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.     

Targeting murine heart and brain: visualisation conditions for multi-pinhole SPECT with <Superscript>99m</Superscript>Tc- and <Superscript>123</Superscript>I-labelled probes

Purpose  The study serves to optimise conditions for multi-pinhole SPECT small animal imaging of ¹²³I- and ^99mTc-labelled radiopharmaceuticals with different distributions in murine heart and brain and to investigate detection and dose range thresholds for verification of differences in tracer uptake. Methods  A Triad 88/Trionix system with three 6-pinhole collimators was used for investigation of dose requirements for imaging of the dopamine D₂ receptor ligand [¹²³I]IBZM and the cerebral perfusion tracer [^99mTc]HMPAO (1.2–0.4 MBq/g body weight) in healthy mice. The fatty acid [¹²³I]IPPA (0.94 ± 0.05 MBq/g body weight) and the perfusion tracer [^99mTc]sestamibi (3.8 ± 0.45 MBq/g body weight) were applied to cardiomyopathic mice overexpressing the prostaglandin EP₃ receptor. Results  In vivo imaging and in vitro data revealed 45 kBq total cerebral uptake and 201 kBq cardiac uptake as thresholds for visualisation of striatal [¹²³I]IBZM and of cardiac [^99mTc]sestamibi using 100 and 150 s acquisition time, respectively. Alterations of maximal cerebral uptake of [¹²³I]IBZM by >20% (116 kBq) were verified with the prerequisite of 50% striatal of total uptake. The labelling with [^99mTc]sestamibi revealed a 30% lower uptake in cardiomyopathic hearts compared to wild types. [¹²³I]IPPA uptake could be visualised at activity doses of 0.8 MBq/g body weight. Conclusion  Multi-pinhole SPECT enables detection of alterations of the cerebral uptake of ¹²³I- and ^99mTc-labelled tracers in an appropriate dose range in murine models targeting physiological processes in brain and heart. The thresholds of detection for differences in the tracer uptake determined under the conditions of our experiments well reflect distinctions in molar activity and uptake characteristics of the tracers. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

In vivo imaging of serotonin transporters with [99mTc]TRODAT-1 in nonhuman primates

[^99mTc]TRODAT-1 was the first ^99mTc-labeled imaging agent to show specific binding to dopamine transporters (DAT) in the striatum (STR) of human brain. Additionally, in vitro binding and autoradiographic experiments demonstrated that this tracer also binds to serotonin transporters (SERT) in the midbrain/hypothalamus (MB) area. In this study, [^99mTc]TRODAT-1 was investigated as a potentially useful ligand to image SERT in the MB of living brain. A total of eight single-photon emission tomography (SPET) scans were performed in two baboons (Papio anubis) after intravenous (i.v.) injection of 740 MBq (20 mCi) of [^99mTc]TRODAT-1 using a triple-head gamma camera equipped with ultra-high-resolution fan-beam collimators (scan time: 0–210 min). In four blocking studies, baboons were pretreated with (+)McN5652 (1 mg/kg, i.v.) or methylphenidate (1 mg/kg, i.v.) to specifically block SERT or DAT, respectively. After co-registration with magnetic resonance images of the same baboon, a region of interest analysis was performed using predefined templates to calculate specific uptake in the midbrain area and the striatum, with the cerebellum as the background region [(MB–CB)/CB, (STR–CB)/CB]. Additionally, two PET scans of the same baboons were performed after i.v. injections of 74–111 MBq (2–3 mCi) of [¹¹C](+)McN5652 to identify the SERT sites. In [^99mTc]TRODAT-1/SPET scans, the SERT sites in the MB region were clearly visualized. Semiquantitative analysis revealed a specific uptake in MB ([MB–CB]/CB) of 0.30±0.02, which was decreased to 0.040±0.005 after pretreatment with nonradioactive (+)McN5652, a selective SERT ligand. Pretreatment with methylphenidate reduced the specific binding of [^99mTc]TRODAT-1 to DAT sites [(STR-CB)/CB] from 2.45±0.13 to 0.32±0.04 without any effect on its binding to SERT sites [(MB–CB)/CB], which was confirmed by the co-registration of the [¹¹C](+)McN5652/PET scans. This preliminary study suggests that specific binding of [^99mTc]TRODAT-1 to SERT sites can be detected by in vivo SPET imaging despite the low target to background ratio. These findings provide impetus for further development of similar compounds with improved binding affinity and selectivity to SERT sites. Received 15 September and in revised form 15 November 1998     

Pharmacological effects of dopaminergic drugs on in vivo binding of [99mTc]TRODAT-1 to the central dopamine transporters in rats

R -(exo-exo)]-, [^99mTc]TRODAT-1, to DAT. This paper describes the further characterization of [^99mTc]TRODAT-1 binding sites in rats under conditions which may exist in patients receiving various drug treatments. All experiments were carried out using an i.v. injection of [^99mTc]TRODAT-1 into male Sprague-Dawley rats. Measurements of % dose/gram ratio of (striatum–cerebellum)/cerebellum at 1 h post injection were used as an indicator for specific DAT binding. The biodistribution studies were performed in the presence of drugs which compete for the binding site, such as CFT (WIN 35,428) and methylphenidate, drugs which influence dopamine levels, such as l-DOPA, γ-hydroxybutyrolactone, and α-methyl-p-tyrosine, and d-amphetamine, which both acts as a competitor for DAT binding and increases dopamine levels. Additionally, the influence of dopamine receptor agonists, such as apomorphine and (+)bromocriptine, on biodistribution was tested. Binding of [^99mTc]TRODAT-1 to DAT was found to be inhibited by CFT, methylphenidate, and d-amphetamine in a dose-dependent manner. The specific binding of [^99mTc]TRODAT-1 was not altered by dopamine receptor agonists or by drugs which cause minor changes in dopamine levels. When administered in high doses (634 μmol/kg), l-DOPA also decreased the binding of [^99mTc]TRODAT-1. It is likely that a low dose of l-DOPA (normally needed in the treatment of Parkinson’s disease) will not affect the results on [^99mTc]TRODAT-1 single-photon emission tomographic (SPET) imaging studies. In conclusion, the results clearly demonstrate the specificity of [^99mTc]TRODAT-1 binding to DAT in vivo. Competition for [^99mTc]TRODAT-1 binding was observed only with drug treatment that significantly increases dopamine levels or actively competes for binding at DAT. The results suggest that prior knowledge of whether patients are receiving various drug treatments may assist in the interpretation of DAT status as assessed by SPET imaging studies using [^99mTc]TRODAT-1. Received 7 July and in revised form 27 September 1997     

Dopamine transporter imaging with [123I]FP-CIT SPECT: potential effects of drugs

Background [¹²³I]N-ω-fluoropropyl-2β-carbomethoxy-3β-{4-iodophenyl}nortropane ([¹²³I]FP-CIT) single photon emission computed tomography (SPECT) is a frequently and routinely used technique to detect or exclude dopaminergic degeneration by imaging the dopamine transporter (DAT) in parkinsonian and demented patients. This technique is also used in scientific studies in humans, as well as in preclinical studies to assess the availability of DAT binding in the striatum. In routine clinical studies, but also in scientific studies, patients are frequently on medication and sometimes even use drugs of abuse. Moreover, in preclinical studies, animals will be anesthetized. Prescribed drugs, drugs of abuse, and anesthetics may influence the visual interpretation and/or quantification of [¹²³I]FP-CIT SPECT scans. Discussion Here, we discuss the basic principle of how drugs and anesthetics might influence the visual interpretation and/or quantification of [¹²³I]FP-CIT SPECT scans. We also review drugs which are likely to have a significant influence on the visual interpretation and/or quantification of [¹²³I]FP-CIT SPECT scans. Additionally, we discuss the evidence as to whether frequently prescribed drugs in parkinsonian and demented patients may have an influence on the visual interpretation and/or quantification of [¹²³I]FP-CIT SPECT scans. Finally, we discuss our recommendations as to which drugs should be ideally withdrawn before performing a [¹²³I]FP-CIT SPECT scan for routine clinical purposes. The decision to withdraw any medication must always be made by the specialist in charge of the patient’s care and taking into account the pros and cons of doing so.     

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

Technetium-99m is the most commonly used radionuclide in routine nuclear medicine imaging procedures. Development of^99mTc-labeled receptor-specific imaging agents for studying the central nervous system is potentially useful for evaluation of brain function in normal and disease states. A novel^99mTc-labeled tropane derivative, [^99mTc]TRODAT 1, which is useful as a potential CNS dopamine transporter imaging agent, was evaluated and characterized. After i.v. injection into rats, [^99mTc]TRODAT-1 displayed specific brain uptake in the rat striatal region (striatum-cerebellum/cerebellum ratio 1.8 at 60 min), where dopamine neurons are concentrated. The specific striatal uptake could be blocked by pretreating rats with a dose of competing dopamine transporter ligand, ß-CIT (or RTI-55, i.v., 1 mg/kg). However, the specific striatal uptake of [^99mTc]TRODAT-] was not affected by co-injection of excess free ligand (TRODAT-1, up to 200 g per rat) or by pretreating the rats with haloperidol (i.v., 1 mg/kg). The specific uptake in striatal regions of rats that had prior 6-hydroxydopamine lesion in the substantia nigra area showed a dramatic reduction. The radioactive material recovered from the rat striatal homogenates at 60 min after i.v. injection of [^99mTc]TRODAT-1 showed primarily the original compound (>95%), a good indication of in vivo stability in brain tissue. Similar and comparable organ distribution patterns and brain regional uptakes of [^99mTc]TRODAT-1 were obtained for male and female rats. Ex vivo autoradiography results of rat brain sections further confirmed the high uptake and retention of [^99mTc]TRODAT-1 in the striatal region. In vitro binding studies measuring the affinity to dopamine transporters for the free ligand, TRODAT-1, and a nonradioactive rhenium derivative, Re-TRODAT-1, showed K _i values of 9.7 nM and 14.1 nM, respectively. Behavioral studies in rats using the free ligand, TRODAT-1 and Re-TRODAT-1 indicated that, unlike other tropane derivatives, they displayed no effect on locomotor activity, suggesting low toxicity. These results strongly support the conclusions that this novel^99mTc radioligand binds selectively to dopamine transporters in the brain and that is is potentially useful for in vivo assessment of the loss of dopamine neurons in Parkinson's and other neurodegeneralive diseases.     

99mTc-TRODAT-1 and 123I-IBZM SPECT studies in a patient with extrapontine myelinolysis with parkinsonian features

Osmotic demyelination syndrome can result from the rapid correction of hyponatremia, and is categorized by central pontine myelinolysis and extrapontine myelinolysis (EPM). Magnetic resonance imaging (MRI) is currently the most useful modality for visualizing EPM lesions. However, MRI is unable to delineate the severity of involvement in the nigrostriatal dopaminergic pathway. The authors describe the case of a 48-year-old woman who developed isolated EPM with predominantly right-sided parkinsonian symptoms after rapid correction of hyponatremia. MRI revealed symmetrical demyelinating lesions in the bilateral striatum without central pontine involvement. ^99mTc-TRODAT-1 and ¹²³I-iodobenzamide (¹²³I-IBZM) single-photon emission computed tomography (SPECT) images showed unequally decreased uptake in the bilateral striatum. Treatment with carbidopa/levodopa improved the clinical parkinsonian symptoms. ^99mT_C-TRODAT-1 and ¹²³I-IBZM SPECT images provide presynaptic and postsynaptic molecular information of the nigrostriatal dopaminergic pathway. The lesions demonstrated in the ^99mT_C-TRODAT-1 and ¹²³I-IBZM SPECT images show higher correlation with the severity of clinical features in EPM than MRI, and the modalities may be useful in the evaluation of patients with parkinsonian symptoms.     

Diagnostic value of asymmetric striatal D<Subscript>2</Subscript> receptor upregulation in Parkinson’s disease: an [<Superscript>123</Superscript>I]IBZM and [<Superscript>123</Superscript>I]FP-CIT SPECT study

Introduction Striatal postsynaptic D₂ receptors in Parkinson’s disease (PD) are thought to be upregulated in the first years of the disease, especially contralateral to the clinically most affected side. The aim of this study was to evaluate whether the highest striatal D₂ binding is found contralateral to the most affected side in PD, and whether this upregulation can be used as a diagnostic tool. Methods Cross-sectional survey was undertaken of 81 patients with clinically asymmetric PD, without antiparkinsonian drugs and with a disease duration of ≤5 years and 26 age-matched controls. Striatal D₂ binding was assessed with [¹²³I]IBZM SPECT, and severity of the presynaptic dopaminergic lesion with [¹²³I]FP-CIT SPECT. Results The mean striato-occipital ratio of [¹²³I]IBZM binding was significantly higher in PD patients (1.56 ±0.09) than in controls (1.53 ±0.06). In PD patients, higher values were found contralateral to the clinically most affected side (1.57 ±0.09 vs 1.55 ±0.10 ipsilaterally), suggesting D₂ receptor upregulation, and the reverse was seen using [¹²³I]FP-CIT SPECT. However, on an individual basis only 56% of PD patients showed this upregulation. Conclusion Our study confirms asymmetric D₂ receptor upregulation in PD. However, the sensitivity of contralateral higher striatal [¹²³I]IBZM binding is only 56%. Therefore, the presence of contralateral higher striatal IBZM binding has insufficient diagnostic accuracy for PD, and PD cannot be excluded in patients with parkinsonism and no contralateral upregulation of D₂ receptors, assessed with [¹²³I]IBZM SPECT.     

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 150 MBq [¹²³I]ADAM. Data were acquired from the time of injection until 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.     

Simultaneous dual-isotope imaging based on an artificial neural network for evaluating myocardial perfusion and fatty acid metabolism

Background

We contrived a scatter correction method based on an artificial neural network (ANN) and applied it to the simultaneous evaluation of myocardial perfusion and fatty acid metabolism in single-photon emission computed tomography (SPECT).

Methods

The count data of three energy windows were used as inputs of the ANN. The count ratios of the estimated primary-to-total photons for ^99mTc and ¹²³I, which were used to reconstruct ^99mTc and ¹²³I images, were calculated using the ANN. In a phantom study, single- and dual-isotope imaging with ^99mTc/¹²³I and ²⁰¹Tl/¹²³I was performed by means of a cardiac phantom simulating patients with and without obesity. In a human study, five normal volunteers and ten patients with myocardial infarction underwent myocardial perfusion and fatty acid metabolism imaging with single and dual SPECT with combinations of ^99mTc-methoxyisobutylisonitrile/¹²³I-beta-methyl(p-iodophenyl)pentadecanoic acid (BMIPP) and ²⁰¹Tl/¹²³I-BMIPP as tracers.

Results

Technetium-99m yielded more homogeneous images than ²⁰¹Tl because of the lower degree of photon attenuation, especially in the condition of obese patients, resulting in clearer visualization of the perfusion-metabolism mismatch. Dual ^99mTc/¹²³I SPECT offered comparable images with single SPECT in assessing myocardial damage.

Conclusions

The method effectively separated ^99mTc and ¹²³I primary photons and proved applicable to ^99mTc/¹²³I dual-isotope myocardial SPECT.     

“Luxury perfusion” with99mTc-HMPAO and123I-IMP SPECT imaging during the subacute phase of stroke

To compare the merits of¹²³I-isopropyl-iodoam-phetamine (¹²³I-IMP) and^99mTc-HMPAO in showing abnormal brain uptake distribution during cerebral ischemia, we studied ten patients during the subacute phase of their stroke, a period where metabolism and blood flow are frequently uncoupled. SPECT imaging was performed using both radiopharmaceuticals in the 10 patients from 48 h to 4 weeks after onset of symptoms. Two patients out of the 10 had similar defects with¹²³I-IMP and^99mTc-HMPAO SPECT, the location of the defects corresponding to the area of infarction observed on CT. Six patients had normal^99mTc-HMPAO SPECT and abnormal¹²³I-IMP SPECT with defects in the area of infarction shown by CT. The remaining 2 patients had hyperactive abnormalities on^99mTc-HMPAO in areas corresponding to defects on the¹²³I-IMP images. Two of the patients with SPECT mismatches were studied again more than 1 month after onset. On reexamination,^99mTc-HMPAO SPECT which was previously normal or hyperactive became hypoactive with a focal area of decreased activity corresponding to the defect on¹²³I-IMP. Crossed cerebellar diaschisis was found in 7 patients with^99mTc-HMPAO and was absent for both¹²³I-IMP and^99mTc-HMPAO in 3. We suggest that SPECT with⁹⁹mTc-HMPAO could show transient hyperemia not demonstrated by¹²³I-IMP whereas in some cases cerebral infarction would be more difficult to demonstrate with^99mTc-HMPAO than with¹²³I-IMP. SPECT with both tracers is recommended to follow the evolution of strokes in terms of regional cerebral blood flow and tissue metabolism.     

[99mTc]Demotate 2 in the detection of sst2-positive tumours: a preclinical comparison with [111In]DOTA-tate

Purpose The aim of this study was to evaluate [^99mTc]Demotate 2 ([^99mTc-N₄ ^0-1,Asp⁰,Tyr³]octreotate) as a candidate for in vivo imaging of sst₂-positive tumours and to compare it with [¹¹¹In]DOTA-tate ([¹¹¹In-DOTA⁰,Tyr³]octreotate). Methods Labelling of Demotate 2 with ^99mTc was performed at room temperature using SnCl₂ as reductant in the presence of citrate at alkaline pH. Radiochemical analysis involved ITLC and HPLC methods. Peptide conjugate affinities for sst₂ were determined by receptor autoradiography on rat brain cortex sections using [DOTA⁰,¹²⁵I-Tyr³]octreotate as the radioligand. The affinity profile of Demotate 2 for human sst₁–sst₅ was studied by receptor autoradiography in cell preparations using the universal somatostatin radioligand [¹²⁵I][Leu⁸,(D)Trp²²,Tyr²⁵]somatostatin-28. The internalisation rates of [^99mTc]Demotate 2 and [¹¹¹In]DOTA-tate were compared in sst₂-positive and -negative control cell lines. Biodistribution of radiopeptides was studied in male Lewis rats bearing CA20948 tumours. Results Peptide conjugates showed selectivity and a high affinity binding for sst₂ (Demotate 2 IC₅₀=3.2 nM and DOTA-tate IC₅₀=5.4 nM). [^99mTc]Demotate 2, like [¹¹¹In]DOTA-tate, internalised rapidly in all sst₂-positive cells tested, but not in sst₂-negative control cells. After injection in CA20948 tumour-bearing rats both radiopeptides showed high and specific uptake in the sst₂-positive organs and in the implanted tumour and rapid excretion from non-target tissues via the kidneys. Conclusion [^99mTc]Demotate 2, similarly to the known sst₂-targeting agent [¹¹¹In]DOTA-tate, showed promising biological qualities for application in the scintigraphy of sst₂-positive tumours.     

Simultaneous SPECT studies of pre- and postsynaptic dopamine binding sites in baboons.

The central nervous system dopamine transporters (DATs) and dopamine D2/D3 receptors are implicated in a variety of neurological disorders. Both sites are also targets for drug treatment. With the successful development of [99mTc]TRODAT-1, single-isotope imaging studies using this ligand for DAT imaging can be complemented by additional use of 123I-labeled D2/D3 receptor ligand co-injected to assess both pre- and postsynaptic sites of the dopaminergic system simultaneously. METHODS: Twelve SPECT scans of the brain were obtained in two baboons after intravenous administration of 740 MBq (20 mCi) [99mTc]-TRODAT-1 (technetium, [2-[[2-[[[3-(4-chlorophenyl)-8-methyl-8-azabicyclo[3,2,1]oct-2-yl]methyl ](2-mercaptoethyl) amino]ethyl]-amino]ethanethiolato (3-)]- oxo-[1R-(exo-exo)]) and 185 MBq (5 mCi) [123I]iodobenzamide or [123I]iodobenzofuran. SPECT data were acquired by a triple-head gamma camera equipped with ultra-high-resolution fanbeam collimators (scan duration = 210 min). Two sets of SPECT data were obtained using energy windows of 15% centered on 140 keV for 99mTc and 10% asymmetric with a lower bound at 159 keV for 123I. After coregistration with MRI, region-of-interest analysis was performed using predefined templates from coregistered MRI. In blocking studies, baboons were pretreated with N-methyl-2beta-carbomethoxy-3beta-(4-fluorophenyl)tropane (CFT, 14 mg) or raclopride (14 mg) to block DAT or D2/D3 binding site, respectively. RESULTS: Image quality of dual-isotope studies was similar to that obtained from single-isotope studies. When one site was blocked with CFT or raclopride, the binding of the respective ligand to the other site was not affected. CONCLUSION: This is the first example that clearly demonstrates the feasibility of simultaneous imaging of both pre- and postsynaptic sites of the dopaminergic system in baboons with dual-isotope SPECT studies. With or without corrections for cross-contamination of 123I into the 99mTc window, striatum-to-cerebellum ratios (target-to-nontarget) of dual-isotope experiments did not differ significantly from single-isotope experiments. This method may be a valuable and cost-effective tool for gaining comprehensive information about the dopaminergic system in one SPECT imaging session.     

Efficacy of <Superscript>99m</Superscript>Tc pertechnetate and <Superscript>131</Superscript>I radioisotope therapy in sodium/iodide symporter (NIS)-expressing neuroendocrine tumors in vivo

Purpose There is growing interest in the human sodium/iodide symporter (NIS) gene both as a molecular imaging reporter gene and as a therapeutic gene. Here, we show the feasibility of radioisotope therapy of neuroendocrine tumors. As a separate application of NIS gene transfer, we image NIS-expressing tumors with pinhole SPECT in living subjects. Methods Biodistribution studies and in vivo therapy experiments were performed in nude mice carrying stably NIS-expressing neuroendocrine tumor xenografts following i.v. injection of ¹³¹I and ^99mTc pertechnetate. To show the usefulness of NIS as an imaging reporter gene, ^99mTc pertechnetate uptake was imaged in vivo using a clinical gamma camera in combination with a custom-made single pinhole collimator, followed by SPECT/small animal MRI data coregistration. Results NIS-expressing neuroendocrine tumors strongly accumulated ¹³¹I and ^99mTc pertechnetate, as did thyroid, stomach, and salivary gland. The volume of NIS-expressing neuroendocrine tumors decreased significantly after therapeutic administration of ¹³¹I or ^99mTc pertechnetate, whereas control tumors continued to grow. NIS-mediated uptake of ^99mTc pertechnetate could be imaged in vivo at high resolution with a clinical gamma camera equipped with a custom-made single pinhole collimator. High-resolution functional and morphologic information could be combined in a single three-dimensional data set by coregistration of SPECT and small animal MRI data. Lastly, we demonstrated a therapeutic effect of ^99mTc pertechnetate on NIS-expressing neuroendocrine tumors in cell culture and, for the first time, in vivo, thought to be due to emitted Auger and conversion electrons. Conclusions NIS-expressing neuroendocrine tumors efficiently concentrate radioisotopes, allowing for in vivo high-resolution small animal SPECT imaging as well as rendering possible successful radioisotope therapy of neuroendocrine tumors.     

Differences in [99mTc]TRODAT-1 SPECT binding to dopamine transporters in patients with multiple system atrophy and Parkinson’s disease

Purpose Multiple system atrophy (MSA), a disorder causing autonomic dysfunction, parkinsonism, and cerebellar dysfunction, is difficult to differentiate from other movement disorders, particularly early in the course of disease. This study evaluated whether [^99mTc]TRODAT-1 binding to the dopamine transporter differentiates MSA from other movement disorders.Methods Single-photon emission computed tomographic brain scans were acquired in 25 MSA patients, 48 age-matched controls, and 130 PD patients, 3 h after the injection of 740 MBq (20 mCi) of [^99mTc]TRODAT-1. Regions of interest (ROIs) were placed manually on subregions of both basal ganglia and distribution volume ratios (DVRs) were calculated. Regional DVRs were compared between study groups in MSA patients. Students t tests were used to compare MSA patients with other study groups. Spearman correlations were used to compare DVRs with NP measures.Results Based upon various motor scores, MSA and PD patients had comparable motor impairment, and were significantly impaired compared with controls. Mean DVRs in the basal ganglia of MSA patients were significantly less than those of controls, but generally higher (p<0.05) than in PD patients. In particular, the MSA patients had significantly increased DVRs in the posterior putamen (mean 0.49±0.30) compared with PD patients (0.74±0.25).Conclusion Movement disorder patients could be differentiated from controls, but MSA and PD patients could not be easily differentiated from each other. As a group, MSA patients had significantly higher mean [^99mTc]TRODAT-1 binding, particularly in the posterior putamen, compared with PD patients and significantly lower binding compared with controls. This may reflect different pathophysiological processes of the two neurodegenerative diseases.     

Imaging of dopamine transporters in humans with technetium-99m TRODAT 1

Technetium-99m TRODAT-1, a tropane derivative, has shown promise as a tracer for the imaging of dopamine transporters in preliminary studies in rats and baboons. The present report concerns the first study of the use of [^99mTc]TRODAT 1 for the same purpose in humans. The specific uptake of [^99mTc]TRODAT1 in dopamine transporter sites located in the basal ganglia area was confirmed: the best contrast between the basal ganglia and the occipital area, which is devoid of dopamine transporters, was achieved at 120–140 min following injection. The development of a^99mTc-based agent bypasses the need for cyclotron-produced radionuclides, which will be of benefit for routine clinical studies.     

Effect of cyclosporin A administration on the biodistribution and multipinhole μSPECT imaging of [<Superscript>123</Superscript>I]R91150 in rodent brain

Purpose  P-glycoprotein (Pgp) is an efflux protein found amongst other locations in the blood–brain barrier. It is important to investigate the effect of Pgp modulation on clinically used brain tracers, because brain uptake of the tracer can be altered by blocking of the Pgp efflux transporter. The function of Pgp can be blocked with cyclosporin A. Methods  We investigated the effect of cyclosporin A administration on the biodistribution of [¹²³I]R91150 in rodents, and the effect of Pgp blocking on the quality of multipinhole μSPECT imaging with [¹²³I]R91150. The influence of increasing doses of cyclosporin A on the brain uptake of [¹²³I]R91150 was investigated in NMRI mice. A biodistribution study with [¹²³I]R91150 was performed in male Sprague-Dawley rats pretreated with cyclosporin A and not pretreated. Brain uptake of [¹²³I]R91150 after cyclosporin A injection was compared to the brain uptake in untreated animals, and a displacement study with ketanserin was performed in both groups. A multipinhole μSPECT brain imaging study was also performed using a Milabs U-SPECT-II camera in male Sprague-Dawley rats. To exclude the effect of possible metabolites, a metabolite study was also performed. Results  At the highest cyclosporin A dose (50 mg/kg), a sevenfold increase in brain radioactivity concentration was observed in NMRI mice. Also, a dose-response relationship was established between the dose of cyclosporin A and the brain uptake of [¹²³I]R91150 in mice. Compared to the control group, a five-fold increase in [¹²³I]R91150 radioactivity concentration was observed in the brain of Sprague-Dawley rats after cyclosporin A treatment (50 mg/kg). Radioactivity concentration in the frontal cortex increased from 0.24±0.0092 to 1.58±0.097% injected dose per gram of tissue after treatment with cyclosporin A (at the 1-h time-point). Blood radioactivity concentrations did not increase to the same extent. The cortical activity was displaced by administration of ketanserin. A metabolite study confirmed that there was no increased metabolism of [¹²³I]R91150 due to cyclosporin A. The visual quality of multipinhole μSPECT images with [¹²³I]R91150 in Sprague-Dawley rats improved markedly after cyclosporin A pretreatment. Conclusion  From the results obtained in the biodistribution studies, it can be concluded that [¹²³I]R91150 is a substrate for Pgp in rodents. A relationship between the administered dose of cyclosporin A and the increase in [¹²³I]R91150 brain radioactivity concentration was established. The overall quality of our multipinhole μSPECT images with [¹²³I]R91150 in rats improved markedly after pretreatment of the animals with cyclosporin A.     

Quantitation of dopamine transporter blockade by methylphenidate: first in vivo investigation using [123I]FP-CIT and a dedicated small animal SPECT

Purpose The aim of this study was to investigate the feasibility of assessing dopamine transporter binding after treatment with methylphenidate in the rat using a recently developed high-resolution small animal single-photon emission computed tomograph (TierSPECT) and [¹²³I]FP-CIT.Methods [¹²³I]FP-CIT was administered intravenously 1 h after intraperitoneal injection of methylphenidate (10 mg/kg) or vehicle. Animals underwent scanning 2 h after radioligand administration. The striatum was identified by superimposition of [¹²³I]FP-CIT scans with bone metabolism and perfusion scans obtained with ^99mTc-DPD and ^99mTc-tetrofosmin, respectively. As these tracers do not pass the blood–brain barrier, their distribution permits the identification of extracerebral anatomical landmarks such as the orbitae and the harderian glands. The cerebellum was identified by superimposing [¹²³I]FP-CIT scans with images of brain perfusion obtained with ^99mTc-HMPAO.Results Methylphenidate-treated animals and vehicle-treated animals yielded striatal equilibrium ratios (V₃) of 0.24±0.26 (mean ± SD) and 1.09±0.42, respectively (t test, two-tailed, p<0.0001). Cortical V₃ values amounted to 0.05±0.28 (methylphenidate) and 0.3±0.39 (saline, p=0.176). This first in vivo study of rat dopamine transporter binding after pre-treatment with methylphenidate showed a mean reduction of 78% in striatal [¹²³I]FP-CIT accumulation.Conclusion The results can be interpreted in terms of a pharmacological blockade in the rat striatum and show that in vivo quantitation of dopamine transporter binding is feasible with [¹²³I]FP-CIT and the TierSPECT. This may be of future relevance for in vivo investigations on rat models of attention deficit/hyperactivity disorder. Furthermore, our findings suggest that investigations in other animal models, e.g. of Parkinsons and Huntingtons disease, may be feasible using SPECT radioligands and small animal imaging systems.     

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

BACKGROUND: A novel radioiodine ligand [(123)I] ADAM (2-((2-((dimethylamino)methyl)phenyl)thio)-5-iodophenylamine) has been suggested as a promising serotonin transporter (SERT) imaging agent for the central nervous system. In this study, the biodistribution of SERTs in the rabbit brain was investigated using [(123)I] ADAM and mapping images of the same animal produced by both single-photon emission computed tomography (SPECT) and microautoradiography. A semiquantification method was adopted to deduce the optimum time for SPECT imaging, whereas the input for a simple fully quantitative tracer kinetic model was provided from arterial blood sampling data. METHODS: SPECT imaging was performed on female rabbits postinjection of 185 MBq [(123)I] ADAM. The time-activity curve obtained from the SPECT images was used to quantify the SERTs, for which the binding potential was calculated from the kinetic modeling of [(123)I] ADAM. The kinetic data were analyzed by the nonlinear least squares method. The effects of the selective serotonin reuptake inhibitors fluoxetine and p-chloroamphetamine (PCA) on rabbits were also evaluated. After scanning, the same animal was sacrificed and the brain was removed for microautoradiography. Regions-of-interest were analyzed using both SPECT and microautoradiography images. The SPECT images were coregistered manually with the corresponding microautoradiography images for comparative study. RESULTS: During the time interval 90-100 min postinjection, the peak specific binding levels in different brain regions were compared and the brain stem was shown to have the highest activity. The target-to-background ratio was 1.89+/-0.02. Similar studies with fluoxetine and PCA showed a background level for SERT occupation. Microautoradiography demonstrated a higher level of anatomical details of the [(123)I] ADAM distribution than that obtained by SPECT imaging of the rabbit brain. CONCLUSION: SPECT imaging of the rabbit brain with [(123)I] ADAM showed high affinity, high specificity, and favorable kinetics. The time-activity curve showed that the accumulation of the [(123)I] ADAM in the brain stem reached a maximum between 90 and 100 min postinjection. The microautoradiography provides high-resolution images of the rabbit brain. Our results for the [(123)I] ADAM biodistribution in the rabbit brains demonstrate that this new radioligand is suitable as a selective SPECT imaging agent for SERTs.     

Is [99mTc]glucarate uptake mediated by fructose transporter GLUT-5?

PurposeThere is growing interest in the ability of [^99mTc]Glucarate ([^99mTc]GLA) to accumulate in viable tumor cells. Recent vivo studies suggest that [^99mTc]Glucarate could be helpful for tumor detection. Fructose transport is thought to be implicated. It is clearly established that facilitated fructose transport in tumor cells is related to the GLUT-5 transporter. This study therefore investigated whether [^99mTc]GLA uptake is mediated by GLUT-5 transporter.MethodsDifferent tumor cell lines were used. Modulation of GLUT-5 expression was assessed with and without antisense oligonucleotides directed against GLUT-5. GLUT-5 expression was assessed by indirect cell ELISA. To correlate GLUT-5 expression with tracer accumulation, [^99mTc]GLA uptake was determined after antisense treatment. A competition with fructose was also monitored.ResultsInhibition of GLUT-5 expression by antisense oligonucleotides directed against GLUT-5 was effective after 24 h. An optimal of 10 μM antisense oligonucleotides directed against GLUT-5 produced a 30%–40% decrease in protein expression. Modulation of [^99mTc]GLA uptake was monitored either by use of specific antisense oligonucleotides or by competition with fructose. Both of them produced a significant decrease of [^99mTc]GLA accumulation in all tested cell lines.ConclusionOur results clearly demonstrate that [^99mTc]GLA uptake is related to GLUT-5 transporter expression and transport. In tumor imaging, [^99mTc]GLA may be a useful tool for non-invasive detection of malignant tumors expressing high levels of GLUT-5 transporter as, for example, breast cancers.