Quantitative analyses of regional [11C]PE2I binding to the dopamine transporter in the human brain: a PET study

Challenges and innovations in brain PET analysis of neurodegenerative disorders: a mini-review on partial volume effects, small brain region studies, and reference region selection

Positron Emission Tomography (PET) brain imaging is increasingly utilized in clinical and research settings due to its unique ability to study biological processes and subtle changes in living subjects. However, PET imaging is not without its limitations. Currently, bias introduced by partial volume effect (PVE) and poor signal-to-noise ratios of some radiotracers can hamper accurate quantification. Technological advancements like ultra-high-resolution scanners and improvements in radiochemistry are on the horizon to address these challenges. This will enable the study of smaller brain regions and may require more sophisticated methods (e.g., data-driven approaches like unsupervised clustering) for reference region selection and to improve quantification accuracy. This review delves into some of these critical aspects of PET molecular imaging and offers suggested strategies for improvement. This will be illustrated by showing examples for dopaminergic and cholinergic nerve terminal ligands.

Striatal dopamine transporter and receptor availability correlate with relative cerebral blood flow measured with [11C]PE2I, [18F]FE-PE2I and [11C]raclopride PET in healthy individuals

The aim of this retrospective study was to investigate relationships between relative cerebral blood flow and striatal dopamine transporter and dopamine D2/3 availability in healthy subjects. The data comprised dynamic PET scans with two dopamine transporter tracers [11C]PE2I (n = 20) and [18F]FE-PE2I (n = 20) and the D2/3 tracer [11C]raclopride (n = 18). Subjects with a [11C]PE2I scan also underwent a dynamic scan with the serotonin transporter tracer [11C]DASB. Binding potential (BPND) and relative tracer delivery (R1) values were calculated on regional and voxel-level. Striatal R1 and BPND values were correlated, using either an MRI-based volume of interest (VOI) or an isocontour VOI based on the parametric BPND image. An inter-tracer comparison between [11C]PE2I BPND and [11C]DASB R1 was done on a VOI-level and simulations were performed to investigate whether the constraints of the modeling could cause correlation of the parameters. A positive association was found between BPND and R1 for all three dopamine tracers. A similar correlation was found for the inter-tracer correlation between [11C]PE2I BPND and [11C]DASB R1. Simulations showed that this relationship was not caused by cross-correlation between parameters in the kinetic model. In conclusion, these results suggest an association between resting-state striatal dopamine function and relative blood flow in healthy subjects.

PET Molecular Imaging in Drug Development: The Imaging and Chemistry Perspective

Positron emission tomography with selective radioligands advances the drug discovery and development process by revealing information about target engagement, proof of mechanism, pharmacokinetic and pharmacodynamic profiles. Positron emission tomography (PET) is an essential and highly significant tool to study therapeutic drug development, dose regimen, and the drug plasma concentrations of new drug candidates. Selective radioligands bring up target-specific information in several disease states including cancer, cardiovascular, and neurological conditions by quantifying various rates of biological processes with PET, which are associated with its physiological changes in living subjects, thus it reveals disease progression and also advances the clinical investigation. This study explores the major roles, applications, and advances of PET molecular imaging in drug discovery and development process with a wide range of radiochemistry as well as clinical outcomes of positron-emitting carbon-11 and fluorine-18 radiotracers.

Limbic Serotonergic Plasticity Contributes to the Compensation of Apathy in Early Parkinson's Disease

BACKGROUND

De novo Parkinson's disease (PD) patients with apathy exhibit prominent limbic serotonergic dysfunction and microstructural disarray. Whether this distinctive lesion profile at diagnosis entails different prognosis remains unknown.

OBJECTIVES

To investigate the progression of dopaminergic and serotonergic dysfunction and their relation to motor and nonmotor impairment in PD patients with or without apathy at diagnosis.

METHODS

Thirteen de novo apathetic and 13 nonapathetic PD patients were recruited in a longitudinal double-tracer positron emission tomography cohort study. We quantified the progression of presynaptic dopaminergic and serotonergic pathology using [¹¹ C]PE2I for dopamine transporter and [¹¹ C]DASB for serotonin transporter at baseline and 3 to 5 years later, using linear mixed-effect models and mediation analysis to compare the longitudinal evolution between groups for clinical impairment and region-of-interest-based analysis.

RESULTS

After the initiation of dopamine replacement therapy, apathy, depression, and anxiety improved at follow-up in patients with apathy at diagnosis (n = 10) to the level of patients without apathy (n = 11). Patients had similar progression of motor impairment, whereas mild impulsive behaviors developed in both groups. Striato-pallidal and mesocorticolimbic presynaptic dopaminergic loss progressed similarly in both groups, as did serotonergic pathology in the putamen, caudate nucleus, and pallidum. Contrastingly, serotonergic innervation selectively increased in the ventral striatum and anterior cingulate cortex in apathetic patients, contributing to the reversal of apathy besides dopamine replacement therapy.

CONCLUSION

Patients suffering from apathy at diagnosis exhibit compensatory changes in limbic serotonergic innervation within 5 years of diagnosis, with promising evidence that serotonergic plasticity contributes to the reversal of apathy. The relationship between serotonergic plasticity and dopaminergic treatments warrants further longitudinal investigations. © 2022 International Parkinson and Movement Disorder Society.

Single Photon Emission Computed Tomography/Positron Emission Tomography Molecular Imaging for Parkinsonism: A Fast-Developing Field

The early differential diagnosis of Parkinson disease and atypical parkinsonism is a major challenge. The use of single photon emission computed tomography (SPECT)/positron emission tomography (PET) molecular imaging to investigate parkinsonism is a fast‐developing field. Imaging biomarker research may potentially lead to more accurate disease detection, enabling earlier diagnosis and treatment. This review summarizes recent SPECT/PET advances in radiopharmaceuticals and imaging technologies/analyses that improve the diagnosis of neurodegenerative parkinsonism. We are currently witnessing a turning point in the field. Integrating molecular imaging as a diagnostic technique represents an opportunity to reassess the strategies for diagnosing neurodegenerative parkinsonism. ANN NEUROL 2021;90:711–719

D1- and D2-like receptors differentially mediate the effects of dopaminergic transmission on cost-benefit evaluation and motivation in monkeys

It has been widely accepted that dopamine (DA) plays a major role in motivation, yet the specific contribution of DA signaling at D₁-like receptor (D₁R) and D₂-like receptor (D₂R) to cost–benefit trade-off remains unclear. Here, by combining pharmacological manipulation of DA receptors (DARs) and positron emission tomography (PET) imaging, we assessed the relationship between the degree of D₁R/D₂R blockade and changes in benefit- and cost-based motivation for goal-directed behavior of macaque monkeys. We found that the degree of blockade of either D₁R or D₂R was associated with a reduction of the positive impact of reward amount and increasing delay discounting. Workload discounting was selectively increased by D₂R antagonism. In addition, blocking both D₁R and D₂R had a synergistic effect on delay discounting but an antagonist effect on workload discounting. These results provide fundamental insight into the distinct mechanisms of DA action in the regulation of the benefit- and cost-based motivation, which have important implications for motivational alterations in both neurological and psychiatric disorders.

Using quantitatively controlled pharmacological manipulations, this study teases apart the role of D1- and D2-like dopamine receptors in motivation and goal-directed behavior in monkeys, revealing complementary roles of two dopamine receptor subtypes in the computation of the cost/benefit trade-off to guide action.

Parkinson's disease laterality: a 11C-PE2I PET imaging study

Asymmetry of striatal dopaminergic deficits and motor symptoms is a typical characteristic of idiopathic Parkinson's disease (PD). This study aims to characterise the trend of asymmetry in moderate-stage PD. We performed a 19-month longitudinal study in 27 patients with PET-CT imaging and appropriate clinical assessments. 11C-PE2I non-displaceable binding potential (BPND) was calculated bilaterally for the striatum at baseline and follow-up to estimate the in vivo density of striatal dopamine transporters (DAT). Changes in striatal 11C-PE2I BPND over time were more prominent in the ipsilateral as compared to contralateral side. Changes in MDS-UPDRS-III (motor component of the Movement Disorders Society Unified PD Rating Scale) were not different between the clinically most and least affected body sides. Our data support that the asymmetry in striatal dopaminergic degeneration becomes less prominent in moderate-stage PD. In contrast, during the above period, the asymmetry of motor symptoms was maintained between the clinically most and least affected body sides.

Increased microglial activation in patients with Parkinson disease using [18F]-DPA714 TSPO PET imaging

INTRODUCTION

Increasing evidence suggests that neuroinflammation is active in Parkinson disease (PD) and contributes to neurodegeneration. This process can be studied in vivo with PET and radioligands targeting TSPO, upregulated in activated microglia. Initial PET studies investigating microglial activation in PD with the [¹¹C]-PK11195 have provided inconclusive results. Here we assess the presence and distribution of neuroinflammatory response in PD patients using [¹⁸F]-DPA714 and to correlate imaging biomarkers to dopamine transporter imaging and clinical status.

METHODS

PD patients (n = 24, Hoehn and Yahr I-III) and 28 healthy controls were scanned with [¹⁸F]-DPA714 and [¹¹C]-PE2I and analyzed. They were all genotyped for TSPO polymorphism. Regional binding parameters were estimated (reference Logan graphical approach with supervised cluster analysis). Impact of TSPO genotype was analyzed using Wilcoxon signed-rank test. Differences between groups were investigated using a two-way ANOVA and Tukey post hoc tests.

RESULTS

PD patients showed significantly higher [¹⁸F]-DPA714 binding compared to healthy controls bilaterally in the midbrain (p < 0.001), the frontal cortex (p = 0.001), and the putamen contralateral to the more clinically affected hemibody (p = 0.038). Microglial activation in these regions did not correlate with the severity of motor symptoms, disease duration nor putaminal [¹¹C]-PE2I uptake. However, there was a trend toward a correlation between cortical TSPO binding and disease duration (p = 0.015 uncorrected, p = 0.07 after Bonferroni correction).

CONCLUSION

[¹⁸F]-DPA714 binding confirmed that there is a specific topographic pattern of microglial activation in the nigro-striatal pathway and the frontal cortex of PD patients.

TRIAL REGISTRATION

Trial registration: INFLAPARK, NCT02319382. Registered 18 December 2014- Retrospectively registered, https://clinicaltrials.gov/ct2/show/NCT02319382.

Neurorestorative Effects of a Novel Fas-Associated Factor 1 Inhibitor in the MPTP Model: An [18F]FE-PE2I Positron Emission Tomography Analysis Study

Fas-associated factor 1 (FAF1), a Fas-binding protein, is implicated in neuronal cell death in Parkinson’s disease (PD). We examined the effects of a novel FAF1 inhibitor, KM-819, in dopaminergic neurons in a 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) mouse model using [¹⁸F]FE-PE2I positron emission tomography (PET). The MPTP model was generated with subacute MPTP treatment (20 mg/kg/day, i.p.) for 5 consecutive days in C57bl/6J mice. This study included three groups: the control group (treatment with saline only), the MPTP model group with KM-819 treatment (20 mg/kg/day p.o.) for 6 days, and the MPTP model group without KM-819 treatment. [¹⁸F]FE-PE2I PET studies were conducted in the same animals before and after MPTP with or without KM-819 treatment to monitor changes in striatal dopamine transporter activity indicated by non-displaceable binding potential (BP_ND) of [¹⁸F]FE-PE2I, and the expression levels of tyrosine hydroxylase were assessed using immunohistochemistry before and after KM-819 treatment. After MPTP injection, decreased striatal BP_ND was observed in the MPTP model group compared with the control group. Striatal BP_ND increased in the MPTP model group with KM-819 treatment, but not in the MPTP model group without KM-819 treatment. The tyrosine hydroxylase expression levels also significantly increased in the MPTP model group with KM-819 treatment compared with the control group. This study indicates that inhibition of the Fas-mediated cell death pathway by KM-819 has neurorestorative effects in striatal dopamine neurons in the MPTP model. Further studies would be needed to investigate the potential of KM-819 as a therapeutic drug for PD treatment.

EANM practice guideline/SNMMI procedure standard for dopaminergic imaging in Parkinsonian syndromes 1.0

Purpose

This joint practice guideline or procedure standard was developed collaboratively by the European Association of Nuclear Medicine (EANM) and the Society of Nuclear Medicine and Molecular Imaging (SNMMI). The goal of this guideline is to assist nuclear medicine practitioners in recommending, performing, interpreting, and reporting the results of dopaminergic imaging in parkinsonian syndromes.

Methods

Currently nuclear medicine investigations can assess both presynaptic and postsynaptic function of dopaminergic synapses. To date both EANM and SNMMI have published procedural guidelines for dopamine transporter imaging with single photon emission computed tomography (SPECT) (in 2009 and 2011, respectively). An EANM guideline for D2 SPECT imaging is also available (2009). Since the publication of these previous guidelines, new lines of evidence have been made available on semiquantification, harmonization, comparison with normal datasets, and longitudinal analyses of dopamine transporter imaging with SPECT. Similarly, details on acquisition protocols and simplified quantification methods are now available for dopamine transporter imaging with PET, including recently developed fluorinated tracers. Finally, [¹⁸F]fluorodopa PET is now used in some centers for the differential diagnosis of parkinsonism, although procedural guidelines aiming to define standard procedures for [¹⁸F]fluorodopa imaging in this setting are still lacking.

Conclusion

All these emerging issues are addressed in the present procedural guidelines for dopaminergic imaging in parkinsonian syndromes.

Lower midbrain dopamine transporter availability in depressed patients: Report from high-resolution PET imaging

BACKGROUND

A reduced presynaptic dopamine neurotransmission has long been implicated in major depressive disorder (MDD). However, molecular imaging studies that assessed the dopamine transporter (DAT) availability have led to inconsistent results, partly due to methodological considerations, and to exclusive focus on the striatum, precluding findings in extra-striatal regions.

METHODS

Herein, we leveraged our database of high-resolution Positron Emission Tomography (PET) images acquired with a highly selective radiotracer, [¹¹C]PE2I, to assess striatal and extra-striatal DAT availability in eight patients treated for depression compared to twenty-four healthy controls.

RESULTS

Statistical parametric mapping and voxel-based analyses of PET images detected a significant lower DAT availability in depressed patients within the superior part of the midbrain (right, p_FWE = 0.002; left, p_FWE = 0.006), a region including the ventral tegmental area and the substantia nigra from where the mesocorticolimbic and nigrostriatal dopamine pathways originate. A similar difference was found in the right dorsal putamen (p_FWE = 0.012).

LIMITATIONS

The statistical power was limited to detect only large effects, due to the size of the patients' sample.

CONCLUSIONS

The findings support the hypothesis that a reduced presynaptic dopamine function plays a role in the pathophysiology of depression, and that extra-striatal dopamine function should be further investigated.

PET imaging of Dopamine Transporter with [18F]LBT-999: initial evaluation in healthy volunteers

BACKGROUND

To evaluate in healthy human brain the distribution, uptake, and kinetics of [18F]LBT-999, a PET ligand targeting the dopamine transporter, to assess its ability to explore dopaminergic innervation, using a shorter protocol, more convenient for patients than currently with [123I]ioflupane.

METHODS

After intravenous injection of [18F]LBT-999, 8 healthy subjects (53-80y) underwent a dynamic PET-scan. Venous samples were concomitantly obtained for metabolites analysis. Time activity curves (TACs) were generated for several ROIs (caudate, putamen, occipital cortex, substantia nigra and cerebellum). Cerebellum was used as reference region to calculate binding potentials (BPND).

RESULTS

No adverse events or detectable pharmacological effects were reported. [18F]LBT-999 PET revealed a good cerebral distribution, with an intense and symmetric uptake in both putamen and caudate (BPND of 6.75±1.17 and 6.30±1.17, respectively), without other brain abnormal tracer accumulation. Regional TACs showed a plateau from the maximal uptake, 20min pi, to the end of the acquisition for both caudate and putamen, whereas uptake in substantia nigra decreased progressively. A faster clearance and lowest BPND values were observed in both cortex and cerebellum. Ratios to the cerebellum exhibit value of about 3 in substantia nigra, close to 10 for both caudate and putamen, and remained around the value of 1 in cortex. The parent fraction of [18F]LBT-999 in plasma was 80%, 60% and 45% at 15, 30 and 45 min pi, respectively.

CONCLUSIONS

These findings support the usefulness of [18F]LBT-999 for a quantitative clinical evaluation of presynaptic dopaminergic innervation.

Relationship between neuromelanin and dopamine terminals within the Parkinson's nigrostriatal system

Parkinson's disease is characterized by the progressive loss of pigmented dopaminergic neurons in the substantia nigra and associated striatal deafferentation. Neuromelanin content is thought to reflect the loss of pigmented neurons, but available data characterizing its relationship with striatal dopaminergic integrity are not comprehensive or consistent, and predominantly involve heterogeneous samples. In this cross-sectional study, we used neuromelanin-sensitive MRI and the highly specific dopamine transporter PET radioligand, 11C-PE2I, to assess the association between neuromelanin-containing cell levels in the substantia nigra pars compacta and nigrostriatal terminal density in vivo, in 30 patients with bilateral Parkinson's disease. Fifteen healthy control subjects also underwent neuromelanin-sensitive imaging. We used a novel approach taking into account the anatomical and functional subdivision of substantia nigra into dorsal and ventral tiers and striatal nuclei into pre- and post-commissural subregions, in accordance with previous animal and post-mortem studies, and consider the clinically asymmetric disease presentation. In vivo, Parkinson's disease subjects displayed reduced neuromelanin levels in the ventral (-30 ± 28%) and dorsal tiers (-21 ± 24%) as compared to the control group [F(1,43) = 11.95, P = 0.001]. Within the Parkinson's disease group, nigral pigmentation was lower in the ventral tier as compared to the dorsal tier [F(1,29) = 36.19, P < 0.001] and lower in the clinically-defined most affected side [F(1,29) = 4.85, P = 0.036]. Similarly, lower dopamine transporter density was observed in the ventral tier [F(1,29) = 76.39, P < 0.001] and clinically-defined most affected side [F(1,29) = 4.21, P = 0.049]. Despite similar patterns, regression analysis showed no significant association between nigral pigmentation and nigral dopamine transporter density. However, for the clinically-defined most affected side, significant relationships were observed between pigmentation of the ventral nigral tier with striatal dopamine transporter binding in pre-commissural and post-commissural striatal subregions known to receive nigrostriatal projections from this tier, while the dorsal tier correlated with striatal projection sites in the pre-commissural striatum (P < 0.05, Benjamini-Hochberg corrected). In contrast, there were no statistically significant relationships between these two measures in the clinically-defined least affected side. These findings provide important insights into the topography of nigrostriatal neurodegeneration in Parkinson's disease, indicating that the characteristics of disease progression may fundamentally differ across hemispheres and support post-mortem data showing asynchrony in the loss of neuromelanin-containing versus tyrosine hydroxylase positive nigral cells.

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

BACKGROUND

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.

METHODS

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.

RESULTS

Standard multiple regression at baseline indicated that ¹¹ C-PE2I BP_ND predicted UPDRS-III and bradykinesia-rigidity scores (P < 0.05), whereas ¹⁸ F-dopa K_i did not make significant unique explanatory contributions. Voxel-wise analysis showed negative correlations between ¹¹ C-PE2I BP_ND and motor severity across the whole striatum bilaterally. ¹⁸ F-Dopa K_i clusters were restricted to the most affected putamen and caudate. Longitudinally, negative correlations were found between striatal Δ¹¹ C-PE2I BP_ND , ΔUPDRS-III, and Δbradykinesia-rigidity, whereas no significant associations were found for Δ¹⁸ F-dopa K_i . One cluster in the most affected putamen was identified in the longitudinal voxel-wise analysis showing a negative relationship between Δ¹¹ C-PE2I BP_ND and Δbradykinesia-rigidity.

CONCLUSIONS

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.

Revisiting the Logan plot to account for non-negligible blood volume in brain tissue

Background

Reference tissue-based quantification of brain PET data does not typically include correction for signal originating from blood vessels, which is known to result in biased outcome measures. The bias extent depends on the amount of radioactivity in the blood vessels. In this study, we seek to revisit the well-established Logan plot and derive alternative formulations that provide estimation of distribution volume ratios (DVRs) that are corrected for the signal originating from the vasculature.

Results

New expressions for the Logan plot based on arterial input function and reference tissue were derived, which included explicit terms for whole blood radioactivity. The new methods were evaluated using PET data acquired using [¹¹C]raclopride and [¹⁸F]MNI-659. The two-tissue compartment model (2TCM), with which signal originating from blood can be explicitly modeled, was used as a gold standard.

DVR values obtained for [¹¹C]raclopride using the either blood-based or reference tissue-based Logan plot were systematically underestimated compared to 2TCM, and for [¹⁸F]MNI-659, a proportionality bias was observed, i.e., the bias varied across regions. The biases disappeared when optimal blood-signal correction was used for respective tracer, although for the case of [¹⁸F]MNI-659 a small but systematic overestimation of DVR was still observed.

Conclusions

The new method appears to remove the bias introduced due to absence of correction for blood volume in regular graphical analysis and can be considered in clinical studies. Further studies are however required to derive a generic mapping between plasma and whole-blood radioactivity levels.

Electronic supplementary material

The online version of this article (doi:10.1186/s13550-017-0314-z) contains supplementary material, which is available to authorized users.

A hybrid deconvolution approach for estimation of in vivo non-displaceable binding for brain PET targets without a reference region

BACKGROUND AND AIM

Estimation of a PET tracer's non-displaceable distribution volume (VND) is required for quantification of specific binding to its target of interest. VND is generally assumed to be comparable brain-wide and is determined either from a reference region devoid of the target, often not available for many tracers and targets, or by imaging each subject before and after blocking the target with another molecule that has high affinity for the target, which is cumbersome and involves additional radiation exposure. Here we propose, and validate for the tracers [11C]DASB and [11C]CUMI-101, a new data-driven hybrid deconvolution approach (HYDECA) that determines VND at the individual level without requiring either a reference region or a blocking study.

METHODS

HYDECA requires the tracer metabolite-corrected concentration curve in blood plasma and uses a singular value decomposition to estimate the impulse response function across several brain regions from measured time activity curves. HYDECA decomposes each region's impulse response function into the sum of a parametric non-displaceable component, which is a function of VND, assumed common across regions, and a nonparametric specific component. These two components differentially contribute to each impulse response function. Different regions show different contributions of the two components, and HYDECA examines data across regions to find a suitable common VND. HYDECA implementation requires determination of two tuning parameters, and we propose two strategies for objectively selecting these parameters for a given tracer: using data from blocking studies, and realistic simulations of the tracer. Using available test-retest data, we compare HYDECA estimates of VND and binding potentials to those obtained based on VND estimated using a purported reference region.

RESULTS

For [11C]DASB and [11C]CUMI-101, we find that regardless of the strategy used to optimize the tuning parameters, HYDECA provides considerably less biased estimates of VND than those obtained, as is commonly done, using a non-ideal reference region. HYDECA test-retest reproducibility is comparable to that obtained using a VND determined from a non-ideal reference region, when considering the binding potentials BPP and BPND.

CONCLUSIONS

HYDECA can provide subject-specific estimates of VND without requiring a blocking study for tracers and targets for which a valid reference region does not exist.

The Effects of Methylphenidate on Resting-State Functional Connectivity of the Basal Nucleus of Meynert, Locus Coeruleus, and Ventral Tegmental Area in Healthy Adults

Background: Methylphenidate (MPH) influences catecholaminergic signaling. Extant work examined the effects of MPH on the neural circuits of attention and cognitive control, but few studies have investigated the effect of MPH on the brain's resting-state functional connectivity (rsFC).

Methods: In this observational study, we compared rsFC of a group of 24 healthy adults who were administered an oral 45 mg dose of MPH with a group of 24 age and gender matched controls who did not receive MPH. We focused on three seed regions: basal nucleus of Meynert (BNM), locus coeruleus (LC), and ventral tegmental area/substantia nigra, pars compacta (VTA/SNc), each providing cholinergic, noradrenergic and dopaminergic inputs to the cerebral cortex. Images were pre-processed and analyzed as in our recent work (Li et al., 2014; Zhang et al., 2015). We used one-sample t-test to characterize group-specific rsFC of each seed region and two-sample t-test to compare rsFC between groups.

Results: MPH reversed negative connectivity between BNM and precentral gyri. MPH reduced positive connectivity between LC and cerebellum, and induced positive connectivity between LC and right hippocampus. MPH decreased positive VTA/SNc connectivity to the cerebellum and putamen, and reduced negative connectivity to left middle occipital gyrus.

Conclusion: MPH had distinct effects on the rsFC of BNM, LC, and VTA/SNc in healthy adults. These new findings may further our understanding of the role of catecholaminergic signaling in Attention Deficit Hyperactivity Disorder (ADHD) and Parkinson's disease and provide insights into the therapeutic mechanisms of MPH in the treatment of clinical conditions that implicate catecholaminergic dysfunction.

PET imaging of dopamine transporters with [(18)F]FE-PE2I: Effects of anti-Parkinsonian drugs

PURPOSE

This study aimed to assess the striatal [(18)F]FE-PE2I binding and the immunohistochemical stain of tyrosine hydroxylase (TH) in the striatum, and to evaluate the effects of therapeutic drugs on [(18)F]FE-PE2I binding.

METHODS

Dynamic PET/CT of [(18)F]FE-PE2I was performed in Parkinson's disease (PD) rat models, induced by the unilateral injection of 6-OHDA into the striatum. A simplified reference tissue model method was used to calculate the striatal binding potential (striatal BPND). Each of the four normal rats was pretreated with pramipexole, amantadine, and escitalopram 30 min before [(18)F]FE-PE2I injection. The effect of L-DOPA combined with benserazide was assessed in the normal and PD rats.

RESULTS

The BPND was significantly lower in the lesioned striatum than in the striatum of the normal rats. After the pretreatment with pramipexole, amantadine, and escitalopram, the values of the striatal BPND did not differ from those of the controls. The pretreatment with L-DOPA/benserazide, however, significantly reduced the striatal BPND. The striatal BPND of the PD rats with L-DOPA/benserazide pretreatment was not different from that of the same PD rats with placebo treatment.

CONCLUSION

[(18)F]FE-PE2I may be used as a radioligand for the in-vivo imaging of the DAT. In the normal rats, [(18)F]FE-PE2I binding is unaffected by pramipexole, amantadine, and escitalopram. L-DOPA/benserazide does not affect the striatal [(18)F]FE-PE2I binding in PD rats.

Evaluation of semi-quantitative method for quantification of dopamine transporter in human PET study with ¹⁸F-FE-PE2I

OBJECTIVES

Positron emission tomography (PET) with ¹⁸F-FE-PE2I is useful for investigating the function of dopamine transporter, and kinetics of ¹⁸F-FE-PE2I could be described by standard two-tissue compartment model (2CM) using plasma input function. In this study, we investigated the feasibility of semi-quantitative methods for estimating binding potential (BPND) and transporter occupancy to shorten the scan period and to reduce the effect of statistical noise on quantitative outcomes using computer simulation and human PET studies with ¹⁸F-FE-PE2I.

METHODS

In the simulations, time-activity curves (TACs) for the putamen with a wide range of BPND were generated. In these TACs, BPNDs were estimated by standardized uptake value ratio (SUVR) using various integration intervals and the simplified reference tissue model (SRTM) with the cerebellum as reference region, and reduction of BPND assuming transporter occupancy by antipsychotics was calculated from BPND obtained from TACs with various BPND values. These estimates were evaluated by comparison with those of 2CM. In human studies with normal volunteers, BPNDs were estimated in the caudate and putamen using SUVR and SRTM with the cerebellar reference region, and compared with BPND by standard 2CM.

RESULTS

In the simulations, BPND estimated by SUVR with late time frames and SRTM showed linear correlation with those by 2CM, although the estimates by SUVR were overestimated and affected by the cerebral blood flow as BPND became higher. As for transporter occupancy, SRTM showed higher linearity with 2CM and less effect of statistical noise than the SUVR method. In human studies, BPND by SRTM and SUVR with late time frames showed good correlation with BPND by 2CM.

CONCLUSIONS

Although SRTM is more reliable than the SUVR method for BPND and occupancy estimation, SUVR using late time frames has the potential to provide practical indices of BPND and occupancy with a shorter scan period.

Evaluation of safety and tolerability, pharmacokinetics, and pharmacodynamics of BMS-820836 in healthy subjects: a placebo-controlled, ascending single-dose study

RATIONALE

BMS-820836, a novel triple monoamine reuptake inhibitor, is an experimental monotherapy for sufferers of major depressive disorder who have had an inadequate response to an existing antidepressant treatment.

OBJECTIVES

This study was conducted to evaluate the safety and tolerability, pharmacokinetics (PK), and serotonin transporter (SERT) and dopamine transporter (DAT) occupancy for single doses of BMS-820836 in healthy subjects.

METHODS

Healthy subjects were assigned to seven BMS-820836 dose panels (0.025, 0.1, 0.5, 1, 2, 3, and 5 mg; n = 8 each), in which subjects were randomly allocated 3:1 to a single BMS-820836 dose or matched placebo. Serial blood samples were collected on Days 1, 2, 3, 4, 7, and 14 to characterize the PK of BMS-820836. Following evaluation of the maximum tolerated dose, SERT occupancy was determined by applying [(11)C]DASB positron emission tomography (PET) after single-dose BMS-820836 (0.5 or 3 mg; n = 3 each) and DAT occupancy by applying [(11)C]PE2I PET after single-dose BMS-820836 (3 mg; n = 6).

RESULTS

Single oral doses of BMS-820836 (0.025-3 mg) were generally safe and well tolerated. BMS-820836 had a median T max of 5.0-7.2 h and a mean apparent terminal T 1/2 of 34-57 h. Mean striatal SERT occupancies were 19 ± 9 % and 82 ± 8 % after single doses of 0.5 and 3 mg BMS-820836, respectively. The mean striatal DAT occupancy was 19 ± 9 % after a single 3 mg BMS-820836 dose.

CONCLUSIONS

Single doses of BMS-820836 have meaningful SERT and DAT occupancy and demonstrate an acceptable safety and tolerability profile in healthy control subjects.

In vivo activity of modafinil on dopamine transporter measured with positron emission tomography and [¹⁸F]FE-PE2I

Modafinil, a wake-promoting drug used to treat narcolepsy, is a dopamine transporter inhibitor and is said to have very low abuse liability; this, however, is still up for debate. We conducted a dopamine transporter (DAT) occupancy study with modafinil (200 or 300 mg) in ten healthy volunteers using positron emission tomography (PET) with [¹⁸F]FE-PE2I, a new PET radioligand with high affinity and selectivity for the dopamine transporter, to characterize its relation to abuse liability. Mean striatal DAT occupancies were 51.4% at 200 mg and 56.9% at 300 mg. There was a significant correlation between occupancy and plasma concentration, indicating dose dependency of DAT inhibition by modafinil in the striatum, and especially in the nucleus accumbens. This study showed that DAT occupancy by modafinil was close to that of methylphenidate, indicating that modafinil may be near the same level as methylphenidate in relation to abuse liability in terms of dopaminergic transmission.

Age-related decline in dopamine transporter in human brain using PET with a new radioligand [¹⁸F]FE-PE2I

OBJECTIVE

Dopamine transporter (DAT) density is considered as a marker of pre-synaptic function. Numerous neuroimaging studies have consistently demonstrated an age-related decrease in DAT density in normal human brain. However, the precise degree of the regional decline is not yet clear. The purpose of this study was to evaluate the effect of the normal aging process on DAT densities in human-specific brain regions including the substantia nigra and thalamus using positron emission tomography (PET) with [(18)F]FE-PE2I, a new PET radioligand with high affinity and selectivity for DAT.

METHODS

Thirty-six healthy volunteers ranging in age from 22 to 80 years were scanned with PET employing [(18)F]FE-PE2I for measuring DAT densities. Region of interest (ROI)-based analysis was used, and ROIs were manually defined for the caudate, putamen, substantia nigra, thalamus, and cerebellar cortex. DAT binding was quantified using a simplified reference tissue model, and the cerebellum was used as reference region. Estimations of binding potential in the caudate, putamen, substantia nigra, and thalamus were individually regressed according to age using simple regression analysis. Estimates of DAT loss per decade were obtained using the values from the regression slopes.

RESULTS

There were 7.6, 7.7, and 3.4% per-decade declines in DAT in the caudate, putamen, and substantia nigra, respectively. By contrast, there was no age-related decline of DAT in the thalamus.

CONCLUSIONS

[(18)F]FE-PE2I allowed reliable quantification of DAT, not only in the caudate and putamen but also in the substantia nigra. From the results, we demonstrated the age-related decline in the caudate and putamen as reported in previous studies, and additionally for those in the substantia nigra for the first time.

Quantification of dopamine transporter density with [18F]FECNT PET in healthy humans

INTRODUCTION

Fluorine-18 labeled 2β-carbomethoxy-3β-(4-chlorophenyl)-8-(2-fluoroethyl)nortropane ([(18)F]FECNT) binds reversibly to the dopamine transporter (DAT) with high selectivity. [(18)F]FECNT has been used extensively in the quantification of DAT occupancy in non-human primate brain and can distinguish between Parkinson's and healthy controls in humans. The purpose of this work was to develop a compartment model to characterize the kinetics of [(18)F]FECNT for quantification of DAT density in healthy human brain.

METHODS

Twelve healthy volunteers underwent 180 min dynamic [(18)F]FECNT PET imaging including sampling of arterial blood. Regional time-activity curves were extracted from the caudate, putamen and midbrain including a reference region placed in the cerebellum. Binding potential, BPND, was calculated for all regions using kinetic parameters estimated from compartmental and Logan graphical model fits to the time-activity data. Simulations were performed to determine whether the compartment model could reliably fit time-activity data over a range of BPND values.

RESULTS

The kinetics of [(18)F]FECNT were well-described by the reversible 2-tissue arterial input and full reference tissue compartment models. Calculated binding potentials in the caudate, putamen and midbrain were in good agreement between the arterial input model, reference tissue model and the Logan graphical model. The distribution volume in the cerebellum did not reach a plateau over the duration of the study, which may be a result of non-specific binding in the cerebellum. Simulations that included non-specific binding show that the reference and arterial input models are able to estimate BPND for DAT densities well below that observed in normal volunteers.

CONCLUSION

The kinetics of [(18)F]FECNT in human brain are well-described by arterial input and reference tissue compartment models. Measured and simulated data show that BPND calculated with reference tissue model is proportional to BPND calculated from the arterial input model.

A simple algorithm for subregional striatal uptake analysis with partial volume correction in dopaminergic PET imaging

OBJECTIVE

In positron emission tomography (PET) of the dopaminergic system, quantitative measurements of nigrostriatal dopamine function are useful for differential diagnosis. A subregional analysis of striatal uptake enables the diagnostic performance to be more powerful. However, the partial volume effect (PVE) induces an underestimation of the true radioactivity concentration in small structures. This work proposes a simple algorithm for subregional analysis of striatal uptake with partial volume correction (PVC) in dopaminergic PET imaging.

METHODS

The PVC algorithm analyzes the separate striatal subregions and takes into account the PVE based on the recovery coefficient (RC). The RC is defined as the ratio of the PVE-uncorrected to PVE-corrected radioactivity concentration, and is derived from a combination of the traditional volume of interest (VOI) analysis and the large VOI technique. The clinical studies, comprising 11 patients with Parkinson's disease (PD) and 6 healthy subjects, were used to assess the impact of PVC on the quantitative measurements. Simulations on a numerical phantom that mimicked realistic healthy and neurodegenerative situations were used to evaluate the performance of the proposed PVC algorithm. In both the clinical and the simulation studies, the striatal-to-occipital ratio (SOR) values for the entire striatum and its subregions were calculated with and without PVC.

RESULTS

In the clinical studies, the SOR values in each structure (caudate, anterior putamen, posterior putamen, putamen, and striatum) were significantly higher by using PVC in contrast to those without. Among the PD patients, the SOR values in each structure and quantitative disease severity ratings were shown to be significantly related only when PVC was used. For the simulation studies, the average absolute percentage error of the SOR estimates before and after PVC were 22.74% and 1.54% in the healthy situation, respectively; those in the neurodegenerative situation were 20.69% and 2.51%, respectively.

CONCLUSIONS

We successfully implemented a simple algorithm for subregional analysis of striatal uptake with PVC in dopaminergic PET imaging. The PVC algorithm provides an accurate measure of the SOR in the entire striatum and its subregions, and improves the correlation between the SOR values and the clinical disease severity of PD patients.

MPTP animal model of Parkinsonism: dopamine cell death or only tyrosine hydroxylase impairment? A study using PET imaging, autoradiography, and immunohistochemistry in the cat

AIMS

1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) is a neurotoxin widely used to produce experimental models of Parkinson's disease in laboratory animals. It is believed to cause a selective destruction of substantia nigra dopamine neurons, mainly based on a large reduction of tyrosine hydroxylase (TH), the catecholamine's synthesizing enzyme. Unlike Parkinson's disease in humans, however, all animal models are able to recover more or less rapidly from the MPTP induced Parkinsonian syndrome. This raises the question as whether MPTP causes a cell death with a decrease in dopamine transporter or a simple impairment of TH.

METHODS

To respond to this question, we quantified in a cat model of Parkinson's disease (MPTP 5 mg/kg i.p. during 5 days) the dopamine transporter using positron emission tomography (PET) imaging and autoradiography of [(11) C]PE2I and compared the data with the TH-immunoreactivity.

RESULTS

We found no changes in [(11) C]PE2I PET binding either 5 or 26 days after MPTP treatment when compared to baseline levels. Similarly, there were no significant changes in [(11) C]PE2I autoradiographic binding in the cat brain one week after MPTP treatment. In sharp contrast, MPTP treated cats exhibited severe Parkinson-like motor syndrome during the acute period with a marked decrease in TH-immunoreactivity in the striatum.

CONCLUSION

These data suggest that MPTP toxicity impairs efficiently TH and that such an effect is not necessarily accompanied by significant reduction of dopamine transporter seen with in vitro or in vivo [(11) C]PE2I binding.

Correlation between decreased motor activity and dopaminergic degeneration in the ventrolateral putamen in monkeys receiving repeated MPTP administrations: a positron emission tomography study

Parkinson's disease (PD) patients have remarkably reduced levels of dopaminergic biomarkers in the caudal putamen. However, the relationship between motor impairments and the localization of intrastriatal dopaminergic degeneration in monkey PD models remains unclear. To identify the striatal areas with dopaminergic dysfunction responsible for motor impairments, we measured changes in both general motor activity and in vivo dopaminergic biomarkers in three cynomolgus monkeys that repeatedly received 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), starting in the normal state and continuing until after tremor appearance. Binding of dopamine transporters (DAT) and D(2) receptors were measured by positron emission tomography (PET) using [(11)C]PE2I and [(11)C]raclopride, respectively. Region-of-interest-based regression analysis demonstrated the degree of general motor activity reduction to be explained by striatal DAT binding but not by D(2) receptor binding. Furthermore, voxel-based analysis revealed a significant correlation between reduced general motor activity and decreased DAT binding, specifically in the ventrolateral putamen, which corresponds to the area receiving upper body motor inputs from the primary motor cortex. These results suggest that specific functional deficits in PD models are closely related to the degeneration of dopaminergic terminals in the striatal subregion responsible for these functions and that the level of deficit is dependent on the degree of degeneration.

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

BACKGROUND

LBT-999 (E)-N-(4-fluorobut-2-enyl)-2β-carbomethoxy-3β-(4'-tolyl)nortropane is a dopamine transporter (DAT) ligand. [(18)F]LBT-999 was first labelled with carbon-11; we will now describe its in vivo behaviour in comparison to that of [(11)C]LBT-999.

METHODS/RESULTS

Positron emission tomography (PET) experiments (baboons) confirmed the high affinity/specificity of [(18)F]LBT-999 for DAT. The brain regional distribution was in accordance with that of DAT. Pre-treatment with LBT-999 (1 mg/kg iv), but not with desipramine, a norepinephrine (NET) antagonist, reduced the striatum-to-cerebellum ratio by 96%, confirming the specificity for DAT vs. NET. The parent compound decreased rapidly and represented 24.3 ± 5.0% of plasma radioactivity at 30 min pi. Whole-body scans showed an important bone uptake of free fluorine following metabolism of [(18)F]LBT-999. In the cerebellum and striatum, distribution volumes increased by 30-40% between 80 and 230 min, suggesting the polluting role of a radiometabolite(s). [(11)C]LBT-999 exhibited a 40% higher standardized uptake value in the striata. This difference is likely due to N-dealkylation followed by [(18)F]fluoride release. 2β-Carbomethoxy-3β-(4'-tolyl) nortropane is then formed, while [(11)C]2β-carbomethoxy-3β-(4'-tolyl) nortropane is formed following injection of [(11)C]LBT-999. This metabolite has high affinity for the DAT. In one specific PET experiment, intravenous injection of this metabolite induced a strong displacement of [(18)F]LBT-999 in the striata, confirming that this metabolite readily crosses the blood-brain barrier (BBB) and binds to DAT.

CONCLUSIONS

[(18)F]LBT-999 is N-dealkylated in vivo to yield (1) a nonradioactive metabolite that crosses the BBB and has a high affinity for the DAT and (2) a [(18)F]fluoro-alkyl chain which is further defluorinated. The temporal changes in distribution volumes are consistent with the accumulation of a radiometabolite(s) in the brain. Therefore, the quantification of DAT density with [(18)F]LBT-999 is rather difficult.

Characterisation of [¹¹C]PR04.MZ in Papio anubis baboon: a selective high-affinity radioligand for quantitative imaging of the dopamine transporter

N-(4-fluorobut-2-yn-1-yl)-2β-carbomethoxy-3β-(4'-tolyl)nortropane (PR04.MZ, 1) is a PET radioligand for the non-invasive exploration of the function of the cerebral dopamine transporter (DAT). A reliable automated process for routine production of the carbon-11 labelled analogue [(11)C]PR04.MZ ([(11)C]-1) has been developed using GMP compliant equipment. An adult female Papio anubis baboon was studied using a test-retest protocol with [(11)C]-1 in order to assess test-retest reliability, metabolism and CNS distribution profile of the tracer in non-human primates. Blood sampling was performed throughout the studies for determination of the free fraction in plasma (f(P)), plasma input functions and metabolic degradation of the radiotracer [(11)C]-1. Time-activity curves were derived for the putamen, the caudate nucleus, the ventral striatum, the midbrain and the cerebellum. Distribution volumes (V(T)) and non-displaceable binding potentials (BP(ND)) for various brain regions and the blood were obtained from kinetic modelling. [(11)C]-1 shows promising results as a selective marker of the presynaptic dopamine transporter. With the reliable visualisation of the extra-striatal dopaminergic neurons and no indication on labelled metabolites, the tracer provides excellent potential for translation into man.

Early presymptomatic and long-term changes of rest activity cycles and cognitive behavior in a MPTP-monkey model of Parkinson's disease

Background

It is increasingly recognized that non-motor symptoms are a prominent feature of Parkinson's disease and in the case of cognitive deficits can precede onset of the characteristic motor symptoms. Here, we examine in 4 monkeys chronically treated with low doses of the neurotoxin MPTP the early and long-term alterations of rest-activity rhythms in relationship to the appearance of motor and cognitive symptoms.

Methodology/Principal Findings

Behavioral activity recordings as well as motor and cognitive assessments were carried out continuously and in parallel before, during and for several months following MPTP-treatment (12–56 weeks). Cognitive abilities were assessed using a task that is dependent on the functional integrity of the fronto-striatal axis. Rest-activity cycles were monitored continuously using infrared movement detectors of locomotor activity. Motor impairment was evaluated using standardized scales for primates. Results show that MPTP treatment led to an immediate alteration (within one week) of rest-activity cycles and cognitive deficits. Parkinsonian motor deficits only became apparent 3 to 5 weeks after initiating chronic MPTP administration. In three of the four animals studied, clinical scores returned to control levels 5–7 weeks following cessation of MPTP treatment. In contrast, both cognitive deficits and chronobiological alterations persisted for many months. Levodopa treatment led to an improvement of cognitive performance but did not affect rest-activity rhythms in the two cases tested.

Conclusions/Significance

Present results show that i) changes in the rest activity cycles constituted early detectable consequences of MPTP treatment and, along with cognitive alterations, characterize the presymptomatic stage; ii) following motor recovery there is a long-term persistence of non-motor symptoms that could reflect differential underlying compensatory mechanisms in these domains; iii) the progressive MPTP-monkey model of presymptomatic ongoing parkinsonism offers possibilities for in-depth studies of early non-motor symptoms including sleep alterations and cognitive deficits.

Monitoring therapeutic effects in Parkinson's disease by serial imaging of the nigrostriatal dopaminergic pathway

PET and SPECT are very sensitive techniques to detect in-vivo nigrostriatal degeneration in Parkinson's disease, even in the pre-motor phase of the disease. Furthermore, these techniques are able to measure disease progression. However, caution must be used in the interpretation of studies in which therapeutic effects in Parkinson's disease were also monitored by serial imaging of nigrostriatal neurons, as disparity between imaging and clinical outcomes has been reported in several clinical studies.

The dopamine system in idiopathic generalized epilepsies: identification of syndrome-related changes

The present study tests the hypothesis that the dopamine system is altered in idiopathic generalized epilepsy (IGE), and that the pattern of possible changes differs between juvenile myoclonic epilepsy (JME) and epilepsy with tonic-clonic seizures only (GTCS). The dopamine (DA) system was investigated with PET and a DA transporter (DAT) ligand [(11)C]PE2I in 13 patients with JME, 13 with GTCS, and 12 healthy controls. The binding potential (BP) to DAT was quantified in the caudate, putamen, and midbrain. The possible impact on function was evaluated by correlating regional BP with test performance in a battery of neuropsychological tests. Both patient groups showed a reduced BP compared to controls, albeit in different locations. JME patients had a lower tracer binding than controls in the midbrain (0.8+/-0.1 vs. 1.0+/-0.2, p=0.019), whereas GTCS patients had reduced tracer binding in the putamen (5.9+/-1.6 vs. 7.1+/-1.2, p=0.023). While GTCS patients showed impaired performance in motor functions and on one test of executive function, JME patients performed poorly also in tests of working memory and several tests of executive function. Alterations in the DA system seem to exist in both GTCS and JME. However, the regional distribution of these changes differs between the two syndromes, as does their association with psychomotor and working memory performance. The present data suggest that the two forms of IGE have different neuronal substrates.

In vitro autoradiography and in vivo evaluation in cynomolgus monkey of [18F]FE-PE2I, a new dopamine transporter PET radioligand

This study evaluated the in vitro and in vivo characteristics of a new dopamine transporter (DAT) radioligand, [(18)F]fluoroethyl(FE)PE2I, by autoradiography from postmortem human brain and by positron emission tomography (PET) in three cynomolgus monkeys. In the autoradiography experiments, high [18F]FE-PE2I accumulation was observed in caudate and putamen that was selectively abolished by GBR12909 or beta-CIT but not by maprotiline. High doses of citalopram (>5 microM) also inhibited [18F]FE-PE2I binding in the striatum. In vitro Ki of the radioligand was 12 nM at rodent dopamine transporter. [18F]FE-PE2I brain uptake measured by PET was approximately 4-5% of the injected dose, with highest uptake in striatum followed by midbrain and thalamus, lower uptake in neocortex, and lowest in cerebellum. Peak specific binding in striatum was reached approximately 40 min and in midbrain 20-30 min postinjection. The ratio-to-cerebellum was 7-10 in striatum and 1.5-2.3 in midbrain. BP(ND) measured with simplified reference tissue method using the cerebellum as reference region was 4.5 in striatum and 0.6 in midbrain. No displacement was shown after citalopram or maprotiline administration, while GBR12909 decreased the binding in striatum and midbrain to the level of cerebellum. [18F]FE-PE2I showed relatively fast elimination and metabolism with the presence of two metabolite peaks with similar retention time as the labeled metabolites of [11C]PE2I. [18F]FE-PE2I showed in vivo selectivity for the DAT and compared with [11C]PE2I, it showed faster kinetics and earlier peak equilibrium. The potential influence of the two radiometabolites on PET quantification requires further evaluation.

Increase in thalamic binding of [(11)C]PE2I in patients with schizophrenia: a positron emission tomography study of dopamine transporter

Previous in vivo imaging studies reported no difference in dopamine transporter (DAT) bindings in the striatum between control subjects and patients with schizophrenia. However, as the signals of radioligands with moderate affinity were insufficient for allowing the evaluation of small amounts of DAT, DAT binding in extrastriatal regions has not been determined. Positron emission tomography scanning using [(11)C]PE2I was performed on eight patients with schizophrenia and twelve normal control subjects. Binding potential (BP(ND)) for DAT in the caudate, putamen, thalamus and substantia nigra was calculated, using the cerebellum as reference region. In patients with schizophrenia, clinical symptoms were evaluated by Positive and Negative Syndrome Scale (PANSS). BP(ND) in the thalamus of patients with schizophrenia was significantly higher than in control subjects (P=0.044). In patients with schizophrenia, there were significantly positive correlations between BP(ND) in the thalamus and total (r=0.75), positive (r=0.78) and negative PANSS scores (r=0.82). Altered DAT in the thalamus might be related to the pathophysiology and clinical symptoms of schizophrenia.

Modeling considerations for in vivo quantification of the dopamine transporter using [(11)C]PE2I and positron emission tomography

The dopamine transporter (DAT) is an important imaging target as changes in DAT have been implicated in a variety of neurologic and psychiatric disorders and can result from certain classes of medications. [(11)C]N-(3-iodoprop-2E-enyl)-2beta-carbomethoxy-3beta-(4-methylphenyl)nortropane ([(11)C]PE2I), a radioligand with high specificity for DAT, has been shown to exhibit favorable kinetics and to produce high contrast positron emission tomography (PET) images. To better characterize this ligand and to assess its measurement reliability, PET images of seven subjects were acquired in a test-retest paradigm. For optimal model performance, each subject was scanned for 120 mins, ensuring that high binding regions could reach equilibrium, a validated coregistration method was performed for accurate anatomic delineations and an exhaustive search for a reference region having one-tissue compartment kinetics was undertaken. Eleven modeling methods were tested and six metrics were used for method evaluation. A noniterative two-tissue compartment method with 100 mins of scanning time was found to be optimal for characterizing [(11)C]PE2I.

Synthesis, radiolabeling and preliminary in vivo evaluation of [18F]FE-PE2I, a new probe for the dopamine transporter

A new dopamine transporter (DAT) ligand, (E)-N-(3-iodoprop-2-enyl)-2beta-carbofluoroethoxy-3beta-(4'-methyl-phenyl) nortropane (FE-PE2I, 6), derived from PE2I (1), was prepared and found to be a potent inhibitor of rodent DAT in vitro. Compound 6 was radiolabelled with fluorine-18 (t(1/2)=109.8 min) for PET studies in monkeys. In vivo PET measurements showed a regional distribution in brain that corresponds to the known distribution of DAT. This binding was specific, reversible and the kinetics of [(18)F]6 binding in brain were faster than for its lead compound, [(11)C]1. The possible presence of a hydroxymethyl-radiometabolite formed by oxidation in the 3beta-benzylic position of [(18)F]6 warrants further detailed evaluation of the metabolism of [(18)F]6. [(18)F]6 is a potential radioligand for imaging DATs in the human brain with PET.

PE2I: a radiopharmaceutical for in vivo exploration of the dopamine transporter

The membrane dopamine transporter (DAT) has a pivotal role in the regulation of dopamine (DA) neurotransmission involved in a number of physiological functions and brain disorders. Molecular imaging techniques, such as positron emission tomography (PET) and single photon emission computerized tomography (SPECT), are relevant tools to explore the DAT, and we developed the cocaine derivative N-(3-iodopro-2E-enyl)-2beta-carbomethoxy-3beta-(4'-methylphenyl) nortropane (PE2I) that has proved to be a very potent radiopharmaceutical to image the DAT by these techniques. Several methods are available to obtain PE2I labeled with iodine-123 or -125, carbon-11 and tritium. The pharmacological properties of PE2I have demonstrated that it has good affinity for the DAT (4 nM) and is one of the most selective DAT ligands. [(125)I]PE2I characterized postmortem in human brains has revealed very intense and selective binding in the basal ganglia. Ex vivo autoradiography in rats has shown that high level of [(125)I]PE2I accumulates in the striatum and also in the substantia nigra and ventral tegmental area. [(125)I]PE2I accumulation in the rat striatum is rapid, high, and selective, providing a maximum striatum/cerebellum ratio of 10 during the first 30 min post injection. Using SPECT or PET, rapid, high, and selective accumulation of PE2I was found in the caudate nucleus and putamen in monkeys, whereas rapid wash out from the cerebellum was observed. In vivo investigations in healthy humans have demonstrated that PE2I has high striatal uptake, low nonspecific binding, low radiation exposure, and a fairly short scanning time. A number of findings in various animal models of Parkinson's disease in rats and monkeys have demonstrated the high efficacy of PE2I for detection of reduction in the density of DAT, thus showing the potential value of PE2I for early diagnosis and evaluation of treatment of this disease. The excellent properties of PE2I are basis for the development of new DAT tracers for use in future PET explorations using fluor-18.

Applying neuroimaging ligands to study major depressive disorder

The recent increase in radioligands available for neuroimaging major depressive disorder has led to advancements in our understanding of the pathophysiology of this illness and improved antidepressant development. Major depressive disorder can be defined as an illness of recurrent major depressive episodes of persistently low mood, dysregulated sleep, appetite and weight, anhedonia, cognitive impairment, and suicidality. The main target sites investigated with radioligand neuroimaging include receptor sites that regulate in response to lowered monoamine levels, targets related to removal of monoamines, uptake of ligands related to regional brain function, and target sites of antidepressants.