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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 4decline 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.

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: 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.

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.

Selective modulation of metabotropic glutamate receptor 2 (mGlu2) represents a novel therapeutic approach for treating brain disorders, including schizophrenia, depression, Parkinson’s disease (PD), Alzheimer’s disease (AD), drug abuse and addiction. Imaging mGlu2 using positron emission tomography (PET) would allow in vivo quantification under physiological and pathological conditions and facilitate drug discovery by enabling target engagement studies. However, to date, few successful efforts have been disclosed to visualize mGlu2 in vivo, indicating an unmet and urgent clinical need. Herein we report the synthesis and preclinical evaluation of a focused library of mGlu2 modulators amenable for radiolabeling. The promising candidate 4-(2,4-difluorophenyl)-7-((2-methoxypyridin -4-yl)methyl)-5,6,7,8-tetrahydro-1,7-naphthyridine-2-carboxamide (21b), exhibited high potency and excellent subtype selectivity, which was then evaluated by in vitro autoradiography and in vivo PET imaging studies after labeling with carbon-11. Ligand [11C]21b (also known as [11C]MG2-1904) demonstrated high brain uptake and excellent in vitro/in vivo specific binding towards mGlu2 with high metabolic stability in the brain. As proof-of-concept, our preliminary work demonstrated a successful example of visualizing mGlu2 in vivo derived from negative allosteric modulators (NAMs), which represents a promising chemotype for further development and optimization aimed for clinical translation.

N-methyl-D-aspartate receptors (NMDARs) play critical roles in the physiological function of the mammalian central nervous system (CNS), including learning, memory, and synaptic plasticity, through modulating excitatory neurotransmission. Attributed to etiopathology of various CNS disorders and neurodegenerative diseases, GluN2B is one of the most well-studied subtypes in preclinical and clinical studies on NMDARs. Herein, we report the synthesis and preclinical evaluation of two 11C-labeled GluN2B-selective negative allosteric modulators (NAMs) containing N,N-dimethyl-2-(1H-pyrrolo[3,2-b]pyridin-1-yl)acetamides for positron emission tomography (PET) imaging. Two PET ligands, namely [11C]31 and [11C]37 (also called N2B-1810 and N2B-1903, respectively) were labeled with [11C]CH3I in good radiochemical yields (decay-corrected 28% and 32% relative to starting [11C]CO2, respectively), high radiochemical purity (>99%) and high molar activity (>74 GBq/μmol). In particular, PET ligand [11C]31 demonstrated moderate specific binding to GluN2B subtype by in vitro autoradiography studies. However, because in vivo PET imaging studies showed limited brain uptake of [11C]31 (up to 0.5 SUV), further medicinal chemistry and ADME optimization are necessary for this chemotype attributed to low binding specificity and rapid metabolism in vivo.

Selective modulation of metabotropic glutamate receptor 2 (mGlu2) represents a novel therapeutic approach for treating brain disorders, including schizophrenia, depression, Parkinson's disease (PD), Alzheimer's disease (AD), drug abuse and addiction. Imaging mGlu2 using positron emission tomography (PET) would allow for in vivo quantification under physiological and pathological conditions and facilitate drug discovery by enabling target engagement studies. In this paper, we aimed to develop a novel specific radioligand derived from negative allosteric modulators (NAMs) for PET imaging of mGlu2. Methods. A focused small molecule library of mGlu2 NAMs with tetrahydro naphthyridine scaffold was synthesized for pharmacology and physicochemical evaluation. GIRK dose-response assays and CNS panel binding selectivity assays were performed to study the affinity and selectivity of mGlu2 NAMs, among which compounds 14a and 14b were selected as PET ligand candidates. Autoradiography in SD rat brain sections was used to confirm the in vitro binding specificity and selectivity of [11C]14a and [11C]14b towards mGlu2. In vivo binding specificity was then studied by PET imaging. Whole body biodistribution study and radiometabolite analysis were conducted to demonstrate the pharmacokinetic properties of [11C]14b as most promising PET mGlu2 PET ligand. Results. mGlu2 NAMs 14a-14g were synthesized in 14%-20% yields in five steps. NAMs 14a and 14b were selected to be the most promising ligands due to their high affinity in GIRK dose-response assays. [11C]14a and [11C]14b displayed similar heterogeneous distribution by autoradiography, consistent with mGlu2 expression in the brain. While PET imaging study showed good brain permeability for both tracers, compound [11C]14b demonstrated superior binding specificity compared to [11C]14a. Further radiometabolite analysis of [11C]14b showed excellent stability in the brain. Conclusions. Compound 14b exhibited high affinity and excellent subtype selectivity, which was then evaluated by in vitro autoradiography and in vivo PET imaging study after labeling with carbon-11. Ligand [11C]14b, which we named [11C]MG2-1904, demonstrated high brain uptake and excellent in vitro/in vivo specific binding towards mGlu2 with high metabolic stability in the brain. As proof-of-concept, our preliminary work demonstrated a successful example of visualizing mGlu2 in vivo derived from NAMs, which represents a promising chemotype for further development and optimization aimed for clinical translation.

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.

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.

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.

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

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 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.

Muscarinic acetylcholine receptors (mAChRs) have five distinct subunits (M1 -M5 ) and are involved in the action of the neurotransmitter acetylcholine in the central and peripheral nervous system. Attributed to the promising clinical efficacy of xanomeline, an M1 /M4 -preferring agonist, in patients of schizophrenia and Alzheimer's disease, M1 - or M4 -selective mAChR modulators have been developed that target the topographically distinct allosteric sites. Herein we report the synthesis and preliminary evaluation of 11 C-labeled positron emission tomography (PET) ligands based on a validated M4 R positive allosteric modulator VU0467485 (AZ13713945) to facilitate drug discovery. [11 C]VU0467485 and two other ligands were prepared in high radiochemical yields (>30 %, decay-corrected) with high radiochemical purity (>99 %) and high molar activity (>74 GBq μmol-1 ). In vitro autoradiography studies indicated that these three ligands possess moderate-to-high in vitro specific binding to M4 R. Nevertheless, further physiochemical property optimization is necessary to overcome the challenges associated with limited brain permeability.

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...

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.

To introduce the 3-[18 F]fluoro-2-hydroxypropyl moiety into positron emission tomography (PET) radiotracers, we performed automated synthesis of (rac)-, (R)-, and (S)-[18 F]epifluorohydrin ([18 F]1) by nucleophilic displacement of (rac)-, (R)-, or (S)-glycidyl tosylate with 18 F- and purification by distillation. The ring-opening reaction of (R)- or (S)-[18 F]1 with phenol precursors gave enantioenriched [18 F]fluoroalkylated products without racemisation. We then synthesised (rac)-, (R)-, and (S)- 2-{5-[4-(3-[18 F]fluoro-2-hydroxypropoxy)phenyl]-2-oxobenzo[d]oxazol-3(2H)-yl}-N-methyl-N-phenylacetamide ([18 F]6) as novel radiotracers for the PET imaging of translocator protein (18 kDa) and showed that (R)- and (S)-[18 F]6 had different radioactivity uptake in mouse bone and liver. Thus, (rac)-, (R)-, and (S)-[18 F]1 are effective radiolabelling reagents and can be used to develop PET radiotracers by examining the effects of chirality on their in vitro binding affinities and in vivo behaviour.

The imdazoline I2 receptor (I2R) has been found in the feeding centers of the brain, such as the hypothalamus, and certain I2R ligands have been reported to stimulate food intake. Thus, it has been proposed that I2R may play a role in feeding control. [11C]BU99008 was developed as a positron emission tomography (PET) tracer for imaging of I2R. [11C]BU99008 displayed relatively high brain penetration and specific binding by brain PET studies in preclinical studies. Here, we evaluated a pathological condition caused by obesity related to I2R function by quantitative PET study using [11C]BU99008. PET scans were acquired in the Zucker (ZUC) lean and fatty rats, radioactivity and metabolites of plasma were measured, and the kinetic parameters were estimated. Radioactivity levels after the injection of [11C]BU99008 in the hypothalamus of both ZUC lean and fatty rats were highly accumulated, and then gradually decreased until 60 min after the injection. The accumulated radioactivity from 30 to 60 min after the injection in the hypothalamus of the ZUC fatty rats was 1.3 times greater than that of lean rats. The volume of distribution (VT) estimated by Logan graphical analysis in the hypothalamus of the ZUC fatty rats was 1.8 times greater than that in the ZUC lean rats. In metabolite analysis, the percentages of the unchanged form in the plasma of the ZUC fatty rats at 60 min after the injection (5.0 %) was significantly lower than that of lean rats (9.1 %). By PET imaging using [11C]BU99008, we demonstrated that the accumulated radioactivity and estimated VT value in the feeding center of ZUC lean rats was lower than that in fatty rats. PET studies using [11C]BU99008 may contribute to elucidate a pathological condition caused by obesity related to I2R function.

Translocator Protein (18 kDa, TSPO) is regarded as a useful biomarker for neuroinflammation imaging. TSPO PET imaging could be used to understand the role of neuroinflammation in brain diseases and as a tool for evaluating novel therapeutic effects. As a promising TSPO probe, [18F]DPA-714 is highly specific and offers reliable quantification of TSPO in vivo. In this study, we further radiosynthesized and evaluated another novel TSPO probe, 2-(7-butyl-2-(4-(2-[18F]fluoroethoxy)phenyl)-5-methylpyrazolo[1,5-a]pyrimidin-3-yl)-N,N-diethylacetamide ([18F]VUIIS1018A), which features a 700-fold higher binding affinity for TSPO than that of [18F]DPA-714. We evaluated the performance of [18F]VUIIS1018A using dynamic in vivo PET imaging, radiometabolite analysis, in vitro autoradiography assays, biodistribution analysis, and blocking assays. In vivo study using this probe demonstrated high signal-to-noise ratio, binding potential (BPND), and binding specificity in preclinical neuroinflammation studies. Taken together, these findings indicate that [18F]VUIIS1018A may serve as a novel TSPO PET probe for neuroinflammation imaging.

Two novel radiotracers, namely, N-(4-[18F]fluorobenzyl)-N-methyl-2-(7-methyl-8-oxo-2-phenyl-7,8-dihydro-9H-purin-9-yl)acetamide ([18F]5) and 2-(5-(4-[18F]fluorophenyl)-2-oxobenzo[d]oxazol-3(2H)-yl)-N-methyl-N-phenylacetamide ([18F]6), were developed for positron emission tomography (PET) imaging of translocator protein (18 kDa) (TSPO) in ischemic brain in this study. The two radiotracers with a [18F]fluorobenzene ring were derived from the corresponding [18F]fluoroethyl tracers [18F]7 and [18F]8 which underwent [18F]defluoroethylation in vivo easily. [18F]5 or [18F]6 was synthesized by the radiofluorination of the spirocyclic iodonium ylide precursor 10 or 17 with [18F]F− in 23 ± 10% (n = 7) or 56 ± 9% (n = 7) radiochemical yields (decay-corrected, based on [18F]F−). [18F]5 and [18F]6 showed high in vitro binding affinities (Ki = 0.70 nM and 5.9 nM) for TSPO and moderate lipophilicities (log D = 2.9 and 3.4). Low uptake of radioactivity for both radiotracers was observed in mouse bones. Metabolite analysis showed that the in vivo stability of [18F]5 and [18F]6 was improved in comparison to the parent radiotracers [18F]7 and [18F]8. In particular, no radiolabelled metabolite of [18F]5 was found in the mouse brains at 60 min after the radiotracer injection. PET studies with [18F]5 on ischemic rat brains revealed a higher binding potential (BPND = 3.42) and maximum uptake ratio (4.49) between the ipsilateral and contralateral sides. Thus, [18F]5 was shown to be a useful PET radiotracer for visualizing TSPO in neuroinflammation models.

Metabotropic glutamate 2 receptors (mGlu2) are involved in the pathogenesis of several CNS disorders and neurodegenerative diseases. Pharmacological modulation of this target represents a potential disease-modifying approach for the treatment of substance abuse, depression, schizophrenia, and dementias. While quantification of mGlu2 receptors in the living brain by positron emission tomography (PET) would help us better understand signaling pathways relevant to these conditions, few successful examples have been demonstrated to image mGlu2 in vivo, and a suitable PET tracer is yet to be identified. Herein we report the design and synthesis of a radiolabeled negative allosteric modulator (NAM) for mGlu2 PET tracer development based on a quinoline 2-carboxamide scaffold. The most promising candidate, 7-((2,5-dioxopyrrolidin-1-yl)methyl)-4-(2-fluoro-4-[11C]methoxyphenyl) quinoline-2-carboxamide ([11C]QCA) was prepared in 13% radiochemical yield (non-decay-corrected at the end of synthesis) with >99% radioche...

A suitable TSPO PET ligand may visualize and quantify neuroinflammation in a living brain. Herein we report a 18F-ligand, [18F]2 ([18F]FDPA), is radiolabeled in high yield and high specific activity based on our spirocyclic iodonium ylide (SCIDY) strategy. [18F]2 demonstrated saturable specific binding to TSPO, substantially elevated brain uptake, and slow washout of bound PET signal in the preclinical models of brain neuroinflammation (cerebral ischemia and Alzheimer’s disease).

Brain metabotropic glutamate receptor 2 (mGluR2) has been proposed as a therapeutic target for the treatment of schizophrenia-like symptoms arising from increased glutamate transmission in the forebrain. However, there does not exist a reliable tool for the study of mGluR2 in human neuroimaging. The purpose of this study was to radiosynthesize 1-(cyclopropylmethyl)-4-(4-[11C]methoxyphenyl)piperidin-1-yl-2-oxo-1,2-dihydropyridine-3-carbonitrile ([11C]CMDC) and evaluate its potential as a positron emission tomography (PET) radiotracer for imaging mGluR2 in the rat brain. CMDC, a positive allosteric modulator of mGluR2, showed potent functional activity (EC50: 98 nM) for human mGluR2 in vitro. [11C]CMDC was synthesized by O-[11C]methylation of 1-(cyclopropylmethyl)-4-(4-hydroxyphenyl)piperidin-1-yl-2-oxo-1,2-dihydropyridine-3-carbonitrile (1) with [11C]methyl iodide. [11C]CMDC (2.2 ± 0.9 GBq; n = 20) was obtained from [11C]CO2 of 14.0–17.8 GBq with >98% radiochemical purity and 86–150 GBq/μmol specific activity at the end of synthesis. In vitro autoradiography indicated that [11C]CMDC binding was expressed (>50% of total binding) in mGluR2-rich brain regions including the cerebral cortex, striatum and hippocampus. However, small-animal PET showed low in vivo specific binding of [11C]CMDC in the rat brain. While [11C]CMDC has limited potential as a PET tracer for brain mGluR2, it can be used to develop new radiotracers with improved behaviors.

Selective modulation of metabotropic glutamate receptor 2 (mGlu2) represents a novel therapeutic approach for treating brain disorders, including schizophrenia, depression, Parkinson's disease (PD), Alzheimer's disease (AD), drug abuse and addiction. Imaging mGlu2 using positron emission tomography (PET) would allow for in vivo quantification under physiological and pathological conditions and facilitate drug discovery by enabling target engagement studies. In this paper, we aimed to develop a novel specific radioligand derived from negative allosteric modulators (NAMs) for PET imaging of mGlu2. Methods. A focused small molecule library of mGlu2 NAMs with tetrahydro naphthyridine scaffold was synthesized for pharmacology and physicochemical evaluation. GIRK dose-response assays and CNS panel binding selectivity assays were performed to study the affinity and selectivity of mGlu2 NAMs, among which compounds 14a and 14b were selected as PET ligand candidates. Autoradiography in SD rat brain sections was used to confirm the in vitro binding specificity and selectivity of [11C]14a and [11C]14b towards mGlu2. In vivo binding specificity was then studied by PET imaging. Whole body biodistribution study and radiometabolite analysis were conducted to demonstrate the pharmacokinetic properties of [11C]14b as most promising PET mGlu2 PET ligand. Results. mGlu2 NAMs 14a-14g were synthesized in 14%-20% yields in five steps. NAMs 14a and 14b were selected to be the most promising ligands due to their high affinity in GIRK dose-response assays. [11C]14a and [11C]14b displayed similar heterogeneous distribution by autoradiography, consistent with mGlu2 expression in the brain. While PET imaging study showed good brain permeability for both tracers, compound [11C]14b demonstrated superior binding specificity compared to [11C]14a. Further radiometabolite analysis of [11C]14b showed excellent stability in the brain. Conclusions. Compound 14b exhibited high affinity and excellent subtype selectivity, which was then evaluated by in vitro autoradiography and in vivo PET imaging study after labeling with carbon-11. Ligand [11C]14b, which we named [11C]MG2-1904, demonstrated high brain uptake and excellent in vitro/in vivo specific binding towards mGlu2 with high metabolic stability in the brain. As proof-of-concept, our preliminary work demonstrated a successful example of visualizing mGlu2 in vivo derived from NAMs, which represents a promising chemotype for further development and optimization aimed for clinical translation.

Transmembrane AMPA receptor regulatory proteins (TARPs) are a recently discovered family of proteins that modulate AMPA receptors activity. Based on a potent and selective TARP subtype γ−8 antagonist, 6-(methyl(4-(pyridin-2-yl)thiazol-2-yl)amino)benzo[d]thiazol-2(3H)-one (compound 9), we perform the radiosynthesis of its (11)C-isotopologue 1 and conduct preliminary PET evaluation to test the feasibility of imaging TARP γ−8 dependent receptors in vivo.

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-[

Background Neuropeptide Y (NPY) has been implicated in a wide variety of physiological processes, including feeding, learning, memory, emotion, cardiovascular homeostasis, hormone secretion, and circadian rhythms. NPY Yl receptor (NPY Y1-R) is the most widely studied NPY receptor, and is involved in many of these processes. BMS-193885 (1) was previously developed as a potent and selective NPY Y1-R antagonist, which has good systemic bioavailability and brain penetration. To evaluate the characteristics of 1 in vivo, we developed 11C-labeled BMS-193885 ([11C]1) and its desmethyl analog ([11C]2) for potential use as two new positron emission tomography (PET) tracers. Results [11C]1 was synthesized from [11C]methyl iodide using 2. [11C]2 was synthesized from [11C]phosgene using its aniline and amine derivatives. The mean ± S.D. decay-corrected radiochemical yields of [11C]1 and [11C]2 from 11CO2 at the end of radionuclide production were 23 ± 3.2% (n = 6) and 24 ± 1.5% (n = 4), respectively. In biodistribution on mice, radioactivity levels for both tracers were relatively high in the kidney, small intestine, and liver at 60 min post-injection. The radioactivity levels in the kidney, lung, and spleen of mice at 30 min post-injection with [11C]1 were significantly reduced by pretreatment with 1 (10 mg/kg), and levels of [11C]1 in the brain of mice were significantly increased by pretreatment with the P-glycoprotein and breast cancer resistance protein inhibitor elacridar (10 mg/kg). In metabolite analysis using mouse plasma, [11C]1 and [11C]2 were rapidly metabolized within 30 min post-injection, and [11C]1 was mainly metabolized into unlabeled 2 and radiolabeled components. Conclusion [11C]1 and [11C]2 were successfully synthesized with sufficient amount of radioactivity and high quality for use in vivo. Our study of [11C]1 and its desmethyl analog [11C]2 was useful in that it helped to elucidate the in vivo characteristics of 1. Electronic supplementary material The online version of this article (10.1186/s41181-019-0056-5) contains supplementary material, which is available to authorized users.

Introduction A positron emission tomography (PET) probe with ultra-high specific radioactivity (SA) enables measuring high receptor specific binding in brain regions by avoiding mass effect of the PET probe itself. It has been reported that PET probe with ultra-high SA can detect small change caused by endogenous or exogenous ligand. Recently, Kealey et al. developed [ 11 C]BU99008, a more potent PET probe for I 2 -imidazoline receptors (I 2 Rs) imaging, with a conventional SA (mean 76GBq/μmol) showed higher specific binding in the brain. Here, to detect small change of specific binding for I 2 Rs caused by endogenous or exogenous ligand in an extremely small region, such as hypothalamus in the brain, we synthesized and evaluated [ 11 C]BU99008 with ultra-high SA as a useful PET probe for small-animal PET imaging of I 2 Rs. Methods [ 11 C]BU99008 was prepared by [ 11 C]methylation of N -desmethyl precursor with [ 11 C]methyl iodide. Biodistribution, metabolite analysis, and brain PET studies were conducted in rats. Results [ 11 C]BU99008 with ultra-high SA in the range of 5400–16,600GBq/μmol were successfully synthesized ( n =7), and had appropriate radioactivity for in vivo study. In the biodistribution study, the mean radioactivity levels in all investigated tissues except for the kidney did not show significant difference between [ 11 C]BU99008 with ultra-high SA and that with conventional SA. In the metabolite analysis, the percentage of unchanged [ 11 C]BU99008 at 30min after the injection of probes with ultra-high and conventional SA was similar in rat brain and plasma. In the PET study of rats' brain, radioactivity level (AUC 30–60 min ) in the hypothalamus of rats injected with [ 11 C]BU99008 with ultra-high SA (64 [SUV ∙ min]) was significantly higher than that observed for that with conventional SA (50 [SUV ∙ min]). The specific binding of [ 11 C]BU99008 with ultra-high SA (86% of total binding) for I 2 R was higher than that of conventional SA (76% of total binding). Conclusion A PET study using [ 11 C]BU99008 with ultra-high SA would thus contribute to the detection of small changes in or small regions with I 2 R expression and hence may be useful in elucidating new functions of I 2 R.

Here, we describe the very first attempt to visualize in vivo formyl peptide receptors (FPRs) in mouse brain by positron emission tomography (PET). FPRs are expressed in microglial cells where they mediate chemotactic activity of β-amyloid peptide in Alzheimer disease and, thus, are involved in neuroinflammatory processes. To this purpose, we have selected (2S)-3-(1H-Indol-3-yl)-2-{[(4-methoxyphenyl)carbamoyl]amino}-N-{[1-(5-methoxypyridin-2-yl)cyclohexyl]methyl}propanamide ((S)-1), that we have previously identified as a potent non-peptidic FPR agonist. (S)-[(11) C]-1 has been prepared in high radiochemical yield. (S)-[(11) C]-1 showed very low penetration of blood-brain barrier and, thus, was unable to accumulate into the brain. In addition, (S)-[(11) C]-1 was not able to label FPRs receptors in brain slices of PS19 and APP23 mice, two animal models of Alzheimer disease. Although (S)-[(11) C]-1 was not suitable to visualize FPRs in the brain, this study provides useful information for the design and characterization of future potential PET radioligands for visualization of brain FPRs by PET.

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.

To visualize fatty acid amide hydrolase (FAAH) in brain in vivo, we developed a novel positron emission tomography (PET) ligand N-(3,4-dimethylisoxazol-5-yl)piperazine-4-[4-(2-fluoro-4-[(11)C]methylphenyl)thiazol-2-yl]-1-carboxamide ([(11)C]DFMC, [(11)C]1). DFMC (1) was shown to have high binding affinity (IC50: 6.1nM) for FAAH. [(11)C]1 was synthesized by C-(11)C coupling reaction of arylboronic ester 2 with [(11)C]methyl iodide in the presence of Pd catalyst. At the end of synthesis, [(11)C]1 was obtained with a radiochemical yield of 20±10% (based on [(11)C]CO2, decay-corrected, n=5) and specific activity of 48-166GBq/μmol. After the injection of [(11)C]1 in mice, high uptake of radioactivity (>2% ID/g) was distributed in the lung, liver, kidney, and brain, organs with high FAAH expression. PET images of rat brains for [(11)C]1 revealed high uptakes in the cerebellar nucleus (SUV=2.4) and frontal cortex (SUV=2.0), two known brain regions with high FAAH expression. Pretreatment with the FAAH-selective inhibitor URB597 reduced the brain uptake. Higher than 90% of the total radioactivity in the rat brain was irreversible at 30min after the radioligand injection. The present results indicate that [(11)C]1 is a promising PET ligand for imaging of FAAH in living brain.

Parkinson's disease (PD) is a prevalent degenerative disorder affecting the CNS that is primarily characterized by resting tremor and movement deficits. Group I metabotropic glutamate receptor subtypes 1 and 5 (mGluR1 and mGluR5, respectively) are important targets for investigation in several CNS disorders. In the present study, we investigated thein vivoroles of mGluR1 and mGluR5 in chronic PD pathology by performing longitudinal positron emission tomography (PET) imaging inA53Ttransgenic (A53T-Tg) rats expressing an abnormal human α-synuclein (ASN) gene.A53T-Tgrats showed a dramatic decline in general motor activities with age, along with abnormal ASN aggregation and striatal neuron degeneration. In longitudinal PET imaging, striatal nondisplaceable binding potential (BPND) values for [11C]ITDM (N-[4-[6-(isopropylamino) pyrimidin-4-yl]-1,3-thiazol-2-yl]-N-methyl-4-[11C]methylbenzamide), a selective PET ligand for mGluR1, temporarily increased before PD symptom onset and dramatically decreased afterward with age. However, striatal BPNDvalues for (E)-[11C]ABP688 [3-(6-methylpyridin-2-ylethynyl)-cyclohex-2-enone-(E)-O-[11C]methyloxime], a specific PET ligand for mGluR5, remained constant during experimental terms. The dynamic changes in striatal mGluR1 BPNDvalues also showed a high correlation in pathological decreases in general motor activities. Furthermore, declines in mGluR1 BPNDvalues were correlated with decreases in BPNDvalues for [18F]FE-PE2I [(E)-N-(3-iodoprop-2E-enyl)-2β-carbo-[18F]fluoroethoxy-3β-(4-methylphenyl) nortropane], a specific PET ligand for the dopamine transporter, a biomarker for dopaminergic neurons. In conclusion, our results have demonstrated for the first time that dynamic changes occur in mGluR1, but not mGluR5, that accompany pathological progression in a PD animal model.SIGNIFICANCE STATEMENTSynaptic signaling by glutamate, the principal excitatory neurotransmitter in the brain, is modulated by group I metabotropic glutamate receptors, including the mGluR1 and mGluR5 subtypes. In the brain, mGluR1 and mGluR5 have distinct functional roles and regional distributions. Their roles in brain pathology, however, are not well characterized. Using longitudinal PET imaging in a chronic rat model of PD, we demonstrated that expression of mGluR1, but not mGluR5, dynamically changed in the striatum accompanying pathological PD progression. These findings imply that monitoring mGluR1in vivomay provide beneficial information to further understand central nervous system disorders.

Positive allosteric modulators of muscarinic acetylcholine receptor subtype 4 have been identified as promising activators for the treatment of neurological disorders and neurodegenerative diseases, including schizophrenia, Alzheimer's disease (AD) and dementia with Lewy bodies (DLB). Herein we report the synthesis and preliminary evaluation of a 18F-labeled positron emission tomography ligand based on a M4 activator (7). 18F-Isotopologue of 7 was prepared in a reasonable radiochemical yield with high radiochemical purity (>99%) and high molar activity (>37 GBq/µmol). In vitro autoradiography studies indicated that the ligand possessed moderate in vitro specific binding. Dynamic PET studies in vivo demonstrated that [18F]7 (also named as [18F]M4R-1911) failed to cross the blood–brain barrier. Therefore, further chemical scaffold optimization in chemotype of 7 is necessary to overcome limited brain permeability and improve specific binding.

Transmembrane AMPA receptor regulatory proteins (TARPs) are a recently discovered family of proteins that modulate AMPA receptors activity. Based on a potent and selective TARP subtype γ-8 antagonist, 6-(methyl(4-(pyridin-2-yl)thiazol-2-yl)amino)benzo[d]thiazol-2(3H)-one (compound 9), we perform the radiosynthesis of its 11C-isotopologue 1 and conduct preliminary PET evaluation to test the feasibility of imaging TARP γ-8 dependent receptors in vivo.

Phosphodiesterase 10A (PDE10A) is a newly identified therapeutic target for central-nervous-system disorders. 2-(2-(3-(4-([

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-[