Proceedings of the National Academy of Sciences, 2019
Significance Posttraumatic stress disorder (PTSD) is an important risk factor for suicidal ideati... more Significance Posttraumatic stress disorder (PTSD) is an important risk factor for suicidal ideation, attempts, and death by suicide. Understanding of the biology underlying suicidality in PTSD is limited. In this study, we used positron emission tomography to evaluate the metabotropic glutamate receptor type 5 (mGluR5) as a potential treatment target and biomarker of suicidal ideation in individuals with PTSD and major depressive disorder (MDD). We found higher availability of mGluR5 in individuals with PTSD relative to healthy control and MDD groups. Furthermore, higher mGluR5 availability was associated with scan-day suicidal ideation among individuals with PTSD, but not MDD. Findings identify mGluR5 as a biomarker for intervention and potentially suicide risk management in PTSD.
Purpose Neuronal damage and synapse loss in the spinal cord (SC) have been implicated in spinal c... more Purpose Neuronal damage and synapse loss in the spinal cord (SC) have been implicated in spinal cord injury (SCI) and neurodegenerative disorders such as Amyotrophic Lateral Sclerosis (ALS). Current standards of diagnosis for SCI include CT or MRI imaging to evaluate injury severity. The current study explores the use of PET imaging with [11C]UCB-J, which targets the synaptic vesicle protein 2A (SV2A), in the human spinal cord, as a way to visualize synaptic density and integrity in vivo. Results First, simulations of baseline and blocking [11C]UCB-J HRRT scans were performed, based on SC dimensions and SV2A distribution to predict VT, VND, and VS values. Next, human baseline and blocking [11C]UCB-J HRRT images were used to estimate these values in the cervical SC (cSC). Simulation results had excellent agreement with observed values of VT, VND, and VS from the real human data, with baseline VT, VND, and VS of 3.07, 2.15, and 0.92 mL/cm3, respectively, with a BPND of 0.43. Lastly, w...
Acetylcholine (ACh) has distinct functional roles in striatum compared with cortex, and imbalance... more Acetylcholine (ACh) has distinct functional roles in striatum compared with cortex, and imbalance between these systems may contribute to neuropsychiatric disease. Preclinical studies indicate markedly higher ACh concentrations in the striatum. The goal of this work was to leverage positron emission tomography (PET) imaging estimates of drug occupancy at cholinergic receptors to explore ACh variation across the human brain, because these measures can be influenced by competition with endogenous neurotransmitter. PET scans were analyzed from healthy human volunteers (n = 4) and nonhuman primates (n = 2) scanned with the M1-selective radiotracer [11C]LSN3172176 in the presence of muscarinic antagonist scopolamine, and human volunteers (n = 10) scanned with the α4β2* nicotinic ligand (−)-[18F]flubatine during nicotine challenge. In all cases, occupancy estimates within striatal regions were consistently lower (M1/scopolamine human scans, 31 ± 3.4% occupancy in striatum, 43 ± 2.9% in ex...
Purpose: As stereotactic radiosurgery (SRS) and immunotherapy are increasingly used to treat brai... more Purpose: As stereotactic radiosurgery (SRS) and immunotherapy are increasingly used to treat brain metastases, incidence of radiation necrosis (RN) is consequently rising. Differentiating tumor regrowth (TR) from RN is vital in management but difficult to assess using MRI. We hypothesized that tumor methionine levels would be elevated given increased metabolism and high amino acid uptake, whereas RN would increase inflammation marked by upregulated translocator protein (PBR-TSPO), which can be quantified with specific PET tracers. Procedures: We performed a feasibility study to prospectively evaluate [11C]methionine and [11C]PBR28 using PET in 5 patients with 7 previously SRS-treated brain metastases demonstrating regrowth to differentiate TR from RN. Results: Sequential imaging with dual tracers was well-tolerated. [11C]methionine was accurate for detecting pathologically confirmed TR in 7/7 lesions, whereas [11C]PBR28 was only accurate in 3/7 lesions. Tumor PBR-TSPO expression was...
ABSTRACTPurposeTo investigate the synaptic vesicle glycoprotein 2A (SV2A) expression in the whole... more ABSTRACTPurposeTo investigate the synaptic vesicle glycoprotein 2A (SV2A) expression in the whole central nervous system and peripheral tissues, a metabolically stable SV2A radiotracer is desirable to minimize a potential confounding effect of radiometabolites. The aim of this study was to develop and evaluate a metabolically stable SV2A radiotracer, [18F]SDM-16, in nonhuman primate brains.MethodsThe racemic SDM-16 (4-(3,5-difluorophenyl)-1-((2-methyl-1H-imidazol-1-yl)methyl)pyrrolidin-2-one) was synthesized and assayed for in vitro SV2A binding affinity. We synthesized the enantiopure [18F]SDM-16 using the corresponding arylstannane precursor. Nonhuman primate brain PET was performed on a FOCUS 220 system. Arterial blood was drawn for metabolite analysis and construction of plasma input function. Regional time-activity curves (TACs) were evaluated with the one-tissue compartment (1TC) model to obtain the volume of distribution (VT). Binding potential (BPND) was calculated using eit...
BACKGROUND. Investigations of stress dysregulation in posttraumatic stress disorder (PTSD) have f... more BACKGROUND. Investigations of stress dysregulation in posttraumatic stress disorder (PTSD) have focused on peripheral cortisol, but none have examined cortisol in the human brain. This study used positron emission tomography (PET) to image 11β-hydroxysteroid dehydrogenase type 1 (11β-HSD1), a cortisol-producing enzyme, as a putative brain cortisol marker in PTSD. METHODS. Sixteen individuals with PTSD and 17 healthy, trauma-exposed controls (TCs) underwent PET imaging with [ 18 F] AS2471907, a radioligand for 11β-HSD1. RESULTS. Prefrontal-limbic 11β-HSD1 availability, estimated as [ 18 F]AS2471907 volume of distribution (V T), was significantly higher in the PTSD group compared with the TC group (β = 1.16, P = 0.0057). Lower prefrontal-limbic 11β-HSD1 availability was related to greater overall PTSD severity (R 2 = 0.27, P = 0.038) in the PTSD group. 11β-HSD1 availability was not related to plasma cortisol levels (R 2 = 0.026, P = 0.37). In a PTSD subset (n = 10), higher 11β-HSD1 availability was associated with higher availability of translocator protein (TSPO), a microglial marker (β = 4.40, P = 0.039). CONCLUSION. Higher brain cortisol-producing 11β-HSD1 in the PTSD group may represent a resilience-promoting neuroadaptation resulting in lower PTSD symptoms. Along with preliminary associations between 11β-HSD1 and TSPO, corroborating previous evidence of immune suppression in PTSD, these findings collectively challenge previous hypotheses of the deleterious effects of both excessive brain glucocorticoid and brain immune signaling in PTSD.
BackgroundAttempts to associate amyloid-β (Aβ) pathogenesis with synaptic loss in Alzheimer’s dis... more BackgroundAttempts to associate amyloid-β (Aβ) pathogenesis with synaptic loss in Alzheimer’s disease (AD) have thus far been limited to small numbers of postmortem studies. Aβ plaque burden is not well-correlated with indices of clinical severity or neurodegeneration—at least in the dementia stage—as deposition of Aβ reaches a ceiling. In this study, we examined in vivo the association between fibrillar Aβ deposition and synaptic density in early AD using positron emission tomography (PET). We hypothesized that global Aβ deposition would be more strongly inversely associated with hippocampal synaptic density in participants with amnestic mild cognitive impairment (aMCI; a stage of continued Aβ accumulation) compared to those with dementia (a stage of relative Aβ plateau).MethodsWe measured SV2A binding ([11C]UCB-J) and Aβ deposition ([11C]PiB) in 14 participants with aMCI due to AD and 24 participants with mild AD dementia. Distribution volume ratios (DVR) with a cerebellar referen...
The use of synaptic vesicle glycoprotein 2A radiotracers with PET imaging could provide a way to ... more The use of synaptic vesicle glycoprotein 2A radiotracers with PET imaging could provide a way to measure synaptic density quantitatively in living humans. 11 C-UCB-J ((R)-1-((3-(11 C-methyl-11 C)pyridin-4-yl) methyl)-4-(3,4,5-trifluorophenyl)pyrrolidin-2-one), previously developed and assessed in nonhuman primates and humans, showed excellent kinetic properties as a PET radioligand. However, it is labeled with the short half-life isotope 11 C. We developed a new tracer, an 18 F-labeled difluoro-analog of UCB-J (18 F-SynVesT-1, also known as 18 F-SDM-8), which displayed favorable properties in monkeys. The purpose of this first-inhuman study was to assess the kinetic and binding properties of 18 F-SynVesT-1 and compare with 11 C-UCB-J. Methods: Eight healthy volunteers participated in a baseline study of 18 F-SynVesT-1. Four of these subjects were also scanned after a blocking dose of the antiepileptic drug levetiracetam (20 mg/kg). Metabolite-corrected arterial input functions were measured. Regional time-activity curves were analyzed using 1-tissue-compartment (1TC) and 2-tissue-compartment (2TC) models and multilinear analysis 1 to compute total distribution volume (V T) and binding potential (BP ND). The centrum semiovale was used as a reference region. The Lassen plot was applied to compute levetiracetam occupancy and nondisplaceable distribution volume. SUV ratio-1 (SUVR-1) over several time windows was compared with BP ND. Results: Regional time-activity curves were fitted better with the 2TC model than the 1TC model, but 2TC V T estimates were unstable. The 1TC V T values matched well with those from the 2TC model (excluding the unstable values). Thus, 1TC was judged as the most useful model for quantitative analysis of 18 F-SynVesT-1 imaging data. The minimum scan time for stable V T measurement was 60 min. The rank order of V T and BP ND was similar between 18 F-SynVesT-1 and 11 C-UCB-J. Regional V T was slightly higher for 11 C-UCB-J, but BP ND was higher for 18 F-SynVesT-1, though these differences were not significant. Levetiracetam reduced the uptake of 18 F-SynVesT-1 in all regions and produced occupancy of 85.7%. The SUVR-1 of 18 F-SynVesT-1 from 60 to 90 min matched best with 1TC BP ND. Conclusion: The novel synaptic vesicle glycoprotein 2A tracer, 18 F-SynVesT-1, displays excellent kinetic and in vivo binding properties in humans and holds great potential for the imaging and quantification of synaptic density in neuropsychiatric disorders.
This was a first-inhuman study of the PET radiotracer 11 C-LSN3172176 for the muscarinic acetylch... more This was a first-inhuman study of the PET radiotracer 11 C-LSN3172176 for the muscarinic acetylcholine receptor subtype M1. The objectives of the study were to determine the appropriate kinetic model to quantify binding of the tracer to M1 receptors, and the reliability of the chosen quantification method. Methods: Six healthy subjects completed the test-retest protocol, and 5 healthy subjects completed the baseline-scopolamine blocking protocol. Multiple modeling methods were applied to calculate total distribution volume (V T) and nondisplaceable binding potential (BP ND) in various brain regions. The reference region was selected from the blocking study. The occupancy plot was applied to compute receptor occupancy by scopolamine and nondisplaceable distribution volume. Results: Tracer uptake was highest in the striatum, followed by neocortical regions and white matter, and lowest in the cerebellum. Regional time-activity curves were fitted well by all models. The 2-tissue-compartment (2TC) model fits were good, but the 2TC parameters often could not be reliably estimated. Because V T correlated well between the 2TC and 1-tissue-compartment (1TC) models after exclusion of unreliable estimates, the 1TC model was chosen as the most appropriate. The cerebellum showed the lowest V T , consistent with preclinical studies showing little to no specific binding in the region. Further, cerebellar V T did not change between baseline and blocking scans, indicating that the cerebellum is a suitable reference region. The simplified reference tissue model (SRTM) slightly underestimated 1TC BP ND , and the simplified reference tissue model 2 (SRTM2) improved BP ND estimation. An 80-min scan was sufficient to quantify V T and BP ND. The test-retest study showed excellent absolute test-retest variability for 1TC V T (#5%) and BP ND (#10%). In the baseline and blocking studies, occupancy values were lower in the striatum than in nonstriatal regions, as may be attributed to differences in regional acetylcholine concentrations. Conclusion: The 1TC and SRTM2 models are appropriate for quantitative analysis of 11 C-LSN3172176 imaging data. 11 C-LSN3172176 displayed excellent test-retest reproducibility and is a highly promising ligand to quantify M1 receptors in the human brain.
11 C-UCB-J is a PET tracer for synaptic vesicle glycoprotein 2A which may be a marker of synaptic... more 11 C-UCB-J is a PET tracer for synaptic vesicle glycoprotein 2A which may be a marker of synaptic density. To simplify the scan protocol, standardized uptake value ratios (SUVR) were compared to model-based binding potential (BP ND) to select the optimal time window in healthy and neuropsychiatric subjects. Methods: A total of 141 scans were acquired for 90min. Arterial blood sampling and metabolite analysis were conducted. SUVR-1 (centrum semiovale reference region) was computed for six 30min windows and compared with 1-tissue compartment model BP ND. Simulations were performed to assess the time dependency of SUVR-1. Results: Greater correlation and less bias were observed for SUVR-1 at later time windows for all subjects. Simulations showed that the agreement between SUVR-1 and BP ND is time-dependent. Conclusion: The 60-90min period provided the best match between SUVR-1 and BP ND (-1±7%), thus, a short scan is sufficient for accurate quantification of 11 C-UCB-J specific binding.
The κ-opioid receptor (KOR) is implicated in various neuropsychiatric disorders. We previously ev... more The κ-opioid receptor (KOR) is implicated in various neuropsychiatric disorders. We previously evaluated an agonist tracer, 11 C-GR103545, for PET imaging of KOR in humans. Although 11 C-GR103545 showed high brain uptake, good binding specificity, and selectivity for KOR, it displayed slow kinetics and relatively large test-retest variability of total distribution volume (V T) estimates (15%). Therefore, we set out to develop 2 novel KOR agonist radiotracers, 11 C-EKAP and 11 C-FEKAP. In nonhuman primates, both tracers exhibited faster kinetics than 11 C-GR103545 and comparable binding parameters to 11 C-GR103545. The aim of this study was to assess their kinetic and binding properties in humans. Methods: Six healthy subjects underwent 120-min test-retest PET scans with both 11 C-EKAP and 11 C-FEKAP. Metabolite-corrected arterial input functions were measured. Regional time-activity curves were generated for 14 regions of interest. One-tissue-compartment and 2tissue-compartment (2TC) models and the multilinear analysis-1 (MA1) method were applied to the regional time-activity curves to calculate V T. The time stability of V T and test-retest reproducibility were evaluated. Levels of specific binding, as measured by the nondisplaceable binding potential (BP ND) for the 3 tracers (11 C-EKAP, 11 C-FEKAP, and 11 C-GR103545), were compared using a graphical method. Results: For both tracers, regional time-activity curves were fitted well with the 2TC model and MA1 method (t* 5 20 min) but not with the 1-tissue-compartment model. Given the unreliably estimated parameters in several fits with the 2TC model and a good V T match between MA1 and 2TC, MA1 was chosen as the appropriate model for both tracers. Mean MA1 V T was highest for 11 C-GR103545, followed by 11 C-EKAP and then 11 C-FEKAP. The minimum scan time for stable V T measurement was 90 and 110 min for 11 C-EKAP and 11 C-FEKAP, respectively, compared with 140 min for 11 C-GR103545. The mean absolute test-retest variability in MA1 V T estimates was 7% and 18% for 11 C-EKAP and 11 C-FEKAP, respectively. BP ND levels were similar for 11 C-FEKAP and 11 C-GR103545 but were about 25% lower for 11 C-EKAP. Conclusion: The 2 novel KOR agonist tracers showed faster tissue kinetics than 11 C-GR103545. Even with a slightly lower BP ND , 11 C-EKAP is judged to be a better tracer for imaging and quantification of KOR in humans, on the basis of the shorter minimum scan time and the excellent test-retest reproducibility of regional V T .
ObjectiveParkinson disease is characterized by motor and nonmotor symptoms, reduced striatal dopa... more ObjectiveParkinson disease is characterized by motor and nonmotor symptoms, reduced striatal dopamine signaling, and loss of dopamine neurons in the substantia nigra. It is now known that the pathological process in Parkinson disease may begin decades before the clinical diagnosis and include a variety of neuronal alterations in addition to the dopamine system.MethodsThis study examined the density of all synapses with synaptic vesicle glycoprotein 2A (SV2A) in Parkinson disease subjects with mild bilateral disease (n = 12) and matched normal controls (n = 12) using in vivo high‐resolution positron emission tomographic imaging as well as postmortem autoradiography in an independent sample with Parkinson disease (n = 15) and normal controls (n = 13) in the substantia nigra and putamen.ResultsA group‐by‐brain region interaction effect (F10, 22 = 3.52, p = 0.007) was observed in the primary brain areas with in vivo SV2A binding. Post hoc analyses revealed that the Parkinson disease gro...
Background Metabotropic glutamate subtype 5 receptors (mGluR5) modulate synaptic transmission and... more Background Metabotropic glutamate subtype 5 receptors (mGluR5) modulate synaptic transmission and may constitute an important therapeutic target in Alzheimer’s disease (AD) by mediating the synaptotoxic action of amyloid-β oligomers. We utilized the positron emission tomography (PET) radioligand [18F]FPEB to investigate mGluR5 binding in early AD. Methods Sixteen individuals with amnestic mild cognitive impairment (MCI) due to AD or mild AD dementia who were positive for brain amyloid were compared to 15 cognitively normal (CN) participants who were negative for brain amyloid. Diagnostic groups were well balanced for age, sex, and education. Dynamic PET scans were acquired for 60 min, starting at 60 min after the initial administration of up to 185 MBq of [18F]FPEB using a bolus-plus-constant-infusion method (Kbol = 190 min). Equilibrium modeling with a cerebellum reference region was used to estimate [18F]FPEB binding (BPND) to mGluR5. Analyses were performed with and without corre...
BACKGROUND: Naltrexone is a nonselective opioid receptor antagonist used as a treatment for alcoh... more BACKGROUND: Naltrexone is a nonselective opioid receptor antagonist used as a treatment for alcohol use disorder. However, only modest clinical effects have been observed, possibly because of limited knowledge about the biological variables affecting the efficacy of naltrexone. We investigated the potential role of the kappa opioid receptor (KOR) in the therapeutic effect of naltrexone. METHODS: A total of 48 non-treatment-seeking heavy drinkers (16 women) who met DSM-IV criteria for alcohol dependence participated in two alcohol drinking paradigms (ADPs) separated by a week of open-label naltrexone (100 mg daily). Craving, assessed with the Alcohol Urge Questionnaire and the Yale Craving Scale, and drinking behavior were recorded in each ADP. Prior to naltrexone initiation, KOR availability was determined in the amygdala, hippocampus, pallidum, striatum, cingulate cortex, and prefrontal cortex using positron emission tomography with [ 11 C]LY2795050. RESULTS: Participants reported lower levels of craving (Yale Craving Scale: 211 6 1, p , .0001; Alcohol Urge Questionnaire: 26 6 0.6, p , .0001) and consumed fewer drinks (23.7 6 4, p , .0001) during the second ADP following naltrexone therapy. The observed reduction in drinking was negatively associated with baseline KOR availability in the striatum (p = .005), pallidum (p = .023), and cingulate cortex (p = .018). Voxelwise analysis identified clusters in the bilateral insula, prefrontal, and cingulate cortex associated with the reduction in drinking (p , .0001). In addition, KOR availability in all evaluated brain regions was associated with craving measured in both ADPs. CONCLUSIONS: The KOR is implicated in drinking and craving following naltrexone therapy in alcohol use disorder.
The positron emission tomography (PET) radioligand (−)-[ 18 F]flubatine is specific to nicotinic ... more The positron emission tomography (PET) radioligand (−)-[ 18 F]flubatine is specific to nicotinic acetylcholine receptors (nAChRs) and has promise for future investigation of the acetylcholine system in neuropathologies such as Alzheimer's disease, schizophrenia, and substance use disorders. The two goals of this work were to develop a simplified method for nAChR quantification with bolus plus constant infusion (B/I) (−)-[ 18 F]flubatine administration, and to assess the radioligand's sensitivity to acetylcholine fluctuations in humans. Healthy human subjects were imaged following either bolus injection (n = 8) or B/I (n = 4) administration of (−)-[ 18 F]flubatine. The metabolite-corrected input function in arterial blood was measured. Freefraction corrected distribution volumes (V T /f P) were estimated with modeling and graphical analysis techniques. Next, sensitivity to acetylcholine was assessed in two ways: 1. A bolus injection paradigm with two scans (n = 6), baseline (scan 1) and physostigmine challenge (scan 2; 1.5 mg over 60 min beginning 5 min prior to radiotracer injection); 2. A single scan B/I paradigm (n = 7) lasting up to 240 min with 1.5 mg physostigmine administered over 60 min beginning at 125 min of radiotracer infusion. Changes in V T /f P were measured. Baseline V T /f P values were 33.8 ± 3.3 mL/cm 3 in thalamus, 12.9 ± 1.6 mL/cm 3 in cerebellum, and ranged from 9.8 to 12.5 ☆ Funding: This research was supported in part by National Institutes of Health Grants T32 DA022975 (Hillmer), K01 MH092681
Animal studies indicate that the kappa-opioid receptor/dynorphin system plays an important role i... more Animal studies indicate that the kappa-opioid receptor/dynorphin system plays an important role in cocaine binges and stressinduced relapse. Our goal was to investigate changes in kappa-opioid receptor (KOR) availability in the human brain using positron emission tomography (PET), before and after a cocaine binge. We also investigated the correlation between KOR and stress-induced cocaine self-administration. PET imaging was performed with the KOR selective agonist [ 11 C]GR103545. Subjects with cocaine-use disorder (CUD) underwent PET scans and performed two types of cocaine self-administration sessions in the laboratory as follows: (1) choice sessions following a cold pressor test, to induce stress, and (2) binge dosing of cocaine. This allowed us investigate the following: (1) the association between KOR binding and a laboratory model of stress-induced relapse and (2) the change in KOR binding following a 3-day cocaine binge, which is thought to represent a change in endogenous dynorphin. A group of matched healthy controls was included to investigate between group differences in KOR availability. A significant association between [ 11 C] GR103545 binding and cocaine self-administration was seen: greater KOR availability was associated with more choices for cocaine. In addition, the 3-day cocaine binge significantly reduced [ 11 C]GR103545 binding by 18% in the striatum and 14% across brain regions. No difference in [ 11 C]GR103545 binding was found between the CUD subjects and matched controls. In the context of previous studies, these findings add to the growing evidence that pharmacotherapies targeting the KOR have the potential to significantly impact treatment development for cocaine-use disorder.
SummaryObjectiveBrivaracetam (BRV) and levetiracetam (LEV) are antiepileptic drugs that bind syna... more SummaryObjectiveBrivaracetam (BRV) and levetiracetam (LEV) are antiepileptic drugs that bind synaptic vesicle glycoprotein 2A (SV2A). In vitro and in vivo animal studies suggest faster brain penetration and SV2A occupancy (SO) after dosing with BRV than LEV. We evaluated human brain penetration and SO time course of BRV and LEV at therapeutically relevant doses using the SV2A positron emission tomography (PET) tracer 11C‐UCB‐J (EP0074; NCT02602860).MethodsHealthy volunteers were recruited into three cohorts. Cohort 1 (n = 4) was examined with PET at baseline and during displacement after intravenous BRV (100 mg) or LEV (1500 mg). Cohort 2 (n = 5) was studied during displacement and 4 hours postdose (BRV 50‐200 mg or LEV 1500 mg). Cohort 3 (n = 4) was examined at baseline and steady state after 4 days of twice‐daily oral dosing of BRV (50‐100 mg) and 4 hours postdose of LEV (250‐600 mg). Half‐time of 11C‐UCB‐J signal change was computed from displacement measurements. Half‐saturation...
Changes in the mesolimbic dopamine (DA) system are implicated in a range of neuropsychiatric cond... more Changes in the mesolimbic dopamine (DA) system are implicated in a range of neuropsychiatric conditions including addiction, depression and schizophrenia. Dysfunction of the neuroimmune system is often comorbid with such conditions and affects similar areas of the brain. The goal of this study was to use positron emission tomography with the dopamine D2 antagonist tracer, 11C-raclopride, to explore the effect of acute immune activation on striatal DA levels. DA transmission was modulated by an oral methylphenidate (MP) challenge in order to reliably elicit DA elevation. Elevation in DA concentration due to MP was estimated via change in 11C-raclopride binding potential from the baseline scan. Prior to the post-MP scan, subjects were pre-treated with either the immune activator lipopolysaccharide (LPS) or placebo (PBO) in a cross-over design. Immune activation was confirmed by measuring tumor necrosis factor alpha (TNFα), interleukin (IL)-6 and IL-8 concentration in plasma. Eight hea...
Proceedings of the National Academy of Sciences, 2019
Significance Posttraumatic stress disorder (PTSD) is an important risk factor for suicidal ideati... more Significance Posttraumatic stress disorder (PTSD) is an important risk factor for suicidal ideation, attempts, and death by suicide. Understanding of the biology underlying suicidality in PTSD is limited. In this study, we used positron emission tomography to evaluate the metabotropic glutamate receptor type 5 (mGluR5) as a potential treatment target and biomarker of suicidal ideation in individuals with PTSD and major depressive disorder (MDD). We found higher availability of mGluR5 in individuals with PTSD relative to healthy control and MDD groups. Furthermore, higher mGluR5 availability was associated with scan-day suicidal ideation among individuals with PTSD, but not MDD. Findings identify mGluR5 as a biomarker for intervention and potentially suicide risk management in PTSD.
Purpose Neuronal damage and synapse loss in the spinal cord (SC) have been implicated in spinal c... more Purpose Neuronal damage and synapse loss in the spinal cord (SC) have been implicated in spinal cord injury (SCI) and neurodegenerative disorders such as Amyotrophic Lateral Sclerosis (ALS). Current standards of diagnosis for SCI include CT or MRI imaging to evaluate injury severity. The current study explores the use of PET imaging with [11C]UCB-J, which targets the synaptic vesicle protein 2A (SV2A), in the human spinal cord, as a way to visualize synaptic density and integrity in vivo. Results First, simulations of baseline and blocking [11C]UCB-J HRRT scans were performed, based on SC dimensions and SV2A distribution to predict VT, VND, and VS values. Next, human baseline and blocking [11C]UCB-J HRRT images were used to estimate these values in the cervical SC (cSC). Simulation results had excellent agreement with observed values of VT, VND, and VS from the real human data, with baseline VT, VND, and VS of 3.07, 2.15, and 0.92 mL/cm3, respectively, with a BPND of 0.43. Lastly, w...
Acetylcholine (ACh) has distinct functional roles in striatum compared with cortex, and imbalance... more Acetylcholine (ACh) has distinct functional roles in striatum compared with cortex, and imbalance between these systems may contribute to neuropsychiatric disease. Preclinical studies indicate markedly higher ACh concentrations in the striatum. The goal of this work was to leverage positron emission tomography (PET) imaging estimates of drug occupancy at cholinergic receptors to explore ACh variation across the human brain, because these measures can be influenced by competition with endogenous neurotransmitter. PET scans were analyzed from healthy human volunteers (n = 4) and nonhuman primates (n = 2) scanned with the M1-selective radiotracer [11C]LSN3172176 in the presence of muscarinic antagonist scopolamine, and human volunteers (n = 10) scanned with the α4β2* nicotinic ligand (−)-[18F]flubatine during nicotine challenge. In all cases, occupancy estimates within striatal regions were consistently lower (M1/scopolamine human scans, 31 ± 3.4% occupancy in striatum, 43 ± 2.9% in ex...
Purpose: As stereotactic radiosurgery (SRS) and immunotherapy are increasingly used to treat brai... more Purpose: As stereotactic radiosurgery (SRS) and immunotherapy are increasingly used to treat brain metastases, incidence of radiation necrosis (RN) is consequently rising. Differentiating tumor regrowth (TR) from RN is vital in management but difficult to assess using MRI. We hypothesized that tumor methionine levels would be elevated given increased metabolism and high amino acid uptake, whereas RN would increase inflammation marked by upregulated translocator protein (PBR-TSPO), which can be quantified with specific PET tracers. Procedures: We performed a feasibility study to prospectively evaluate [11C]methionine and [11C]PBR28 using PET in 5 patients with 7 previously SRS-treated brain metastases demonstrating regrowth to differentiate TR from RN. Results: Sequential imaging with dual tracers was well-tolerated. [11C]methionine was accurate for detecting pathologically confirmed TR in 7/7 lesions, whereas [11C]PBR28 was only accurate in 3/7 lesions. Tumor PBR-TSPO expression was...
ABSTRACTPurposeTo investigate the synaptic vesicle glycoprotein 2A (SV2A) expression in the whole... more ABSTRACTPurposeTo investigate the synaptic vesicle glycoprotein 2A (SV2A) expression in the whole central nervous system and peripheral tissues, a metabolically stable SV2A radiotracer is desirable to minimize a potential confounding effect of radiometabolites. The aim of this study was to develop and evaluate a metabolically stable SV2A radiotracer, [18F]SDM-16, in nonhuman primate brains.MethodsThe racemic SDM-16 (4-(3,5-difluorophenyl)-1-((2-methyl-1H-imidazol-1-yl)methyl)pyrrolidin-2-one) was synthesized and assayed for in vitro SV2A binding affinity. We synthesized the enantiopure [18F]SDM-16 using the corresponding arylstannane precursor. Nonhuman primate brain PET was performed on a FOCUS 220 system. Arterial blood was drawn for metabolite analysis and construction of plasma input function. Regional time-activity curves (TACs) were evaluated with the one-tissue compartment (1TC) model to obtain the volume of distribution (VT). Binding potential (BPND) was calculated using eit...
BACKGROUND. Investigations of stress dysregulation in posttraumatic stress disorder (PTSD) have f... more BACKGROUND. Investigations of stress dysregulation in posttraumatic stress disorder (PTSD) have focused on peripheral cortisol, but none have examined cortisol in the human brain. This study used positron emission tomography (PET) to image 11β-hydroxysteroid dehydrogenase type 1 (11β-HSD1), a cortisol-producing enzyme, as a putative brain cortisol marker in PTSD. METHODS. Sixteen individuals with PTSD and 17 healthy, trauma-exposed controls (TCs) underwent PET imaging with [ 18 F] AS2471907, a radioligand for 11β-HSD1. RESULTS. Prefrontal-limbic 11β-HSD1 availability, estimated as [ 18 F]AS2471907 volume of distribution (V T), was significantly higher in the PTSD group compared with the TC group (β = 1.16, P = 0.0057). Lower prefrontal-limbic 11β-HSD1 availability was related to greater overall PTSD severity (R 2 = 0.27, P = 0.038) in the PTSD group. 11β-HSD1 availability was not related to plasma cortisol levels (R 2 = 0.026, P = 0.37). In a PTSD subset (n = 10), higher 11β-HSD1 availability was associated with higher availability of translocator protein (TSPO), a microglial marker (β = 4.40, P = 0.039). CONCLUSION. Higher brain cortisol-producing 11β-HSD1 in the PTSD group may represent a resilience-promoting neuroadaptation resulting in lower PTSD symptoms. Along with preliminary associations between 11β-HSD1 and TSPO, corroborating previous evidence of immune suppression in PTSD, these findings collectively challenge previous hypotheses of the deleterious effects of both excessive brain glucocorticoid and brain immune signaling in PTSD.
BackgroundAttempts to associate amyloid-β (Aβ) pathogenesis with synaptic loss in Alzheimer’s dis... more BackgroundAttempts to associate amyloid-β (Aβ) pathogenesis with synaptic loss in Alzheimer’s disease (AD) have thus far been limited to small numbers of postmortem studies. Aβ plaque burden is not well-correlated with indices of clinical severity or neurodegeneration—at least in the dementia stage—as deposition of Aβ reaches a ceiling. In this study, we examined in vivo the association between fibrillar Aβ deposition and synaptic density in early AD using positron emission tomography (PET). We hypothesized that global Aβ deposition would be more strongly inversely associated with hippocampal synaptic density in participants with amnestic mild cognitive impairment (aMCI; a stage of continued Aβ accumulation) compared to those with dementia (a stage of relative Aβ plateau).MethodsWe measured SV2A binding ([11C]UCB-J) and Aβ deposition ([11C]PiB) in 14 participants with aMCI due to AD and 24 participants with mild AD dementia. Distribution volume ratios (DVR) with a cerebellar referen...
The use of synaptic vesicle glycoprotein 2A radiotracers with PET imaging could provide a way to ... more The use of synaptic vesicle glycoprotein 2A radiotracers with PET imaging could provide a way to measure synaptic density quantitatively in living humans. 11 C-UCB-J ((R)-1-((3-(11 C-methyl-11 C)pyridin-4-yl) methyl)-4-(3,4,5-trifluorophenyl)pyrrolidin-2-one), previously developed and assessed in nonhuman primates and humans, showed excellent kinetic properties as a PET radioligand. However, it is labeled with the short half-life isotope 11 C. We developed a new tracer, an 18 F-labeled difluoro-analog of UCB-J (18 F-SynVesT-1, also known as 18 F-SDM-8), which displayed favorable properties in monkeys. The purpose of this first-inhuman study was to assess the kinetic and binding properties of 18 F-SynVesT-1 and compare with 11 C-UCB-J. Methods: Eight healthy volunteers participated in a baseline study of 18 F-SynVesT-1. Four of these subjects were also scanned after a blocking dose of the antiepileptic drug levetiracetam (20 mg/kg). Metabolite-corrected arterial input functions were measured. Regional time-activity curves were analyzed using 1-tissue-compartment (1TC) and 2-tissue-compartment (2TC) models and multilinear analysis 1 to compute total distribution volume (V T) and binding potential (BP ND). The centrum semiovale was used as a reference region. The Lassen plot was applied to compute levetiracetam occupancy and nondisplaceable distribution volume. SUV ratio-1 (SUVR-1) over several time windows was compared with BP ND. Results: Regional time-activity curves were fitted better with the 2TC model than the 1TC model, but 2TC V T estimates were unstable. The 1TC V T values matched well with those from the 2TC model (excluding the unstable values). Thus, 1TC was judged as the most useful model for quantitative analysis of 18 F-SynVesT-1 imaging data. The minimum scan time for stable V T measurement was 60 min. The rank order of V T and BP ND was similar between 18 F-SynVesT-1 and 11 C-UCB-J. Regional V T was slightly higher for 11 C-UCB-J, but BP ND was higher for 18 F-SynVesT-1, though these differences were not significant. Levetiracetam reduced the uptake of 18 F-SynVesT-1 in all regions and produced occupancy of 85.7%. The SUVR-1 of 18 F-SynVesT-1 from 60 to 90 min matched best with 1TC BP ND. Conclusion: The novel synaptic vesicle glycoprotein 2A tracer, 18 F-SynVesT-1, displays excellent kinetic and in vivo binding properties in humans and holds great potential for the imaging and quantification of synaptic density in neuropsychiatric disorders.
This was a first-inhuman study of the PET radiotracer 11 C-LSN3172176 for the muscarinic acetylch... more This was a first-inhuman study of the PET radiotracer 11 C-LSN3172176 for the muscarinic acetylcholine receptor subtype M1. The objectives of the study were to determine the appropriate kinetic model to quantify binding of the tracer to M1 receptors, and the reliability of the chosen quantification method. Methods: Six healthy subjects completed the test-retest protocol, and 5 healthy subjects completed the baseline-scopolamine blocking protocol. Multiple modeling methods were applied to calculate total distribution volume (V T) and nondisplaceable binding potential (BP ND) in various brain regions. The reference region was selected from the blocking study. The occupancy plot was applied to compute receptor occupancy by scopolamine and nondisplaceable distribution volume. Results: Tracer uptake was highest in the striatum, followed by neocortical regions and white matter, and lowest in the cerebellum. Regional time-activity curves were fitted well by all models. The 2-tissue-compartment (2TC) model fits were good, but the 2TC parameters often could not be reliably estimated. Because V T correlated well between the 2TC and 1-tissue-compartment (1TC) models after exclusion of unreliable estimates, the 1TC model was chosen as the most appropriate. The cerebellum showed the lowest V T , consistent with preclinical studies showing little to no specific binding in the region. Further, cerebellar V T did not change between baseline and blocking scans, indicating that the cerebellum is a suitable reference region. The simplified reference tissue model (SRTM) slightly underestimated 1TC BP ND , and the simplified reference tissue model 2 (SRTM2) improved BP ND estimation. An 80-min scan was sufficient to quantify V T and BP ND. The test-retest study showed excellent absolute test-retest variability for 1TC V T (#5%) and BP ND (#10%). In the baseline and blocking studies, occupancy values were lower in the striatum than in nonstriatal regions, as may be attributed to differences in regional acetylcholine concentrations. Conclusion: The 1TC and SRTM2 models are appropriate for quantitative analysis of 11 C-LSN3172176 imaging data. 11 C-LSN3172176 displayed excellent test-retest reproducibility and is a highly promising ligand to quantify M1 receptors in the human brain.
11 C-UCB-J is a PET tracer for synaptic vesicle glycoprotein 2A which may be a marker of synaptic... more 11 C-UCB-J is a PET tracer for synaptic vesicle glycoprotein 2A which may be a marker of synaptic density. To simplify the scan protocol, standardized uptake value ratios (SUVR) were compared to model-based binding potential (BP ND) to select the optimal time window in healthy and neuropsychiatric subjects. Methods: A total of 141 scans were acquired for 90min. Arterial blood sampling and metabolite analysis were conducted. SUVR-1 (centrum semiovale reference region) was computed for six 30min windows and compared with 1-tissue compartment model BP ND. Simulations were performed to assess the time dependency of SUVR-1. Results: Greater correlation and less bias were observed for SUVR-1 at later time windows for all subjects. Simulations showed that the agreement between SUVR-1 and BP ND is time-dependent. Conclusion: The 60-90min period provided the best match between SUVR-1 and BP ND (-1±7%), thus, a short scan is sufficient for accurate quantification of 11 C-UCB-J specific binding.
The κ-opioid receptor (KOR) is implicated in various neuropsychiatric disorders. We previously ev... more The κ-opioid receptor (KOR) is implicated in various neuropsychiatric disorders. We previously evaluated an agonist tracer, 11 C-GR103545, for PET imaging of KOR in humans. Although 11 C-GR103545 showed high brain uptake, good binding specificity, and selectivity for KOR, it displayed slow kinetics and relatively large test-retest variability of total distribution volume (V T) estimates (15%). Therefore, we set out to develop 2 novel KOR agonist radiotracers, 11 C-EKAP and 11 C-FEKAP. In nonhuman primates, both tracers exhibited faster kinetics than 11 C-GR103545 and comparable binding parameters to 11 C-GR103545. The aim of this study was to assess their kinetic and binding properties in humans. Methods: Six healthy subjects underwent 120-min test-retest PET scans with both 11 C-EKAP and 11 C-FEKAP. Metabolite-corrected arterial input functions were measured. Regional time-activity curves were generated for 14 regions of interest. One-tissue-compartment and 2tissue-compartment (2TC) models and the multilinear analysis-1 (MA1) method were applied to the regional time-activity curves to calculate V T. The time stability of V T and test-retest reproducibility were evaluated. Levels of specific binding, as measured by the nondisplaceable binding potential (BP ND) for the 3 tracers (11 C-EKAP, 11 C-FEKAP, and 11 C-GR103545), were compared using a graphical method. Results: For both tracers, regional time-activity curves were fitted well with the 2TC model and MA1 method (t* 5 20 min) but not with the 1-tissue-compartment model. Given the unreliably estimated parameters in several fits with the 2TC model and a good V T match between MA1 and 2TC, MA1 was chosen as the appropriate model for both tracers. Mean MA1 V T was highest for 11 C-GR103545, followed by 11 C-EKAP and then 11 C-FEKAP. The minimum scan time for stable V T measurement was 90 and 110 min for 11 C-EKAP and 11 C-FEKAP, respectively, compared with 140 min for 11 C-GR103545. The mean absolute test-retest variability in MA1 V T estimates was 7% and 18% for 11 C-EKAP and 11 C-FEKAP, respectively. BP ND levels were similar for 11 C-FEKAP and 11 C-GR103545 but were about 25% lower for 11 C-EKAP. Conclusion: The 2 novel KOR agonist tracers showed faster tissue kinetics than 11 C-GR103545. Even with a slightly lower BP ND , 11 C-EKAP is judged to be a better tracer for imaging and quantification of KOR in humans, on the basis of the shorter minimum scan time and the excellent test-retest reproducibility of regional V T .
ObjectiveParkinson disease is characterized by motor and nonmotor symptoms, reduced striatal dopa... more ObjectiveParkinson disease is characterized by motor and nonmotor symptoms, reduced striatal dopamine signaling, and loss of dopamine neurons in the substantia nigra. It is now known that the pathological process in Parkinson disease may begin decades before the clinical diagnosis and include a variety of neuronal alterations in addition to the dopamine system.MethodsThis study examined the density of all synapses with synaptic vesicle glycoprotein 2A (SV2A) in Parkinson disease subjects with mild bilateral disease (n = 12) and matched normal controls (n = 12) using in vivo high‐resolution positron emission tomographic imaging as well as postmortem autoradiography in an independent sample with Parkinson disease (n = 15) and normal controls (n = 13) in the substantia nigra and putamen.ResultsA group‐by‐brain region interaction effect (F10, 22 = 3.52, p = 0.007) was observed in the primary brain areas with in vivo SV2A binding. Post hoc analyses revealed that the Parkinson disease gro...
Background Metabotropic glutamate subtype 5 receptors (mGluR5) modulate synaptic transmission and... more Background Metabotropic glutamate subtype 5 receptors (mGluR5) modulate synaptic transmission and may constitute an important therapeutic target in Alzheimer’s disease (AD) by mediating the synaptotoxic action of amyloid-β oligomers. We utilized the positron emission tomography (PET) radioligand [18F]FPEB to investigate mGluR5 binding in early AD. Methods Sixteen individuals with amnestic mild cognitive impairment (MCI) due to AD or mild AD dementia who were positive for brain amyloid were compared to 15 cognitively normal (CN) participants who were negative for brain amyloid. Diagnostic groups were well balanced for age, sex, and education. Dynamic PET scans were acquired for 60 min, starting at 60 min after the initial administration of up to 185 MBq of [18F]FPEB using a bolus-plus-constant-infusion method (Kbol = 190 min). Equilibrium modeling with a cerebellum reference region was used to estimate [18F]FPEB binding (BPND) to mGluR5. Analyses were performed with and without corre...
BACKGROUND: Naltrexone is a nonselective opioid receptor antagonist used as a treatment for alcoh... more BACKGROUND: Naltrexone is a nonselective opioid receptor antagonist used as a treatment for alcohol use disorder. However, only modest clinical effects have been observed, possibly because of limited knowledge about the biological variables affecting the efficacy of naltrexone. We investigated the potential role of the kappa opioid receptor (KOR) in the therapeutic effect of naltrexone. METHODS: A total of 48 non-treatment-seeking heavy drinkers (16 women) who met DSM-IV criteria for alcohol dependence participated in two alcohol drinking paradigms (ADPs) separated by a week of open-label naltrexone (100 mg daily). Craving, assessed with the Alcohol Urge Questionnaire and the Yale Craving Scale, and drinking behavior were recorded in each ADP. Prior to naltrexone initiation, KOR availability was determined in the amygdala, hippocampus, pallidum, striatum, cingulate cortex, and prefrontal cortex using positron emission tomography with [ 11 C]LY2795050. RESULTS: Participants reported lower levels of craving (Yale Craving Scale: 211 6 1, p , .0001; Alcohol Urge Questionnaire: 26 6 0.6, p , .0001) and consumed fewer drinks (23.7 6 4, p , .0001) during the second ADP following naltrexone therapy. The observed reduction in drinking was negatively associated with baseline KOR availability in the striatum (p = .005), pallidum (p = .023), and cingulate cortex (p = .018). Voxelwise analysis identified clusters in the bilateral insula, prefrontal, and cingulate cortex associated with the reduction in drinking (p , .0001). In addition, KOR availability in all evaluated brain regions was associated with craving measured in both ADPs. CONCLUSIONS: The KOR is implicated in drinking and craving following naltrexone therapy in alcohol use disorder.
The positron emission tomography (PET) radioligand (−)-[ 18 F]flubatine is specific to nicotinic ... more The positron emission tomography (PET) radioligand (−)-[ 18 F]flubatine is specific to nicotinic acetylcholine receptors (nAChRs) and has promise for future investigation of the acetylcholine system in neuropathologies such as Alzheimer's disease, schizophrenia, and substance use disorders. The two goals of this work were to develop a simplified method for nAChR quantification with bolus plus constant infusion (B/I) (−)-[ 18 F]flubatine administration, and to assess the radioligand's sensitivity to acetylcholine fluctuations in humans. Healthy human subjects were imaged following either bolus injection (n = 8) or B/I (n = 4) administration of (−)-[ 18 F]flubatine. The metabolite-corrected input function in arterial blood was measured. Freefraction corrected distribution volumes (V T /f P) were estimated with modeling and graphical analysis techniques. Next, sensitivity to acetylcholine was assessed in two ways: 1. A bolus injection paradigm with two scans (n = 6), baseline (scan 1) and physostigmine challenge (scan 2; 1.5 mg over 60 min beginning 5 min prior to radiotracer injection); 2. A single scan B/I paradigm (n = 7) lasting up to 240 min with 1.5 mg physostigmine administered over 60 min beginning at 125 min of radiotracer infusion. Changes in V T /f P were measured. Baseline V T /f P values were 33.8 ± 3.3 mL/cm 3 in thalamus, 12.9 ± 1.6 mL/cm 3 in cerebellum, and ranged from 9.8 to 12.5 ☆ Funding: This research was supported in part by National Institutes of Health Grants T32 DA022975 (Hillmer), K01 MH092681
Animal studies indicate that the kappa-opioid receptor/dynorphin system plays an important role i... more Animal studies indicate that the kappa-opioid receptor/dynorphin system plays an important role in cocaine binges and stressinduced relapse. Our goal was to investigate changes in kappa-opioid receptor (KOR) availability in the human brain using positron emission tomography (PET), before and after a cocaine binge. We also investigated the correlation between KOR and stress-induced cocaine self-administration. PET imaging was performed with the KOR selective agonist [ 11 C]GR103545. Subjects with cocaine-use disorder (CUD) underwent PET scans and performed two types of cocaine self-administration sessions in the laboratory as follows: (1) choice sessions following a cold pressor test, to induce stress, and (2) binge dosing of cocaine. This allowed us investigate the following: (1) the association between KOR binding and a laboratory model of stress-induced relapse and (2) the change in KOR binding following a 3-day cocaine binge, which is thought to represent a change in endogenous dynorphin. A group of matched healthy controls was included to investigate between group differences in KOR availability. A significant association between [ 11 C] GR103545 binding and cocaine self-administration was seen: greater KOR availability was associated with more choices for cocaine. In addition, the 3-day cocaine binge significantly reduced [ 11 C]GR103545 binding by 18% in the striatum and 14% across brain regions. No difference in [ 11 C]GR103545 binding was found between the CUD subjects and matched controls. In the context of previous studies, these findings add to the growing evidence that pharmacotherapies targeting the KOR have the potential to significantly impact treatment development for cocaine-use disorder.
SummaryObjectiveBrivaracetam (BRV) and levetiracetam (LEV) are antiepileptic drugs that bind syna... more SummaryObjectiveBrivaracetam (BRV) and levetiracetam (LEV) are antiepileptic drugs that bind synaptic vesicle glycoprotein 2A (SV2A). In vitro and in vivo animal studies suggest faster brain penetration and SV2A occupancy (SO) after dosing with BRV than LEV. We evaluated human brain penetration and SO time course of BRV and LEV at therapeutically relevant doses using the SV2A positron emission tomography (PET) tracer 11C‐UCB‐J (EP0074; NCT02602860).MethodsHealthy volunteers were recruited into three cohorts. Cohort 1 (n = 4) was examined with PET at baseline and during displacement after intravenous BRV (100 mg) or LEV (1500 mg). Cohort 2 (n = 5) was studied during displacement and 4 hours postdose (BRV 50‐200 mg or LEV 1500 mg). Cohort 3 (n = 4) was examined at baseline and steady state after 4 days of twice‐daily oral dosing of BRV (50‐100 mg) and 4 hours postdose of LEV (250‐600 mg). Half‐time of 11C‐UCB‐J signal change was computed from displacement measurements. Half‐saturation...
Changes in the mesolimbic dopamine (DA) system are implicated in a range of neuropsychiatric cond... more Changes in the mesolimbic dopamine (DA) system are implicated in a range of neuropsychiatric conditions including addiction, depression and schizophrenia. Dysfunction of the neuroimmune system is often comorbid with such conditions and affects similar areas of the brain. The goal of this study was to use positron emission tomography with the dopamine D2 antagonist tracer, 11C-raclopride, to explore the effect of acute immune activation on striatal DA levels. DA transmission was modulated by an oral methylphenidate (MP) challenge in order to reliably elicit DA elevation. Elevation in DA concentration due to MP was estimated via change in 11C-raclopride binding potential from the baseline scan. Prior to the post-MP scan, subjects were pre-treated with either the immune activator lipopolysaccharide (LPS) or placebo (PBO) in a cross-over design. Immune activation was confirmed by measuring tumor necrosis factor alpha (TNFα), interleukin (IL)-6 and IL-8 concentration in plasma. Eight hea...
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Papers by Nabeel Nabulsi