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.

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.

The Search for a Subtype-Selective PET Imaging Agent for the GABAA Receptor Complex: Evaluation of the Radiotracer [11C]ADO in Nonhuman Primates.

The myriad physiological functions of γ-amino butyric acid (GABA) are mediated by the GABA-benzodiazepine receptor complex comprising of the GABAA, GABAB, and GABAC groups. The various GABAA subunits with region-specific distributions in the brain subserve different functional and physiological roles. For example, the sedative and anticonvulsive effects of classical benzodiazepines are attributed to the α1 subunit, and the α2 and α3 subunits mediate the anxiolytic effect. To optimize pharmacotherapies with improved efficacy and devoid of undesirable side effects for the treatment of anxiety disorders, subtype-selective imaging radiotracers are required to assess target engagement at GABA sites and determine the dose-receptor occupancy relationships. The goal of this work was to characterize, in nonhuman primates, the in vivo binding profile of a novel positron emission tomography (PET) radiotracer, [11C]ADO, which has been indicated to have functional selectivity for the GABAA α2/α3 subunits. High specific activity [11C]ADO was administrated to 3 rhesus monkeys, and PET scans of 120-minute duration were performed on the Focus-220 scanner. In the blood, [11C]ADO metabolized at a fairly rapid rate, with ∼36% of the parent tracer remaining at 30 minutes postinjection. Uptake levels of [11C]ADO in the brain were high (peak standardized uptake value of ∼3.0) and consistent with GABAA distribution, with highest activity levels in cortical areas, intermediate levels in cerebellum and thalamus, and lowest uptake in striatal regions and amygdala. Tissue kinetics was fast, with peak uptake in all brain regions within 20 minutes of tracer injection. The one-tissue compartment model provided good fits to regional time-activity curves and reliable measurement of kinetic parameters. The absolute test-retest variability of regional distribution volumes ( VT) was low, ranging from 4.5% to 8.7%. Pretreatment with flumazenil (a subtype nonselective ligand, 0.2 mg/kg, intravenous [IV], n = 1), Ro15-4513 (an α5-selective ligand, 0.03 mg/kg, IV, n = 2), and zolpidem (an α1-selective ligand, 1.7 mg/kg, IV, n = 1) led to blockade of [11C]ADO binding by 96.5%, 52.5%, and 76.5%, respectively, indicating the in vivo binding specificity of the radiotracer. Using the nondisplaceable volume of distribution ( VND) determined from the blocking studies, specific binding signals, as measured by values of regional binding potential ( BPND), ranged from 0.6 to 4.4, which are comparable to those of [11C]flumazenil. In conclusion, [11C]ADO was demonstrated to be a specific radiotracer for the GABAA receptors with several favorable properties: high brain uptake, fast tissue kinetics, and high levels of specific binding in nonhuman primates. However, subtype selectivity in vivo is not obvious for the radiotracer, and thus, the search for subtype-selective GABAA radiotracers continues.

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.