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Purpose: Synapse loss is a hallmark of Alzheimer's disease (AD) and correlates with cognitive decline. The validation of a noninvasive in vivo imaging approach to quantify synapse would greatly facilitate our understanding of AD pathogenesis and assist drug developments for AD. As animal models of neurodegenerative and neuropsychiatric disorders play a critical role in the drug discovery and development process, a robust, objective, and translational method for quantifying therapeutic drug efficacy in animal models will facilitate the drug development process. In this study, we tested the quantification reliability of the SV2A PET tracer, [ 18 F]SynVesT-1, in a mouse model of AD (APP/PS1) and wild-type controls, and developed a simplified quantification method to facilitate large cohort preclinical imaging studies., Procedures: We generated nondisplaceable binding potential (BP ND ) and distribution volume ratio (DVR) values using the simplified reference tissue model (SRTM) on the 90-min dynamic PET imaging data, with brain stem and cerebellum as the reference region, respectively. Then, we correlated the standardized uptake value ratio (SUVR)-1 and SUVR averaged from different imaging windows with BP ND and DVR, using brain stem and cerebellum as the reference region, respectively. We performed homologous competitive binding assay and autoradiographic saturation binding assay using [ 18 F]SynVesT-1 to calculate the B max and K d ., Results: Using brain stem as the reference region, the averaged SUVR-1 from 30 to 60 min postinjection correlated well with the BP ND calculated using SRTM. Using cerebellum as the reference region, the averaged SUVR from 30 to 60 min postinjection correlated well with the SRTM DVR. From the homologous competitive binding assay and autoradiographic saturation binding assay, the calculated the B max and K d were 4.5-18 pmol/mg protein and 9.8-19.6 nM, respectively, for rodent brain tissue., Conclusions: This simplified SUVR method provides reasonable SV2A measures in APP/PS1 mice and their littermate controls. Our data indicate that, in lieu of a full 90-min dynamic scan, a 30-min static PET scan (from 30 to 60 min postinjection) would be sufficient to provide quantification data on SV2A expression, equivalent to the data generated from kinetic modeling. The methods developed here are readily applicable to the evaluation of therapeutic effects of novel drugs in this rodent model using [ 18 F]SynVesT-1 and small animal PET.

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-in-human 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., (© 2021 by the Society of Nuclear Medicine and Molecular Imaging.)

11 C-UCB-J (( R )-1-((3-( 11 C-methyl- 11 C)pyridin-4-yl)methyl)-4-(3,4,5-trifluorophenyl)pyrrolidin-2-one) is a PET tracer for synaptic vesicle glycoprotein 2A, which may be a marker of synaptic density. To simplify the scan protocol, SUV ratios (SUVRs) were compared with model-based nondisplaceable binding potential ( BP ND ) to select the optimal time window in healthy and neuropsychiatric subjects. Methods: In total, 141 scans were acquired for 90 min. Arterial blood sampling and metabolite analysis were conducted. SUVR-1 (centrum semiovale reference region) was computed for six 30-min 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- to 90-min 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., (© 2021 by the Society of Nuclear Medicine and Molecular Imaging.)

Objective: In this positron emission tomography (PET) study with [ 11 C]UCB-J, we evaluated synaptic vesicle glycoprotein 2A (SV2A) binding, which is decreased in resected brain tissues from epilepsy patients, in subjects with temporal lobe epilepsy (TLE) and compared the regional binding pattern to [ 18 F]fluorodeoxyglucose (FDG) uptake., Methods: Twelve TLE subjects and 12 control subjects were examined. Regional [ 11 C]UCB-J binding potential (BP ND ) values were estimated using the centrum semiovale as a reference region. [ 18 F]FDG uptake in TLE subjects was quantified using mean radioactivity values. Asymmetry in outcome measures was assessed by comparison of ipsilateral and contralateral regions. Partial volume correction (PVC) with the iterative Yang algorithm was applied based on the FreeSurfer segmentation., Results: In 11 TLE subjects with medial temporal lobe sclerosis (MTS), the hippocampal volumetric asymmetry was 25 ± 11%. After PVC, [ 11 C]UCB-J BP ND asymmetry indices were 37 ± 19% in the hippocampus, with very limited asymmetry in other brain regions. Reductions in [ 11 C]UCB-J BP ND values were restricted to the sclerotic hippocampus when compared to control subjects. The corresponding asymmetry in hippocampal [ 18 F]FDG uptake was 22 ± 7% and correlated with that of [ 11 C]UCB-J BP ND across subjects (R 2  = .38). Hippocampal asymmetries in [ 11 C]UCB-J binding were 1.7-fold larger than those of [ 18 F]FDG uptake., Significance: [ 11 C]UCB-J binding is reduced in the seizure onset zone of TLE subjects with MTS. PET imaging of SV2A may be a promising biomarker approach in the presurgical selection and evaluation of TLE patients and may improve the sensitivity of molecular imaging for seizure focus detection., (© 2020 International League Against Epilepsy.)

11 C-UCB-J is a new PET tracer for synaptic density imaging. Recently, we conducted 11 C-UCB-J PET on patients with mild cognitive impairment or early Alzheimer disease (AD) and found a 41% decrease in specific binding in the hippocampus compared with healthy subjects. We hypothesized that 11 C-UCB-J may have potential to be a general biomarker for evaluating AD treatment effects via monitoring of synaptic density changes. In this study, we performed longitudinal 11 C-UCB-J PET on AD mice to measure the treatment effects of saracatinib, which previously demonstrated synaptic changes with postmortem methods. Methods: Nine wild-type (WT) mice and 9 amyloid precursor protein and presenilin 1 double-transgenic (APPswe/PS1ΔE9 [APP/PS1]) mice underwent 3 11 C-UCB-J PET measurements: at baseline, after treatment, and during drug washout. After baseline measurements, saracatinib, a Fyn kinase inhibitor currently in clinical development for AD treatment, was administered by oral gavage for 41 ± 11 d. Treatment-phase measurements were performed on the last day of treatment, and washout-phase measurements occurred more than 27 d after the end of treatment. SUVs from 30 to 60 min after injection of 11 C-UCB-J were calculated and normalized by the whole-brain (WB) or brain stem (BS) average values as SUV ratio (SUVR (WB) or SUVR-1 (BS) ). Results: Hippocampal SUVR (WB) at baseline was significantly lower in APP/PS1 than WT mice (APP/PS1: 1.11 ± 0.04, WT: 1.15 ± 0.02, P = 0.033, unpaired t test). Using SUVR-1 (BS) in the hippocampus, there was also a significant difference at baseline (APP/PS1: 0.48 ± 0.13, WT: 0.65 ± 0.10, P = 0.017, unpaired t test). After treatment with saracatinib, hippocampal SUVR (WB) in APP/PS1 mice was significantly increased ( P = 0.037, paired t test). A trend-level treatment effect was seen with hippocampal SUVR-1 (BS). Saracatinib treatment effects may persist, as there were no significant differences between WT and APP/PS1 mice after drug washout. Conclusion: On the basis of the 11 C-UCB-J PET results, hippocampal synaptic density was lower in APP/PS1 mice than in WT mice at baseline, and this deficit was normalized by treatment with saracatinib. These results support the use of 11 C-UCB-J PET to identify disease-specific synaptic deficits and to monitor treatment effects in AD., (© 2019 by the Society of Nuclear Medicine and Molecular Imaging.)

Purpose: Synaptic abnormalities have been implicated in a variety of neuropsychiatric disorders, including epilepsy, Alzheimer's disease, and schizophrenia. Hence, PET imaging of the synaptic vesicle glycoprotein 2A (SV2A) may be a valuable in vivo biomarker for neurologic and psychiatric diseases. We previously developed [ 11 C]UCB-J, a PET radiotracer with high affinity and selectivity toward SV2A; however, the short radioactive half-life (20 min for 11 C) places some limitations on its broader application. Herein, we report the first synthesis of the longer-lived 18 F-labeled counterpart (half-life: 110 min), [ 18 F]UCB-J, and its evaluation in nonhuman primates., Methods: [ 18 F]UCB-J was synthesized from the iodonium precursors. PET imaging experiments with [ 18 F]UCB-J were conducted in rhesus monkeys to assess the pharmacokinetic and in vivo binding properties. Arterial samples were taken for analysis of radioactive metabolites and generation of input functions. Regional time-activity curves were analyzed using the one-tissue compartment model to derive regional distribution volumes and binding potentials for comparison with [ 11 C]UCB-J., Results: [ 18 F]UCB-J was prepared in high radiochemical and enantiomeric purity, but low radiochemical yield. Evaluation in nonhuman primates indicated that the radiotracer displayed pharmacokinetic and imaging characteristics similar to those of [ 11 C]UCB-J, with moderate metabolism rate, high brain uptake, fast and reversible binding kinetics, and high specific binding signals., Conclusion: We have accomplished the first synthesis of the novel SV2A radiotracer [ 18 F]UCB-J. [ 18 F]UCB-J is demonstrated to be an excellent imaging agent and may prove to be useful for imaging and quantification of SV2A expression, and synaptic density, in humans.

Synaptic vesicle glycoprotein 2A (SV2A) is ubiquitously present in presynaptic terminals. Here we report kinetic modeling and test-retest reproducibility assessment of the SV2A positron emission tomography (PET) radioligand [ 11 C]UCB-J in humans. Five volunteers were examined twice on the HRRT after bolus injection of [ 11 C]UCB-J. Arterial blood samples were collected for measurements of radiometabolites and free fraction. Regional time-activity curves were analyzed with 1-tissue (1T) and 2-tissue (2T) compartment models to estimate volumes of distribution ( V T ). Parametric maps were generated using the 1T model. [ 11 C]UCB-J metabolized fairly quickly, with parent fraction of 36 ± 13% at 15 min after injection. Plasma free fraction was 32 ± 1%. Regional time-activity curves displayed rapid kinetics and were well described by the 1T model, except for the cerebellum and hippocampus. V T values estimated with the 2T model were similar to 1T values. Parametric maps were of high quality and V T values correlated well with time activity curve (TAC)-based estimates. Shortening of acquisition time from 120 min to 60 min had a negligible effect on V T values. The mean absolute test-retest reproducibility for V T was 3-9% across regions. In conclusion, [ 11 C]UCB-J exhibited excellent PET tracer characteristics and has potential as a general purpose tool for measuring synaptic density in neurodegenerative disorders.

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 β-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%] β-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%] β-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.

Positron emission tomography tracers [ 11 C]ABP688 and [ 18 F]FPEB target the metabotropic glutamate receptor subtype 5 providing quantification of the brain glutamatergic system in vivo. Previous [ 11 C]ABP688 positron emission tomography human test-retest studies indicate that, when performed on the same day, significant binding increases are observed; however, little deviation is reported when scans are >7 days apart. Due to the small cohorts examined previously (eight and five males, respectively), we aimed to replicate the same-day test-retest studies in a larger cohort including both males and females. Results confirmed large within-subject binding differences (ranging from -23% to 108%), suggesting that measurements are greatly affected by study design. We further investigated whether this phenomenon was specific to [ 11 C]ABP688. Using [ 18 F]FPEB and methodology that accounts for residual radioactivity from the test scan, four subjects were scanned twice on the same day. In these subjects, binding estimates increased between 5% and 39% between scans. Consistent with [ 11 C]ABP688, mean absolute test-retest variability was previously reported as <12% when scans were >21 days apart. This replication study and pilot extension to [ 18 F]FPEB suggest that observed within-day binding variation may be due to characteristics of mGluR5; for example, diurnal variation in mGluR5 may affect measurement of this receptor.

Unlabelled: The synaptic vesicle glycoprotein 2A (SV2A) is found in secretory vesicles in neurons and endocrine cells. PET with a selective SV2A radiotracer will allow characterization of drugs that modulate SV2A (e.g., antiepileptic drugs) and potentially could be a biomarker of synaptic density (e.g., in neurodegenerative disorders). Here we describe the synthesis and characterization of the SV2A PET radiotracer (11)C-UCB-J ((R)-1-((3-((11)C-methyl-(11)C)pyridin-4-yl)methyl)-4-(3,4,5-trifluorophenyl)pyrrolidin-2-one) in nonhuman primates, including whole-body biodistribution., Methods: (11)C-UCB-J was prepared by C-(11)C-methylation of the 3-pyridyl trifluoroborate precursor with (11)C-methyl iodide via the Suzuki-Miyaura cross-coupling method. Rhesus macaques underwent multiple scans including coinjection with unlabeled UCB-J (17, 50, and 150 μg/kg) or preblocking with the antiepileptic drug levetiracetam at 10 and 30 mg/kg. Scans were acquired for 2 h with arterial sampling and metabolite analysis to measure the input function. Regional volume of distribution (VT) was estimated using the 1-tissue-compartment model. Target occupancy was assessed using the occupancy plot; the dissociation constant (Kd) was determined by fitting self-blocking occupancies to a 1-site model, and the maximum number of receptor binding sites (Bmax) values were derived from baseline VT and from the estimated Kd and the nondisplaceable distribution volume (VND)., Results: (11)C-UCB-J was synthesized with greater than 98% purity. (11)C-UCB-J exhibited high free fraction (0.46 ± 0.02) and metabolized at a moderate rate (39% ± 5% and 24% ± 3% parent remaining at 30 and 90 min) in plasma. In the monkey brain, (11)C-UCB-J displayed high uptake and fast kinetics. VT was high (∼25-55 mL/cm(3)) in all gray matter regions, consistent with the ubiquitous expression of SV2A. Preblocking with 10 and 30 mg/kg of levetiracetam resulted in approximately 60% and 90% occupancy, respectively. Analysis of the self-blocking scans yielded a Kd estimate of 3.4 nM and Bmax of 125-350 nM, in good agreement with the in vitro inhibition constant (Ki) of 6.3 nM and regional Bmax in humans. Whole-body biodistribution revealed that the liver and the brain are the dose-limiting organs for males and females, respectively., Conclusion: (11)C-UCB-J exhibited excellent characteristics as an SV2A PET radiotracer in nonhuman primates. The radiotracer is currently undergoing first-in-human evaluation., (© 2016 by the Society of Nuclear Medicine and Molecular Imaging, Inc.)

Purpose: [(18)F]FPEB is a promising PET radioligand for the metabotropic glutamate receptor 5 (mGluR5), a potential target for the treatment of neuropsychiatric diseases. The purpose of this study was to evaluate the test-retest reproducibility of [(18)F]FPEB in the human brain., Methods: Seven healthy male subjects were scanned twice, 3 - 11 weeks apart. Dynamic data were acquired using bolus plus infusion of 162 ± 32 MBq [(18)F]FPEB. Four methods were used to estimate volume of distribution (V T): equilibrium analysis (EQ) using arterial (EQA) or venous input data (EQV), MA1, and a two-tissue compartment model (2 T). Binding potential (BP ND) was also estimated using cerebellar white matter (CWM) or gray matter (CGM) as the reference region using EQ, 2 T and MA1. Absolute test-retest variability (aTRV) of V T and BP ND were calculated for each method. Venous blood measurements (C V) were compared with arterial input (C A) to examine their usability in EQ analysis., Results: Regional V T estimated by the four methods displayed a high degree of agreement (r (2) ranging from 0.83 to 0.99 among the methods), although EQA and EQV overestimated V T by a mean of 9 % and 7 %, respectively, compared to 2 T. Mean values of aTRV of V T were 11 % by EQA, 12 % by EQV, 14 % by MA1 and 14 % by 2 T. Regional BP ND also agreed well among the methods and mean aTRV of BP ND was 8 - 12 % (CWM) and 7 - 9 % (CGM). Venous and arterial blood concentrations of [(18)F]FPEB were well matched during equilibrium (C V = 1.01 · C A, r (2) = 0.95)., Conclusion: [(18)F]FPEB binding shows good TRV with minor differences among analysis methods. Venous blood can be used as an alternative for input function measurement instead of arterial blood in EQ analysis. Thus, [(18)F]FPEB is an excellent PET imaging tracer for mGluR5 in humans.

Unlabelled: The κ-opioid receptors (KORs) are implicated in several neuropsychiatric diseases and addictive disorders. PET with radioligands provides a means to image the KOR in vivo and investigate its function in health and disease. The purpose of this study was to develop the selective KOR antagonist (11)C-LY2459989 as a PET radioligand and characterize its imaging performance in nonhuman primates., Methods: LY2459989 was synthesized and assayed for in vitro binding to opioid receptors. Ex vivo studies in rodents were conducted to assess its potential as a tracer candidate. (11)C-LY2459989 was synthesized by reaction of its iodophenyl precursor with (11)C-cyanide, followed by partial hydrolysis of the resulting (11)C-cyanophenyl intermediate. Imaging experiments with (11)C-LY2459989 were performed in rhesus monkeys with arterial input function measurement. Imaging data were analyzed with kinetic models to derive in vivo binding parameters., Results: LY2459989 is a full antagonist with high binding affinity and selectivity for KOR (0.18, 7.68, and 91.3 nM, respectively, for κ, μ, and δ receptors). Ex vivo studies in rats indicated LY2459989 as an appropriate tracer candidate with high specific binding signals and confirmed its KOR binding selectivity in vivo. (11)C-LY2459989 was synthesized in high radiochemical purity and good specific activity. In rhesus monkeys, (11)C-LY2459989 displayed a fast rate of peripheral metabolism. Similarly, (11)C-LY2459989 displayed fast uptake kinetics in the brain and an uptake pattern consistent with the distribution of KOR in primates. Pretreatment with naloxone (1 mg/kg, intravenously) resulted in a uniform distribution of radioactivity in the brain. Further, specific binding of (11)C-LY2459989 was dose-dependently reduced by the selective KOR antagonist LY2456302 and the unlabeled LY2459989. Regional binding potential values derived from the multilinear analysis-1 (MA1) method, as a measure of in vivo specific binding signal, were 2.18, 1.39, 1.08, 1.04, 1.03, 0.59, 0.51, and 0.50, respectively, for the globus pallidus, cingulate cortex, insula, caudate, putamen, frontal cortex, temporal cortex, and thalamus., Conclusion: The novel PET radioligand (11)C-LY2459989 displayed favorable pharmacokinetic properties, a specific and KOR-selective binding profile, and high specific binding signals in vivo, thus making it a promising PET imaging agent for KOR., (© 2014 by the Society of Nuclear Medicine and Molecular Imaging, Inc.)

Background: Preclinical studies demonstrate that glutamate homeostasis in the striatum is disrupted following cocaine exposure, including a decrease in metabotropic glutamate receptor type 5 (mGluR5) expression and reduced glutamate turnover. The goal of this study was to use imaging of the human brain to investigate alterations in the glutamate signaling in cocaine addiction., Methods: Positron emission tomography imaging with the radiotracer [(11)C]ABP688 was used to measure mGluR5 binding and magnetic resonance spectroscopy was used to measure glutamate-glutamine levels in the striatum of cocaine-addicted participants (n = 15) compared with healthy control subjects (n = 15). Following the scans, the cocaine-addicted volunteers performed cocaine self-administration sessions to investigate the correlation between cocaine-seeking behavior and mGluR5 receptor binding., Results: The results of the study showed that cocaine addiction was associated with a 20% to 22% reduction in [(11)C]ABP688 binding in the striatum. A secondary analysis of cortical and subcortical regions other than the striatum showed a similar reduction in [(11)C]ABP688 binding, suggesting that the decrease was widespread. No between-group differences were seen in the magnetic resonance spectroscopy measures of glutamate-glutamine in the left striatum. In addition, no correlation was seen between [(11)C]ABP688 binding in the striatum and the choice to self-administer cocaine., Conclusions: Overall, these results show that long-term cocaine use is associated with a decrease in mGluR5 availability compared with matched healthy control subjects and suggests that this receptor may serve as a viable target for treatment development for this disorder., (Copyright © 2014 Society of Biological Psychiatry. Published by Elsevier Inc. All rights reserved.)

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)., (Copyright © 2013 Wiley Periodicals, Inc.)

Unlabelled: Microglia play an essential role in many brain diseases. Microglia are activated by local tissue damage or inflammation, but systemic inflammation can also activate microglia. An important clinical question is whether the effects of systemic inflammation on microglia mediate the deleterious effects of systemic inflammation in diseases such as Alzheimer's dementia, multiple sclerosis, and stroke. Positron Emission Tomography (PET) imaging with ligands that bind to Translocator Protein (TSPO) can be used to detect activated microglia. The aim of this study was to evaluate whether the effect of systemic inflammation on microglia could be measured with PET imaging in nonhuman primates, using the TSPO ligand [(11)C]PBR28., Methods: Six female baboons (Papio anubis) were scanned before and at 1h and/or 4h and/or 22 h after intravenous administration of E. coli lipopolysaccharide (LPS; 0.1mg/kg), which induces systemic inflammation. Regional time-activity data from regions of interest (ROIs) were fitted to the two-tissue compartmental model, using the metabolite-corrected arterial plasma curve as input function. Total volume of distribution (V(T)) of [(11)C]PBR28 was used as a measure of total ligand binding. The primary outcome was change in V(T) from baseline. Serum levels of tumor necrosis factor alpha (TNFα), interleukin-1 beta (IL-1β), interleukin-6 (IL-6), and interleukin-8 (IL-8) were used to assess correlations between systemic inflammation and microglial activation. In one baboon, immunohistochemistry was used to identify cells expressing TSPO., Results: LPS administration increased [(11)C]PBR28 binding (F(3,6)=5.1, p=.043) with a 29 ± 16% increase at 1h (n=4) and a 62 ± 34% increase at 4h (n=3) post-LPS. There was a positive correlation between serum IL-1β and IL-6 levels and the increase in [(11)C]PBR28 binding. TSPO immunoreactivity occurred almost exclusively in microglia and rarely in astrocytes., Conclusion: In the nonhuman-primate brain, LPS-induced systemic inflammation produces a robust increase in the level of TSPO that is readily detected with [(11)C]PBR28 PET. The effect of LPS on [(11)C]PBR28 binding is likely mediated by inflammatory cytokines. Activation of microglia may be a mechanism through which systemic inflammatory processes influence the course of diseases such as Alzheimer's, multiple sclerosis, and possibly depression., (Copyright © 2012 Elsevier Inc. All rights reserved.)

Reference tissue methods to estimate neuroreceptor binding are not applicable to [(18)F]FP-TZTP (a muscarinic-2 cholinergic receptor ligand), because there is no suitable receptor-free reference region. We evaluated a new method to estimate, without using arterial data or a receptor-free reference region, a receptor parameter called the normalized distribution volume, V(T)(*), using a region containing receptors as the input tissue. V(T)(*) is defined as V(T)/K'(1) (distribution volume (V(T)) normalized by K'(1) of the input region). We used a two-parameter multilinear reference tissue model (MRTM2) to generate parametric images of V(T)(*) and R(1) (R(1)=K(1)/K'(1)) from [(18)F]FP-TZTP PET data of healthy aged subjects (10 with apolipoprotein E-epsilon4 alleles (APOE-epsilon4(+)) and nine without (APOE-epsilon4(-)). V(T)(*) and V(T) were normalized by plasma-free fraction, f(P). By one-tissue kinetic analysis (1TKA) with metabolite-corrected plasma data, V(T) was previously reported as higher in the APOE-epsilon4(+) group. The noise magnitude of MRTM2 V(T)(*) and R(1) images were nearly identical to those of 1TKA V(T) and K(1) images. K'(1) or f(P) was not different between the two groups. V(T)(*) (mins) (1,659+/-497) and V(T) (mL/cm(3)) (701+/-99) in APOE-epsilon4(+) were higher by 38 and 22% than those (1,209+/-233 and 577+/-112) in APOE-epsilon4(-), respectively. The statistical significance for V(T)(*) (0.041) was lower than that for V(T) (0.025), due to the higher intersubject variability of V(T)(*) (25%) than that of V(T) (17%). We conclude that MRTM2 V(T)(*) allows detection of group differences in receptor binding without arterial blood or a receptor-free reference region.

The apolipoprotein E-epsilon4 allele (APOE-epsilon4) confers greater susceptibility to age-related memory disorders. Abnormalities in the cholinergic system are likely contributors to these disorders with both age and APOE-epsilon4 genotype modifying behavioral and physiological responses to drugs that alter cholinergic pathway function. Recently, we reported a greater in vivo distribution volume of the F-18 labeled muscarinic-2 (M2) selective agonist, 3-(3-(3-[18F]Flouropropyl)thio)-1,2,5-thiadiazol-4-yl)-1,2,5,6-tetrahydro-1-methylpyridine ([18F]FP-TZTP), in aging healthy subjects with an APOE-epsilon4 allele. To examine the effects of aging and the APOE-epsilon4 allele on the response of the muscarinic component of cholinergic pathway to pharmacologic augmentation, two [18F]FP-TZTP PET scans were conducted in 19 subjects varying in age from 22 to 74 years, the first served as baseline for the second scan that was performed while the subjects were either infused with saline (n = 6) or with the acetylcholinesterase inhibitor physostigmine (6 with an APOE-epsilon4 allele and 7 without an APOE-epsilon4 allele). Using a multiple regression analysis, both AGE (beta = 0.621 +/- 0.135, B = 0.353 +/- 0.077, t(10) = 4.61, P < 0.001) and APOE-epsilon4 genotype (beta = 0.742, B = 14.8 +/- 2.69, t(10) = 5.51, P < 0.0003) were found to be significant contributors to subject response to physostigmine. The adjusted R2 for the model as a whole was 0.786 (F(2,10) = 23.00, P < 0.0002) with both increasing age and the presence of the APOE-epsilon4 allele modifying the response to physostigmine in the direction of larger decreases in [18F]FP-TZTP distribution volumes in all brain regions examined. The findings, particularly the absence of an interaction between AGE and APOE-epsilon4 genotype, contribute to the growing body of evidence that suggests that the APOE-epsilon4 genotype is likely to contribute to brain structure and function prior to aging., (Published 2006 Wiley-Liss, Inc.)

The precise effects of normal aging on the cholinergic system are unknown, as both in vitro and PET studies have shown conflicting results. In vivo determination of muscarinic receptor distribution and density has been hampered by both poor subtype selectivity and/or blood-brain barrier permeability of known ligands. Previous in vitro and in vivo work with the F-18 labeled muscarinic agonist, 3-(3- (3-[(18)F]Flouropropyl)thio)-1,2,5-thiadiazol-4-yl)-1,2,5,6-tetrahydro-1-methylpyridine ((18)FP-TZTP) suggested the use of (18)FP-TZTP to selectively quantify M2 receptors in humans. In this study, we used (18)FP-TZTP to infer M2 receptor avidity in the brains of 15 healthy younger subjects (mean age = 28.3 +/- 5.5 years) and 20 healthy older subjects (mean age = 62.1 +/- 7.7 years). Corrections for subject motion during the 120-min acquisition and partial voluming (PVC) were performed. A one-tissue compartment model was used to estimate the volumes of distribution (V(T)) of (18)FP-TZTP. Within both groups of subjects, volumes of distribution (K(1)/k(2)) in cortical, subcortical, and cerebellar areas were consistent with M2 receptor topography. Compared to younger subjects older subjects had significantly higher means and standard deviations for the volumes of distribution of (18)FP-TZTP throughout much of the cerebellum, cortex, and subcortex (Global Gray V(T) = 742 +/- 163 in older subjects and 645 +/- 74 in younger subjects, P < 0.03). Across all subjects (18)FP-TZTP, regional, and Global Gray distribution volumes were significantly correlated to age (Global Gray V(T,) r = 0.41, P < 0.01). A lower concentration of acetylcholine in the synapse of some older subjects is one possible explanation for the data.

Liquid-liquid and solid phase extraction methods were developed for the accurate and rapid quantitation of radioactive components in human plasma following injection of two PET ligands. A solid phase extraction (SPE) method was developed for the determination of the 5HT(1A) receptor ligand [N-[2-[4-(2-methoxyphenyl) piperazino]ethyl]-N-(2-pyridinyl) trans-4-[(18)F]fluorocyclohexanecarboxamide (FCWAY), and its acidic metabolite, 4-[(18)F]fluorocyclohexane carboxylic acid (FC). In both cases, the extraction method was much faster and easier to use, yet provided results comparable to HPLC and TLC methods. In addition, an easy to perform two-step liquid-liquid extraction was developed for quantitation of 3-(3-((3-[(18)F]fluoropropyl)thio)-1,2,5-thiadiazol-4-yl)-1,2,5,6-tetrahydro-1-methylpyridine ([(18)F]FP-TZTP), a selective M2 muscarinic agonist.

We synthesized [(18)F]FCWAY, an analog of [carbonyl-(11)C]WAY-100635 ¿[(11)C]N-(2-(1-(4-(2-methoxyphenyl)-piperazinyl)ethyl))-N-(2-(pyridi nyl))cyclohexanecarboxamide¿, by replacing the cyclohexanecarbonyl group acid with a trans-4-fluorocyclohexanecarbonyl group (FC). Control and preblocking studies were performed in anesthetized monkeys. Plasma radioactive metabolite analysis showed the presence of [(18)F]FC and [(18)F]fluoride. Tissue time-radioactivity curves were corrected for metabolite contamination based on separate positron-emission tomography studies of these two labeled metabolites. Analysis using a two-tissue compartment model gave distribution volume (V) estimates (mL/mL) ranging from 33 in frontal cortex to 4 in cerebellum. Preblocking data showed uniform V of 2-3 mL/mL. These studies demonstrate that [(18)F]FCWAY has very similar kinetic characteristics to [(11)C]WAY-100635.

We have synthesized five fluorinated derivatives of WAY 100635, N-{2-[4-(2-methoxyphenyl)piperazino]ethyl}-N-(2-pyridyl)cyclohe xaneca rboxamide (4a), using various acids in place of the cyclohexanecarboxylic acid (CHCA, 2a) in the reaction scheme. The five acids are 4-fluorobenzoic acid (FB, 2b), 4-fluoro-3-methylbenzoic acid (MeFB, 2c), trans-4-fluorocyclohexanecarboxylic acid (FC, 2d), 4-(fluoromethyl)benzoic acid (FMeB, 2e), and 3-nitro-4-(fluoromethyl)benzoic acid (NFMeB, 2f) (see Scheme 1). These compounds were radiolabeled with fluorine-18, and their biological properties were evaluated in rats and compared with those of [11C]carbonyl WAY 100635 ([carbonyl-11C]4a). [Carbonyl-11C]4a cleared the brain with a biological half-life averaging 41 min. The metabolite-corrected blood radioactivity had a half-life of 29 min. [18F]FCWAY ([18F]4d) gave half-lives and intercepts comparable to [carbonyl-11C]4a in the brain, but the blood clearance was faster. [18F]FBWAY ([18F]4b) showed an early rapid net efflux from the whole brain, clearing with a biological half-life of 35 min. The metabolite-corrected blood half-life was 41 min. The comparable whole brain and blood half-lives for Me[18F]FBWAY ([18F]4c) were 16 and 18 min, respectively. For each compound, the corresponding carboxylic acid was identified as a major metabolite in blood. Fluoride was also found after injection of [18F]4d. However, for all compounds there was a good correlation (R > 0.97) between the differential uptake ratio (DUR, (%ID/g) x body weight (g)/100) in individual rat brain regions at 30 min after injection and the concentration of receptors as determined by in vitro quantitative autoradiography in rat. Specific binding ratios [region of interest (ROI)/cerebellum-1] in control studies for cortex (Ctx) and hippocampus (H) were higher for [carbonyl-11C]4a and [18F]4d compared to [18F]4b and [18F]4c. [18F]4d has similar pharmacokinetic properties and comparable specific binding ratios to [carbonyl-11C]4a. Fifty nanomoles of 4a blocked only 30% of the specific binding of [18F]4d, while complete blockade was obtained from co-injection of 200 nmol of 4a (H/Cb-1 from 17.2 to 0.6). [18F]4b and [18F]4c showed lower specific binding ratios than [carbonyl-11C]4a and [18F]4d. [18F]4c was superior to [18F]4b since its specific binding was more readily blocked by 4a. These studies suggest that [18F]4c should be a useful compound to assess dynamic changes in serotonin levels while [18F]4d, with its high contrast and F-18 label, should provide better statistics and quantification for static measurement of 5-HT1A receptor distribution.

Based on encouraging in vitro data indicating M2 subtype selectivity, we synthesized, radiolabeled with 18F, and evaluated 3-(3-(2-fluoroethylthio)-1,2,5-thiadiazol-4-yl)-1,2,5,6-tetr ahydro-1-methylpyridine [FE-TZTP], and 3-(3-(3-fluoropropylthio)-1,2,5-thiadiazol-4-yl)-1,2,5,6-tet rahydro-1-methylpyridine [FP-TZTP] for muscarinic subtype selectivity in vivo. [18F]FE-TZTP displays high uptake in vivo but is inhibited only weakly by coinjecting unlabeled P-TZTP. Contrarily, [18F]FP-TZTP shows significant inhibition of uptake by coinjecting unlabeled P-TZTP or the muscarinic agonist L-687,306 (3-(3-cyclopropyl-1,2,4-oxadiazol-5-yl)-1-azabicyclo[2.2.1]heptane ). Using in vivo autoradiography, [18F]FP-TZTP displays regional distribution consistent with M2 subtype distribution. In addition, [18F]FP-TZTP shows specific uptake in the heart at 5 min. Analysis of metabolites in the awake rat brain revealed that the parent compound represents >95% of the extractable activity at 30 min. In vivo studies in rhesus monkeys revealed rapid brain uptake of [18F]FP-TZTP, with clearance sustained over 2 h. Administration of P-TZTP or FP-TZTP (80 nmol/kg) at 60 min after injection of [18F]FP-TZTP results in a significant displacement of brain activity in all regions. Metabolite analysis in monkey plasma shows that parent compound represents 20% of the extractable radioactivity at 40 min postinjection. One metabolite, which increases with time, has similar lipophilicity to the parent. However, based on metabolism in rat we believe metabolites are not in the brain to any significant extent in monkeys during the time of imaging studies. Regional uptake, autoradiographic distribution, and clearance rates in the brain are consistent with the hypothesis that [18F]FP-TZTP is M2 selective in vivo.

Two [18F] labeled ligands for the mAChR were prepared and evaluated in rodents and nonhuman primates. The properties of both compounds, one an agonist and the other an antagonist, were consistent with M2 subtype specificity.

[18F]Fluoropropyl-TZTP (FP-TZTP) is a subtype-selective muscarinic cholinergic ligand with potential suitability for studying Alzheimer's disease. Positron emission tomography studies in isofluorane-anesthetized rhesus monkeys were performed to assess the in vivo behavior of this radiotracer. First, control studies (n = 11) were performed to characterize the tracer kinetics and to choose an appropriate model using a metabolite-corrected arterial input function. Second, preblocking studies (n = 4) with unlabeled FP-TZTP were used to measure nonspecific binding. Third, the sensitivity of [18F]FP-TZTP binding to changes in brain acetylcholine (ACh) was assessed by administering physostigmine, an acetylcholinesterase (AChE) inhibitor, by intravenous infusion (100 to 200 microg x kg(-1) x h(-1)) beginning 30 minutes before tracer injection (n = 7). Tracer uptake in the brain was rapid with K1 values of 0.4 to 0.6 mL x min(-1) x mL(-1) in gray matter. A model with one tissue compartment was chosen because reliable parameter estimates could not be obtained with a more complex model. Volume of distribution (V) values, determined from functional images created by pixel-by-pixel fitting, were very similar in cortical regions, basal ganglia, and thalamus, but significantly lower (P < 0.01) in the cerebellum, consistent with the distribution of M2 cholinergic receptors. Preblocking studies with unlabeled FP-TZTP reduced V by 60% to 70% in cortical and subcortical regions. Physostigmine produced a 35% reduction in cortical specific binding (P < 0.05), consistent with increased ACh competition. The reduction in basal ganglia (12%) was significantly smaller (P < 0.05), consistent with its markedly higher AChE activity. These studies indicate that [18F]FP-TZTP should be useful for the in vivo measurement of muscarinic receptors with positron emission tomography.