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.

UNLABELLED: 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). METHODS: 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. RESULTS: 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. CONCLUSION: 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.

2-(2'-((Dimethylamino)methyl)-4'-(3-[(18)F]fluoropropoxy)-phenylthio)benzenamine for positron emission tomography imaging of serotonin transporters.

INTRODUCTION: A new (18)F ligand, 2-(2'-((dimethylamino)methyl)-4'-(3-[(18)F]fluoropropoxy)-phenylthio)benzenamine ([(18)F]1), for positron emission tomography (PET) imaging of serotonin transporters (SERT) was evaluated. METHODS: Binding affinity was determined through in vitro binding assays with LLC-PK1 cells overexpressing SERT, NET or DAT (LLC-SERT, LLC-NET and LLC-DAT) and with rat cortical homogenates. Localization and selectivity of [(18)F]1 binding in vivo were evaluated by biodistribution, autoradiography and A-PET imaging studies in rats. RESULTS: This compound displayed excellent binding affinity for SERT in vitro with K(i)=0.33 and 0.24 nM in LLC-SERT and rat cortical homogenates, respectively. Biodistribution studies with [(18)F]1 showed good brain uptake (1.61% dose/g at 2 min postinjection), high uptake into the hypothalamus (1.22% dose/g at 30 min) and a high target-to-nontarget (hypothalamus to cerebellum) ratio of 9.66 at 180 min postinjection. Pretreatment with a SERT selective inhibitor considerably inhibited [(18)F]1 binding in biodistribution studies. Ex vivo autoradiography reveals [(18)F]1 localization to brain regions with high SERT density, and this binding was blocked by pretreatment with SERT selective inhibitors. Small animal PET (A-PET) imaging in rats provided clear images of tracer localization in the thalamus, midbrain and striatum. In A-PET chasing experiments, injecting a SERT selective inhibitor 75 min post-tracer injection causes a dramatic reduction in regional radioactivity and the target-to-nontarget ratio. CONCLUSION: The results of the biological studies and the ease of radiosynthesis with moderately good radiochemical yield (RCY=10-35%) make [(18)F]1 an excellent candidate for SERT PET imaging.

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.

The stressors and demands of peacekeeping in Kosovo: predictors of mental health response.

U.S. soldiers' appraisal and experience of the Kosovo peacekeeping mission is described. Using a prospective design, we evaluated the prevalence, severity, and predictors of several mental health outcomes at redeployment. We found that peacekeepers frequently were exposed to potentially traumatizing and other stressful events while in Kosovo, but on average, their appraisal of those events was moderate. Postdeployment psychopathology was also low--soldiers endorsed more severe mental health difficulties at predeployment, which suggests anticipatory negative affect. After controlling for the impact of predeployment stressors, we examined the contribution of potentially traumatizing events, general overseas military duty stressors, negative aspects of peacekeeping roles, and generic positive military experiences, including morale, to explain variance in four outcomes: post-traumatic stress disorder, depression, hostility and aggression problems, and problems with alcohol abuse. Findings indicate that hostility and drinking may be more chronic problems that emerge during stressful times, whereas depression and post-traumatic stress disorder symptoms may be more apt to fluctuate and are associated with potentially traumatizing experiences during peacekeeping. The implications and limitations of the study are discussed.

FlipADAM: a potential new SPECT imaging agent for the serotonin transporter.

INTRODUCTION: Single photon emission computed tomography (SPECT) imaging of the serotonin transporter (SERT) in the brain is a useful tool for examining normal physiological functions and disease states involving the serotonergic system. The goal of this study was to develop an improved SPECT radiotracer with faster kinetics than the current leading SPECT tracer, [(123)I]ADAM, for selective SERT imaging. METHODS: The in vitro binding affinities of (2-(2'-((dimethylamino)methyl)-4'-iodophenylthio)benzenamine) (FlipADAM) (1c), were determined using Hampshire pig kidney cells stably overexpressing the serotonin, norepinephrine (NET) or dopamine transporter (DAT). Localization of [(125)I]FlipADAM (1c) was evaluated through biodistribution and autoradiography in male Sprague Dawley rats, and the specificity of binding was assessed by injecting selective SERT or NET inhibitors prior to [(125)I]FlipADAM (1c). RESULTS: FlipADAM (1c) displayed a high binding affinity for SERT (K(i)=1.0 nM) and good selectivity over NET and DAT binding (43-fold and 257-fold, respectively). [(125)I]FlipADAM (1c) successfully penetrated the blood brain barrier, as evidenced by the brain uptake at 2 min (1.75% dose/g). [(125)I]FlipADAM(1c) also had a good target to non-target (hypothalamus/cerebellum) ratio of 3.35 at 60 min post-injection. In autoradiography studies, [(125)I]FlipADAM (1c) showed selective localization in SERT-rich brain regions such as the thalamic nuclei, amygdala, dorsal raphe nuclei and other areas. CONCLUSION: [(125)I]FlipADAM (1c) exhibited faster clearance from the brain and time to binding equilibrium when compared to [(125)I]2-(2'-((dimethylamino)methyl)-phenylthio)-5-iodophenylamine [(125)I]ADAM (1b) and a higher target to non-target ratio when compared to [(125)I]5-iodo-2-(2'-((dimethylamino)methyl)-phenylthio)benzyl alcohol [(125)I]IDAM (1a). Therefore, [(123)I]FlipADAM (1c) may be an improved SPECT tracer for imaging SERT.

In vivo studies of the SERT-selective [18F]FPBM and VMAT2-selective [18F]AV-133 radiotracers in a rat model of Parkinson's disease.

INTRODUCTION: The utility of [(18)F]FPBM [2-(2'-((dimethylamino)methyl)-4'-(3-[(18)F]-fluoropropoxy)phenylthio)benzenamine], a selective serotonin transporter (SERT) tracer, and [(18)F]AV-133 [(+)-2-Hydroxy-3-isobutyl-9-(3-fluoropropoxy)-10-methoxy-1,2,3,4,6,7-hexahydro-11bH-benzo[a]quinolizine], a selective vesicular monoamine transporter 2 (VMAT2) tracer, were tested in the 6-hydroxydopamine (6-OHDA) unilateral lesioned rat model. METHODS: Positron emission tomography (PET) imaging of three 6-OHDA unilateral lesioned male Sprague Dawley rats (Rats 1-3) were performed with [(18)F]FPBM and [(18)F]AV-133 to examine whether changes in SERT and VMAT2 binding, respectively, could be detected in the brain. The brains of the three rats were then removed and examined by in vitro autoradiography with [(18)F]FPBM and the dopamine transporter ligand, [(125)I]IPT [N-(3'-[(125)I]-iodopropen-2'-yl)-2-beta-carbomethoxy-3-beta-(4-chloro phenyl) tropane, for confirmation. Biodistribution of [(18)F]FPBM in a separate group of p-chloroamphetamine (PCA) treated rats were also performed. RESULTS: PET image analysis showed varying levels of SERT binding reduction (Rat 1=-11%, Rat 2=-4%, Rat 3=-43%; n=2) and a clear and definitive loss of VMAT2 binding (Rat 1=-87%, Rat 2=-72%, and Rat 3=-91%; n=1) in the left striatum when compared to the right (non-lesioned side) striatum. The results from PET imaging were corroborated with quantitative in vitro autoradiography. Rats treated with a selective serotonin toxin (p-chloroamphetamine) showed a significant reduction of [(18)F]FPBM uptake in the cortex and hypothalamus regions of the brain. CONCLUSION: The preliminary data suggest that [(18)F]FPBM and [(18)F]AV-133 may be useful for the examination of serotonergic and dopaminergic neuron integrity, respectively, in the living brain.