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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.

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.

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

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

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

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

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

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.

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.

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

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.

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.

Monoacylglycerol lipase (MAGL) is a main regulator of the endocannabinoid system within the central nervous system (CNS). Recently, [11C]SAR127303 was developed as a promising radioligand for MAGL imaging. In this study, we aimed to quantify regional MAGL concentrations in the rat brain using positron emission tomography (PET) with [11C]SAR127303. An irreversible two-tissue compartment model (2-TCMi, k4 = 0) analysis was conducted to estimate quantitative parameters (k3, Ki2-TCMi, and λk3). These parameters were successfully obtained with high identifiability ( striatum > hippocampus > thalamus > cerebellum > hypothalamus ≈ pons. In vitro autoradiographs using [11C]SAR127303 showed a heterogeneous distribution of radioactivity, as seen in the PET images. The Ki2-TCMi and λk3 values correlated relatively highly with in vitro binding (r > 0.4, P

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.

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.

Introduction 2-(2-(3-(4-(2-[ 18 F]Fluoroethoxy)phenyl)-7-methyl-4-oxo-3,4-dihydroquinazolin-2-yl)ethyl)-4-isopropoxyisoindoline-1,3-dione ([ 18 F]MNI-659, [ 18 F] 1 ) is a useful PET radiotracer for imaging phosphodiesterase 10A (PDE10A) in human brain. [ 18 F] 1 has been previously prepared by direct [ 18 F]fluorination of a tosylate precursor 2 with [ 18 F]F − . The aim of this study was to determine the conditions for the [ 18 F]fluorination reaction to obtain [ 18 F] 1 of high quality and with sufficient radioactivity for clinical use in our institute. Moreover, we synthesized [ 18 F] 1 by [ 18 F]fluoroethylation of a phenol precursor 3 with [ 18 F]fluoroethyl bromide ([ 18 F]FEtBr), and the outcomes of [ 18 F]fluorination and [ 18 F]fluoroethylation were compared. Methods We performed the automated synthesis of [ 18 F] 1 by [ 18 F]fluorination and [ 18 F]fluoroethylation using a multi-purpose synthesizer. We determined the amounts of tosylate precursor 2 and potassium carbonate as well as the reaction temperature for direct [ 18 F]fluorination. Results The efficiency of the [ 18 F]fluorination reaction was strongly affected by the amount of 2 and potassium carbonate. Under the determined reaction conditions, [ 18 F] 1 with 0.82±0.2GBq was obtained in 13.6%±3.3% radiochemical yield ( n =8, decay-corrected to EOB and based on [ 18 F]F − ) at EOS, starting from 11.5±0.4GBq of cyclotron-produced [ 18 F]F − . On the other hand, the [ 18 F]fluoroethylation of 3 with [ 18 F]FEtBr produced [ 18 F] 1 with 1.0±0.2GBq and in 22.5±2.5 % radiochemical yields ( n =7, decay-corrected to EOB and based on [ 18 F]F − ) at EOS, starting from 7.4GBq of cyclotron-produced [ 18 F]F − . Clearly, [ 18 F]fluoroethylation resulted in a higher radiochemical yield of [ 18 F] 1 than [ 18 F]fluorination. Conclusion [ 18 F] 1 of high quality and with sufficient radioactivity was successfully radiosynthesized by two methods. [ 18 F] 1 synthesized by direct [ 18 F]fluorination has been approved and will be provided for clinical use in our institute.

The purpose of this study was to synthesize a new positron emission tomography radiotracer, N-(4-tertiarybutylphenyl)-4-(3-chloropyridin-2-yl)tetrahydropyrazine-1(2H)-[11C]carboxamide ([11C]BCTC, [11C]1), and assess its in vivo binding to the transient receptor potential vanilloid subfamily member 1 (TRPV1) receptor in mice. [11C]BCTC was synthesized by reacting the hydrochloride of 4-tertiarybutylaniline (2·HCl) with [11C]phosgene ([11C]COCl2) to give isocyanate [11C]4, followed by reaction with 4-(3-chloropyridin-2-yl)tetrahydropyrazine (3). [11C]BCTC was obtained at a 16-20% radiochemical yield, based on the cyclotron-produced [11C]CO2 (decay-corrected to the end of bombardment), with >98% radiochemical purity and 65-110GBq/μmol specific activity at the end of synthesis. An ex vivo biodistribution study in mice confirmed the specific binding of [11C]BCTC to TRPV1 in the trigeminal nerve, which is a tissue with high TRPV1 expression.

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.

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.

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.

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.

Fatty acid amide hydrolase (FAAH) is one of the principle enzymes for metabolizing endogenous cannabinoid neurotransmitters such as anandamide, and thus regulates endocannabinoid (eCB) signaling. Selective pharmacological blockade of FAAH has emerged as a potential therapy to discern the endogenous functions of anandamide-mediated eCB pathways in anxiety, pain, and addiction. Quantification of FAAH in the living brain by positron emission tomography (PET) would help our understanding of the endocannabinoid system in these conditions. While most FAAH radiotracers operate by an irreversible ("suicide") binding mechanism, a FAAH tracer with reversibility would facilitate quantitative analysis. We have identified and radiolabeled a reversible FAAH inhibitor, 7-(2-[(11)C]methoxyphenyl)-1-(5-(pyridin-2-yl)oxazol-2-yl)heptan-1-one ([(11)C]MPPO) in 13% radiochemical yield (nondecay corrected) with >99% radiochemical purity and 2 Ci/μmol (74 GBq/μmol) specific activity. The tracer showed moderate brain uptake (0.8 SUV) with heterogeneous brain distribution. However, blocking studies with a potent FAAH inhibitor URB597 demonstrated a low to modest specificity to the target. Measurement of lipophilicity, metabolite, and efflux pathway analysis were also performed to study the pharmacokinetic profile of [(11)C]MPPO. In all, we reported an efficient radiolabeling and preliminary evaluation of the first-in-class FAAH inhibitor [(11)C]MPPO with α-ketoheterocyclic scaffold.

Cannabinoid receptor type 1 (CB1) is mainly expressed in the brain, as well as being expressed in functional relevant concentrations in various peripheral tissues. 1-(4-Chlorophenyl)-3-(3-(6-(pyrrolidin-1-yl)pyridin-2-yl)phenyl)urea (PSNCBAM-1, 1) was developed as a potent allosteric antagonist for CB1 and its oral administration led to reductions in the appetite and body weight of rats. Several analogs of 1 (compounds 2 and 3) were recently identified through a series of structure-activity relationship studies. Herein, we report the synthesis of radiolabeled analogs of these compounds using [11C]COCl2, and an evaluation of their potential as PET ligands for CB1 imaging using in vitro and in vivo techniques. [11C]2 and [11C]3 were successfully synthesized in two steps using [11C]COCl2. The radiochemical yields of [11C]2 and [11C]3 were 17 ± 8% and 20 ± 9% (decay-corrected to the end of bombardment, based on [11C]CO2). The specific activities of [11C]2 and [11C]3 were 42 ± 36 and 37 ± 13 GBq/μmol, respectively. The results of an in vitro binding assay using brown adipose tissue (BAT) homogenate showed that the binding affinity of 2 for CB1 (KD = 15.3 µM) was much higher than that of 3 (KD = 26.0 µM). PET studies with [11C]2 showed a high uptake of radioactivity in BAT, which decreased considerably in animals pretreated with AM281 (a selective allosteric antagonist for CB1). In conclusion, [11C]2 may be a useful PET ligand for imaging peripheral CB1 in BAT. 2009 Elsevier Ltd. All rights reserved.

CEP-32496, also known as RXDX-105 or Agerafenib, is a new orally active inhibitor for the mutated v-raf murine sarcoma viral oncogene homolog B1 (BRAFV600E), which has attracted considerable attention in clinical trials for the treatment of human cancers. Here, we used carbon-11-labeled CEP-32496 ([11C]CEP-32496) as a positron emission tomography (PET) radiotracer to evaluate its pharmacokinetic properties and explore its potential for in vivo imaging. Following radiotracer synthesis, we performed in vitro binding assays and autoradiography of [11C]CEP-32496 in the A375 melanoma cell line and on tumor tissue sections from mice harboring the BRAFV600E mutation. These were followed by PET scans and biodistribution studies on nude mice bearing subcutaneous A375 cell-induced melanoma. [11C]CEP-32496 showed high binding affinity for BRAFV600E-positive A375 melanoma cells and densely accumulated in the respective tissue sections; this could be blocked by the BRAFV600E selective antagonist sorafenib and by unlabeled CEP-32496. The PET and biodistribution results revealed that [11C]CEP-32496 accumulated continuously but slowly into the tumor within a period of 0 to 60 minutes postinjection in A375-melanoma-bearing nude mice. Metabolite analysis showed high in vivo stability of [11C]CEP-32496 in plasma. Our results indicate that [11C]CEP-32496 has excellent specificity and affinity for the BRAFV600E mutation in vitro, while its noninvasive personalized diagnostic role needs to be studied further.