Development of Novel 11C-Labeled Selective Orexin-2 Receptor Radioligands for Positron Emission Tomography Imaging

Orexin 2 receptors (OX2R) represent a vital subtype of orexin receptors intricately involved in the regulation of wakefulness, arousal, and sleep-wake cycles. Despite their importance, there are currently no positron emission tomography (PET) tracers available for imaging the OX2R in vivo. Herein, we report [11C]1 ([11C]OX2-2201) and [11C]2 ([11C]OX2-2202) as novel PET ligands. Both compounds 1 (Ki = 3.6 nM) and 2 (Ki = 2.2 nM) have excellent binding affinity activities toward OX2R and target selectivity (OX2/OX1 > 600 folds). In vitro autoradiography in the rat brain suggested good to excellent in vitro binding specificity for [11C]1 and [11C]2. PET imaging in rat brains indicated that the low brain uptake of [11C]2 may be due to P-glycoprotein and/or breast cancer resistance protein efflux interaction and/or low passive permeability. Continuous effort in medicinal chemistry optimization is necessary to improve the brain permeability of this scaffold.

Multimodal Imaging for Validation and Optimization of Ion Channel-Based Chemogenetics in Nonhuman Primates

Chemogenetic tools provide an opportunity to manipulate neuronal activity and behavior selectively and repeatedly in nonhuman primates (NHPs) with minimal invasiveness. Designer Receptors Exclusively Activated by Designer Drugs (DREADDs) are one example that is based on mutated muscarinic acetylcholine receptors. Another channel-based chemogenetic system available for neuronal modulation in NHPs uses pharmacologically selective actuator modules (PSAMs), which are selectively activated by pharmacologically selective effector molecules (PSEMs). To facilitate the use of the PSAM/PSEM system, the selection and dosage of PSEMs should be validated and optimized for NHPs. To this end, we used a multimodal imaging approach. We virally expressed excitatory PSAM (PSAM4-5HT3) in the striatum and the primary motor cortex (M1) of two male macaque monkeys, and visualized its location through positron emission tomography (PET) with the reporter ligand [18F]ASEM. Chemogenetic excitability of neurons triggered by two PSEMs (uPSEM817 and uPSEM792) was evaluated using [18F]fluorodeoxyglucose-PET imaging, with uPSEM817 being more efficient than uPSEM792. Pharmacological magnetic resonance imaging (phMRI) showed that increased brain activity in the PSAM4-expressing region began ∼13 min after uPSEM817 administration and continued for at least 60 min. Our multimodal imaging data provide valuable information regarding the manipulation of neuronal activity using the PSAM/PSEM system in NHPs, facilitating future applications.SIGNIFICANCE STATEMENT Like other chemogenetic tools, the ion channel-based system called pharmacologically selective actuator module/pharmacologically selective effector molecule (PSAM/PSEM) allows remote manipulation of neuronal activity and behavior in living animals. Nevertheless, its application in nonhuman primates (NHPs) is still limited. Here, we used multitracer positron emission tomography (PET) imaging and pharmacological magnetic resonance imaging (phMRI) to visualize an excitatory chemogenetic ion channel (PSAM4-5HT3) and validate its chemometric function in macaque monkeys. Our results provide the optimal agonist, dose, and timing for chemogenetic neuronal manipulation, facilitating the use of the PSAM/PSEM system and expanding the flexibility and reliability of circuit manipulation in NHPs in a variety of situations.

Automated radiosynthesis and in vivo evaluation of 18F-labeled analog of the photosensitizer ADPM06 for planning photodynamic therapy

BACKGROUND

A family of BF₂-chelated tetraaryl-azadipyrromethenes was developed as non-porphyrin photosensitizers for photodynamic therapy. Among the developed photosensitizers, ADPM06 exhibited excellent photochemical and photophysical properties. Molecular imaging is a useful tool for photodynamic therapy planning and monitoring. Radiolabeled photosensitizers can efficiently address photosensitizer biodistribution, providing helpful information for photodynamic therapy planning. To evaluate the biodistribution of ADPM06 and predict its pharmacokinetics on photodynamic therapy with light irradiation immediately after administration, we synthesized [¹⁸F]ADPM06 and evaluated its in vivo properties.

RESULTS

[¹⁸F]ADPM06 was automatically synthesized by Lewis acid-assisted isotopic ¹⁸F-¹⁹F exchange using ADPM06 and tin (IV) chloride at room temperature for 10 min. Radiolabeling was carried out using 0.4 μmol of ADPM06 and 200 μmol of tin (IV) chloride. The radiosynthesis time was approximately 60 min, and the radiochemical purity was > 95% at the end of the synthesis. The decay-corrected radiochemical yield from [¹⁸F]F^- at the start of synthesis was 13 ± 2.7% (n = 5). In the biodistribution study of male ddY mice, radioactivity levels in the heart, lungs, liver, pancreas, spleen, kidney, small intestine, muscle, and brain gradually decreased over 120 min after the initial uptake. The mean radioactivity level in the thighbone was the highest among all organs investigated and increased for 120 min after injection. Upon co-injection with ADPM06, the radioactivity levels in the blood and brain significantly increased, whereas those in the heart, lung, liver, pancreas, kidney, small intestine, muscle, and thighbone of male ddY mice were not affected. In the metabolite analysis of the plasma at 30 min post-injection in female BALB/c-nu/nu mice, the percentage of radioactivity corresponding to [¹⁸F]ADPM06 was 76.3 ± 1.6% (n = 3). In a positron emission tomography study using MDA-MB-231-HTB-26 tumor-bearing mice (female BALB/c-nu/nu), radioactivity accumulated in the bone at a relatively high level and in the tumor at a moderate level for 60 min after injection.

CONCLUSIONS

We synthesized [¹⁸F]ADPM06 using an automated ¹⁸F-labeling synthesizer and evaluated the initial uptake and pharmacokinetics of ADPM06 using biodistribution of [¹⁸F]ADPM06 in mice to guide photodynamic therapy with light irradiation.

A 211At-labelled mGluR1 inhibitor induces cancer senescence to elicit long-lasting anti-tumor efficacy

Metabotropic glutamate receptor 1 (mGluR1), a key mediator of glutamatergic signaling, is frequently overexpressed in tumor cells and is an attractive drug target for most cancers. Here, we present a targeted radiopharmaceutical therapy strategy that antagonistically recognizes mGluR1 and eradicates mGluR1⁺ human tumors by harnessing a small-molecule alpha (α)-emitting radiopharmaceutical, ²¹¹At-AITM. A single dose of ²¹¹At-AITM (2.96 MBq) in mGluR1⁺ cancers exhibits long-lasting in vivo antitumor efficacy across seven subtypes of four of the most common tumors, namely, breast cancer, pancreatic cancer, melanoma, and colon cancers, with little toxicity. Moreover, complete regression of mGluR1⁺ breast cancer and pancreatic cancer is observed in approximate 50% of tumor-bearing mice. Mechanistically, the functions of ²¹¹At-AITM are uncovered in downregulating mGluR1 oncoprotein and inducing senescence of tumor cells with a reprogrammed senescence-associated secretory phenotype. Our findings suggest α-radiopharmaceutical therapy with ²¹¹At-AITM can be a useful strategy for mGluR1⁺ pan-cancers, regardless of their tissue of origin.

A novel monoacylglycerol lipase-targeted 18F-labeled probe for positron emission tomography imaging of brown adipose tissue in the energy network

Monoacylglycerol lipase (MAGL) constitutes a serine hydrolase that orchestrates endocannabinoid homeostasis and exerts its function by catalyzing the degradation of 2-arachidonoylglycerol (2-AG) to arachidonic acid (AA). As such, selective inhibition of MAGL represents a potential therapeutic and diagnostic approach to various pathologies including neurodegenerative disorders, metabolic diseases and cancers. Based on a unique 4-piperidinyl azetidine diamide scaffold, we developed a reversible and peripheral-specific radiofluorinated MAGL PET ligand [18F]FEPAD. Pharmacokinetics and binding studies on [18F]FEPAD revealed its outstanding specificity and selectivity towards MAGL in brown adipose tissue (BAT) - a tissue that is known to be metabolically active. We employed [18F]FEPAD in PET studies to assess the abundancy of MAGL in BAT deposits of mice and found a remarkable degree of specific tracer binding in the BAT, which was confirmed by post-mortem tissue analysis. Given the negative regulation of endocannabinoids on the metabolic BAT activity, our study supports the concept that dysregulation of MAGL is likely linked to metabolic disorders. Further, we now provide a suitable imaging tool that allows non-invasive assessment of MAGL in BAT deposits, thereby paving the way for detailed mechanistic studies on the role of BAT in endocannabinoid system (ECS)-related pathologies.

Imaging of Transmembrane AMPA Receptor Regulatory Proteins by Positron Emission Tomography

The transmembrane α-amino-3-hydroxyl-5-methyl-4-isoxazolepropionic acid (AMPA) receptor regulatory protein γ-8 (TARP γ-8) constitutes an auxiliary subunit of AMPA receptors, which mediates various brain functions including learning and memory. TARP γ-8 has emerged as a promising therapeutic target for central nervous system disorders. Despite considerable efforts, previously reported TARP γ-8 PET radioligands, such as [11C]TARP-1903 and [11C]TARP-1811 series, were plagued by limited brain uptake and/or high nonspecific binding in vivo. Herein, we developed two novel 11C-labeled probes, [11C]8 and [11C]15 (also named as [11C]TARP-2105), of which the latter exhibited a reasonable brain uptake as well as specific binding toward TARP γ-8 both in vitro and in vivo, as confirmed by blocking experiments with the commercially available TARP γ-8 inhibitor, JNJ-55511118 in the TARP γ-8-rich hippocampus. Overall, [11C]15 exhibited promising tracer characteristics and proved to be a lead positron-emission tomography ligand for the non-invasive quantification of TARP γ-8 in the mammalian brain.

Scandium Triflate-Catalyzed N-[18F]Fluoroalkylation of Aryl- Or Heteroaryl-Amines with [18F]Epifluorohydrin under Mild Conditions

The scandium triflate-catalyzed N-[¹⁸F]fluoroalkylation of aryl- or heteroaryl-amines with [¹⁸F]epifluorohydrin ([¹⁸F]2) was investigated. This reaction is mild and provides one-step access to N-[¹⁸F]fluoroalkylated aryl- or heteroaryl-amines, which are used for positron emission tomography imaging. The use of 2,2,2-trifluoroethanol as a cosolvent improved the reaction efficiency. The use of (S)- and (R)-[¹⁸F]2 produced the corresponding enantiomeric N-[¹⁸F]fluoroalkylated anilines.

Upregulation of Striatal Metabotropic Glutamate Receptor Subtype 1 (mGluR1) in Rats with Excessive Glutamate Release Induced by N-Acetylcysteine

The aim of this study is to investigate the changes in expression of metabotropic glutamate (Glu) receptor subtype 1 (mGluR1), a key molecule involved in neuroexcitetoxicity, during excessive Glu release in the brain by PET imaging. An animal model of excessive Glu release in the brain was produced by intraperitoneally implanting an Alzet osmotic pump containing N-acetylcysteine (NAC), an activator of the cysteine/Glu antiporter, into the abdomen of rats. Basal Glu concentration in the brain was measured by microdialysis, which showed that basal Glu concentration in NAC-treated rats (0.31 µM) was higher than that in saline-treated rats (0.17 µM) at day 7 after the implantation of the osmotic pump. Similarly, PET studies with [¹¹C]ITDM, a useful radioligand for mGluR1 imaging exhibited that the striatal binding potential (BP_ND) of [¹¹C]ITDM for mGluR1 in PET assessments was increased in NAC-treated animals at day 7 after implantation (2.30) compared with before implantation (1.92). The dynamic changes in striatal BP_ND during the experimental period were highly correlated with basal Glu concentration. In conclusion, density of mGluR1 is rapidly upregulated by increases in basal Glu concentration, suggesting that mGluR1 might to be a potential biomarker of abnormal conditions in the brain.

A genetically targeted reporter for PET imaging of deep neuronal circuits in mammalian brains

Positron emission tomography (PET) allows biomolecular tracking but PET monitoring of brain networks has been hampered by a lack of suitable reporters. Here, we take advantage of bacterial dihydrofolate reductase, ecDHFR, and its unique antagonist, TMP, to facilitate in vivo imaging in the brain. Peripheral administration of radiofluorinated and fluorescent TMP analogs enabled PET and intravital microscopy, respectively, of neuronal ecDHFR expression in mice. This technique can be used to the visualize neuronal circuit activity elicited by chemogenetic manipulation in the mouse hippocampus. Notably, ecDHFR-PET allows mapping of neuronal projections in non-human primate brains, demonstrating the applicability of ecDHFR-based tracking technologies for network monitoring. Finally, we demonstrate the utility of TMP analogs for PET studies of turnover and self-assembly of proteins tagged with ecDHFR mutants. These results establish opportunities for a broad spectrum of previously unattainable PET analyses of mammalian brain circuits at the molecular level.

Novel Reversible-Binding PET Ligands for Imaging Monoacylglycerol Lipase Based on the Piperazinyl Azetidine Scaffold

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.

PET imaging of colony-stimulating factor 1 receptor: A head-to-head comparison of a novel radioligand, 11C-GW2580, and 11C-CPPC, in mouse models of acute and chronic neuroinflammation and a rhesus monkey

Colony-stimulating factor 1 receptor (CSF1R) is a specific biomarker for microglia. In this study, we developed a novel PET radioligand for CSF1R, ¹¹C-GW2580, and compared it to a reported CSF1R tracer, ¹¹C-CPPC, in mouse models of acute and chronic neuroinflammation and a rhesus monkey. Dynamic ¹¹C-GW2580- and ¹¹C-CPPC-PET images were quantified by reference tissue-based models and standardized uptake value ratio. Both tracers exhibited increased uptake in the lesioned striata of lipopolysaccharide-injected mice and in the forebrains of App^{NL-G-F/NL-G-F}-knock-in mice, spatially in agreement with an increased 18-kDa translocator protein radioligand retention. Moreover, ¹¹C-GW2580 captured changes in CSF1R availability more sensitively than ¹¹C-CPPC, with a larger dynamic range and a smaller inter-individual variability, in these model animals. PET imaging of CSF1R in a rhesus monkey displayed moderate-to-high tracer retention in the brain at baseline. Homologous blocker (i. e. unlabeled tracer) treatment reduced the uptake of ¹¹C-GW2580 by ∼30% in all examined brain regions except for centrum semi-ovale white matter, but did not affect the retention of ¹¹C-CPPC. In summary, our results demonstrated that ¹¹C-GW2580-PET captured inflammatory microgliosis in the mouse brain with higher sensitivity than a reported radioligand, and displayed saturable binding in the monkey brain, potentially providing an imaging-based quantitative biomarker for reactive microgliosis.

Design, Synthesis, and Evaluation of 11C-Labeled 3-Acetyl-Indole Derivatives as a Novel Positron Emission Tomography Imaging Agent for Diacylglycerol Kinase Gamma (DGKγ) in Brain

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 (IC50 = 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 [11C]9 ([11C]T-278) specifically binds to DGKγ and yields a high signal-to-noise ratio for DGKγ in rodent brains.

Detection of Alzheimer's disease-related neuroinflammation by a PET ligand selective for glial versus vascular translocator protein

A substantial and constitutive expression of translocator protein (TSPO) in cerebral blood vessels hampers the sensitive detection of neuroinflammation characterized by greatly induced TSPO expression in activated glia. Here, we conducted in vivo positron emission tomography (PET) and in vitro autoradiographic imaging of normal and TSPO-deficient mouse brains to compare the binding properties of ¹⁸F-FEBMP, a relatively novel TSPO radioligand developed for human studies based on its insensitivity to a common polymorphism, with ¹¹C-PK11195, as well as other commonly used TSPO radioligands including ¹¹C-PBR28, ¹¹C-Ac5216 and ¹⁸F-FEDAA1106. TSPO in cerebral vessels of normal mice was found to provide a major binding site for ¹¹C-PK11195, ¹¹C-PBR28 and ¹⁸F-FEDAA1106, in contrast to no overt specific binding of ¹⁸F-FEBMP and ¹¹C-Ac5216 to this vascular component. In addition, ¹⁸F-FEBMP yielded PET images of microglial TSPO with a higher contrast than ¹¹C-PK11195 in a tau transgenic mouse modeling Alzheimer's disease (AD) and allied neurodegenerative tauopathies. Moreover, TSPO expression examined by immunoblotting was significantly increased in AD brains compared with healthy controls, and was well correlated with the autoradiographic binding of ¹⁸F-FEBMP but not ¹¹C-PK11195. Our findings support the potential advantage of comparatively glial TSPO-selective radioligands such as ¹⁸F-FEBMP for PET imaging of inflammatory glial cells.

Automated radiosynthesis of two 18F-labeled tracers containing 3-fluoro-2-hydroxypropyl moiety, [18F]FMISO and [18F]PM-PBB3, via [18F]epifluorohydrin

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.

Supplementary Information

The online version contains supplementary material available at 10.1186/s41181-021-00138-9.

Development of a highly-specific 18F-labeled irreversible positron emission tomography tracer for monoacylglycerol lipase mapping

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 S_NAr 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.

Synthesis and preliminary evaluation of novel 11C-labeled GluN2B-selective NMDA receptor negative allosteric modulators

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.

3-(Cyclopropylmethyl)-7-((4-(4-[11C]methoxyphenyl)piperidin-1-yl)methyl)-8-(trifluoromethyl)-[1,2,4]triazolo[4,3-a]pyridine: Synthesis and preliminary evaluation for PET imaging of metabotropic glutamate receptor subtype 2

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

Identification and Development of a New Positron Emission Tomography Ligand 4-(2-Fluoro-4-[11C]methoxyphenyl)-5-((1-methyl-1H-pyrazol-3-yl)methoxy)picolinamide for Imaging Metabotropic Glutamate Receptor Subtype 2 (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.

Synthesis and preliminary evaluation of 4-hydroxy-6-(3-[11C]methoxyphenethyl)pyridazin-3(2H)-one, a 11C-labeled d-amino acid oxidase (DAAO) inhibitor for PET imaging

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.

Isolated Pancreatic Sarcoidosis Diagnosed by Endoscopic Ultrasound-guided Fine-needle Aspiration

We herein report a 52-year-old man with multiple hypoechoic lesions in the body and tail of the pancreas detected during a screening ultrasound. Computed tomography (CT) showed no lesions other than those in the pancreas and peripheral lymph nodes. Contrast-enhanced CT identified hypovascular tumors in the pancreas. Endoscopic ultrasound-guided fine-needle aspiration (EUS-FNA) demonstrated partial fibrosis and noncaseating granulomas with Langhans giant cells. To our knowledge, this is the first report of isolated pancreatic sarcoidosis diagnosed by EUS-FNA. Although pancreatic sarcoidosis is very rare, clinicians should be aware of this possibility in patients presenting with multiple hypovascular pancreatic tumors.

Development of an In Vivo Method to Estimate Effective Drug Doses and Quantify Fatty Acid Amide Hydrolase in Rodent Brain using Positron Emission Tomography Tracer [11C]DFMC

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 (K_i), an indirect index for FAAH, in the FAAH-richest region (the cingulate cortex). Subsequently, to noninvasively estimate K_i 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 K_i value (K^REF) was concisely estimated with high correlation (r > 0.95) to K_i values based on 2TCMi. Using estimated K^REF 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.

Radiosynthesis and evaluation of 4-(6-[18F]Fluoro-4-(5-isopropoxy-1H-indazol-3-yl)pyridin-2-yl)morpholine as a novel radiotracer candidate targeting leucine-rich repeat kinase 2

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 (¹⁸F), 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 (IC₅₀ = 8.0 nM). [¹⁸F]FIPM was prepared in 5% radiochemical yield (n = 5), by inserting ¹⁸F into a pyridine ring, followed by removal of the protecting group. After HPLC separation and formulation, [¹⁸F]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 [¹⁸F]FIPM. While [¹⁸F]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.

In vivo positron emission tomography imaging of mitochondrial abnormalities in a mouse model of tauopathy

Damaged mitochondria may be one of the earliest manifestations of Alzheimer's disease. Because oxidative phosphorylation is a primary source of neuronal energy, unlike glycolysis-dependent energy production in inflamed glia, mitochondrial respiration could provide a selective biomarker of neuronal deterioration in Alzheimer's disease. Here we used a recently developed positron emission tomography (PET) probe targeting mitochondrial complex I (MC-I), ¹⁸F-BCPP-EF, to non-invasively visualize mitochondrial abnormalities in the brains of tau transgenic mice (rTg4510). Tauopathy and neuroinflammation were visualized by PET using a tau probe ¹¹C-PBB3 and a translocator protein probe, ¹⁸F-FEBMP, respectively. A marked reduction in ¹⁸F-BCPP-EF uptake was observed in hippocampal and forebrain regions of tau transgenic mice, colocalizing with regions of tauopathy, neuronal damage, and neuroinflammation. MC-I signals were highly correlated with atrophy assayed by magnetic resonance imaging, but negatively associated with inflammatory signals, indicating that neuronal metabolic signals measured by MC-I PET were robust to inflammatory interference. MC-I may be a useful imaging biomarker to detect neuronal damage and metabolic changes with minimal interference from concomitant glial hypermetabolism.

[11C]phosgene: Synthesis and application for development of PET radiotracers

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.

Radiosynthesis and evaluation of acetamidobenzoxazolone based radioligand [11C]N′-MPB for visualization of 18 kDa TSPO in brain

Modified acetamidobenzoxazolone radioligand [11C]N′-MPB for visualization of 18 kDa TSPO.

Design, Synthesis, and Evaluation of 18F-Labeled Monoacylglycerol Lipase Inhibitors as Novel Positron Emission Tomography 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.

131I-IITM and 211At-AITM: Two Novel Small-Molecule Radiopharmaceuticals Targeting Oncoprotein Metabotropic Glutamate Receptor 1

Targeted radionuclide therapy (TRT) targeting oncoproteins facilitates the delivery of therapeutic radionuclides to tumor tissues with high precision. Herein, we developed 2 new radiopharmaceuticals, 4-¹³¹I-iodo- and 4-²¹¹At-astato-N-[4-(6-(isopropylamino)pyridine-4-yl)-1,3-thiazol-2-yl]-N-methylbenzamide (¹³¹I-IITM and ²¹¹At-AITM), targeting the ectopic metabotropic glutamate receptor 1 (mGluR1) in melanomas for TRT studies. Methods: ¹³¹I-IITM and ²¹¹At-AITM were synthesized by reacting a stannyl precursor with ¹³¹I-NaI and ²¹¹At in the presence of an oxidizing agent. The therapeutic efficacy and safety of the 2 radiopharmaceuticals were investigated using mGluR1-expressing B16F10 melanoma cells and melanoma-bearing mice. Results: ¹³¹I-IITM and ²¹¹At-AITM were obtained with a radiochemical purity of greater than 99% and radiochemical yields of 42.7% ± 10.4% and 45.7% ± 6.5%, respectively, based on the total radioactivity of used radionuclides. ¹³¹I-IITM and ²¹¹At-AITM exhibited a maximum uptake of 4.66% ± 0.70 and 7.68% ± 0.71 percentage injected dose per gram (%ID/g) in the targeted melanomas, respectively, and were rapidly cleared from nontarget organs after intravenous injection. Both agents markedly inhibited melanoma growth compared with the controls (61.00% and 95.68%, respectively). In the melanoma model, considerably greater therapeutic efficacy with negligible toxicity was observed using ²¹¹At-AITM. Conclusion: The nontoxic radiopharmaceuticals ¹³¹I-IITM and ²¹¹At-AITM are useful high-precision TRT agents that can be used to target the oncoprotein mGluR1 for melanoma therapy.

Radiosynthesis and evaluation of a novel monoacylglycerol lipase radiotracer: 1,1,1,3,3,3-hexafluoropropan-2-yl-3-(1-benzyl-1H-pyrazol-3-yl)azetidine-1-[11C]carboxylate

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.

Design, Synthesis, and Evaluation of Reversible and Irreversible Monoacylglycerol Lipase Positron Emission Tomography (PET) Tracers Using a "Tail Switching" Strategy on a Piperazinyl Azetidine Skeleton

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 ¹¹C or ¹⁸F. [¹¹C]8 ([¹¹C]MAGL-2-11) exhibited high brain uptake and excellent binding specificity in the brain toward MAGL. Reversible radioligands [¹¹C]17 ([¹¹C]PAD) and [¹⁸F]37 ([¹⁸F]MAGL-4-11) also demonstrated excellent in vivo binding specificity toward MAGL in peripheral organs. This work may pave the way for the development of MAGL-targeted positron emission tomography tracers with tunability in reversible and irreversible binding mechanisms.

Synthesis of two novel [18F]fluorobenzene-containing radiotracers via spirocyclic iodonium ylides and positron emission tomography imaging of translocator protein (18 kDa) in ischemic brain

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.

Radiosynthesis and in vivo evaluation of 11C-labeled BMS-193885 and its desmethyl analog as PET tracers for neuropeptide Y1 receptors

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.

Pharmacokinetic Evaluation of [11C]CEP-32496 in Nude Mice Bearing BRAFV600E Mutation-Induced Melanomas

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 (BRAF^V600E), 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 BRAF^V600E 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 BRAF^V600E-positive A375 melanoma cells and densely accumulated in the respective tissue sections; this could be blocked by the BRAF^V600E 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 BRAF^V600E mutation in vitro, while its noninvasive personalized diagnostic role needs to be studied further.

Development of 2-(2-(3-(4-([18F]Fluoromethoxy- d2)phenyl)-7-methyl-4-oxo-3,4-dihydroquinazolin-2-yl)ethyl)-4-isopropoxyisoindoline-1,3-dione for Positron-Emission-Tomography Imaging of Phosphodiesterase 10A in the Brain

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 ( K_i = 2.9 nM) to PDE10A and suitable lipophilicity (log D = 2.2). In PET studies, the binding potential (BP_ND) 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 contamination with radiolabeled metabolites in the brains of rats given [¹⁸F]9 than in those given [¹⁸F]5. In conclusion, [¹⁸F]9 is a useful PET ligand for PDE10A imaging in brain.

Synthesis and evaluation of 4-(2-fluoro-4-[11C]methoxyphenyl)-5-((2-methylpyridin-4-yl)methoxy)picolinamide for PET imaging of the metabotropic glutamate receptor 2 in the rat brain

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.

[18F]DAA1106: Automated radiosynthesis using spirocyclic iodonium ylide and preclinical evaluation for positron emission tomography imaging of translocator protein (18 kDa)

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 (K_i = 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.

Synthesis, pharmacology and preclinical evaluation of 11C-labeled 1,3-dihydro-2H-benzo[d]imidazole-2-ones for imaging γ8-dependent transmembrane AMPA receptor regulatory protein

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.

Automated Synthesis of (rac)-, (R)-, and (S)-[18 F]Epifluorohydrin and Their Application for Developing PET Radiotracers Containing a 3-[18 F]Fluoro-2-hydroxypropyl Moiety

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.

First demonstration of in vivo mapping for regional brain monoacylglycerol lipase using PET with [11C]SAR127303

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, K_i^2-TCMi, and λk₃). These parameters were successfully obtained with high identifiability (<10 %COV) for the following regions ranked in order from highest to lowest: cingulate cortex > 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 K_i^2-TCMi and λk₃ values correlated relatively highly with in vitro binding (r > 0.4, P < 0.005). The K_i^2-TCMi values showed high correlation and low underestimation (<10%) compared with the slope of a Patlak plot analysis with linear regression (K_i^Patlak). In conclusion, we successfully estimated regional net uptake value of [¹¹C]SAR127303 reflecting MAGL concentrations in rat brain regions for the first time.

In Vitro and in Vivo Evaluation of 11C-Labeled Azetidinecarboxylates for Imaging Monoacylglycerol Lipase by PET Imaging Studies

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.

Evaluation of the novel TSPO radiotracer 2-(7-butyl-2-(4-(2-([18F]fluoroethoxy)phenyl)-5-methylpyrazolo[1,5-a]pyrimidin-3-yl)-N,N-diethylacetamide in a preclinical model of neuroinflammation

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 (BP_ND), 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.

Comparison between [18F]fluorination and [18F]fluoroethylation reactions for the synthesis of the PDE10A PET radiotracer [18F]MNI-659

INTRODUCTION

2-(2-(3-(4-(2-[¹⁸F]Fluoroethoxy)phenyl)-7-methyl-4-oxo-3,4-dihydroquinazolin-2-yl)ethyl)-4-isopropoxyisoindoline-1,3-dione ([¹⁸F]MNI-659, [¹⁸F]1) is a useful PET radiotracer for imaging phosphodiesterase 10A (PDE10A) in human brain. [¹⁸F]1 has been previously prepared by direct [¹⁸F]fluorination of a tosylate precursor 2 with [¹⁸F]F^-. The aim of this study was to determine the conditions for the [¹⁸F]fluorination reaction to obtain [¹⁸F]1 of high quality and with sufficient radioactivity for clinical use in our institute. Moreover, we synthesized [¹⁸F]1 by [¹⁸F]fluoroethylation of a phenol precursor 3 with [¹⁸F]fluoroethyl bromide ([¹⁸F]FEtBr), and the outcomes of [¹⁸F]fluorination and [¹⁸F]fluoroethylation were compared.

METHODS

We performed the automated synthesis of [¹⁸F]1 by [¹⁸F]fluorination and [¹⁸F]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 [¹⁸F]fluorination.

RESULTS

The efficiency of the [¹⁸F]fluorination reaction was strongly affected by the amount of 2 and potassium carbonate. Under the determined reaction conditions, [¹⁸F]1 with 0.82±0.2GBq was obtained in 13.6%±3.3% radiochemical yield (n=8, decay-corrected to EOB and based on [¹⁸F]F^-) at EOS, starting from 11.5±0.4GBq of cyclotron-produced [¹⁸F]F^-. On the other hand, the [¹⁸F]fluoroethylation of 3 with [¹⁸F]FEtBr produced [¹⁸F]1 with 1.0±0.2GBq and in 22.5±2.5 % radiochemical yields (n=7, decay-corrected to EOB and based on [¹⁸F]F^-) at EOS, starting from 7.4GBq of cyclotron-produced [¹⁸F]F^-. Clearly, [¹⁸F]fluoroethylation resulted in a higher radiochemical yield of [¹⁸F]1 than [¹⁸F]fluorination.

CONCLUSION

[¹⁸F]1 of high quality and with sufficient radioactivity was successfully radiosynthesized by two methods. [¹⁸F]1 synthesized by direct [¹⁸F]fluorination has been approved and will be provided for clinical use in our institute.