Gil Leurquin-Sterk

Positive Association Between Limbic Metabotropic Glutamate Receptor 5 Availability and Novelty-Seeking Temperament in Humans: a 18F-FPEB PET study

Heritable temperament traits have been linked to several neuropsychiatric illnesses, including disorders associated with metabotropic glutamate receptor 5 (mGluR5) and dopaminergic dysfunctions. Considering its modulating effect on neurotransmission, we hypothesized that cerebral mGluR5 availability is associated with temperament traits in healthy humans. Methods: Forty-four nonsmoking healthy volunteers (mean age ± SD, 40 ± 14 y; age range, 22–66 y; 22 women) were included in this cross-sectional investigation. Brain mGluR5 availability was quantified on both a voxel-by-voxel and a volume-of-interest basis using the total distribution volume of the radioligand 18F-3-fluoro-5-[(pyridin-3-yl)ethynyl]benzonitrile (18F-FPEB) with 90-min dynamic PET and arterial input function. Moreover, glutamate–glutamine concentrations in the anterior cingulate cortex were measured using MR spectroscopy. These measures were related to the temperament traits of the 240-item Cloninger temperament and character inventory using a regression analysis with age and sex as nuisance variables. Results: High novelty-seeking temperament was robustly associated with increased mGluR5 availability in various regions including the thalamus (r = 0.71; the strongest association), amygdala, parahippocampus, insula, anterior and posterior cingulate cortex, and several primary sensory areas (all r > 0.58; P < 0.05, corrected for familywise error). These associations were specific because no correlations were found with other temperament scales or with spectroscopic measures of glutamatergic transmission. Conclusion: Overall, these data posit mGluR5 in key paralimbic areas as a strong determinant of the temperament trait novelty seeking. These data add to our understanding of how brain neurochemistry accounts for the variation in human behavior and strongly support further research on mGluR5 as a potential therapeutic target in neuropsychiatric disorders associated with abnormal novelty-seeking behaviors.

Kinetic modeling and long‐term test‐retest reproducibility of the mGluR5 PET tracer 18F‐FPEB in human brain

18F‐FPEB is a promising PET tracer for studying the metabotropic glutamate subtype 5 receptor (mGluR5) expression in neuropsychiatric disorders. To assess the potential of 18F‐FPEB for longitudinal mGluR5 evaluation in patient studies, we evaluated the long‐term test‐retest reproducibility using various kinetic models in the human brain. Nine healthy volunteers underwent consecutive scans separated by a 6‐month period. Dynamic PET was combined with arterial sampling and radiometabolite analysis. Total distribution volume (VT) and nondisplaceable binding potential (BPND) were derived from a two‐tissue compartment model without constraints (2TCM) and with constraining the K1/k2 ratio to the value of either cerebellum (2TCM‐CBL) or pons (2TCM‐PONS). The effect of fitting different functions to the tracer parent fractions and reducing scan duration were assessed. Regional absolute test‐retest variability (aTRV), coefficient of repeatability (CR) and intraclass correlation coefficient (ICC) were computed. The 2TCM‐CBL showed best fits. The mean 6‐month aTRV of VT ranged from 8 to 13% (CR < 25%) with ICC > 0.6 for all kinetic models. BPND from 2TCM‐CBL with a sigmoid fit for the parent fractions showed the best reproducibility, with aTRV ≤ 7% (CR < 16%) and ICC > 0.9 in most regions. Reducing the scan duration from 90 to 60 min did not affect reproducibility. These results demonstrate for the first time that 18F‐FPEB brain PET has good long‐term reproducibility, therefore validating its use to monitor mGluR5 expression in longitudinal clinical studies. We suggest a 2TCM‐CBL with fitting a sigmoid function to the parent fractions to be optimal for this tracer. Synapse, 2016.

Preclinical Evaluation and Quantification of 18F-FPEB as a Radioligand for PET Imaging of the Metabotropic Glutamate Receptor 5

The metabotropic glutamate receptor 5 (mGluR5) is a high-interest target for PET imaging because it plays a role in several pathologies, including addiction, schizophrenia, and fragile X syndrome. Methods: We studied the pharmacokinetics of 18F-FPEB (3-18F-fluoro-5-(2-pyridinylethynyl)benzonitrile), a selective PET radioligand for mGluR5, and used it to quantify mGluR5 in rat brain. Quantification was performed using both arterial sampling in combination with compartment models and simplified reference methods. The simplified reference tissue model (SRTM), Ichises original multi-linear reference tissue model (MRTMO), and Logan noninvasive were tested as reference models with nondisplaceable binding (BPND) as outcome parameter. Additionally, test–retest scans were obtained in 6 animals. Results: 18F-FPEB uptake in rat brain was consistent with its known distribution. No radiometabolites were present in the brain, and binding was specific as shown in blocking experiments, which also confirmed the cerebellum as a viable reference region. A 2-tissue-compartment model was used to determine BPND for the striatum (11.7 ± 1.5), nucleus accumbens (10.6 ± 2.0), hippocampus (9.0 ± 1.2), cortex (7.2 ± 1.0), and thalamus (4.0 ± 0.9). Reference methods were able to estimate these values with small bias (<2%). Test–retest analysis showed high repeatability between scans below 6%, also for shorter scan durations of 30 and 60 min. Conclusion: Because of its favorable reversible kinetics, high specificity, and absence of brain radiometabolites 18F-FPEB proves a highly useful tracer for in vivo visualization of the mGluR5 in rat brain. Moreover, reference tissue models allow noninvasive, rapid scanning with good test–retest.

What we observe in vivo is not always what we see in vitro: development and validation of 11C-JNJ-42491293, a novel radioligand for mGluR2

Positive allosteric modulators (PAM) of metabotropic glutamate receptor 2 (mGluR2) are a potential therapy for anxiety, schizophrenia, and addiction. Aside from pathophysiologic imaging studies, an mGluR2 PET tracer would enable confirmation of sufficient central target engagement and assist dose selection for proof-of-concept studies of PAM compounds. 11C-JNJ-42491293, a novel high-affinity radioligand (human 50% inhibitory concentration = 9.6 nM) for the PAM site of mGluR2, was evaluated as a selective mGluR2 PAM PET tracer. Methods: In vitro and ex vivo autoradiography binding experiments in Wistar and in mGluR2 knockout and wildtype rats as well as in vivo biodistribution and brain PET imaging studies in wildtype and mGluR2 knockout rats in a primate and in humans were performed. Results: In vitro binding studies and in vivo imaging studies in Wistar rats showed moderate brain uptake, with a distribution pattern fully consistent with the reported intracerebral distribution of mGluR2. Given these promising findings, biodistribution, dosimetry, and brain kinetic modeling of 11C-JNJ-42491293 were determined in humans. Because of an unexpected high myocardial retention, additional 11C-JNJ-42491293 imaging studies were performed in recently available mGluR2 knockout and wildtype rats and in a monkey using a structurally distinct mGluR2 PAM ligand with affinity for the same site, demonstrating off-target binding in vivo that could not have been anticipated from previous in vitro experiments. To date, the target of this non-mGluR2 tracer binding remains unknown. Conclusion: On the basis of in vivo selectivity issues suggested by human distribution and demonstrated in knockout rat models, 11C-JNJ-42491293 was considered unsuitable as a specific PET ligand for in vivo imaging of mGluR2. These results emphasize the importance of elaborated in vitro/in vivo comparative studies and, when available, validation with knockout animal models or structurally distinct ligands with affinity for the same site, in radiotracer development.

Ethyl Glucuronide in Hair Is an Accurate Biomarker of Chronic Excessive Alcohol Use in Patients With Alcoholic Cirrhosis

*Department of Gastroenterology & Hepatology, University Hospitals KU Leuven, Leuven, Belgium; Division of Gastroenterology & Hepatology, Department of Internal Medicine, Maastricht University Medical Center, Maastricht, the Netherlands; Toxicological Center, University of Antwerp, Antwerp, Belgium; kDepartment of Psychiatry, University Hospital Brussels, Brussels, Belgium; Department of Nuclear Medicine and Molecular Imaging, University Hospitals KU Leuven, Leuven, Belgium; Department of Gastroenterology & Hepatology, University Hospital of Antwerp, Antwerp, Belgium; **TDM and Toxicology Laboratory, ZNA Stuivenberg, Antwerp, Belgium; Department of Abdominal Transplantation Surgery, University Hospitals KU Leuven, Leuven, Belgium; Department of Pathology, University Hospitals KU Leuven, Leuven, Belgium

Lower Limbic Metabotropic Glutamate Receptor 5 Availability in Alcohol Dependence

Animal studies suggest an important role for the metabotropic glutamate receptor subtype 5 (mGlu5) in the pathophysiology of alcohol dependence, but direct human evidence is lacking. The goal of this study was to investigate cerebral mGlu5 availability in alcohol-dependent subjects versus controls using 18F-3-fluoro-5-[(pyridin-3-yl)ethynyl]benzonitrile (18F-FPEB) PET. Methods: Dynamic 90-min 18F-FPEB scans combined with arterial blood sampling were acquired for 16 recently abstinent alcohol-dependent subjects and 32 age-matched controls. Regional mGlu5 availability was quantified by the 18F-FPEB total distribution volume using both a voxel-by-voxel and a volume-of-interest analysis with partial-volume effect correction. Alcohol consumption within the last 3 mo was assessed by questionnaires and by hair ethyl glucuronide analysis. Craving was assessed using the Desire for Alcohol Questionnaire. Results: mGlu5 availability was lower in mainly limbic regions of alcohol-dependent subjects than in controls (P < 0.05, familywise error–corrected), ranging from 14% in the posterior cingulate cortex to 36% in the caudate nucleus. Lower mGlu5 availability was associated with higher hair ethyl glucuronide levels for most regions and was related to a lower level of craving specifically in the middle frontal gyrus, cingulate cortex, and inferolateral temporal lobe. Conclusion: These findings provide human in vivo evidence that limbic mGlu5 has a role in the pathophysiology of alcohol dependence, possibly involved in a compensatory mechanism helping to reduce craving during abstinence.

Glutamatergic Biomarkers for Cocaine Addiction: A Longitudinal Study Using MR Spectroscopy and mGluR5 PET in Self-Administering Rats

Cocaine addiction is a disorder that still lacks diagnostic biomarkers or effective pharmacotherapy. We present findings on a rat model of cocaine self-administration that was followed up longitudinally using the metabotropic glutamate receptor type 5 (mGluR5) tracer 18F-3-fluoro-5-[(pyridin-3-yl)ethynyl]benzonitrile (18F-FPEB) PET, proton MR spectroscopy (1H-MRS), and behavioral tests. Methods: Forty-two Wistar rats were scanned with 18F-FPEB PET and 1H-MRS before and after sucrose or intravenous cocaine self-administration, during withdrawal, and during relapse. All animals performed a rodent Iowa Gambling Task (rIGT) at baseline to evaluate decision making. Baseline values were used in a mixed model to assess associations with later cocaine use, and follow-up measurements were compared with the values before drug exposure. Results: Preexposure rIGT scores were significantly related to both cocaine and sucrose use during the drug-exposure phase. However, only cocaine self-administration induced a decrease in 18F-FPEB binding. This decrease was most pronounced bilaterally in the hippocampus, where mGluR5 availability correlated with the amount of cocaine used during relapse. Compared with the sucrose group, a larger decrease was observed in the hippocampo–prefrontal cortex pathway. Preexposure glutamate and glycine concentrations in the prefrontal cortex were significantly associated with cocaine use during the drug-exposure phase. Moreover, prefrontal glutamate exhibited a distinct, reversible decrease when animals had access to cocaine but not sucrose. Conclusion: Baseline values of prefrontal glutamate and glycine are associated with future cocaine use. Furthermore, baseline rIGT scores are associated with both sucrose and cocaine. Finally, both glutamate concentration and mGluR5 availability decrease during exposure to cocaine.

Effects of alcohol exposure on the glutamatergic system: a combined longitudinal 18F-FPEB and 1H-MRS study in rats: Biomarkers for alcohol use

To study the role of the glutamatergic system in alcohol abuse disorders, we conducted a longitudinal study of alcohol exposure in rats. Rats were followed with 1H‐magnetic resonance spectroscopy during a baseline, exposure and abstinence phase to measure prefrontal glutamate and glycine concentrations. Additionally, 18F‐FPEB PET was used to assess metabotropic glutamate receptor 5 (mGluR5) availability before and after exposure to alcohol or saccharin. Finally, individual anxiety levels were assessed with an open field test at baseline. We found that the total distance traveled in the open field test and mGluR5 availability in the nucleus accumbens were associated with future alcohol preference (P = 0.02). Furthermore, prefrontal glutamate concentration decreased significantly during alcohol exposure (> −1.25 ± 0.55 mmol/L, P = 0.038), but normalized during abstinence. Finally, 18F‐FPEB PET showed that self‐administration of alcohol resulted in decreased mGluR5 availability in the hippocampus (>14 ± 3 percent, pCluster = 0.047) and amygdala (>16 ± 2 percent, pCluster = 0.004), whereas saccharin induced decreases in the prefrontal cortex (>11 ± 1 percent, pCluster = 0.035) and hippocampus (>15 ± 2 percent, pCluster = 0.003). A direct comparison of both groups showed differences in mGluR5 availability in the bilateral striatum (−2 ± 4 percent versus +2 ± 2 percent, pCluster = <0.0001) and hippocampus (−16 ± 4 percent versus −5 ± 4 percent, pCluster = <0.0001). In conclusion, this study corroborates the anxiolytic effect of alcohol and provides evidence for mGluR5 downregulation in the amygdala as potential underlying mechanism. Saccharin and alcohol differentially affect prefrontal glutamate concentrations, but both induce an mGluR5 decrease, potentially to modulate the dopamine release elicited by these drugs.

Cerebral dopaminergic and glutamatergic transmission relate to different subjective responses of acute alcohol intake: an in vivo multimodal imaging study

Converging preclinical evidence links extrastriatal dopamine release and glutamatergic transmission via the metabotropic glutamate receptor 5 (mGluR5) to the rewarding properties of alcohol. To date, human evidence is lacking on how and where in the brain these processes occur. Mesocorticolimbic dopamine release upon intravenous alcohol administration and mGluR5 availability were measured in 11 moderate social drinkers by single‐session [18F]fallypride and [18F]FPEB positron emission tomography, respectively. Additionally, baseline and postalcohol glutamate and glutamine levels in the anterior cingulate cortex (ACC) were measured by using proton‐magnetic resonance spectroscopy. To investigate differences in reward domains linked to both neurotransmitters, regional imaging data were related to subjective alcohol responses. Alcohol induced significant [18F]fallypride displacement in the prefrontal cortex (PFC), temporal and parietal cortices and thalamus (P < 0.05, corrected for multiple comparisons). Dopamine release in the ACC and orbitofrontal and ventromedial PFCs were correlated with subjective ‘liking’ and ‘wanting’ effects (P < 0.05). In contrast, baseline mGluR5 availability was positively correlated with the ‘high’ effect of alcohol in dorsolateral, ventrolateral and ventromedial PFCs and in the medial temporal lobe, thalamus and caudate nucleus (P < 0.05). Although neither proton‐magnetic resonance spectroscopy glutamate nor glutamine levels were affected by alcohol, baseline ACC glutamate levels were negatively associated with the alcohol ‘liking’ effect (P < 0.003). These data reveal new mechanistic understanding and differential neurobiological underpinnings of the effects of acute alcohol consumption on human behavior. Specifically, prefrontal dopamine release may encode alcohol ‘liking’ and ‘wanting’ effects in specific areas underlying value processing and motivation, whereas mGluR5 availability in distinct prefrontal–temporal–subcortical regions is more related to the alcohol ‘high’ effect.