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We designed four novel acetamidobenzoxazolone compounds 7a-d as candidates for positron emission tomography (PET) radiotracers for imaging the translocator protein (18 kDa, TSPO) in ischemic brain and glioma. Among these compounds, 2-(5-(6-fluoropyridin-3-yl)-2-oxobenzo[d]oxazol-3(2H)-yl)-N-methyl-N-phenylacetamide (7d) exhibited high binding affinity (K i = 13.4 nM) with the TSPO and moderate lipophilicity (log D = 1.92). [18F]7d was radiosynthesized by [18F]fluorination of the bromopyridine precursor 7h with [18F]F- in 12 ± 5% radiochemical yield (n = 6, decay-corrected). In vitro autoradiography and PET studies of ischemic rat brain revealed higher binding of [18F]7d with TSPO on the ipsilateral side, as compared to the contralateral side, and improved brain kinetics compared with our previously developed radiotracers. Metabolite study of [18F]7d showed 93% of unchanged form in the ischemic brain at 30 min after injection. Moreover, PET study with [18F]7d provided a clear tumor image in a glioma-bearing rat model. We demonstrated that [18F]7d is a useful PET radiotracer for visualizing not only neuroinflammation but also glioma and will translate this radiotracer to a "first-in-human" study in our facility. [ABSTRACT FROM AUTHOR]

Metabotropic glutamate 1 (mGlu1)receptor is found not only inthe brain but also in melanomas and breast cancers. mGlu1 is a promisingtarget for molecular imaging-based diagnosis and treatment of melanomabecause its overexpression induces melanocyte carcinogenesis. Herewe developed three PET tracers: 4-halogeno-N-[4-[6-(isopropylamino)pyrimidin-4-yl]-1,3-thiazol-2-yl]-N-[11C]methylbenzamide ([11C]4–6), which exhibited high uptakein target tumor and decreased uptake in nontarget brain tissues. Invitro binding assay indicated high to moderate binding affinitiesof 4–6(Ki, 22–143 nM) for mGlu1 receptor. In vivo biodistribution studiesin mice implanted with B16F10 melanoma cells confirmed high radioactiveuptake in tumor and low uptake in blood, skin, and muscles. Inhibitionof mGlu1 receptor using an mGlu1-selective ligand led to reduced radioactiveuptake in the tumor. [11C]6displayed thehighest ratio of uptake between tumor and nontarget tissue and mayprove useful as a PET tracer for mGlu1 imaging in melanoma. [ABSTRACT FROM AUTHOR]

Three novel 4-substituted benzamides have been synthesizedas potentialligands for the positron emission tomography (PET) imaging of metabotropicglutamate 1 (mGlu1) receptor in the brain. Of these compounds, N-(4-(6-(isopropylamino)pyrimidin-4-yl)-1,3-thiazol-2-yl)-N,4-dimethylbenzamide (4) exhibited the highestbinding affinity (Ki= 13.6 nM) for mGlu1and was subsequently labeled with carbon-11. In vitro autoradiographyusing rat brain sections showed that [11C]4binding was consistent with the distribution of mGlu1, with highspecific binding in the cerebellum and thalamus. PET studies with[11C]4in monkey showed a high brain uptakeand a kinetic profile suitable for quantitative analysis. Pretreatmentwith a mGlu1-selective ligand 16largely decreased thebrain uptake, indicating high in vivo specific binding of [11C]4to mGlu1. In metabolite analysis, only unchanged[11C]4was found in the brain. [11C]4is a useful PET ligand for the imaging and quantitativeanalysis of mGlu1 in monkey brain and merits further evaluation inhumans. [ABSTRACT FROM AUTHOR]

We designed three novel positron emission tomographyligands, N-(4-(6-(isopropylamino)pyrimidin-4-yl)-1,3-thiazol-2-yl)-4-[11C]methoxy-N-methylbenzamide ([11C]6), 4-[18F]fluoroethoxy-N-[4-[6-(isopropylamino)pyrimidin-4-yl]-1,3-thiazol-2-yl]-N-methylbenzamide ([18F]7), and4-[18F]fluoropropoxy-N-[4-[6-(isopropylamino)pyrimidin-4-yl]-1,3-thiazol-2-yl]-N-methylbenzamide ([18F]8), forimaging metabotropic glutamate receptor type 1 (mGluR1) in rodentbrain. Unlabeled compound 6was synthesized by benzoylationof 4-pyrimidinyl-2-methylaminothiazole 10, followed byreaction with isopropylamine. Removal of the methyl group in 6gave phenol precursor 12for radiosynthesis.Two fluoroalkoxy analogues 7and 8wereprepared by reacting 12with tosylates 13and 14. Radioligands [11C]6, [18F]7, and [18F]8were synthesized by O-[11C]methylationor [18F]fluoroalkylation of 12. Compound 6showed high in vitro binding affinity for mGluR1, whereas 7and 8had weak affinity. Autoradiography usingrat brain sections showed that [11C]6bindingis aligned with the reported distribution of mGluR1 with high specificbinding in the cerebellum and thalamus. PET study with [11C]6in rats showed high brain uptake and a similar distributionpattern to that in autoradiography, indicating the usefulness of [11C]6for imaging brain mGluR1. [ABSTRACT FROM AUTHOR]

Diacylglycerol kinase gamma (DGKγ) is a subtype of DGK enzyme, which catalyzes ATP-dependent conversion of diacylglycerol to phosphatidic acid. DGKγ, localized in the brain, plays an important role in the central nervous system. However, its function has not been widely investigated. Positron emission tomography (PET) imaging of DGKγ validates target engagement of therapeutic DGKγ inhibitors and investigates DGKγ levels under normal and disease conditions. In this study, we designed and synthesized a series of 3-acetyl indole derivatives as candidates for PET imaging agents for DGKγ. Among the synthesized compounds, 2-((3-acetyl-1-(6-methoxypyridin-3-yl)-2-methyl-1H-indol-5-yl)oxy)-N-methylacetamide ( 9 ) exhibited potent inhibitory activity (IC 50 = 30 nM) against DGKγ and desirable physicochemical properties allowing efficient blood-brain barrier penetration and low levels of undesirable nonspecific binding. The radiolabeling of 9 followed by PET imaging of wild-type and DGKγ-deficient mice and rats indicated that [ 11 C] 9 ([ 11 C] T-278 ) specifically binds to DGKγ and yields a high signal-to-noise ratio for DGKγ in rodent brains.

Metabotropic glutamate receptor 2 (mGlu 2 ) is a known target for treating several central nervous system (CNS) disorders. To develop a viable positron emission tomography (PET) ligand for mGlu 2 , we identified new candidates 5a - i that are potent negative allosteric modulators (NAMs) of mGlu 2 . Among these candidates, 4-(2-fluoro-4-methoxyphenyl)-5-((1-methyl-1 H -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 .

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 cytosolic serine hydrolase involved in endocannabinoid and inflammatory signaling. Positron-emission tomography (PET) imaging of MAGL serves to validate target engagement of therapeutic MAGL inhibitors as well as to investigate MAGL levels under normal and disease conditions. However, PET radioligands with reversible binding kinetics for MAGL, which allow quantitative assessment of MAGL, are hitherto unavailable. In this study, we designed and synthesized fluoro-containing PET probes starting from a recently identified piperazinyl pyrrolidine-2-one derivative with reversible binding to MAGL. By tailoring the lipophilicity of the molecule to optimize nonspecific binding and blood-brain barrier permeability, we successfully identified two compounds that show high uptake to regions enriched with MAGL. PET imaging of wild-type and MAGL-deficient mice as well as a macaque monkey indicated that [ 18 F]5 ((4 R)-1-{3-[2-( 18 F)fluoro-4-methylpyridin-3-yl]phenyl}-4-[4-(1,3-thiazol-2-ylcarbonyl)piperazin-1-yl]pyrrolidin-2-one, [ 18 F]T-401) specifically binds to MAGL with adequate reversibility, yielding a high contrast for MAGL within an appropriate imaging time.

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

Monoacylglycerol lipase (MAGL) is the principle enzyme for metabolizing endogenous cannabinoid ligand 2-arachidonoyglycerol (2-AG). Blockade of MAGL increases 2-AG levels, resulting in subsequent activation of the endocannabinoid system, and has emerged as a novel therapeutic strategy to treat drug addiction, inflammation, and neurodegenerative diseases. Herein we report a new series of MAGL inhibitors, which were radiolabeled by site-specific labeling technologies, including 11 C-carbonylation and spirocyclic iodonium ylide (SCIDY) radiofluorination. The lead compound [ 11 C]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.

A suitable TSPO PET ligand may visualize and quantify neuroinflammation in a living brain. Herein we report a 18 F-ligand, [ 18 F]2 ([ 18 F]FDPA), is radiolabeled in high yield and high specific activity based on our spirocyclic iodonium ylide (SCIDY) strategy. [ 18 F]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).

We designed four novel acetamidobenzoxazolone compounds 7a-d as candidates for positron emission tomography (PET) radiotracers for imaging the translocator protein (18 kDa, TSPO) in ischemic brain and glioma. Among these compounds, 2-(5-(6-fluoropyridin-3-yl)-2-oxobenzo[d]oxazol-3(2H)-yl)-N-methyl-N-phenylacetamide (7d) exhibited high binding affinity (K i = 13.4 nM) with the TSPO and moderate lipophilicity (log D = 1.92). [ 18 F]7d was radiosynthesized by [ 18 F]fluorination of the bromopyridine precursor 7h with [ 18 F]F - in 12 ± 5% radiochemical yield (n = 6, decay-corrected). In vitro autoradiography and PET studies of ischemic rat brain revealed higher binding of [ 18 F]7d with TSPO on the ipsilateral side, as compared to the contralateral side, and improved brain kinetics compared with our previously developed radiotracers. Metabolite study of [ 18 F]7d showed 93% of unchanged form in the ischemic brain at 30 min after injection. Moreover, PET study with [ 18 F]7d provided a clear tumor image in a glioma-bearing rat model. We demonstrated that [ 18 F]7d is a useful PET radiotracer for visualizing not only neuroinflammation but also glioma and will translate this radiotracer to a "first-in-human" study in our facility.