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

Multidrug resistance-associated protein 4 (MRP4) and organic anion transporter 3 (OAT3) mediate the efflux of organic anions from the brain and heart. In this study, we have developed a probe for estimating the activity of these transporters in these tissues using positron emission tomography. Several 11C-labeled hippuric acid ester derivatives were screened with the expectation that they would be hydrolyzed in situ to form the corresponding 11C-labeled organic acids in target tissues. Among the compounds screened, benzyl [11C]hippurate showed favorable hydrolysis rates and uptake properties in the target tissues of mice. Subsequent evaluation using transporter knockout mice revealed that radioactivity was retained in the brain and heart of Oat3(-/-) and Mrp4(-/-) mice, respectively, compared with that of control mice after the intravenous administration of benzyl [11C]hippurate. Benzyl [11C]hippurate could therefore be used as a probe for estimating the activities of OAT3 and MRP4 in mouse brain and heart, respectively.

We found out 3-[5-(pyridin-2-yl)-2H-tetrazol-2-yl]benzonitrile analogues as the candidate for positron emission tomography (PET) imaging agents of metabotropic glutamate receptor subtype 5 (mGluR5). Among these compounds, 3-methyl-5-(5-(pyridin-2-yl)-2H-tetrazol-2-yl)benzonitrile (10) exhibited high binding affinity (Ki = 9.4 nM) and moderate lipophilicity (cLogD, 2.4). Subsequently, [11C]10 was radiosynthesized at 25 ± 14% radiochemical yield (n = 11) via C-[11C]methylation of the arylstannyl precursor 15 with [11C]methyl iodide. In vitro autoradiography and PET assessments using [11C]10 showed high specific binding in the striatum and hippocampus, two brain regions enriched with mGluR5. Moreover, test–retest PET studies with [11C]10 indicated high reliability to quantify mGluR5 density, such as the intraclass correlation coefficient (0.90) and Pearson r (0.91) in the striatum of rat brain. We demonstrated that [11C]10 is a useful PET ligand for imaging and quantitative analysis of mGluR5. Furthermore, [11C]10 might be modified using its skeleton as a lead compound.

ADX88178 (1) has been recently developed as a potent positive allosteric modulator for metabotropic glutamate receptor 4 (mGluR4). The aim of this study was to develop [(11)C]1 as a novel positron emission tomography ligand and to evaluate its binding ability for mGluR4. Using stannyl precursor 3, [(11)C]1 was efficiently synthesized by introducing an [(11)C]methyl group into a pyrimidine ring via C-(11)C coupling and deprotection reactions, in 16±6% radiochemical yield (n=10). At the end of synthesis, 0.54-1.10GBq of [(11)C]1 was acquired with >98% radiochemical purity and 90-120GBq/μmol of specific activity. In vitro autoradiography and ex vivo biodistribution study in rat brains showed specific binding of [(11)C]1 in the cerebellum, striatum, thalamus, cerebral cortex, and medulla oblongata, which showed dose-dependent decreases by administration with multi-dose of unlabeled 1.

The five transmembrane translocator protein (18 kDa, TSPO) is abundantly expressed in the mitochondria of activated microglia (brain) and peripheral tissues, including those of the heart, lung and kidney. We recently developed the 18F-labelled molecule 2-[5-(4-[18F]fluoropropyloxyphenyl)-2-oxo-1,3-benzoxazol-3(2H)-yl]-N-methyl-N-phenylacetamide ([18F]FPBMP) as a novel positron emission tomography (PET) radioligand for imaging TSPO. In this study, we have evaluated the pharmacokinetics of this radioligand based on its biodistribution in mice, as well as the results of PET and metabolite studies in rats. The specificity of [18F]FPBMP towards TSPO was assessed by blocking experiments involving the intravenous injection of 1 mg kg−1 of unlabeled PK11195. A metabolite study was performed in the plasma and peripheral organs of rats by HPLC methods. The ex vivo biodistribution of [18F]FPBMP in mice showed a high uptake of radioactivity in TSPO enriched peripheral organs, especially in the lung, heart and kidney. The in vivo biodistribution of this compound was evaluated through PET summation images of rats 1–10, 10–20, 20–30 and 50–60 min after the injection of the radioligand. The TSPO-enriched organs, including the heart, kidney and lung, were clearly visualized. Pre-treatment with TSPO-specific PK11195 minimized the uptake of [18F]FPBMP in the TSPO-enriched tissues, thereby confirming its selectivity for TSPO. Metabolite analysis in rats confirmed the presence of [18F]FPBMP in the heart, lung and kidney up to 60 min. In summary, these data demonstrate that [18F]FPBMP is a suitable PET ligand for the imaging of TSPO in peripheral tissues.

We developed a novel positron emission tomography (PET) radiotracer N-(3,4-dimethylisoxazol-5-yl) piperazine-4-[4-(4-fluorophenyl)thiazol-2-yl]-1-[11C]carboxamide ([11C]DPFC, [11C]1) for in vivo imaging of fatty acid amide hydrolase (FAAH) in rat brain. Compound 1 showed a high binding affinity for FAAH (IC50: 3.3 nM). [11C]1 was synthesized by reaction of 5-amino-3,4-dimethylisoxazole (2) with [11C] phosgene ([11C]COCl2), followed by reaction with 4-(4-fluorophenyl)-2-(piperazin-1-yl)thiazole (3), with a 9 4% radiochemical yield (decay-corrected, n ¼ 9) based on [11C]CO2. A biodistribution study in mice showed a high uptake of radioactivity in FAAH-rich organs, including the lung, liver, and kidney. PET summation images of rat brains showed high radioactivity (>2 SUV) in the cerebellar nuclei and frontal cortex. This pattern was consistent with the known regional distribution pattern of FAAH in the rodent brain. Pretreatment with the FAAH-selective inhibitor URB597 significantly reduced the whole brain uptake of [11C]1. At 30 min after the radiotracer injection, more than 95% of the total radioactivity was found to be irreversible in the brain homogenate of rats. Our results indicate that [11C]1 is a promising PET tracer for in vivo visualization of FAAH in living brains.

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

We evaluated the efficacy of 2-[5-(4-[(18)F]fluoroethoxy-2-oxo-1,3-benzoxazol-3(2H)-yl)-N-methyl-N-phenylacetamide] ([(18)F]FEBMP) for positron emission tomography (PET) imaging of translocator protein (18 kDa, TSPO). Dissection was used to determine the distribution of [(18)F]FEBMP in mice, while small-animal PET and metabolite analysis were used for a rat model of focal cerebral ischemia. [(18)F]FEBMP showed high radioactivity uptake in mouse peripheral organs enriched with TSPO, and relatively high initial brain uptake (2.67 ± 0.12% ID/g). PET imaging revealed an increased accumulation of radioactivity in the infarcted striatum, with a maximum ratio of 3.20 ± 0.12, compared to non-injured striatum. Displacement with specific TSPO ligands lowered the accumulation levels in infarcts to those on the contralateral side. This suggests that the increased accumulation reflected TPSO-specific binding of [(18)F]FEBMP in vivo. Using a simplified reference tissue model, the binding potential on the infarcted area was 2.72 ± 0.27. Metabolite analysis in brain tissues showed that 83.2 ± 7.4% and 76.4 ± 2.1% of radioactivity was from intact [(18)F]FEBMP at 30 and 60 min, respectively, and that this ratio was higher than in plasma (8.6 ± 1.9% and 3.9 ± 1.1%, respectively). In vitro autoradiography on postmortem human brains showed that TSPO rs6971 polymorphism did not affect binding sites for [(18)F]FEBMP. These findings suggest that [(18)F]FEBMP is a promising new tool for visualization of neuroinflammation.

The 5-HT7 receptor is a recently cloned G-protein-coupled receptor (GPCR) that is important in regulating sleep, depression, and circadian rhythms. However, the potential pathophysiological roles of 5-HT7 have not been fully elucidated, and no 5-HT7 positron emission tomography (PET) radioligands are available, thus limiting imaging studies of this receptor in humans. Here, we present the radiosynthesis and biological evaluation of 5-(4-([11C]methoxyphenyl)-1-methyl-4-nitro-1H-imidazole ([11C]1) as a new PET ligand for 5-HT7. Three-dimensional pharmacophore evaluation and docking studies confirmed its high affinity for 5-HT7, and in vitro binding assays showed that the binding affinity was 16.8 0.9 nM. The specific activity was found to be 48 29 GBq mmol 1 for [11C]1 in a synthetic time of 26 3 min (n ¼ 8), having 38 7% radiochemical yield (decay-corrected) based on [11C]CO2. Ligand interactions with human serum albumin were studied by fluorescence quenching to obtain a Stern–Volmer plot, which showed a binding constant of 1.15 104 M 1. Whole-body biodistribution patterns were evaluated in normal mice by 1 h dynamic PET imaging; this analysis showed rapid clearance of radioactivity from the main peripheral organs, with the exception of the liver. Preliminary PET studies in rat brains showed rapid accumulation of radioactivity in the brain. The regional radioactivity reached a maximum within 0–2 min after the radioligand injection and then decreased rapidly, resulting in minimal radiation burden in the brain during the scan. In summary, this specific biaryl system has shown potential as a 5-HT7 ligand and further optimization and longitudinal studies may yield the first small molecule-based PET ligand for 5-HT7 in clinical settings.

We developed 2-methylpyridin-3-yl-4-(5-(2-fluorophenyl)-4H-1,2,4-triazol-3-yl)piperidine-1-[11C]carboxylate ([11C]MFTC) as a promising PET tracer for in vivo imaging of fatty acid amide hydrolase (FAAH) in rat and monkey brains. [11C]MFTC was synthesized by reacting 3-hydroxy-2-methylpyridine (2) with [11C]phosgene ([11C]COCl2), followed by reacting with 4-(5-(2-fluorophenyl)-4H-1,2,4-triazol-3-yl)piperidine (3), with a 20 ± 4.6% radiochemical yield (decay-corrected, n = 30) based on [11C]CO2 and 40 min synthesis time from the end of bombardment. A biodistribution study in mice showed high uptake of radioactivity in FAAH-rich organs, including the lung, liver, and kidneys. Positron emission tomography (PET) summation images of rat brains showed high radioactivity in the frontal cortex, cerebellum, and hippocampus, which was consistent with the regional distribution pattern of FAAH in rodent brain. Pretreatment with MFTC or FAAH-selective URB597 significantly reduced the uptake in the brain. PET imaging of monkey brain showed relatively high uptake in the whole brain, particularly in the occipital cortex, which was also inhibited by treatment with MFTC or URB597. More than 96% of the total radioactivity was irreversible in the brain homogenate of rats 5 min after the radiotracer injection. The specific in vivo FAAH binding indicates that [11C]MFTC is a promising PET tracer for visualizing FAAH in the brain.

Metabotropic glutamate 1 (mGlu1) receptor is found not only in the brain but also in melanomas and breast cancers. mGlu1 is a promising target for molecular imaging-based diagnosis and treatment of melanoma because its overexpression induces melanocyte carcinogenesis. Here we developed three PET tracers: 4-halogeno-N-[4-[6-(isopropylamino)- pyrimidin-4-yl]-1,3-thiazol- 2-yl]-N-[11C]methylbenzamide ([11C]4−6), which exhibited high uptake in target tumor and decreased uptake in nontarget brain tissues. In vitro binding assay indicated high to moderate binding affinities of 4−6 (Ki, 22− 143 nM) for mGlu1 receptor. In vivo biodistribution studies in mice implanted with B16F10 melanoma cells confirmed high radioactive uptake in tumor and low uptake in blood, skin, and muscles. Inhibition of mGlu1 receptor using an mGlu1-selective ligand led to reduced radioactive uptake in the tumor. [11C]6 displayed the highest ratio of uptake between tumor and nontarget tissue and may prove useful as a PET tracer for mGlu1 imaging in melanoma.

α5β1 Integrin, a fibronectin receptor, is becoming a pertinent therapeutic target and a promising prognostic biomarker for cancer patients. The aim of this study was to functionalize an α5β1-specific fibronectin-mimetic peptide sequence KSSPHSRN(SG)5RGDSP (called PR_b) as a positron emission tomography (PET) probe. PR_b was modified by addition of a β-alanine residue, conjugated with 2-S-(4-isothiocyanatobenzyl)-1,4,7-triazacyclononane-1,4,7-triacetic acid (p-SCN-Bn-NOTA), and radiolabeled with (18)F based on the chelation of (18)F-aluminum fluoride. A control probe was produced by glycine to alanine substitution in the RGD motif of PR_b. Cell binding and blocking assays, autoradiographic evaluation of tissue binding and blocking, dynamic PET scans, and a biodistribution study were conducted using cell lines and murine tumor models with determined expression levels of α5β1 and other related integrins. (18)F-PR_b was produced with a labeling yield of 22.3±1.9% based on (18)F-F(-), a radiochemical purity of >99%, and a specific activity of 30-70 GBq/µmol; it exhibited α5β1-binding activity and specificity in vitro, ex vivo, and in vivo, and had a rapid blood clearance and a predominant renal excretion pathway. In vivo α5β1-positive tumors could be clearly visualized by (18)F-PR_b PET imaging. Both imaging and biodistribution studies suggested higher uptake of (18)F-PR_b in α5β1-positive tumors than in α5β1-negative tumors and higher α5β1-positive tumor uptake of (18)F-PR_b than the control probe. In contrast, there was no significant difference seen in the contralateral muscle uptake. A PET radioprobe, (18)F-PR_b, was developed de novo and potentially can be used for noninvasive detection of α5β1 expression in tumors.

Objectives: Disulfiram (DSF) has been widely used to treat alcoholism for more than half a century and recent studies suggest that DSF may have potential therapeutic effect for several cancers. However, the detail reaction mechanism for the anti-cancer effect of DSF is still not clear. Therefore, the development of [11C]DSF and its in vivo and in vitro studies might be helpful for the understanding of the role of DSF. Here we present the first synthesis of [11C]DSF using [11C]carbon disulfide ([11C]CS2) as a labelling agent. Methods: [11C]DSF was synthesized via iodine (I2) oxidation of [11C]diethylcarbamodithioic acid ([11C]DETC), which was prepared by the reaction of [11C]CS2 and diethylamine in acetonitrile (MeCN) at room temperature (r.t.) Results: The (decay-corrected isolated) radiochemical yield (RCY) of [11C]DSF was greatly affected by the addition of non-radioactive carbon disulfide (CS2). In the presence of CS2, RCY was increased up to 22% with low molar radioactivity (0.27 GBq/mol). On the other hand, in the absence of CS2, [11C]DSF was obtained in 0.4% RCY with high molar radioactivity (95 GBq/mol). The radiochemical purities (determined by radio-HPLC analysis of the isolated product) of [11C]DSF were always greater than 98%. The first PET study of [11C]DSF was performed on mice. High uptake of radioactivity in the liver, kidney and gallbladder were observed and the metabolite analysis of mouse serum showed the rapid decomposition of [11C]DSF. Conclusions: [11C]DSF was successfully synthesized by the reaction of [11C]CS2 with diethylamine followed by I2 oxidation. Regardless of the molar radioactivity, PET studies of [11C]DSF showed similar uptake in mouse organ and rapid decomposition in serum., 22nd International symposium on radiopharmaceutical science (ISRS)

The visualization of the activated microglia/TSPO is one of the main aspects of neuroimaging. Here we describe two new (18)F-labelled molecules, 2-[5-(4-[(18)F]fluoroethoxyphenyl)- ([(18)F]2) and 2-[5-(4-[(18)F]fluoropropyloxyphenyl)- ([(18)F]3) -2-oxo-1,3-benzoxazol-3(2H)-yl]-N-methyl-N-phenylacetamide as novel PET ligands for imaging the translocator protein (18 kDa, TSPO) in the brain. The three-D pharmacophore evaluation and docking studies suggested their high affinity for the TSPO and in vitro binding assays of the TSPO showed binding affinities 6.6 ± 0.7 nM and 16.7 ± 2.5 nM for 2 and 3, respectively. The radiochemical yields for [(18)F]2 and [(18)F]3 were found to be 22 ± 4% (n = 8) and 5 ± 2% (n = 5), respectively at EOB. The radiochemical purity for both was found ≥98% and the specific activity was in the range of 98-364 GBq μmol(-1) at EOS. In vitro autoradiography with an ischemic rat brain showed significantly increased binding on the ipsilateral side compared to the contralateral side. The specificity of [(18)F]2 and [(18)F]3 for binding TSPO was confirmed using the TSPO ligands PK11195 and MBMP. The biodistribution patterns of both PET ligands were evaluated in normal mice by 1 h dynamic PET imaging. In the brain, regional radioactivity reached the maximum very rapidly within 0-4 min for both ligands, similar to (R)[(11)C]PK11195. The metabolite study of [(18)F]2 also favoured a more favourable profile for quantification in comparison to (R)[(11)C]PK11195. In summary, these data indicated that [(18)F]2 and [(18)F]3 have good potential to work as PET ligands, therefore there are merits to use these radioligands for the in vivo evaluation in animal models to see their efficacy in the living brain.

2-((1E,3E)-4-(6-((11)C-methylamino)pyridin-3-yl)buta-1,3-dienyl)benzo[d]thiazol-6-ol ((11)C-PBB3) is a clinically useful PET probe that we developed for in vivo imaging of tau pathology in the human brain. To ensure the availability of this probe among multiple PET facilities, in the present study we established protocols for the radiosynthesis and quality control of (11)C-PBB3 and for the characterization of its photoisomerization, biodistribution, and metabolism.

CEP-32496 is a novel, orally active serine/threonine-protein kinase B-raf (BRAF) (V600E) kinase inhibitor that is being investigated in clinical trials for the treatment of some cancers in patients. In this study, we developed [11C-carbonyl]CEP-32496 as a novel positron emission tomography (PET) probe to study its biodistribution in the whole bodies of mice. [11C]CEP-32496 was synthesized by the reaction of 5-(1,1,1-trifluoro-2-methylpropan-2-yl)isoxazol-3-amine hydrochloride (1·HCl) with [11C]phosgene, followed by treatment with 3-(6,7-dimethoxyquinozolin-4-yloxy)aniline (2). Small-animal PET studies with [11C]CEP-32496 indicated that radioactivity levels (AUC0–90 min, SUV × min) accumulated in the brains of P-gp/BCRP knockout mice at a 8-fold higher rate than in the brains of wild-type mice.

The metabotropic glutamate receptor type 1 (mGluR1) is a novel target protein for the development of new drugs against central nervous system disorders. Recently, we have developed (11)C-labeled PET probes (11)C-ITMM and (11)C-ITDM, which demonstrate similar profiles, for imaging of mGluR1. In the present study, we compared (11)C-ITMM and (11)C-ITDM PET imaging and quantitative analysis in the monkey brain. Respective PET images showed similar distribution of uptake in the cerebellum, thalamus, and cingulate cortex. Slightly higher uptake was detected with (11)C-ITDM than with (11)C-ITMM. For the kinetic analysis using the two-tissue compartment model (2-TCM), the distribution volume (VT) in the cerebellum, an mGluR1-rich region in the brain, was 2.5 mL∙cm(-3) for (11)C-ITMM and 3.6 mL∙cm(-3) for (11)C-ITDM. By contrast, the VT in the pons, a region with negligible mGluR1 expression, was similarly low for both radiopharmaceuticals. Based on these results, we performed noninvasive PET quantitative analysis with general reference tissue models using the time-activity curve of the pons as a reference region. We confirmed the relationship and differences between the reference tissue models and 2-TCM using correlational scatter plots and Bland-Altman plots analyses. Although the scattergrams of both radiopharmaceuticals showed over- or underestimations of reference tissue model-based the binding potentials against 2-TCM, there were no significant differences between the two kinetic analysis models. In conclusion, we first demonstrated the potentials of (11)C-ITMM and (11)C-ITDM for noninvasive PET quantitative analysis using reference tissue models. In addition, our findings suggest that (11)C-ITDM may be superior to (11)C-ITMM as a PET probe for imaging of mGluR1, because regional VT values in PET with (11)C-ITDM were higher than those of (11)C-ITMM. Clinical studies of (11)C-ITDM in humans will be necessary in the future.

Because of its role in multiple central nervous system (CNS) pathways, metabotropic glutamate receptor type 1 (mGluR1) is a crucial target in the development of pharmaceuticals for CNS disorders. N-[4-[6- (isopropylamino)-pyrimidin-4-yl]-1,3-thiazol-2-yl]-Nmethyl- 4-[11C]-methylbenzamide ([11C]ITDM) was recently developed as a positron emission tomography (PET) ligand for mGluR1. To devise a method for measurement of the binding potential (BPND) of [11C]ITDM to mGluR1, reference tissue methods aimed at replacing measurement of the arterial input function are desirable. In this study, we evaluated a noninvasive quantification method of mGluR1 with [11C]ITDM, demonstrating its accuracy using Huntington disease model R6/2 mice. The BPND measurements based on the Logan reference (Logan Ref) method have closely approximated that based on the arterial input method. We performed PET analysis with Logan Ref to assess its accuracy in quantifying the decline of mGluR1 expression in R6/2 mice. Significant decreases in BPND values in R6/2 mice were detected in cerebellum, thalamus, striatum, and cingulate cortex. We compared autoradiographs of R6/2 mouse brain sections with immunohistochemical images, and found a close correlation between changes in radioactive signal intensity and degree of mGluR1 expression. In conclusion, [11C]ITDM-PET is a promising tool for in vivo quantification of mGluR1 expression.

Aim: Epilepsy is known as one of neurological diseases characterized by epileptic seizures induced via overstimulation of glutamatergic neurons. The metabotropic glutamate receptors (mGluRs) moderate in excitatory neurotransmission upon the central nerves system (CNS). Among mGluRs, mGluR1 is known as a trigger of neurotoxic cascades. In this study, we aimed to in vivo monitor regional changes of mGluR1 in pilocarpine-induced status epilepticus (PISE) model rat brains via longitude PET assessments. Methods: PISE model rats were induced by repeated pilocarpine hydrochloride treatments (10 mg/kg) after administration with lithium chloride (127 mg/kg). PET assessments with N-[4-[6-(isopropylamino)-pyrimidin-4-yl]-1,3-thiazol-2-yl]-N-methyl-4-[11C]-methylbezamide ([11C]ITDM), a selective radioligand for mGluR1, were scheduled in acute (1 day), subacute (1 week), and chronic (3 weeks) periods after status epilepticus (SE). Acquired PET data were analysed with reference tissue models by PMOD software. To validate results of PET assessments, in vitro autoradiography with [11C]ITDM was performed using PISE rat brain sections prepared in same experimental period used in PET experiments. Results: In PET assessment with [11C]ITDM, binding potential (BPND) in the 1-3 cerebellar lobes significantly decreased (P < 0.001) in acute period (BPND = 3.2), subsequently recovering to normal level over subacute period (BPND = 4.6). Additionally, thalamic and hippocampal BPNDs showed gradual declines with time after SE over chronic period. However, in vitro autoradiography showed no change of specifically bindings of [11C]ITDM in the cerebellum in acute period, whereas bindings of [11C]ITDM in the thalamus and hippocampus indicated a tendency of decrease with time after SE. Conclusion: Our study indicates that gradual declines of mGluR1 expressions in the thalamus and hippocampus of PISE model rat brain via longitude PET imaging. In vivo monitoring of mGluR1 using PET with [11C]ITDM may enable to further understand mechanism involved in dysfunction of CNS., EANM 2016 Annual Congress

OBJECTIVES: Metabotropic glutamate receptor 5 (mGluR5) is characterized as a G protein coupled receptor and is known as one of regulators for excitatory neurotransmission, which is involved in central nervous system (CNS) disorders, such as fragile X syndrome, Parkinson’s disease, and depression. Therefore, mGluR5 is an important target for development of new pharmaceuticals against these CNS disorders. The most widely used PET radiotracer for mGluR5 was (E)-11C-ABP688 containing alkyne moiety. However, some test-retest PET studies with this radiotracer indicated low reliability for quantification of mGluR5 density. Here, we describe the synthesis of a new radiotracer 11C-MPTB not containing alkyne moiety and evaluation for PET imaging of mGluR5 in brain. METHODS: MPTB and its arylstannyl precursor for radiolabeling were synthesized using commercially available compounds. 11C-MPTB was synthesized via C-11C-methylation of the precursor with 11C-methyl iodide. In vitro binding of 11C-MPTB to mGluR5 was characterized by homogenate assay and autoradiography using rat brain. Further, in vivo brain regional distribution and test-retest reliability of 11C-MPTB were determined by small-animal PET assessments. RESULTS: 11C-MPTB was synthesized with 25 ± 14% (n = 11) radiochemical yield at 32 min after end of bombardment and 52–190 GBq/µmol specific activity at the end of synthesis. MPTB showed high in vitro binding affinity (Ki = 9.4 nM) for mGluR5. In vitro autoradiography with 11C-MPTB exhibited heterogeneous distribution pattern of radioactivity in the rat brain sections, which corresponded with the biological localization of mGluR5. These radioactive accumulations were totally diminished by co-incubation with mGluR5-selective antagonist MPEP. In the PET study, the time-activity curves of 11C-MPTB immediately reached maximum levels after the injection and then quickly decreased. The uptake ratio of each region against cerebellum, a reference region for mGluR5, under the equilibrium state was 3.0 ± 0.1 in the striatum, the richest brain region with mGluR5. By treatment with MPEP, the ratio decreased to 1.3. The parametric PET images scaled with binding potential visualized regional differences in specific binding to mGluR5, indicating that PET with 11C-MPTB could provide significant signal for mGluR5 in brain regions. Moreover, test-retest PET studies with this tracer exhibited high reliability for quantification of mGluR5 density. CONCLUSIONS: 11C-MPTB is a useful PET radiotracer for the imaging and quantitative analysis of mGluR5 in brain and may merit further evaluation in humans., SNMMI 2016 Annual Meeting