of normal controls. 4-[(18)F]-ADAM appears to be a highly selective radioligand for imaging SERT in monkey brain.

Development of Effective PET and SPECT Imaging Agents for the Serotonin Transporter: Has a Twenty-Year Journey Reached its Destination?

The serotonin transporter (SERT) has been implicated in a variety of neuropsychiatric disorders including depression, schizophrenia, substance abuse, alcoholism, and Alzheimer's disease. Radiotracer-based in vivo imaging techniques such as Positron Emission Tomography (PET) and Single-Photon Emission Computed Tomography (SPECT) are important tools to investigate the functions of SERT in the living brain under normal conditions and its dysfunction in diseases. In this report we review the development and validation of effective PET and SPECT radiotracers in the last twenty years. First, the requirements for an effective imaging tracer, and factors influencing a tracer's in vivo imaging performance are discussed. PET and SPECT radiotracers for SERT are then categorized and reviewed according to their chemical scaffolds: 1) SSRIs and related compounds; 2) tropane-based ligands; 3) isoquinolines; and 4) substituted diarylsulfides. Critical evaluation and comments are provided for promising radiotracers, if any, emerging from each chemical scaffold. Based on experimental data gathered from radiotracer development for SERT, an examination of the relationship between an imaging tracer's in vitro physicochemical and pharmacological properties and its in vivo performance parameters is provided. Finally, tracers available for imaging applications in humans are assessed and compared in terms of tissue binding kinetics, non-specific binding, and specific binding signals in vivo. From these assessments, we conclude that, after twenty years of development efforts, a number of effective PET and SPECT radiotracers have now been validated and are available for imaging SERT in humans. The applications of these efficacious SERT imaging agents will further advance our understanding of this important transporter in psychiatric and neurodegenerative disorders.

Validation of two fluoro-analogues of N,N-dimethyl-2-(2′-amino-4′-hydroxymethyl-phenylthio)benzylamine as serotonin transporter imaging agents using microPET

Carbon-11 (C-11) N,N-dimethyl-2-(2'-amino-4'-hydroxymethyl-phenylthio)benzylamine ([(11)C]HOMADAM) has been reported as highly specific and selective positron emission tomography (PET) radiotracer showing fast kinetics for the human brain serotonin transporter (SERT). In our continued effort to develop appropriate PET SERT radioligand that can be labeled with either C-11 or fluorine-18 (F-18), two new C-11 labeled analogues of HOMADAM, [(11)C]-N,N-dimethyl-2-(2'-amino-5'-fluoro-4'-hydroxymethyl-phenylthio)benzylamine ([(11)C]-(2)) and [(11)C]-N,N-dimethyl-2-(2'-amino-4-fluoro-4'-hydroxymethyl-phenylthio)benzylamine ([(11)C]-(3)) have been synthesized and evaluated along the previously reported [(11)C]-N,N-dimethyl-2-(2'-amino-5-fluoro-4'-hydroxymethyl-phenylthio)benzylamine ([(11)C]-(1)). The in vitro competitive binding assays were performed in cells transfected with human SERT (hSERT), human dopamine transporter (hDAT), and human norepinephrine transporter (hNET). [(11)C]-(2) and [(11)C]-(3) were prepared by methylation of their monomethylbenzylamine precursors 13 and 22 with cyclotron produced [(11)C]iodomethane ([(11)C]CH(3)I), respectively. Uptake and kinetics of [(11)C]-(2) and [(11)C]-(3) in the brain regions of interest were determined in anesthetized rhesus monkeys using Concorde microPET P4. 2 and 3 displayed moderate and high affinity for the SERT with Kis (SERT) = 5.45 and 1.10 nM (vs [(3)H]citalopram), respectively. After High Performance Liquid Chromatography (HPLC) purification, [(11)C]-(2) and [(11)C]-(3) were obtained in 23 and 9% radiochemical yield (RCY) and log Ps(7.4) of 1.77 and 1.91, respectively. The microPET images of [(11)C]-(2) and [(11)C]-(3) showed clear localization in the monkey brain regions rich in SERT with midbrain to cerebellum ratios of 1.75 and 3.86 at 85 min post injection, respectively, comparing to 3.40 for [(11)C]-(1), at the same time. [(11)C]-(3) was selected for further examination and showed to be specific to the SERT as displacement with citalopram (a potent SERT ligand) reduced radioactivity in SERT rich regions, such as midbrain, to the cerebellum level. Compound 2, the 5'-fluoro-analogue of HOMADAM, had the lowest brain uptake and target to non-target ratios. Compound 3, the 4-fluoro-analogue of HOMADAM, had good brain uptake and higher midbrain and thalamus to cerebellum ratios than compound 1, the 5-fluoro-analogue of HOMADAM. Although 1 and 3 presented better imaging properties than 2, none of the three candidates was suitable to surpass the binding or distributional qualities of the parent HOMADAM. Alternative fluoro-analogues of HOMADAM will soon be characterized, in future work, as SERT radioimaging agents.

In Vivo Characterization of a Series of 18F-Diaryl Sulfides (18F-2-(2′-((Dimethylamino)Methyl)-4′-(Fluoroalkoxy)Phenylthio)Benzenamine) for PET Imaging of the Serotonin Transporter

PET of the serotonin transporter (SERT) in the brain is a useful tool for examining normal physiologic functions as well as disease states involving the serotonergic system. The goal of this study was to further develop and refine a series of 4'-fluoroalkoxy-substituted, (18)F-radiolabeled SERT imaging agents. 2-(2'-((Dimethylamino)methyl)-4'-(4-(18)F-fluorobutoxy)phenylthiol)benzenamine (3) and 2-(2'-((dimethylamino)methyl)-4'-(5-(18)F-fluoropentoxy)phenylthiol)benzenamine (4) were synthesized and evaluated along with 2 previously reported compounds of this series, 2-(2'-((dimethylamino)methyl)-4'-(2-(18)F-fluoroethoxy)phenylthiol)benzenamine (1) and 2-(2'-((dimethylamino)methyl)-4'-(3-(18)F-fluoropropoxy)phenylthiol)benzenamine (2). The in vitro binding affinities of compounds 3 and 4 were determined in monoamine transporter-transfected LLC-PK(1) cell homogenates. In vivo localization of the respective (18)F-labeled compounds was evaluated by biodistribution studies in male Sprague-Dawley rats. Compound 3 was selected for further examination by autoradiographic and PET studies in rats. The corresponding mesylate precursors of 3 and 4 were radiolabeled with (18)F within 75-90 min. Radiochemical yield was 6%-35%, specific activity was 15-170 GBq/mumol, and radiochemical purity was greater than 97% (end of synthesis). The compounds showed subnanomolar binding affinities for SERT (inhibition constants, 0.51 and 0.76 nM, respectively), had brain uptake at 2 min of 1.25 and 0.68 percentage injected dose per gram, respectively, and possessed high target-to-nontarget (hypothalamus-to-cerebellum) ratios at 120 min after injection (6.51 and 5.70, respectively). Autoradiographic studies of (18)F-3 showed selective localization in SERT-rich brain regions. PET studies of (18)F-3 showed clear localization in the midbrain, thalamus, and striatum. This compound series was found to have potential for producing a suitable (18)F-radiolabeled PET radiotracer for SERT. Compound 4, the pentoxy derivative, had the lowest brain uptake and target-to-nontarget ratio. Compound 3, the butoxy derivative, had a lower target-to-nontarget ratio than compounds 1 (ethoxy derivative) and 2 (propoxy derivative). Compounds 1 and 2 both hold promise as SERT radioimaging agents, but because of cost limitations, only compound 2 will be evaluated in further studies.

Study on the neuroprotective effect of fluoxetine against MDMA-induced neurotoxicity on the serotonin transporter in rat brain using micro-PET

3, 4-Methylenedioxymethamphetamine (MDMA, “ecstasy”) has toxic effects on serotonergic neurons in the brain. Our aim was to determine whether N,N-dimethyl-2-(2-amino-4-[18F]-fluorophenylthio) benzylamine (4-[18F]-ADAM; a serotonin transporter imaging agent) and micropositron emission tomography (micro-PET) can be used to examine in vivo the effect of fluoxetine on MDMA-induced loss of serotonin transporters in rat brain. Male Sprague–Dawley rats were injected with fluoxetine [1 dose, 5 mg/kg, subcutaneously (s.c.)] followed by MDMA (twice a day for 4 consecutive days, 10 mg/kg, s.c.). Micro-PET with 4-[18F]-ADAM was performed on days 4, 10, 17, 24, and 31. In addition, the time course of occupancy by fluoxetine at 4-[18F]-ADAM sites was measured. Specific 4-[18F]-ADAM uptake ratios (SURs) were calculated from the micro-PET imaging data for various brain regions. Immunohistochemistry was performed 7 days after the last micro-PET scan. From day 4 to day 31, SURs were markedly decreased (by approximately 55–75% compared to control values) in all brain regions in MDMA-treated rats. The effect of MDMA was markedly attenuated (approximately 30–50%) by fluoxetine. The fluoxetine-induced decrease in uptake in different brain regions was 40–75% at 90-min postinjection, and this decrease returned to baseline values in most brain regions by day 31. The distribution and intensity of serotonin transporter (SERT) immunostaining in the brain paralleled the PET imaging results, suggesting that a single dose of fluoxetine provides long-lasting protection against MDMA-induced loss of SERT and that such neuroprotection is detectable in vivo by 4-[18F]-ADAM micro-PET.

Dual-isotope single-photon emission computed tomography for dopamine and serotonin transporters in normal and parkinsonian monkey brains

Parkinson's disease (PD) affects both dopaminergic and serotonergic systems. In this study, we simultaneously evaluated dopamine and serotonin transporters in primates using dual-isotope single-photon emission computed tomography (SPECT) imaging and compared the results with traditional single-isotope imaging. Four healthy and one 6-OHDA-induced PD monkeys were used for this study. SPECT was performed over 4 h after individual or simultaneous injection of [(99m)Tc]TRODAT-1 (a dopamine transporter imaging agent) and [(123)I]ADAM (a serotonin transporter imaging agent). The results showed that the image quality and uptake ratios in different brain regions were comparable between single- and dual-isotope studies. The striatal [(99m)Tc]TRODAT-1 uptake in the PD monkey was markedly lower than that in normal monkeys. The uptake of [(123)I]ADAM in the midbrain of the PD monkey was comparable to that in the normal monkeys, but there were decreased uptakes in the thalamus and striatum of the PD monkey. Our results suggest that dual-isotope SPECT using [(99m)Tc]TRODAT-1 and [(123)I]ADAM can simultaneously evaluate changes in dopaminergic and serotonergic systems in a PD model.

Synthesis and biological evaluation of one novel technetium-99m-labeled nitroquipazine derivative as an imaging agent for serotonin transporter

Imaging of serotonin transporter (SERT) by positron emission tomography (PET) or single-photon emission-computed tomography (SPECT) in humans would provide useful information in diagnosis and therapy of several neurodegenerative and neuropsychiatric disorders. 6-Nitroquipazine is a highly potent and selective inhibitor of the SERT. For the development of new 99mTc-labeled 6-nitroquipazine derivatives as SERT imaging agents, novel [ N-[2-((3-(4-(6-nitroquinolin-2-yl)piperazin-1-yl)propyl)(2-mercaptoethyl)amino]-acetyl-2-aminoethanethiolato] [ 99mTc]technetium (V) oxide ( 99mTc-MAMA-3-PQ) and its rhenium analog were synthesized and characterized. 99mTc-MAMA-3-PQ displayed high initial brain uptake (0.52% ID/organ at 2 min post-injection (pi)) and relatively fast washout in mice (0.09% ID/organ at 60 min pi). The regional brain distribution studies in rats showed high-specific binding ratios at 60 min pi. Maximum regional contrast ratio observed for thalamus/cerebellum was 2.94, followed by 2.62 for hypothalamus/cerebellum. These encouraging results lead us to further explore its derivatives as new imaging agents for the SERT in the brain.

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.

2-(2′-((Dimethylamino)methyl)-4′-(fluoroalkoxy)-phenylthio)benzenamine Derivatives as Serotonin Transporter Imaging Agents

A novel series of ligands with substitutions at the 5-position on phenyl ring A and at the 4'-position on phenyl ring B of 2-(2'-((dimethylamino)methyl)-4'-(fluoroalkoxy)phenylthio)benzenamine (4'-2-fluoroethoxy derivatives 28-31 and 4'-3-fluoropropoxy derivatives 40-42) were prepared and tested as serotonin transporter (SERT) imaging agents. The new ligands displayed high binding affinities to SERT (Ki ranging from 0.03 to 1.4 nM). The corresponding 18F labeled compounds, which can be prepared readily, showed excellent brain uptake and retention after iv injection in rats. The hypothalamus region showed high uptake values between 0.74% and 2.2% dose/g at 120 min after iv injection. Significantly, the hypothalamus to cerebellum ratios (target to nontarget ratios) at 120 min were 7.8 and 7.7 for [18F]28 and [18F]40, respectively. The selective uptake and retention in the hypothalamus, which has a high concentration of SERT binding sites, demonstrated that [18F]28 and [18F]40 are promising positron emission computed tomography imaging agents for mapping SERT binding sites in the brain.

Synthesis and in Vivo Evaluation of Fluorine-18 and Iodine-123 Labeled 2β-Carbo(2-fluoroethoxy)-3β-(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.

In vivo characterization in non-human primates of βFE mZIENT and βFE mZBrENT: New serotonin transporter imaging agents: effects of selective monoamine transporter displacement using microPET

In vivo characterization in non-human primates of βFE mZIENT and βFE mZBrENT: New serotonin transporter imaging agents: effects of selective monoamine transporter displacement using microPET

Synthesis of N-methyl-2-(2-amino-4-[18F]fluorophenylthio)benzylamine as serotonin transporter imaging agent

Synthesis of N-methyl-2-(2-amino-4-[¹⁸F]fluorophenylthio)benzylamine as serotonin transporter imaging agent

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.

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).