2-((2-((dimethylamino)methyl)phenyl)thio)-5-iodophenylamine (ADAM): an improved serotonin transporter ligand

Association between serotonin transporter availability and overall rating scores of quality of life in healthy volunteers

Depression and impaired quality of life (QOL) are frequently observed in patients suffering from a variety of diseases. In addition, it has been reported that an enhanced degradation of the serotonin precursor tryptophan may contribute to QOL deterioration in some diseases. However, it is unclear whether the correlation between the QOL scores and the central serotonergic tone is only mediated by the severity of either the depression symptoms or the physical illness itself. The present study examined the relationship between serotonin transporter (SERT) availability and life quality as measured by the World Health Organization Quality of Life brief version questionnaire (WHO-QOL) in healthy participants in order to exclude the influence of depressive mood and disease. The SERT availability in the midbrain was approximated using SPECT with [(123)I] ADAM ligand in fifty-eight healthy volunteers. The overall rating sub scores of the WHO-QOL correlated positively with serotonin transporter availability in the males. Central serotoninergic activity may play a role in the overall rating scores of the WHO-QOL.

Imaging the serotonin transporter using (123)I-ADAM in the human brain

The aim of this study was to examine the feasibility of (123)I-ADAM to image the serotonin transporter (SERT) in Asian (Taiwanese) subjects. Single photon emission computed tomography (SPECT) scans were performed on nine healthy volunteers who were s-allele carriers at the polymorphism within the serotonin transporter promoter region (SERTPR) after intravenous bolus injection of (123)I-ADAM. Quantification of (123)I-ADAM binding was performed using the ratio equilibrium method (REM) with specific uptake ratio (SUR) and a simplified reference tissue model (SRTM). Curve-fitting techniques were used to obtain the peak equilibrium point from 241 to 301 min (average 264+/-20 min) after injection of (123)I-ADAM for the midbrain and from 215 to 270 min (average 235+/-18 min) after injection of (123)I-ADAM for the striatum. Two sets of SUR were obtained by either curve fitting (estimated values) or integrated period from 240 to 270 min (observed values). The estimated values of SUR were 2.11+/-0.51 for the midbrain and 1.50+/-0.44 for the striatum, whereas the observed values were 2.11+/-0.83 for the midbrain and 1.24+/-0.31 for the striatum. The SRTM showed that the binding potential (BP) was 2.10+/-0.66 for the midbrain and 1.35+/-0.25 for the striatum. There was a good correlation between estimated SUR, observed SUR and SRTM in the midbrain but not in the striatum. The optimal scanning duration for both the midbrain and the striatum should be 220 to 280 min similar to that suggested by previous studies in Caucasians. However, due to the low signal-to-noise ratio in the striatum, (123)I-ADAM could be an ideal tracer for imaging SERT in the midbrain but not in the striatum.

Simultaneous [99mTc]TRODAT-1 and [123I]ADAM brain SPECT in nonhuman primates

PURPOSE

This study examined the feasibility of simultaneous dopamine and serotonin transporter imaging using [(123)I]ADAM and [(99m)Tc]TRODAT-1 single photon emission computed tomography (SPECT).

PROCEDURES

Simultaneous [(123)I]ADAM (185 MBq) and [(99m)Tc]TRODAT-1 (740 MBq) SPECT was performed in three age-matched female Formosan rock monkeys. An asymmetric energy window was used for dual, and symmetric energy windows were used for single-isotope imaging. Oral fluoxetine (20 mg) and intravenous methylphenidate HCl (1 mg/kg) were given 24 h and 10 min, respectively, before dual-isotope SPECT to test imaging specificities of [(123)I]ADAM and [(99m)Tc]TRODAT-1.

RESULTS

Comparable image quality and uptake ratios between dual- and single-isotope SPECT scans were found. Dual-isotope SPECT in fluoxetine-pretreated monkeys showed decreased uptake of [(123)I]-ADAM, but not of [(99m)Tc]TRODAT-1. Dual-isotope SPECT in methylphenidate-pretreated monkeys showed decreased [(99m)Tc]TRODAT-1 uptake without affecting [(123)I]-ADAM uptake.

CONCLUSION

Simultaneous [(123)I]-ADAM and [(99m)Tc]TRODAT-1 SPECT appears promising in nonhuman primates and may provide a suitable preclinical model with further clinical implications.

Evaluation of reference tissue model and tissue ratio method for 5-HTT using [(123)I] ADAM tracer

The serotonin (5-hydroxytryptamine, or 5-HT) transporters (5-HTT) are target-sites for commonly used antidepressants. [(123)I] ADAM is a novel radiotracer that selectively binds the 5-HTT of the central nervous system. The aim for this study was to compare four-parameter model (FPM) with three-parameter model (TPM) from non-invasive reference tissue model (RTM) for 5-HTT quantification using the cerebellum as indirect input function. Furthermore, we compared tracer kinetic model with the tissue ratio (TR) method. The binding potential (BP) values derived from both models were almost the same, but ratio of delivery (R(1)) in TPM had smaller standard deviation than the FPM. There was also significant correlation between BP and specific uptake ratio (SUR). In conclusion, simplified reference tissue model (SRTM) was the better choice because of its stability and convenient implementation for non-invasive quantification of brain SPECT studies. The correlation found between BP and SUR supports the use of TR method for quantification of 5-HTT to avoid arterial sampling in dynamic SPECT scan.

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.

Docking study, synthesis, and in vitro evaluation of fluoro-MADAM derivatives as SERT ligands for PET imaging

In order to predict affinity of new diphenylsulfides for the serotonin transporter (SERT), a molecular modeling model was used to compare potential binding affinity of new compounds with known potent ligands. The aim of this study is to identify a suitable PET radioligand for imaging the SERT, new derivatives, and their precursors for a C-11 or F-18 radiolabeling, were synthesized. Two fluorinated derivatives displayed good in vitro affinity for the SERT (K(i)=14.3+/-1 and 10.1+/-2.7 nM) and good selectivity toward the other monoamine transporters as predicted by the docking study.

Brain single-photon emission CT physics principles

The basic principles of scintigraphy are reviewed and extended to 3D imaging. Single-photon emission computed tomography (SPECT) is a sensitive and specific 3D technique to monitor in vivo functional processes in both clinical and preclinical studies. SPECT/CT systems are becoming increasingly common and can provide accurately registered anatomic information as well. In general, SPECT is affected by low photon-collection efficiency, but in brain imaging, not all of the large FOV of clinical gamma cameras is needed: The use of fan- and cone-beam collimation trades off the unused FOV for increased sensitivity and resolution. The design of dedicated cameras aims at increased angular coverage and resolution by minimizing the distance from the patient. The corrections needed for quantitative imaging are challenging but can take advantage of the relative spatial uniformity of attenuation and scatter. Preclinical systems can provide submillimeter resolution in small animal brain imaging with workable sensitivity.

Applying neuroimaging ligands to study major depressive disorder

The recent increase in radioligands available for neuroimaging major depressive disorder has led to advancements in our understanding of the pathophysiology of this illness and improved antidepressant development. Major depressive disorder can be defined as an illness of recurrent major depressive episodes of persistently low mood, dysregulated sleep, appetite and weight, anhedonia, cognitive impairment, and suicidality. The main target sites investigated with radioligand neuroimaging include receptor sites that regulate in response to lowered monoamine levels, targets related to removal of monoamines, uptake of ligands related to regional brain function, and target sites of antidepressants.

Imaging serotonin transporters using [123I]ADAM SPECT in a parkinsonian primate model

Parkinson's disease (PD) affects multiple neurotransmitter systems. The purpose of this study was to investigate differences in the serotonin transport system between normal and parkinsonian monkeys using 2-([2-([di-methylamino]methyl)phenyl]thio)-5-[(123)I] iodophenyl-amine([(123)I]ADAM), a serotonin transporters (SERT) radioligand. The brain single photon emission computed tomography (SPECT) was performed on two normal and one parkinsonian monkey. The parkinsonian monkey was induced by bilateral injection of 6-hydroxydopamine (6-OHDA) into the medial forebrain bundle under magnetic resonance imaging (MRI) guidance. Each monkey underwent two [(99m)Tc] TRODAT-1 (a dopamine transporters imaging agent) and two [(123)I] ADAM brain SPECT scans. After a bolus injection of the radioligand, the SPECT data were acquired over 4h using a dual-head gamma camera equipped with ultra-high resolution fan-beam collimators. The striatal uptake of [(99m)Tc]TRODAT-1 was 46% lower in the parkinsonian monkey than those of normal monkeys at 210-240 min post-injection. [(123)I]ADAM uptake in the midbrain of the parkinsonian monkey was comparable to those of the controls. The uptakes of [(123)I]ADAM in the striatum, thalamus, and frontal cortex of the parkinsonian monkey, were 31%, 31%, and 23% lower than those of normal monkeys at 210-240 min post-injection, respectively. Our results suggest that [(123)I]ADAM SPECT has potential for evaluating the serotonin transporter changes in human PD.

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 (99m)Tc-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] [(99m)Tc]technetium (V) oxide ((99m)Tc-MAMA-3-PQ) and its rhenium analog were synthesized and characterized. (99m)Tc-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.

[123I] ADAM brainstem binding correlates with the loudness dependence of auditory evoked potentials

The in vivo assessment of brain serotonergic function might be of clinical relevance in neuropsychiatry. The loudness dependence of auditory evoked potentials (LD) has been proposed as an indirect indicator of cortical serotonergic activity, whereas single photon emission computed tomography (SPECT) and [123I]ADAM allow the selective assessment of brain serotonin transporters (SERT). The aim of this study was to investigate LD and SERT availability as independent variables of the brain serotonergic system in healthy volunteers. Fifteen (six male, nine female) subjects received both neurophysiological and imaging investigations. Evoked potentials were recorded following the application of acoustic stimuli with increasing intensities; the LD was analyzed using dipole source analysis. SPECT was performed four hours after injection of 137 +/- 11.4 MBq [123I]ADAM. As a measure of SERT availability specific ADAM brainstem binding was used. LD correlated significantly with SERT availability (Pearson's correlations: rho = -0.57, p < 0.05). The correlations remained significant after controlling for the effects of age or gender (partial correlations: rho = -0.60, p < 0.05) but were pronounced in the female group (rho = -0.83, p < 0.01). Associations between LD and SERT availability contribute to the understanding of the central serotonergic system and further validate the use of neurophysiological approaches as indirect measures of neurochemical brain activity.

Evaluation of the Serotonin Transporter Ligand 123I-ADAM for SPECT Studies on Humans

Imaging serotonin transporters in the living human brain is important in several fields, such as normal psychophysiology, mood disorders, eating disorders, and neurodegenerative disorders. The aim of this study was to compare different kinetic and semiquantitative methods for assessing serotonin transporters using (123)I-labeled 2-((2-((dimethylamino)methyl)phenyl)thio)-5-iodophenylamine (ADAM) in humans: an arterial plasma input model, simplified and Logan reference tissue models, and standardized uptake value ratios.

METHODS

Nine subjects were scanned with dynamic (123)I-ADAM SPECT (mean age, 31 y; range, 24-43 y), and metabolite-corrected arterial input was measured. Tissue reference models (simplified reference tissue model, Logan reference tissue model, and ratio method) were validated against the outcome of a 1-tissue-compartment model, and performance with decreasing scan length was evaluated. The specificity of (123)I-ADAM binding was investigated in a blocking experiment.

RESULTS

Binding estimates from the simplified reference tissue and Logan reference tissue models correlated tightly with full kinetic modeling when based on a 240- or 360-min dynamic acquisition (r = 0.99); however, there were slight underestimations (3%-5%), especially in high-binding regions. Application of the ratio method to data from 200 to 240 min overestimated specific binding (on average, by 10% +/- 28%) and correlated only moderately with estimates from the 1-tissue-compartment model (r = 0.94). With an acquisition time of 0-120 min, the Logan model still yielded an acceptable outcome when a fixed clearance rate constant (k2') from the cerebellum was applied. Intravenously injected citalopram was not associated with a decrease in cerebellar binding. A lipophilic metabolite that did not seem to bind specifically to serotonin transporter was seen in 2 of 7 subjects.

CONCLUSION

Serotonin transporter binding with (123)I-ADAM SPECT can be assessed with the Logan model based on a 120-min acquisition when a constant k2' is applied. This model, because it allows for more accurate and less biased binding estimates and thus reduces the required sample size, is advantageous over the ratio method used in clinical studies so far. A single blocking experiment supported the use of the cerebellum as a reference region.

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.

New fluoro-diphenylchalcogen derivatives to explore the serotonin transporter by PET

A series of fluorinated diphenylchalcogen derivatives, possessing a sulfur or an oxygen bridge, has been prepared with the aim to get a suitable radiotracer to image the SERT in vivo using positron emission tomography (PET). The compounds were synthesized and assayed toward the serotonin (SERT), dopamine (DAT), and norepinephrine (NET) transporters. Among the developed series, five compounds display a high SERT affinity (K(i): 0.27-2.91 nM range) and can be labeled either with carbon-11 or fluorine-18.

Imaging the availability of serotonin transporter in rat brain with 123I-ADAM and small-animal SPECT

BACKGROUND

Imaging serotonin transporters during antidepressant treatment in small animals is a useful tool for preclinical study during drug development. In this work, we aimed to demonstrate the feasibility of using 123I-ADAM and small-animal SPECT to monitor serotonin transporter availabilities in rat brains prior to and after administration of a selective serotonin re-uptake inhibitor.

METHODS

Male Sprague-Dawley rats with and without administration of citalopram (4 mg x kg body weight) were examined in this study. During the process rat brains were scanned using a double-headed microSPECT system equipped with pinhole collimators. SPECT tomographic images and X-ray computed tomography (CT) were acquired after introducing 123I-ADAM via the tail vein. The 123I-ADAM specific binding was assessed by SPECT/CT fused image to draw regions of interest in the midbrain and cerebellum. Ex-vivo autoradiography was carried out as a parallel investigation to validate the SPECT technique.

RESULTS

SPECT images displayed specific binding ratio in midbrain to be 0.91+/-0.30 averaged from three rats. Drug occupancies (95.47+/-1.56)% were shown after administration of citalopram in a dosage of 4 mg x kg.

CONCLUSION

This study demonstrated that the serotonin transporter availability during antidepressant treatment in small animals can be assessed semi-quantitatively by using 123I-ADAM and SPECT.

5-Chloro-2-(2'-((dimethylamino)methyl)-4'-iodophenylthio)benzenamine: a new serotonin transporter ligand

Two novel ligands with 4' substitution on the Phenyl Ring B of biphenylthiol, 5-chloro-2-(2'-((dimethylamino)methyl)-4'-iodophenylthio)benzenamine (7) and 2-(2'-((dimethylamino)methyl)-4'-methoxyphenylthio)-5-iodobenzenamine (8), were prepared and tested as potential serotonin transporter (SERT) imaging agents. The new ligands displayed extremely high binding affinities to SERT (K(i)=0.22+/-0.09 and 0.11+/-0.04 nM, respectively), with very low binding affinities to dopamine and norepinephrine transporters (K(i)>1000 nM). The corresponding [(125)I]7 and [(125)I]8 were successfully prepared from tri-n-butyltin derivatives. They showed good brain uptakes and prolonged retention after intravenous injection in rats (brain uptake was 1.77% and 0.98% dose/g for [(125)I]7, and 0.92% and 0.29% dose/g for [(125)I]8, at 2 and 120 min, respectively). Significantly, [(125)I]7 showed excellent uptake and prolonged retention in the hypothalamus, where SERT concentration was highest. The hypothalamus/cerebellum (HY/CB) ratios (target/background ratios) were 4.24, 7.10, 8.24 and 12.6 at 2, 4, 6 and 12 h, respectively. The HY/CB ratios for [(125)I]8 were 3.97, 5.57 and 5.06 at 1, 2 and 4 h, respectively. Adding the 4'-iodo group to the Phenyl Ring B of Compound (7) appeared to reduce the rate of clearance from the brain, and kinetics favored uptake and retention in the hypothalamus. The localization of [(125)I]7 in the hypothalamus region in the rat brain could be blocked by pretreatment with (+)McN5652, escitalopram and ADAM (2), which are all selective SERT ligands (at 2 mg/kg iv, 5 min pretreatment). Ex vivo autoradiograms of rat brain sections (at 4 h after intravenous injection of [(125)I]7) showed intense labeling in regions of the brain known to have high SERT density. The excellent selective uptake and retention in the hypothalamus region suggest that [(123)I]7 is a potential lead compound for developing new imaging agents targeting SERT-binding sites with single-photon emission computed tomography.

Serotonin transporter binding of [123I]ADAM in bulimic women, their healthy twin sisters, and healthy women: a SPET study

BACKGROUND

Bulimia Nervosa (BN) is believed to be caused by an interaction of genetic and environmental factors. Previous studies support the existence of a bulimia-related endophenotype as well as disturbances in serotonin (5-HT) transmission. We studied serotonin transporter (SERT) binding in BN, and to investigate the possibility of a SERT-related endophenotype for BN, did this in a sample of female twins. We hypothesized clearly reduced SERT binding in BN women as opposed to healthy women, and intermediate SERT binding in unaffected co-twins.

METHODS

We studied 13 female twins with BN (9 with purging and 4 with non-purging BN) and 25 healthy women, including 6 healthy twin sisters of BN patients and 19 women from 10 healthy twin pairs. [123I]ADAM, a selective SERT radioligand for single photon emission tomography (SPET) imaging, was used to assess SERT availability in the midbrain and the thalamus.

RESULTS

No differences in SERT binding were evident when comparing the BN women, their unaffected co-twins and the healthy controls (p = 0.14). The healthy sisters of the BN patients and the healthy control women had similar SERT binding in both brain regions. In a post hoc subgroup analysis, the purging bulimics had higher SERT binding than the healthy women in the midbrain (p = 0.03), but not in the thalamus.

CONCLUSION

Our finding of increased SERT binding in the midbrain in the purging BN women raises the possibility that this subgroup of bulimics might differ in serotonergic function from the non-purging ones. The similarity of the unaffected co-twins and the healthy controls doesn't support our initial assumption of a SERT-related endophenotype for BN. Due to the small sample size, our results need to be interpreted with caution and verified in a larger sample.

Higher serotonin transporter occupancy after multiple dose administration of escitalopram compared to citalopram: an [123I]ADAM SPECT study

OBJECTIVES

Previous studies have investigated the occupancy of the serotonin reuptake transporter (SERT) after clinical doses of citalopram and other selective serotonin reuptake inhibitors. In the present study, the occupancies of SERT after multiple doses of escitalopram and citalopram were compared using the radioligand [(123)I]ADAM and single photon emission computed tomography (SPECT).

METHODS

Fifteen healthy subjects received escitalopram 10 mg/day (n = 6) or citalopram 20 mg/day (n = 9) for a total of 10 days. SERT occupancies in midbrain were determined with SPECT and [(123)I]ADAM at three different time points: at baseline (no medication) and at 6 and 54 h after last drug intake.

RESULTS

At 6 h after the last dose, mean SERT occupancies were 81.5 +/- 5.4% (mean+/-SD) for escitalopram and 64.0 +/- 12.7% for citalopram (p < 0.01). At 54 h after the last dose, mean SERT occupancies were 63.3 +/- 12.1% for escitalopram and 49.0 +/- 11.7% for citalopram (p < 0.05). The plasma concentrations of the S-enantiomer were of the same magnitude in both substances. For both drugs, the elimination rate of the S-enantiomer in plasma was markedly higher than the occupancy decline rate in the midbrain.

CONCLUSION

The significantly higher occupancy of SERT after multiple doses of escitalopram compared to citalopram indicates an increased inhibition of SERT by escitalopram. The results can also be explained by an attenuating effect of R-citalopram on the occupancy of S-citalopram at the SERT.

Increased serotonin transporter availability in the brainstem of migraineurs

For decades, serotonin has been speculated to play a major role in migraine pathophysiology. The central serotonergic system is located in the raphe nuclei and the adjacent reticular formation in the brainstem. Recently, radioligands targeting the brain serotonin transport protein (SERT) have been developed. We used the highly specific SERT-radioligand (123)I-ADAM [2-((2-((dimethylamino) methyl)phenyl)thio)-5-iodophenylamine] to test the hypothesis of the mesopontine serotonergic system being involved in the pathophysiology of migraine. Nineteen migraine patients and 10 healthy, age- and sex-matched controls were enrolled. The neuroimaging study was performed interictally during the pain-free interval. Single Photon Emission Computed Tomography (SPECT)-images were coregistered with MRI-scans. Region of interest (ROI)-analysis revealed a highly significant increase of (123)I-ADAM uptake in the mesopontine brainstem of migraineurs (p < 0.001). In contrast, (123)IADAM uptake in the thalamus did not differ significantly between migraineurs and controls. Our study demonstrates for the first time a significant increase of brainstem SERT-availability in migraineurs, suggesting a dysregulation of the brainstem serotonergic system. It remains to be elucidated whether the altered SERT-availability is causally related to migraine pathophysiology or whether it reflects secondary pathophysiological mechanisms.

In vivo imaging of serotonin transporter occupancy by means of SPECT and [123I]ADAM in healthy subjects administered different doses of escitalopram or citalopram

BACKGROUND

Escitalopram is a dual serotonin reuptake inhibitor (SSRI) approved for the treatment of depression and anxiety disorders. It is the S-enantiomer of citalopram, and is responsible for the serotonin reuptake activity, and thus for its pharmacological effects. Previous studies pointed out that clinically efficacious doses of other SSRIs produce an occupancy of the serotonin reuptake transporter (SERT) of about 80% or more. The novel radioligand [123I]ADAM and single photon emission computer tomography (SPECT) were used to measure midbrain SERT occupancies for different doses of escitalopram and citalopram.

METHODS

Twenty-five healthy subjects received a single dose of escitalopram [5 mg (n=5), 10 mg (n=5), and 20 mg (n=5)] or citalopram [(10 mg (n=5) and 20 mg (n=5)]. Midbrain SERT binding was measured with [(123)I]ADAM and SPECT on two study days, once without study drug and once 6 h after single dose administration of the study drug. The ratio of midbrain-cerebellum/cerebellum was the outcome measure (V3") for specific binding to SERT in midbrain. Subsequently, SERT occupancy levels were calculated using the untreated baseline level for each subject. An Emax model was used to describe the relationship between S-citalopram concentrations and SERT occupancy values. Additionally, four subjects received placebo to determine test-retest variability.

RESULTS

Single doses of 5, 10, or 20 mg escitalopram led to a mean SERT occupancy of 60+/-6, 64+/-6, and 75+/-5%, respectively. SERT occupancies for subjects treated with single doses of 10 and 20 mg citalopram were 65+/-10 and 70+/-6%, respectively. A statistically significant difference was found between SERT occupancies after application of 10 and 20 mg escitalopram, but not for 10 and 20 mg citalopram. There was no statistically significant difference between the SERT occupancies of either 10 mg citalopram or 10 mg escitalopram, or between 20 mg citalopram and 20 mg escitalopram. Emax was slightly higher after administration of citalopram (84%) than escitalopram (79%). In the test-retest study, a mean SERT "occupancy" of 4% was found after administration of placebo, the intraclass correlation coefficient was 0.92, and the repeatability coefficient was 0.25.

CONCLUSION

SPECT and [123I]ADAM were used to investigate SERT occupancies after single doses of escitalopram or citalopram. The test-retest study revealed good reproducibility of SERT quantification. Similar SERT occupancies were found after administration of equal doses (in respect to mg) of escitalopram and citalopram, giving indirect evidence for a fractional blockade of SERT by the inactive R-citalopram.

Mass effect of injected dose in small rodent imaging by SPECT and PET

This paper discusses the effect of mass (chemical quantity) of injected dose on positron emission tomography (PET) and single-photon emission computed tomography (SPECT). Commonly, PET or SPECT imaging study uses a "no-carrier added" dose, which contains a small amount of radioactive imaging agent (in picogram to microgram). For small animal (rodent) imaging studies, specifically targeting binding sites or biological processes, the mass (chemical quantity) in the dose may significantly modify the binding, pharmacokinetics and, ultimately, the imaging outcome. Due to differences in size and other physiological factors between humans and rodents, there is a dramatic divergence of mass effect between small animal and human imaging study. In small animal imaging studies, the mass, or effective dose (ED(50)), a dose required for 50% of receptor or binding site occupancy, is usually not directly related to binding potential (B(max)/K(d)) (measured by in vitro binding assay). It is likely that dynamic interplays between specific and nonspecific binding in blood circulation, transient lung retention, kidney excretion, liver-gallbladder flow, soft tissue retention as well as metabolism could each play a significant role in determining the concentration of the tracer in the target regions. When using small animal imaging for studying drug occupancy (either by a pretreatment, coinjection or chasing dose), the mass effects on imaging outcome are important factors for consideration.

[(11)C]MADAM, a new serotonin transporter radioligand characterized in the monkey brain by PET

The aim of this study was to explore the potential of a new selective serotonin transporter (5-HTT) inhibitor, N,N-dimethyl-2-(2-amino-4-methylphenylthio)benzylamine (MADAM, K(i)=1.65 nM), as a PET radioligand for examination of 5-HTT in the nonhuman primate brain. MADAM was radiolabeled by an N-methylation reaction using [(11)C]methyl triflate and the binding was characterized by PET in four cynomolgus monkeys. Metabolite levels in plasma were measured by gradient high-performance liquid chromatography (HPLC). The radiochemical incorporation yield of [(11)C]MADAM was 75-80% and the specific radioactivity at the time of administration was 34-652 GBq/micromol (n=8). The highest uptake of radioactivity was observed in striatum, thalamus, mesencephalon, and the lower brainstem. Lower binding was detected in neocortex and the lowest radioactive uptake was found in the cerebellum. This distribution is in accordance with the known expression of 5-HTT in vitro. The fraction of the total radioactivity in monkey plasma representing unchanged [(11)C]MADAM was 20% at 45 min after injection, as measured by gradient HPLC. Pretreatment measurements, using unlabeled citalopram, GBR 12909, and maprotiline, as well as a displacement measurement, using unlabeled MADAM, confirmed that [(11)C]MADAM binds selectively and reversibly to 5-HTT, and support the use of the cerebellum as reference region. The present characterization of binding in the monkey brain suggests that [(11)C]MADAM is a potential PET radioligand for quantitative studies of 5-HTT binding in the human brain.

Imaging of serotonin transporters and its blockade by citalopram in patients with major depression using a novel SPECT ligand [123I]-ADAM

We studied the midbrain SERT availability in patients with major depression and assessed the relation of SERT occupancy by citalopram to the treatment response. 21 non-medicated patients with major depression and 13 healthy controls were examined by [(123)I]-ADAM SPECT. The midbrain SERT availability (SERT V(3)'') was calculated using individual MRI scans. In 13/21 patients SPECT was repeated 7 days after oral medication with citalopram (10 mg/day). We found no significant difference in the mean midbrain SERT availability between the studied patients with major depression and healthy controls (0.86 +/- 0.27 vs. 0.71 +/- 0.44, p = 0.069). The mean SERT occupancy accounted to 61%. The degree of SERT blockade by citalopram did not correlate with the reduction in HAMD total score. Treatment with low-dosed citalopram caused individually variable occupancy of the midbrain-SERT and a rapid clinical improvement in 54% of the investigated patients.

Brain kinetics of the new selective serotonin transporter tracer [123I]ADAM in healthy young adults

Recently, the tracer (123)I-2-([2-({dimethylamino}methyl)phenyl]thio)-5-iodophenylamine ([(123)I]ADAM) has been developed for selective imaging of serotonin transporters (SERTs) with single photon emission computed tomography (SPECT). The purpose of this study was to develop an [(123)I]ADAM SPECT protocol for clinical studies in young adults.

METHODS

We examined the time course of [(123)I]ADAM binding to central SERTs in eight healthy young volunteers up to 6 h postinjection.

RESULTS

We found that the time of peak-specific [(123)I]ADAM binding was highly variable among subjects, but specific binding in the SERT-rich (hypo)thalamus peaked within 5 h postinjection in all subjects. Moreover, in this brain area, binding ratios of specific to nonspecific binding did not significantly change between 3 and 6 h postinjection, and peaked 5 h postinjection.

CONCLUSIONS

Five hours postinjection may be optimal for single-scan [(123)I]ADAM SPECT studies in humans, but more work is needed to assess the accuracy of the 5-h tissue ratio as a measure of SERT in the brain.

Brain SPECT imaging and whole-body biodistribution with [123I]ADAM — a serotonin transporter radiotracer in healthy human subjects

INTRODUCTION

[(123)I]-2-((2-((dimethylamino)methyl)phenyl)thio)-5-iodophenylamine ([(123)I]ADAM), a novel radiotracer, has promising application in the imaging of the serotonin transporter (SERT) in the human brain. In this study, the optimal scanning time for acquiring brain single photon emission computed tomography (SPECT) images was determined by performing dynamic SPECT studies at intervals from 0 to 6 h postinjection of [(123)I]ADAM. Additionally, radiation-absorbed doses were determined for three healthy human subjects using attenuation-corrected images.

METHODS

Twelve subjects were randomized into one of three study groups as follows: whole-body distribution imaging (n=3), dynamic SPECT imaging (n=3) and brain SPECT imaging (n=6). The radiation-absorbed dose was calculated using MIRDOSE 3.0 software with attenuation-corrected data. The specific binding (SB) ratio of the brain stem was measured from dynamic SPECT images to determine the optimal scanning time.

RESULTS

Dynamic SPECT images showed that the SB of the brain stem gradually increased to a maximum 4 h postinjection. Single photon emission computed tomography images at 4 h postinjection showed a high uptake of the radiotracer (SB) in the hypothalamus (1.40+/-0.12), brain stem (1.44+/-0.16), pons (1.13+/-0.14) and medial temporal lobe (0.59+/-0.10). The mean adult male value of effective dose was 3.37 x 10(-2) mSv/MBq with a 4.8-h urine-voiding interval. Initial high uptake in SERT-rich sites was demonstrated in the lung and brain. A prominent washout of the radiotracer from the lung further increased brain radioactivity that reached a peak value of 5.03% of injected dose 40 min postinjection.

CONCLUSIONS

[(123)I]ADAM is a promising radiotracer for SPECT imaging of SERT in humans with acceptable dosimetry and high uptake in SERT-rich regions. Brain SPECT images taken within 4 h following injection show optimal levels of radiotracer uptake in known SERT sites. However, dynamic changes in lung SERT distribution must be carefully evaluated.

Synthesis of novel, potentially biologically active dibenzosuberone derivatives

Novel representatives of the important group of biologically active dibenzosuberone derivatives were prepared: 3,7-dibromo-5-(dimethylaminoethyl-oxyimino)-10,11-dihydro-5H-dibenzo[a,d]cyclohepta-1,4-diene (1), 3,7-dibromo-5-(3-dimethylaminopropylidene)-10,11-dihydro-5H-dibenzo[a,d]cycloheptene (2) and 1,7-dibromo-5-(3-dimethylaminopropylidene)-10,11-dihydro-5H-dibenzo-[a,d]-cycloheptene (3). These compounds are potential tricyclic antidepressants (TCAs), which are still the most frequently prescribed antidepressants in many countries.

Measuring SSRI occupancy of SERT using the novel tracer [123I]ADAM: a SPECT validation study

PURPOSE

Serotonergic brain regions play a crucial role in the modulation of emotion, and serotonergic dysfunction may contribute to several neurological disorders. [123I]ADAM is a novel SPECT tracer which binds with high affinity to serotonin transporters (SERT). The objective of this study was to compare different methods for the quantification of tracer binding and to develop a simplified single-scan protocol for this tracer, as well as to investigate its potential for characterisation of the transporter occupancy versus plasma concentration curve of a selective serotonin re-uptake inhibitor (SSRI).

METHODS

Dynamic SPECT scans were performed on 16 healthy volunteers after administration of approximately 150 MBq [123I]ADAM. Data were acquired from the time of injection until approximately 5.5 h after injection in 30- or 45-min sessions. Each subject was scanned twice: with and without pre-treatment with the SSRI citalopram in various dosage regimens. The plasma concentration of citalopram (C(p)) was determined from venous samples. Images were reconstructed by filtered back-projection with scatter and attenuation correction. Tracer binding was quantified for midbrain, striatum and thalamus using cerebellum as a reference region. Quantification was done by kinetic modelling, graphical analysis and multi-linear regression, as well as by the ratio method, with binding potential (BP2) as the outcome measure. The SERT occupancy by citalopram was determined relative to the baseline scan for each subject, and the occupancy versus C(p) curve was fitted with the E(max) model.

RESULTS

The highest binding of [123I]ADAM was in midbrain (mean baseline BP2+/-SD=1.31+/-0.29), with lower binding in thalamus (0.79+/-0.16) and striatum (0.66+/-0.13). There was good agreement between BP2 values obtained by different quantification methods. Using the ratio method, the best agreement with kinetic modelling was obtained with data from the time interval [200,260] min after injection. The fitting of the midbrain occupancy curve yielded a maximum occupancy of 84% and a plasma concentration required to reach 50% of the maximum of 2.5 ng/ml, with a goodness-of-fit variability of 13% (SD).

CONCLUSION

Binding of [123I]ADAM to SERT in midbrain can be quantified with a single scan starting 200 min after injection. However, the variability of estimated occupancy values may be too high for critical assessment of occupancy of SERT by SSRI.

The new PET imaging agent [11C]AFE is a selective serotonin transporter ligand with fast brain uptake kinetics

A new positron emission tomography (PET) radioligand for the serotonin transporter (SERT), [(11)C]2-[2-[[(dimethylamino)methyl]phenyl]thio]-5-(2-fluoroethyl)phenylamine ([(11)C]AFE, 12), was synthesized and evaluated in vivo in rats and baboons. [(11)C]AFE (12) was prepared from its monomethylamino precursor 11 by reaction with high specific activity [(11)C]methyl triflate. Radiochemical yield was 32+/-17% based on [(11)C]methyl triflate (n=6) and specific activity was 1670+/-864 Ci/mmol at end of synthesis (EOS, n=6). Binding assays indicated that AFE displays high affinity for SERT (K(i)=1.80 nM for hSERT) and lower affinity for norepinephrine transporter (K(i)=946 nM for hNET) or dopamine transporter (K(i)>10,000 nM for hDAT). In addition, AFE displays negligible binding affinities for other serotonin and dopamine receptors, indicating an excellent binding selectivity in vitro. Biodistribution studies in rats indicated that [(11)C]AFE enters the brain readily and localizes in regions known to contain high concentrations of SERT, such as the thalamus, hypothalamus, frontal cortex and striatum. Moreover, such binding in SERT-rich brain regions is reduced significantly by pretreatment with either citalopram or the cold compound itself, but not by nisoxetine or GBR 12935, thus demonstrating that [(11)C]AFE binding in the rat brain is saturable, specific and selective for the SERT. Imaging experiments in baboons indicated that the uptake pattern of [(11)C]AFE is consistent with the known distribution of SERT in the baboon brain, with high levels of radioactivity detected in the midbrain and thalamus, moderate levels in the hippocampus and striatum and low levels in the cortical regions. The uptake kinetics of [(11)C]AFE in the baboon brain is rapid, with activity in the midbrain and thalamus peaking at 15-40 min postinjection. Pretreatment of the baboon with citalopram (4 mg/kg) 20 min before radioactivity injection reduced the binding of [(11)C]AFE in all SERT-containing brain regions to the level in the cerebellum. Kinetic analysis revealed that in all brain regions examined, [(11)C]AFE specific-to-nonspecific partition coefficients (V(3)'') are similar to those of [(11)C]McN5652 and [(11)C]2-[2-[[(dimethylamino)methyl]phenyl]thio]-5-fluorophenylamine ([(11)C]AFA), but lower than those of [(11)C]2-[2-[[(dimethylamino)methyl]phenyl]thio]-5-fluoromethylphenylamine ([(11)C]AFM) or [(11)C]DASB. In summary, [(11)C]AFE appears to be a PET radioligand with fast brain uptake kinetics and can be used for the visualization and quantification of SERT in vivo.

Serotonin transporter availability in patients with schizophrenia: a positron emission tomography imaging study with [11C]DASB

BACKGROUND

Postmortem studies have reported several alterations in serotonin transporter (SERT) binding parameters in patients with schizophrenia. The aim of this study was to compare SERT availability in vivo in patients with schizophrenia and matched control subjects.

METHODS

Ten medication-free patients with schizophrenia and 10 healthy subjects underwent positron emission tomography (PET) scans for 90 min after 11C-3-amino-4-(2-dimethylaminomethylphenylthio)benzonitrile ([11C]DASB) injection. Metabolite-corrected arterial input function was measured. Regional distribution volumes (mL/g) were derived with a two tissue compartment kinetic model. Outcome measures for SERT availability included binding potential (BP) and the specific-to-nonspecific equilibrium partition coefficient (V3''). Ten brain regions with high density of SERT and where SERT availability can be reliably quantified with [11C]DASB were included in the analysis.

RESULTS

No significant differences were observed in regional BP or V3'' between patients and control subjects. No significant relationships were observed between regional SERT availability and severity of positive, negative, and depressive symptoms.

CONCLUSIONS

This study failed to detect alterations of SERT availability in patients with schizophrenia; however, this study does not rule out the possibility that schizophrenia might be associated with alterations of SERT density in the cortical regions, where the [11C]DASB-specific binding signal is too low for reliable quantification of SERT.

Weaving single photon imaging into new drug development

The specific aim of this review is to assess the potential contribution of single photon emitting radiopharmaceutical technologies to new drug development. For each phase of therapeutic drug development, published literature was sought that shows single photon emitters can add value by quantifying pharmacokinetics, visualizing mechanisms of drug action, estimating therapeutic safety indices, or measuring dose-dependent pharmacodynamic effects. Not any published reports were found that describe using nuclear medicine techniques to help manage the progress of a new drug development program. As a consequence, most of the case in favor of weaving single photon imaging into the process had to be built on extrapolations from studies that showed feasibility post hoc. The strongest evidence of potential value was found for drug candidates that hope to influence diseases characterized by cell proliferation or cell death, particularly in the fields of oncology, cardiology, nephrology, and inflammation. Receptor occupancy studies were observed to occasionally offer unique advantages over analogous studies with positron emission tomography (PET). Enough hard data sets were found to justify the costs of using single photon imaging in a variety of new drug development paradigms.

Validation of [(123)I]beta-CIT SPECT to assess serotonin transporters in vivo in humans: a double-blind, placebo-controlled, crossover study with the selective serotonin reuptake inhibitor citalopram

Disturbances in the serotonin (5-HT) system are associated with various neuropsychiatric disorders. The 5-HT system can be studied in vivo by measuring 5-HT transporter (SERT) densities using (123)iodine-labeled 2beta-carbomethoxy-3beta(4-iodophenyl)tropane ([(123)I]beta-CIT) and single photon emission computed tomography (SPECT). Validation of this technique is important because [(123)I]beta-CIT does not bind selectively to SERTs. Some studies have validated this technique in vivo in the human brain in SERT-rich areas, but the technique has not been validated yet in SERT-low cortical areas. The aim of this study was to further validate [(123)I]beta-CIT SPECT in assessing SERTs in vivo in humans in both SERT-rich and SERT-low areas. A double-blind, placebo-controlled, crossover design was used with the selective 5-HT reuptake inhibitor (SSRI) citalopram. Six male subjects underwent two [(123)I]beta-CIT SPECT sessions: one after pretreatment with citalopram and one after placebo. Scans were acquired 4 h and 22-27 h p.i., and both region-of-interest and voxel-by-voxel analyses were performed. Citalopram reduced [(123)I]beta-CIT binding ratios in SERT-rich midbrain and (hypo)thalamus. Binding ratios were also lower after citalopram in SERT-low cortical areas, but statistical significance was only reached in several cortical areas using voxel-by-voxel analysis. In addition, citalopram increased binding ratios in the DAT-rich striatum and increased absolute uptake in the cerebellum. The results show that [(123)I]beta-CIT SPECT is a valid technique to study SERT binding in vivo in human brain in SERT-rich areas. Although we provide some evidence that [(123)I]beta-CIT SPECT may be used to measure SERTs in SERT-low cortical areas, these measurements must be interpreted with caution.

Carbon-11 HOMADAM: A novel PET radiotracer for imaging serotonin transporters

Carbon-11-labeled N,N-dimethyl-2-(2'-amino-4'-hydroxymethylphenylthio)benzylamine (HOMADAM) was synthesized as a new serotonin transporter (SERT) imaging agent.

METHODS

Carbon-11 was introduced into HOMADAM by preparation of N-methyl-2-(2'-amino-4'-hydroxymethylphenylthio)benzylamine followed by alkylation with carbon-11 iodomethane. Binding affinities of HOMADAM and the radiolabeling substrate, N-methyl-2-(2'-amino-4'-hydroxymethylphenylthio)benzylamine, were determined in cDNA transfected cells expressing human SERT, dopamine transporters (DAT) and norepinephrine transporters NET using [3H]citalopram, [(125)I]RTI-55 and [3H]nisoxetine, respectively. MicroPET brain imaging was performed in monkeys. Arterial plasma metabolites of HOMADAM were analyzed in a rhesus monkey by high-performance liquid chromatography (HPLC).

RESULTS

HOMADAM displayed high affinity for the SERT (Ki = 0.6 nM). N-methyl-2-(2'-amino-4'-hydroxymethylphenylthio)benzylamine displayed moderate affinity for the SERT (Ki = 15.11 nM). The affinities of HOMADAM for the DAT and NET were 2000- and 253-fold lower, respectively, than for the SERT. [11C]HOMADAM was prepared from [11C]iodomethane in approximately 25% radiochemical yield (decay-corrected to end of bombardment). MicroPET brain imaging studies in monkeys demonstrated that [11C]HOMADAM uptake was selectively localized in the midbrain, thalamus, pons, caudate, putamen and medulla. The midbrain-to-cerebellum, pons-to-cerebellum, thalamus-to-cerebellum and putamen-to-cerebellum ratios at 85 min were 4.2, 2.8, 2.3 and 2.0, respectively. HOMADAM binding achieved quasi-equilibrium at 45 min. Radioactivity in the SERT-rich regions of monkey brain was displaceable with R,S-citalopram. Radioactivity in the DAT-rich regions of monkey brain was not displaceable with the DAT ligand RTI-113. Radioactivity in the SERT-rich regions of monkey brain was displaceable with the R,S-reboxetine, a NET ligand with a high nanomolar affinity for SERT. Arterial plasma metabolites of HOMADAM were analyzed in a rhesus monkey by HPLC and displayed a single peak that corresponded to unmetabolized HOMADAM.

CONCLUSION

HOMADAM is an excellent candidate for PET primate imaging of brain SERTs.

A PET imaging agent with fast kinetics: synthesis and in vivo evaluation of the serotonin transporter ligand [11C]2-[2-dimethylaminomethylphenylthio)]-5-fluorophenylamine ([11C]AFA)

A new serotonin transporter (SERT) ligand, [11C]2-[2-(dimethylaminomethylphenylthio)]-5-fluorophenylamine (10, [11C]AFA), was synthesized and evaluated as a candidate PET radioligand in pharmacological and pharmacokinetic studies. As a PET radioligand, AFA (8) can be labeled with either C-11 or F-18. In vitro, AFA displayed high affinity for SERT (Ki 1.46 +/- 0.15 nM) and lower affinity for norepinephrine transporter (NET, Ki 141.7 +/- 47.4 nM) or dopamine transporter (DAT, Ki > 10,000 nM). [11C]AFA (10) was prepared from its monomethylamino precursor 9 by reaction with high specific activity [11C]methyl iodide. Radiochemical yield was 43 +/- 20% based on [11C]methyl iodide at end of bombardment (EOB, n = 10) and specific activity was 2,129 +/- 1,369 Ci/mmol at end of synthesis (EOS, n = 10). Biodistribution studies in rats indicated that [11C]AFA accumulated in brain regions known to contain high concentrations of SERT. Binding in SERT-rich brain regions was reduced significantly by pretreatment with either the cold compound 8 or with the selective serotonin reuptake inhibitor (SSRI) citalopram, but not by the selective norepinephrine reuptake inhibitor nisoxetine, thus underlining its in vivo binding selectivity and specificity for SERT. Imaging experiments in baboons demonstrated that the uptake pattern of [11C]AFA in the baboon brain is consistent with the known distribution of SERT, with highest activity levels in the midbrain and thalamus, followed by striatum, hippocampus, and cortical regions. Activity levels in the baboon brain peaked at 15-40 min after radioligand injection, indicating a fast uptake kinetics for [11C]AFA. Pretreatment of the baboon with citalopram (4 mg/kg) significantly reduced the specific binding of [11C]AFA in all SERT-containing brain regions. Kinetic analysis revealed that the regional equilibrium specific to non-specific partition coefficients (V3") of [11C]AFA are similar to those of [11C]McN5652, but lower than those of [11C]AFM or [11C]DASB. In summary, [11C]AFA appears to be an appropriate PET radioligand with a fast brain uptake kinetics:

Synthesis and characterization of EADAM: a selective radioligand for mapping the brain serotonin transporters by positron emission tomography

[11C]N,N-Dimethyl-2-(2'-amino-4'-ethylphenylthio)benzylamine ([11C]EADAM) was synthesized in the development of a serotonin transporter (SERT) imaging ligand for positron emission tomography (PET). The methods of ligand synthesis, results of in vitro characterization, 11C labeling and in vivo micro-PET imaging studies of [11C]EADAM in cynomolgus monkey brain are described. 11C was introduced into N,N-dimethyl-2-(2'-amino-4'-ethylphenylthio)benzylamine (5) by alkylation of N-methyl-2-(2'-amino-4'-ethylphenylthio)benzylamine (10) in 32% radiochemical yield (end of bombardment [EOB], decay-corrected from [11C]methyl iodide). Competition binding assays in cells stably expressing the transfected human dopamine transporter (DAT), SERT and norepinephrine transporter (NET) labeled with [3H]WIN 35428 or [(125)I]RTI-55, [3H]citalopram and [3H]nisoxetine, respectively, indicated the following order of SERT affinity: ADAM>EADAM>>fluvoxamine. The affinity of EADAM for DAT and NET was 500- and >1000-fold lower, respectively, than for SERT. Micro-PET brain imaging studies in a cynomolgus monkey demonstrated high [11C]EADAM uptake in the striatum, thalamus and brainstem. [11C]EADAM uptake in these brain regions peaked in less than 60 min following administration of [11C]EADAM. The tissue-to-cerebellum ratios of the striatum, thalamus and brainstem were 1.67, 1.71 and 1.63, respectively, at 120 min postinjection of [11C]EADAM. Analysis of monkey arterial plasma samples using high-pressure liquid chromatography determined there was no detectable formation of lipophilic radiolabeled metabolites capable of entering the brain. In a displacement experiment with citalopram in a cynomolgus monkey, radioactivity in the striatum, thalamus and brainstem was displaced 20-60 min after administration of citalopram. In a blocking experiment with citalopram in a cynomolgus monkey, radioactivity in the striatum, thalamus and brainstem was significantly reduced. These results support the candidacy of [11C]EADAM as a radioligand for visualizing brain SERT using PET.

Synthesis and evaluation of N,N-dimethyl-2-(2-amino-5-[18F]fluorophenylthio)benzylamine (5-[18F]-ADAM) as a serotonin transporter imaging agent

The synthesis and evaluation of a new serotonin transporter (SERT) imaging agent, N,N-dimethyl-2-(2-amino-5-[18F]fluorophenylthio)benzylamine (5-[18F]-ADAM) is reported. Nucleophilic substitution of N,N-dimethyl-2-(2-nitro-5-bromophenylthio)benzylamine with K[18F]/Kryptofix 2.2.2 in DMSO at 125 degrees C followed by reduction with NaBH4-Cu(OAc)2 in EtOH at 78 degrees C and purification with HPLC produces the desired compound with an unoptimized yield of approximately 5-10% in a synthesis time of 150 min from EOB. The biodistribution of 5-[18F]-ADAM in rats showed a high initial uptake and relatively rapid clearance in the brain (3.221+/-0.762, 0.440+/-0.059, 0.160+/-0.035 and 0.028+/-0.003% injected dose/organ at 2, 30, 60 and 120 min after I.V. injection, respectively) with the specific binding peaking at 1 h postinjection (hypothalamus/cerebellum and hippocampus/cerebellum were 2.97 and 3.59, respectively). The initial uptake in blood, lung, kidney and heart were also high, but it cleared rapidly. The radioactivity in the femur increased with time for 5-[18F]-ADAM indicating that in vivo defluorination may occur. Metabolism studies in rats showed that 5-[18F]-ADAM was not metabolized in rat brain, but was metabolized rapidly in the blood. Blocking experiments showed that there were significant decreases in the uptake of 5-[18F]-ADAM in the brain regions (hypothalamus, hippocampus and striatum) where SERT concentrations are high when rats were pretreated with (+)McN 5652 (2 mg/kg, 5 min prior to IV injection of 5-[18F]-ADAM). These results suggest that 5-[18F]-ADAM may be a potential new serotonin transporter PET imaging agent. However, due to its rapid wash-out from the brain, defluorination in vivo and lower uptake in the brain than 4-[18F]-ADAM, 5-[18F]-ADAM may not be as useful as 4-[18F]-ADAM as a SERT imaging agent.

In vivo quantification by SPECT of [123I] ADAM bound to serotonin transporters in the brains of rabbits

BACKGROUND

A novel radioiodine ligand [(123)I] ADAM (2-((2-((dimethylamino)methyl)phenyl)thio)-5-iodophenylamine) has been suggested as a promising serotonin transporter (SERT) imaging agent for the central nervous system. In this study, the biodistribution of SERTs in the rabbit brain was investigated using [(123)I] ADAM and mapping images of the same animal produced by both single-photon emission computed tomography (SPECT) and microautoradiography. A semiquantification method was adopted to deduce the optimum time for SPECT imaging, whereas the input for a simple fully quantitative tracer kinetic model was provided from arterial blood sampling data.

METHODS

SPECT imaging was performed on female rabbits postinjection of 185 MBq [(123)I] ADAM. The time-activity curve obtained from the SPECT images was used to quantify the SERTs, for which the binding potential was calculated from the kinetic modeling of [(123)I] ADAM. The kinetic data were analyzed by the nonlinear least squares method. The effects of the selective serotonin reuptake inhibitors fluoxetine and p-chloroamphetamine (PCA) on rabbits were also evaluated. After scanning, the same animal was sacrificed and the brain was removed for microautoradiography. Regions-of-interest were analyzed using both SPECT and microautoradiography images. The SPECT images were coregistered manually with the corresponding microautoradiography images for comparative study.

RESULTS

During the time interval 90-100 min postinjection, the peak specific binding levels in different brain regions were compared and the brain stem was shown to have the highest activity. The target-to-background ratio was 1.89+/-0.02. Similar studies with fluoxetine and PCA showed a background level for SERT occupation. Microautoradiography demonstrated a higher level of anatomical details of the [(123)I] ADAM distribution than that obtained by SPECT imaging of the rabbit brain.

CONCLUSION

SPECT imaging of the rabbit brain with [(123)I] ADAM showed high affinity, high specificity, and favorable kinetics. The time-activity curve showed that the accumulation of the [(123)I] ADAM in the brain stem reached a maximum between 90 and 100 min postinjection. The microautoradiography provides high-resolution images of the rabbit brain. Our results for the [(123)I] ADAM biodistribution in the rabbit brains demonstrate that this new radioligand is suitable as a selective SPECT imaging agent for SERTs.

A new positron emission tomography imaging agent for the serotonin transporter: synthesis, pharmacological characterization, and kinetic analysis of [11C]2-[2-(dimethylaminomethyl)phenylthio]-5-fluoromethylphenylamine ([11C]AFM)

The synthesis, radiolabeling, and in vitro and in vivo evaluation of a new positron emission tomography (PET) radioligand for the serotonin transporter (SERT), [(11)C]2-[2-(dimethylaminomethyl)phenylthio]-5-fluoromethylphenylamine ([(11)C]AFM) is reported. AFM was prepared from 4-chloro-3-nitrobenzyl acetate and thiosalicylic acid in a five-step synthetic sequence. In binding studies in vitro with cloned human transporters, AFM displayed high binding affinity (Ki 1.04 nmol/L for hSERT) and good selectivity (Ki 664 nmol/L for hNET and >10,000 nmol/L for hDAT) for SERT. The radiolabled compound [(11)C]AFM was prepared in 30-37 minutes from its monomethylamine precursor by reaction with high specific activity [(11)C]iodomethane. Radiochemical yield was 12.3 +/- 8.1% based on [(11)C]iodomethane and specific activity was 1733 +/- 428 Ci/mmol at end of synthesis (EOS, n = 14). Radiochemical and chemical purity of the final product was >97%. Biodistribution studies in rats indicated that [(11)C]AFM entered the brain readily and localized in regions known to contain high concentrations of SERT, with high specific to nonspecific binding ratios. Furthermore, binding of [(11)C]AFM in SERT-rich regions was blocked by the cold compound AFM and the selective serotonin reuptake inhibitor citalopram but not by the selective norepinephrine reuptake inhibitor nisoxetine or the selective dopamine reuptake inhibitor GBR 12935. At 30 minutes after injection, >95% of the brain activity corresponded to the parent compound, indicating the absence of radiolabeled metabolites in the rat brain. PET imaging experiments in baboons showed a brain distribution pattern of [(11)C]AFM consistent with the regional concentrations of SERT, with the highest levels of radioactivity detected in the midbrain and thalamus, moderate levels in the hippocampus and striatum, and the low levels in the cortical regions. Pretreatment of the baboons with citalopram (4 and 6 mg/kg, intravenously) reduced regional brain distribution volumes to low and homogeneous levels, thus underlining the binding specificity of [(11)C]AFM for SERT in vivo. Analysis of blood samples indicated a fast metabolism of the radioligand into more hydrophilic components, as well as the absence of radiolabeled lipophilic metabolites. Regional time-activity curves were analyzed with kinetic and graphical analysis methods using the arterial concentrations as input function. Both methods returned similar kinetic parameters and documented high specific to nonspecific equilibrium coefficients (V(3)") for [(11)C]AFM. Identical V(3)" values were also derived with the simple reference tissue method, indicating that quantification of SERT with [(11)C]AFM can be achieved without arterial blood sampling. In summary, [(11)C]AFM appears to be an excellent PET radioligand for the visualization and reliable quantification of SERT in vivo.

Serotonin and dopamine transporter availabilities correlate with the loudness dependence of auditory evoked potentials in patients with obsessive-compulsive disorder

Brain monoaminergic function is involved in the pathophysiology of psychiatric disorders. The loudness dependence (LD) of the N1/P2 component of auditory evoked potentials has been proposed as a noninvasive indicator of central serotonergic function, whereas single photon emission computed tomography (SPECT) and [123I]beta-CIT can be used to visualize both serotonin (SERT) and dopamine transporters (DAT). The aim of the study was to correlate LD and SPECT measures in patients with obsessive-compulsive disorder, a condition with evidence for a serotonergic dysfunction. A total of 10 subjects received both neurophysiological and imaging investigations. Evoked potentials were recorded following the application of acoustic stimuli with increasing intensities. The LD of the relevant subcomponents (tangential dipoles) was investigated using dipole source analysis. SPECT was performed 20-24 h after injection of a mean 140 MBq [123I]beta-CIT. As a measure of brain SERT and DAT availabilities, a ratio of specific to nonspecific [123I]beta-CIT binding for the midbrain . pons region (SERT) and the striatum (DAT) was used. The LD of the right tangential dipole correlated significantly with both SERT and DAT availabilities (Pearson's correlations: rho = 0.69, p < 0.05, and rho = 0.80, p < 0.01, respectively). The correlations remained significant after controlling for the effects of age, gender, and severity of clinical symptoms. Associations between LD and both SERT and DAT availabilities further validate the use of neurophysiological approaches as noninvasive indirect measures of neurochemical brain function and point at a hypothesized interconnection of central monoaminergic systems.

Microautoradiography of [123I]ADAM in mice treated with fluoxetine and serotonin reuptake inhibitors

A radiopharmaceutical, (123)I-labeled 2-((2-((dimethylamino)methyl)phenyl)thio)-5-iodophenylamine ([(123)I]ADAM), has been developed recently for evaluation of how serotonin transporters (SERT) function in the brain. However, the detailed biodistribution and specific binding in certain brain areas are not well investigated. In this study, both phosphor plate imaging and microautoradiography were applied to explore the binding characteristics of [(123)I]ADAM in SERT neurons. The effect of two psychotropics and one narcotic on the binding of [(123)I]ADAM to SERT was also studied. Fluoxetine and desipramine, both are psychotropics and specific SERT ligands and decreased the affinity of [(123)I]ADAM, while p-chloroamphetamine (PCA), a narcotic, destroyed most of serotonergic neurons, as well as reducing the concentration of serotonin and the number of SERT in the brain as shown by the biodistribution of [(123)I]ADAM. Significant and selective accumulation of [(123)I]ADAM in the areas from midbrain to brain stem in normal mice with maximum target-to-background ratio was found at 90 minutes postinjection. A rapid clearance of [(131)I]ADAM at 120 minutes postinjection was found in the CA1, CA3 and ThN brain areas. In addition, the inhibition effect on binding ability of [(123)I]ADAM to SERT by the psychotropics and the narcotic was found to have the order of: PCA > fluoxetine > desipramine.

Imaging serotonin transporters with 123I-ADAM brain SPECT in healthy non-human primates

BACKGROUND

Serotonin transporters (SERTs) play a major role in modulating serotonergic neuronal function and are the target of many antidepressant drugs used in neuro-psychiatric disorders. To gain more information on the temporal distribution of SERTs, 2-([2-([dimethylamino]methyl)phenoxyl]thio)-5-[I]iodophenylamine (I-ADAM) single photon emission computed tomography (SPECT) was utilized in an in vivo imaging study using non-human primates.

METHODS

Two female monkeys (Macaca cyclopis) were studied. Eight brain SPECT imaging examinations, each 30 min in duration, were obtained after injection of 185 MBq of I-ADAM. Images were obtained using a dual-head gamma camera equipped with ultra-high resolution fan-beam collimators. In addition to visual inspection, the radio-uptake and specific uptake ratios (SURs) of midbrain (MB), thalamus (TH), striatum (ST), temporal and frontal cortices and the whole brain in reference to the corresponding magnetic resonance image at the eight time points were measured. The SUR of MB, using cerebellum (CB) as the reference tissue, was calculated as (MB - CB)/CB, in mean counts/pixel. The SURs of the other brain regions were similarly measured.

RESULTS

There was relatively high uptake of I-ADAM in the MB and TH, moderate uptake in ST, lower uptake in the cerebral cortex, and almost no uptake in the CB. The image of MB could be easily identified at the first 30 min time point. It appeared that the SURs of MB, TH and ST reached equilibrium around 210 min after injection. No adverse reactions of the primates were found during and after imaging. Brain distribution of I-ADAM in the primate appeared consistent with the known distribution of SERTs.

CONCLUSION

In conjunction with a high SUR in MB, TH and ST, we speculate that I-ADAM may be a potential radioligand for SPECT studies of serotonin transporters in humans.

Positron emission tomography radiochemistry

Factors that place constraints on radio-chemists who are seeking to design and develop radiopharmaceuticals for PET imaging studies include the short half-lives of 11C and 18F, minimum radiochemical yield and specific activity requirements, and high radiation fields that are associated with multi-Curie quantities of PET radionuclides. Nevertheless, during the past 20 years, considerable progress has been made in the development and application of a variety of PET radiotracers for a range of imaging studies in human subjects. We have highlighted a few areas of radiochemistry that focused on PET radiotracers that are described in this issue. Although the number of PET radiotracers synthesized is in the hundreds [6], much work remains to develop specific and useful PET radiotracers for a host of new and exciting noninvasive imaging applications.

ADAM is an effective tool for in vivo study of serotonergic function: Validation in rat models

ADAM, 2-((2-((dimethylamino)methyl)phenyl)thio)-5-iodophenylamine, is a recently described SPECT tracer for exploration of the serotonin transporter. We evaluated its potential to detect abnormalities in serotonergic function in the rat using 1) a model of serotonergic neuron lesion induced with 5,7-dihydroxytryptamine (5,7-DHT), and 2) experimental induction of acute decrease in endogenous brain serotonin levels. Cerebral biodistribution studies of [125I]ADAM were performed in normal conditions, in 5,7-DHT-lesioned rats, and after acute serotonin depletion obtained with p-chlorophenylalanine (pCPA). Around 50% reduction in accumulation of ADAM was observed in the hypothalamus and hippocampus 3 weeks after lesion of serotonergic neurons, whereas a more modest decrease of 15-30% occurred in the thalamus, frontal cortex, and striatum. This demonstrated the ability of the tracer to detect serotonergic neuron loss in vivo. After inducing acute 5-HT depletion with pCPA, we observed an increase in in vivo [125I]ADAM binding in all brain areas studied. The higher in vivo binding of [125I]ADAM in pCPA-treated rats than in controls was mainly due to an increase in specific binding to the SERT, as demonstrated by greatly reduced binding in the presence of a saturating dose of paroxetine. This may indicate in vivo competition between ADAM and 5-HT for binding to the SERT. The present findings thus demonstrate that ADAM is a specific SERT radioligand which can be used for in vivo study of central serotonin systems, and supports its use as a tracer for SPECT studies in human disorders involving dysfunction of serotonergic neurotransmission.

Elevated brain serotonin transporter availability in patients with obsessive-compulsive disorder

BACKGROUND

A central serotonergic dysfunction is considered to be involved in the pathophysiology of obsessive-compulsive disorder (OCD). The aim of this study was to investigate the serotonin transporter availability in patients with OCD as an in vivo marker of the central serotonergic system.

METHODS

Nine unmedicated (7 drug-naive) patients with OCD and 10 healthy control subjects were included and received single photon emission computed tomography (SPECT) 20.75 +/- 1.51 hours after injection of a mean 147.20 +/- 6.74 MBq [(123)I]-2beta-carbomethoxy-3beta-(4-iodophenyl)tropane ([(123)I]beta-CIT). As a measure of brain serotonin transporter availability, a ratio of specific-to-nonspecific [(123)I]beta-CIT binding for the midbrain-pons (V(3)" = [midbrain/pons-occipital]/occipital) was used.

RESULTS

Mean specific-to-nonspecific ratios showed a 25% higher midbrain-pons [(123)I]beta-CIT binding in the patients as compared with healthy controls (2.26 +/-.37 vs. 1.81 +/-.23, p <.01). The difference remained significant after adjustment for clinical variables and controlling for age and gender. Stratification of the patients according to onset of the disorder revealed significant differences between controls and patients with early (childhood, adolescence) but not late (adult) onset of OCD.

CONCLUSIONS

The study provides evidence of a serotonergic dysfunction in patients with OCD and suggests a serotonergic component in the pathophysiology of the disorder.

Synthesis and evaluation of iodine-123 labelled tricyclic tropanes as radioligands for the serotonin transporter

The tricyclic tropane analogues (1S,3S,6R,10S)-(Z)-10-(benzoyloxymethyl)-9-(3-chloro-4-iodobenzylidene)-7-azatricyclo[4.3.1.0(3,7)]decane, 1, and (1S,3S,6R,10S)-(Z)-9-(3-chloro-4-iodobenzylidene)-7-azatricyclo[4.3.1.0(3,7)]decane-10-carboxylic acid methyl ester, 2, have been shown to be potent and selective serotonin transporter (SERT) ligands. They possess nanomolar affinity for the SERT (Ki = 0.06 nM and 1.8 nM respectively) and are suitable for radiolabelling using iodine-123. In the present study we prepared [(123)I]1 and [(123)I]2 from the appropriate tributylstannane precursors using acidic media with chloramine-T as the oxidising agent. The radiochemical yield obtained for [(123)I]1 varied between 50-60% while for [(123)I]2 the range was 65-80%. Both radioligands were obtained with radiochemical purity > 97% and specific activity estimated to be > 185 GBq/micromol. The biodistribution of [(123)I]1 demonstrated low degree of brain penetration at 5 min (0.14%ID/g) with a homogeneous distribution. The radioactivity cleared quickly from all brain regions with no preferential localization. In comparison, [(123)I]2 demonstrated on average a higher brain uptake at 5 min (0.5%ID/g). However the distribution of radioactivity was homogeneous and cleared to levels similar to [(123)I]1 at 1 hr post-injection. Pre-administration of citalopram failed to show any significant inhibition of [(123)I]2 uptake in the rat brain. The high lipophilicity of 1 and 2 (HPLC-derived log P(7.4) values of 6.41 and 4.25 respectively) and in vivo metabolism, seen by high thyroid uptake would explain the absence of any specific binding observed in the rat brain. In view of these results [(123)I]1 and [(123)I]2 do not appear to be suitable radioligands for in vivo studies of the SERT.

Specific in vivo binding to the norepinephrine transporter demonstrated with the PET radioligand, (S,S)-[11C]MeNER

(S,S)-2-(alpha-(2-Methoxyphenoxy)benzyl)morpholine (MeNER), an O-methyl analog of the selective and potent norepinephrine transporter (NET) inhibitor, (S,S)-reboxetine, and its less active enantiomer, (R,R)-MeNER, have each been radiolabeled by O-methylation of their corresponding phenolic precursors in good yields from [(11)C]methyl iodide or [(11)C]methyl triflate. Radiochemical purities were >99% and specific radioactivity at time of injection was about 74 GBq/micromol. Autoradiographic examination of (S,S)-[(11)C]MeNER binding to human brain slices post mortem indicated specific binding in a brain region including the locus coeruleus. PET examination of both [(11)C]MeNER enantiomers in a cynomolgus monkey demonstrated a higher specific binding of the (S,S)-enantiomer with ratios of 1.4-1.6 in the lower brainstem, mesencephalon and thalamus to striatum. Pretreatment with the NET ligand, desipramine, decreased the specific binding of (S,S)-[(11)C]MeNER. Labeled metabolites of [(11)C]MeNER were all more polar. (S,S)-[(11)C]MeNER is a good lead compound in the search for a selective radioligand for quantitation of NET in the human brain in vivo.