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

Recent progress of imaging agents for Parkinson's disease

Parkinson's disease (PD) is a common progressive, neurodegenerative brain disease that is promoted by mitochondrial dysfunction, oxidative stress, protein aggregation and proteasome dysfunction in the brain. Compared with computer tomography (CT) or magnetic resonance imaging (MRI), non-invasive nuclear radiopharmaceuticals have great significance for the early diagnosis of PD due to their high sensitivity and specificity in atypical and preclinical cases. Based on the development of coordination chemistry and chelator design, radionuclides may be delivered to lesions by attaching to PD-related transporters and receptors, such as dopamine, serotonin, and others. In this review, we comprehensively detailed the current achievements in radionuclide imaging in Parkinson's disease.

Association study of serotonin transporter availability and SLC6A4 gene polymorphisms in patients with major depression

The serotonin transporter (SERT) is hypothesized to be an important component of the pathophysiology of major depression (MD). The aim of this study was to use [(123)I]ADAM single-photon emission computed tomography (SPECT) to explore whether SERT availability in four regions of the brain (striatum, thalamus, midbrain and pons) is different in patients with MD and healthy individuals. The effects of three genetic variants (rs25531, rs6354 and STin2) of the serotonin transporter gene (SLC6A4) on SERT availability were also investigated. This study included 40 MD patients and 12 controls. The mean specific uptake ratio (SUR) values in the thalamus differed significantly between MD patients and controls. Genetic variants of SLC6A4, age, gender, severity of depression, and smoking behavior did not influence SERT availability. SERT availability might be a useful biomarker of the development of MD; however, a larger sample size is needed to provide more concrete evidence.

5-HT radioligands for human brain imaging with PET and SPECT

The serotonergic system plays a key modulatory role in the brain and is the target for many drug treatments for brain disorders either through reuptake blockade or via interactions at the 14 subtypes of 5-HT receptors. This review provides the history and current status of radioligands used for positron emission tomography (PET) and single photon emission computerized tomography (SPECT) imaging of human brain serotonin (5-HT) receptors, the 5-HT transporter (SERT), and 5-HT synthesis rate. Currently available radioligands for in vivo brain imaging of the 5-HT system in humans include antagonists for the 5-HT(1A), 5-HT(1B), 5-HT(2A), and 5-HT(4) receptors, and for SERT. Here we describe the evolution of these radioligands, along with the attempts made to develop radioligands for additional serotonergic targets. We describe the properties needed for a radioligand to become successful and the main caveats. The success of a PET or SPECT radioligand can ultimately be assessed by its frequency of use, its utility in humans, and the number of research sites using it relative to its invention date, and so these aspects are also covered. In conclusion, the development of PET and SPECT radioligands to image serotonergic targets is of high interest, and successful evaluation in humans is leading to invaluable insight into normal and abnormal brain function, emphasizing the need for continued development of both SPECT and PET radioligands for human brain imaging.

Availability of the serotonin transporter in patients with alcohol dependence

OBJECTIVES

Evidence has suggested that the serotonin transporter (SERT) plays a role in the pathogenesis of alcohol dependence, anxiety and depression and that polymorphisms of the serotonin-transporter-linked promoter region (5-HTTLPR) may influence the SERT. This study evaluated the differences in SERT availability between healthy controls and alcoholic patients and the impact of 5-HTTLPR polymorphisms on SERT availability.

METHODS

Eleven healthy controls and 28 alcoholic patients were recruited. SERT availability was measured in vivo with single photon emission computed tomography and (123)I-labelled 2-((2-((dimethyl-amino)methyl)phenyl)thio)-5-iodophenylamine in the midbrain, thalamus and striatum. Each subject was genotyped for the 5-HTTLPR polymorphism.

RESULTS

Compared to healthy controls, there was a significantly lower availability of SERT in the midbrain among patients with pure alcohol dependence (pure ALC). Of patients with anxiety, depression and alcohol dependence (ANX/DEPALC), the carriers of one L(A) allele showed a significantly higher availability of SERT in the striatum compared to non-L(A) carriers. After Bonferroni correction, these significances vanished. There were no significant differences in SERT availability between controls and ANX/DEP ALC.

CONCLUSIONS

The results suggest that pure alcoholics may have lower SERT availability in the midbrain; the 5HTTLPR polymorphism may influence SERT availability in ANX/DEP ALC. These findings may serve as a springboard for future large-scale studies.

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.

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.

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.

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.

A review of the literature on neuroimaging of serotoninergic function in Alzheimer's disease and related disorders

Behavioural and psychological disorders are frequent not only in frontotemporal dementia (FTD), but also in Alzheimer's disease (AD), and many of them are related to serotoninergic dysfunction. In vitro biochemical measurements on brain samples show both pre- and post-synaptic impaired brain serotoninergic function in degenerative dementia, sometimes related to hyperactivity or aggressive behaviour. To date, few studies have explored in vivo 5HT2A and 5HT1A brain receptors in AD and FTD. They suggest that brain cells are lost in the associative cortices (5HT2A) and hippocampus (5HT1A) of AD patients, and in the medial prefrontal and orbitofrontal cortices of FTD subjects (5HT2A). Apart from reflecting a loss of local neurons, the meaning of the decrease in 5HT receptors is not yet clear and larger populations are required to establish relationships with clinical symptoms such as dementia severity and search for possible consequences for patients' behavioural and affective status.

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 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.

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.