Dopamine Transporter Radioligand Research Articles

Synthesis and initial evaluation of radioiodine-labelled deuterated tropane derivatives targeting dopamine transporter

The dopamine transporter (DAT) is closely related to a variety of neurological disorders including Parkinson’s disease (PD) and other neurodegenerative diseases. In vivo imaging of DAT with radio-labelled tracers has become a powerful technique in related disorders. The radioiodine-labelled tropane derivative [123I]FP-CIT ([123I]1a) is widely used in clinical single photon emission computed tomography (SPECT) imaging as a DAT imaging agent. To develop more metabolically stable DAT radioligands for accurate imaging, this work compared two novel deuterated tropane derivatives ([131I]1c-d) with non-deuterated tropane derivatives ([131I]1a-b). [131I]1a-d were obtained in high radiochemical purity (RCP) above 99 % with molar activities of 7.0–10.0 GBq/μmol. The [131I]1a and [131I]1c exhibited relatively higher affinity to DAT (Ki: 2.0–3.12 nM) than [131I]1b and [131I]1d. Biodistribution results showed that [131I]1c consistently exhibited a higher ratio of the target to non-target (striatum/cerebellum) than [131I]1a. Furthermore, metabolism studies indicated that the in vivo metabolic stability of [131I]1c was superior to that of [131I]1a. Ex vivo autoradiography showed that [131I]1c selectively localized on DAT-rich striatal regions and the specific signal could be blocked by DAT inhibitor. These results indicated that [131I]1c might be a potential probe for DAT SPECT imaging in the brain.

Microsome Mediated in Vitro Metabolism: A Convenient Method for the Preparation of the PET Radioligand Metabolite [18F]FE-PE2I-OH for Translational Dopamine Transporter Imaging.

Undesired radiometabolites can be detrimental to the development of positron emission tomography (PET) radioligands. Methods for quantifying radioligand metabolites in brain tissue include ex vivo studies in small animals or labeling and imaging of the radiometabolite(s) of interest. The latter is a time- and resource-demanding process, which often includes multistep organic synthesis. We hypothesized that this process could be replaced by making use of liver microsomes, an in vitro system that mimics metabolism. In this study, rat liver microsomes were used to prepare radiometabolites of the dopamine transporter radioligand [18F]FE-PE2I for in vitro imaging using autoradiography and in vivo imaging using PET in rats and nonhuman primates. The primary investigated hydroxy-metabolite [18F]FE-PE2I-OH ([18F]2) was obtained in a 2% radiochemical yield and >99% radiochemical purity. In vitro and in vivo imaging demonstrated that [18F]2 readily crossed the blood-brain barrier and bound specifically and reversibly to the dopamine transporter. In conclusions, the current study demonstrates the potential of liver microsomes in the production of radiometabolites for translational imaging studies and radioligand discovery.

Deuterated [18F]fluoroethyl tropane analogs as dopamine transporter probes: Synthesis and biological evaluation

IntroductionThe dopamine transporter (DAT) is vitally correlated with Parkinson's disease (PD) and other neurodegenerative diseases. Non-invasive imaging of DAT contributes to early diagnosis and monitoring of related diseases. Recently, we reported a deuterated [18F]fluoroethyl tropane analogue [18F]FECNT-d4 as a potential DAT PET imaging agent. The objective of this work was to extend the investigation by comparing four deuterated [18F]fluoroethyl tropane derivatives ([18F]2a-d) to develop metabolically stable DAT radioligands. MethodsFour fluoroethyl substituted phenyl-tropane compounds 1a-d and deuterated compounds 2a-d were synthesized and their IC50 values to DAT were evaluated. The [18F]fluoroethyl ligands [18F]1a-d and [18F]2a-d were obtained from corresponding labeling precursors by one-step radio-labeling reactions and investigated in terms of lipophilicity and in vitro binding affinity studies. [18F]1d and [18F]2d were then selected for further evaluations by in vivo metabolism study, biodistribution, ex vivo autoradiography, and microPET imaging studies. Results[18F]1a-d and [18F]2a-d were obtained in radiochemical yield of 11–32 % with molar activities of 28–54 GBq/μmol. The 1d and 2d exhibited relatively high affinity to DAT (IC50: 1.9–2.1 nM). Ex vivo autoradiography and microPET studies showed that [18F]2d selectively localized on DAT-rich striatal regions and the specific signal could be blocked by DAT inhibitor. Biodistribution results showed that [18F]2d consistently exhibited a higher ratio of the target to non-target (striatum/cerebellum) than [18F]1d. Furthermore, metabolism study indicated that the in vivo metabolic stability of [18F]2d was superior to that of [18F]1d. ConclusionOur findings suggested that the deuterated compound [18F]2d might be a potential probe for DAT PET imaging in the brain.

N-acetylcysteine decreases dopamine transporter availability in the non-lesioned striatum of the 6-OHDA hemiparkinsonian rat

This study aimed to explore the beneficial effects of the antioxidant N-acetylcysteine (NAC) on the degenerated dopamine system. The short- and long-term regulatory mechanisms of NAC on the 6-OHDA hemiparkinsonian rat model were longitudinally investigated by performing positron emission tomography (PET) imaging using the specific dopamine transporter (DAT) radioligand [18F]FE-PE2I.The results demonstrate that after a unilateral dopamine insult NAC has a strong influence on the non-lesioned hemisphere by decreasing the levels of DAT in the striatum early after the lesion. We interpret this early and short-term decrease of DAT in the healthy striatum of NAC-treated animals as a beneficial compensatory effect induced by NAC.

Accuracy and precision of zero-echo-time, single- and multi-atlas attenuation correction for dynamic [11C]PE2I PET-MR brain imaging

BackgroundA valid photon attenuation correction (AC) method is instrumental for obtaining quantitatively correct PET images. Integrated PET/MR systems provide no direct information on attenuation, and novel methods for MR-based AC (MRAC) are still under investigation. Evaluations of various AC methods have mainly focused on static brain PET acquisitions. In this study, we determined the validity of three MRAC methods in a dynamic PET/MR study of the brain.MethodsNine participants underwent dynamic brain PET/MR scanning using the dopamine transporter radioligand [11C]PE2I. Three MRAC methods were evaluated: single-atlas (Atlas), multi-atlas (MaxProb) and zero-echo-time (ZTE). The 68Ge-transmission data from a previous stand-alone PET scan was used as reference method. Parametric relative delivery (R1) images and binding potential (BPND) maps were generated using cerebellar grey matter as reference region. Evaluation was based on bias in MRAC maps, accuracy and precision of [11C]PE2I BPND and R1 estimates, and [11C]PE2I time-activity curves. BPND was examined for striatal regions and R1 in clusters of regions across the brain.ResultsFor BPND, ZTE-MRAC showed the highest accuracy (bias < 2%) in striatal regions. Atlas-MRAC exhibited a significant bias in caudate nucleus (− 12%) while MaxProb-MRAC revealed a substantial, non-significant bias in the putamen (9%). R1 estimates had a marginal bias for all MRAC methods (− 1.0–3.2%). MaxProb-MRAC showed the largest intersubject variability for both R1 and BPND. Standardized uptake values (SUV) of striatal regions displayed the strongest average bias for ZTE-MRAC (~ 10%), although constant over time and with the smallest intersubject variability. Atlas-MRAC had highest variation in bias over time (+10 to − 10%), followed by MaxProb-MRAC (+5 to − 5%), but MaxProb showed the lowest mean bias. For the cerebellum, MaxProb-MRAC showed the highest variability while bias was constant over time for Atlas- and ZTE-MRAC.ConclusionsBoth Maxprob- and ZTE-MRAC performed better than Atlas-MRAC when using a 68Ge transmission scan as reference method. Overall, ZTE-MRAC showed the highest precision and accuracy in outcome parameters of dynamic [11C]PE2I PET analysis with use of kinetic modelling.

Usefulness of PET With [18F]LBT-999 for the Evaluation of Presynaptic Dopaminergic Neuronal Loss in a Clinical Environment.

Purpose: The density of the neuronal dopamine transporter (DAT) is directly correlated with the presynaptic dopaminergic system injury. In a first study, we evaluated the brain distribution and kinetics of [18F]LBT-999, a DAT PET radioligand, in a group of eight healthy subjects. Taking into account the results obtained in healthy volunteers, we wanted to evaluate whether the loss of presynaptic striatal dopaminergic fibers could be estimated, under routine clinical conditions, using [18F]LBT-999 and a short PET acquisition.Materials and methods: Six patients with Parkinson's disease (PD) were compared with eight controls. Eighty-nine minutes of dynamic PET following an intravenous injection of [18F]LBT-999 were acquired. Using regions of interest for striatal nuclei, substantia nigra (SN), cerebellum, and occipital cortex, defined over each T1 3D MRI, time–activity curves (TACs) were obtained. From TACs, binding potential (BPND) using the simplified reference tissue model and distribution volume ratios (DVRs) using Logan graphical analysis were calculated. Ratios obtained for a 10-min image, acquired between 30 and 40 min post-injection, were also calculated. Cerebellum activity was used as non-specific reference region.Results: In PD patients and as expected, striatal uptake was lower than in controls which is confirmed by BPND, DVR, and ratios calculated for both striatal nuclei and SN, significantly inferior in PD patients compared with controls (p < 0.001).Conclusions: PET with [18F]LBT-999 could be an alternative to assess dopaminergic presynaptic injury in a clinical environment using a single 10 min acquisition.

PET imaging of [11C]PBR28 in Parkinson\u2019s disease patients does not indicate increased binding to TSPO despite reduced dopamine transporter binding

PurposeTo examine the hypothesis that cerebral binding to the 18 kDa translocator protein (TSPO), a marker of microglia activation, is elevated in Parkinson’s disease (PD), and to assess the relationship between brain TSPO binding and dopaminergic pathology in PD.MethodsThe radioligand [11C]PBR28 was used for quantitative assessment of brain TSPO in 16 control subjects and 16 PD patients. To analyse the relationship between dopaminergic pathology and brain TSPO binding, PET studies of the dopamine transporter (DAT) were undertaken in PD patients using the DAT radioligand [18F]FE-PE2I. The total distribution volume of [11C]PBR28 was quantified in nigrostriatal regions, limbic cortices and thalamus, and the binding potential of [18F]FE-PE2I was quantified in nigrostriatal regions.ResultsBased on genotype analysis of the TSPO rs6971 polymorphism, 16 subjects (8 control subjects and 8 PD patients) were identified as high-affinity binders, and the remaining subjects were identified as mixed-affinity binders. A two-way ANOVA showed a strong main effect of TSPO genotype on the cerebral binding of [11C]PBR28, whereas no statistically significant main effect of diagnostic group, or a group by genotype interaction was found for any of the regions analysed. [18F]FE-PE2I PET studies in patients indicated a marked reduction in nigrostriatal binding to DAT. However, no correlations between the binding parameters were found for [11C]PBR28 and [18F]FE-PE2I.ConclusionThe findings do not support the hypothesis of elevated cerebral TSPO binding or a relationship between TSPO binding and dopaminergic pathology in PD.

11 C-PE2I and 18 F-Dopa PET for assessing progression rate in Parkinson's: A longitudinal study.

18 F-dopa PET measuring aromatic l-amino acid decarboxylase activity is regarded as the gold standard for evaluating dopaminergic function in Parkinson's disease. Radioligands for dopamine transporters are also used in clinical trials and for confirming PD diagnosis. Currently, it is not clear which imaging marker is more reliable for assessing clinical severity and rate of progression. The objective of this study was to directly compare 18 F-dopa with the highly selective dopamine transporter radioligand 11 C-PE2I for the assessment of motor severity and rate of progression in PD. Thirty-three mild-moderate PD patients underwent 18 F-dopa and 11 C-PE2I PET at baseline. Twenty-three were followed up for 18.8 ± 3.4 months. Standard multiple regression at baseline indicated that 11 C-PE2I BPND predicted UPDRS-III and bradykinesia-rigidity scores (P < 0.05), whereas 18 F-dopa Ki did not make significant unique explanatory contributions. Voxel-wise analysis showed negative correlations between 11 C-PE2I BPND and motor severity across the whole striatum bilaterally. 18 F-Dopa Ki clusters were restricted to the most affected putamen and caudate. Longitudinally, negative correlations were found between striatal Δ11 C-PE2I BPND , ΔUPDRS-III, and Δbradykinesia-rigidity, whereas no significant associations were found for Δ18 F-dopa Ki . One cluster in the most affected putamen was identified in the longitudinal voxel-wise analysis showing a negative relationship between Δ11 C-PE2I BPND and Δbradykinesia-rigidity. Striatal 11 C-PE2I appears to show greater sensitivity for detecting differences in motor severity than 18 F-dopa. Furthermore, dopamine transporter decline is closely associated with motor progression over time, whereas no such relationship was found with aromatic l-amino acid decarboxylase. 11 C-PE2I may be more effective for evaluating the efficacy of neuroprotective treatments in PD. © 2017 International Parkinson and Movement Disorder Society.

Striatal and Extrastriatal Dopamine Transporter Availability in Schizophrenia and Its Clinical Correlates: A Voxel-Based and High-Resolution PET Study.

Neuroimaging studies investigating dopamine (DA) function widely support the hypothesis of presynaptic striatal DA hyperactivity in schizophrenia. However, published data on the striatal DA transporter (DAT) appear less consistent with this hypothesis, probably partly due to methodological limitations. Moreover, DAT in extrastriatal regions has been very poorly investigated in the context of schizophrenia. In order to address these issues, we used a high resolution positron emission tomograph and the selective DAT radioligand [11C]PE2I, coupled with a whole brain voxel-based analysis method to investigate DAT availability in striatal but also extra-striatal regions in 21 male chronic schizophrenia patients compared to 30 healthy male controls matched by age. We found higher DAT availability in schizophrenia patients in midbrain, striatal, and limbic regions. DAT availability in amygdala/hippocampus and putamen/pallidum was positively correlated with hallucinations and suspiciousness/persecution, respectively. These results are consistent with an increase of presynaptic DA function in patients with schizophrenia, and support the involvement of both striatal and extrastriatal DA dysfunction in positive psychotic symptoms. The study also highlights the whole brain voxel-based analysis method to explore DA dysfunction in schizophrenia.

Pharmacological treatment with L-DOPA may reduce striatal dopamine transporter binding in invivo imaging studies.

Numerous neurologic and psychiatric conditions are treated with pharmacological compounds, which lead to an increase of synaptic dopamine (DA) levels. One example is the DA precursor L-3,4-dihydroxyphenylalanine (L-DOPA), which is converted to DA in the presynaptic terminal. If the increase of DA concentrations in the synaptic cleft leads to competition with exogenous radioligands for presynaptic binding sites, this may have implications for DA transporter (DAT) imaging studies in patients under DAergic medication. This paper gives an overview on those findings, which, so far, have been obtained on DAT binding in human Parkinson's disease after treatment with L-DOPA. Findings, moreover, are related to results obtained on rats, mice or non-human primates. Results indicate that DAT imaging may be reduced in the striata of healthy animals, in the unlesioned striata of animal models of unilateral Parkinson's disease and in less severly impaired striata of Parkinsonian patients, if animal or human subjects are under acute or subchronic treatment with L-DOPA. If also striatal DAT binding is susceptible to alterations of synaptic DA levels, this may allow to quantify DA reuptake in analogy to DA release by assessing the competition between endogenous DA and the administered exogenous DAT radioligand.

Screening for dopa-responsive dystonia in patients with Scans Without Evidence of Dopaminergic Deficiency (SWEDD).

The clinical diagnosis of Parkinson's Disease (PD) is not supported by Single Photon Emission Computed Tomography (SPECT) using dopamine transporter radioligand in 4-15 % of patients. It has been hypothesized that this phenomenon, named "Scans Without Evidence of Dopaminergic Deficiency" (SWEDD), may be an adult-onset dystonia. We investigated the hypothesis that these patients might be affected by Dopa-Responsive Dystonia (DRD). We enrolled eleven unrelated patients (8 F and 3 M) with clinical parkinsonism and normal [(123)I]FP-CIT SPECT. The GTP-cyclohydrolase1 (GCH1) gene was sequenced in all patients; urine biopterin and neopterin analysis was carried out in nine and oral phenylalanine (Phe) loading in seven. Neurological examination showed bradykinesia and resting/postural tremor in all patients, and rigidity in ten, suggesting a clinical diagnosis of PD. We detected mild dystonic signs in eight cases. In particular, five of them presented cranial dyskinesias. No mutation of the GCH1 gene was found. The results of the urine biopterin and neopterin analysis and the oral Phe loading did not reveal biochemical abnormalities suggestive of reduced GCH1 activity. We confirm that some clinical features, namely the presence of focal or segmental dystonia, suggest an adult-onset dystonia in SWEDD cases. However, we exclude DRD caused by GCH1 gene mutations in the present series.

Diagnoses behind patients with hard-to-classify tremor and normal DaT-SPECT: a clinical follow up study.

The [123I]ioflupane—a dopamine transporter radioligand—SPECT (DaT-SPECT) has proven to be useful in the differential diagnosis of tremor. Here, we investigate the diagnoses behind patients with hard-to-classify tremor and normal DaT-SPECT. Therefore, 30 patients with tremor and normal DaT-SPECT were followed up for 2 years. In 18 cases we were able to make a diagnosis. The residual 12 patients underwent a second DaT-SPECT, were then followed for additional 12 months and thereafter the diagnosis was reconsidered again. The final diagnoses included cases of essential tremor, dystonic tremor, multisystem atrophy, vascular parkinsonism, progressive supranuclear palsy, corticobasal degeneration, fragile X–associated tremor ataxia syndrome, psychogenic parkinsonism, iatrogenic parkinsonism and Parkinson's disease. However, for 6 patients the diagnosis remained uncertain. Larger series are needed to better establish the relative frequency of the different conditions behind these cases.

Long-Term Stimulant Treatment Affects Brain Dopamine Transporter Level in Patients with Attention Deficit Hyperactive Disorder

ObjectiveBrain dopamine dysfunction in attention deficit/hyperactivity disorder (ADHD) could explain why stimulant medications, which increase dopamine signaling, are therapeutically beneficial. However while the acute increases in dopamine induced by stimulant medications have been associated with symptom improvement in ADHD the chronic effects have not been investigated.MethodWe used positron emission tomography and [11C]cocaine (dopamine transporter radioligand) to measure dopamine transporter availability in the brains of 18 never-medicated adult ADHD subjects prior to and after 12 months of treatment with methylphenidate and in 11 controls who were also scanned twice at 12 months interval but without stimulant medication. Dopamine transporter availability was quantified as non-displaceable binding potential using a kinetic model for reversible ligands.ResultsTwelve months of methylphenidate treatment increased striatal dopamine transporter availability in ADHD (caudate, putamen and ventral striatum: +24%, p<0.01); whereas there were no changes in control subjects retested at 12-month interval. Comparisons between controls and ADHD participants revealed no significant difference in dopamine transporter availability prior to treatment but showed higher dopamine transporter availability in ADHD participants than control after long-term treatment (caudate: p<0.007; putamen: p<0.005).ConclusionUpregulation of dopamine transporter availability during long-term treatment with methylphenidate may decrease treatment efficacy and exacerbate symptoms while not under the effects of the medication. Our findings also suggest that the discrepancies in the literature regarding dopamine transporter availability in ADHD participants (some studies reporting increases, other no changes and other decreases) may reflect, in part, differences in treatment histories.

Molecular Imaging of Conscious, Unrestrained Mice with AwakeSPECT

We have developed a SPECT imaging system, AwakeSPECT, to enable molecular brain imaging of untrained mice that are conscious, unanesthetized, and unrestrained. We accomplished this with head tracking and motion correction techniques. The capability of the system for motion-corrected imaging was demonstrated with a (99m)Tc-pertechnetate phantom, (99m)Tc-methylene diphosphonate bone imaging, and measurement of the binding potential of the dopamine transporter radioligand (123)I-ioflupane in mouse brain in the awake and anesthetized (isoflurane) states. Stress induced by imaging in the awake state was assessed through measurement of plasma corticosterone levels. AwakeSPECT provided high-resolution bone images reminiscent of those obtained from CT. The binding potential of (123)I-ioflupane in the awake state was on the order of 50% of that obtained with the animal under anesthesia, consistent with previous studies in nonhuman primates. Levels of stress induced were on the order of those seen in other behavioral tasks and imaging studies of awake animals. These results demonstrate the feasibility of SPECT molecular brain imaging of mice in the conscious, unrestrained state and demonstrate the effects of isoflurane anesthesia on radiotracer uptake.

Preparation of [3H]fluoroethyl tosylate and its use in the labelling of the dopamine transporter radioligand [3H]FE‐PE2I

[(3) H]Fluoroethyl tosylate, a novel alkylating tritium labelling agent, was synthesized from tritium gas with high specific activity and with 99% radiochemical purity. [(3) H]Fluoroethyl tosylate was applied in the tritium labelling of the dopamine transporter radioligand [(3) H]FE-PE2I.

A New Era of Clinical Dopamine Transporter Imaging Using 123I-FP-CIT

(123)I-labeled 2β-carbomethoxy-3β-(4-iodophenyl)-N-(3-fluoropropyl)nortropane ((123)I-FP-CIT) was approved for clinical use in 2011 by the Food and Drug Administration. (123)I-FP-CIT is a radioligand for brain dopamine transporter (DAT) imaging that is useful for the differential diagnosis of Parkinson disease (PD) and other diseases that mimic PD. The sensitivity and specificity of (123)I-FP-CIT SPECT for PD diagnosis are more than 90% and equivalent to those of other DAT SPECT methods. In the near future, the clinical indications of DAT imaging are expected to be broadened; for example, including treatment response assessment, disease progression monitoring, and early diagnosis of premotor PD in each individual patient.

Parkinson's disease: the role of dopamine transporter neuroimaging in clinically unclear cases (Abstract)

INTRODUCTION: Misdiagnosis in Parkinson’s disease (PD) can reach 25%. Recently, single-photon emission computer tomography (SPECT), using dopamine transporter (DAT) radioligands, has become useful to help diagnose clinically unclear cases of Parkinsonism (CUCP). Parkinson’s disease: the role of dopamine transporter neuroimaging in clinically unclear cases (Abstract)

Greater striatal dopamine transporter density may be associated with major depressive episode.

We examined striatal dopamine transporter (DAT) distribution volume ratio (DVR) values in subjects with unipolar or bipolar major depressive episode (versus non-depressed healthy volunteers) using the selective DAT radioligand [(99m)Tc]TRODAT-1 and single photon emission computed tomography (SPECT). We hypothesized that striatal DVR values would be greater in depressed versus non-depressed subjects, and that greater DVR values may represent a possible clinical biomarker of depression. [(99m)Tc]TRODAT-1 spect images were acquired from 39 depressed and 103 non-depressed drug-free subjects. The primary outcome measure was the DVR value of [(99m)Tc]TRODAT-1 binding for the putamen region and the combined putamen plus caudate region. DVR values were significantly correlated across all striatal regions within both subject groups (p<0.005). Depressed subjects had significantly greater DVR values (versus non-depressed subjects) in the putamen (p<0.0005) and the combined putamen plus caudate (p<0.0005) regions. There was no difference in DVR values between unipolar (n=24) and bipolar (n=15) depressed subjects, and no difference in DVR values for depressed subjects with or without prior antidepressant exposure. The predictive probability of the putamen or combined putamen plus caudate DVR value to distinguish depressed from non-depressed subjects was significant (p<0.0005). DAT values could potentially be influenced by age, gender, diagnosis, prior psychotropic dug exposure, illness length, or symptom severity. Results confirm prior observations of greater striatal DAT density in depressed versus non-depressed subjects, and suggest that greater DVR values may possibly represent a potential diagnostic biomarker for distinguish depressed from non-depressed individuals.

To scan or not to scan

The Food and Drug Administration (FDA) recently approved [123I]ioflupane ([123I]-fluoropropyl βCIT), a dopamine transporter (DAT) radioligand, for SPECT to “assist in the evaluation of adult patients with suspected parkinsonian syndromes (PS).”1 This permits physicians to prescribe [123I]ioflupane SPECT scans as part of clinical practice. Advertisements encourage clinicians to order this and patients to request such scans. The key questions for neurologists are as follows: Do [123I]ioflupane SPECT scans provide useful data beyond clinical evaluation? Does this information improve patient care? SPECT measurement of the striatal uptake of [123I]ioflupane reflects the integrity of terminal fields of nigrostriatal neurons. People with Parkinson disease (PD) have reduced striatal uptake.2,3 In fact, such scans may be more sensitive than the clinical examination to detect nigrostriatal defects.4 However, loss of striatal DAT occurs in most degenerative PS, including multisystem atrophy (MSA), progressive supranuclear palsy (PSP), and corticobasal disease (CBD).5 Thus, in individual cases, reduced striatal uptake of [123I]ioflupane does not distinguish these different conditions. Most clinical studies focused on whether these scans reliably discriminate PS from disorders that clinically resemble PS but without striatal DAT deficiency. Multiple studies report that DAT SPECT can distinguish PS from essential tremor (ET). ET is chosen as the comparator disorder for many studies since it has normal striatal DAT. The first published study compared DAT SPECT with initial and 6-month follow-up examinations by a movement disorders expert.6 The goal was to determine whether SPECT predicts the follow-up clinical diagnosis. The subjects had either PS …

Difficulties in dopamine transporter radioligand PET analysis: the example of LBT-999 using [18F] and [11C] labelling

LBT-999, (E)-N-(4-fluorobut-2-enyl)-2β-carbomethoxy-3β-(4'-tolyl)nortropane, has been developed for PET imaging of the dopamine transporter. [(18)F]LBT-999 PET studies in baboons showed a lower brain uptake than [(11)C]LBT-999 and a high bone uptake, suggesting the presence of interfering metabolites. Therefore, in vitro and in vivo metabolism of these radiotracers was investigated. Rat and human liver microsomal incubations, baboon plasma and rat brain extracts were analyzed by radio-HPLC and LC-MS-MS. In vitro experiments demonstrated the formation by P450s of five polar metabolites. The main routes of LBT-999 metabolism proposed were N-dealkylation, tolyl-hydroxylation and dealkylation plus tolyl-hydroxylation. In vivo in baboons, [(18)F]LBT-999 was rapidly converted into a [(18)F]hydroxylated metabolite likely oxidized in plasma into a [(18)F]carboxylic acid and into unlabeled N-dealkyl-LBT-999. The latter was detected in baboon plasma and in rat brain by LC-MS-MS. The time course of unchanged [(18)F]LBT-999 decreased rapidly in plasma and was higher than that of [(11)C]LBT-999 due to the formation of unlabeled N-dealkyl-LBT-999. In rats, striatum-to-cerebellum ratios of [(18)F]LBT-999, [(18)F]hydroxylated and [(18)F]acidic metabolite were 20, 4.2 and 1.65, respectively, suggesting a possible accumulation of the hydroxylated compound in the striatum. P450s catalyzed the formation of dealkylated and hydroxylated metabolites of LBT-999. In baboons, an extensive metabolism of [(18)F]LBT-999, with formation of unlabeled N-dealkyl-LBT-999, [(18)F]fluorobutenaldehyde (or its oxidation product) and [(18)F]hydroxy-LBT-999 able to penetrate the brain, prevented an easy and accurate estimation of the input function of the radiotracer. CYP3A4 being the main P450 involved in the metabolism of LBT-999, a similar pathway may occur in humans and confound PET quantification.