Potential for in vivo visualization of intracellular pH gradient in the brain using PET imaging.

Intracellular pH is a valuable index for predicting neuronal damage and injury. However, no PET probe is currently available for monitoring intracellular pH in vivo. In this study, we developed a new approach for visualizing the hydrolysis rate of monoacylglycerol lipase, which is widely distributed in neurons and astrocytes throughout the brain. This approach uses PET with the new radioprobe [11C]QST-0837 (1,1,1,3,3,3-hexafluoropropan-2-yl-3-(1-phenyl-1H-pyrazol-3-yl)azetidine-1-[11C]carboxylate), a covalent inhibitor containing an azetidine carbamate skeleton for monoacylglycerol lipase. The uptake and residence of this new radioprobe depends on the intracellular pH gradient, and we evaluated this with in silico, in vitro and in vivo assessments. Molecular dynamics simulations predicted that because the azetidine carbamate moiety is close to that of water molecules, the compound containing azetidine carbamate would be more easily hydrolyzed following binding to monoacylglycerol lipase than would its analogue containing a piperidine carbamate skeleton. Interestingly, it was difficult for monoacylglycerol lipase to hydrolyze the azetidine carbamate compound under weakly acidic (pH 6) conditions because of a change in the interactions with water molecules on the carbamate moiety of their complex. Subsequently, an in vitro assessment using rat brain homogenate to confirm the molecular dynamics simulation-predicted behaviour of the azetidine carbamate compound showed that [11C]QST-0837 reacted with monoacylglycerol lipase to yield an [11C]complex, which was hydrolyzed to liberate 11CO2 as a final product. Additionally, the 11CO2 liberation rate was slower at lower pH. Finally, to indicate the feasibility of estimating how the hydrolysis rate depends on intracellular pH in vivo, we performed a PET study with [11C]QST-0837 using ischaemic rats. In our proposed in vivo compartment model, the clearance rate of radioactivity from the brain reflected the rate of [11C]QST-0837 hydrolysis (clearance through the production of 11CO2) in the brain, which was lower in a remarkably hypoxic area than in the contralateral region. In conclusion, we indicated the potential for visualization of the intracellular pH gradient in the brain using PET imaging, although some limitations remain. This approach should permit further elucidation of the pathological mechanisms involved under acidic conditions in multiple CNS disorders.

Novel synthesis of 11 C-labeled imidazolines via Pd(0)-mediated 11 C-carbomethoxylation using [11 C]CO and arylborons.

A labeling technique was developed for the imidazoline I2 receptor ligand 2-(3-fluoro-tolyl)-4, 5-dihydro-1H-imidazole (FTIMD) using Pd(0)-mediated 11 C-carbomethoxylation with [11 C]CO, followed by imidazoline ring formation with ethylenediamine-trimethylaluminium (EDA-AlMe3 ). To achieve this, [11 C]CO was passed through a methanol (MeOH) solution containing 3-fluoro-4-methylphenylboronic acid (1), palladium (II) acetate (Pd [OAc]2 ), triphenylphosphine (PPh3 ), and p-benzoquinone (PBQ). The mixture was then heated at 65°C for 5 min. EDA was introduced into the reaction mixture, and MeOH was completely evaporated at temperatures exceeding 100°C. The dried reaction mixture was combined with an EDA-AlMe (1:1) toluene solution and heated at 145°C for 10 min. Portions of the reaction mixture were analyzed through high-performance liquid chromatography, resulting in [11 C]FTIMD with 26% (n = 2) decay-corrected radiochemical yield (RCY). This method could be utilized for various arylborons to produce [2-11 C]imidazolines 4a-h with RCYs ranging from low to moderate. Notably, [2-11 C]benazoline was obtained with a moderate RCY of 65%. The proposed technique serves as an alternative to the Grignard method, which uses [11 C]CO to generate a [2-11 C]-labeled imidazoline ring.

The Glutaminase-1 Inhibitor [11C-carbony]BPTES: Synthesis and Positron Emission Tomography Study in Mice.

Bis-2-(5-phenylacetamido-1,3,4-thiadiazol-2-yl)ethyl sulfide (BPTES) is a selective inhibitor of glutaminase-1 (GLS1), consequently inhibiting glutaminolysis. BPTES is known for its potent antitumor activity and plays a significant role in senescent cell removal. In this study, we synthesized [11C-carbonyl]BPTES ([11C]BPTES) as a positron emission tomography (PET) probe for the first time and assessed its biodistribution in mice using PET. [11C]BPTES was synthesized by the reaction of an amine precursor () with [11C-carbonyl]phenylacetyl acid anhydride ([11C]2), which was prepared from [11C]CO2 and benzyl magnesium chloride, followed by in situ treatment with isobutyl chloroformate. The decay-corrected isolated radiochemical yield of [11C]BPTES was 9.5% (based on [11C]CO2) during a synthesis time of 40 min. A PET study with [11C]BPTES showed high uptake levels of radioactivity in the liver, kidney, and small intestine of mice.

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.

Evaluation and improvement of CuI-mediated 11 C-cyanation.

CuI-mediated 11 C-cyanation was evaluated by synthesizing [11 C]perampanel ([11 C]5) as a model compound and compared with previous reports. To a DMF solution with 5'-(2-bromophenyl)-1'-phenyl-[2,3'-bipyridin]-6'(1'H)-one (4) and CuI, [11 C]NH4 CN in a stream of ammonia/nitrogen (5:95, v/v) gas was bubbled. Subsequently, the reaction mixture was heated at 180°C for 5 min. After HPLC purification, [11 C]5 was obtained in 7.2 ± 1.0% (n = 4) non-decay corrected radiochemical yield with >99% radiochemical purity and a molar activity of 98 ± 28 GBq/µmol. In vivo evaluations of [11 C]5 were performed using small animals. PET scans to check the kinetics of [11 C]5 in the whole body of mice suggested that [11 C]5 spreads rapidly into the brain, heart, and lungs and then accumulates in the small intestine. To evaluate the performance of CuI-mediated 11 C-cyanation reaction, bromobenzene (6a) was selected as the model compound; however, it failed. Therefore, optimization of the reaction conditions has been performed, and consequently, the addition of K2 CO3 and prolonging the reaction time improved the radiochemical yield about double. With this improved method, CuI-mediated 11 C-cyanation of various (hetero)aromatic bromides was performed to exhibit the tolerance of most functional groups and to provide 11 C-cyanated products in good to moderate radiochemical yields.

Small-animal PET study for noninvasive quantification of transmembrane AMPA receptor regulatory protein γ-8 (TARP γ-8) in the brain.

Transmembrane AMPA receptor regulatory protein γ-8 (TARP γ-8) mediates various AMPA receptor functions. Recently, [11C]TARP-2105 was developed as a PET ligand for TARP γ-8 imaging. We performed a full kinetic analysis of [11C]TARP-2105 using PET with [11C]TARP-2105 for the first time. The distribution volume (VT), which is a macro parameter consisting of the K1-k4 rate constants in the two-tissue compartment model analysis, exhibited the following rank order: hippocampus (1.4 ± 0.3) > amygdala (1.0 ± 0.2) > frontal cortex (0.9 ± 0.2) > striatum (0.8 ± 0.2) ≫ cerebellum (0.5 ± 0.1) ≈ thalamus (0.5 ± 0.1) > pons (0.4 ± 0.1 mL/cm3). These heterogenous VT values corresponded with the order of biological distribution of TARP γ-8 in the brain. To validate the reference tissue model, the binding potential (BPND) of [11C]TARP-2105 for TARP γ-8 was estimated using general methods (SRTM, MRTM0, Logan reference model, and ratio method). These BPNDs based on reference models indicated excellent correlation (R2 > 0.9) to the indirect BPNDs based on 2TCM with moderate reproducibility (%variability ≈ 10). PET with [11C]TARP-2105 enabled noninvasive BPND estimation and visual mapping of TARP γ-8 in the living rat brain.

11C-Labeled Radiotracer for Noninvasive and Quantitative Assessment of the Thiocyanate Efflux System in the Brain.

Thiocyanate (SCN-) alters the potency of certain agonists for the α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor, and dysfunctions in AMPA receptor signaling are considered to underlie a number of neurological diseases. While humans may be exposed to SCN- from the environment, including food sources, a carrier-mediated system transports SCN- from the brain into the blood and is an important regulator of SCN- distribution in the central nervous system. The assessment of this SCN- efflux system in the brain would thus be useful for understanding the mechanisms underlying the neurotoxicity of SCN- and for elucidating the relationship between the efflux system and brain diseases. However, the currently available technique for studying SCN- efflux is severely limited by its invasiveness. Here, we describe the development of a SCN- protracer, 9-pentyl-6-[11C]thiocyanatopurine ([11C]1), to overcome this limitation. [11C]1 was synthesized by the reaction of the iodo-precursor and [11C]SCN- or the reaction of the disulfide precursor with [11C]NH4CN. The protracer [11C]1 entered the brain after intravenous injection into mice and was rapidly metabolized to [11C]SCN-, which was then eliminated from the brain. The efflux of [11C]SCN- was dose-dependently inhibited by perchlorate, a monovalent anion, and the highest dose caused an 82% reduction in the efflux rate. Our findings demonstrate that [11C]1 can be used for the noninvasive and quantitative assessment of the SCN- efflux system in the 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.

Synthesis of [11C]carbonyl-labeled cyclohexyl (5-(2-acetamidobenzo[d]thiazol-6-yl)-2-methylpyridin-3-yl)carbamate ([11C-carbonyl]PK68) as a potential PET tracer for receptor-interacting protein 1 kinase.

BACKGROUND: Receptor-interacting protein 1 kinase (RIPK1) is a key enzyme in the regulation of cellular necroptosis. Recently, cyclohexyl (5-(2-acetamidobenzo[d]thiazol-6-yl)-2-methylpyridin-3-yl)carbamate (PK68, 5) has been developed as a potent inhibitor of RIPK1. Herein, we synthesized [11C]carbonyl-labeled PK68 ([11C-carbonyl]PK68, [11C]PK68) as a potential PET tracer for imaging RIPK1 and evaluated its brain uptake in vivo. RESULTS: We synthesized [11C]PK68 by reacting amine precursor 14 with [11C]acetyl chloride. At the end of synthesis, we obtained [11C]PK68 of 1200-1790 MBq with a radiochemical yield of 9.1 ± 5.9% (n = 10, decay-corrected to the end of irradiation) and radiochemical purity of > 99%, and a molar activity of 37-99 GBq/µmol starting from 18-33 GBq of [11C]CO2. The fully automated synthesis took 30 min from the end of irradiation. In a small-animal PET study, [11C]PK68 was rapidly distributed in the liver and kidneys of healthy mice after injection, and subsequently cleared from their bodies via hepatobiliary excretion and the intestinal reuptake pathway. Although there was no obvious specific binding of RIPK1 in the PET study, [11C]PK68 demonstrated relatively high stability in vivo and provided useful structural information further candidate development. CONCLUSIONS: In the present study, we successfully radiosynthesized [11C]PK68 as a potential PET tracer and evaluated its brain uptake. We are planning to optimize the chemical structure of [11C]PK68 and conduct further PET studies on it using pathological models.

Automated radiosynthesis of [11 C]MTP38-a phosphodiesterase 7 imaging tracer-using [11 C]hydrogen cyanide for clinical applications.

We have developed 8-amino-3-(2S,5R-dimethyl-1-piperidyl)-[1,2,4]triazolo[4,3-a]pyrazine-5-[11 C]carbonitrile ([11 C]MTP38) as a positron emission tomography (PET) tracer for the imaging of phosphodiesterase 7. For the fully automated production of [11 C]MTP38 routinely and efficiently for clinical applications, we determined the radiosynthesis procedure of [11 C]MTP38 using [11 C]hydrogen cyanide ([11 C]HCN) as a PET radiopharmaceutical. Radiosynthesis of [11 C]MTP38 was performed using an automated 11 C-labeling synthesizer developed in-house within 40 min after the end of irradiation. [11 C]MTP38 was obtained with a relatively high radiochemical yield (33 ± 5.5% based on [11 C]CO2 at the end of irradiation, decay-corrected, n = 15), radiochemical purity (>97%, n = 15), and molar activity (47 ± 12 GBq/µmol at the end of synthesis, n = 15). All the results of the quality control (QC) testing for the [11 C]MTP38 injection complied with our in-house QC and quality assurance specifications. We successfully automated the radiosynthesis of [11 C]MTP38 for clinical applications using an 11 C-labeling synthesizer and sterile isolator. Taken together, this protocol provides a new radiopharmaceutical [11 C]MTP38 suitable for clinical applications.

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

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.

Off-tumor IDO1 target engagements determine the cancer-immune set point and predict the immunotherapeutic efficacy.

BACKGROUND: Indoleamine-2,3-dioxygenase 1 (IDO1) has been intensively pursued as a therapeutic target to reverse the immunosuppressive cancer-immune milieu and promote tumor elimination. However, recent failures of phase III clinical trials with IDO1 inhibitors involved in cancer immunotherapies highlight the urgent need to develop appropriate methods for tracking IDO1 when the cancer-immune milieu is therapeutically modified. METHODS: We utilized a small-molecule radiotracer, 11C-l-1MTrp, to quantitatively and longitudinally visualize whole-body IDO1 dynamics. Specifically, we first assessed 11C-l-1MTrp in mice-bearing contralateral human tumors with distinct IDO1 expression patterns. Then, we applied 11C-l-1MTrp to longitudinally monitor whole-body IDO1 variations in immunocompetent melanoma-bearing mice treated with 1-methyl-l-tryptophan plus either chemotherapeutic drugs or antibodies targeting programmedcell death 1 and cytotoxic T-lymphocyte-associated protein 4. RESULTS: 11C-l-1MTrp positron emission tomography (PET) imaging accurately delineated IDO1 expression in xenograft mouse models. Moreover, we were able to visualize dynamic IDO1 regulation in the mesenteric lymph nodes (MLNs), an off-tumor IDO1 target, where the percentage uptake of 11C-l-1MTrp accurately annotated the therapeutic efficacy of multiple combination immunotherapies in preclinical models. Remarkably, 11C-l-1MTrp signal intensity in the MLNs was inversely related to the specific growth rates of treated tumors, suggesting that IDO1 expression in the MLNs can serve as a new biomarker of the cancer-immune set point. CONCLUSIONS: PET imaging of IDO1 with 11C-l-1MTrp is a robust method to assess the therapeutic efficacy of multiple combinatorial immunotherapies, improving our understanding of the merit and challenges of IDO1 regimens. Further validation of this animal data in humans is ongoing. We envision that our results will provide a potential precision medicine paradigm for noninvasive visualizing each patient's individual response in combinatorial cancer immunotherapy, and tailoring optimal personalized combination strategies.

Radiosynthesis of 18F-fluoroethylated tracers via a simplified one-pot 18F-fluoroethylation method using [18F]fluoroethyl tosylate.

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.

Radiosynthesis and quality control testing of the tau imaging positron emission tomography tracer [18 F]PM-PBB3 for clinical applications.

Recently, we produced 11 C-labeled 2-((1E,3E)-4-(6-(methylamino)pyridin-3-yl)buta-1,3-dienyl)benzo[d]thiazol-6-ol ([11 C]PBB3) as a clinically useful positron emission tomography (PET) tracer for in vivo imaging of tau pathologies in the human brain. To overcome the limitations (i.e., rapid in vivo metabolism and short half-life) of [11 C]PBB3, we further synthesized 18 F-labeled 1-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 ([18 F]PM-PBB3). [18 F]PM-PBB3 is also a useful tau PET tracer for imaging tau pathologies. In this study, we developed a routine radiosynthesis and quality control testing of [18 F]PM-PBB3 for clinical applications. [18 F]PM-PBB3 was synthesized by direct 18 F-fluorination of the tosylated derivative, followed by removal of the protecting group. [18 F]PM-PBB3 was obtained with sufficient radioactivity (25 ± 6.0% of the nondecay-corrected radiochemical yield at the end of synthesis, EOS), radiochemical purity (98 ± 0.6%), and molar activity (350 ± 94 GBq/µmol at EOS; n = 53). Moreover, [18 F]PM-PBB3 consistently retained >95% of radiochemical purity for 60 min without undergoing photoisomerization using a new UV-cutoff light (yellow light) fixed in the hot cell to monitor the synthesis. All the results of the quality control testing for the [18 F]PM-PBB3 injection complied with our in-house quality control and quality assurance specifications. We have accomplished >200 production runs of [18 F]PM-PBB3 in our facility for various research purposes.

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.

Simultaneous measurements of the molar radioactivity, radiochemical purity and chemical impurity in the [11C]choline injection using radio-HPLC with a corona-charged aerosol detector.

[11C]choline has a weak UV absorption chromophore, and it is challenging to detect less than 1 µg/mL choline using radio-HPLC-UV. In this study, we established an analytical procedure of [11C]choline using the radio-HPLC coupled with the corona-charged aerosol detector. As a result, we achieved more than 100 GBq/µmol (over 0.1 µg/mL of choline) of molar radioactivity at the end of synthesis, over 95% of radiochemical purity, and more than 0.1 µg/mL of 2-dimethyoaminoethanol as a chemical impurity.

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 (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.

Radiosynthesis of [thiocarbonyl-11C]disulfiram and its first PET study in mice.

[Thiocarbonyl-11C]disulfiram ([11C]DSF) was synthesized via iodine oxidation of [11C]diethylcarbamodithioic acid ([11C]DETC), which was prepared from [11C]carbon disulfide and diethylamine. The decay-corrected isolated radiochemical yield (RCY) of [11C]DSF was greatly affected by the addition of unlabeled carbon disulfide. In the presence of carbon disulfide, the RCY was increased up to 22% with low molar activity (Am, 0.27 GBq/µmol). On the other hand, [11C]DSF was obtained in 0.4% RCY with a high Am value (95 GBq/µmol) in the absence of carbon disulfide. The radiochemical purity of [11C]DSF was always >98%. The first PET study on [11C]DSF was performed in mice. A high uptake of radioactivity was observed in the liver, kidneys, and gallbladder. The uptake level and distribution pattern in mice were not significantly affected by the Am value of the [11C]DSF sample used. In vivo metabolite analysis showed the rapid decomposition of [11C]DSF in mouse plasma.