Age, gender and body-mass-index relationships with in vivo CB1 receptor availability in healthy humans measured with [11C]OMAR PET.

Brain cannabinoid 1 receptors (CB1Rs) contribute importantly to the regulation of autonomic tone, appetite, mood and cognition. Inconsistent results have been reported from positron emission tomography (PET) studies using different radioligands to examine relationships between age, gender and body mass index (BMI) and CB1R availability in healthy individuals. In this study, we examined these variables in 58 healthy individuals (age range: 18-55 years; 44 male; BMI=27.01±5.56), the largest cohort of subjects studied to date using the CB1R PET ligand [11C]OMAR. There was a significant decline in CB1R availability (VT) with age in the pallidum, cerebellum and posterior cingulate. Adjusting for BMI, age-related decline in VT remained significant in the posterior cingulate among males, and in the cerebellum among women. CB1R availability was higher in women compared to men in the thalamus, pallidum and posterior cingulate. Adjusting for age, CB1R availability negatively correlated with BMI in women but not men. These findings differ from those reported using [11C]OMAR and other radioligands such as [18F]FMPEP-d2 and [18F]MK-9470. Although reasons for these seemingly divergent findings are unclear, the choice of PET radioligand and range of BMI in the current dataset may contribute to the observed differences. This study highlights the need for cross-validation studies using both [11C]OMAR and [18F]FMPEP-d2 within the same cohort of subjects.

Systemic inflammation enhances stimulant-induced striatal dopamine elevation in tobacco smokers.

Immune-brain interactions influence the pathophysiology of addiction. Lipopolysaccharide (LPS)-induced systemic inflammation produces effects on reward-related brain regions and the dopamine system. We previously showed that LPS amplifies dopamine elevation induced by methylphenidate (MP), compared to placebo (PBO), in eight healthy controls. However, the effects of LPS on the dopamine system of tobacco smokers have not been explored. The goal of Study 1 was to replicate previous findings in an independent cohort of tobacco smokers. The goal of Study 2 was to combine tobacco smokers with the aforementioned eight healthy controls to examine the effect of LPS on dopamine elevation in a heterogenous sample for power and effect size determination. Eight smokers were each scanned with [11C]raclopride positron emission tomography three times-at baseline, after administration of LPS (0.8 ng/kg, intravenously) and MP (40 mg, orally), and after administration of PBO and MP, in a double-blind, randomized order. Dopamine elevation was quantified as change in [11C]raclopride binding potential (ΔBPND) from baseline. A repeated-measures ANOVA was conducted to compare LPS and PBO conditions. Smokers and healthy controls were well-matched for demographics, drug dosing, and scanning parameters. In Study 1, MP-induced striatal dopamine elevation was significantly higher following LPS than PBO (p = 0.025, 18 ± 2.9 % vs 13 ± 2.7 %) for smokers. In Study 2, MP-induced striatal dopamine elevation was also significantly higher under LPS than under PBO (p < 0.001, 18 ± 1.6 % vs 11 ± 1.5 %) in the combined sample. Smoking status did not interact with the effect of condition. This is the first study to translate the phenomenon of amplified dopamine elevation after experimental activation of the immune system to an addicted sample which may have implications for drug reinforcement, seeking, and treatment.

Preliminary in vivo evidence of lower hippocampal synaptic density in cannabis use disorder.

Cannabis is one of the most commonly and widely used psychoactive drugs. The rates of cannabis misuse have been increasing. Therefore, understanding the effects of cannabis use on the brain is important. Adolescent and adult rodents exposed to repeated administration of cannabinoids show persistent microstructural changes in the hippocampus both pre- and post-synaptically. Whether similar alterations exist in human cannabis users, has not yet been demonstrated in vivo. Positron emission tomography (PET) and [11C]UCB-J, a radioligand for the synaptic vesicle glycoprotein 2A (SV2A), were used to study hippocampal synaptic integrity in vivo in an equal number (n = 12) of subjects with DSM-5 cannabis use disorder (CUD) and matched healthy controls (HC). Arterial sampling was used to measure plasma input function. [11C]UCB-J binding potential (BPND) was estimated using a one-tissue (1T) compartment model with centrum semiovale as the reference region. Hippocampal function was assessed using a verbal memory task. Relative to HCs, CUDs showed significantly lower [11C]UCB-J BPND in the hippocampus (~10%, p = 0.008, effect size 1.2) and also performed worse on the verbal memory task. These group differences in hippocampal BPND persisted after correction for volume differences (p = 0.013), and correction for both age and volume (p = 0.03). We demonstrate, for the first time, in vivo evidence of lower hippocampal synaptic density in cannabis use disorder. These results are consistent with the microstructural findings from experimental studies with cannabinoids in animals, and studies of hippocampal macrostructure in human with CUD. Whether the lower hippocampal synaptic density resolves with abstinence warrants further study.

Nicotine Patch Alters Patterns of Cigarette Smoking-Induced Dopamine Release: Patterns Relate to Biomarkers Associated With Treatment Response.

INTRODUCTION: Tobacco smoking is a major public health burden. The first-line pharmacological treatment for tobacco smoking is nicotine replacement therapy (eg, the nicotine patch (NIC)). Nicotine acts on nicotinic-acetylcholine receptors on dopamine terminals to release dopamine in the ventral and dorsal striatum encoding reward and habit formation, respectively. AIMS AND METHODS: To better understand treatment efficacy, a naturalistic experimental design combined with a kinetic model designed to characterize smoking-induced dopamine release in vivo was used. Thirty-five tobacco smokers (16 female) wore a NIC (21 mg, daily) for 1-week and a placebo patch (PBO) for 1-week in a randomized, counter-balanced order. Following 1-week under NIC and then overnight abstinence, smokers participated in a 90-minute [11C]raclopride positron emission tomography scan and smoked a cigarette while in the scanner. Identical procedures were followed for the PBO scan. A time-varying kinetic model was used at the voxel level to model transient dopamine release peaking instantaneously at the start of the stimulus and decaying exponentially. Magnitude and spatial extent of dopamine release were estimated. Smokers were subcategorized by nicotine dependence level and nicotine metabolism rate. RESULTS: Dopamine release magnitude was enhanced by NIC in ventral striatum and diminished by NIC in dorsal striatum. More-dependent smokers activated more voxels than the less-dependent smokers under both conditions. Under PBO, fast metabolizers activated more voxels in ventral striatum and fewer voxels in dorsal striatum compared to slow metabolizers. CONCLUSIONS: These findings demonstrate that the model captured a pattern of transient dopamine responses to cigarette smoking which may be different across smoker subgroup categorizations. IMPLICATIONS: This is the first study to show that NIC alters highly localized patterns of cigarette smoking-induced dopamine release and that levels of nicotine dependence and nicotine clearance rate contribute to these alterations. This current work included a homogeneous subject sample with regards to demographic and smoking variables, as well as a highly sensitive model capable of detecting significant acute dopamine transients. The findings of this study add support to the recent identification of biomarkers for predicting the effect of nicotine replacement therapies on dopamine function which could help refine clinical practice for smoking cessation.

Recently Abstinent Smokers Exhibit Mood-Associated Dopamine Dysfunction in the Ventral Striatum Compared to Nonsmokers: A [11C]-(+)-PHNO PET Study.

INTRODUCTION: Chronic nicotine exposure desensitizes dopamine responses in animals, but it is not known if this occurs in human tobacco smokers. Deficits in dopamine function are likely to make smoking cessation difficult. We used positron emission tomography (PET) brain imaging with the dopamine D2/3 receptor agonist radioligand [11C]-(+)-PHNO to determine if abstinent smokers exhibit less amphetamine-induced dopamine release in the ventral striatum than nonsmokers, and whether this was associated with clinical correlates of smoking cessation. METHODS: Baseline [11C]-(+)-PHNO scans were acquired from smokers (n = 22, 7 female, abstinent 11 ± 9 days) and nonsmokers (n = 20, 7 female). A subset of thirty-seven participants (18 smokers) received oral amphetamine (0.5 mg/kg) three hours before a second [11C]-(+)-PHNO scan. Binding potential (BPND) (i.e., D2/3 receptor availability) was estimated at baseline and postamphetamine in the ventral striatum. Amphetamine-induced percent change in BPND was calculated to reflect dopamine release. Subjects also completed the Center for Epidemiologic Studies Depression Scale (CES-D). RESULTS: There were no group differences in baseline BPND. Amphetamine-induced percent change in BPND in the ventral striatum was significantly lower in abstinent smokers compared to nonsmokers (p=0.019; d=0.82). Higher CES-D scores were significantly associated with lower ventral striatal percent change in BPND for abstinent smokers (rs=-0.627, p=0.025). CONCLUSIONS: In conclusion, abstinent smokers exhibited significantly less amphetamine-induced dopamine release in the ventral striatum than nonsmokers. In abstinent smokers, worse mood was significantly associated with less striatal dopamine release. Our findings highlight a potential neural mechanism that may underlie negative mood symptoms during early abstinence. IMPLICATIONS: This study combined quantitative PET imaging and an amphetamine challenge to examine striatal dopamine function during early smoking cessation attempts. The findings demonstrate that recently abstinent tobacco smokers exhibit significant, mood-associated striatal dopamine dysfunction compared to nonsmokers. This study advances our knowledge of the neurobiology underlying early quit attempts, and bridges novel neural findings with clinically relevant symptoms of smoking cessation. These results may explain the challenge of maintaining long-term abstinence from smoking, and can lend insight into the development of treatment strategies for smoking cessation.

PET Imaging Estimates of Regional Acetylcholine Concentration Variation in Living Human Brain.

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 α4ß2* 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 extrastriatal regions, p = 0.0094; nonhuman primate scans, 42 ± 26% vs. 69 ± 28%, p < 0.0001; α4ß2*/nicotine scans, 67 ± 15% vs. 74 ± 16%, p = 0.0065), indicating higher striatal ACh concentration. Subject-level measures of these concentration differences were estimated, and whole-brain images of regional ACh concentration gradients were generated. These results constitute the first in vivo estimates of regional variation in ACh concentration in the living brain and offer a novel experimental method to assess potential ACh imbalances in clinical populations.

Assessment of test-retest reproducibility of [18F]SynVesT-1, a novel radiotracer for PET imaging of synaptic vesicle glycoprotein 2A.

PURPOSE: Synaptic abnormalities are associated with many brain disorders. Recently, we developed a novel synaptic vesicle glycoprotein 2A (SV2A) radiotracer [18F]SynVesT-1 and demonstrated its excellent imaging and binding properties in nonhuman primates. The aim of this study was to perform dosimetry calculations in nonhuman primates and to evaluate this tracer in humans and assess its test-retest reliability in comparison with [11C]UCB-J. METHODS: Three rhesus monkeys underwent whole body dynamic PET scanning to estimate the absorbed dose. PET scans in six healthy human subjects were acquired. Time-activity curves (TACs) were generated with defined regions of interest (ROI). Reproducibility of distribution volume (VT) values and its sensitivity to scan duration were assessed with the one-tissue compartment (1TC) model. Non-displaceable binding potential (BPND) was calculated using centrum semiovale as the reference region. RESULTS: The dosimetry study showed high uptake in the urinary bladder and brain. In humans, [18F]SynVesT-1 displayed high uptake with maximum SUV of ~10 and appropriate kinetics with a quick rise in tracer uptake followed by a gradual clearance. Mean 1TC VT values (mL/cm3) ranged from 3.4 (centrum semiovale) to 19.6 (putamen) and were similar to those of [11C]UCB-J. Regional BPND values were 2.7-4.7 in gray matter areas, and mean BPND values across all ROIs were ~ 21% higher than those of [11C]UCB-J. The absolute test-retest variability of VT and BPND was excellent (< 9%) across all brain regions. CONCLUSIONS: [18F]SynVesT-1 demonstrates outstanding characteristics in humans: fast and high brain uptake, appropriate tissue kinetics, high levels of specific binding, and excellent test-retest reproducibility of binding parameters. As such, [18F]SynVesT-1 is proved to be a favorable radiotracer for SV2A imaging and quantification in humans.

Inverse changes in raphe and cortical 5-HT1B receptor availability after acute tryptophan depletion in healthy human subjects.

Serotonergic neurotransmission plays a key role in the pathophysiology and treatment of various neuropsychiatric diseases. The purpose of this study was to investigate changes in serotonergic neurotransmission after acute tryptophan depletion (ATD) using positron emission tomography (PET) with [11 C]P943, a 5-HT1B receptor radioligand previously shown to be sensitive to changes in 5-HT. Five healthy subjects were scanned on a high resolution PET scanner twice on the same day, before and approximately 5 hours after ingesting capsules containing an amino acid mixture that lacks tryptophan. For each scan, emission data were acquired for 120 min after intravenous bolus injection of [11 C]P943. Binding potential (BPND ) values were estimated from parametric images using the second version of the multilinear reference tissue model (MRTM2, t* = 20 min) with cerebellar grey matter used as a reference region. The change in [11 C]P943 binding (ΔBPND , %) was calculated as (BPND,post  - BPND,pre )/(BPND,pre ) × 100, and correlation analysis was performed to measure linear associations of ΔBPND between raphe and other regions of interest (ROIs). ΔBPND ranged from -6% to 45% in the raphe, with positive values indicating reduced competition from 5-HT. In cortical regions, ΔBPND ranged from -28% to 7%. While these changes did not reach significance, there were significant negative correlations of ΔBPND of the raphe with those of cerebral cortical regions and the thalamus (e.g., r = -.96, p = .011 for average cortex). These findings support the hypothesis that raphe serotonin is a critical modulator of cortical serotonin release via projecting neurons in healthy human subjects.

Further evaluation of [11C]MP-10 as a radiotracer for phosphodiesterase 10A: PET imaging study in rhesus monkeys and brain tissue metabolite analysis.

[(11)C]MP-10 is a potent and specific PET tracer previously shown to be suitable for imaging the phosphodiesterase 10A (PDE10A) in baboons with reversible kinetics and high specific binding. However, another report indicated that [(11)C]MP-10 displayed seemingly irreversible kinetics in rhesus monkeys, potentially due to the presence of a radiolabeled metabolite capable of penetrating the blood-brain-barrier (BBB) into the brain. This study was designed to address the discrepancies between the species by re-evaluating [(11)C]MP-10 in vivo in rhesus monkey with baseline scans to assess tissue uptake kinetics and self-blocking scans with unlabeled MP-10 to determine binding specificity. Ex vivo studies with one rhesus monkey and 4 Sprague-Dawley rats were also performed to investigate the presence of radiolabeled metabolites in the brain. Our results indicated that [(11)C]MP-10 displayed reversible uptake kinetics in rhesus monkeys, albeit slower than in baboons. Administration of unlabeled MP-10 reduced the binding of [(11)C]MP-10 in a dose-dependent manner in all brain regions including the cerebellum. Consequently, the cerebellum appeared not to be a suitable reference tissue in rhesus monkeys. Regional volume of distribution (VT) was mostly reliably derived with the multilinear analysis (MA1) method. In ex vivo studies in the monkey and rats only negligible amount of radiometabolites was seen in the brain of either species. In summary, results from the present study strongly support the suitability of [(11)C]MP-10 as a radiotracer for PET imaging and quantification of PDE10A in nonhuman primates.

First-in-Human Study of 18F-SynVesT-2: An SV2A PET Imaging Probe with Fast Brain Kinetics and High Specific Binding.

PET imaging of synaptic vesicle glycoprotein 2A allows for noninvasive quantification of synapses. This first-in-human study aimed to evaluate the kinetics, test-retest reproducibility, and extent of specific binding of a recently developed synaptic vesicle glycoprotein 2A PET ligand, (R)-4-(3-(18F-fluoro)phenyl)-1-((3-methylpyridin-4-yl)methyl)pyrrolidine-2-one (18F-SynVesT-2), with fast brain kinetics. Methods: Nine healthy volunteers participated in this study and were scanned on a High Resolution Research Tomograph scanner with 18F-SynVesT-2. Five volunteers were scanned twice on 2 different days. Five volunteers were rescanned with preinjected levetiracetam (20 mg/kg, intravenously). Arterial blood was collected to calculate the plasma free fraction and generate the arterial input function. Individual MR images were coregistered to a brain atlas to define regions of interest for generating time-activity curves, which were fitted with 1- and 2-tissue-compartment (1TC and 2TC) models to derive the regional distribution volume (V T). The regional nondisplaceable binding potential (BP ND) was calculated from 1TC V T, using the centrum semiovale (CS) as the reference region. Results: 18F-SynVesT-2 was synthesized with high molar activity (187 ± 69 MBq/nmol, n = 19). The parent fraction of 18F-SynVesT-2 in plasma was 28% ± 8% at 30 min after injection, and the plasma free fraction was high (0.29 ± 0.04). 18F-SynVesT-2 entered the brain quickly, with an SUVpeak of 8 within 10 min after injection. Regional time-activity curves fitted well with both the 1TC and the 2TC models; however, V T was estimated more reliably using the 1TC model. The 1TC V T ranged from 1.9 ± 0.2 mL/cm3 in CS to 7.6 ± 0.8 mL/cm3 in the putamen, with low absolute test-retest variability (6.0% ± 3.6%). Regional BP ND ranged from 1.76 ± 0.21 in the hippocampus to 3.06 ± 0.29 in the putamen. A 20-min scan was sufficient to provide reliable V T and BP ND Conclusion: 18F-SynVesT-2 has fast kinetics, high specific uptake, and low nonspecific uptake in the brain. Consistent with the nonhuman primate results, the kinetics of 18F-SynVesT-2 is faster than the kinetics of 11C-UCB-J and 18F-SynVesT-1 in the human brain and enables a shorter dynamic scan to derive physiologic information on cerebral blood flow and synapse density.

Studies of the metabotropic glutamate receptor 5 radioligand [¹¹C]ABP688 with N-acetylcysteine challenge in rhesus monkeys.

Detecting changes in receptor binding at the metabotropic glutamate receptor 5 (mGluR5) with the PET allosteric antagonist, [¹¹C]ABP688, may be valuable for studying dysfunctional glutamate transmission associated with psychiatric illnesses. This study was designed to validate the findings of a recent pilot study in baboons which reported a significant global decrease from baseline [¹¹C]ABP688 binding after increasing endogenous glutamate with 50 mg/kg N-acetylcysteine (NAC), with no change from test to retest. In rhesus monkeys (n = 5), paired [¹¹C]ABP688 scans were performed on the same day on the Focus-220 as follows (n = 3 per group): test-retest, baseline-NAC (50 mg/kg), and baseline-NAC (100 mg/kg). Multiple modeling methods were evaluated for kinetic analysis to estimate the total volume of distribution (VT ) and non-displaceable binding potential (BP(ND)) in regions-of-interest (ROIs), with the cerebellum gray matter (CGM) as the reference region. There was an increasing trend from test to retest BP(ND) across ROIs (13%). NAC (50 mg/kg and 100 mg/kg) increased VT (5% and 19%) and decreased BP(ND) (3% and 10%), respectively, significant only for VT in ROIs at the 100 mg/kg dose. High intersubject variability in BP(ND) was comparable to that reported in the baboon study. However, interpretability of BP(ND) is difficult with increases in VT in the CGM reference region at the higher NAC dose. Additionally, the net reduction in BP(ND) from the baseline-NAC scans may be obscured due to observed increases in test-retest BP(ND). Thus, we did not strictly replicate the findings in the baboon study based on BP(ND).

Imaging brain cortisol regulation in PTSD with a target for 11ß-hydroxysteroid dehydrogenase type 1.

BACKGROUNDInvestigations 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 11ß-hydroxysteroid dehydrogenase type 1 (11ß-HSD1), a cortisol-producing enzyme, as a putative brain cortisol marker in PTSD.METHODSSixteen individuals with PTSD and 17 healthy, trauma-exposed controls (TCs) underwent PET imaging with [18F]AS2471907, a radioligand for 11ß-HSD1.RESULTSPrefrontal-limbic 11ß-HSD1 availability, estimated as [18F]AS2471907 volume of distribution (VT), was significantly higher in the PTSD group compared with the TC group (ß = 1.16, P = 0.0057). Lower prefrontal-limbic 11ß-HSD1 availability was related to greater overall PTSD severity (R2 = 0.27, P = 0.038) in the PTSD group. 11ß-HSD1 availability was not related to plasma cortisol levels (R2 = 0.026, P = 0.37). In a PTSD subset (n = 10), higher 11ß-HSD1 availability was associated with higher availability of translocator protein (TSPO), a microglial marker (ß = 4.40, P = 0.039).CONCLUSIONHigher brain cortisol-producing 11ß-HSD1 in the PTSD group may represent a resilience-promoting neuroadaptation resulting in lower PTSD symptoms. Along with preliminary associations between 11ß-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.FUNDINGBrain and Behavior Research Foundation Independent Investigator Grant, National Institute of Mental Health grants F30MH116607 and R01MH110674, the Veterans Affairs National Center for PTSD, the Gustavus and Louise Pfeiffer Foundation Fellowship, Clinical and Translational Science Awards grant UL1 TR000142 from the NIH National Center for Advancing Translational Science.

First in-human PET study and kinetic evaluation of [18F]AS2471907 for imaging 11ß-hydroxysteroid dehydrogenase type 1.

11ß-Hydroxysteroid dehydrogenase type 1 (11ß-HSD1) catalyzes enzymatic conversion of cortisone into the stress hormone cortisol. This first-in-human brain imaging study characterizes the kinetic modeling and test-retest reproducibility of [18F]AS2471907, a novel PET radiotracer for 11ß-HSD1. Eight individuals underwent one 180-min (n = 4) or two 240-min (n = 4) [18F]AS2471907 PET brain scans (12 total) acquired on the high-resolution research tomograph (HRRT) scanner with arterial blood sampling. Imaging data were modeled with 1-tissue (1T) and 2-tissue (2T) compartment models and with multilinear analysis (MA1) to estimate [18F]AS2471907 availability (VT). [18F]AS2471907 demonstrated high, heterogeneous uptake throughout the brain. Of the compartment models, 2T best described [18F]AS2471907 data. Estimates of VT were highly correlated between 2T and MA1 (t* = 30 min) with MA1 yielding VT values ranging from 3.2 ± 1.0 mL/cm3 in the caudate to 15.7 ± 4.2 mL/cm3 in the occipital cortex. The median absolute test-retest variability of 16 ± 5% and high intraclass correlation coefficient (ICC) values of 0.67-0.97 across regions indicate fair test-retest reliability but large intersubject variability. VT estimates using 180 min were within 10% of estimates using full acquisition time. In summary, [18F]AS2471907 exhibits reasonable kinetic properties for imaging 11ß-HSD1 in human brain.

PET Imaging of Pancreatic Dopamine D2 and D3 Receptor Density with 11C-(+)-PHNO in Type 1 Diabetes.

Type 1 diabetes mellitus (T1DM) has traditionally been characterized by a complete destruction of ß-cell mass (BCM); however, there is growing evidence of possible residual BCM present in T1DM. Given the absence of in vivo tools to measure BCM, routine clinical measures of ß-cell function (e.g., C-peptide release) may not reflect BCM. We previously demonstrated the potential utility of PET imaging with the dopamine D2 and D3 receptor agonist 3,4,4a,5,6,10b-hexahydro-2H-naphtho[1,2-b][1,4]oxazin-9-ol (11C-(+)-PHNO) to differentiate between healthy control (HC) and T1DM individuals. Methods: Sixteen individuals participated (10 men, 6 women; 9 HCs, 7 T1DMs). The average duration of diabetes was 18 ± 6 y (range, 14-30 y). Individuals underwent PET/CT scanning with a 120-min dynamic PET scan centered on the pancreas. One- and 2-tissue-compartment models were used to estimate pancreas and spleen distribution volume. Reference region approaches (spleen as reference) were also investigated. Quantitative PET measures were correlated with clinical outcome measures. Immunohistochemistry was performed to examine colocalization of dopamine receptors with endocrine hormones in HC and T1DM pancreatic tissue. Results: C-peptide release was not detectable in any T1DM individuals, whereas proinsulin was detectable in 3 of 5 T1DM individuals. Pancreas SUV ratio minus 1 (SUVR-1) (20-30 min; spleen as reference region) demonstrated a statistically significant reduction (-36.2%) in radioligand binding (HCs, 5.6; T1DMs, 3.6; P = 0.03). Age at diagnosis correlated significantly with pancreas SUVR-1 (20-30 min) (R2 = 0.67, P = 0.025). Duration of diabetes did not significantly correlate with pancreas SUVR-1 (20-30 min) (R2 = 0.36, P = 0.16). Mean acute C-peptide response to arginine at maximal glycemic potentiation did not significantly correlate with SUVR-1 (20-30 min) (R2 = 0.57, P = 0.05), nor did mean baseline proinsulin (R2 = 0.45, P = 0.10). Immunohistochemistry demonstrated colocalization of dopamine D3 receptor and dopamine D2 receptor in HCs. No colocalization of the dopamine D3 receptor or dopamine D2 receptor was seen with somatostatin, glucagon, or polypeptide Y. In a separate T1DM individual, no immunostaining was seen with dopamine D3 receptor, dopamine D2 receptor, or insulin antibodies, suggesting that loss of endocrine dopamine D3 receptor and dopamine D2 receptor expression accompanies loss of ß-cell functional insulin secretory capacity. Conclusion: Thirty-minute scan durations and SUVR-1 provide quantitative outcome measures for 11C-(+)-PHNO, a dopamine D3 receptor-preferring agonist PET radioligand, to differentiate BCM in T1DM and HCs.

The Kappa Opioid Receptor Is Associated With Naltrexone-Induced Reduction of Drinking and Craving.

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 [11C]LY2795050. RESULTS: Participants reported lower levels of craving (Yale Craving Scale: -11 ± 1, p < .0001; Alcohol Urge Questionnaire: -6 ± 0.6, p < .0001) and consumed fewer drinks (-3.7 ± 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.

Evaluation of 11C-LSN3172176 as a Novel PET Tracer for Imaging M1 Muscarinic Acetylcholine Receptors in Nonhuman Primates.

The M1 muscarinic acetylcholine receptor (mAChR) plays an important role in learning and memory, and therefore is a target for development of drugs for treatment of cognitive impairments in Alzheimer disease and schizophrenia. The availability of M1-selective radiotracers for PET will help in developing therapeutic agents by providing an imaging tool for assessment of drug dose-receptor occupancy relationship. Here we report the synthesis and evaluation of 11C-LSN3172176 (ethyl 4-(6-(methyl-11C)-2-oxoindolin-1-yl)-[1,4'-bipiperidine]-1'-carboxylate) in nonhuman primates. Methods:11C-LSN3172176 was radiolabeled via the Suzuki-Miyaura cross-coupling method. PET scans in rhesus macaques were acquired for 2 h with arterial blood sampling and metabolite analysis to measure the input function. Blocking scans with scopolamine (50 µg/kg) and the M1-selective agent AZD6088 (0.67 and 2 mg/kg) were obtained to assess tracer binding specificity and selectivity. Regional brain time-activity curves were analyzed with the 1-tissue-compartment model and the multilinear analysis method (MA1) to calculate regional distribution volume. Nondisplaceable binding potential values were calculated using the cerebellum as a reference region. Results:11C-LSN3172176 was synthesized with greater than 99% radiochemical purity and high molar activity. In rhesus monkeys, 11C-LSN3172176 metabolized rapidly (29% ± 6% parent remaining at 15 min) and displayed fast kinetics and extremely high uptake in the brain. Imaging data were modeled well with the 1-tissue-compartment model and MA1 methods. MA1-derived distribution volume values were high (range, 10-81 mL/cm3) in all known M1 mAChR-rich brain regions. Pretreatment with scopolamine and AZD6088 significantly reduced the brain uptake of 11C-LSN3172176, thus demonstrating its binding specificity and selectivity in vivo. The cerebellum appeared to be a suitable reference region for derivation of nondisplaceable binding potential, which ranged from 2.42 in the globus pallidus to 8.48 in the nucleus accumbens. Conclusion:11C-LSN3172176 exhibits excellent in vivo binding and imaging characteristics in nonhuman primates and appears to be the first appropriate radiotracer for PET imaging of human M1 AChR.

Evaluation of PET Brain Radioligands for Imaging Pancreatic ß-Cell Mass: Potential Utility of 11C-(+)-PHNO.

Type 1 diabetes mellitus (T1DM) is characterized by a loss of ß-cells in the islets of Langerhans of the pancreas and subsequent deficient insulin secretion in response to hyperglycemia. Development of an in vivo test to measure ß-cell mass (BCM) would greatly enhance the ability to track diabetes therapies. ß-cells and neurologic tissues have common cellular receptors and transporters, therefore, we screened brain radioligands for their ability to identify ß-cells. Methods: We examined a ß-cell gene atlas for endocrine pancreas receptor targets and cross-referenced these targets with brain radioligands that were available at our institution. Twelve healthy control subjects and 2 T1DM subjects underwent dynamic PET/CT scans with 6 tracers. Results: The D2/D3 receptor agonist radioligand 11C-(+)-4-propyl-9-hydroxynaphthoxazine (PHNO) was the only radioligand to demonstrate sustained uptake in the pancreas with high contrast versus abdominal organs such as the kidneys, liver, and spleen, based on the first 30 min of data. Mean SUV from 20 to 30 min demonstrated high uptake of 11C-(+)-PHNO in healthy controls (SUV, 13.8) with a 71% reduction in a T1DM subject with undetectable levels of C-peptide (SUV, 4.0) and a 20% reduction in a T1DM subject with fasting C-peptide level of 0.38 ng/mL (SUV, 11.0). SUV in abdominal organs outside the pancreas did not show measurable differences between the control and T1DM subjects, suggesting that the changes in SUV of 11C-(+)-PHNO may be specific to changes in the pancreas between healthy controls and T1DM subjects. When D3 and D2 antagonists were used in nonhuman primates, specific pancreatic binding (SUVR-1) of 11C-PHNO was reduced by 57% and 38%, respectively. Conclusion:11C-(+)-PHNO is a potential marker of BCM, with 2:1 binding of D3 receptors over D2 receptors. Further in vitro and in vivo studies to establish D2/D3 receptor specificity to ß-cells is warranted to characterize 11C-(+)-PHNO as a candidate for clinical measurement of BCM in healthy control and diabetic subjects.

Evaluation of the Lysophosphatidic Acid Receptor Type 1 Radioligand 11C-BMT-136088 for Lung Imaging in Rhesus Monkeys.

The lysophosphatidic acid receptor type 1 (LPA1) is 1 of 6 known receptors of the extracellular signaling molecule lysophosphatidic acid. It mediates effects such as cell proliferation, migration, and differentiation. In the lung, LPA1 is involved in pathways leading, after lung tissue injury, to pulmonary fibrosis instead of normal healing, by mediating fibroblast recruitment and vascular leakage. Thus, a LPA1 PET radiotracer may be useful for studying lung fibrosis or for developing LPA1-targeting drugs. We developed and evaluated the radiotracer 11C-BMT-136088 (1-(4'-(3-methyl-4-(((1(R)-(3-11C-methylphenyl)ethoxy)carbonyl)amino)isoxazol-5-yl)-[1,1'-biphenyl]-4-yl)cyclopropane-1-carboxylic acid) in rhesus monkeys to image LPA1 in the lung in vivo with PET. Methods: The study consisted of 3 parts: test-retest scans; self-saturation to estimate the tracer's in vivo dissociation constant, nondisplaceable volume of distribution (VND), and nondisplaceable binding potential (BPND); and dosimetry. In the first 2 parts, the radiotracer was administered using a bolus-plus-infusion protocol, the arterial input function was measured, and the animals underwent 2 scans per day separated by about 4 h. Lung regions of interest were segmented, and the tissue density estimated, from CT images. A fixed blood volume correction was applied. The tracer volume of distribution (VT) was estimated using multilinear analysis 1 (MA1) or equilibrium analysis (EA). Results:11C-BMT-136088 baseline VT was 1.83 ± 0.16 (MA1, n = 5) or 2.1 ± 0.55 (EA, n = 7) mL of plasma per gram of tissue in the left and right lung regions of interest, with a test-retest variability of -6% (MA1, n = 1) or -1% ± 14% (EA, n = 2). For the self-saturation study, 11C-BMT-136088 VND and BPND were estimated to be 0.9 ± 0.08 mL of plasma per gram of tissue and 1.1 ± 0.14, respectively. The unlabeled drug dose and plasma concentration leading to a 50% reduction of 11C-BMT-136088 specific binding were 73 ± 30 nmol/kg and 28 ± 12 nM, respectively. The average plasma free fraction was 0.2%; thus, the tracer's in vivo dissociation constant was estimated to be 55 pM. For the dosimetry study, the highest organ dose was in the liver (43.1 ± 4.9 and 68.9 ± 9.4 µSv/MBq in reference human male and female phantoms, respectively), and the effective dose equivalent was 6.9 ± 0.6 and 8.7 ± 0.6 µSv/MBq, respectively. Conclusion: Specific binding of 11C-BMT-136088 can be reliably measured to quantify LPA1 in the lungs of rhesus monkeys in vivo.

Assessing Synaptic Density in Alzheimer Disease With Synaptic Vesicle Glycoprotein 2A Positron Emission Tomographic Imaging.

Importance: Synaptic loss is well established as the major structural correlate of cognitive impairment in Alzheimer disease (AD). The ability to measure synaptic density in vivo could accelerate the development of disease-modifying treatments for AD. Synaptic vesicle glycoprotein 2A is an essential vesicle membrane protein expressed in virtually all synapses and could serve as a suitable target for synaptic density. Objective: To compare hippocampal synaptic vesicle glycoprotein 2A (SV2A) binding in participants with AD and cognitively normal participants using positron emission tomographic (PET) imaging. Design, Setting, and Participants: This cross-sectional study recruited 10 participants with AD and 11 participants who were cognitively normal between November 2015 and June 2017. We hypothesized a reduction in hippocampal SV2A binding in AD, based on the early degeneration of entorhinal cortical cell projections to the hippocampus (via the perforant path) and hippocampal SV2A reductions that had been observed in postmortem studies. Participants underwent high-resolution PET scanning with ((R)-1-((3-(11C-methyl-11C)pyridin-4-yl)methyl)-4-(3,4,5-trifluorophenyl)pyrrolidin-2-one), a compound more commonly known as 11C-UCB-J, for SV2A. They also underwent high-resolution PET scanning with carbon 11-labeled Pittsburgh Compound B (11C-PiB) for ß-amyloid, magnetic resonance imaging, and cognitive and neurologic evaluation. Main Outcomes and Measures: Outcomes were 11C-UCB-J-specific binding (binding potential [BPND]) via PET imaging in brain regions of interest in participants with AD and participants who were cognitively normal. Results: Ten participants with AD (5 male and 5 female; mean [SD] age, 72.7 [6.3] years; 10 [100%] ß-amyloid positive) were compared with 11 participants who were cognitively normal (5 male and 6 female; mean [SD] age, 72.9 [8.7] years; 11 [100%] ß-amyloid negative). Participants with AD spanned the disease stages from amnestic mild cognitive impairment (n = 5) to mild dementia (n = 5). Participants with AD had significant reduction in hippocampal SV2A specific binding (41%) compared with cognitively normal participants, as assessed by 11C-UCB-J-PET BPND (cognitively normal participants: mean [SD] BPND, 1.47 [0.37]; participants with AD: 0.87 [0.50]; P = .005). These reductions remained significant after correction for atrophy (ie, partial volume correction; participants who were cognitively normal: mean [SD], 2.71 [0.46]; participants with AD: 2.15 [0.55]; P = .02). Hippocampal SV2A-specific binding BPND was correlated with a composite episodic memory score in the overall sample (R = 0.56; P = .01). Conclusions and Relevance: To our knowledge, this is the first study to investigate synaptic density in vivo in AD using 11C-UCB-J-PET imaging. This approach may provide a direct measure of synaptic density, and it therefore holds promise as an in vivo biomarker for AD and as an outcome measure for trials of disease-modifying therapies, particularly those targeted at the preservation and restoration of synapses.

Preclinical Evaluation of 18F-PF-05270430, a Novel PET Radioligand for the Phosphodiesterase 2A Enzyme.

UNLABELLED: The enzyme phosphodiesterase 2A (PF-05270430) is a potential target for development of novel therapeutic agents for the treatment of cognitive impairments. The goal of the present study was to evaluate the PDE2A ligand (18)F-PF-05270430, 4-(3-fluoroazetidin-1-yl)-7-methyl-5-(1-methyl-5-(4-(trifluoromethyl)phenyl)-1H-pyrazol-4-yl)imidazo[1,5-f][1,2,4]triazine, in nonhuman primates. METHODS: (18)F-PF-05270430 was radiolabeled by 2 methods via nucleophilic substitution of its tosylate precursor. Tissue metabolite analysis in rodents and PET imaging in nonhuman primates under baseline and blocking conditions were performed to determine the pharmacokinetic and binding characteristics of the new radioligand. Various kinetic modeling approaches were assessed to select the optimal method for analysis of imaging data. RESULTS: (18)F-PF-05270430 was synthesized in greater than 98% radiochemical purity and high specific activity. In the nonhuman primate brain, uptake of (18)F-PF-05270430 was fast, with peak concentration (SUVs of 1.5-1.8 in rhesus monkeys) achieved within 7 min after injection. The rank order of uptake was striatum > neocortical regions > cerebellum. Regional time-activity curves were well fitted by the 2-tissue-compartment model and the multilinear analysis-1 (MA1) method to arrive at reliable estimates of regional distribution volume (VT) and binding potential (BPND) with 120 min of scan data. Regional VT values (MA1) ranged from 1.28 mL/cm(3) in the cerebellum to 3.71 mL/cm(3) in the putamen, with a BPND of 0.25 in the temporal cortex and 1.92 in the putamen. Regional BPND values estimated by the simplified reference tissue model (SRTM) were similar to those from MA1. Test-retest variability in high-binding regions (striatum) was 4% ± 6% for MA1 VT, 13% ± 6% for MA1 BPND, and 13% ± 7% SRTM BPND, respectively. Pretreatment of animals with the PDE2A inhibitor PF-05180999 resulted in a dose-dependent reduction of (18)F-PF-05270430 specific binding, with a half maximal effective concentration of 69.4 ng/mL in plasma PF-05180999 concentration. CONCLUSION: (18)F-PF-05270430 displayed fast and reversible kinetics in nonhuman primates, as well as specific binding blockable by a PDE2A inhibitor. This is the first PET tracer with desirable imaging properties and demonstrated ability to image and quantify PDE2A in vivo.