[99mTc]TRODAT-1: a novel technetium-99m complex as a dopamine transporter imaging agent

99mTc(CO)3-labeled 1-(2-Pyridyl)piperazine derivatives as radioligands for 5-HT7 receptors

BACKGROUND

The 5-hydroxytryptamine receptor (5-HTR) family includes seven classes of receptors. The 5-HT7R is the newest member of this family and contributes to different physiological and pathological processes. As a pathology, glioblastoma multiform (GBM) overexpresses 5-HT7R; hence, this study aims to develop radiolabeled aryl piperazine derivatives as 5-HT7R imaging agents.  METHODS: Compounds 6 and 7 as 1-(3-nitropyridin-2-yl)piperazine derivatives were radiolabeled with fac-[99mTc(CO)3(H2O)3]+ and 99mTc(CO)3-[6] and 99mTc(CO)3-[7] were obtained with high radiochemical purity (RCP > 94%). The stability of the radiotracers was evaluated in both saline and mouse serum. Specific binding on different cell lines including U-87 MG, MCF-7, SKBR3, and HT-29 was performed. The biodistribution of these radiotracers was evaluated in normal and U-87 MG Xenografted models. Finally, 99mTc(CO)3-[6] and 99mTc(CO)3-[7] were applied for in vivo imaging in U-87 MG Xenografted models.

RESULTS

Specific binding study indicates that 99mTc(CO)3-[6] and 99mTc(CO)3-[7] can recognize 5-HT7R of U87-MG cell line. The biodistribution study in normal mice indicates that the brain uptake of 99mTc(CO)3-[6] and 99mTc(CO)3-[7] is the highest at 30 min post-injection (0.8 ± 0.25 and 0.64 ± 0.18%ID/g, respectively). The data of the biodistribution study in the U87-MG xenograft model revealed that these radiotracers could accumulate in the tumor site, and the highest tumor uptake was observed at 60 min post-injection (3.38 ± 0.65 and 3.27 ± 0.5%ID/g, respectively). The injection of pimozide can block the tumor's radiotracer uptake, indicating the binding of these radiotracers to the 5-HT7R. The imaging study in the xenograft model also confirms the biodistribution data. The acquired images clearly show the tumor site, and the tumor-to-muscle ratio for 99mTc(CO)3-[6] and 99mTc(CO)3-[7] at 60 min was 3.33 and 3.88, respectively.  CONCLUSIONS: 99mTc(CO)3-[6] and 99mTc(CO)3-[7] can visualize tumor in the U87-MG xenograft model  due to their affinity toward 5-HT7R.

Technetium Nitrido Complexes of Tetradentate Thiosemicarbazones: Kit-Based Radiolabeling, Characterization, and In Vivo Evaluation

Bis(thiosemicarbazone) and pyridylhydrazone-thiosemicarbazone chelators have demonstrated utility in nuclear medicine. In particular, the 64Cu2+ complexes have been extensively developed for hypoxia imaging and molecular imaging of peptide and protein markers of disease. However, the chemistry and application of bis(thiosemicarbazone) and pyridylhydrazone-thiosemicarbazone chelators in combination with 99mTc, the most widely used radionuclide in nuclear medicine, is underexplored. Herein, a series of bis(thiosemicarbazone) and pyridylhydrazone-thiosemicarbazone chelators were radiolabeled with nitrido-technetium-99m in an optimized one-pot synthesis from [99mTc]TcO4-. Optimization of the radiochemical syntheses allowed for production of the complexes in >90% radiochemical conversion with apparent molar activities of 3.3-5 GBq/μmol. Competition experiments demonstrated the excellent stability of the complexes. The nitrido-technetium-99 complexes were synthesized, and the chemical identities were investigated using mass spectrometry, spectroscopy, and density functional theory calculations. Complexation of nitrido-rhenium(V) was achieved with the N4-dialkylated bis(thiosemicarbazones). Planar imaging and ex vivo biodistribution studies of the five 99mTc complexes were conducted on healthy BALB/c mice to determine in vivo behavior. The lipophilic nature of the complexes resulted in uptake of 1.6-5.7% ID g-1 in the brain at 2 min postinjection and retention of 0.4-1.7% ID g-1 at 15 min postinjection. The stability of the complexes and the biodistribution data demonstrate that these chelators are ideal platforms for future production of radiopharmaceutical candidates.

Biallelic SHQ1 variants in early infantile hypotonia and paroxysmal dystonia as the leading manifestation

Biallelic SHQ1 variant-related neurodevelopmental disorder is extremely rare. To date, only six affected individuals, from four families, have been reported. Here, we report eight individuals, from seven unrelated families, who exhibited neurodevelopmental disorder and/or dystonia, received whole-genome sequencing, and had inherited biallelic SHQ1 variants. The median age at disease onset was 3.5 months old. All eight individuals exhibited normal eye contact, profound hypotonia, paroxysmal dystonia, and brisk deep tendon reflexes at the first visit. Varying degrees of autonomic dysfunction were observed. One individual had cerebellar atrophy at the initial neuroimaging study, however, three individuals showed cerebellar atrophy at follow-up. Seven individuals who underwent cerebral spinal fluid analysis all had a low level of homovanillic acid in neurotransmitter metabolites. Four individuals who received 99mTc-TRODAT-1 scan had moderate to severe decreased uptake of dopamine in the striatum. Four novel SHQ1 variants in 16 alleles were identified: 9 alleles (56%) were c.997C > G (p.L333V); 4 (25%) were c.195T > A (p.Y65X); 2 (13%) were c.812T > A (p.V271E); and 1 (6%) was c.146T > C (p.L49S). The four novel SHQ1 variants transfected into human SH-SY5Y neuronal cells resulted in a retardation in neuronal migration, suggestive of SHQ1 variant correlated with neurodevelopmental disorders. During the follow-up period, five individuals still exhibited hypotonia and paroxysmal dystonia; two showed dystonia; and one had hypotonia only. The complex interactions among movement disorders, dopaminergic pathways, and the neuroanatomic circuit needs further study to clarify the roles of the SHQ1 gene and protein in neurodevelopment.

Generation of Digital Brain Phantom for Machine Learning Application of Dopamine Transporter Radionuclide Imaging

While machine learning (ML) methods may significantly improve image quality for SPECT imaging for the diagnosis and monitoring of Parkinson's disease (PD), they require a large amount of data for training. It is often difficult to collect a large population of patient data to support the ML research, and the ground truth of lesion is also unknown. This paper leverages a generative adversarial network (GAN) to generate digital brain phantoms for training ML-based PD SPECT algorithms. A total of 594 PET 3D brain models from 155 patients (113 male and 42 female) were reviewed and 1597 2D slices containing the full or a portion of the striatum were selected. Corresponding attenuation maps were also generated based on these images. The data were then used to develop a GAN for generating 2D brain phantoms, where each phantom consisted of a radioactivity image and the corresponding attenuation map. Statistical methods including histogram, Fréchet distance, and structural similarity were used to evaluate the generator based on 10,000 generated phantoms. When the generated phantoms and training dataset were both passed to the discriminator, similar normal distributions were obtained, which indicated the discriminator was unable to distinguish the generated phantoms from the training datasets. The generated digital phantoms can be used for 2D SPECT simulation and serve as the ground truth to develop ML-based reconstruction algorithms. The cumulated experience from this work also laid the foundation for building a 3D GAN for the same application.

Evaluation of dopamine transporter density in healthy Brazilians using Tc-99m TRODAT-1 SPECT

The presynaptic dopamine transporter (DAT) modulates the uptake of dopamine by regulating its concentration in the central nervous system. We aimed to evaluate the DAT binding potential (DAT-BP) in a sample of healthy Brazilians through technetium-99 metastable TRODAT-1 single-photon emission computed tomography imaging.We selected 126 healthy individuals comprising 72 men and 54 women, aged 18 to 80 years. We conducted semi-quantitative evaluation in transaxial slices, following which we identified the regions of interest in the striatal region using the occipital lobe as a region of non-specific DAT-BP.We found a decrease in DAT-BP in healthy individuals aged over 30 years, culminating in a 42% mean reduction after 80 years. There was no difference in the decrease by age group between the right (linear regression test [R2] linear = 0.466) and left striatum (R2 linear = 0.510). Women presented a higher DAT-BP than men (women: R2 linear = 0.431; men: R2 linear = 0.457); nonetheless, their decrease by age group was equal to that in men.Our study sheds light on important DAT-BP findings in healthy Brazilian subjects. Our results will facilitate understanding of brain illnesses that involve the dopamine system, such as neuropsychiatric disorders.

Synthesis and Biological Evaluation of 99mTc-Labeled Phenylpiperazine Derivatives as Selective Serotonin-7 Receptor Ligands for Brain Tumor Imaging

With a poor prognosis, glioblastoma multiforme is the most aggressive tumor of the central nervous system in humans. The aim of this study was to develop novel tracers for the tumor targeting and imaging of overexpressed serotonin-7 receptors (5-HT₇Rs) in U-87 MG glioma xenografted nude mice. Two phenylpiperazine derivatives named as PHH and MPHH were designed, and the corresponding radiotracers ^99mTc-PHH and ^99mTc-MPHH were synthesized in high radiochemical purity (>95%). ^99mTc-MPHH showed a higher affinity to 5-HT₇Rs on U-87 MG cells compared to ^99mTc-PHH. In biodistribution studies, the radiocomplexes showed good brain uptake at 15 min combined with good radioactivity retention in the brain for 240 min. Regional rabbit brain studies indicated a higher radioactivity concentration in the hippocampus and diencephalon than in the cerebellum. Compared to ^99mTc-MPHH, the ^99mTc-PHH exhibited a significantly increased tumor uptake at 15 and 60 min, but the rapid blood clearance of ^99mTc-MPHH led to enhanced tumor-to-muscle ratios at 240 min. A significant reduction in tumor uptake 60 min after an injection of pimozide (5-HT₇ receptor antagonist) confirms the tumor uptake was receptor-mediated specifically. The tumor-to-contralateral muscle tissue ratio of ^99mTc-PHH and ^99mTc-MPHH in nude mice with U-87 MG xenograft was measured (5.25 and 4.65) at 60 min as well as (6.25 and 6.76) at 240 min, respectively.

Low dopamine transporter binding in the nucleus accumbens in geriatric patients with severe depression

AIM

Dysfunction of dopaminergic neurons in the central nervous system is considered to be related to major depressive disorder (MDD). Especially, MDD in geriatric patients is characterized by anhedonia, which is assumed to be associated with reduced dopamine neurotransmission in the reward system. Dopamine transporter (DAT) is considered to reflect the function of the dopamine nerve system. However, previous DAT imaging studies using single photon emission computed tomography or positron emission tomography (PET) have shown inconsistent results. The radioligand [¹⁸ F]FE-PE2I for PET enables more precise evaluation of DAT availability. Hence, we aimed to evaluate the DAT availability in geriatric patients with MDD using [¹⁸ F]FE-PE2I.

METHODS

Eleven geriatric patients with severe MDD and 27 healthy controls underwent PET with [¹⁸ F]FE-PE2I, which has high affinity and selectivity for DAT. Binding potentials (BP_ND ) in the striatum (caudate and putamen), nucleus accumbens (NAc), and substantia nigra were calculated. BP_ND values were compared between MDD patients and healthy controls.

RESULTS

MDD patients showed significantly lower DAT BP_ND in the NAc (P = 0.009), and there was a trend of lower BP_ND in the putamen (P = 0.032) compared to controls.

CONCLUSION

We found low DAT in the NAc and putamen in geriatric patients with severe MDD, which could be related to dysregulation of the reward system.

Innovative Molecular Imaging for Clinical Research, Therapeutic Stratification, and Nosography in Neuroscience

Over the past few decades, several radiotracers have been developed for neuroimaging applications, especially in PET. Because of their low steric hindrance, PET radionuclides can be used to label molecules that are small enough to cross the blood brain barrier, without modifying their biological properties. As the use of 11C is limited by its short physical half-life (20 min), there has been an increasing focus on developing tracers labeled with 18F for clinical use. The first such tracers allowed cerebral blood flow and glucose metabolism to be measured, and the development of molecular imaging has since enabled to focus more closely on specific targets such as receptors, neurotransmitter transporters, and other proteins. Hence, PET and SPECT biomarkers have become indispensable for innovative clinical research. Currently, the treatment options for a number of pathologies, notably neurodegenerative diseases, remain only supportive and symptomatic. Treatments that slow down or reverse disease progression are therefore the subject of numerous studies, in which molecular imaging is proving to be a powerful tool. PET and SPECT biomarkers already make it possible to diagnose several neurological diseases in vivo and at preclinical stages, yielding topographic, and quantitative data about the target. As a result, they can be used for assessing patients' eligibility for new treatments, or for treatment follow-up. The aim of the present review was to map major innovative radiotracers used in neuroscience, and explain their contribution to clinical research. We categorized them according to their target: dopaminergic, cholinergic or serotoninergic systems, β-amyloid plaques, tau protein, neuroinflammation, glutamate or GABA receptors, or α-synuclein. Most neurological disorders, and indeed mental disorders, involve the dysfunction of one or more of these targets. Combinations of molecular imaging biomarkers can afford us a better understanding of the mechanisms underlying disease development over time, and contribute to early detection/screening, diagnosis, therapy delivery/monitoring, and treatment follow-up in both research and clinical settings.

Effects of 99mTc-TRODAT-1 drug template on image quantitative analysis

^99mTc-TRODAT-1 is a type of drug that can bind to dopamine transporters in living organisms and is often used in SPCT imaging for observation of changes in the activity uptake of dopamine in the striatum. Therefore, it is currently widely used in studies on clinical diagnosis of Parkinson’s disease (PD) and movement-related disorders. In conventional ^99mTc-TRODAT-1 SPECT image evaluation, visual inspection or manual selection of ROI for semiquantitative analysis is mainly used to observe and evaluate the degree of striatal defects. However, these methods are dependent on the subjective opinions of observers, which lead to human errors, have shortcomings such as long duration, increased effort, and have low reproducibility. To solve this problem, this study aimed to establish an automatic semiquantitative analytical method for ^99mTc-TRODAT-1. This method combines three drug templates (one built-in SPECT template in SPM software and two self-generated MRI-based and HMPAO-based TRODAT-1 templates) for the semiquantitative analysis of the striatal phantom and clinical images. At the same time, the results of automatic analysis of the three templates were compared with results from a conventional manual analysis for examining the feasibility of automatic analysis and the effects of drug templates on automatic semiquantitative analysis results. After comparison, it was found that the MRI-based TRODAT-1 template generated from MRI images is the most suitable template for ^99mTc-TRODAT-1 automatic semiquantitative analysis.

Protective and restorative effects of the traditional Chinese medicine Jitai tablet against methamphetamine-induced dopaminergic neurotoxicity

Background

Methamphetamine (METH) is a psychostimulant with high abuse liability that affects the monoamine neurotransmitter systems, particularly the dopamine system. Currently there are no effective medications for the treatment of METH abuse to restore METH-induced dopaminergic dysfunction. The Jitai tablet (JTT), a commercial traditional Chinese medicinal preparation, has been shown to modulate the dopaminergic function both in heroin addicts and in morphine-dependent rats. The purpose of this study was to investigate, in a rodent model, whether JTT can protect against METH-induced neurotoxicity, and/or restore METH-damaged dopaminergic function.

Methods

Immunohistochemical staining and/or autoradiography staining were used to detect tyrosine hydroxylase (TH) expression in the substantia nigra, and to examine the levels of dopamine transporter (DAT), dopamine D2 receptor (D2R) and TH levels in the striatum. Using a stereotyped behavior rating scale, we evaluated the inhibitory effect of JTT on METH-induced behavioral sensitization.

Results

Repeated METH administration induced obvious stereotyped behavior and neurotoxicity on the dopaminergic system. Pre-treatment with JTT significantly attenuated METH-induced stereotyped responses, and interdicted METH-induced changes in the levels of DAT, D2R and TH expression. Treatment with JTT after METH administration restored DAT, D2R and TH expression to normal levels.

Conclusions

Our results indicated that JTT protects against METH-induced neurotoxicity and restores the dopaminergic function, and thus might be a potential treatment for the dopaminergic deficits associated with METH abuse.

In-house Preparation and Quality Control of Tc99m TRODAT 1 for Diagnostic Single-photon Emission Computed Tomography/Computed Tomography Imaging in Parkinson's Disease

Purpose of Study:

Loss of dopamine neurons in the brain is a characteristic feature of Parkinson's disease (PD). TRODAT-1 is a tropane derivative that binds to dopamine transporter (DAT) receptors. It can be used for noninvasive in vivo imaging of DAT receptors leading to the early detection of PD. The present study aims to optimize the in-house radiolabeling of TRODAT-1 with Tc-99 m in hospital radiopharmacy set up along with performing single-photon emission computed tomography/computed tomography imaging in patients with PD.

Materials and Methods:

Radiolabeling was performed through transchelation method. For optimization studies, varied amount of glucoheptonate (GHA) and stannous chloride was incubated with Tc-99 m for 10 min at room temperature. TRODAT-1 was added to the reaction mixture followed by incubation at 95°C for various time intervals. Phosphate buffer saline was added to maintain the pH of the final product. After performing the quality checks, whole-body imaging was performed to check the biodistribution in 4 patients at 1 h postinjection of 20–25 mCi (740–925 MBq) of Tc-99 m-TRODAT-1. Regional brain imaging was performed at 3–4 h. Clinical evaluation was done in control (n = 5) and in patients with PD (n = 5).

Results:

Radiolabeling yield of 100% was achieved by incubating TRODAT-1 with Tc-99 m GHA. All the quality control indicated the suitability of radiopharmaceutical for the intravenous administration. Good uptake of Tc-99 m TRODAT-1 was observed in the striatum of normal patients. However, decreased uptake was seen in patients with PD.

Conclusion:

Tc-99 m TRODAT-1 is a potential radiopharmaceutical for the diagnosis and staging PD which can be radiolabeled in-house with good yield leading to its easy availability.

Mapping neuroreceptors with metal-labeled radiopharmaceuticals

The growing epidemiological and economic burden of neurological diseases on society is tremendous. A correct and timely diagnosis can help in lowering the burden and improving the life quality of both the diseased person and the caretaker. Imaging of the brain (neuroimaging) using CT, MRI, and nuclear imaging methods can provide anatomical and functional information. Neuroreceptors are central to neurotransmission and neuromodulation in the CNS. In vivo imaging of receptors in the brain provides powerful tools for the functional study of the central nervous system (CNS) in normal or diseased states. Presently, PET imaging using non-metallic radiotracers dominates the imaging of neuroreceptors. Metal-based probes for SPECT and PET can be economical and logistically easier to use without compromising the information. This review focuses on the development of metallic radiotracers for (99mTc) SPECT and (68Ga) PET along with future directions based on the metallic probes developed for other imaging modalities namely MRI.

Purification-Free Method for Preparing Technetium-99m-Labeled Multivalent Probes for Enhanced in Vivo Imaging of Saturable Systems

Metallic radionuclides provide target-specific radiolabeled probes for molecular imaging in radiochemical yields sufficient for administration to subjects without purification. However, unlabeled ligands in the injectate can compete for targeted molecules with radiolabeled probes, which eventually necessitates postlabeling purification. Herein we describe a "1 to 3" design to circumvent the issue by taking advantage of inherent coordination properties of technetium-99m ((99m)Tc). A monovalent RGD ligand possessing an isonitrile as a coordinating moiety (CN-RGD) was reacted with [(99m)Tc(CO)3(OH2)3](+) to prepare [(99m)Tc(CO)3(CN-RGD)3](+) in over 95% radiochemical yields. This complex exhibited higher integrin αvβ3 binding affinity than its unlabeled monovalent ligand, primarily due to its multivalency. This compound visualized a murine tumor without removing unlabeled ligands, while a (99m)Tc-labeled monovalent probe derived from a monovalent ligand could not. The metal coordination-mediated synthesis of radiolabeled multivalent probes thereby can be a useful approach for preparing ready-to-use target-specific probes labeled with (99m)Tc and other metallic radionuclides of interest.

Considerations in the Development of Reversibly Binding PET Radioligands for Brain Imaging

The development of reversibly binding radioligands for imaging brain proteins in vivo, such as enzymes, neurotransmitter transporters, receptors and ion channels, with positron emission tomography (PET) is keenly sought for biomedical studies of neuropsychiatric disorders and for drug discovery and development, but is recognized as being highly challenging at the medicinal chemistry level. This article aims to compile and discuss the main considerations to be taken into account by chemists embarking on programs of radioligand development for PET imaging of brain protein targets.

Development of (18)F-labeled radiotracers for neuroreceptor imaging with positron emission tomography

Positron emission tomography (PET) is an in vivo molecular imaging tool which is widely used in nuclear medicine for early diagnosis and treatment follow-up of many brain diseases. PET uses biomolecules as probes which are labeled with radionuclides of short half-lives, synthesized prior to the imaging studies. These probes are called radiotracers. Fluorine-18 is a radionuclide routinely used in the radiolabeling of neuroreceptor ligands for PET because of its favorable half-life of 109.8 min. The delivery of such radiotracers into the brain provides images of transport, metabolic, and neurotransmission processes on the molecular level. After a short introduction into the principles of PET, this review mainly focuses on the strategy of radiotracer development bridging from basic science to biomedical application. Successful radiotracer design as described here provides molecular probes which not only are useful for imaging of human brain diseases, but also allow molecular neuroreceptor imaging studies in various small-animal models of disease, including genetically-engineered animals. Furthermore, they provide a powerful tool for in vivo pharmacology during the process of pre-clinical drug development to identify new drug targets, to investigate pathophysiology, to discover potential drug candidates, and to evaluate the pharmacokinetics and pharmacodynamics of drugs in vivo.

Functional effects of dopamine transporter gene genotypes on in vivo dopamine transporter functioning: a meta-analysis

Much psychiatric genetic research has focused on a 40-base pair variable number of tandem repeats (VNTR) polymorphism located in the 3'-untranslated region (3'UTR) of the dopamine active transporter (DAT) gene (SLC6A3). This variant produces two common alleles with 9- and 10-repeats (9R and 10R). Studies associating this variant with in vivo DAT activity in humans have had mixed results. We searched for studies using positron emission tomography (PET) or single-photon emission computed tomography (SPECT) to evaluate this association. Random effects meta-analyses assessed the association of the 3'UTR variant with DAT activity. We also evaluated heterogeneity among studies and evidence for publication bias. We found twelve studies comprising 511 subjects, 125 from PET studies and 386 from SPECT studies. The PET studies provided highly significant evidence that the 9R allele was associated with increased DAT activity in human adults. The SPECT studies were highly heterogeneous. As a group, they suggested no association between the 3'UTR polymorphism and DAT activity. When the analysis was limited to the most commonly used ligand, [123I]β-CIT, stratification by affection status dramatically reduced heterogeneity and revealed a significant association of the 9R allele with increased DAT activity for healthy subjects. In humans, the 9R allele of the 3'UTR polymorphism of SLC6A3 regulates dopamine activity in the striatal brain regions independent of the presence of neuropsychiatric illness. Differences in study methodology account for the heterogeneous results across individual studies.

Salivary DJ-1 could be an indicator of Parkinson's disease progression

Objective: The goal of the current investigation was to explore whether salivary DJ-1 could be a potential biomarker for monitoring disease progression in Parkinson's disease (PD) by evaluating the association between salivary DJ-1 concentrations and nigrostriatal dopaminergic function.

Methods: First, in 74 patients with PD and 12 age-matched normal controls, single photon emission computed tomography (SPECT) imaging with labeled dopamine transporters (DAT) (^99mTc-TRODAT-1), which has been used for measuring DAT density in PD was prformed. Then, the DJ-1 level in their saliva was analyzed by quantitative and sensitive Luminex assay and compared to caudate or putamen DAT density. Finally, based on the above, our cross-section study was carried out in 376 research volunteers (285 patients with PD and 91 healthy controls) to measure salivary DJ-1 level.

Results: From our analysis, we found a correlation between salivary concentration of DJ-1 and putamen nucleus uptake of ^99mTc-TRODAT-1 in the PD group. Although salivary DJ-1 levels were not affected by UPDRS scores, gender, age, and pharmacotherapy, DJ-1 levels in H&Y 4 stage of PD were higher than those in H&Y 1-3 stage as well as those in healthy controls. Salivary DJ-1 also decreased significantly in mixed type PD patients compared to the tremor-dominant type (TDT) and akinetic-rigid dominant type (ARDT) PD patients.

Conclusions: According to the investigation in a large cohort, we reported for the first time the prognostic potential of the salivary DJ-1 as a biomarker for evaluating nigrostriatal dopaminergic function in PD.

Peripheral leukocyte apoptosis in patients with Parkinsonism: correlation with clinical characteristics and neuroimaging findings

Apoptosis of both brain neurons and peripheral blood leukocyte is believed to be an important biomarker for evaluating the functional status of Parkinson's disease (PD). However, their correlation remains unknown. A better understanding of the pathophysiology of neurodegeneration is essential for the treatment and prevention of PD. The present study demonstrated that leukocyte apoptosis is significantly higher in PD patients and is associated with central dopamine neuron loss by using ^99mTc-TRODAT-1 SPECT. The leukocyte apoptosis and striatal dopamine transporter uptake ratios were further associated with increased severity and longer duration of disease. The interaction between brain and systemic inflammation may be responsible for the neurodegenerative disease progression.

Dopamine transporter dysfunction in Han Chinese people with chronic methamphetamine dependence after a short-term abstinence

Single-photon emission-computed tomography (SPECT) after the administration of (99m)Tc-TRODAT-1 was performed on healthy subjects and subjects with methamphetamine (METH)dependence at time 1 (T1) after 24-48 h of abstinence, time 2 (T2) after 2 weeks of abstinence, and time 3 (T3) after 4 weeks of abstinence. In contrast to values in controls, the values of the striatal DAT specific uptake ratios (SURs) in subjects with METH dependence were significantly lower at T1 (n=25), T2 (n=9), and T3 (n=8); a mild increase in SURs was observed at T2 and T3, but values were still significantly lower than those in controls. In subjects with METH dependence, there was a trend for a negative correlation of striatal DAT SURs and craving for METH at T1. METH craving, anxiety and depression scores significantly decreased from T1 to T2 to T3. We conclude that Han Chinese people with METH dependence experience significant striatal DAT dysfunction, and that these changes may be mildly reversible after 4 weeks of abstinence, but that DAT levels still remain significantly lower than those in healthy subjects. The mild recovery of striatal DAT may parallel improvements in craving, anxiety and depression.

Recent advances in imaging of dopaminergic neurons for evaluation of neuropsychiatric disorders

Dopamine is the most intensely studied monoaminergic neurotransmitter. Dopaminergic neurotransmission plays an important role in regulating several aspects of basic brain function, including motor, behavior, motivation, and working memory. To date, there are numerous positron emission tomography (PET) and single photon emission computed tomography (SPECT) radiotracers available for targeting different steps in the process of dopaminergic neurotransmission, which permits us to quantify dopaminergic activity in the living human brain. Degeneration of the nigrostriatal dopamine system causes Parkinson's disease (PD) and related Parkinsonism. Dopamine is the neurotransmitter that has been classically associated with the reinforcing effects of drug abuse. Abnormalities within the dopamine system in the brain are involved in the pathophysiology of attention deficit hyperactivity disorder (ADHD). Dopamine receptors play an important role in schizophrenia and the effect of neuroleptics is through blockage of dopamine D₂ receptors. This review will concentrate on the radiotracers that have been developed for imaging dopaminergic neurons, describe the clinical aspects in the assessment of neuropsychiatric disorders, and suggest future directions in the diagnosis and management of such disorders.

Reduced striatal dopamine transporters in people with internet addiction disorder

In recent years, internet addiction disorder (IAD) has become more prevalent worldwide and the recognition of its devastating impact on the users and society has rapidly increased. However, the neurobiological mechanism of IAD has not bee fully expressed. The present study was designed to determine if the striatal dopamine transporter (DAT) levels measured by ^99mTc-TRODAT-1 single photon emission computed tomography (SPECT) brain scans were altered in individuals with IAD. SPECT brain scans were acquired on 5 male IAD subjects and 9 healthy age-matched controls. The volume (V) and weight (W) of bilateral corpus striatum as well as the ^99mTc-TRODAT-1 uptake ratio of corpus striatum/the whole brain (Ra) were calculated using mathematical models. It was displayed that DAT expression level of striatum was significantly decreased and the V, W, and Ra were greatly reduced in the individuals with IAD compared to controls. Taken together, these results suggest that IAD may cause serious damages to the brain and the neuroimaging findings further illustrate IAD is associated with dysfunctions in the dopaminergic brain systems. Our findings also support the claim that IAD may share similar neurobiological abnormalities with other addictive disorders.

Involvement of nigrostriatal pathway in Japanese encephalitis with movement disorders: evidence from 99mTc-TRODAT-1 and 123I-IBZM SPECT imagings

PURPOSE

The purpose of this study was to evaluate molecular evidence of nigrostriatal pathway involvement in Japanese encephalitis (JE) survivors with movement complications.

METHODS

Three JE patients were recruited. All had cranial magnetic resonance imaging (MRI) and single-photon emission computed tomography (SPECT) studies with (99m)Tc-TRODAT-1 and (123)I-IBZM.

RESULTS

Cranial MRI revealed involvement of bilateral thalami, substantia nigra, and medial temporal lobes in all three patients, but only case 1 had additional bilateral basal ganglia involvement. The (99m)Tc-TRODAT-1 SPECT for presynaptic dopamine transporter imaging disclosed asymmetrical decreases in bilateral striatal uptake in all three patients. However, the (123)I-IBZM SPECT imaging for postsynaptic D2 dopamine receptors (D2Rs) revealed inconsistent abnormalities including asymmetrical bilateral decreases (case 1), unilateral decrease (case 2), and bilateral increases (case 3) in striatal uptakes.

CONCLUSION

Data have suggested that presynaptic dopaminergic neurons in JE patients are more susceptible to JE virus than postsynaptic striatal neurons. The degree of movement impairment was more closely correlated to the degree of D2Rs disruption seen in (123)I-IBZM SPECT imaging.

The posttraumatic stress disorder project in Brazil: neuropsychological, structural and molecular neuroimaging studies in victims of urban violence

BACKGROUND

Life trauma is highly prevalent in the general population and posttraumatic stress disorder is among the most prevalent psychiatric consequences of trauma exposure. Brazil has a unique environment to conduct translational research about psychological trauma and posttraumatic stress disorder, since urban violence became a Brazilian phenomenon, being particularly related to the rapid population growth of its cities. This research involves three case-control studies: a neuropsychological, a structural neuroimaging and a molecular neuroimaging study, each focusing on different objectives but providing complementary information. First, it aims to examine cognitive functioning of PTSD subjects and its relationships with symptomatology. The second objective is to evaluate neurostructural integrity of orbitofrontal cortex and hippocampus in PTSD subjects. The third aim is to evaluate if patients with PTSD have decreased dopamine transporter density in the basal ganglia as compared to resilient controls subjects. This paper shows the research rationale and design for these three case-control studies.

METHODS AND DESIGN

Cases and controls will be identified through an epidemiologic survey conducted in the city of São Paulo. Subjects exposed to traumatic life experiences resulting in posttraumatic stress disorder (cases) will be compared to resilient victims of traumatic life experiences without PTSD (controls) aiming to identify biological variables that might protect or predispose to PTSD. In the neuropsychological case-control study, 100 patients with PTSD, will be compared with 100 victims of trauma without posttraumatic stress disorder, age- and sex-matched controls. Similarly, 50 cases and 50 controls will be enrolled for the structural study and 25 cases and 25 controls in the functional neuroimaging study. All individuals from the three studies will complete psychometrics and a structured clinical interview (the Structured Clinical Interview for DSM-IV and the Clinician-Administered PTSD Scale, Beck Anxiety Inventory, Beck Depression Inventory, Global Assessment of Function, The Social Adjustment Scale, Medical Outcomes Study 36-Item Short-Form Health Survey, Early Trauma Inventory, Clinical global Impressions, and Peritraumatic Dissociative Experiences Questionnaire). A broad neuropsychological battery will be administered for all participants of the neuropsychological study. Magnetic resonance scans will be performed to acquire structural neuroimaging data. Single photon emission computerized tomography with [(99m)Tc]-TRODAT-1 brain scans will be performed to evaluate dopamine transporters.

DISCUSSION

This study protocol will be informative for researchers and clinicians interested in considering, designing and/or conducting translational research in the field of trauma and posttraumatic stress disorder.

Quantitative evaluation of simultaneous reconstruction with model-based crosstalk compensation for 99mTc/123I dual-isotope simultaneous acquisition brain SPECT

A model-based method has been previously developed to estimate and compensate for the crosstalk and downscatter contamination in simultaneous 123I/99mTc dual-isotope SPECT imaging. In this method, photon scatter in the object is modeled using the effective source scatter estimate technique. Photon interactions with the collimator-detector are estimated using precalculated Monte Carlo simulated point response functions. Two different approaches, simultaneous and alternating model-based compensations, have been proposed for iterative reconstruction-based crosstalk and downscatter contamination compensation. In this work, both model-based approaches were evaluated in the context of quantitative accuracy when imaging the dopaminergic system using both Monte Carlo simulated and experimentally acquired data. Results indicate that mddel-based estimates of the crosstalk and downscatter contamination in both energy windows were in good agreement with the truth for the simulated data. The effects of the contamination reduced image contrast and overestimated absolute activity in all structures by up to 66%. Compensation using both model-based approaches improved image contrast. Errors in absolute activity quantitation were also reduced to less than +/-5% for most brain structures. The accuracy of striatal specific binding potentials, calculated as the ratio of activity in various striatal structures to the background, was also greatly improved after model-based compensation. In conclusion, model-based compensation of simultaneously acquired images of 99mTc and 123I labeled brain imaging agents provided image quality and quantitative accuracy that were comparable to the image without crosstalk. Both proposed compensation approaches can potentially be applied clinically, but when reconstruction time is a limiting factor, the alternating model-based compensation may be preferable.

Phasic dysfunction of dopamine transmission in Tourette's syndrome evaluated with 99mTc TRODAT-1 imaging

This study investigated the complex dysregulation of the dopaminergic neurotransmitter system in Tourette's syndrome (TS) patients challenged with methylphenidate (MPH). Eight drug-naïve male patients (aged 21-25 years) who met DSM-IV criteria for TS and had a mean disease severity of 25 on the Yale Global Tic Severity Scale were recruited. Brain (99m)TC TRODAT-1 dopamine transporter (DAT) single photon emission computed tomography (SPECT) was performed 5 days before, and 2 h after 10 mg of orally administered MPH. Eight age-matched healthy males served as controls. Repeated measures analysis of variance was used to measure differences in DAT-binding ratios before and after MPH challenge between the TS patients and controls. The DAT-binding ratios decreased significantly after MPH treatment in both groups. However, a significant interaction between group and MPH effects was found only in the right caudate, which was mainly due to a smaller decline of the DAT-binding ratio after MPH in the TS group than in the controls. Such a distinction was not found in the other striatal sub-regions in the two groups. No correlation, however, was observed between the tic severity score and DAT-binding ratio measured from the whole striatum or its sub-regions. The observed change in the DAT-binding ratio might indicate a functional abnormality of the dopaminergic system in the right caudate nucleus of TS patients. Future studies exploring dopamine transmission are thus needed to understand the pathophysiology of TS.

Dopamine transporters and cognitive function in methamphetamine abuser after a short abstinence: A SPECT study

The purpose of this study was to examine the change of dopamine transporters (DAT) binding in methamphetamine (METH) abusers in a two-week period of abstinence and its association with cognitive function. Seven healthy subjects and seven METH abusers were recruited. At baseline conditions, the values of specific uptake ratio (SUR) of DAT binding measured by single photon emission computed tomography were lower in METH abusers than in controls. After a two-week period of abstinence, DAT binding was partially recovered and there were no statistic differences in SUR between METH abusers and controls. There was a borderline correlation between the changes of DAT binding in the right, but not the left, striatum and the %Error of Wisconsin Card Sorting Test. These findings indicate that DAT binding in METH abusers can be reversed in a short period of abstinence. The recovery of DAT binding was asymmetric and possibly parallel with the improvement of cognitive function.

Selective binding of 2-[125I]iodo-nisoxetine to norepinephrine transporters in the brain

A radioiodinated ligand, (R)-N-methyl-(2-[(125)I]iodo-phenoxy)-3-phenylpropylamine, [(125)I]2-INXT, targeting norepinephrine transporters (NET), was successfully prepared. A no-carrier-added product, [(125)I]2-INXT, displayed a saturable binding with a high affinity (K(d)=0.06 nM) in the homogenates prepared from rat cortical tissues as well as from LLC-PK(1) cells expressing NET. A relatively low number of binding sties (B(max)=55 fmol/mg protein) measured with [(125)I]2-INXT in rat cortical homogenates is consistent with the value reported for a known NET ligand, [(3)H]nisoxetine. Competition studies with various compounds on [(125)I]2-INXT binding clearly confirmed the pharmacological specificity and selectivity for NET binding sites. Following a tail-vein injection of [(125)I]2-INXT in rats, a good initial brain uptake was observed (0.56% dose at 2 min) followed by a slow washout from the brain (0.2% remained at 3 hours post-injection). The hypothalamus (a NET-rich region) to striatum (a region devoid of NET) ratio was 1.5 at 3 hours post-i.v. injection. Pretreatment of rats with nisoxetine significantly inhibited the uptake of [(125)I]2-INXT (70-100% inhibition) in locus coeruleus, hypothalamus and raphe nuclei, regions known to have a high density of NET; whereas escitalopram, a serotonin transporter ligand, did not show a similar effect. Ex vivo autoradiography of rat brain sections of [(125)I]2-INXT (at 3 hours after an i.v. injection) displayed an excellent regional brain localization pattern corroborated to the specific NET distribution in the brain. The specific brain localization was significantly reduced by a dose of nisoxetine pretreatment. Taken together, the data suggest that [(123)I]2-INXT may be useful for mapping NET binding sites in the brain.

Biodistribution study of [99mTc] TRODAT-1 alone or combined with other dopaminergic drugs in mice with macroautoradiography

A 99mTc labeled tropane derivative, [99mTc] TRODAT-1 (2beta-((N,N'-bis(2-mercaptoethyl) ethylene diamino)methyl), 3beta-(4-chlorophenyl) tropane), is a potential dopamine transporter (DAT) imaging agent for the central nervous system. To better understand the binding localization of [99mTc] TRODAT-1 both in the brain and the body, whole-body macroautoradiography (WBAR) was used in this study. The effect of DAT competing drugs, such as levadopa (L-DOPA), N-methyl-2beta-carbomethoxy-3beta-(4fluorophenyl)tropane (CFT, WIN 35,428) and methylphenidate, on the biodistribution of [99mTc] TRODAT-1 were also included in this study. Doses of 150 MBq [99mTc] TRODAT-1 were injected into normal male ICR mice through the caudal veins. For comparison, mice treated with 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), L-DOPA, methylphenidate and CFT, respectively, were also investigated under the similar protocols. One and a half hours after [99mTc] TRODAT-1 injection, the mice were sacrificed. Whole-body autoradiography was performed immediately after sacrifice. Both frontal and sagittal sections showed that the liver and mucosa of stomach had the highest uptake of [99mTc] TRODAT-1. Other binding sites included the periphery of the spinal cord and the epithelium of the intestine. In the brain, autoradiographic imaging obtained from frontal sections showed symmetrical uptakes of [99mTc] TRODAT-1 in bilateral striata. Remaining binding sites include olfactory bulbs, thyroid gland, and salivary gland. The autoradiographic imaging obtained from sagittal sections showed a similar biodistribution. Mice treated with MPTP or L-DOPA showed no significant difference in the uptake of [99mTc] TRODAT-1 in bilateral striata, as compared to those of the control. In CFT or methylphenidate-treated mice, DAT binding sites were almost completely inhibited. These data showed that [99mTc] TRODAT-1 has potential clinical use for neurological investigation, such as Parkinson's and similar diseases.

Dual SPECT of dopamine system using [99mTc]TRODAT-1 and [123I]IBZM in normal and 6-OHDA-lesioned formosan rock monkeys

Parkinson's disease (PD) is a progressive neurodegenerative disorder characterized by a severe loss of the dopaminergic neurons in the substantia nigra pars compacta. In this study, we evaluated pre- and post-synaptic binding sites of the dopamine system in three normal and one parkinsonian monkeys using simultaneous [99mTc]TRODAT-1 and [123I]IBZM imaging. The parkinsonian monkey was induced by injecting 6-hydroxydopamine (6-OHDA) bilaterally into the medial forebrain bundle under MRI guidance. [99mTc]TRODAT-1 (targeting dopamine transporters) and [123I]IBZM (targeting D(2)/D(3) receptors) were administered almost simultaneously and the SPECT images were acquired over 4 h using a dual-headed gamma camera equipped with ultra-high resolution fan-beam collimators. Data were obtained using energy window of 15% centered on 140 keV for 99mTc in conjunction with 10% asymmetric energy window in a lower bound at 159 keV for 123I. Single SPECT studies of [99mTc]TRODAT-1 and [123I]IBZM were also performed. We found a comparable image quality and uptake ratios between single- and dual-isotope studies. There are higher TRODAT-1 uptakes in the control monkeys than the 6-OHDA-lesioned monkey. The uptake of [123I] IBZM showed no significant difference between controls and 6-OHDA-lesioned monkey. Our results suggest that dual isotope imaging using [99mTc]TRODAT-1 and [123I]IBZM may be a useful means in evaluating the changes of both pre- and post-synaptic dopamine system in a primate model of parkinsonism.

The preclinical pharmacologic study of dopamine transporter imaging agent [99mTc]TRODAT-1

The purpose of this study was to investigate the pharmacologic characteristics of TRODAT-1 (2beta-((N,N'-bis(2-mercaptoethyl)ethylene diamino)methyl), 3beta-(4-chlorophenyl)tropane) labeled with [99mTc] as an imaging agent for dopamine transporter (DAT). Radiochemical purity of [99mTc]TRODAT-1 was over 90%. The partition coefficients in octanol and buffer were 2.12 and 2.19 at pH 7.0 and 7.4, respectively. Animal studies have been performed in rats, rabbits, and normal and hemi-Parkinsonian model monkeys. Biodistribution displayed moderate uptake in rat brain (0.28 %ID/organ at 2 min) and the striatal uptake was 0.193, 0.142, and 0.136 %ID/g at 2, 60, and 120 min, respectively. The ratios of striatal/cerebellar (ST/CB) uptake were 2.4, 4.45, and 2.45 %ID/g at 60, 120, and 240 min, respectively. The major radioactivity was excreted by the hepatobiliary system. Blood clearance kinetics was performed in rabbits, and the initial half-life of 1.18 min and late half-life of 367.8 min were obtained. Brain single photon emission computed tomography imaging studies in normal monkeys showed the ratios of ST/CB uptake were 1.56-2.0 %ID/g and indicated that both uptake and retention in the striatal area were associated with the DAT. The imaging of hemi-Parkinsonian model monkeys also displayed the expected selectivity, the highest uptake being observed in the basal ganglia area of the normal side. Thereby, it is suggested that [99mTc]TRODAT-1 is a safe and useful imaging agent for localization of the presynaptic DAT in the brain.

A simplified method to determine [99mTc]TRODAT-1 in human plasma

[99mTc]TRODAT-1 is a useful imaging agent in evaluating changes in presynaptic dopamine transporters (DAT) for Parkinson's disease and other central nervous system (CNS) neurodegenerative diseases, for which a reduction of dopamine neurons is indicated. As part of an effort to establish a quantitative single-photon emission tomography (SPECT) procedure for imaging CNS DAT, measurement of nonmetabolized [99mTc]TRODAT-1 in human plasma was investigated. After an intravenous injection of [99mTc]TRODAT-1, there are three possible radioactive components in human plasma: hydrophilic compounds (pertechnetate, etc.), lipophilic metabolite(s), and unchanged [99mTc]TRODAT-1. Based on the differences in lipophilicity of [99mTc]TRODAT-1 and its lipophilic metabolite [99mTc]BAT, a new quantitative method for measuring [99mTc]TRODAT-1 with a simple solvent extraction method was developed. Various organic solvents or mixtures of solvents were tested, among which cyclohexane gave the best extraction yield for [99mTc]TRODAT-1 (76.06 +/- 3.32%) with a low extraction for [99mTc]BAT (2.43 +/- 0.82%). Extractions of [99mTc]TRODAT-1 and [99mTc]BAT mixtures in different predetermined ratios to simulate the actual human plasma samples with cyclohexane from phosphate buffer (5 mM, pH 8.0) were evaluated. The experimentally obtained ratios were in good agreement with the theoretical ratios. To investigate further the possibility of replacing the previously established high performance liquid chromatography (HPLC) method with the new solvent extraction method for the clinical application, both HPLC and extraction methods were used side by side to determine the unchanged [99mTc]TRODAT-1 in human plasma samples during [99mTc]TRODAT-1/SPECT imaging studies. The results from four human subjects showed that both methods consistently produced similar values for unchanged [99mTc]TRODAT-1 in the plasma samples. This improved solvent extraction method provides an easy and reliable technique to quantify unchanged [99mTc]TRODAT-1 in human plasma, thus making the clinical application of this agent readily available for quantitation of the DAT binding sites in the brain by SPECT imaging.

An improved kit formulation of a dopamine transporter imaging agent: [Tc-99m]TRODAT-1

Recently, [Tc-99m]TRODAT-1, the first Tc-99m-labeled tracer for imaging CNS dopamine transporters in humans, was reported. This tracer displayed excellent specific binding to dopamine transporters in the basal ganglia region of the brain, thus it is potentially useful for the diagnosis of deficit of dopamine transporters in neurodegenerative diseases, such as Parkinson's disease. Preparation of [Tc-99m]TRODAT-1 was previously achieved by a multistep kit formulation. It is highly desirable to further improve the preparation by developing a simplified one-vial formulation with a reduced amount of TRODAT-1 ligand for routine clinical use. To achieve this goal, a series of studies to optimize labeling efficiency by varying a combination of factors (amount of free ligand, reaction reagents, and reaction pH) was carried out. [Tc-99m]TRODAT-1 prepared by this new kit formulation was evaluated by assessing the brain uptake and target (striatum) versus nontarget (cerebellum) ratios in rats. Appropriate amounts of various ingredients for a one-vial kit formulation providing > or =90% radiolabeling yields were identified. The most consistent and reliable formulation contained 10 microg of TRODAT-1 (a reduction of free ligand from 200 microg to 10 microg), 32 microg of SnCl2, 10 mg of sodium glucoheptonate, and 840 microg of disodium EDTA in one vial as a lyophilized kit. It is feasible to reconstitute the vial with [Tc-99m]pertechnetate (0.5-2 mL, < or =1110 MBq, 30 mCi), resulting in a final solution with a pH value of 4.5-5.0. [Tc-99m]TRODAT-1, prepared by this new kit, was stable at room temperature for 6 h. Biodistribution studies of this agent in rats with the new formulation showed similar regional brain distribution as compared with those obtained with the previous preparation (high striatum-to-cerebellum ratio). In conclusion, using this lyophilized one-vial kit formulation, [Tc-99m]TRODAT-1 can be prepared with greater than 90% radiochemical purity. This simplified kit will significantly improve the reliability of preparation of this agent for routine clinical use.

Novel rhenium complexes derived from alpha-tropanol as potential ligands for the dopamine transporter

A series of rhenium complexes was synthesized as model compounds for the corresponding radioactive technetium-99m complexes for preliminary biological investigations. In a '3 + 1' mixed-ligand approach the tropanol molecule was linked with the metal core through a omega-mercaptoester group as monodentate ligand. Bis(thioethyl)sulphide was used as the tridentate dithiol ligand to block the remaining free coordination sites. The omega-mercaptoesters were synthesized via the trityl-protected precursors. Binding tests on the cloned human dopamine transporter revealed moderate binding affinities for some of the prepared compounds. The complexes were characterised by X-ray and the lipophilicity as well as pKa values were determined.

Technetium-99m-labeled receptor-specific small-molecule radiopharmaceuticals: recent developments and encouraging results

The development of technetium-99m-labeled small-molecule radiopharmaceuticals directed at specific high-affinity binding sites, as are found in receptors for hormones and neurotransmitters, transport systems, and certain enzymes, is a natural outgrowth from the successful development of technetium radiopharmaceuticals for imaging flow and metabolism. Although many receptor-specific radiopharmaceuticals labeled with PET and other SPECT isotopes already exist, the low cost and widespread availability of technetium-99m would make their 99mTc-labeled counterparts much more accessible to the medical community. This review has four goals: (a) To survey and analyze critically the results of a flurry of research activity in this area in recent years, which has led to the preparation of a number of novel technetium-labeled radiopharmaceuticals targeted at high-affinity sites, a few of which appear to be very promising; (b) to provide a conceptual analysis of how these agents are being designed; (c) to provide a context in terms of binding and uptake behavior by which these agents should be judged; and (d) to highlight emerging knowledge on the structure of receptors and related high-affinity binding biomolecules and their distribution, which may serve as reference points for understanding the results that have been obtained so far, and may be useful guides for future design.