Preclinical evaluation of a brain penetrant PARP PET imaging probe in rat glioblastoma and nonhuman primates.

PURPOSE: Currently, there are multiple active clinical trials involving poly(ADP-ribose) polymerase (PARP) inhibitors in the treatment of glioblastoma. The noninvasive quantification of baseline PARP expression using positron emission tomography (PET) may provide prognostic information and lead to more precise treatment. Due to the lack of brain-penetrant PARP imaging agents, the reliable and accurate in vivo quantification of PARP in the brain remains elusive. Herein, we report the synthesis of a brain-penetrant PARP PET tracer, (R)-2-(2-methyl-1-(methyl-11C)pyrrolidin-2-yl)-1H-benzo[d]imidazole-4-carboxamide ([11C]PyBic), and its preclinical evaluations in a syngeneic RG2 rat glioblastoma model and healthy nonhuman primates. METHODS: We synthesized [11C]PyBic using veliparib as the labeling precursor, performed dynamic PET scans on RG2 tumor-bearing rats and calculated the distribution volume ratio (DVR) using simplified reference region method 2 (SRTM2) with the contralateral nontumor brain region as the reference region. We performed biodistribution studies, western blot, and immunostaining studies to validate the in vivo PET quantification results. We characterized the brain kinetics and binding specificity of [11C]PyBic in nonhuman primates on FOCUS220 scanner and calculated the volume of distribution (VT), nondisplaceable volume of distribution (VND), and nondisplaceable binding potential (BPND) in selected brain regions. RESULTS: [11C]PyBic was synthesized efficiently in one step, with greater than 97% radiochemical and chemical purity and molar activity of 148 ± 85 MBq/nmol (n = 6). [11C]PyBic demonstrated PARP-specific binding in RG2 tumors, with 74% of tracer binding in tumors blocked by preinjected veliparib (i.v., 5 mg/kg). The in vivo PET imaging results were corroborated by ex vivo biodistribution, PARP1 immunohistochemistry and immunoblotting data. Furthermore, brain penetration of [11C]PyBic was confirmed by quantitative monkey brain PET, which showed high specific uptake (BPND > 3) and low nonspecific uptake (VND < 3 mL/cm3) in the monkey brain. CONCLUSION: [11C]PyBic is the first brain-penetrant PARP PET tracer validated in a rat glioblastoma model and healthy nonhuman primates. The brain kinetics of [11C]PyBic are suitable for noninvasive quantification of available PARP binding in the brain, which posits [11C]PyBic to have broad applications in oncology and neuroimaging.

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.

A metabolically stable PET tracer for imaging synaptic vesicle protein 2A: synthesis and preclinical characterization of [18F]SDM-16.

PURPOSE: To quantify the synaptic vesicle glycoprotein 2A (SV2A) changes in the whole central nervous system (CNS) under pathophysiological conditions, a high affinity SV2A PET radiotracer with improved in vivo stability is desirable to minimize the potential confounding effect of radiometabolites. The aim of this study was to develop such a PET tracer based on the molecular scaffold of UCB-A, and evaluate its pharmacokinetics, in vivo stability, specific binding, and nonspecific binding signals in nonhuman primate brains, in comparison with [11C]UCB-A, [11C]UCB-J, and [18F]SynVesT-1. METHODS: The racemic SDM-16 (4-(3,5-difluorophenyl)-1-((2-methyl-1H-imidazol-1-yl)methyl)pyrrolidin-2-one) and its two enantiomers were synthesized and assayed for in vitro binding affinities to human SV2A. We synthesized the enantiopure [18F]SDM-16 using the corresponding enantiopure arylstannane precursor. Nonhuman primate brain PET scans were performed on FOCUS 220 scanners. Arterial blood was drawn for the measurement of plasma free fraction (fP), radiometabolite analysis, and construction of the plasma input function. Regional time-activity curves (TACs) were fitted with the one-tissue compartment (1TC) model to obtain the volume of distribution (VT). Nondisplaceable binding potential (BPND) was calculated using either the nondisplaceable volume of distribution (VND) or the centrum semiovale (CS) as the reference region. RESULTS: SDM-16 was synthesized in 3 steps with 44% overall yield and has the highest affinity (Ki = 0.9 nM) to human SV2A among all reported SV2A ligands. [18F]SDM-16 was prepared in about 20% decay-corrected radiochemical yield within 90 min, with greater than 99% radiochemical and enantiomeric purity. This radiotracer displayed high specific binding in monkey brains and was metabolically more stable than the other SV2A PET tracers. The fP of [18F]SDM-16 was 69%, which was higher than those of [11C]UCB-J (46%), [18F]SynVesT-1 (43%), [18F]SynVesT-2 (41%), and [18F]UCB-H (43%). The TACs were well described with the 1TC. The averaged test-retest variability (TRV) was 7 ± 3%, and averaged absolute TRV (aTRV) was 14 ± 7% for the analyzed brain regions. CONCLUSION: We have successfully synthesized a novel SV2A PET tracer [18F]SDM-16, which has the highest SV2A binding affinity and metabolical stability among published SV2A PET tracers. The [18F]SDM-16 brain PET images showed superb contrast between gray matter and white matter. Moreover, [18F]SDM-16 showed high specific and reversible binding in the NHP brains, allowing for the reliable and sensitive quantification of SV2A, and has potential applications in the visualization and quantification of SV2A beyond the brain.

Feasibility study of PET dynamic imaging of [18F]DHMT for quantification of reactive oxygen species in the myocardium of large animals.

OBJECTIVES: We aimed to develop a dynamic imaging technique for a novel PET superoxide tracer, [18F]DHMT, to allow for absolute quantification of myocardial reactive oxygen species (ROS) production in a large animal model. METHODS: Six beagle dogs underwent a single baseline dynamic [18F]DHMT PET study, whereas one animal underwent three serial dynamic studies over the course of chronic doxorubicin administration (1 mg·kg-1·week-1 for 15 weeks). During the scans, sequential arterial blood samples were obtained for plasma metabolite correction. The optimal compartment model and graphical analysis method were identified for kinetic modeling. Values for the left ventricular (LV) net influx rate, Ki, were reported for all the studies and compared with the LV standard uptake values (SUVs) and the LV-to-blood pool SUV ratios from the 60 to 90 minute static images. Parametric images were also generated. RESULTS: [18F]DHMT followed irreversible kinetics once oxidized within the myocardium in the presence of superoxide, as evidenced by the fitting generated by the irreversible two-tissue (2Ti) compartment model and the linearity of Patlak analysis. Myocardial Ki values showed a weak correlation with LV SUV (R2 = 0.27), but a strong correlation with LV-to-blood pool SUV ratio (R2 = 0.92). Generation of high-quality parametric images showed superior myocardial to blood contrast compared to static images. CONCLUSIONS: A dynamic PET imaging technique for [18F]DHMT was developed with full and simplified kinetic modeling for absolute quantification of myocardial superoxide production in a large animal model.

Preliminary In Vivo Evidence of Reduced Synaptic Density in Human Immunodeficiency Virus (HIV) Despite Antiretroviral Therapy.

BACKGROUND: Synaptic injury is a pathological hallmark of neurological impairment in people living with human immunodeficiency virus (HIV, PLWH), a common complication despite viral suppression with antiretroviral therapy (ART). Measurement of synaptic density in living humans may allow better understanding of HIV neuropathogenesis and provide a dynamic biomarker for therapeutic studies. We applied novel synaptic vesical protein 2A (SV2A) positron emission tomographic (PET) imaging to investigate synaptic density in the frontostriatalthalamic region in PLWH and HIV-uninfected participants. METHODS: In this cross-sectional pilot study,13 older male PLWH on ART underwent magnetic resonance imaging (MRI) and PET scanning with the SV2A ligand [11C]UCB-J with partial volume correction and had neurocognitive assessments. SV2A binding potential (BPND) in the frontostriatalthalamic circuit was compared to 13 age-matched HIV-uninfected participants and assessed with respect to neurocognitive performance in PLWH. RESULTS: PLWH had 14% lower frontostriatalthalamic SV2A synaptic density compared to HIV-uninfected (PLWH: mean [SD], 3.93 [0.80]; HIV-uninfected: 4.59 [0.43]; P = .02, effect size 1.02). Differences were observed in widespread additional regions in exploratory analyses. Higher frontostriatalthalamic SV2A BPND associated with better grooved pegboard performance, a measure of motor coordination, in PLWH (r = 0.61, P = .03). CONCLUSIONS: In a pilot study, SV2A PET imaging reveals reduced synaptic density in older male PLWH on ART compared to HIV-uninfected in the frontostriatalthalamic circuit and other cortical areas. Larger studies controlling for factors in addition to age are needed to determine whether differences are attributable to HIV or comorbidities in PLWH. SV2A imaging is a promising biomarker for studies of neuropathogenesis and therapeutic interventions in HIV.

Separating dopamine D2 and D3 receptor sources of [11C]-(+)-PHNO binding potential: Independent component analysis of competitive binding.

Development of medications selective for dopamine D2 or D3 receptors is an active area of research in numerous neuropsychiatric disorders including addiction and Parkinson's disease. The positron emission tomography (PET) radiotracer [11C]-(+)-PHNO, an agonist that binds with high affinity to both D2 and D3 receptors, has been used to estimate relative receptor subtype occupancy by drugs based on a priori knowledge of regional variation in the expression of D2 and D3 receptors. The objective of this work was to use a data-driven independent component analysis (ICA) of receptor blocking scans to separate D2-and D3-related signal in [11C]-(+)-PHNO binding data in order to improve the precision of subtype specific measurements of binding and occupancy. Eight healthy volunteers underwent [11C]-(+)-PHNO PET scans at baseline and at two time points following administration of the D3-preferring antagonist ABT-728 (150-1000 â€‹mg). Parametric binding potential (BPND) images were analyzed as four-dimensional image series using ICA to extract two independent sources of variation in [11C]-(+)-PHNO BPND. Spatial source maps for each component were consistent with respective regional patterns of D2-and D3-related binding. ICA-derived occupancy estimates from each component were similar to D2-and D3-specific occupancy estimated from a region-based approach (intraclass correlation coefficients â€‹> â€‹0.95). ICA-derived estimates of D3 receptor occupancy improved quality of fit to a single site binding model. Furthermore, ICA-derived estimates of the regional fraction of [11C]-(+)-PHNO binding related to D3 receptors was generated for each subject and values showed good agreement with region-based model estimates and prior literature values. In summary, ICA successfully separated D2-and D3-related components of the [11C]-(+)-PHNO binding signal, establishing this approach as a powerful data-driven method to quantify distinct biological features from PET data composed of mixed data sources.

Novel Kappa Opioid Receptor Agonist as Improved PET Radiotracer: Development and in Vivo Evaluation.

The kappa opioid receptor (KOR) is involved in depression, alcoholism, and drug abuse. The current agonist radiotracer 11C-GR103545 is not ideal for imaging KOR due to its slow tissue kinetics in human. The aim of our project was to develop novel KOR agonist radiotracers with improved imaging properties. A novel compound FEKAP ((( R))-4-(2-(3,4-dichlorophenyl)acetyl)-3-((ethyl(2-fluoroethyl)amino)methyl) piperazine-1-carboxylate) was designed, synthesized, and assayed for in vitro binding affinities. It was then radiolabeled and evaluated in rhesus monkeys. Baseline and blocking scans were conducted on a Focus-220 scanner to assess binding specificity and selectivity. Metabolite-corrected arterial activities over time were measured and used as input functions to analyze the brain regional time-activity curves and derive kinetic and binding parameters with kinetic modeling. FEKAP displayed high KOR binding affinity ( Ki = 0.43 nM) and selectivity (17-fold over mu opioid receptor and 323-fold over delta opioid receptor) in vitro. 11C-FEKAP was prepared in high molar activity (mean of 718 GBq/µmol, n = 19) and >99% radiochemical purity. In monkeys, 11C-FEKAP metabolized fairly fast, with ∼31% of intact parent fraction at 30 min post-injection. In the brain, it exhibited fast and reversible kinetics with good uptake. Pretreatment with the nonselective opioid receptor antagonist naloxone (1 mg/kg) decreased uptake in high binding regions to the level in the cerebellum, and the selective KOR antagonist LY2456302 (0.02 and 0.1 mg/kg) reduced 11C-FEKAP specific binding in a dose-dependent manner. As a measure of specific binding signals, the mean binding potential ( BPND) values of 11C-FEKAP derived from the multilinear analysis-1 (MA1) method were greater than 0.5 for all regions, except for the thalamus. The novel KOR agonist tracer 11C-FEKAP demonstrated binding specificity and selectivity in vivo and exhibited attractive properties of fast tissue kinetics and high specific binding.

PET imaging of α7 nicotinic acetylcholine receptors: a comparative study of [18F]ASEM and [18F]DBT-10 in nonhuman primates, and further evaluation of [18F]ASEM in humans.

PURPOSE: The α7 nicotinic acetylcholine receptor (nAChR) is implicated in many neuropsychiatric disorders, making it an important target for positron emission tomography (PET) imaging. The first aim of this work was to compare two α7 nAChRs PET radioligands, [18F]ASEM (3-(1,4-diazabicyclo[3.2.2]nonan-4-yl)-6-([18F]fluorodibenzo[b,d]thiophene 5,5-dioxide) and [18F]DBT-10 (7-(1,4-diazabicyclo[3.2.2]nonan-4-yl)-2-([18F]fluorodibenzo[b,d]thiophene 5,5-dioxide), in nonhuman primates. The second aim was to assess further the quantification and test-retest variability of [18F]ASEM in humans. METHODS: PET scans with high specific activity [18F]ASEM or [18F]DBT-10 were acquired in three rhesus monkeys (one male, two female), and the kinetic properties of these radiotracers were compared. Additional [18F]ASEM PET scans with blocking doses of nicotine, varenicline, and cold ASEM were acquired separately in two animals. Next, six human subjects (five male, one female) were imaged with [18F]ASEM PET for 180 min, and arterial sampling was used to measure the parent input function. Different modeling approaches were compared to identify the optimal analysis method and scan duration for quantification of [18F]ASEM distribution volume (V T). In addition, retest scans were acquired in four subjects (three male, one female), and the test-retest variability of V T was assessed. RESULTS: In the rhesus monkey brain [18F]ASEM and [18F]DBT-10 exhibited highly similar kinetic profiles. Dose-dependent blockade of [18F]ASEM binding was observed, while administration of either nicotine or varenicline did not change [18F]ASEM V T. [18F]ASEM was selected for further validation because it has been used in humans. Accurate quantification of [18F]ASEM V T in humans was achieved using multilinear analysis with at least 90 min of data acquisition, resulting in V T values ranging from 19.6 ± 2.5 mL/cm3 in cerebellum to 25.9 ± 2.9 mL/cm3 in thalamus. Test-retest variability of V T was 11.7 ± 9.8%. CONCLUSIONS: These results confirm [18F]ASEM as a suitable radiotracer for the imaging and quantification of α7 nAChRs in humans.

Receptor Occupancy of the κ-Opioid Antagonist LY2456302 Measured with Positron Emission Tomography and the Novel Radiotracer 11C-LY2795050.

The κ-opioid receptor (KOR) is thought to play an important therapeutic role in a wide range of neuropsychiatric and substance abuse disorders, including alcohol dependence. LY2456302 is a recently developed KOR antagonist with high affinity and selectivity and showed efficacy in the suppression of ethanol consumption in rats. This study investigated brain penetration and KOR target engagement after single oral doses (0.5-25 mg) of LY2456302 in 13 healthy human subjects. Three positron emission tomography scans with the KOR antagonist radiotracer (11)C-LY2795050 were conducted at baseline, 2.5 hours postdose, and 24 hours postdose. LY2456302 was well tolerated in all subjects without serious adverse events. Distribution volume was estimated using the multilinear analysis 1 method for each scan. Receptor occupancy (RO) was derived from a graphical occupancy plot and related to LY2456302 plasma concentration to determine maximum occupancy (rmax) and IC50. LY2456302 dose dependently blocked the binding of (11)C-LY2795050 and nearly saturated the receptors at 10 mg, 2.5 hours postdose. Thus, a dose of 10 mg of LY2456302 appears well suited for further clinical testing. Based on the pharmacokinetic (PK)-RO model, the rmax and IC50 of LY2456302 were estimated as 93% and 0.58 ng/ml to 0.65 ng/ml, respectively. Assuming that rmax is 100%, IC50 was estimated as 0.83 ng/ml.

PET imaging evaluation of [(18)F]DBT-10, a novel radioligand specific to α7 nicotinic acetylcholine receptors, in nonhuman primates.

PURPOSE: Positron emission tomography (PET) radioligands specific to α7 nicotinic acetylcholine receptors (nAChRs) afford in vivo imaging of this receptor for neuropathologies such as Alzheimer's disease, schizophrenia, and substance abuse. This work aims to characterize the kinetic properties of an α7-nAChR-specific radioligand, 7-(1,4-diazabicyclo[3.2.2]nonan-4-yl)-2-[(18)F]-fluorodibenzo[b,d]thiophene 5,5-dioxide ([(18)F]DBT-10), in nonhuman primates. METHODS: [(18)F]DBT-10 was produced via nucleophilic substitution of the nitro-precursor. Four Macaca mulatta subjects were imaged with [(18)F]DBT-10 PET, with measurement of [(18)F]DBT-10 parent concentrations and metabolism in arterial plasma. Baseline PET scans were acquired for all subjects. Following one scan, ex vivo analysis of brain tissue was performed to inspect for radiolabeled metabolites in brain. Three blocking scans with 0.69 and 1.24 mg/kg of the α7-nAChR-specific ligand ASEM were also acquired to assess dose-dependent blockade of [(18)F]DBT-10 binding. Kinetic analysis of PET data was performed using the metabolite-corrected input function to calculate the parent fraction corrected total distribution volume (V T/f P). RESULTS: [(18)F]DBT-10 was produced within 90 min at high specific activities of 428 ± 436 GBq/µmol at end of synthesis. Metabolism of [(18)F]DBT-10 varied across subjects, stabilizing by 120 min post-injection at parent fractions of 15-55%. Uptake of [(18)F]DBT-10 in brain occurred rapidly, reaching peak standardized uptake values (SUVs) of 2.9-3.7 within 30 min. The plasma-free fraction was 18.8 ± 3.4%. No evidence for radiolabeled [(18)F]DBT-10 metabolites was found in ex vivo brain tissue samples. Kinetic analysis of PET data was best described by the two-tissue compartment model. Estimated V T/f P values were 193-376 ml/cm(3) across regions, with regional rank order of thalamus > frontal cortex > striatum > hippocampus > occipital cortex > cerebellum > pons. Dose-dependent blockade of [(18)F]DBT-10 binding by structural analog ASEM was observed throughout the brain, and occupancy plots yielded a V ND/f P estimate of 20 ± 16 ml/cm(3). CONCLUSION: These results demonstrate suitable kinetic properties of [(18)F]DBT-10 for in vivo quantification of α7-nAChR binding in nonhuman primates.

Comparative evaluation of two glycine transporter 1 radiotracers [11C]GSK931145 and [18F]MK-6577 in baboons.

Glycine transporter type-1 (GlyT1) has been proposed as a target for drug development for schizophrenia. PET imaging with a GlyT1 specific radiotracer will allow for the measurement of target occupancy of GlyT1 inhibitors, and for in vivo investigation of GlyT1 alterations in schizophrenia. We conducted a comparative evaluation of two GlyT1 radiotracers, [(11) C]GSK931145, and [(18) F]MK-6577, in baboons. Two baboons were imaged with [(11) C]GSK931145 and [(18) F]MK-6577. Blocking studies with GSK931145 (0.3 or 0.2 mg/kg) were conducted to determine the level of tracer specific binding. [(11) C]GSK931145 and [(18) F]MK-6577 were synthesized in good yield and high specific activity. Moderately fast metabolism was observed for both tracers, with ∼ 30% of parent at 30 min post-injection. In the brain, both radiotracers showed good uptake and distribution profiles consistent with regional GlyT1 densities. [(18) F]MK-6577 displayed higher uptake and faster kinetics than [(11) C]GSK931145. Time activity curves were well described by the two-tissue compartment model. Regional volume of distribution (VT ) values were higher for [(18) F]MK-6577 than [(11) C]GSK931145. Pretreatment with GSK931145 reduced tracer uptake to a homogeneous level throughout the brain, indicating in vivo binding specificity and lack of a reference region for both radiotracers. Linear regression analysis of VT estimates between tracers indicated higher specific binding for [(18) F]MK-6577 than [(11) C]GSK931145, consistent with higher regional binding potential (BPND ) values of [(18) F]MK-6577 calculated using VT from the baseline scans and non-displaceable distribution volume (VND ) derived from blocking studies. [(18) F]MK-6577 appears to be a superior radiotracer with higher brain uptake, faster kinetics, and higher specific binding signals than [(11) C]GSK931145.

Highly penetrative, drug-loaded nanocarriers improve treatment of glioblastoma.

Current therapy for glioblastoma multiforme is insufficient, with nearly universal recurrence. Available drug therapies are unsuccessful because they fail to penetrate through the region of the brain containing tumor cells and they fail to kill the cells most responsible for tumor development and therapy resistance, brain cancer stem cells (BCSCs). To address these challenges, we combined two major advances in technology: (i) brain-penetrating polymeric nanoparticles that can be loaded with drugs and are optimized for intracranial convection-enhanced delivery and (ii) repurposed compounds, previously used in Food and Drug Administration-approved products, which were identified through library screening to target BCSCs. Using fluorescence imaging and positron emission tomography, we demonstrate that brain-penetrating nanoparticles can be delivered to large intracranial volumes in both rats and pigs. We identified several agents (from Food and Drug Administration-approved products) that potently inhibit proliferation and self-renewal of BCSCs. When loaded into brain-penetrating nanoparticles and administered by convection-enhanced delivery, one of these agents, dithiazanine iodide, significantly increased survival in rats bearing BCSC-derived xenografts. This unique approach to controlled delivery in the brain should have a significant impact on treatment of glioblastoma multiforme and suggests previously undescribed routes for drug and gene delivery to treat other diseases of the central nervous system.

A Promising PET Tracer for Imaging of α7 Nicotinic Acetylcholine Receptors in the Brain: Design, Synthesis, and in Vivo Evaluation of a Dibenzothiophene-Based Radioligand.

Changes in the expression of α7 nicotinic acetylcholine receptors (α7 nAChRs) in the human brain are widely assumed to be associated with neurological and neurooncological processes. Investigation of these receptors in vivo depends on the availability of imaging agents such as radioactively labelled ligands applicable in positron emission tomography (PET). We report on a series of new ligands for α7 nAChRs designed by the combination of dibenzothiophene dioxide as a novel hydrogen bond acceptor functionality with diazabicyclononane as an established cationic center. To assess the structure-activity relationship (SAR) of this new basic structure, we further modified the cationic center systematically by introduction of three different piperazine-based scaffolds. Based on in vitro binding affinity and selectivity, assessed by radioligand displacement studies at different rat and human nAChR subtypes and at the structurally related human 5-HT3 receptor, we selected the compound 7-(1,4-diazabicyclo[3.2.2]nonan-4-yl)-2-fluorodibenzo-[b,d]thiophene 5,5-dioxide (10a) for radiolabeling and further evaluation in vivo. Radiosynthesis of [18F]10a was optimized and transferred to an automated module. Dynamic PET imaging studies with [18F]10a in piglets and a monkey demonstrated high uptake of radioactivity in the brain, followed by washout and target-region specific accumulation under baseline conditions. Kinetic analysis of [18F]10a in pig was performed using a two-tissue compartment model with arterial-derived input function. Our initial evaluation revealed that the dibenzothiophene-based PET radioligand [18F]10a ([18F]DBT-10) has high potential to provide clinically relevant information about the expression and availability of α7 nAChR in the brain.

Evaluation of the agonist PET radioligand [¹¹C]GR103545 to image kappa opioid receptor in humans: kinetic model selection, test-retest reproducibility and receptor occupancy by the antagonist PF-04455242.

INTRODUCTION: Kappa opioid receptors (KOR) are implicated in several brain disorders. In this report, a first-in-human positron emission tomography (PET) study was conducted with the potent and selective KOR agonist tracer, [(11)C]GR103545, to determine an appropriate kinetic model for analysis of PET imaging data and assess the test-retest reproducibility of model-derived binding parameters. The non-displaceable distribution volume (V(ND)) was estimated from a blocking study with naltrexone. In addition, KOR occupancy of PF-04455242, a selective KOR antagonist that is active in preclinical models of depression, was also investigated. METHODS: For determination of a kinetic model and evaluation of test-retest reproducibility, 11 subjects were scanned twice with [(11)C]GR103545. Seven subjects were scanned before and 75 min after oral administration of naltrexone (150 mg). For the KOR occupancy study, six subjects were scanned at baseline and 1.5 h and 8 h after an oral dose of PF-04455242 (15 mg, n=1 and 30 mg, n=5). Metabolite-corrected arterial input functions were measured and all scans were 150 min in duration. Regional time-activity curves (TACs) were analyzed with 1- and 2-tissue compartment models (1TC and 2TC) and the multilinear analysis (MA1) method to derive regional volume of distribution (V(T)). Relative test-retest variability (TRV), absolute test-retest variability (aTRV) and intra-class coefficient (ICC) were calculated to assess test-retest reproducibility of regional VT. Occupancy plots were computed for blocking studies to estimate occupancy and V(ND). The half maximal inhibitory concentration (IC50) of PF-04455242 was determined from occupancies and drug concentrations in plasma. [(11)C]GR103545 in vivo K(D) was also estimated. RESULTS: Regional TACs were well described by the 2TC model and MA1. However, 2TC VT was sometimes estimated with high standard error. Thus MA1 was the model of choice. Test-retest variability was ~15%, depending on the outcome measure. The blocking studies with naltrexone and PF-04455242 showed that V(T) was reduced in all regions; thus no suitable reference region is available for the radiotracer. V(ND) was estimated reliably from the occupancy plot of naltrexone blocking (V(ND)=3.4±0.9 mL/cm(3)). The IC50 of PF-04455242 was calculated as 55 ng/mL. [(11)C]GR103545 in vivo K(D) value was estimated as 0.069 nmol/L. CONCLUSIONS: [(11)C]GR103545 PET can be used to image and quantify KOR in humans, although it has slow kinetics and variability of model-derived kinetic parameters is higher than desirable. This tracer should be suitable for use in receptor occupancy studies, particularly those that target high occupancy.

Radiolabeling of poly(lactic-co-glycolic acid) (PLGA) nanoparticles with biotinylated F-18 prosthetic groups and imaging of their delivery to the brain with positron emission tomography.

The avidin-biotin interaction permits rapid and nearly irreversible noncovalent linkage between biotinylated molecules and avidin-modified substrates. We designed a biotinylated radioligand intended for use in the detection of avidin-modified polymer nanoparticles in tissue with positron emission tomography (PET). Using an F-18 labeled prosthetic group, [(18)F]4-fluorobenzylamine, and a commercially available biotin derivate, NHS-PEG4-biotin, [(18)F]-fluorobenzylamide-poly(ethylene glycol)4-biotin ([(18)F]NPB4) was prepared with high purity and specific activity. The attachment of the [(18)F]NPB4 radioligand to avidin-modified poly(lactic-co-glycolic acid) (PLGA) nanoparticles was tested by using PET imaging to measure the kinetics of convection-enhanced delivery (CED) of nanoparticles of varying size to the rat brain. PET imaging enabled the direct observation of nanoparticle delivery by measurement of the spatial volume of distribution of radiolabeled nanoparticles as a function of time, both during and after the infusion. This work thus validates new methods for radiolabeling PEG-biotin derivatives and also provides insight into the fate of nanoparticles that have been infused directly into the brain.

Evaluation of the sensitivity of the novel α4ß2* nicotinic acetylcholine receptor PET radioligand 18F-(-)-NCFHEB to increases in synaptic acetylcholine levels in rhesus monkeys.

OBJECTIVE: 18F-(-)-NCFHEB (also known as 18F-(-)-Flubatine) is a new radioligand to image α4ß2* nicotinic acetylcholine receptors in vivo with positron emission tomography (PET), with faster kinetics than previous radioligands such as 18F-2-F-A85380. The goal of this study was to assess the sensitivity of 18F-(-)-NCFHEB-PET to increases in synaptic acetylcholine concentration induced by acetylcholinesterase inhibitors. METHODS: Two rhesus monkeys were scanned four times each on a Focus 220 scanner: first at baseline, then during two bolus plus infusions of physostigmine (0.06-0.28 mg/kg), and finally following a bolus injection of donepezil (0.25 mg/kg). The arterial input function and the plasma free fraction fP were measured. 18F-(-)-NCFHEB volume of distribution VT was estimated using the multilinear analysis MA1 and then normalized by plasma free fraction fP . RESULTS: 18F-(-)-NCFHEB fP was 0.89±0.04. At baseline, 18F-(-)-NCFHEB VT /fP ranged from 7.9±1.3 mL plasma/cm3 tissue in the cerebellum to 34.3±8.4 mL plasma/cm3 tissue in the thalamus. Physostigmine induced a dose-dependent reduction of 18F-(-)-NCFHEB VT /fP of 34±9% in the putamen, 32±8% in the thalamus, 25±8% in the cortex, and 23±10% in the hippocampus. With donepezil, 18F-(-)-NCFHEB VT /fP was reduced by 24±2%, 14+3% and 14±5%, 10±6% in the same regions. CONCLUSION: 18F-(-)-NCFHEB can be used to detect changes in synaptic acetylcholine concentration and is a promising tracer to study acetylcholine dynamics with shorter scan durations than previous radioligands.

PET Imaging of Sphingosine-1-Phosphate Receptor 1 with [18F]TZ4877 in Nonhuman Primates.

Purpose: The sphingosine-1-phosphate receptor-1 (S1PR1) is involved in regulating responses to neuroimmune stimuli. There is a need for S1PR1-specific radioligands with clinically suitable brain pharmcokinetic properties to complement existing radiotracers. This work evaluated a promising S1PR1 radiotracer, [18F]TZ4877, in nonhuman primates. Procedures: [18F]TZ4877 was produced via nucleophilic substitution of tosylate precursor with K[18F]/F- followed by deprotection. Brain PET imaging data were acquired with a Focus220 scanner in two Macaca mulatta (6, 13 years old) for 120-180 min following bolus injection of 118-163 MBq [18F]TZ4877, with arterial blood sampling and metabolite analysis to measure the parent input function and plasma free fraction (f P). Each animal was scanned at baseline, 15-18 min after 0.047-0.063 mg/kg of the S1PR1 inhibitor ponesimod, 33 min after 0.4-0.8 mg/kg of the S1PR1-specific compound TZ82112, and 167-195 min after 1 ng/kg of the immune stimulus endotoxin. Kinetic analysis with metabolite-corrected input function was performed to estimate the free fraction corrected total distribution volume (V T/f P). Whole-body dosimetry scans were acquired in 2 animals (1M, 1F) with a Biograph Vision PET/CT System, and absorbed radiation dose estimates were calculated with OLINDA. Results: [18F]TZ4877 exhibited fast kinetics that were described by the reversible 2-tissue compartment model. Baseline [18F]TZ4877 f P was low (< 1%), and [18F]TZ4877 V T/f P values were 233-866 mL/cm3. TZ82112 dose-dependently reduced [18F]TZ4877 V T/f P, while ponesimod and endotoxin exhibited negligible effects on V T/f P, possibly due to scan timing relative to dosing. Dosimetry studies identified the critical organs of gallbladder (0.42 (M) and 0.31 (F) mSv/MBq) for anesthetized nonhuman primate. Conclusions: [18F]TZ4877 exhibits reversible kinetic properties, but the low f P value limits quantification with this radiotracer. S1PR1 is a compelling PET imaging target, and these data support pursuing alternative F-18 labeled radiotracers for potential future human studies.

Test-retest reproducibility of [11C]-(+)-propyl-hexahydro-naphtho-oxazin positron emission tomography using the bolus plus constant infusion paradigm.

We examined the reproducibility of using the constant infusion paradigm for equilibrium measurement of D2/3 receptors using [11C]-(+)-propyl-hexahydro-naphtho-oxazin (PHNO) positron emission tomography (PET). Six subjects were scanned with a bolus plus constant infusion (Kbol = 80 minutes) of [11C]-(+)-PHNO. Binding potential (BPND) was computed using the equilibrium approach and compared to a simplified reference tissue model (SRTM). The rate of change in the concentration-activity curve from 60 to 90 minutes was -5 ± 13%/h in the caudate, putamen, substantia nigra, thalamus, and cerebellum but was 15 ± 15%/h in the ventral striatum and pallidum. Test-retest variability was lower in striatal compared to extrastriatal regions (4 ± 8% vs -8 ± 22%, respectively) using the equilibrium approach, with comparable results with SRTM. The equilibrium ratio and SRTM yielded reliable BPND estimates (intraclass correlation coefficient = 0.88 and 0.82, respectively). These studies support the reproducibility of the bolus plus constant infusion paradigm with [11C]-(+)-PHNO PET.

In Vivo Imaging and Kinetic Modeling of Novel Glycogen Synthase Kinase-3 Radiotracers [11C]OCM-44 and [18F]OCM-50 in Non-Human Primates.

Glycogen synthase kinase 3 (GSK-3) is a potential therapeutic target for a range of neurodegenerative and psychiatric disorders. The goal of this work was to evaluate two leading GSK-3 positron emission tomography (PET) radioligands, [11C]OCM-44 and [18F]OCM-50, in non-human primates to assess their potential for clinical translation. A total of nine PET scans were performed with the two radiotracers using arterial blood sampling in adult rhesus macaques. Brain regional time-activity curves were extracted and fitted with one- and two-tissue compartment models using metabolite-corrected arterial input functions. Target selectivity was assessed after pre-administration of the GSK-3 inhibitor PF-04802367 (PF-367, 0.03-0.25 mg/kg). Both radiotracers showed good brain uptake and distribution throughout grey matter. [11C]OCM-44 had a free fraction in the plasma of 3% at baseline and was metabolized quickly. The [11C]OCM-44 volume of distribution (VT) values in the brain increased with time; VT values from models fitted to truncated 60-min scan data were 1.4-2.9 mL/cm3 across brain regions. The plasma free fraction was 0.6% for [18F]OCM-50 and VT values (120-min) were 0.39-0.87 mL/cm3 in grey matter regions. After correcting for plasma free fraction increases during blocking scans, reductions in regional VT indicated >80% target occupancy by 0.1 mg/kg of PF-367 for both radiotracers, supporting target selectivity in vivo. [11C]OCM-44 and [18F]OCM-50 warrant further evaluation as radioligands for imaging GSK-3 in the brain, though radio-metabolite accumulation may confound image analysis.

Cholinergic system adaptations are associated with cognitive function in people recently abstinent from smoking: a (-)-[18F]flubatine PET study.

The cholinergic system is a critical mediator of cognition in animals. People who smoke cigarettes exhibit cognitive deficits, especially during quit attempts. Few studies jointly examine the cholinergic system and cognition in people while trying to quit smoking. We used positron emission tomography (PET) brain imaging with the ß2-subunit containing nicotinic acetylcholine receptor (ß2*-nAChR) partial agonist radioligand (-)-[18F]flubatine and the acetylcholinesterase inhibitor physostigmine to jointly examine the cholinergic system, smoking status, and cognition. (-)-[18F]Flubatine scans and cognitive data were acquired from twenty people who recently stopped smoking cigarettes (aged 38 ± 11 years; 6 female, 14 male; abstinent 7 ± 1 days) and 27 people who never smoked cigarettes (aged 29 ± 8 years; 11 female, 16 male). A subset of fifteen recently abstinent smokers and 21 never smokers received a mid-scan physostigmine challenge to increase acetylcholine levels. Regional volume of distribution (VT) was estimated with equilibrium analysis at "baseline" and post-physostigmine. Participants completed a cognitive battery prior to (-)-[18F]flubatine injection and physostigmine administration assessing executive function (Groton Maze Learning test), verbal learning (International Shopping List test), and working memory (One Back test). Physostigmine significantly decreased cortical (-)-[18F]flubatine VT, consistent with increased cortical acetylcholine levels reducing the number of ß2*-nAChR sites available for (-)-[18F]flubatine binding, at comparable magnitudes across groups (p values < 0.05). A larger magnitude of physostigmine-induced decrease in (-)-[18F]flubatine VT was significantly associated with worse executive function in people who recently stopped smoking (p values < 0.05). These findings underscore the role of the cholinergic system in early smoking cessation and highlight the importance of neuroscience-informed treatment strategies.