[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 of 6-[¹⁸F]fluoro-PBR28, a novel radiotracer for imaging the TSPO 18 kDa with PET

6-Fluoro-PBR28 (N-(6-fluoro-4-phenoxypyridin-3-yl)-N-(2-methoxybenzyl)acetamide), a fluorinated analogue of the recently developed TSPO 18 kDa ligand PBR28, was synthesized and labelled with fluorine-18. 6-Fluoro-PBR28 and its 6-chloro/6-bromo counterparts were synthesized in six chemical steps and obtained in 16%, 10% and 19% overall yields, respectively. Labelling with fluorine-18 was performed in one single step (chlorine/bromine-for-fluorine heteroaromatic substitution) using a Zymate-XP robotic system affording HPLC-purified, ready-to-inject, 6-[¹⁸F]fluoro-PBR28 (>95% radiochemically pure). Non-decay-corrected overall yields were 9-10% and specific radioactivities ranged from 74 to 148 GBq/μmol. In vitro binding experiments, dynamic μPET studies performed in a rat model of acute neuroinflammation (unilaterally, AMPA-induced, striatum-lesioned rats) and ex vivo autoradiography on the same model demonstrated the potential of 6-[¹⁸F]fluoro-PBR28 to image the TSPO 18 kDa using PET.     

Synthesis of [¹¹C]PBR06 and [¹⁸F]PBR06 as agents for positron emission tomographic (PET) imaging of the translocator protein (TSPO)

The translocator protein 18 kDa (TSPO) is an attractive target for molecular imaging of neuroinflammation and tumor progression. [¹⁸F]PBR06, a fluorine-18 labeled form of PBR06, is a promising PET TSPO radioligand originally developed at NIMH. [¹¹C]PBR06, a carbon-11 labeled form of PBR06, was designed and synthesized for the first time. The standard PBR06 was synthesized from 2,5-dimethoxybenzaldehyde in three steps with 71% overall chemical yield. The radiolabeling precursor desmethyl-PBR06 was synthesized from 2-hydroxy-5-methoxybenzaldehyde in five steps with 12% overall chemical yield. The target tracer [¹¹C]PBR06 was prepared by O-[¹¹C]methylation of desmethyl-PBR06 with [¹¹C]CH₃OTf in CH₃CN at 80 °C under basic condition and isolated by HPLC combined with SPE purification with 40–60% decay corrected radiochemical yield and 222–740 GBq/μmol specific activity at EOB. On the similar grounds, [¹⁸F]PBR06 was also designed and synthesized. The previously described Br-PBR06 precursor was synthesized from 2,5-dimethoxybenzaldehyde in two steps with 78% overall chemical yield. A new radiolabeling precursor tosyloxy-PBR06, previously undescribed tosylate congener of PBR06, was designed and synthesized from ethyl 2-hydroxyacetate, 4-methylbenzene-1-sulfonyl chloride, and N-(2,5-dimethoxybenzyl)-2-phenoxyaniline in four steps with 50% overall chemical yield. [¹⁸F]PBR06 was prepared by the nucleophilic substitution of either new tosyloxy-PBR06 precursor or known Br-PBR06 precursor in DMSO at 140 °C with K[¹⁸F]F/Kryptofix 2.2.2 for 15 min and HPLC combined with SPE purification in 20–60% decay corrected radiochemical yield, >99% radiochemical purity, 87–95% chemical purity, and 37–222 GBq/μmol specific activity at EOB. Radiosynthesis of [¹⁸F]PBR06 using new tosylated precursor gave similar radiochemical purity, and higher specific activity, radiochemical yield and chemical purity in comparison with radiosynthesis using bromine precursor.     

Synthesis and biological evaluation of positron emission tomography radiotracers targeting serotonin 4 receptors in brain: [18F]MNI-698 and [18F]MNI-699

Two new benzodioxane derivatives were synthesized as candidates to image the serotonin 4 receptors by positron emission tomography (PET) and radiolabeled with fluorine-18 via a two-step procedure. Competition binding assays demonstrated that MNI-698 and MNI-699 had sub-nanomolar binding affinities against rat striatal 5-HT₄ receptors (K_i of 0.20 and 0.07 nM, respectively). PET imaging in rhesus monkey showed that the regional brain distribution of [¹⁸F]MNI-698 and [¹⁸F]MNI-699 were consistent with the known densities of 5-HT₄ in brain. [¹⁸F]MNI-698 and [¹⁸F]MNI-699 are among the first fluorine-18 radiotracers developed for imaging the 5-HT₄ receptors in vivo and are currently under preclinical investigation in primates for future human use.     

Synthesis and preclinical evaluation of [18F]FSL25.1188, a reversible PET radioligand for monoamine oxidase-B

Carbon-11 labeled SL25.1188 is a promising reversible monoamine oxidase-B (MAO-B) radioligand that was recently translated for human positron emission tomography (PET) imaging. Herein, we report the development of a novel fluorinated derivative, namely, [¹⁸F](S)-3-(6-(3-fluoropropoxy)benzo[d]isoxazol-3-yl)-5-(methoxymethyl)oxazolidin-2-one ([¹⁸F]FSL25.1188; [¹⁸F]6), as a candidate ¹⁸F-labeled MAO-B radioligand, and, its subsequent preclinical evaluation in non-human primates (NHP). [¹⁸F]6 was produced and isolated (>6 GBq) with high radiochemical purity (>99%), and molar activity (>100 GBq/µmol at time of injection). Autoradiography studies conducted in post-mortem human brain sections revealed [¹⁸F]6 binding in MAO-B rich regions. PET imaging study of [¹⁸F]6 in NHP showed high brain uptake (SUV > 2.5) as well as a regional brain radioactivity distribution in accordance with MAO-B expression. [¹⁸F]6 displayed favorable in vivo kinetics, with an early peak in the time-activity curve followed by progressive wash-out from the NHP brain. Specificity of [¹⁸F]6 was investigated in a pre-treatment study with l-deprenyl (1.0 mg/kg) wherein reduced radioligand uptake was observed in all MAO-B rich regions. Results from the current preclinical investigation suggests [¹⁸F]6 is a promising MAO-B PET radioligand. Further evaluation of [¹⁸F]6 and structurally related ¹⁸F-analogs are underway to identify an optimized candidate for clinical research studies.     

Preclinical TSPO Ligand PET to Visualize Human Glioma Xenotransplants: A Preliminary Study

Current positron emission tomography (PET) imaging biomarkers for detection of infiltrating gliomas are limited. Translocator protein (TSPO) is a novel and promising biomarker for glioma PET imaging. To validate TSPO as a potential target for molecular imaging of glioma, TSPO expression was assayed in a tumor microarray containing 37 high-grade (III, IV) gliomas. TSPO staining was detected in all tumor specimens. Subsequently, PET imaging was performed with an aryloxyanilide-based TSPO ligand, [¹⁸F]PBR06, in primary orthotopic xenograft models of WHO grade III and IV gliomas. Selective uptake of [¹⁸F]PBR06 in engrafted tumor was measured. Furthermore, PET imaging with [¹⁸F]PBR06 demonstrated infiltrative glioma growth that was undetectable by traditional magnetic resonance imaging (MRI). Preliminary PET with [¹⁸F]PBR06 demonstrated a preferential tumor-to-normal background ratio in comparison to 2-deoxy-2-[¹⁸F]fluoro-D-glucose ([¹⁸F]FDG). These results suggest that TSPO PET imaging with such high-affinity radiotracers may represent a novel strategy to characterize distinct molecular features of glioma growth, as well as better define the extent of glioma infiltration for therapeutic purposes.     

Radiosynthesis and preliminary evaluation of 4-[18F]fluoro-N-[4-[6-(isopropylamino)pyrimidin-4-yl]-1,3-thiazol-2-yl]-N-methylbenzamide as a new positron emission tomography ligand for metabotropic glutamate receptor subtype 1

The purpose of this study was to develop 4-[¹⁸F]fluoro-N-[4-[6-(isopropylamino)pyrimidin-4-yl]-1,3-thiazol-2-yl]-N-methylbenzamide ([¹⁸F]FITM, [¹⁸F]4) as a new PET ligand for imaging metabotropic glutamate receptor subtype 1 (mGluR1). [¹⁸F]4 was synthesized by [¹⁸F]fluorination of a novel nitro precursor 3 with [¹⁸F]KF in the presence of Kryptofix 222. At the end of synthesis, 429-936 MBq (n = 8) of [¹⁸F]4 was obtained with >99% radiochemical purity and 204-559 GBq/μmol specific activity starting from 6.7 to 13.0 GBq of [¹⁸F]F⁻. The brain distribution of [¹⁸F]4 was determined by the in vitro and ex vivo autoradiography using rat brain sections. The in vitro and in vivo specific binding of [¹⁸F]4 to mGluR1 was detected in the cerebellum, thalamus, hippocampus, and striatum. These results suggest that [¹⁸F]4 is a promising PET ligand for the in vivo evaluation of mGluR1.     

[18F]DPA-714: Direct Comparison with [11C]PK11195 in a Model of Cerebral Ischemia in Rats

Purpose

Neuroinflammation is involved in several brain disorders and can be monitored through expression of the translocator protein 18 kDa (TSPO) on activated microglia. In recent years, several new PET radioligands for TSPO have been evaluated in disease models. [¹⁸F]DPA-714 is a TSPO radiotracer with great promise; however results vary between different experimental models of neuroinflammation. To further examine the potential of [¹⁸F]DPA-714, it was compared directly to [¹¹C]PK11195 in experimental cerebral ischaemia in rats.

Methods

Under anaesthesia, the middle cerebral artery of adult rats was occluded for 60 min using the filament model. Rats were allowed recovery for 5 to 7 days before one hour dynamic PET scans with [¹¹C]PK11195 and/or [¹⁸F]DPA-714 under anaesthesia.

Results

Uptake of [¹¹C]PK11195 vs [¹⁸F]DPA-714 in the ischemic lesion was similar (core/contralateral ratio: 2.84±0.67 vs 2.28±0.34 respectively), but severity of the brain ischemia and hence ligand uptake in the lesion appeared to vary greatly between animals scanned with [¹¹C]PK11195 or with [¹⁸F]DPA-714. To solve this issue of inter-individual variability, we performed a direct comparison of [¹¹C]PK11195 and [¹⁸F]DPA-714 by scanning the same animals sequentially with both tracers within 24 h. In this direct comparison, the core/contralateral ratio (3.35±1.21 vs 4.66±2.50 for [¹¹C]PK11195 vs [¹⁸F]DPA-714 respectively) showed a significantly better signal-to-noise ratio (1.6 (1.3–1.9, 95%CI) fold by linear regression) for [¹⁸F]DPA-714.

Conclusions

In a clinically relevant model of neuroinflammation, uptake for both radiotracers appeared to be similar at first, but a high variability was observed in our model. Therefore, to truly compare tracers in such models, we performed scans with both tracers in the same animals. By doing so, our result demonstrated that [¹⁸F]DPA-714 displayed a higher signal-to-noise ratio than [¹¹C]PK11195. Our results suggest that, with the longer half-life of [¹⁸F] which facilitates distribution of the tracer across PET centre, [¹⁸F]DPA-714 is a good alternative for TSPO imaging.     

Synthesis and preclinical characterization of 1-(6′-deoxy-6′-[18F]fluoro-β-d-allofuranosyl)-2-nitroimidazole (β-6′-[18F]FAZAL) as a positron emission tomography radiotracer to assess tumor hypoxia

Positron emission tomography (PET) using fluorine-18 (¹⁸F)-labeled 2-nitroimidazole radiotracers has proven useful for assessment of tumor oxygenation. However, the passive diffusion-driven cellular uptake of currently available radiotracers results in slow kinetics and low tumor-to-background ratios. With the aim to develop a compound that is actively transported into cells, 1-(6′-deoxy-6′-[¹⁸F]fluoro-β-d-allofuranosyl)-2-nitroimidazole (β-[¹⁸F]1), a putative nucleoside transporter substrate, was synthetized by nucleophilic [¹⁸F]fluoride substitution of an acetyl protected labeling precursor with a tosylate leaving group (β-6) in a final radiochemical yield of 12 ± 8% (n = 10, based on [¹⁸F]fluoride starting activity) in a total synthesis time of 60 min with a specific activity at end of synthesis of 218 ± 58 GBq/μmol (n = 10). Both radiolabeling precursor β-6 and unlabeled reference compound β-1 were prepared in multistep syntheses starting from 1,2:5,6-di-O-isopropylidene-α-d-allofuranose. In vitro experiments demonstrated an interaction of β-1 with SLC29A1 and SLC28A1/2/3 nucleoside transporter as well as hypoxia specific retention of β-[¹⁸F]1 in tumor cell lines. In biodistribution studies in healthy mice β-[¹⁸F]1 showed homogenous tissue distribution and excellent metabolic stability, which was unaffected by tissue oxygenation. PET studies in tumor bearing mice showed tumor-to-muscle ratios of 2.13 ± 0.22 (n = 4) at 2 h after administration of β-[¹⁸F]1. In ex vivo autoradiography experiments β-[¹⁸F]1 distribution closely matched staining with the hypoxia marker pimonidazole. In conclusion, β-[¹⁸F]1 shows potential as PET hypoxia radiotracer which merits further investigation.     

Synthesis and in vitro evaluation of three novel radiotracers for imaging of metabotropic glutamate receptor subtype 2 in rat brain

The purpose of this study was to develop three new radiotracers, 1-(cyclopropylmethyl)-4-([¹¹C/¹⁸F]substituted-phenyl)piperidin-1-yl-2-oxo-1,2-dihydropyridine-3-carbonitrile ([¹¹C]1, [¹¹C]2, and [¹⁸F]4), and to examine their specific bindings with metabotropic glutamate receptor subtype 2 (mGluR2) in rat brain sections by using in vitro autoradiography. These compounds were found to possess potent in vitro binding affinities (K_i: 8.0–34.1 nM) for mGluR2 in rat brain homogenate. [¹¹C]1, [¹¹C]2, and [¹⁸F]4 were synthesized by [¹¹C/¹⁸F]alkylation of the corresponding phenol precursors with [¹¹C]methyl iodide or [¹⁸F]fluoroethyl bromide with >98% radiochemical purity and 80–130 GBq/μmol specific activity at the end of synthesis. In vitro autoradiography indicated that these radiotracers showed heterogeneous specific bindings in mGluR2-rich brain regions, such as the cerebral cortex, striatum, hippocampus, and granular layer of the cerebellum.     

Synthesis and evaluation of a 18F-curcumin derivate for β-amyloid plaque imaging

IntroductionCurcumin is a neuroprotective compound that inhibits the formation of amyloid oligomers and fibrils and binds to β-amyloid plaques in Alzheimer’s disease (AD). We aimed to synthesize an ¹⁸F-labeled curcumin derivate ([¹⁸F]4) and to characterize its positron emission tomography (PET) tracer-binding properties to β-amyloid plaques in a transgenic APP23 mouse model of AD.MethodsWe utilized facile one-pot synthesis of [¹⁸F]4 using nucleophilic ¹⁸F-fluorination and click chemistry. Binding of [¹⁸F]4 to β-amyloid plaques in the transgenic APP23 mouse brain cryosections was studied in vitro using heterologous competitive binding against PIB. [¹⁸F]4 uptake was studied ex vivo in rodents and in vivo using PET/computed tomography of transgenic APP23 and wild-type control mice.ResultsThe radiochemical yield of [¹⁸F]4 was 21 ± 11%, the specific activity exceeded 1 TBq/μmol, and the radiochemical purity exceeded 99.3% at the end of synthesis. In vitro studies of [¹⁸F]4 with the transgenic APP23 mouse revealed high β-amyloid plaque binding. In vivo and ex vivo studies demonstrated that [¹⁸F]4 has fast clearance from the blood, moderate metabolism but low blood–brain barrier (BBB) penetration.Conclusions[¹⁸F]4 was synthesized in high yield and excellent quality. In vitro studies, metabolite profile, and fast clearance from the blood indicated a promising tracer for Aβ imaging. However, [¹⁸F]4 has low in vivo BBB penetration and thus further studies are needed to reveal the reason for this and to possibly overcome this issue.     

Stoichiometry-focused (18)F-labeling of alkyne-substituted oligodeoxynucleotides using azido([(18)F]fluoromethyl)benzenes by Cu-catalyzed Huisgen reaction

A novel method for ¹⁸F-radiolabeling of oligodeoxynucleotides (ODNs) by a Cu-catalyzed Huisgen reaction has been developed by using the lowest possible amount of the precursor biomolecule for the realization of stoichiometry-oriented PET (positron emission tomography) chemistry. Under the optimized cyclization conditions of p- or m-azido([¹⁸F]fluoromethyl)benzene and alkyne-substituted ODN (20 nmol) at 40 °C for 15 min in the presence of CuSO₄, TBTA [tris((1-benzyl-1H-1,2,3-triazol-4-yl)methyl)amine], and sodium ascorbate (2:1:2), the synthesis of ¹⁸F-labeled ODNs with sufficiently high radioactivities of 2.1-2.5 GBq and specific radioactivities of 1800-2400 GBq/??mol have been accomplished for use in animal and human PET studies.     

Synthesis and evaluation of NLRP3-inhibitory sulfonylurea [11C]MCC950 in healthy animals

The diaryl sulfonylurea MCC950/CRID3 is a potent NLRP3 inhibitor (IC₅₀ = 8 nM) and, in animal models, MCC950 protects against numerous NLRP3-related neurodegenerative disorders. To evaluate the brain uptake and investigate target engagement of MCC950, we synthesised [¹¹C-urea]MCC950 via carrier added [¹¹C]CO₂ fixation chemistry (activity yield = 237 MBq; radiochemical purity >99%; molar activity = 7 GBq/µmol; radiochemical yield (decay-corrected from [¹¹C]CO₂) = 1.1%; synthesis time from end-of-bombardment = 31 min; radiochemically stable for >1 h). Despite preclinical efficacy in neurodegeneration studies, preclinical positron emission tomography (PET) imaging studies in mouse, rat and rhesus monkey revealed poor brain uptake of low molar activity [¹¹C]MCC950 and rapid washout. In silico prediction tools suggest efflux transporter liabilities for MCC950 at microdoses, and this information should be taken into account when developing next generation NLRP3 inhibitors and/or PET radiotracers.     

Synthesis and evaluation of novel potent TSPO PET ligands with 2-phenylpyrazolo[1,5-a]pyrimidin-3-yl acetamide

Translocator protein (TSPO) expression is closely related with neuroinflammation and neuronal damage which might cause several central nervous system diseases. Herein, a series of TSPO ligands (11a–c and 13a–d) with a 2-phenylpyrazolo[1,5-a]pyrimidin-3-yl acetamide structure were prepared and evaluated via an in vitro binding assay. Most of the novel ligands exhibited a nano-molar affinity for TSPO, which was better than that of DPA-714. Particularly, 11a exhibited a subnano-molar TSPO binding affinity with suitable lipophilicity for in vivo brain studies. After radiolabeling with fluorine-18, [¹⁸F]11a was used for a dynamic positron emission tomography (PET) study in a rat LPS-induced neuroinflammation model; the inflammatory lesion was clearly visualized with a superior target-to-background ratio compared to [¹⁸F]DPA-714. An immunohistochemical examination of the dissected brains confirmed that the uptake location of [¹⁸F]11a in the PET study was consistent with a positively activated microglia region. This study proved that [¹⁸F]11a could be employed as a potential PET tracer for detecting neuroinflammation and could give possibility for diagnosis of other diseases, such as cancers related with TSPO expression.     

Characterization of a novel acetamidobenzoxazolone‐based PET ligand for translocator protein (18 kDa) imaging of neuroinflammation in the brain

We developed the novel positron emission tomography (PET) ligand 2‐[5‐(4‐[¹¹C]methoxyphenyl)‐2‐oxo‐1,3‐benzoxazol‐3(2H)‐yl]‐N‐methyl‐N‐phenylacetamide ([¹¹C]MBMP) for translocator protein (18 kDa, TSPO) imaging and evaluated its efficacy in ischemic rat brains. [¹¹C]MBMP was synthesized by reacting desmethyl precursor ( 1 ) with [¹¹C]CH₃I in radiochemical purity of ≥ 98% and specific activity of 85 ± 30 GBq/μmol (n = 18) at the end of synthesis. Biodistribution study on mice showed high accumulation of radioactivity in the TSPO‐rich organs, e.g., the lungs, heart, kidneys, and adrenal glands. The metabolite analysis in mice brain homogenate showed 80.1 ± 2.7% intact [¹¹C]MBMP at 60 min after injection. To determine the specific binding of [¹¹C]MBMP with TSPO in the brain, in vitro autoradiography and PET studies were performed in an ischemic rat model. In vitro autoradiography indicated significantly increased binding on the ipsilateral side compared with that on the contralateral side of ischemic rat brains. This result was supported firmly by the contrast of radioactivity between the ipsilateral and contralateral sides in PET images. Displacement experiments with unlabelled MBMP or PK11195 minimized the difference in uptake between the two sides. In summary, [¹¹C]MBMP is a potential PET imaging agent for TSPO and, consequently, for the up‐regulation of microglia during neuroinflammation.

     

Facile synthesis of SSR180575 and discovery of 7-chloro-N,N,5-trimethyl-4-oxo-3(6-[18F]fluoropyridin-2-yl)-3,5-dihydro-4H-pyridazino[4,5-b]indole-1-acetamide,a potent pyridazinoindole ligand for PET imaging of TSPO in cancer

A novel synthesis of the translocator protein (TSPO) ligand 7-chloro-N,N,5-trimethyl-4-oxo-3-phenyl-3,5-dihydro-4H-pyridazino[4,5-b]indole-1-acetamide (SSR180575, 3) was achieved in four steps from commercially available starting materials. Focused structure–activity relationship development about the pyridazinoindole ring at the N3 position led to the discovery of 7-chloro-N,N,5-trimethyl-4-oxo-3(6-fluoropyridin-2-yl)-3,5-dihydro-4H-pyridazino[4,5-b]indole-1-acetamide (14), a novel ligand of comparable affinity. Radiolabeling with fluorine-18 (¹⁸F) yielded 7-chloro-N,N,5-trimethyl-4-oxo-3(6-[¹⁸F]fluoropyridin-2-yl)-3,5-dihydro-4H-pyridazino[4,5-b]indole-1-acetamide ([¹⁸F]-14) in high radiochemical yield and specific activity. In vivo studies of [¹⁸F]-14 revealed this agent as a promising probe for molecular imaging of glioma.     

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 (A_m, 0.27 GBq/μmol). On the other hand, [¹¹C]DSF was obtained in 0.4% RCY with a high A_m 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 A_m value of the [¹¹C]DSF sample used. In vivo metabolite analysis showed the rapid decomposition of [¹¹C]DSF in mouse plasma.     

Radiosynthesis of a carbon-11-labeled AMPAR allosteric modulator as a new PET radioligand candidate for imaging of Alzheimer’s disease

To develop PET tracers for imaging of Alzheimer’s disease, a new carbon-11-labeled AMPAR allosteric modulator 4-cyclopropyl-7-(3-[¹¹C]methoxyphenoxy)-3,4-dihydro-2H-benzo[e][1,2,4]thiadiazine 1,1-dioxide ([¹¹C]8) has been synthesized. The reference standard 4-cyclopropyl-7-(3-methoxyphenoxy)-3,4-dihydro-2H-benzo[e][1,2,4]thiadiazine 1,1-dioxide (8) and its corresponding desmethylated precursor 4-cyclopropyl-7-(3-hydroxyphenoxy)-3,4-dihydro-2H-benzo[e][1,2,4]thiadiazine 1,1-dioxide (9) were synthesized from 4-methoxyabiline and chlorosulfonyl isocyanate in eight and nine steps with 3% and 1% overall chemical yield, respectively. The target tracer [¹¹C]8 was prepared from the precursor 9 with [¹¹C]CH₃OTf through O-[¹¹C]methylation and isolated by HPLC combined with SPE in 10–15% radiochemical yield, based on [¹¹C]CO₂ and decay corrected to end of bombardment (EOB). The radiochemical purity was >99%, and the molar activity (A_M) at EOB was 370–740?GBq/μmol with a total synthesis time of 35–40-minutes from EOB.     

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.     

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.