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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.

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 [11C]ITDM, a useful radioligand for mGluR1 imaging exhibited that the striatal binding potential (BPND) of [11C]ITDM for mGluR1 in PET assessments was increased in NAC-treated animals at day 7 after implantation (2.30) compared with before implantation (1.92). The dynamic changes in striatal BPND during the experimental period were highly correlated with basal Glu concentration. In conclusion, density of mGluR1 is rapidly upregulated by increases in basal Glu concentration, suggesting that mGluR1 might to be a potential biomarker of abnormal conditions in the brain.

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.

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.

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: 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.

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.

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.

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.

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.

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.

Background [ 18 F]Fluoromisonidazole ([ 18 F]FMISO) and 1-[ 18 F]fluoro-3-((2-((1 E ,3 E )-4-(6-(methylamino)pyridine-3-yl)buta-1,3-dien-1-yl)benzo[d]thiazol-6-yl)oxy)propan-2-ol ([18 F]PM-PBB3 or [ 18 F]APN-1607) are clinically used radiotracers for imaging hypoxia and tau pathology, respectively. Both radiotracers were produced by direct 18 F-fluorination using the corresponding tosylate precursors 1 or 2 and [ 18 F]F - , followed by the removal of protecting groups. In this study, we synthesized [ 18F]FMISO and [ 18 F]PM-PBB3 by 18 F-fluoroalkylation using [ 18 F]epifluorohydrin ([ 18 F] 5 ) for clinical applications. Results First, [ 18 F] 5 was synthesized by the reaction of 1,2-epoxypropyl tosylate ( 8 ) with [ 18 F]F - and was purified by distillation. Subsequently, [ 18 F] 5 was reacted with 2-nitroimidazole ( 6 ) or PBB3 ( 7 ) as a precursor for 18 F-labeling, and each reaction mixture was purified by preparative high-performance liquid chromatography and formulated to obtain the [ 18 F]FMISO or [ 18 F]PM-PBB3 injection. All synthetic sequences were performed using an automated 18 F-labeling synthesizer. The obtained [ 18 F]FMISO showed sufficient radioactivity (0.83 ± 0.20 GBq at the end of synthesis (EOS); n = 8) with appropriate radiochemical yield based on [ 18F]F - (26 ± 7.5% at EOS, decay-corrected; n = 8). The obtained [ 18 F]PM-PBB3 also showed sufficient radioactivity (0.79 ± 0.10 GBq at EOS; n = 11) with appropriate radiochemical yield based on [ 18 F]F - (16 ± 3.2% at EOS, decay-corrected; n = 11). Conclusions Both [ 18 F]FMISO and [ 18 F]PM-PBB3 injections were successfully synthesized with sufficient radioactivity by 18 F-fluoroalkylation using [ 18 F] 5 .

Recently, a straightforward one-pot method for 18F-fluoroethylation without azeotropic drying of cyclotron-produced [18F]F– was developed. In this study, we have attempted to simplify the automated radiosynthesis of two [18F]fluoroethylated tracers, [18F]FEDAC and [18F]FET, using a desmethyl labeling precursor and [18F]fluoroethyl tosylate, based on the above-mentioned method. The radiochemical yields of [18F]FEDAC and [18F]FET were 26 ± 3.7% (n = 5) and 14 ± 2.2% (n = 4), respectively, based on total [18F]F– at the end of irradiation.

Monoacylglycerol lipase (MAGL) is a 33 kDa member of the serine hydrolase superfamily that preferentially degrades 2-arachidonoylglycerol (2-AG) to arachidonic acid in the endocannabinoid system. Inhibition of MAGL is not only of interest for probing the cannabinoid pathway but also as a therapeutic and diagnostic target for neuroinflammation. Limited attempts have been made to image MAGL in vivo and a suitable PET ligand for this target has yet to be identified and is urgently sought to guide small molecule drug development in this pathway. Herein we synthesized and evaluated the physiochemical properties of an array of eleven sulfonamido-based carbamates and ureas with a series of terminal aryl moieties, linkers and leaving groups. The most potent compounds were a novel MAGL inhibitor, N-((1-(1H-1,2,4-triazole-1-carbonyl)piperidin-4-yl) methyl)-4-chlorobenzenesulfonamide (TZPU; IC50 = 35.9 nM), and the known inhibitor 1,1,1,3,3,3-hexafluoropropan-2-yl 4-(((4-chlorophenyl)sulfonamido) methyl)piperidine-1-carboxylate (SAR127303; IC50 = 39.3 nM), which were also shown to be selective for MAGL over fatty acid amide hydrolase (FAAH), and cannabinoid receptors (CB1 & CB2). Both of these compounds were radiolabeled with carbon-11 via [(11)C]COCl2, followed by comprehensive ex vivo biodistribution and in vivo PET imaging studies in normal rats to determine their brain permeability, specificity, clearance and metabolism. Whereas TZPU did not show adequate specificity to warrant further evaluation, [(11)C]SAR127303 was advanced for preliminary PET neuroimaging studies in nonhuman primate. The tracer showed good brain permeability (ca. 1 SUV) and heterogeneous regional brain distribution which is consistent with the distribution of MAGL.

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 11C-labeled positron emission tomography (PET) tracer originating from a mGluR2 inhibitor, 3-(cyclopropylmethyl)-7-((4-(4-methoxyphenyl)piperidin-1-yl)methyl)-8-(trifluoromethyl)-[1,2,4]triazolo[4,3-a]pyridine (CMTP, 1a). [11C]CMTP ([11C]1a) was synthesized by O-[11C]methylation of desmethyl precursor 1b with [11C]methyl iodide in 19.7 ± 8.9% (n = 10) radiochemical yield (based on [11C]CO2) with >98% radiochemical purity and >74 GBq/μmol molar activity. Autoradiography study showed that [11C]1a possessed moderate in vitro specific binding to mGluR2 in the rat brain, with a heterogeneous distribution of radioactive accumulation in the mGluR2-rich brain tissue sections, such as the cerebral cortex and striatum. PET study indicated that [11C]1a was able to cross the blood–brain barrier and enter the brain, but had very low specific binding in the rat brain. Further optimization for the chemical structure of 1a is necessary to increase binding affinity to mGluR2 and then improve in vivo specific binding in brain.

Monoacylglycerol lipase (MAGL) is the principle enzyme for metabolizing endogenous cannabinoid ligand 2-arachidonoyglycerol (2-AG). Blockade of MAGL increases 2-AG levels, resulting in subsequent activation of the endocannabinoid system, and has emerged as a novel therapeutic strategy to treat drug addiction, inflammation, and neurodegenerative diseases. Herein we report a new series of MAGL inhibitors, which were radiolabeled by site-specific labeling technologies, including 11C-carbonylation and spirocyclic iodonium ylide (SCIDY) radiofluorination. The lead compound [11C]10 (MAGL-0519) demonstrated high specific binding and selectivity in vitro and in vivo. We also observed unexpected washout kinetics with these irreversible radiotracers, in which in vivo evidence for turnover of the covalent residue was unveiled between MAGL and azetidine carboxylates. This work may lead to new directions for drug discovery and PET tracer development based on azetidine carboxylate inhibitor scaffold.