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Background A family of BF2-chelated tetraaryl-azadipyrromethenes was developed as non-porphyrin photosensitizers for photodynamic therapy. Among the developed photosensitizers, ADPM06 exhibited excellent photochemical and photophysical properties. Molecular imaging is a useful tool for photodynamic therapy planning and monitoring. Radiolabeled photosensitizers can efficiently address photosensitizer biodistribution, providing helpful information for photodynamic therapy planning. To evaluate the biodistribution of ADPM06 and predict its pharmacokinetics on photodynamic therapy, we synthesized [18F]ADPM06 and evaluated its in vivo properties. Results [18F]ADPM06 was automatically synthesized by Lewis acid-assisted isotopic 18F-19F exchange using ADPM06 and tin (IV) chloride at room temperature for 10 min. Radiolabeling was carried out using 0.4 µmol of ADPM06 and 200 µmol of tin (IV) chloride. The radiosynthesis time was approximately 60 min, and the radiochemical purity was > 95% at the end of the synthesis. The decay-corrected radiochemical yield from [18F]F- at the end of irradiation was 13 ± 2.7% (n = 5). In the biodistribution study, radioactivity levels in the heart, lungs, liver, pancreas, spleen, kidney, small intestine, muscle, and brain gradually decreased over 120 min after the initial uptake. The mean radioactivity level in the bone was the highest among all organs investigated and increased for 120 min after injection. Upon co-injection with ADPM06, the radioactivity levels in the blood, heart, and brain significantly increased, whereas those in the lung, liver, pancreas, kidney, small intestine, muscle, and bone were not affected. In the metabolite study of the plasma in mice, the percentage of radioactivity corresponding to [18F]ADPM06 was 76.3 ± 1.6% (n = 3). In a positron emission tomography study using MDA-MB-231-HTB-26 tumor-bearing mice, radioactivity accumulated in the bone at a relatively high level and in the tumor at a moderate level for 60 min after injection. Conclusions We synthesized [18F]ADPM06 using an automated 18F-labeling synthesizer and evaluated the biodistribution of [18F]ADPM06 in mice, which may be useful for predicting the pharmacokinetics of ADPM06 in photodynamic therapy.

The transmembrane α-amino-3-hydroxyl-5-methyl-4-isoxazolepropionic acid (AMPA) receptor regulatory protein γ-8 (TARP γ-8) constitutes an auxiliary subunit of AMPA receptors, which mediates various brain functions including learning and memory. TARP γ-8 has emerged as a promising therapeutic target for central nervous system disorders. Despite considerable efforts, previously reported TARP γ-8 PET radioligands, such as [ 11C]TARP-1903 and [11C]TARP-1811 series, were plagued by limited brain uptake and/or high nonspecific binding in vivo. Herein, we developed two novel 11C-labeled probes, [11C]8 and [11C]15 (also named as [11C]TARP2105), of which the latter exhibited a reasonable brain uptake as well as specific binding toward TARP γ-8 both in vitro and in vivo, as confirmed by blocking experiments with the commercially available TARP γ-8 inhibitor, JNJ-55511118 in the TARP γ-8-rich hippocampus. Overall, [11C]15 exhibited promising tracer characteristics and proved to be a lead positron-emission tomography ligand for the non-invasive quantification of TARP γ-8 in the mammalian brain.

The aim of this study is to investigate the changes in expression of metabotropic glutamate (Glu) receptor subtype 1 (mGluR1), a key molecule involved in neuroexcitetoxicity, during excessive Glu release in the brain by PET imaging. An animal model of excessive Glu release in the brain was produced by intraperitoneally implanting an Alzet osmotic pump containing N-acetylcysteine (NAC), an activator of the cysteine/Glu antiporter, into the abdomen of rats. Basal Glu concentration in the brain was measured by microdialysis, which showed that basal Glu concentration in NAC-treated rats (0.31 µM) was higher than that in saline-treated rats (0.17 µM) at day 7 after the implantation of the osmotic pump. Similarly, PET studies with [

Monoacylglycerol lipase (MAGL) is a 33 kDa serine protease primarily responsible for hydrolyzing 2-arachidonoylglycerol into the proinflammatory eicosanoid precursor arachidonic acid in the central nervous system. Inhibition of MAGL constitutes an attractive therapeutic concept for treating psychiatric disorders and neurodegenerative diseases. Herein, we present the design and synthesis of multiple reversible MAGL inhibitor candidates based on a piperazinyl azetidine scaffold. Compounds 10 and 15 were identified as the best-performing reversible MAGL inhibitors by pharmacological evaluations, thus channeling their radiolabeling with fluorine-18 in high radiochemical yields and favorable molar activity. Furthermore, evaluation of [18F]10 and [18F]15 ([18F]MAGL-2102) by autoradiography and positron emission tomography (PET) imaging in rodents and nonhuman primates demonstrated favorable brain uptakes, heterogeneous radioactivity distribution, good specific binding, and adequate brain kinetics, and [18F]15 demonstrated a better performance. In conclusion, [18F]15 was found to be a suitable PET radioligand for the visualization of MAGL, harboring potential for the successful translation into humans.

Monoacylglycerol lipase (MAGL) is a 33 kDa serine protease primarily responsible for hydrolyzing 2-arachidonoylglycerol into the proinflammatory eicosanoid precursor arachidonic acid in the central nervous system. Inhibition of MAGL constitutes an attractive therapeutic concept for treating psychiatric disorders and neurodegenerative diseases. Herein, we present the design and synthesis of multiple reversible MAGL inhibitor candidates based on a piperazinyl azetidine scaffold. Compounds 10 and 15 were identified as the best-performing reversible MAGL inhibitors by pharmacological evaluations, thus channeling their radiolabeling with fluorine-18 in high radiochemical yields and favorable molar activity. Furthermore, evaluation of [18F]10 and [18F]15 ([18F]MAGL-2102) by autoradiography and positron emission tomography (PET) imaging in rodents and nonhuman primates demonstrated favorable brain uptakes, heterogeneous radioactivity distribution, good specific binding, and adequate brain kinetics, and [18F]15 demonstrated a better performance. In conclusion, [18F]15 was found to be a suitable PET radioligand for the visualization of MAGL, harboring potential for the successful translation into humans.

Background/Aims: [18F]Fluoroalkyl groups are essential labeling units because they are considered as surrogates for [11C]methyl moieties, and are coupled to the same functional units as the [11C]methyl group. Many [18F]fluoroalkyated PET tracers have been developed. In general, 18F-fluoroalkylation using [18F]fluoroalkyl reagents requires multi-step radiosynthesis procedures and a multi-pot 18F-labeling synthesizer. To overcome these limitations, a straightforward one-pot method for 18F-fluoroethylation without azeotropic drying of [18F]F- was developed [1]. We have used an improved one-pot 18F-fluoroethylaion method to synthesize 18F-fluoroethylated tracers [2]. In this study, we further modified this one-pot method suitable for 18F-fluoromethylation, and simplified the automated radiosynthesis of two [18F]fluoromethylated tracers using this method. Methods: We synthesized [18F]fluoromethyl tosylate in a mixture of 18F- in K222/K2CO3 acetonitrile solution including 2% water, bis(tosyloxy)methane and cesium carbonate. Without purification of [18F]fluoromethyl tosylate, we directly added a labeling precursor to this mixture for the simplified one-pot 18F-fluoromethylation. Using this procedure equipped to a 18F-labeling synthesizer, [18F]FCho (a PET tracer for imaging tumor) and [18F]FMeNER-D2 (a PET tracer for imaging norepinephrine transporter) were automatically synthesized by the reactions of their corresponding labeling precursors with [18F]fluoromethyl tosylate, respectively. Results: Using the simplified one-pot 18F-fluoromethylation procedure in the automated radiosynthesis, we achieved [18F]FCho and [18F]FMeNER-D2 in approximately 10% of radiochemical yield from 18F- at the end of irradiation (EOI). Radiosynthesis times and radiochemical purities of two 18F-labeled tracers were approximately 60 min after EOI and over 95%, respectively. Conclusions: We have successively synthesized [18F]FCho and [18F]FMeNER-D2 using the simplified one-pot 18F-fluoromethylation method, and achieved automation for all radiosynthesis processes using an 18F-labeling synthesizer. The present method provides a shorter synthesis time and automated procedures with one-pot for the 18F-fluoromethylation strategy., 第13回世界核医学会(13th Congress of the World Federation of Nuclear Medicine and Biology)

Background/Aims: Metabotropic glutamate receptor 1 (mGluR1), a key mediator of glutamatergic signaling, is frequently expressed as an oncoprotein and has been an attractive target to overcome most solid tumors, such as melanoma (1). Here, a novel small-molecular radiopharmaceutical pair, 3-iodo-N-[4-[6-(methylamino)pyrimidin-4-yl]-1,3-thiazol-2-yl]-N-[11C]methylbenzamide ([11C]1) and 3-211At-astato-N-[4-[6-(methylamino)pyrimidin-4-yl]-1,3-thiazol-2-yl]-N-methylbenzamide ([211At]1), was designed and developed to target the mGluR1 for theranostics of melanomas. Methods: [ 11C]1 was synthesized by reacting a N-desmethyl precursor with [11C]CH3OTf in the presence of NaOH at room temperature for 5 min. Radiolabeling with [211At]1 was performed by reaction of aryl tin precursor with NCS-containing 211At/MeOH solution according to the method reported by our laboratory (2). The theranostic potentials of the radiopharmaceutical pair were explored for PET imaging and radiotherapy in mGluR1-positive B16F10 melanoma-bearing mice. Results: [ 11C]1 and [211At]1 were obtained with a radiochemical purity of greater than 99% and radiochemical yields of 19 ± 8 % and 46 ± 3%, respectively, based on the total radioactivity of used radionuclides. In vivo PET imaging of [ 11C]1 clearly visualized the targeted melanomas with a good tumor-to-background contrast. Ex vivo biodistribution study verified the persistent increase of [ 11C]1, which reached 12.29 ± 2.44 %ID/g tissue at 90 min in the targeted melanomas, and rapidly cleared from nontarget organs after intravenous injection. In the therapeutic studies, [211At]1 exhibited unequivocal and durable antitumor efficacy with only a single treatment (2.96 MBq) in the melanoma model, compared to the controls (0.31 ± 0.08 cm3 vs. 10.44 ± 1.61 cm3 at 16 days post-therapy). No decrease in body weight and no liver and kidney damage were observed through the examination period in melanoma mice injected with 2.96 MBq of [211At]1. Conclusions: The novel small-molecular radiopharmaceutical pair successfully visualized the mGluR1-positive melanomas by [ 11C]1 with high contrast PET images, and further treated the melanoma by [211At]1 without significant toxicity. The results highlight the good potential of using [ 11C]1 and [211At]1 as theranostic agents for the management of mGluR1-positive tumors and should be further investigated in the theranostic field of oncology in the clinic., 第13回世界核医学会（13th orld Federation of Nuclear Medicine and Biology)

As a serine hydrolase, monoacylglycerol lipase (MAGL) is principally responsible for the metabolism of 2-arachidonoylglycerol (2-AG) in the central nervous system (CNS), leading to the formation of arachidonic acid (AA). Dysfunction of MAGL has been associated with multiple CNS disorders and symptoms, including neuroinflammation, cognitive impairment, epileptogenesis, nociception and neurodegenerative diseases. Inhibition of MAGL provides a promising therapeutic direction for the treatment of these conditions, and a MAGL positron emission tomography (PET) probe would greatly facilitate preclinical and clinical development of MAGL inhibitors. Herein, we design and synthesize a small library of fluoropyridyl-containing MAGL inhibitor candidates. Pharmacological evaluation of these candidates by activity-based protein profiling identified 14 as a lead compound, which was then radiolabeled with fluorine-18 via a facile SNAr reaction to form 2-[18F]fluoropyridine scaffold. Good blood–brain barrier permeability and high in vivo specific binding was demonstrated for radioligand [18F]14 (also named as [18F]MAGL-1902). This work may serve as a roadmap for clinical translation and further design of potent 18F-labeled MAGL PET tracers.

Objectives: Metabotropic glutamate receptor 1 (GRM1) is aberrantly overexpressed in a wide variety of human solid tumors, such as melanoma，but not in normal peripheral organs (1). Therefore, GRM1 can be a widely–applicable target for the theranostics in oncology. Here, a novel small-molecular radiopharmaceutical pair, 3-iodo- N-[4-[6-(methylamino)pyrimidin-4-yl]-1,3-thiazol-2-yl]-N-[11C]methylbenzamide ([11C]1) and 3-211At-astato-N-[4-[6-(methylamino)pyrimidin-4-yl]-1,3-thiazol-2-yl]-N-methylbenzamide ([211At]1), was designed and developed to target the GRM1 for the theranostics in melanomas (Fig.1). Method: [ 11C]1 was synthesized by reacting a N-desmethyl precursor with [11C]CH3OTf in the presence of NaOH at room temperature for 5 min. Radiolabeling of [211At]1 was performed by reaction of aryl tin precursor with NCS-containing 211At/MeOH solution. The theranostic potentials of the radiopharmaceutical pair were explored for PET imaging and radiotherapy in GRM1-positive B16F10 melanoma-bearing mice. Results: [ 11C]1 and [211At]1 were obtained with a radiochemical purity of greater than 99% and radiochemical yields of 19 ± 8 % and 46 ± 3%, respectively, based on the total radioactivity of used radionuclides. In vivo PET imaging of [ 11C]1 clearly visualized the targeted melanomas with a good tumor-to-background contrast (Fig. 1A). Ex vivo biodistribution study verified the persistent increase of [ 11C]1, which reached 12.29 ± 2.44 %ID/g tissue at 90 min in the targeted melanomas, and rapidly cleared from nontarget organs after intravenous injection. In the therapeutic studies, [211At]1 exhibited unequivocal and durable antitumor efficacy with only a single treatment (2.96 MBq) in the melanoma model, compared to the controls (0.38 ± 0.02 cm3 vs. 7.88 ± 1.28 cm3 at 13 days post-therapy) (Fig. 1B). No decrease in body weight, and no liver and kidney damage were observed through the examination period in melanoma mice injected with 2.96 MBq of [211At]1. Conclusion: The novel small-molecular radiopharmaceutical pair successfully visualized the GRM1-positive melanomas by [ 11C]1 with high contrast PET images, and further treated by [211At]1 without significant toxicity. The results highlight the good potential of using [ 11C]1 and [211At]1 as theranostic agents for the management of GRM1-positive tumors and should be further investigated in the theranostic field of oncology in the clinic., SNMMI2022

Objectives: A family of the BF2-chelated tetraaryl-azadipyrromethenes (ADPMs) was developed as a nonporphyrin photosensitizer (PS) for photodynamic therapy (PDT) [1]. Among ADPMs, ADPM06 displayed excellent photochemical and photophysical properties [1]. In addition, PDT using ADPM06 elicited impressive complete response rates in various tumor models when a short drug-light interval was applied [2]. Molecular imaging is a promising PDT planning and monitoring tool, and the PS biodistribution is a relevant issue for PDT planning that radiolabeled PSs may address efficiently. To evaluate efficiency for PDT using ADPM06 and also side effects of ADPM, we synthesized [18F]ADPM06 using an automated 18F-labeling synthesizer, and evaluated its in vivo properties. Methods: [18F]ADPM06 was synthesized using an automated 18F-labeling synthesizer by Lewis acid-assisted isotopic 18F-19F exchange [3]. The [18F]F– was extracted from a Sep-Pak Accell Plus QMA Carbonate Plus Light cartridge with a mixture of tetrabutylanmmonium bicarbonate aqueous solution and acetonitrile. The solution was concentrated by evaporation at 100 °C for 10 min under nitrogen gas flow. After the reaction vessel was cooled, the mixture of ADPM06 in acetonitrile and tin(IV) chloride (SnCl4) solution was added to the reaction vessel, and then was agitated using magnetic stirrer at room temperature for 10 min. After the reaction, the mixture was diluted with water for injection, and transferred to the injector for semi-preparative radio-HPLC. The HPLC fractions were collected in a flask, to which Tween 80 in ethanol was added prior to radiosynthesis. The solution was subsequently evaporated to dryness and the residue was dissolved in physiological saline. The product was analyzed by HPLC with radioactivity and UV-VIS detection. The in vivo biodistribution study was performed using mice. Results: To radio-synthesize efficiently [18F]ADPM06, we performed semi-automated radiosynthesis. By increasing the concentration of SnCl4 from 100 to 400 μmol, the radiochemical conversion (RCC) of [18F]ADPM06 from [18F]TBAF was increased until 60%. Also, the RCC using 0.8 μmol of ADPM06 was 1.2-fold higher than that using 0.4 μmol of ADPM06. In the radiosynthesis using an automated 18F-labeling synthesizer, we successfully synthesized [18F]ADPM06 for in vivo applications. The radiochemical yield (RCY) from [18F]F- was 13 ± 2.7 % (n = 5; 0.4 μmol of ADPM06, 200 μmol of SnCl4) at the end of irradiation. The radiosynthesis time was within 60 min, and radiochemical purity remained >95% after maintaining it for 1 hour after the end of synthesis. In the biodistribution study within 120 min after the injection, radioactivity levels in heart, lung, liver, pancreas, spleen, kidney, small intestine, muscle, and brain gradually decreased after initial uptake. Conclusions: We enabled to synthesize [18F]ADPM06 using an automated 18F-labeling synthesizer, and to evaluate biodistribution of [18F]ADPM06 in mice.

Objectives: The brain acidosis is caused by intracellular hyper-accumulation of acidic sources (H+, lactate, and carbonic acid) by switching the cellular energy metabolism from aerobic to anaerobic by the hypoxia. Such intra cellular acidosis give curucial injury to central nurves system of the brain. Therefore, monitoring intracellular pH would be very important to diagnosis neuronal condition. Recently, covalent inhibitors for monoacylglycerol lipase (MAGL), an enzyme intracellur lacated on neuron and astrocyte in the brain and regulates endocannabinoid system, were identified by Bulter et al [1]. Among these inhibitors, 1,1,1,3,3,3-hexafluoropropan-2-yl 3-(1-phenyl-1H-pyrazol-3-yl)azetidine-1-carboxylate (1) showed reversible inhibitory effect to MAGL. The purpose of this study is to establish quantification method of hydrolysis rate of compound 1 mediated by MAGL and to demonstrate pH-weighted PET imaging in the brain of ischemic rat. Methods: To estimate interaction between compound 1 and MAGL, docking simulations were conducted comparing to similar chemical structural irreversible-type inhibitor (2). In addition, to evaluate influents of pH shifts, molecular dynamics (MD) simulations of compound 1 were also performed under the neutral (pH 7) or acidic (pH 6) conditions. Radiosynthesis of [11C]1 and [11C]2 was described in another presentation in this meeting (Mori W, et a1.). To confirm MAGL-hydrolysis of [11C]1, in vitro assessments using rat brain homogenates were conducted. PET imaging with [11C]1 was carried out using middle cerebral artery occlusion (MCAO) rat as an acute hypoxia model and hydrolysis rate (KH) of [11C]1 with MAGL was estimated by monoexponential fitting on time-activity curves of ipsilateral region. Results: MD simulations predicted that azetidine carbamate moiety of 1 was easily hydrolyzed by MAGL due to close distance from water molecule, compared to 2 containing piperidine carbamate. Moreover, the acylated azetidine in 1 has been shown to react differently than piperidine ring size amide in 2 due to decreased planarity in the amide moiety itself, conveyed by the ring strain associated with the azetidine itself. Additionally, it was simulated that the hydrolysis rate of 1 would be slower under the acidic condition because of changing interaction of 1 against water molecule. In vitro assessments showed that generation rate of 11CO2, the final product derived from hydrolysis of [11C]1, would become slower depending on pH shifts. In PET study with [11C]1 using MCAO rat, KH value in ipsilateral region was significantly slower than that in contralateral region. Conclusions: We successfully established the method for quantifying hydrolysis rate of MAGL using new-type PET probe and demonstrated pH-weighted imaging in vivo.

Objectives: Monoacylglycerol lipase (MAGL) is an attractive target in development of pharmaceuticals for central nervous system disorders and neuroinflammation. So far, to visualize MAGL, we have developed several irreversible-type PET ligands [1,2]. Of these, PET ligands containing azetidine moieties have showed unexpected brain kinetics different from that of the typical irreversible-type PET ligand. Here, to understand relationship between the chemical structure and brain kinetics, we synthesized three new ligands including the azetidine ([11C]1), pyrrolidine ([11C]2) and piperidine ([11C]3) moieties and compared their kinetics in the brain. Methods: Compounds 1–3 and their precursors 5–7 for radiolabeling were synthesized from various pyrazole heterocyclic compounds in 6–7 steps. Synthesis of [11C]1–3 was performed using a home-made automated synthesizer equipped with [11C]COCl2 that was prepared from cyclotron-produced [11C]CO2[3]. As a general procedure exemplified for [11C]1 (Scheme 1), [11C]COCl2 was trapped into a solution of 1,1,1,3,3,3-hexafluoro-2-propanol (4; 3.1 μL) and 1,2,2,6,6-pentamethylpiperidine (PMP; 5.4 μL) in THF (200 μL) at 0 °C. The reaction mixture was heated at 30 °C for 3 min. A solution of corresponding pyrazole 5–7 (1.0 mg) and PMP (2.2 μL) in THF (200 μL) was added into the mixture and this mixture was heated at 30 °C for 3 min and then the solvent was removed at 80 °C for 5 min. The residue dissolved in the preparative HPLC solvent was applied to the semi-preparative HPLC column. Brain kinetics of these radioligands were evaluated by small-animal PET imaging using rats. Results: Starting from 22–24 GBq of [11C]CO2, [11C]1–3 were synthesized with 16–24% radiochemical yields (decay-corrected), 49–60 GBq/μmol molar activity, and ≥98% radiochemical purity. The average total synthesis time from the end of bombardment was 44 min. PET imaging with [11C]1 showed high radioactive uptake with a moderate clearance in MAGL-rich brain regions. In contrast, although radioactive uptake of [11C]3 in the brain indicated similar distribution to that of [11C]1, radioactive clearance from brain was not detected during PET scan. Conclusions: We successfully synthesized several 11C-labeled pyrazole heterocyclic compounds and compared their brain kinetics by PET imaging using rats. The result of present work suggested that the big heterocycle PET ligand for MAGL was more stable in vivo.

Carbon-11-labeled phosgene ([11C]phosgene, [11C]COCl2) is a useful labeling agent that connects two heteroatoms by inserting [11C]carbonyl (11C=O) function in carbamates, ureas, and carbonates, which are components of biologically important heterocyclic compounds and functional groups in drugs as a linker of fragments with in vivo stability. Development of 11C-labeled PET tracers has been performed using [11C]phosgene as a labeling agent. However, [11C]phosgene has not been frequently used for 11C-labeling because preparation of [11C]phosgene required dedicated synthesis apparatus (not commercially available) and had problems in reproducibility and reliability. In our laboratory, an improved method for synthesizing [11C]phosgene using a carbon tetrachloride detection tube kit in environmental air analysis and the automated synthesis system for preparing [11C]phosgene have been developed in 2009. This apparatus has been used for routine synthesis of 11C-labeled tracers 1–4 times/week. Using [11C]phosgene we have developed and produced many PET radiotracers containing [11C]urea and [11C]carbamate moieties. In this review, we report the performance of our method for preparing [11C]phosgene, including automated synthesis apparatus developed in house, and the application of [11C]phosgene for development and production of 11C-labeled PET tracers.

Carbon-11-labeled phosgene ([11C]phosgene, [11C]COCl2) is a useful labeling agent that connects two heteroatoms by inserting [11C]carbonyl (11C=O) function in carbamates, ureas, and carbonates, which are components of biologically important heterocyclic compounds and functional groups in drugs as a linker of fragments with in vivo stability. Development of 11C-labeled PET tracers has been performed using [11C]phosgene as a labeling agent. However, [11C]phosgene has not been frequently used for 11C-labeling because preparation of [11C]phosgene required dedicated synthesis apparatus (not commercially available) and had problems in reproducibility and reliability. In our laboratory, an improved method for synthesizing [11C]phosgene using a carbon tetrachloride detection tube kit in environmental air analysis and the automated synthesis system for preparing [11C]phosgene have been developed in 2009. This apparatus has been used for routine synthesis of 11C-labeled tracers 1-4 times/week. Using [11C]phosgene we have developed and produced many PET radiotracers containing [11C]urea and [11C]carbamate moieties. In this review, we report the performance of our method for preparing [11C]phosgene, including automated synthesis apparatus developed in house, and the application of [11C]phosgene for development and production of 11C-labeled PET tracers.

Monoacylglycerol lipase (MAGL) is a serine hydrolase, which metabolizes 2-arachidonoylglycerol (2-AG) to arachidonic acid (AA) in the central nervous system (CNS). Dysfunction of MAGL has been associated with multiple CNS disorders and symptoms, including neuroinflammation, cognitive impairment, epileptogenesis, nociception and neurodegenerative diseases. Inhibition of MAGL provides a promising therapeutic direction for the treatment of these conditions, and a MAGL positron emission tomography (PET) probe would greatly facilitate preclinical and clinical development of MAGL inhibitors. Herein, we design and synthesize a small library of fluoropyridyl-containing MAGL inhibitor candidates. Pharmacological evaluation of these candidates by activity-based protein profiling identified 14 as a lead compound, which was then radiolabeled with fluorine-18 via a facile SNAr reaction to form 2-[18F]fluoropyridinyl scaffold. Excellent blood−brain barrier penetration and high in vivo binding specificity was validated for radioligand [18F]14 (also named as [18F]MAGL-1902). This work may serve as a roadmap for clinical translation and further design of potent 18F-labeled MAGL PET tracers. KEY WORDS Monoacylglycerol lipase (MAGL); Central nervous system (CNS); 2-Arachidonylglycerol (2-AG); Arachidonic acid (AA); Positron emission tomography (PET); Fluorine-18

Metabotropic glutamate receptor 2 (mGlu2) is a known target for treating several central nervous system (CNS) disorders. To develop a viable positron emission tomography (PET) ligand for mGlu2, we identified new candidates 5a-i that are potent negative allosteric modulators (NAMs) of mGlu2. Among these candidates, 4-(2-fluoro-4-methoxyphenyl)-5-((1-methyl-1H-pyrazol-3-yl)methoxy)picolinamide (5i, also named as [11C]MG2-1812) exhibited high potency, high subtype selectivity, and favorable lipophilicity. Compound 5i was labeled with positron-emitting carbon-11 (11C) to obtain [11C]5i in high radiochemical yield and high molar activity by O-[11C]methylation of the phenol precursor 12 with [11C]CH3I. In vitro autoradiography with [11C]5i showed heterogeneous radioactive accumulation in the brain tissue sections, ranked in the order: cortex > striatum > hippocampus > cerebellum ≫ thalamus > pons. PET study of [11C]5i indicated in vivo specific binding of mGlu2 in the rat brain. Based on the [11C]5i scaffold, further optimization for new candidates is underway to identify a more suitable ligand for imaging mGlu2.

Metabotropic glutamate receptor 2 (mGlu2) is a known target for treating several central nervous system (CNS) disorders. To develop a viable positron emission tomography (PET) ligand for mGlu2, we identified new candidates 5a–i that are potent negative allosteric modulators (NAMs) of mGlu2. Among these candidates, 4-(2-fluoro-4-methoxyphenyl)-5-((1-methyl-1H-pyrazol-3-yl)methoxy)picolinamide (5i, also named as [11C]MG2-1812) exhibited high potency, high subtype selectivity, and favorable lipophilicity. Compound 5i was labeled with positron-emitting carbon-11 (11C) to obtain [11C]5i in high radiochemical yield and high molar activity by O-[11C]methylation of the phenol precursor 12 with [11C]CH3I. In vitro autoradiography with [11C]5i showed heterogeneous radioactive accumulation in the brain tissue sections, ranked in the order: cortex > striatum > hippocampus > cerebellum ≫ thalamus > pons. PET study of [11C]5i indicated in vivo specific binding of mGlu2 in the rat brain. Based on the [11C]5i scaffold, further optimization for new candidates is underway to identify a more suitable ligand for imaging mGlu2.

Fatty acid amide hydrolase (FAAH) is a key enzyme in the endocannabinoid system. N-(3,4-Dimethylisoxazol-5-yl)piperazine-4-[4-(2-fluoro-4-[11C]methylphenyl)thiazol-2-yl]-1-carboxamide ([11C]DFMC) was developed as an irreversible-type positron emission tomography (PET) tracer for FAAH. Here, we attempted to noninvasively estimate rate constant k3 (rate of transfer to the specifically-bound compartment) as a direct index for FAAH in the rat brain. First, the two-tissue compartment model analysis including three parameters [K1−k3, two-tissue compartment model for the irreversible-type radiotracer (2TCMi)] in PET study with [11C]DFMC was conducted, which provided 0.21 ± 0.04 ml·cm−3·min−1 of the net uptake value (Ki), an indirect index for FAAH, in the FAAH-richest region (the cingulate cortex). Subsequently, to noninvasively estimate Ki value, the reference model analysis (Patlak graphical analysis reference model) was tried using a time-activity curve of the spinal cord. In that result, the noninvasive Ki value (KREF) was concisely estimated with high correlation (r > 0.95) to Ki values based on 2TCMi. Using estimated KREF value, we tried to obtain calculated-k3 based on previously defined equations. The calculated k3 was successfully estimated with high correlation (r = 0.95) to direct k3 in 2TCMi. Finally, the dose relationship study using calculated k3 demonstrated that in vivo ED50 value of [3-(3-carbamoylphenyl)phenyl] N-cyclohexylcarbamate, a major inhibitor of FAAH, was 66.4 µg/kg in rat brain. In conclusion, we proposed the calculated k3 as an alternative index corresponding to regional FAAH concentrations and suggested that PET with [11C]DFMC enables occupancy study for new pharmaceuticals targeting FAAH. SIGNIFICANCE STATEMENT In the present study, we proposed calculated k3 as an alternative index corresponding with fatty acid amide hydrolase concentration. By using calculated k3, in vivo ED50 of [3-(3-carbamoylphenyl)phenyl] N-cyclohexylcarbamate was successfully estimated to be 66.4 µg/kg for rats. Thus, we demonstrated the pharmacological utility of positron emission tomography with N-(3,4-dimethylisoxazol-5-yl)piperazine-4-[4-(2-fluoro-4-[11C]methylphenyl)thiazol-2-yl]-1-carboxamide.

Fatty acid amide hydrolase (FAAH) is a key enzyme in the endocannabinoid system. N-(3,4-Dimethylisoxazol-5-yl)piperazine-4-[4-(2-fluoro-4-[11C]methylphenyl)thiazol-2-yl]-1-carboxamide ([11C]DFMC) was developed as an irreversible-type positron emission tomography (PET) ligand for FAAH. Here, we attempted to noninvasively estimate rate constant k3 as a direct index for FAAH in the rat brain. First, the two-tissue compartment model analysis including three parameters (K1–k3, 2TCMi) in PET study with [11C]DFMC was conducted, which provided 0.21 ± 0.04 mL·cm−3·min−1 of the net uptake value (Ki), an indirect index for FAAH, in the FAAH-richest region (the cingulate cortex). Subsequently, to noninvasively estimate Ki value, the reference model analysis (Patlak Reference, PGAREF) was tried using a time-activity curve of the spinal cord. In that result, the noninvasive Ki value (KREF) was concisely estimated with high correlation (r > 0.95) to Ki values based on 2TCMi. Using estimated KREF value, we tried to obtain calculated-k3 based on previously defined equations. The caluculated-k3 was successfully estimated with high correlation (r = 0.95) to direct k3 in 2TCMi. Finally, the dose relationship study using calculated-k3 demonstrated that in vivo ED50 value of URB597, a major inhibitor of FAAH, was 66.4 µg/kg in rat brain. In conclusion, we proposed the calculated-k3 as an alternative index corresponding to regional FAAH concentrations and suggested that PET with [11C]DFMC enables occupancy study for new pharmaceuticals targeting FAAH.

Mutations that increase leucine-rich repeat kinase 2 (LRRK2) activity in the brain are associated with Parkinson's disease. Here, we synthesized a novel compound 4-(6-fluoro-4-(5-isopropoxy-1H-indazol-3-yl)pyridin-2-yl)morpholine (FIPM) and labeled it with fluorine-18 ((18)F), to develop a positron emission tomography (PET) tracer for in vivo visualization of LRRK2 in the brain. FIPM showed high in vitro binding affinity for LRRK2 (IC(50) = 8.0 nM). [(18)F]FIPM was prepared in 5% radiochemical yield (n = 5), by inserting (18)F into a pyridine ring, followed by removal of the protecting group. After HPLC separation and formulation, [(18)F]FIPM was acquired with >97% radiochemical purity and 103–300 GBq μmol(–1) of molar activity at the end of radiosynthesis. Biodistribution and small-animal PET studies in mice indicated a low in vivo specific binding of [(18)F]FIPM. While [(18)F]FIPM presented limited potential as an in vivo PET tracer for LRRK2, we suggested that it can be used as a lead compound for developing new radiotracers with improved in vivo brain properties.

The 18 kDa translocator protein (TSPO) is a viable target for imaging of inflammation in brain. In the recent past we have explored a pharmacophore skeleton acetamidobenzoxazolone for positron emission tomography (PET) imaging of TSPO expression in brain. Here we evaluated a new radioligand for visualization of TSPO, namely [11C]N-(2-methoxyoxyphenyl)-N-methyl-2-(5-nitro-2-oxobenzo[d]oxazol-3(2H)-yl)acetamide ([11C]N′-MPB). This PET ligand exhibited high binding affinity towards TSPO (Ki = 4.9 nM) and a suitable lipophilicity (log D) of 2.08 for brain imaging. A biodistribution study on mice showed high accumulation of radioactivity in TSPO-rich organs, such as the lungs, heart, kidneys, and adrenal glands. Metabolite analysis of rat brain homogenate showed 98% intact [11C]N′-MPB at 30 min after injection. To determine the specific binding of the radioligand with TSPO on neuroinflammation of the brain, in vitro autoradiography and PET studies were performed in an ischemic rat model. In vitro autoradiography indicated significantly increased binding on the ipsilateral side compared with that on the contralateral side of ischemic rat brains. This result was supported firmly by the contrast of radioactivity in PET images. Displacement experiments with PK11195 minimized the difference in radioactivity uptake between the two sides. In summary, [11C]N′-MPB is a potential PET imaging radioligand for TSPO and, consequently, for gaining more insight for up-regulation of microglia during neuroinflammation.

Hypoxia caused by ischemia induces acidosis and neuroexcitotoxicity, resulting in neuronal death in the central nervous system (CNS). Monoacylglycerol lipase (MAGL) is a modulator of 2-arachidonoylglycerol (2-AG), which is involved in retrograde inhibition of glutamate release in the endocannabinoid system. In the present study, we used positron emission tomography (PET) to monitor MAGL-positive neurons and neuroinflammation in the brains of ischemic rats. Additionally, we performed PET imaging to evaluate the neuroprotective effects of an MAGL inhibitor in an ischemic injury model. Methods: Ischemic-injury rat models were induced by intraluminal right middle cerebral artery occlusion (MCAO). PET studies of the brains of the ischemic rats were performed at several experimental time points (pre-occlusion, days 2, 4, and 7 after the MCAO surgery) using [11C]SAR127303 for MAGL and [18F]FEBMP for 18 kDa translocator protein (TSPO, a hall-mark of neuroinflammation). Medication using minocycline (a well-known neuroprotective agent) or KML29 (a potent MAGL inhibitor) was given immediately after the MCAO surgery and then daily over the subsequent three days. Results: PET imaging of the ischemic rats using [11C]SAR127303 showed an acute decline of radioactive accumulation in the ipsilateral side at two days after MCAO surgery (ratio of the area under the curve between the ipsilateral and contralateral sides: 0.49 ± 0.04 in the cortex and 0.73 ± 0.02 in the striatum). PET imaging with [18F]FEBMP, however, showed a moderate increase in accumulation of radioactivity in the ipsilateral hemisphere on day 2 (1.36 ± 0.11), and further increases on day 4 (1.72 ± 0.15) and day 7 (1.99 ± 0.06). Treatment with minocycline or KML29 eased the decline in radioactive accumulation of [11C]SAR127303 for MAGL (minocycline-treated group: 0.82 ± 0.06 in the cortex and 0.81 ± 0.05 in the striatum; KML29-treated group: 0.72 ± 0.07 in the cortex and 0.88 ± 0.04 in the striatum) and increased uptake of [18F]FEBMP for TSPO (minocycline-treated group: 1.52 ± 0.21 in the cortex and 1.56 ± 0.11 in the striatum; KML29-treated group: 1.63 ± 0.09 in the cortex and 1.50 ± 0.17 in the striatum). In MCAO rats, minocycline treatment showed a neuroprotective effect in the sensorimotor cortex suffering from severe hypoxic injury, whereas KML29 treatment saved neurons in the striatum, including bundles of myelinated axons. Conclusions: PET imaging allowed visualization of the different neuroprotective effects of minocycline and KML29, and indicated that combination pharmacotherapy using these drugs may be an effective therapy in acute ischemia.

Dysfunction of monoacylglycerol lipase (MAGL) is associated with several psychopathological disorders, including drug addiction and neurodegenerative diseases. Herein we design, synthesize and evaluate several irreversible fluorine-containing MAGL inhibitors for positron emission tomography (PET) ligand development. Compound 6 (identified from a therapeutic agent) was advanced for (18)F-labeling via a novel spirocyclic iodonium ylide (SCIDY) strategy, which demonstrated high brain permeability and excellent specific binding. This work supports further development of novel (18)F-labeled MAGL PET probes.

Dysfunction of monoacylglycerol lipase (MAGL) is associated with several psychopathological disorders, including drug addiction and neurodegenerative diseases. Herein we design, synthesize, and evaluate several irreversible fluorine-containing MAGL inhibitors for positron emission tomography (PET) ligand development. Compound 6 (identified from a therapeutic agent) was advanced for 18F-labeling via a novel spirocyclic iodonium ylide (SCIDY) strategy, which demonstrated high brain permeability and excellent specific binding. This work supports further development of novel 18F-labeled MAGL PET probes.

Monoacylglycerol lipase (MAGL) is a major serine hydrolase that hydrolyses 2-arachidonoylglycerol (2-AG) into arachidonic acid (AA) and glycerol in the brain. Because 2-AG and AA are endogenous biologically active ligands in the brain, the inhibition of MAGL is an attractive therapeutic target for neurodegenerative diseases. In this study, to visualize MAGL via positron emission tomography (PET), we report a new carbon-11-labeled radiotracer, namely 1,1,1,3,3,3-hexafluoropropan-2-yl-3-(1-benzyl-1H-pyrazol-3-yl)azetidine-1-[11C]carboxylate ([11C]6). Compound 6 exhibited high in vitro binding affinity (IC50 = 0.41 nM) to MAGL in the brain with a suitable lipophilicity (cLogD = 3.29). [11C]6 was synthesized by reacting 1,1,1,3,3,3-hexafluoropropanol (7) with [11C]phosgene ([11C]COCl2), followed by a reaction with 3-(1-benzyl-1H-pyrazol-3-yl)azetidine hydrochloride (8), which resulted in a 15.0 ± 6.8% radiochemical yield (decay-corrected, n = 7) based on [11C]CO2 and a 45 min synthesis time from the end of bombardment. A biodistribution study in mice showed high uptake of radioactivity in MAGL-rich organs, including the lungs, heart, and kidneys. More than 90% of the total radioactivity was irreversibly bound in the brain homogenate of rats 5 min and 30 min after the radiotracer injection. PET summation images of rat brains showed high radioactivity in all brain regions. Pretreatment with 6 or MAGL-selective inhibitor JW642 significantly reduced the uptake of radioactivity in the brain. [11C]6 is a promising PET tracer which offers in vivo specific binding and selectivity for MAGL in rodent brains.

Monoacylglycerol lipase (MAGL) is a serine hydrolase that degrades 2-arachidonoylglycerol (2-AG) in the endocannabinoid system (eCB). Selective inhibition of MAGL has emerged as a potential therapeutic approach for the treatment of diverse pathological conditions including chronic pain, inflammation, cancer and neurodegeneration. Herein we disclose a novel array of reversible and irreversible MAGL inhibitors by means of tail switching on a piperazinyl azetidine scaffold. We developed a lead irreversible-binding MAGL inhibitor 8, and reversible-binding compounds 17 and 37 which are amenable for radiolabeling with (11)C or (18)F. [(11)C]8 ([(11)C]MAGL-2–11) exhibited high brain uptake and excellent binding specificity in the brain towards MAGL. Reversible radioligands [(11)C]17 ([(11)C]PAD) and [(18)F]37 ([(18)F]MAGL-4–11) also demonstrated excellent in vivo binding specificity towards MAGL in peripheral organs. This work may pave the way for the development of MAGL-targeted positron emission tomography (PET) tracers with tunability in reversible and irreversible binding mechanism.

Metabotropic glutamate receptor 2 (mGluR2) has been suggested as a therapeutic target for treating schizophrenia-like symptoms arising from increased glutamate transmission in the human forebrain. However, no reliable positron emission tomography (PET) radiotracer allowing for in vivo visualization of mGluR2 in the human brain is currently available. In this study, we synthesized 4-(2-fluoro-4-[11C]methoxyphenyl)-5-((2-methylpyridin-4-yl)methoxy)picolinamide ([11C]1) and evaluated its potential as a PET tracer for imaging mGluR2 in the rodent brain. Compound 1, a negative allosteric modulator (NAM) of mGluR2, showed high in vitro binding affinity (IC50: 26 nM) for mGluR2 overexpressed in human cells. [11C]1 was synthesized by O-[11C]methylation of the phenol precursor 2 with [11C]methyl iodide. After the reaction, HPLC purification and formulation, [11C]1 of 7.4 ± 2.8 GBq (n = 8) was obtained from [11C]carbon dioxide of 22.5 ± 4.8 GBq (n = 8) with >99% radiochemical purity and 70 ± 32 GBq/μmol (n = 8) molar activity at the end of synthesis. In vitro autoradiography for rat brains showed that [11C]1 binding was heterogeneously distributed in the cerebral cortex, striatum, hippocampus, and cerebellum. This pattern is consistent with the regional distribution pattern of mGluR2 in the rodent brain. The radioactivity was significantly reduced by self- or MNI-137 (a mGluR2 NAM) blocking. Small-animal PET studies indicated a low in vivo specific binding of [11C]1 in the rat brain. The brain uptake was increased in a P-glycoprotein and breast cancer resistant protein double knockout mouse, when compared to a wild-type mouse. While [11C]1 presented limited potential as an in vivo PET tracer for mGluR2, we suggested that it can be used as a lead compound for developing new radiotracers with improved in vivo brain properties.

Phosphodiesterase 10A (PDE10A) is a newly identified therapeutic target for central-nervous-system disorders. 2-(2-(3-(4-([18F]Fluoroethoxy)phenyl)-4-oxo-3,4-dihydroquinazolin-2-yl)ethyl)-4-isopropoxyisoindoline-1,3-dione ([18F]MNI-659, [18F]5) is a useful positron-emission-tomography (PET) ligand for imaging of PDE10A in the human brain. However, the radiolabeled metabolite of [18F]5 can accumulate in the brain. In this study, using [18F]5 as a lead compound, we designed four new 18F-labeled ligands ([18F]6–9) to find one more suitable than [18F]5. Of these, 2-(2-(3-(4-([18F]fluoromethoxy-d2)phenyl)-4-oxo-3,4-dihydroquinazolin-2-yl)ethyl)-4-isopropoxyisoindoline-1,3-dione ([18F]9) exhibited high in vitro binding affinity (Ki = 2.9 nM) to PDE10A and suitable lipophilicity (log D = 2.2). In PET studies, the binding potential (BPND) of [18F]9 (5.8) to PDE10A in the striatum of rat brains was significantly higher than that of [18F]5 (4.6). Furthermore, metabolite analysis showed much lower levels of contamin...

Deposition of intracellular α-synuclein fibrils is implicated in neurodegenerative parkinsonian disorders, while high-contrast in vivo detection of α-synuclein depositions has been unsuccessful in animal models and humans. Here, we have developed a bimodal imaging probe, C05-05, for visualizing α-synuclein inclusions in the brains of living animals modeling α-synuclein propagation. In vivo optical and PET imaging of a mouse model demonstrated visualization of α-synuclein aggregates by C05-05, revealing a dynamic propagation of fibrillogenesis along neural pathways followed by disruptions of these structures. Moreover, longitudinal 18F-C05-05-PET of a marmoset model captured widespread dissemination of fibrillary pathologies accompanied by neurodegeneration detected by dopamine transporter PET. In addition, in vitro assays demonstrated the high-affinity binding of 18F-C05-05 to α-synuclein versus other protein pathologies in human brain tissues. Collectively, we propose a new imaging technology enabling etiological and therapeutic assessments of α-synuclein pathogenesis at non-clinical levels, highlighting the applicability of C05-05 to clinical PET.

As a serine hydrolase, monoacylglycerol lipase (MAGL) is principally responsible for the metabolism of 2-arachidonoylglycerol (2-AG) in the central nervous system (CNS), leading to the formation of arachidonic acid (AA). Dysfunction of MAGL has been associated with multiple CNS disorders and symptoms, including neuroinflammation, cognitive impairment, epileptogenesis, nociception and neurodegenerative diseases. Inhibition of MAGL provides a promising therapeutic direction for the treatment of these conditions, and a MAGL positron emission tomography (PET) probe would greatly facilitate preclinical and clinical development of MAGL inhibitors. Herein, we design and synthesize a small library of fluoropyridyl-containing MAGL inhibitor candidates. Pharmacological evaluation of these candidates by activity-based protein profiling identified 14 as a lead compound, which was then radiolabeled with fluorine-18 via a facile SNAr reaction to form 2-[18F]fluoropyridine scaffold. Good blood–brain barrier permeability and high in vivo specific binding was demonstrated for radioligand [18F]14 (also named as [18F]MAGL-1902). This work may serve as a roadmap for clinical translation and further design of potent 18F-labeled MAGL PET tracers., Graphical abstract A highly potent irreversible MAGL PET tracer [18F]14 was described, which exhibited favorable in vitro and in vivo characteristics, including excellent affinity, high brain uptake, and good binding specificity.Image 1

Recently, a straightforward one-pot method for

Metabotropic glutamate receptor 2 (mGlu(2)) is a known target for treating several central nervous system (CNS) disorders. To develop a viable PET ligand for mGlu(2), we identified new candidates 5a–5i as potent negative allosteric modulators (NAMs) of mGlu(2). Among these candidates, 4-(2-fluoro-4-methoxyphenyl)-5-((1-methyl-1H-pyrazol-3-yl)methoxy)picolinamide (5i, also named as [(11)C]MG2-1812) exhibited high potency, high subtype selectivity, and favorable lipophilicity. Compound 5i was labeled with positron-emitting carbon-11 ((11)C) to obtain [(11)C]5i in high radiochemical yield and high molar activity by O-[(11)C]methylation of the phenol precursor 12 with [(11)C]CH(3)I. In vitro autoradiography with [(11)C]5i showed heterogeneous radioactive accumulation in the brain tissue sections, ranked in the order: cortex > striatum > hippocampus > cerebellum ≫ thalamus > pons. PET study of [(11)C]5i indicated in vivo specific binding of mGlu(2) in the rat brain. Based on the [(11)C]5i scaffold, further optimization for new candidates is underway to identify a more suitable ligand for imaging mGlu(2).

Selective metabotropic glutamate receptor 2 (mGluR2) inhibitors have been demonstrated to show therapeutic effects by improving alleviating symptoms of schizophrenic patients in clinical studies. Herein we report the synthesis and preliminary evaluation of a

Selective DAAO inhibitors have demonstrated promising therapeutic effects in clinical studies, including clinically alleviating symptoms of schizophrenic patients and ameliorating cognitive function in Alzheimer’s patients with early phase. Herein we report the synthesis and preliminary evaluation of a 11C-labeled positron emission tomography ligand based on a DAAO inhibitor, DAO-1903 (8). 11C-Isotopologue of 8 was prepared in high radiochemical yield with high radiochemical purity (>99%) and high molar activity (>37 GBq/µmol). In vitro autoradiography studies indicated that the ligand possessed high in vitro specific binding to DAAO, while in vivo dynamic PET studies demonstrated that [11C]8 failed to cross the blood–brain barrier possibly due to moderate brain efflux mechanism. Further chemical scaffold optimization is necessary to overcome limited brain permeability and improve specific binding.

a-Amino-3-hydroxyl-5-methyl-4-isoxazolepropionic acid (AMPA) receptors are implicated in the pathology of neurological diseases such as epilepsy and schizophrenia. As pan antagonists for this target are often accompanied with undesired effects at high doses, one of the recent drug discovery approaches has shifted to subtype-selective AMPA receptor (AMPAR) antagonists, specifically, via modulating transmembrane AMPAR regulatory proteins (TARPs). The quantification of AMPARs by positron emission tomography (PET) would help obtain insights into disease conditions in the living brain and advance the translational development of AMPAR antagonists. Herein we report the design, synthesis and preclinical evaluation of a series of TARP γ-8 antagonists, amenable for radiolabeling, for the development of subtype-selective AMPAR PET imaging agents. Based on the pharmacology evaluation, molecular docking studies and physiochemical properties, we have identified several promising lead compounds 3, 17–19 and 21 for in vivo PET studies. All candidate compounds were labeled with [11C]COCl2 in high radiochemical yields (13–31% RCY) and high molar activities (35–196 GBq/μmol). While tracers 30 ([11C]17) & 32 ([11C]21) crossed the blood-brain barrier and showed heterogeneous distribution in PET studies, consistent with TARP γ-8 expression, high nonspecific binding prevented further evaluation. To our delight, tracer 31 ([11C]3) showed good in vitro specific binding and characteristic high uptake in the hippocampus in rat brain tissues, which provides the guideline for further development of a new generation subtype selective TARP γ-8 dependent AMPAR tracers.

DAA1106 (N-(2,5-dimethoxybenzyl)-N-(5-fluoro-2-phenoxyphenyl)acetamide), is a potent and selective ligand for the translocator protein (18 kDa, TSPO) in brain mitochondrial fractions of rats and monkey (Ki = 0.043 and 0.188 nM, respectively). In this study, to translate [18F]DAA1106 for clinical studies, we performed automated syntheses of [18F]DAA1106 using the spirocyclic iodonium ylide (1) as a radiolabelling precursor and conducted preclinical studies including positron emission tomography (PET) imaging of TSPO in ischemic rat brains. Radiofluorination of the ylide precursor 1 with [18F]F−, followed by HPLC separation and formulation, produced the [18F]DAA1106 solution for injection in 6% average (n = 10) radiochemical yield (based on [18F]F−) with >98% radiochemical purity and molar activity of 60–100 GBq/μmol at the end of synthesis. The synthesis time was 87 min from the end of bombardment. The automated synthesis achieved [18F]DAA1106 with sufficient radioactivity available for preclinical and clinical use. Biodistribution study of [18F]DAA1106 showed a low uptake of radioactivity in the mouse bones. Metabolite analysis showed that >96% of total radioactivity in the mouse brain at 60 min after the radiotracer injection was unmetabolized [18F]DAA1106. PET study of ischemic rat brains visualized ischemic areas with a high uptake ratio (1.9 ± 0.3) compared with the contralateral side. We have provided evidence that [18F]DAA1106 could be routinely produced for clinical studies.

Fatty acid amide hydrolase (FAAH) is a key enzyme in the endocannabinoid system.