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

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

OBJECTIVE

Diabetes 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).

METHODS

In 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.

RESULTS

In 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.

CONCLUSIONS

The 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.

Radiotracers for the Central Serotoninergic System

This review lists the most important radiotracers described so far for imaging the central serotoninergic system. Single-photon emission computed tomography and positron emission tomography radiotracers are reviewed and critically discussed for each receptor.

Borderline personality disorder and its symptom clusters: A review of positron emission tomography and single photon emission computed tomography studies

Positron emission tomography (PET) and single photon emission computed tomography (SPECT) are two neuroimaging techniques that have led to important insights into the pathophysiology of borderline personality disorder (BPD) and its symptom clusters. This paper provides the most up-to-date review of PET and SPECT studies targeting BPD. A systematic search of PET and SPECT studies using the databases PsycINFO, PubMed, and Embase was implemented, which yielded 30 articles in the end. Earlier PET studies largely reported decreased glucose metabolism in prefrontal brain regions. More recent PET research has pointed to alterations in monoaminergic systems, greater activation of the opioid system during sadness induction, as well as abnormalities of the brain endocannabinoid system in BPD. SPECT studies of BPD mainly identified changes in regional cerebral blood flow and alterations of the serotonergic system. Such PET and SPECT study abnormalities have been suggested to relate to the symptomatology of BPD, including impulsivity, aggression, and mood changes. The implications of these neuroimaging studies in terms of shedding new light on the pathophysiology of BPD and providing new avenues for drug development are discussed.

A study of tryptophan, kynurenine and serotonin transporter in first-episode drug-naïve major depressive disorder

Major depressive disorder (MDD) is associated with the disharmonic functioning of the serotonin system. The serotonin system is mainly modulated by the serotonin transporter (SERT) which regulates serotonin uptake and the metabolism of its precursor, tryptophan and following kynurenine pathway. Currently, there is a lack of research examining both markers concurrently in MDD. This study evaluated the alterations and inter-relationships of both markers in first-episode drug-naïve MDD patients. Thirty-three MDD patients and 33 age- and sex-matched healthy controls (HC) were recruited. The SERT availability were comparable between two groups in the midbrain, thalamus, caudate, and putamen. The kynurenine/tryptophan ratio which indicates tryptophan metabolism was lower in MDD than HC with no group difference in the tryptophan or kynurenine concentration. A negative correlation between the midbrain SERT availability and kynurenine concentration in HC was found. For the subgroup of HC with high kynurenine/tryptophan ratio, the SERT availability was positively associated with the kynurenine/tryptophan ratio and negatively correlated with tryptophan or kynurenine concentration. This study demonstrated the altered tryptophan metabolism and the relationship between tryptophan metabolism and the SERT availability in first-episode drug-naïve MDD patients, which gave a new insight towards the future investigation of the pathophysiology of MDD.

Serotonergic imaging in Parkinson's disease

Parkinson's disease (PD) is a neurodegenerative disorder characterized by the progressive degeneration of monoaminergic central pathways such as the serotonergic. The degeneration of serotonergic signaling in striatal and extrastriatal brain regions is an early feature of PD and is associated with several motor and non-motor complications of the disease. Molecular imaging techniques with Positron Emission Tomography (PET) have greatly contributed to the investigation of biological changes in vivo and to the understanding of the extent of serotonergic pathology in patients or individuals at risk for PD. Such discoveries provide with opportunities for the identification of new targets that can be used for the development of novel disease-modifying drugs or symptomatic treatments. Future studies of imaging serotonergic molecular targets will better clarify the importance of serotonergic pathology in PD, including progression of pathology, target-identification for pharmacotherapy, and relevance to endogenous synaptic serotonin levels. In this article, we review the current status and understanding of serotonergic imaging in PD.

The Correlation Between Mid-Brain Serotonin Transporter Availability and Intelligence Quotient in Healthy Volunteers

Purpose:

This study was performed to investigate the association between the mid-brain serotonin transporter (SERT) availability and intelligence quotient (IQ).

Methods:

One hundred and thirteen healthy participants, including 52 male and 61 female subjects, were recruited. We used SPECT with [123I]ADAM images to determine the SERT availability in the mid-brain, and measured the subjects’ IQ using the WAIS-R.

Results:

We found a significant positive correlation between the mid-brain SERT availability and the IQ of the participants. Even when controlling for age and sex, the significant association still existed.

Conclusion:

This result implied that the higher the SERT binding in the mid-brain, the better the IQ in healthy participants.

Synthesis and evaluation of 2-(2'-((dimethylamino)methyl)-4'-(2-fluoroethoxy-substituted)phenylthio)benzenamine derivatives as potential positron emission tomography imaging agents for serotonin transporters

A series of diphenylsulfide derivatives with various substitutions at the 4-position on phenyl ring A and different lengths of the 2-fluoroethoxy-substituted side-chain at the 4'-position on ring B were synthesized and evaluated as potential positron emission tomography (PET) imaging agents for serotonin transporters (SERT). These ligands exhibited high SERT binding affinities (K_i = 0.11-1.3 nM) and the 4-methyl-substituted (4-Me) compounds 7a and 8a displayed excellent selectivity for SERT versus norepinephrine transporters (NET) (392- and 700-fold, respectively). In the parallel artificial membrane permeability assay (PAMPA), these ligands demonstrated moderate to high brain penetration, and the 4-Me analogs showed higher BBB permeability than the corresponding 4-F analogs. The 2-fluoroethoxy-substituted ligands showed higher metabolic stability and lower lipophilicity than 4-F-ADAM. [¹⁸F]7a-c were readily prepared using an automatic synthesizer and exhibited significant uptake and slow washout in rat brains. At 120 min after iv injection, [¹⁸F]7a exhibited the highest uptake in the midbrain, whereas [¹⁸F]7b exhibited the highest uptake in the hypothalamus and midbrain. After treatment with citalopram, a SERT-selective ligand, the uptake of [¹⁸F]7a in the hypothalamus and striatum was significantly decreased. The potent and highly selective SERT binding and the selective and reversible accumulation in SERT-rich brain regions suggested that [¹⁸F]7a is a promising lead for the further development of novel [¹⁸F]-labeled PET imaging agents for SERT binding sites in the brain.

Serotonin transporter binding is increased in Tourette syndrome with Obsessive Compulsive Disorder

While the importance of the serotonergic system in obsessive compulsive disorder (OCD) is well established, its role in Tourette syndrome (TS) is uncertain. Particularly in TS patients with comorbid OCD (TS + OCD), decreased serotonin transporter (SERT) binding has been suggested. Here, we investigated for the first time SERT binding in TS patients with and without OCD (TS - OCD) compared to both healthy controls (HC) and OCD patients as well as the influence of escitalopram using the potent SERT imaging ligand [¹²³I]2-((2-((dimethylamino)methyl)phenyl)thio)-5-iodophenylamine ([¹²³I]ADAM) and single-photon emission tomography (SPECT). We included 33 adult subjects (10 HC, 10 TS - OCD, 8 TS + OCD and 5 OCD). In patients with OCD and TS + OCD [¹²³I]ADAM SPECT was repeated after 12-16 weeks treatment with escitalopram. SERT binding was normal in patients with OCD and TS - OCD, but significantly increased (p < 0.05) in those with TS + OCD, particularly in caudate and midbrain compared to both HC and TS - OCD. Treatment with escitalopram resulted in a significant overall reduction in SERT binding (range, 19 to 79%, p values between 0.0409 and <0.0001) without any correlation with clinical improvement. Our results provide further evidence that alterations in the serotonergic system in TS are related to comorbid OCD and do not represent the primary cause of the disease.

The role of central dopamine and serotonin in human obesity: lessons learned from molecular neuroimaging studies

Obesity results from an imbalance between energy intake and expenditure, and many studies have aimed to determine why obese individuals continue to (over)consume food under conditions of caloric excess. The two major "neurotransmitter hypotheses" of obesity state that increased food intake is partially driven by decreased dopamine-mediated reward and decreased serotonin-mediated homeostatic feedback in response to food intake. Using molecular neuroimaging studies to visualize and quantify aspects of the central dopamine and serotonin systems in vivo, recent PET and SPECT studies have also implicated alterations in these systems in human obesity. The interpretation of these data, however, is more complex than it may appear. Here, we discuss important characteristics and limitations of current radiotracer methods and use this framework to comprehensively review the available human data on central dopamine and serotonin in obesity. On the basis of the available evidence, we conclude that obesity is associated with decreased central dopaminergic and serotonergic signaling and that future research, especially in long-term follow-up and interventional settings, is needed to advance our understanding of the neuronal pathophysiology of obesity in humans.

Design of Radioiodinated Pharmaceuticals: Structural Features Affecting Metabolic Stability towards in Vivo Deiodination

Radioiodinated pharmaceuticals are convenient tracers for clinical and research investigations because of the relatively long half-lives of radioactive iodine isotopes (i.e., 123I, 124I, and 131I) and the ease of their chemical insertion. Their application in radionuclide imaging and therapy may, however, be hampered by poor in vivo stability of the C-I bond. After an overview of the use of iodine in biology and nuclear medicine, we present here a survey of the catabolic pathways for iodinated xenobiotics, including their biodistribution, accumulation, and biostability. We summarize successful rational improvements in the biostability and conclude with general guidelines for the design of stable radioiodinated pharmaceuticals. It appears to be necessary to consider the whole molecule, rather than the radioiodinated fragment alone. Iodine radionuclides are generally retained in vivo on sp2 carbon atoms in iodoarenes and iodovinyl moieties, but not in iodinated heterocycles or on sp3 carbon atoms. Iodoarene substituents also have an influence, with increased in vivo deiodination in the cases of iodophenols and iodoanilines, whereas methoxylation and difluorination improve biostability.

Comparison of brain serotonin transporter using [I-123]-ADAM between obese and non-obese young adults without an eating disorder

Cerebral serotonin metabolism has an important but controversial role in obesity. However, it is not given enough attention in morbidly obese young adults. We used single photon emission computed tomography (SPECT) with [I-123]-labeled 2-((2-((dimethylamino)methyl)phenyl)thio)-5-iodophenylamine (ADAM) to investigate changes in serotonin transporter (SERT) availability in 10 morbidly obese young adults without an eating disorder (M/F = 5/5, body mass index (BMI): 40.3 ± 4.1 kg/m2, percentage of body fat (BF%): 46.0 ± 3.9%) and 10 age- and sex-matched non-obese controls (BMI: 20.3 ± 1.2 kg/m2, BF%: 20.6 ± 8.9%). All participants underwent SPECT at 10 min and 6 h after an injection of 200 MBq of [I-123]-ADAM. The SERT binding site (midbrain) was drawn with cerebellum normalization. The BF% and fat distribution were measured using dual-energy X-ray absorptiometry. The midbrain/cerebellum SERT binding ratios (2.49 ± 0.46 vs. 2.47 ± 0.47; p = 0.912) at 6 h were not significantly different between groups, nor was the distribution of the summed images at 10 min (1.36 ± 0.14 vs. 1.35 ± 0.11; p = 0.853). There were no significant correlations between midbrain/cerebellum SERT binding ratio and age, BMI, BF%, or fat distribution. No significant difference in SERT availability in the midbrain between morbidly obese and non-obese young adults without an eating disorder indicates an unmet need for investigating the role of cerebral serotonin in obesity.

Effects of dextromethorphan on MDMA-induced serotonergic aberration in the brains of non-human primates using [123I]-ADAM/SPECT

3,4-Methylenedioxymethamphetamine (MDMA), a common recreational drug, is known to cause serotonergic neurotoxicity in the brain. Dextromethorphan (DM) is a widely used antitussive reported to exert anti-inflammatory effect in vivo. In this study, we examined the long-term effect of MDMA on the primate serotonergic system and the protective property of DM against MDMA-induced serotonergic abnormality using single photon emission computed tomography (SPECT). Nine monkeys (Macaca cyclopis) were divided into three groups, namely control, MDMA and co-treatment (MDMA/DM). [¹²³I]-ADAM was used as the radioligand for serotonin transporters (SERT) in SPECT scans. SERT levels of the brain were evaluated and presented as the uptake ratios (URs) of [¹²³I]-ADAM in several regions of interest of the brain including midbrain, thalamus and striatum. We found that the URs of [¹²³I]-ADAM were significantly lower in the brains of MDMA than control group, indicating lower brain SERT levels in the MDMA-treated monkeys. This MDMA-induced decrease in brain SERT levels could persist for over four years. However, the loss of brain SERT levels was not observed in co-treatment group. These results suggest that DM may exert a protective effect against MDMA-induced serotonergic toxicity in the brains of the non-human primate.

Fluorine-18 Radiolabeled PET Tracers for Imaging Monoamine Transporters: Dopamine, Serotonin, and Norepinephrine

This review focuses on the development of fluorine-18 radiolabeled PET tracers for imaging the dopamine transporter (DAT), serotonin transporter (SERT), and norepinephrine transporter (NET). All successful DAT PET tracers reported to date are members of the 3β-phenyl tropane class and are synthesized from cocaine. Currently available carbon-11 SERT PET tracers come from both the diphenylsulfide and 3β-phenyl nortropane class, but so far only the nortropanes have found success with fluorine-18 derivatives. NET imaging has so far employed carbon-11 and fluorine-18 derivatives of reboxetine but due to defluorination of the fluorine-18 derivatives further research is still necessary.

Neither cortisol nor brain-derived neurotrophic factor is associated with serotonin transporter in bipolar disorder

Converging evidence indicates the hypothalamus-pituitary-adrenal axis and serotonergic neurons exert reciprocal modulatory actions. Likewise, brain-derived neurotrophic factor (BDNF) has been implicated as a growth and differentiation factor in the development of serotonergic neurons. The aim of this study was to examine the interaction of cortisol and BDNF on serotonin transporter (SERT) in bipolar disorder (BD). Twenty-eight BD and 28 age- and gender-matched healthy controls (HCs) were recruited. (123)I-ADAM with single-photon emission computed tomography (SPECT) was applied for measurement of SERT availability in the brain, which included the midbrain, thalamus, putamen and caudate. Ten milliliters of venous blood was withdrawn, when the subject underwent SPECT, for the measurement of the plasma concentration of cortisol and BDNF. SERT availability was significantly decreased in the midbrain and caudate of BD compared with HCs, whereas plasma concentration of cortisol and BDNF did not show a significant difference. The linear mixed-effect model revealed that there was a significant interaction of group and cortisol on SERT availability of the midbrain, but not BDNF. Linear regression analyses by groups revealed that cortisol was associated with SERT availability in the midbrain in the HCs, but not in BD. Considering previous studies, which showed a significant association of cortisol with SERT availability in the HCs and major depressive disorder (MDD), our result replicated a similar finding in HCs. However, the negative finding of the association of cortisol and SERT availability in BD, which was different from MDD, suggests a different role for cortisol in the pathophysiology of mood disorder.

Association between the serotonin transporter and cytokines: Implications for the pathophysiology of bipolar disorder

BACKGROUND

Reduced brain serotonin transporter (SERT) has been demonstrated in bipolar disorder (BD). The aim of this study was to explore the potential role of cytokines on reduced SERT in BD.

METHODS

Twenty-eight BD type I patients and 28 age- and gender-matched healthy controls (HCs) were recruited. Single photon emission computed tomography with the radiotracer 123I ADAM was used for SERT imaging. Regions of interest included the midbrain, thalamus, putamen and caudate. Seven cytokines, including tumor necrosis factor-α (TNF-α), interferon-γ (IFN-γ), interleukin-1α (IL-1α), IL-1β, IL-4, IL-6 and IL-10, were measured using an enzyme linked immune-sorbent assay.

RESULTS

SERT availability in the midbrain and caudate was significantly lower in BD compared to HCs. IL-1β was significantly lower, whereas IL-10 was significantly higher in BD compared to HCs. Multiple linear regression analyses revealed that there were associations between cytokines, IL-1α, IL-1β, IL-6 and SERT availability in the midbrain but not in the thalamus, putamen and caudate. Furthermore, linear mixed effect analyses demonstrated that these associations were not different between HCs and BD.

CONCLUSION

While many cytokines have been proposed to be important in the pathophysiology of BD, our results demonstrated that significant associations between cytokines and SERT availability may explain the role of cytokines in mood regulation. However, these associations were not different between HCs and BD, which imply the role of these cytokines is not specific for BD.

Probing the Serotonin Transporter Availability Among Male Cigarette Smokers: A SPECT Study With [123I] ADAM

OBJECTIVE

Genetic studies have suggested that the serotonin transporter (SERT) could be associated with cigarette smoking. However, evidence from neuroimaging is scarce. The aim of the present study was to examine the SERT availability among cigarette smokers by using single-photon emission computed tomography (SPECT).

METHODS

Sixteen male smokers and 32 controls were enrolled. The SERT availability was measured by SPECT with a radiotracer, [I] ADAM, which is highly sensitive and specific to SERT.

RESULTS

No significant difference in SERT availability was found between 2 groups in the midbrain (smokers: 2.12 ± 0.70, nonsmokers: 2.13 ± 0.63; P = 0.86), basal ganglia (smokers: 0.83 ± 0.30, nonsmokers:0.90 ± 0.39; P = 0.95), or thalamus (smokers: 1.14 ± 0.41, nonsmokers: 1.20 ± 0.38; P = 0.88). No significant association was found between the SERT availability, and either the breath carbon monoxide level or the score of the Fagerström Test for Nicotine Dependence.

CONCLUSIONS

Whether the SERT availability in the brain is altered in smokers remains unclear.

Serotonin transporter availability may moderate the association between perceiving stress and depressive tendencies - A SPECT with 5-HTTLPR genotyping study

It was found that serotonin transporter (SERT) gene (5-HTTLPR) polymorphism may moderate the association between perceiving stress and depressive tendency. Although SERT availability in the central nervous system could be associated with 5-HTTLPR polymorphism, whether SERT availability moderates the association between stress and depressive tendency is unclear. This study aimed to investigate whether there is a SERT availability×environmental stress interaction effect, as well as a gene-by-environmental (G×E) interaction effect, using single-photon emission computed tomography (SPECT) with a serotonin transporter radiotracer, [(123)I]ADAM. 87 healthy volunteers were enrolled. The SERT availability was approximated using SPECT with [(123)I]ADAM. Stress and depressive tendencies were measured by the Recent Life Change Questionnaire (RLCQ) and the Taiwanese Depression Questionnaire (TDQ), respectively. A significant interaction of sex×RLCQ×thalamic SERT availability on the TDQ was found, and this effect was robust after controlling for the effect of the SS genotype. The interaction of RLCQ×thalamic SERT availability on the TDQ was significant among males. In particular, a significant association between RLCQ and TDQ (Spearman correlation, ρ=0.64, p<0.01) was found among male subjects with a lower level of thalamic SERT availability. SERT availability may play a role in depressive tendency when under perceived stress among healthy individuals, independent of G×E. This finding provides new evidence that confirms the role of the serotonergic system in the association between stress and depression. Males with lower levels of SERT availability may be more vulnerable to the effects of negative life events.

Revisiting the Serotonin Hypothesis: Implications for Major Depressive Disorders

Major depressive disorder (MDD) is a heritable neuropsychiatric disease associated with severe changes at cellular and molecular levels. Its diagnosis mainly relies on the characterization of a wide range of symptoms including changes in mood and behavior. Despite the availability of antidepressant drugs, 10 to 30 % of patients fail to respond after a single or multiple treatments, and the recurrence of depression among responsive patients is very high. Evidence from the past decades suggests that the brain neurotransmitter serotonin (5-HT) is incriminated in MDD, and that a dysfunction of 5-HT receptors may play a role in the genesis of this disease. The 5-HT membrane transporter protein (SERT), which helps regulate the serotonergic transmission, is also implicated in MDD and is one of the main targets of antidepressant therapy. Although a number of behavioral tests and animal models have been developed to study depression, little is known about the neurobiological bases of MDD. Understanding the role of the serotonergic pathway will significantly help improve our knowledge of the pathophysiology of depression and may open up avenues for the development of new antidepressant drugs. The overarching goal of this review is to present recent findings from studies examining the serotonergic pathway in MDD, with a focus on SERT and the serotonin 1A (5-HT1A), serotonin 1B (5-HT1B), and serotonin 2A (5-HT2A) receptors. This paper also describes some of the main molecules involved in the internalization of 5-HT receptors and illustrates the changes in 5-HT neurotransmission in knockout mice and animal model of depression.

Which demographic variables are necessary to correct in neuroimaging studies of serotonin transporter availability? A SPECT study with [¹²³I]ADAM

Previous studies have shown that many demographic variables influence serotonin transporter (SERT) availability as assessed by single photon emission computed tomography (SPECT). The aim of this study was to explore which demographic variables influenced the SERT availability most in a SPECT study with [(123)I]ADAM. Ninety-five healthy volunteers were recruited. Age, sex, smoking, alcohol intake, educational level, body mass index, seasonal change, and SERT availability were recorded and then analyzed by multivariate linear regression. Age was the only variable that was significantly associated with SERT availability (calculate: (midbrain-cerebellum)/cerebellum). Furthermore, the inverse correlation of age and SERT availability may be present only before the age of 47. Age should be a covariate in SERT-related neuroimaging analyses, particularly in participants under the age of 47 years.

Association of thalamic serotonin transporter and interleukin-10 in bipolar I disorder: a SPECT study

OBJECTIVES

The serotonin hypothesis plays a critical role in the etiology of bipolar disorder (BD). Although many studies have demonstrated reciprocal relationships between serotonin metabolism and immune-inflammatory pathways that occur in depression, studies linking serotonergic function and cytokines are still limited concerning BD. The aim of this study was to investigate the interaction of brain serotonin transporter (SERT) and cytokines in BD.

METHODS

Twenty patients with euthymic BD and 20 age- and sex-matched healthy controls (HC) were recruited. Single photon emission computed tomography with the radiotracer (123) I-ADAM was used for the SERT imaging. The specific uptake ratio, which represents SERT availability, was the primary measured outcome. Cytokines included the pro-inflammatory cytokine tumor necrosis factor-α (TNF-α) and the anti-inflammatory cytokine interleukin-10 (IL-10). Cytokine concentration was measured using an enzyme-linked immunosorbent assay.

RESULTS

SERT availability was significantly lower in the midbrain and caudate of patients with BD compared with HC, but not in the thalamus and putamen. IL-10 was significantly higher, whereas TNF-α was not different in euthymic patients with BD compared with HC. There was a significant association of SERT availability and IL-10 in the thalamus, but not in the midbrain, caudate, or putamen.

CONCLUSIONS

Our results demonstrate the interaction of SERT availability and IL-10 in euthymic BD. This result may further explain the role of SERT and cytokines in the etiology of BD.

123I-ADAM SPET imaging of serotonin transporter in patients with epilepsy and comorbid depression

Background

Purpose of the study was to investigate alterations in midbrain serotonin transporter (SERT) binding in patients with epilepsy and symptoms of depression compared to patients with epilepsy with no symptoms of depression.

Methods

We studied 12 patients with epilepsy (7 patients had focal and 5 had generalized epilepsy syndromes). The presence of self-reported symptoms of depression was assessed using Beck Depression Inventory (BDI) and the Emotional State Questionnaire (EST-Q). The binding potential of the SERT was assessed by performing brain single photon emission tomography (SPET) using the SERT radioligand 2-((2-((dimethylamino)methyl)phenyl)thio)-5-(123)iodophenylamine (¹²³I-ADAM).

Results

Seven patients had BDI and EST-Q subscale scores greater than 11 points, which was interpreted as the presence of symptoms of depression. We found that ¹²³I-ADAM binding was not significantly different between patients with epilepsy with and without symptoms of depression. In addition, ¹²³I-ADAM binding did not show a significant correlation to either BDI or EST-Q depression subscale scores and did not differ between patients with focal vs. generalized epilepsy.

Conclusion

The results of our study failed to demonstrate alterations of SERT binding properties in patients with epilepsy with or without symptoms of depression.

Serotonin transporter availability in the midbrain and perceived social support in healthy volunteers

OBJECTIVE

Serotonin modulates human behavior and emotion. Recent evidence implies that a higher level of serotonergic activity could be associated with a higher level of perceived social support. This study aimed to examine the correlation between serotonin transporter (SERT) availability and perceived social support scores in healthy volunteers.

METHODS

111 healthy participants, 50 males and 61 females, were enrolled from the community and completed the Measurement of Support Function questionnaire. Single photon emission computed tomography (SPECT) with [(123)I] ADAM was performed to examine SERT availability.

RESULTS

Perceived social support was positively correlated with SERT availability (Spearman's ρ=0.29, p<0.01; χ(2)=7.57, p<0.01), particularly in males (Spearman's ρ=0.37, p<0 .01; χ(2)=11.77, p<0.01). Censored regressions indicated that these associations are not influenced by a ceiling effect and remained significant after controlling the effect of age.

CONCLUSIONS

This result confirmed the correlation between perceived social support and central serotonergic activity. However, this correlation was present only in males.

Characterization of FlipIDAM as a SERT-selective SPECT imaging agent

INTRODUCTION

Biological evaluation of ([(125)I]4), a new single-photon emission computed tomography (SPECT) radioligand for imaging the serotonin transporter (SERT) which displayed improved in vivo kinetics for mapping SERT binding sites in the brain.

METHODS

In vitro binding studies of [(125)I]4 were performed with membrane homogenates of LLC-PK1 cells stably transfected and overexpressing one of the monoamine transporter (SERT, DAT or NET) and rat cortical homogenates. Biodistribution and ex vivo autoradiography studies were carried out in rats. In vivo competition experiments were evaluated to determine the SERT selectivity of [(125)I]4 vs. ([(125)I]1).

RESULTS

In vitro binding studies of 4 showed excellent binding affinity (Ki,SERT=0.90 ± 0.05 nM) and excellent selectivity over the other monoamine transporters (100 fold and >4000 fold for NET and DAT respectively). Scatchard analysis of saturation binding of [(125)I]4 to rat cortical homogenates gave a Kd value of 0.5 ± 0.09 nM and a Bmax value of 801.4 ± 58.08 fmol/mg protein. The biodistribution study showed rapid high brain uptake (3.09 ± 0.11% dose/organ at 2 min) and a good target to non-target ratio (hypothalamus to cerebellum) at 30 min (2.62) compared to [(125)I]1 (2.19). Ex vivo autoradiography showed that FlipIDAM localizes in accordance with SERT distribution patterns in the brain. In vivo and ex vivo competition experiments with specific and non-specific SERT compounds also showed that [(125)I]4 binds specifically to SERT rich regions.

CONCLUSIONS

The biological evaluation of [(125)I]4 demonstrates that [(123)I]4 would be a good candidate for SPECT imaging of SERT.

Association of brain serotonin transporter availability and brain-derived neurotrophic factor in models of serotonin transporter genotypes in healthy subjects

The S-allele of functional polymorphisms of the serotonin transporter (SERT) gene has been demonstrated to have lower transcriptional activity compared with the L-allele, which shows low expression of SERT in the brain. However, this finding cannot be consistently replicated in vivo. The aim of this study was to determine the availability of SERT based on SERT genotype. We also examined the relationship between brain-derived neurotrophic factor (BDNF) and the availability of SERT. Sixty-two healthy subjects were recruited. Each subject underwent single-photon emission computed tomography with I-ADAM (I-labeled 2-([2-({dimethylamino}methyl)phenyl]thio)-5-iodophenylamine) for imaging SERT in the brain. The specific uptake ratio was measured, and venous blood was drawn when the subject underwent single-photon emission computed tomography to evaluate BDNF levels and SERT genotype. All subjects expressed SERT genotypes that were consistent with a biallelic model, and 26 subjects had SERT genotypes that were consistent with a triallelic model. No differences in specific uptake ratio were detected in the midbrain, putamen, caudate, and thalamus based on the SERT genotype using the biallelic and triallelic models. Interestingly, The Pearson correlation coefficient revealed a positive correlation between BDNF and SERT availability. In particular, this relationship was observed in homozygous S-allele expression and a genotype with low functional expression (SaSa/SaLg) in the biallelic and triallelic models of SERT genotypes, respectively. This finding might explain why the SS genotype of SERT did not increase the risk of major depressive disorder in Asian populations and implicate an important role of BDNF in the patients, who has the SS genotype of the SERT gene.

Aggression in bipolar II disorder and its relation to the serotonin transporter

BACKGROUND

Aggression is frequently observed in patients diagnosed with bipolar disorder (BD). Previous studies found a negative association between aggression and serotoninergic function in patients with BD, as well as in healthy subjects. The objective of this study was to determine whether there is an association between aggression and the availability of the serotonin transporter (SERT) in euthymic BD II patients.

METHODS

Thirty-eight age-matched healthy controls (HCs) and 24 BD II patients were recruited. BD II patients were under stable treatment in the euthymic state. The Overt Aggression Scale (OAS) was used for the assessment of aggression. Single photon emission computed tomography with (123)I-ADAM was used for SERT imaging. A specific uptake ratio, which represents availability, was the primary measured outcome.

RESULTS

The total OAS scores, as well as the scores on all of the sub-items, were significantly higher in BD II patients than in the HCs group. There was no significant difference in SERT availability between BD II and HCs subjects in different brain regions. The Pearson's correlation between the total OAS scores and the sub-item aggression and SERT availability was significant.

LIMITATION

The OAS was used for the assessment of the past week of the patients' condition and thus did not reflect their trait status.

CONCLUSIONS

The higher total scores of OAS in euthymic BD II patients than in HCs support the idea that aggression might be a trait marker for BD. Although SERT availability in euthymic BD II patients and in HCs did not differ significantly, the correlation of SERT availability and total OAS provides the possible explanation of aggression in BD II.

A new single-photon emission computed tomography (SPECT) imaging agent for serotonin transporters: [(125)I]Flip-IDAM, (2-((2-((dimethylamino)methyl)-4-iodophenyl)thio)phenyl)methanol

New ligands for in vivo brain imaging of serotonin transporter (SERT) with single photon emission tomography (SPECT) were prepared and evaluated. An efficient synthesis and radiolabeling of a biphenylthiol, FLIP-IDAM, 4, was accomplished. The affinity of FLIP-IDAM was evaluated by an in vitro inhibitory binding assay using [(125)I]-IDAM as radioligand in rat brain tissue homogenates (K(i) = 0.03 nM). New [(125)I]Flip-IDAM exhibited excellent binding affinity to SERT binding sites with a high hypothalamus to cerebellum ratio of 4 at 30 min post iv injection. The faster in vivo kinetics for brain uptake and a rapid washout from non-specific regions provide excellent signal to noise ratio. This new agent, when labeled with (123)I, may be a useful imaging agent for mapping SERT binding sites in the human brain.

Impaired cognition in bipolar I disorder: the roles of the serotonin transporter and brain-derived neurotrophic factor

BACKGROUND

Studies have proposed that cognitive deficits are present in a variety of mood states in bipolar disorder (BD). The goal of this study was to find the cognitive deficits in euthymic BD patients and to further explore possible underlying mechanisms of the deficits.

METHODS

Thirty-three healthy controls (HCs) and twenty-three euthymic BD type I patients were recruited. Single photon emission computed tomography (SPECT) with (123)I-ADAM was used to image the serotonin transporter (SERT). Ten milliliters of venous blood was drawn for the measurement of brain derived neurotrophic factor (BDNF). Cognitive functions were tested included attention, memory, and executive function.

RESULTS

We found that the SERT availability in both the midbrain and striatal regions was decreased in the BD patients compared with the HCs; however, the BDNF were not different between the two groups. There was no correlation between the SERT availability and the BDNF. Interestingly, there were statistically significant differences in sub-items of the facial memory test and the Wisconsin Card Sorting Test between the BD patients and the HCs, which showed that there was a cognitive deficit in the BD patients. However, the overall deficits in cognition were not significantly correlated with the SERT availability or the BDNF.

LIMITATION

The effect of medications on cognitive function and BDNF should be considered.

CONCLUSIONS

We replicated previous findings that showed cognitive deficits in euthymic BD patients. However, the underlying mechanism of cognitive deficits in euthymic BD patients cannot be entirely explained by SERT and BDNF.

Effects of [123I]ADAM, a serotonin transporter radiopharmaceutical, on pregnant Sprague-Dawley rats

Serotonin transport abnormalities are implicated in neuropsychiatric disorders. [(123)I]ADAM ([(123)I]-2-([2-({dimethylamino}methyl)phenyl]thio)-5-iodophenylamine) is a novel radiotracer that targets serotonin transporters. We assessed the toxicity of [(123)I]ADAM (18.5 MBq) administered in early- and late-phases (8 and 14 day postfertilization, respectively) of pregnancy. The mortality, clinical status, and gross necropsy were measured in pregnant rats, and the fertility index was measured in rat offspring (weight, clinical observations). We found no dosing-related clinical signs. In conclusion, [(123)I]ADAM was not toxic in an animal pregnancy model.

Low brain serotonin transporter binding in major depressive disorder

We examined midbrain, medial temporal lobe, and basal ganglia serotonin transporter (SERT) distribution volume ratio (DVR) values in subjects with major depressive disorder versus healthy volunteers using a selective SERT radioligand and single photon emission computed tomography (SPECT). We hypothesized that the DVR value for SERT binding would be lower in depressed versus non-depressed subjects. [(123)I]-ADAM SPECT scans were acquired from 20 drug free, depressed subjects and 20 drug-free depressed subjects and 10 drug-free healthy volunteers. The primary outcome measure was the DVR value for [(123)I]-ADAM uptake in the midbrain, medial temporal lobe, and basal ganglia regions. Depressed subjects demonstrated significantly lower DVR values in the midbrain, right and left medial temporal lobe, and right and left basal ganglia. There was significant probability that lower DVR values could distinguish between depressed and non-depressed subjects in the midbrain, medial temporal lobe, and the right and left basal ganglia. These findings confirm prior observations of lower SERT binding in depression, and suggest that low SERT binding may represent a putative biomarker of depression. Future studies are needed to confirm these observations.

Association between somatization subscale score and serotonin transporter availability in healthy volunteers--a single photon emission computed tomography study with [¹²³I] ADAM

RATIONALE AND OBJECTIVE

Serotonin is one of the key neuromodulators involved in fundamental cerebral functions and behaviors. Previous study has demonstrated that somatization symptoms are probably associated with central serotonergic circuits, which are implicated in anxiety and nociception regulation. This study aims to examine the correlation between somatization subscale score and serotonin transporter (SERT) availability in healthy volunteers.

METHODS

Sixty-four healthy participants, 26 males and 38 females, were enrolled from the community and were administered the single somatization subscale of the Chinese symptom checklist 90 revised (SCL90-R). Single photon emission computed tomography with [(123)I] 2-((2-((dimethylamino)methyl)phenyl)thio)-5-iodophenylamine was also performed to examine SERT availability.

RESULTS

The somatization scores were negatively correlated with SERT availability (Spearman's ρ = -0.35, p = 0.005), particularly in males (Spearman's ρ = -0.54, p = 0.004).

CONCLUSION

This result reconfirmed the correlation between central serotonergic activity and the intensity of somatization symptoms, even in healthy participants. However, a gender difference exists in this correlation.

Short term vs. long term test-retest reproducibility of ¹²³I-ADAM for the binding of serotonin transporters in the human brain

Previous brain imaging studies have demonstrated a seasonal difference of serotonin transporter (SERT) binding in the human brain. However, the results were somewhat contradictory. We conducted test-retest study with single photon emission computed tomography (SPECT) with ¹²³I-ADAM as ligand in 28 healthy subjects. Ten of the subjects were studied within 1 month, whereas 18 were randomly assigned to be studied over a period of up to 1 year. The primary measure was the specific uptake ratio (SUR). Regions of interest included the midbrain, thalamus, putamen and caudate. The intra-class correlation coefficient (ICC) was 0.52-0.94 across different brain regions over 1 month, whereas the ICC was -0.24-0.63 over a 1-year period. The 1-month variability ranged from 6.5 ± 5.1% to 12.5 ± 10.6% across different brain regions, and the 1-year variability ranged from 16.5 ± 9.6% to 41.9 ± 35.5%. The Kruskal-Wallis test revealed a significant difference of variability across months. The Wilcoxon Signed Ranks Test showed the SUR between test-retest scans was of borderline significance. Curve fitting, using a 4th degree polynomial model, revealed a significant circadian correlation between the variability and interval of test-retest measurements. Our findings demonstrate the test-retest reproducibility of ¹²³I-ADAM in different time periods and suggest that circadian variation of SERT levels in the human brain might exist.

Development of acute and subacute toxicity with the serotonin transporter radiopharmaceutical, ADAM

It is predicted that depression will become the most common neurological disease in the new millennium. Its incidence is currently about 3% of diseases worldwide. Serotonin is an essential neurotransmitter for the central and peripheral nervous systems and plays a crucial role in neuropsychiatric disorders. (123)I-labeled ADAM was developed to facilitate an early diagnosis of serotonin transporter (SERT) abnormalities in the brain. Many studies have confirmed that the binding of this radiotracer to SERTs is associated with depression. The aim of this study was to evaluate the acute and subacute toxicity of ADAM and to determine its no observed adverse effect level (NOAEL) by administering it via intravenous injection to Sprague-Dawley rats for 14 consecutive days. None of the animals died, and no treatment-related clinical signs were observed. Urinalysis, hematology, and clinical chemistry analysis revealed that daily administration of ADAM (2-2-dimethylaminomethylphenylthio-5-iodophenylamine) for 2 weeks had no toxicological effects. It is concluded that ADAM exerts no adverse toxic effects on this animal model. The NOAEL was 155 microg/kg/day.

Decreased brain serotonin transporter binding in the euthymic state of bipolar I but not bipolar II disorder: a SPECT study

OBJECTIVES

Previous positron emission tomography studies have demonstrated that serotonin transporter (SERT) binding in the midbrain is decreased in the depressive state of bipolar disorder (BD). The aim of this study was to assess SERT binding in the midbrain of patients in a euthymic state of BD.

METHODS

Twenty-eight healthy controls and 24 patients in a euthymic state of medicated BD were recruited. Euthymic state was defined as Montgomery-Asberg Depression Rating Scale scores < 10 and Young Mania Rating Scale scores < 7 within a consecutive eight-week period. Single photon emission computed tomography with the radiotracer (123)I-ADAM was used to measure SERT binding in the midbrain. An equilibrium ratio model was used for data analysis. Specific uptake ratio (SUR), which represents availability of SERT binding in the midbrain, was the primary measurement outcome.

RESULTS

The averaged SURs were not different between healthy controls and BD patients in euthymic state (p = 0.27). However, a three-way ANCOVA analysis comparing SURs in healthy controls, bipolar I disorder (BD I) patients, and bipolar II disorder (BD II) patients, covarying education duration and sex, showed that the averaged SURs were significantly lower in BD I than BD II patients and healthy controls (p = 0.042). The decreased SURs in BD I patients were well correlated with duration of illness (R = -0.742, p = 0.014) only.

CONCLUSIONS

Our findings demonstrate that there is differential biological regulation in BD I and BD II patients after stable treatment, which may support the existence of a dichotomy in BD.

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

INTRODUCTION

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.

METHODS

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).

RESULTS

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.

CONCLUSIONS

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.

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.