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Introduction We recently reported 11C-NR2B-SMe ([S-methyl-11C](R,S)-7-thiomethoxy-3-(4-(4-methyl-phenyl)butyl)-2,3,4,5-tetrahydro-1H-benzo[d]azepin-1-ol) and its enantiomers as candidate radioligands for imaging the GluN2B subunit within rat N-methyl-D-aspartate receptors. However, these radioligands gave unexpectedly high and displaceable binding in rat cerebellum, possibly due to cross-reactivity with sigma-1 (σ1) receptors. This study investigated 11C-labeled enantiomers of a close analogue (7-methoxy-3-(4-(p-tolyl)butyl)-2,3,4,5-tetrahydro-1H-benzo[d]azepin-1-ol; NR2B-Me) of 11C-NR2B-SMe as new candidate GluN2B radioligands. PET was used to evaluate these radioligands in rats and to assess potential cross-reactivity to σ1 receptors. Methods NR2B-Me was assayed for binding affinity and selectivity to GluN2B in vitro. 11C-NR2B-Me and its enantiomers were prepared by Pd-mediated treatment of boronic ester precursors with 11C-iodomethane. Brain PET scans were conducted after radioligand intravenous injection into rats. Various ligands for GluN2B receptors or σ1 receptors were administered at set doses in pre-blocking or displacement experiments to assess their impact on imaging data. 18F-FTC146 and enantiomers of 11C-NR2B-SMe were used for comparison. Radiometabolites from brain and plasma were measured ex vivo and in vitro. Results NR2B-Me enantiomers showed high GluN2B affinity and selectivity in vitro. 11C-NR2B-Me enantiomers gave high early whole rat brain uptake of radioactivity, including high uptake in cerebellum, followed by slower decline. Radioactivity in brain at 30 min ex vivo was virtually all unchanged radioligand. Only less lipophilic radiometabolites appeared in plasma. When 11C-(R)-NR2B-Me was used, three high-affinity GluN2B ligands—NR2B-SMe, Ro25-6981, and CO101,244—showed increasing pre-block of whole brain radioactivity retention with increasing dose. Two σ1 receptor antagonists, FTC146 and BD1407, were ineffective pre-blocking agents. Together, these results strongly resemble those obtained with 11C-NR2B-SMe enantiomers, except that 11C-NR2B-Me enantiomers showed faster reversibility of binding. When 18F-FTC146 was used as a radioligand, FTC146 and BD1407 showed strong pre-blocking effects whereas GluN2B ligands showed only weak blocking effects. Conclusion 11C-NR2B-Me enantiomers showed specific binding to GluN2B receptors in rat brain in vivo. High unexpected specific binding in cerebellum was not due to σ1 receptors. Additional investigation is needed to identify the source of the high specific binding. Graphical Abstract

Purpose: 18F-SF51 was previously found to have high binding affinity and selectivity for 18kDa translocator protein (TSPO) in mouse brain. This study sought to further evaluate the suitability of 18F-SF51 for absolute quantification of TSPO in monkey brain. Methods: Positron emission tomography (PET) imaging was performed in monkey brain (n=3) at baseline and after pre-blockade with the TSPO ligands PK11195 and PBR28. TSPO binding was calculated as total distribution volume corrected for free parent fraction in plasma (VT/fP) using a two-tissue compartment model. Receptor occupancy and nondisplaceable uptake were determined via Lassen plot. Binding potential (BPND) was calculated as the ratio of specific binding to nondisplaceable uptake. Time stability of VT was used as an indirect probe to detect radiometabolite accumulation in the brain. In vivo and ex vivo experiments were performed in mice to determine the distribution of the radioligand. Results: After 18F-SF51 injection, the concentration of brain radioactivity peaked at 2.0 standardized uptake value (SUV) at ~10 minutes and declined to 30% of the peak at 180 minutes. VT/fP at baseline was generally high (203±15 mL· cm-3) and decreased by ~90% after blockade with PK11195. BPND of the whole brain was 7.6 ± 4.3. VT values reached levels similar to terminal 180-minute values by 70 minutes and remained relatively stable thereafter with excellent identifiability (standard errors < 5%), suggesting that no significant radiometabolites accumulated in the brain. Ex vivo experiments in mouse brain showed that 96% of radioactivity was parent. No significant uptake was observed in the skull, suggesting a lack of defluorination in vivo. Conclusion: The results demonstrate that 18F-SF51 is an excellent radioligand with a good ratio of specific to nondisplaceable uptake as well as good time stability of total receptor binding. Collectively, the results suggest that 18F-SF51 warrants further evaluation in humans.

Radiotracers labeled with carbon-11 (t1/2 = 20.4 min) are widely used with positron emission tomography for biomedical research. Radiotracers must be produced for positron emission tomography studies in humans according to prescribed time schedules while also meeting current good manufacturing practice. Translation of an experimental radiosynthesis to a current good manufacturing practice environment is challenging. Here we exemplify such translation with a protocol for the production of an emerging radiotracer for imaging brain translocator protein 18 kDa, namely [11C]ER176. This radiotracer is produced by rapid conversion of cyclotron-produced [11C]carbon dioxide into [11C]iodomethane, which is then used to treat N-desmethyl-ER176 in the presence of base (tBuOK) at room temperature for 5 min. [11C]ER176 is separated in high purity by reversed-phase HPLC and formulated for intravenous injection in sterile ethanol-saline. The radiosynthesis is reliable and takes 50 min. Quality control takes another 20 min. All aspects of the protocol, including quality control, are discussed.

Positron emission tomography (PET) uses many tracers labeled with fluorine-18 (

Previous work found that [11C]deschloroclozapine ([11C]DCZ) is superior to [11C]clozapine ([11C]CLZ) for imaging Designer Receptors Exclusively Activated by Designer Drugs (DREADDs). This study used PET to quantitatively and separately measure the signal from transfected receptors, endogenous receptors/targets, and non-displaceable binding in other brain regions to better understand this superiority. A genetically-modified muscarinic type-4 human receptor (hM4Di) was injected into the right amygdala of a male rhesus macaque. [11C]DCZ and [11C]CLZ PET scans were conducted 2–24 months later. Uptake was quantified relative to the concentration of parent radioligand in arterial plasma at baseline (n = 3 scans/radioligand) and after receptor blockade (n = 3 scans/radioligand). Both radioligands had greater uptake in the transfected region and displaceable uptake in other brain regions. Displaceable uptake was not uniformly distributed, perhaps representing off-target binding to endogenous receptor(s). After correction, [11C]DCZ signal was 19% of that for [11C]CLZ, and background uptake was 10% of that for [11C]CLZ. Despite stronger [11C]CLZ binding, the signal-to-background ratio for [11C]DCZ was almost two-fold greater than for [11C]CLZ. Both radioligands had comparable DREADD selectivity. All reference tissue models underestimated signal-to-background ratio in the transfected region by 40%–50% for both radioligands. Thus, the greater signal-to-background ratio of [11C]DCZ was due to its lower background uptake.

Cyclooxygenase-1 (COX-1) and its isozyme COX-2 are key enzymes in the syntheses of prostanoids. Imaging of COX-1 and COX-2 selective radioligands with positron emission tomography (PET) may clarify how these enzymes are involved in inflammatory conditions and assist in the discovery of improved anti-inflammatory drugs. We have previously labeled the selective high-affinity COX-1 ligand, 1,5-bis(4-methoxyphenyl)-3-(2,2,2-trifluoroethoxy)-1H-1,2,4-triazole (PS13), with carbon-11 (t1/2 = 20.4 min). This radioligand ([11C]PS13) has been successful for PET imaging of COX-1 in monkey and human brain and in periphery. [11C]PS13 is being used in clinical investigations. Alternative labeling of PS13 with fluorine-18 (t1/2 = 109.8 min) is desirable to provide a longer-lived radioligand in high activity that might be readily distributed among imaging centers. However, labeling of PS13 in its 1,1,1-trifluoroethoxy group is a radiochemical challenge. Here we assess two labeling approaches based on nucleophilic addition of cyclotron-produced [18F]fluoride ion to gem-difluorovinyl precursors, either to label PS13 in one step or to produce [18F]2,2,2-trifluoroethyl p-toluenesulfonate for labeling a hydroxyl precursor. From the latter two-step approach, we obtained [18F]PS13 ready for intravenous injection in a decay-corrected radiochemical yield of 7.9% and with a molar activity of up to 7.9 GBq/μmol. PET imaging of monkey brain with [18F]PS13 shows that this radioligand can specifically image and quantify COX-1 without radiodefluorination but with some radioactivity uptake in skull, ascribed to red bone marrow. The development of a new procedure for labeling PS13 with fluorine-18 at a higher molar activity is, however, desirable to suppress occupancy of COX-1 by carrier at baseline.

Positron emission tomography (PET) uses radiotracers to quantify important biochemical parameters in human subjects. A radiotracer arterial input function (AIF) is often essential for converting brain PET data into robust output measures. For radiotracers labeled with carbon-11 (t1/2 = 20.4 min), AIF is routinely determined with radio-HPLC of blood sampled frequently during the PET experiment. There has been no alternative to this logistically demanding method, neither for regular use nor validation. A 11C-labeled tracer is always accompanied by a large excess of non-radioactive tracer known as carrier. In principle, AIF might be obtained by measuring the molar activity (Am; ratio of radioactivity to total mass; Bq/mol) of a radiotracer dose and the time-course of carrier concentration in plasma after radiotracer injection. Here, we implement this principle in a new method for determining AIF, as shown by using [11C]PBR28 as a representative tracer. The method uses liquid chromatography-tandem mass spectrometry for measuring radiotracer Am and then the carrier in plasma sampled regularly over the course of a PET experiment. Am and AIF were determined radiometrically for comparison. The new non-radiometric method is not constrained by the short half-life of carbon-11 and is an attractive alternative to conventional AIF measurement.

1212 Objectives: The pretargeting strategy, which has been proposed as an alternative to radioimmunotherapy, increases the absorbed dose to the tumor while reducing the side effects of normal tissues, resulting in a high therapeutic effect. From the early development stage, the combination of streptavidin and biotin for in vivo binding, which controls the pharmacokinetics and therapeutic effects, has been expected to have clinical application owing to its high affinity for the recognition pair. However, due to the strong immunogenicity of streptavidin in vivo, the streptavidin/biotin pair has not been studied extensively. Hence, we designed Cupid, which is a mutated streptavidin with low immunogenicity and can fuse to four single-chain Fv fragments (scFv), and synthesized Psyche, which is a corresponding biotin variant with a specific affinity to Cupid (Sugiyama et al., Prc. Jpn. Acad. Ser. B 95:602, 2019). In this study, in addition to using these combinations, we labeled Psyche with an alpha-emitting radionuclide 211At, which is expected to have high therapeutic effects in the tumor and fewer side effects on the surrounding tissue in vivo. The aim of this study was to evaluate the labeling, stability, and pharmacokinetics of 211At labeled Psyche and scFV-Cupid in mice. Methods: 211At was produced using 209Bi(α, 2n)211At reaction and isolated through dry distillation. 80 MBq of 211At was labeled to Psyche derivatives (Psyche-B) and purified as a no-carrier-added product using Reversed-Phase HPLC. For the stability evaluation, 211At-Psyche-B was incubated with PBS and serum for 24 h. In total, 1×107 human gastric cancer cells (MKN-45) with a high expression of CEA were xenografted subcutaneously in male, 9-week-old BALB/c nu/nu mice. Thirteen days after the transplantation, 100 pmol of anti-CEACAM5 scFv-Cupid was intravenously administered to mice. After an interval of 24 h, 250 kBq of 211At labeled Psyche-B was intravenously administered to mice. The mice were sacrificed 1 min, 1 h, 6 h, and 24 h after the 211At-Psyche-B injection (n = 3 in each group). Samples were collected from the blood, brain, thyroid, heart, lung, liver, spleen, stomach, small intestine, pancreas, kidney, adrenal gland, muscle, bone, urine, and feces. Subsequently, they were weighed and measured for radioactivity using a gamma counter. Results: The radiochemical yield of 211At-Psyche-B was 96%, as determined by a radio-HPLC analysis of the crude product. 211At-Psyche-B showed high stability in PBS after 24 h but dissociated in serum within 1 h. In the biodistribution study, the clearance of 211At-Psyche-B from blood was 46.4 ± 1.4%ID/g after 1 min, 40.6 ± 3.2 %ID/g after 1 h, 29.4 ± 0.9 %ID/g after 6 h, and 6.2 ± 1.3 %ID/g after 24 h. The accumulation of 211At-Psyche-B in blood decreased with time, and approximately 94% of the injected dose was excreted within 24 h. The accumulation of 211At-Psyche-B in the tumor was 0.6 ± 0.1 %ID/g in 1 min, 3.2 ± 0.4 %ID/g in 1 h, 11.8 ± 1.5 %ID/g in 6 h, and 16.3 ± 4.1 %ID/g in 24 h. The tumor-to-blood ratios were 0.01 ± 0.00 after 1 min, 0.08 ± 0.01 after 1 h, 0.40 ± 0.05 after 6 h, and 2.63 ± 0.10 after 24 h. 211At-Psyche-B showed high liver uptakes in the experiment period: 17.4 ± 2.3 %ID/g in 1 min, 13.6 ± 1.4 %ID/g in 1 h, 16.0 ± 1.4 %ID/g in 6 h, and 15.5 ± 0.1 %ID/g in 24 h. 211At-Psyche-B uptakes in the spleen were 4.9 ± 1.7 %ID/g in 1 min, 7.9 ± 1.1 %ID/g in 1 h, 10.1 ± 0.3 %ID/g in 6 h, and 9.6 ± 2.4 %ID/g in 24 h. 211At-Psyche-B uptakes in the kidney were 29.3 ± 1.5 %ID/g in 1 min, 14.2 ± 1.3 %ID/g in 1 h, 9.8 ± 0.7 %ID/g in 6 h, and 4.7 ± 0.8 %ID/g in 24 hours. Conclusion: Although 211At-Psyche-B showed low serum stability in vitro, tumor uptake was confirmed and accumulation increased over time in vivo. Thus, this study suggested that our pre-targeting strategy of low-immunogenic CEA scFv-Cupid and 211At labeled Psyche might work for advanced cancer treatment. Acknowledgments: This work was financially supported by AMED under Grant Number JP19ck0106414.

Positron emission tomography (PET) is an important imaging modality for biomedical research and drug development. PET requires biochemically selective radiotracers to realize full potential. Fluorine-18 (t1/2 = 109.8 min) is a major radionuclide for labeling such radiotracers but is only readily available in high activities from cyclotrons as [18F]fluoride ion. [18F]fluoroform has emerged for labeling tracers in trifluoromethyl groups. Prior methods of [18F]fluoroform synthesis used difluoro precursors in solution and led to high dilution with carrier and low molar activity (Am). We explored a new approach for the synthesis of [18F]fluoroform based on the radiosynthesis of [18F]fluoromethane from [18F]fluoride ion and then cobaltIII fluoride mediated gas phase fluorination. We estimate that carrier dilution in this process is limited to about 3-fold and find that moderate to high Am values can be achieved. We show that [18F]fluoroform so produced is highly versatile for rapidly and efficiently labeling various chemotypes that carry trifluoromethyl groups, thereby expanding prospects for developing new PET radiotracers.

This study used 11C‐PBR28 positron emission tomography (PET) imaging to determine whether levels of 18‐kDa translocator protein (TSPO), an inflammation‐specific biomarker, are increased in frontotemporal lobar degeneration (FTLD) patients. 11C‐PBR28, 18F‐FDG, and 11C‐PIB brain PET scans, as well as magnetic resonance imaging (MRI), were conducted in four FTLD patients and 22 healthy controls. 11C‐PBR28 scans revealed that all FTLD patients showed increased TSPO binding versus controls. Significantly greater increases in TSPO were observed in the frontal, lateral temporal, parietal, and occipital cortices, topographically consistent with individual clinical phenotypes and with brain MRI and 18F‐FDG PET. Amyloid burden was not increased.

INTRODUCTION: The serotonin 1B receptor subtype is of interest in the pathophysiology and treatment of depression, anxiety, and migraine. Over recent years 5-HT(1B) receptor binding in human brain has been examined with PET using radio ligands that are partial but not full agonists. To explore how the intrinsic activity of a PET radio ligand may affect imaging performance, two high-affinity full 5-HT(1B) receptor agonists (AZ11136118, 4; and AZ11895987, 5) were selected from a large compound library and radiolabeled for PET examination in non-human primates. METHODS: [(11)C]4 was obtained through Pd(O)-mediated insertion of [(11)C]carbon monoxide between prepared iodoarene and homochiral amine precursors. [(11)C]5 was obtained through N-(11)C-methylation of N-desmethyl precursor6 with [(11)C] methyl triflate. [(11)C]4 and [(11)C]5 were studied with PET in rhesus or cynomolgus monkey. [(11)C]4 was studied with PET in mice and rats to measure brain uptake and specific binding. Ex-vivo experiments in rats were performed to identify whether there were radiometabolites in brain. Physiochemical parameters for [(11)C]4 (pKa, logD and conformational energetics) were evaluated. RESULTS: Both [(11)C]4 and [(11)C]5 were successfully produced in high radiochemical purity and in adequate amounts for PET experiments. After intravenous injection of [(11)C]4, brain radioactivity peaked at a low level (0.2 SUV). Pretreatment with tariquidar. an inhibitor of the brain P-gp efflux transporter. increased brain exposure four-fold whereas pretreatment with a high pharmacological dose of the 5-HT(1B) antagonist, AR-A000002, had no effect on the binding. Ex-vivo experiments in rats showed no radiometabolites entering brain. [(11)C]5 also failed to enter monkey brain under baseline conditions. CONCLUSIONS: [(11)C]4 and [(11)C]5 show too low brain uptake and specific binding to be useful PET radio ligands. Low brain uptake is partly ascribed to efflux transporter action as well as unfavorable conformations.

Background Previous studies found that the positron emission tomography (PET) radioligand [18F]LSN3316612 accurately quantified O-GlcNAcase in human brain using a two-tissue compartment model (2TCM). This study sought to assess kinetic model(s) as an alternative to 2TCM for quantifying [18F]LSN3316612 binding, particularly in order to generate good-quality parametric images. Methods The current study reanalyzed data from a previous study of 10 healthy volunteers who underwent both test and retest PET scans with [18F]LSN3316612. Kinetic analysis was performed at the region level with 2TCM using 120-min PET data and arterial input function, which was considered as the gold standard. Quantification was then obtained at both the region and voxel levels using Logan plot, Ichise's multilinear analysis-1 (MA1), standard spectral analysis (SA), and impulse response function at 120 min (IRF120). To avoid arterial sampling, a noninvasive relative quantification (standardized uptake value ratio (SUVR)) was also tested using the corpus callosum as a pseudo-reference region. Venous samples were also assessed to see whether they could substitute for arterial ones. Results Logan and MA1 generated parametric images of good visual quality and their total distribution volume (VT) values at both the region and voxel levels were strongly correlated with 2TCM-derived VT (r = 0.96–0.99) and showed little bias (up to − 8%). SA was more weakly correlated to 2TCM-derived VT (r = 0.93–0.98) and was more biased (~ 16%). IRF120 showed a strong correlation with 2TCM-derived VT (r = 0.96) but generated noisier parametric images. All techniques were comparable to 2TCM in terms of test–retest variability and reliability except IRF120, which gave significantly worse results. Noninvasive SUVR values were not correlated with 2TCM-derived VT, and arteriovenous equilibrium was never reached. Conclusions Compared to SA and IRF, Logan and MA1 are more suitable alternatives to 2TCM for quantifying [18F]LSN3316612 and generating good-quality parametric images.

Cyclooxygenase-1 (COX-1) and its isozyme COX-2 are key enzymes in the syntheses of prostanoids. Imaging of COX-1 and COX-2 selective radioligands with positron emission tomography (PET) may clarify how these enzymes are involved in inflammatory conditions and assist in the discovery of improved anti-inflammatory drugs. We have previously labeled the selective high-affinity COX-1 ligand, 1,5-bis(4-methoxyphenyl)-3-(2,2,2-trifluoroethoxy)-1H-1,2,4-triazole (PS13), with carbon-11 (t(1/2) = 20.4 min). This radioligand ([(11)C]PS13) has been successful for PET imaging of COX-1 in monkey and human brain and in periphery. [(11)C]PS13 is being used in clinical investigations. Alternative labeling of PS13 with fluorine-18 (t(1/2) = 109.8 min) is desirable to provide a longer-lived radioligand in high activity that might be readily distributed among imaging centers. However, labeling of PS13 in its 1,1,1-trifluoroethoxy group is a radiochemical challenge. Here we assess two labeling approaches based on nucleophilic addition of cyclotron-produced [(18)F]fluoride ion to gem-difluorovinyl precursors, either to label PS13 in one step or to produce [(18)F]2,2,2-trifluoroethyl p-toluenesulfonate for labeling a hydroxyl precursor. From the latter two-step approach, we obtained [(18)F]PS13 ready for intravenous injection in a decay-corrected radiochemical yield of 7.9% and with a molar activity of up to 7.9 GBq/μmol. PET imaging of monkey brain with [(18)F]PS13 shows that this radioligand can specifically image and quantify COX-1 without radiodefluorination but with some radioactivity uptake in skull, ascribed to red bone marrow. The development of a new procedure for labeling PS13 with fluorine-18 at a higher molar activity is, however, desirable to suppress occupancy of COX-1 by carrier at baseline.

Radiotracers labeled with carbon-11 (t

We aimed to develop effective radioligands for quantifying brain O-linked-β-N-acetyl-glucosamine (O-GlcNAc) hydrolase (OGA) using positron emission tomography in living subjects as tools for evaluating drug target engagement. Posttranslational modifications of tau, a biomarker of Alzheimer's disease, by O-GlcNAc through the enzyme pair OGA and O-GlcNAc transferase (OGT) are inversely related to the amounts of its insoluble hyperphosphorylated form. Increase in tau O-GlcNAcylation by OGA inhibition is believed to reduce tau aggregation. LSN3316612, a highly selective and potent OGA ligand [half-maximal inhibitory concentration (IC50) = 1.9 nM], emerged as a lead ligand after in silico analysis and in vitro evaluations. [3H]LSN3316612 imaged and quantified OGA in postmortem brains of rat, monkey, and human. The presence of fluorine and carbonyl functionality in LSN3316612 enabled labeling with positron-emitting fluorine-18 or carbon-11. Both [18F]LSN3316612 and [11C]LSN3316612 bound reversibly to OGA in vivo, and such binding was blocked by pharmacological doses of thiamet G, an OGA inhibitor of different chemotype, in monkeys. [18F]LSN3316612 entered healthy human brain avidly (~4 SUV) without radiodefluorination or adverse effect from other radiometabolites, as evidenced by stable brain total volume of distribution (VT) values by 110 min of scanning. Overall, [18F]LSN3316612 is preferred over [11C]LSN3316612 for future human studies, whereas either may be an effective positron emission tomography radioligand for quantifying brain OGA in rodent and monkey.

We aimed to develop radioligands for PET imaging of brain phosphodiesterase subtype 4D (PDE4D), a potential target for developing cognition enhancing or anti-depressive drugs. Exploration of several chemical series gave four leads with high PDE4D inhibitory potency and selectivity, optimal lipophilicity, and good brain uptake. These leads featured alkoxypyridinyl cores. They were successfully labeled with carbon-11 (t(1/2) = 20.4 min) for evaluation with PET in monkey. Whereas two of these radioligands did not provide PDE4D-specific signal in monkey brain, two others, [(11)C]T1660 and [(11)C]T1650, provided sizeable specific signal, as judged by pharmacological challenge using rolipram or a selective PDE4D inhibitor (BPN14770) and subsequent biomathematical analysis. Specific binding was highest in prefrontal cortex, temporal cortex, and hippocampus, regions that are important for cognitive function. [(11)C]T1650 was progressed to evaluation in humans with PET, but the output measure of brain enzyme density (V(T)) increased with scan duration. This instability over time suggests that radiometabolite(s) were accumulating in the brain. BPN14770 blocked PDE4D uptake in human brain after a single dose, but the percentage occupancy was difficult to estimate because of the unreliability of measuring V(T). Overall, these results show that imaging of PDE4D in primate brain is feasible, but that further radioligand refinement is needed, most likely to avoid problematic radiometabolites.

Background Previous studies found that [18F]LSN3316612 was a promising positron emission tomography (PET) radioligand for imaging O-GlcNAcase in nonhuman primates and human volunteers. This study sought to further evaluate the suitability of [18F]LSN3316612 for human clinical research. Methods Kinetic evaluation of [18F]LSN3316612 was conducted in a combined set of baseline brain scans from 17 healthy human volunteers and test-retest imaging was conducted in 10 of these volunteers; another 6 volunteers had whole-body scans to measure radiation exposure to body organs. Total distribution volume (VT) estimates were compared for the one- and two-tissue compartment models with the arterial input function. Test-retest variability and reliability were evaluated via mean difference and intraclass correlation coefficient (ICC). The time stability of VT was assessed down to a 30-min scan time. An alternative quantification method for [18F]LSN3316612 binding without blood was also investigated to assess the possibility of eliminating arterial sampling. Results Brain uptake was generally high and could be quantified as VT with excellent identifiability using the two-tissue compartment model. [18F]LSN3316612 exhibited good absolute test-retest variability (12.5%), but the arithmetic test-retest variability was far from 0 (11.3%), reflecting a near-uniform increase of VT on the retest scan in nine of 10 volunteers. VT values were stable after 110 min in all brain regions, suggesting that no radiometabolites accumulated in the brain. Measurements obtained using only brain activity (i.e., area under the curve (AUC) from 150–180 min) correlated strongly with regional VT values during test-retest conditions (R2 = 0.84), exhibiting similar reliability to VT (ICC = 0.68 vs. 0.64). Estimated radiation exposure for [18F]LSN3316612 PET was 20.5 ± 2.1 μSv/MBq, comparable to other 18F-labeled radioligands for brain imaging. Conclusions [18F]LSN3316612 is an excellent PET radioligand for imaging O-GlcNAcase in the human brain. Alternative quantification without blood is possible, at least for within-subject repeat studies. However, the unexplained increase of VT under retest conditions requires further investigation.

In our preceding paper (Part 1), we identified three 1,5-bis-diaryl-1,2,4-triazole-based compounds that merited evaluation as potential positron emission tomography (PET) radioligands for selectively imaging cyclooxygenase-1 (COX-1) in monkey and human brain, namely, 1,5-bis(4-methoxyphenyl)-3-(alkoxy)-1 H-1,2,4-triazoles bearing a 3-methoxy (PS1), a 3-(2,2,2-trifluoroethoxy) (PS13), or a 3-fluoromethoxy substituent (PS2). PS1 and PS13 were labeled from phenol precursors by O-11C-methylation with [11C]iodomethane and PS2 by O-18F-fluoroalkylation with [2H2,18F]fluorobromomethane. Here, we evaluated these PET radioligands in monkey. All three radioligands gave moderately high uptake in brain, although [2H2,18F]PS2 also showed undesirable radioactivity uptake in skull. [11C]PS13 was selected for further evaluation, mainly based on more favorable brain kinetics than [11C]PS1. Pharmacological preblock experiments showed that about 55% of the radioactivity uptake in brain was specifically bound to COX-1. An index of enzyme density, VT, was well identified from serial brain scans and from the concentrations of parent radioligand in arterial plasma. In addition, VT values were stable within 80 min, suggesting that brain uptake was not contaminated by radiometabolites. [11C]PS13 successfully images and quantifies COX-1 in monkey brain, and merits further investigation for imaging COX-1 in monkey models of neuroinflammation and in healthy human subjects.

Translocator protein 18 kDa (TSPO) has been widely imaged as a marker of neuroinflammation using several radioligands, including [11C]PBR28. In order to study the effects of age, sex, and obesity on TSPO binding and to determine whether this binding can be accurately assessed using fewer radio high-performance liquid chromatography (radio-HPLC) measurements of arterial blood samples, we created a database of 48 healthy subjects who had undergone [11C]PBR28 scans (23 high-affinity binders (HABs) and 25 mixed-affinity binders (MABs), 20 F/28 M, age: 40.6 ± 16.8 years). After analysis by Logan plot using 23 metabolite-corrected arterial samples, total distribution volume ( VT) was found to be 1.2-fold higher in HABs across all brain regions. Additionally, the polymorphism plot estimated nondisplaceable uptake ( VND) as 1.40 mL · cm−3, which generated a specific-to-nondisplaceable ratio ( BPND) of 1.6 ± 0.6 in HABs and 1.1 ± 0.6 in MABs. VT increased significantly with age in nearly all regions and was well estimated with radio-HPLC measurements from six arterial samples. However, VT did not correlate with body mass index and was not affected by sex. These results underscore which patient characteristics should be accounted for during [11C]PBR28 studies and suggest ways to perform such studies more easily and with fewer blood samples.

Neuroinflammation appears to contribute to neurotoxicity observed with heavy alcohol consumption. To assess whether chronic alcohol results in neuroinflammation we used PET and [11C]PBR28, a ligand that binds to the 18-kDa translocator protein (TSPO), to compare participants with an alcohol use disorder (AUD: n = 19) with healthy controls (HC: n = 17), and alcohol-dependent (n = 9) with -nondependent rats (n = 10). Because TSPO is implicated in cholesterol’s transport for steroidogenesis, we investigated whether plasma cholesterol levels influenced [11C]PBR28 binding. [11C]PBR28 binding did not differ between AUD and HC. However, when separating by TSPO genotype rs6971, we showed that medium-affinity binders AUD participants showed lower [11C]PBR28 binding than HC in regions of interest (whole brain, gray and white matter, hippocampus, and thalamus), but no group differences were observed in high-affinity binders. Cholesterol levels inversely correlated with brain [11C]PBR28 binding in combined groups, due to a correlation in AUD participants. In rodents, we observed no differences in brain [11C]PBR28 uptake between alcohol-dependent and -nondependent rats. These findings, which are consistent with two previous [11C]PBR28 PET studies, may indicate lower activation of microglia in AUD, whereas failure to observe alcohol effects in the rodent model indicate that species differences do not explain the discrepancy with prior rodent autoradiographic studies reporting increases in TSPO binding with chronic alcohol. However, reduced binding in AUD participants could also reflect competition from endogenous TSPO ligands such as cholesterol; and since the rs6971 polymorphism affects the cholesterol-binding domain of TSPO this could explain why differences were observed only in medium-affinity binders.

Cyclooxygenase-1 (COX-1) is a key enzyme in the biosynthesis of proinflammatory thromboxanes and prostaglandins and is found in glial and neuronal cells within brain. COX-1 expression is implicated in numerous neuroinflammatory states. We aim to find a direct-acting positron emission tomography (PET) radioligand for imaging COX-1 in human brain as a potential biomarker of neuroinflammation and for serving as a tool in drug development. Seventeen 3-substituted 1,5-diaryl-1H-1,2,4-triazoles were prepared as prospective COX-1 PET radioligands. From this set, three 1,5-(4-methoxyphenyl)-lH-1,2,4-triazoles, carrying a 3-methoxy (5), 3-(1,1,1-trifluoroethoxy) (20), or 3-fluoromethoxy substituent (6), were selected for radioligand development, based mainly on their high affinities and selectivities for inhibiting human COX-1, absence of carboxyl group, moderate computed lipophilicities, and scope for radiolabeling with carbon-11 (t(1/2) = 20.4 min) or fluorine-18 (t(1/2) = 109.8 min). Methods were developed for producing [(11)C]5, [(11)C]20, and [d(2)-(18)F]6 from hydroxy precursors in a form ready for intravenous injection for prospective evaluation in monkey with PET.

Cytosolic phospholipase A2α (cPLA2α) may play a critical role in neuropsychiatric and neurodegenerative disorders associated with oxidative stress and neuroinflammation. An effective PET radioligand for imaging cPLA2α in living brain might prove useful for biomedical research, especially on neuroinflammation. We selected four high-affinity (IC50 2.1-12 nm) indole-5-carboxylic acid-based inhibitors of cPLA2α, namely 3-isobutyryl-1-(2-oxo-3-(4-phenoxyphenoxy)propyl)-1H-indole-5-carboxylic acid (1); 3-acetyl-1-(2-oxo-3-(4-(4-(trifluoromethyl)phenoxy)phenoxy)propyl)-1H-indole-5-carboxylic acid (2); 3-(3-methyl-1,2,4-oxadiazol-5-yl)-1-(2-oxo-3-(4-phenoxyphenoxy)propyl)-1H-indole-5-carboxylic acid (3); and 3-(3-methyl-1,2,4-oxadiazol-5-yl)-1-(3-(4-octylphenoxy)-2-oxopropyl)-1H-indole-5-carboxylic acid (4), for labelling in carboxyl position with carbon-11 (t1/2 =20.4 min) to provide candidate PET radioligands for imaging brain cPLA2α. Compounds [11 C]1-4 were obtained for intravenous injection in adequate overall yields (1.1-5.5 %) from cyclotron-produced [11 C]carbon dioxide and with moderate molar activities (70-141 GBq μmol-1 ) through the use of Pd0 -mediated [11 C]carbon monoxide insertion on iodo precursors. Measured logD7.4 values were within a narrow moderate range (1.9-2.4). After intravenous injection of [11 C]1-4 in mice, radioactivity uptakes in brain peaked at low values (≤0.8 SUV) and decreased by about 90 % over 15 min. Pretreatments of the mice with high doses of the corresponding non-radioactive ligands did not alter brain time-activity curves. Brain uptakes of radioactivity after administration of [11 C]1 to wild-type and P-gp/BCRP dual knock-out mice were similar (peak 0.4 vs. 0.5 SUV), indicating that [11 C]1 and others in this structural class, are not substrates for efflux transporters.

Background Previous studies showed reduction of brain cannabinoid CB1 receptors in adults with cannabis and alcohol use disorders. Preclinical data suggest that these receptors also contribute to nicotine reward and dependence. Tobacco smoking may confound clinical studies of psychiatric disorders because many patients with such disorders smoke tobacco. Whether human subjects who smoke tobacco but are otherwise healthy have altered CB1 receptor binding in brain is unknown. Methods We measured CB1 receptors in brains of 18 healthy men who smoke tobacco (frequent chronic cigarette smokers), and 28 healthy men who do not smoke tobacco, using positron emission tomography and [18F]FMPEP-d2, a radioligand for CB1 receptors. We collected arterial blood samples during scanning to calculate the distribution volume (VT), which is nearly proportional to CB1 receptor density. Repeated-measures analysis of variance compared VT between groups in various brain regions. Results Brain CB1 receptor VT was about 20% lower in subjects who smoke tobacco than in subjects who do not. Decreased VT was found in all brain regions, but reduction did not correlate with years of smoking, number of cigarettes smoked per day, or measures of nicotine dependence. Conclusions Tobacco-smoking healthy men have a widespread reduction of CB1 receptor density in brain. Reduction of CB1 receptors appears to be a common feature of substance use disorders. Future clinical studies on the CB1 receptor should control for tobacco smoking.

We sought to determine whether patients with posterior cortical atrophy (PCA) demonstrate a pattern of binding to translocator protein 18 kDa, a marker of microglial activation, that is distinct from that in patients with amnestic presentation of Alzheimer's disease (AD). Eleven PCA patients, 11 amnestic AD patients, and 15 age-matched controls underwent positron emission tomography with 11C-PBR28 to measure translocator protein 18 kDa. PCA patients showed greater 11C-PBR28 binding than controls in occipital, posterior parietal, and temporal regions. In contrast, amnestic AD patients showed greater 11C-PBR28 binding in inferior and medial temporal cortex. Increased 11C-PBR28 binding overlapped with reduced cortical volume for both PCA and amnestic AD patients, and with areas of reduced glucose metabolism in PCA patients. While both patient groups showed diffuse amyloid binding, PCA patients showed greater binding than amnestic AD patients in bilateral occipital cortex. These results suggest that microglial activation is closely associated with neurodegeneration across different subtypes of AD.

Accumulation of hyperphosphorylated tau, a microtubule-associated protein, plays an important role in the progression of Alzheimer disease. Animal studies suggest that one strategy for treating Alzheimer disease and related tauopathies may be inhibition of O-GlcNAcase (OGA), which may subsequently decrease pathologic tau phosphorylation. Here, we report the pharmacokinetics of a novel PET radioligand, (18)F-LSN3316612, which binds with high affinity and selectivity to OGA. Methods: PET imaging was performed on rhesus monkeys at baseline and after administration of either thiamet-G, a potent OGA inhibitor, or nonradioactive LSN3316612. The density of the enzyme was calculated as distribution volume using a 2-tissue-compartment model and serial concentrations of parent radioligand in arterial plasma. The radiation burden for future studies was based on whole-body imaging of monkeys. Oga(∆Br), a mouse brain-specific knockout of Oga, was also scanned to assess the specificity of the radioligand for its target enzyme. Results: Uptake of radioactivity in monkey brain was high (∼5 SUV) and followed by slow washout. The highest uptake was in the amygdala, followed by striatum and hippocampus. Pretreatment with thiamet-G or nonradioactive LSN3316612 reduced brain uptake to a low and uniform concentration in all regions, corresponding to an approximately 90% decrease in distribution volume. Whole-body imaging of rhesus monkeys showed high uptake in kidney, spleen, liver, and testes. In Oga(∆Br) mice, brain uptake of (18)F-LSN3316612 was reduced by 82% compared with control mice. Peripheral organs were unaffected in Oga(∆Br) mice, consistent with loss of OGA expression exclusively in the brain. The effective dose of (18)F-LSN3316612 in humans was calculated to be 22 μSv/MBq, which is typical for (18)F-labeled radioligands. Conclusion: These results show that (18)F-LSN3316612 is an excellent radioligand for imaging and quantifying OGA in rhesus monkeys and mice. On the basis of these data, (18)F-LSN3316612 merits evaluation in humans.

Of the two 18F-labeled PET ligands currently available to image metabotropic glutamate receptor 5 (mGluR5), [18F]FPEB is reportedly superior because [18F]SP203 undergoes glutathionlyation, generating [18F]-fluoride ion that accumulates in brain and skull. To allow multiple PET studies on the same day with lower radiation exposure, we prepared [11C]FPEB and [11C]SP203 from [11C]hydrogen cyanide and compared their abilities to accurately quantify mGluR5 in human brain, especially as regards radiometabolite accumulation. Genomic plot was used to estimate the ratio of specific-to-nondisplaceable uptake ( BPND) without using a receptor blocking drug. Both tracers quantified mGluR5; however [11C]SP203, like [18F]SP203, had radiometabolite accumulation in brain, as evidenced by increased distribution volume ( VT) over the scan period. Absolute VT values were ∼30% lower for 11C-labeled compared with 18F-labeled radioligands, likely caused by the lower specific activities (and high receptor occupancies) of the 11C radioligands. The genomic plot indicated ∼60% specific binding in cerebellum, which makes it inappropriate as a reference region. Whole-body scans performed in healthy subjects demonstrated a low radiation burden typical for 11C-ligands. Thus, the evidence suggests that [11C]FPEB is superior to [11C]SP203. If prepared in higher specific activity, [11C]FPEB would presumably be as effective as [18F]FPEB for quantifying mGluR5 in human brain.

Treatment of (diacetoxyiodo)arenes (1a–1u) with cyclotron-produced [18F]fluoride ion rapidly affords no-carrier-added [18F]fluoroarenes (2a–2u) in useful yields, and constitutes a new method for converting substituted iodoarenes into substituted [18F]fluoroarenes in just two steps.

Cyclooxygenase 2 (COX-2) is an inducible enzyme responsible for the conversion of arachidonic acid into the prostaglandins, PGG2 and PGH2. Expression of this enzyme increases in inflammation. Therefore, the development of probes for imaging COX-2 with positron emission tomography (PET) has gained interest because they could be useful for the study of inflammation in vivo, and for aiding anti-inflammatory drug development targeting COX-2. Nonetheless, effective PET radioligands are still lacking. We synthesized eleven COX-2 inhibitors based on a 2(4-methylsulfonylphenyl)pyrimidine core from which we selected three as prospective PET radioligands based on desirable factors, such as high inhibitory potency for COX-2, very low inhibitory potency for COX-1, moderate lipophilicity, and amenability to labeling with a positron-emitter. These inhibitors, namely 6-methoxy-2-(4-(methylsulfonyl)phenyl-N-(thiophen-2ylmethyl)pyrimidin-4-amine (17), the 6-fluoromethyl analogue (20), and the 6-(2-fluoroethoxy) analogue (27), were labeled in useful yields and with high molar activities by treating the 6-hydroxy analogue (26) with [11C]iodomethane, [18F]2-fluorobromoethane, and [d2-18F]fluorobromomethane, respectively. [11C]17, [18F]20, and [d2-18F]27 were readily purified with HPLC and formulated for intravenous injection. These methods allow these radioligands to be produced for comparative evaluation as PET radioligands for measuring COX-2 in healthy rhesus monkey and for assessing their abilities to detect inflammation.

We aimed to develop effective radioligands for quantifying brain O-linked-β-N-acetyl-glucosamine (O-GlcNAc) hydrolase (OGA) using positron emission tomography in living subjects as tools for evaluating drug target engagement. Post-translational modifications of tau, a biomarker of Alzheimer’s disease, by O-GlcNAc through the enzyme pair OGA and O-GlcNAc transferase (OGT) are inversely related to the amounts of its insoluble hyper-phosphorylated form. Increase in tau O-GlcNAcylation by OGA inhibition is believed to reduce tau aggregation. LSN3316612, a highly selective and potent OGA ligand (IC(50) = 1.9 nM), emerged as a lead ligand after in silico analysis and in vitro evaluations. [(3)H]LSN3316612 imaged and quantified OGA in postmortem brains of rat, monkey, and human. The presence of fluorine and carbonyl functionality in LSN3316612 enabled labeling with positron-emitting fluorine-18 or carbon-11. Both [(18)F]LSN3316612 and [(11)C]LSN3316612 bound reversibly to OGA in vivo and such binding was blocked by pharmacological doses of thiamet G, an OGA inhibitor of different chemotype, in monkeys. [(18)F]LSN3316612 entered healthy human brain avidly (~4 SUV) without radiodefluorination or adverse effect from other radiometabolites, as evidenced by stable brain V(T) values by 110-min of scanning. Overall, [(18)F]LSN3316612 is preferred over [(11)C]LSN3316612 for future human studies, whereas either may be an effective PET radioligand for quantifying brain OGA in rodent and monkey.

In our preceding paper (Part 1), we identified three 1,5-bis-diaryl-1,2,4-triazole-based compounds that merited evaluation as potential positron emission tomography (PET) radioligands for selectively imaging cyclooxygenase-1 (COX-1) in monkey and human brain, namely, 1,5-bis(4-methoxyphenyl)-3-(alkoxy)-1H-1,2,4-triazoles bearing a 3-methoxy (PS1), a 3-(2,2,2-trifluoroethoxy) (PS13), or a 3-fluoromethoxy substituent (PS2). PS1 and PS13 were labeled from phenol precursors by O-11C-methylation with [11C]-iodomethane and PS2 by O-18F-fluoroalkylation with [2H2, 18F]fluorobromomethane. Here, we evaluated these PET radioligands in monkey. All three radioligands gave moderately high uptake in brain, although [2H 2,18F]PS2 also showed undesirable radioactivity uptake in skull. [11C]PS13 was selected for further evaluation, mainly based on more favorable brain kinetics than [11C]PS1. Pharmacological preblock experiments showed that about 55% of the radioactivity uptake in brain was specifically bound to COX-1. An index of enzyme density, VT, was well identified from serial brain scans and from the concentrations of parent radioligand in arterial plasma. In addition, VT values were stable within 80 min, suggesting that brain uptake was not contaminated by radiometabolites. [11C]PS13 successfully images and quantifies COX-1 in monkey brain, and merits further investigation for imaging COX-1 in monkey models of neuroinflammation and in healthy human subjects.

The permeability-glycoprotein (P-gp) efflux transporter is densely expressed at the blood–brain barrier, and its resultant spare capacity requires substantial blockade to increase the uptake of avid substrates, blunting the ability of investigators to measure clinically meaningful alterations in P-gp function. This study, conducted in humans, examined 2 P-gp inhibitors (tariquidar, a known inhibitor, and disulfiram, a putative inhibitor) and 2 routes of administration (intravenous and oral) to maximally increase brain uptake of the avid and selective P-gp substrate 11C-N-desmethyl-loperamide (dLop) while avoiding side effects associated with high doses of tariquidar. Methods: Forty-two 11C-dLop PET scans were obtained from 37 healthy volunteers. PET was performed with 11C-dLop under the following 5 conditions: injected under baseline conditions without P-gp inhibition, injected 1 h after intravenous tariquidar infusion, injected during intravenous tariquidar infusion, injected after oral tariquidar, and injected after disulfiram. 11C-dLop uptake was quantified with kinetic modeling using metabolite-corrected arterial input function or by measuring the area under the time–activity curve in the brain from 10 to 30 min. Results: Neither oral tariquidar nor oral disulfiram increased brain uptake of 11C-dLop. Injecting 11C-dLop during tariquidar infusion, when plasma tariquidar concentrations reach their peak, resulted in a brain uptake of the radioligand approximately 5-fold greater than baseline. Brain uptake was similar with 2 and 4 mg of intravenous tariquidar per kilogram; however, the lower dose was better tolerated. Injecting 11C-dLop after tariquidar infusion also increased brain uptake, though higher doses (up to 6 mg/kg) were required. Brain uptake of 11C-dLop increased fairly linearly with increasing plasma tariquidar concentrations, but we are uncertain whether maximal uptake was achieved. Conclusion: We sought to increase the dynamic range of P-gp function measured after blockade. Performing 11C-dLop PET during peak plasma concentrations of tariquidar, achieved with concurrent administration of intravenous tariquidar, resulted in greater P-gp inhibition at the human blood–brain barrier than delayed administration and allowed the use of a lower, more tolerable dose of tariquidar. On the basis of prior monkey studies, we suspect that plasma concentrations of tariquidar did not fully block P-gp; however, higher doses of tariquidar would likely be associated with unacceptable side effects.

Cytosolic phospholipase A2α (cPLA2α) may play a critical role in neuropsychiatric and neurodegenerative disorders associated with oxidative stress and neuroinflammation. An effective PET radioligand for imaging cPLA2α in living brain might prove useful for biomedical research, especially on neuroinflammation. We selected four high-affinity (IC50 2.1 to 12 nM) indole-5-carboxylic acid-based inhibitors of cPLA2α, namely 3-isobutyryl-1-(2-oxo-3-(4-phenoxyphenoxy)propyl)-1H-indole-5-carboxylic acid (1); 3-acetyl-1-(2-oxo-3-(4-(4-(trifluoromethyl)-phenoxy)phenoxy)propyl)-1H-indole-5-carboxylic acid (2); 3-(3-methyl-1,2,4-oxadiazol-5-yl)-1-(2-oxo-3-(4-phenoxyphenoxy)propyl)-1H-indole-5-carboxylic acid (3); and 3-(3-methyl-1,2,4-oxadiazol-5-yl)-1-(3-(4-octylphenoxy)-2-oxopropyl)-1H-indole-5-carboxylic acid (4), for labelling in carboxyl position with carbon-11 (t1/2 = 20.4 min) to provide candidate PET radioligands for imaging brain cPLA2α. [11C]1–4 were obtained for intravenous injection in adequate overall yields (1.1−5.5%) from cyclotron-produced [11C]carbon dioxide and with moderate molar activities (70−141 GBq/μmol) through the use of Pd(0)-mediated [11C]carbon monoxide insertion on iodo precursors. Measured logD7.4 values were within a narrow moderate range (1.9−2.4). After intravenous injection of [11C]1–4 in mice, radioactivity uptakes in brain peaked at low values (≤ 0.8 SUV) and decreased by about 90% over 15 min. Pre-treatments of the mice with high doses of the corresponding non-radioactive ligands did not alter brain time-activity curves. Brain uptakes of radioactivity after administration of [11C]1 to wild type and P-gp/BCRP dual knock-out mice were similar (peak 0.4 vs. 0.5 SUV), indicating that [11C]1 and others in this structural class, are not efflux transporters substrates.

This study examined the long-term effects of fluoxetine administered to juvenile rhesus monkeys who, as young adults, were imaged with positron emission tomography for two serotonergic markers: serotonin transporter (SERT) and serotonin 1A (5-HT1A) receptor. An equal number of monkeys separated from their mothers at birth-an animal model of human childhood stress-were also studied.At birth, 32 male rhesus monkeys were randomly assigned to either maternal separation or normal rearing conditions. At age 2, half (N=8) of each group was randomly assigned to fluoxetine (3 mg/kg) or placebo for 1 year. To eliminate the confounding effects of residual drug in the brain, monkeys were scanned at least 1.5 years after drug discontinuation. Social interactions were assessed both during and after drug administration.Fluoxetine persistently upregulated SERT, but not 5-HT1A receptors, in both the neocortex and the hippocampus. Whole-brain voxel-wise analysis revealed that fluoxetine had a significant effect in the lateral temporal and cingulate cortices. In contrast, neither maternal separation by itself nor the rearing-by-drug interaction was significant for either marker. Fluoxetine had no significant effect on the behavioral measures.Fluoxetine administered to juvenile monkeys upregulates SERT into young adulthood. Implications regarding the efficacy or potential adverse effects of SSRIs in patients cannot be directly drawn from this study. Its purpose was to investigate effects of SSRIs on brain development in nonhuman primates using an experimental approach that randomly assigned long-term SSRI treatment or placebo.

Neuroinflammation is a pathological hallmark of Alzheimer’s disease, but its role in cognitive impairment and its course of development during the disease are largely unknown. To address these unknowns, we used positron emission tomography with 11C-PBR28 to measure translocator protein 18 kDa (TSPO), a putative biomarker for inflammation. Patients with Alzheimer’s disease, patients with mild cognitive impairment and older control subjects were also scanned with 11C-Pittsburgh Compound B to measure amyloid burden. Twenty-nine amyloid-positive patients (19 Alzheimer’s, 10 mild cognitive impairment) and 13 amyloid-negative control subjects were studied. The primary goal of this study was to determine whether TSPO binding is elevated in patients with Alzheimer’s disease, and the secondary goal was to determine whether TSPO binding correlates with neuropsychological measures, grey matter volume, 11C-Pittsburgh Compound B binding, or age of onset. Patients with Alzheimer’s disease, but not those with mild cognitive impairment, had greater 11C-PBR28 binding in cortical brain regions than controls. The largest differences were seen in the parietal and temporal cortices, with no difference in subcortical regions or cerebellum. 11C-PBR28 binding inversely correlated with performance on Folstein Mini-Mental State Examination, Clinical Dementia Rating Scale Sum of Boxes, Logical Memory Immediate (Wechsler Memory Scale Third Edition), Trail Making part B and Block Design (Wechsler Adult Intelligence Scale Third Edition) tasks, with the largest correlations observed in the inferior parietal lobule. 11C-PBR28 binding also inversely correlated with grey matter volume. Early-onset (

Of the two 18F-labeled PET ligands currently available to image metabotropic glutamate receptor 5 (mGluR5), [18F]FPEB is reportedly superior because [18F]SP203 undergoes glutathionlyation, generating [18F]-fluoride ion that accumulates in brain and skull. To allow multiple PET studies on the same day with lower radiation exposure, we prepared [11C]FPEB and [11C]SP203 from [11C]hydrogen cyanide and compared their abilities to accurately quantify mGluR5 in human brain, especially as regards radiometabolite accumulation. Genomic plot was used to estimate the ratio of specific-to-nondisplaceable uptake (BPND) without using a receptor blocking drug. Both tracers quantified mGluR5; however [11C]SP203, like [18F]SP203, had radiometabolite accumulation in brain, as evidenced by increased distribution volume (VT) over the scan period. Absolute VT values were ∼30% lower for 11C-labeled compared with 18F-labeled radioligands, likely caused by the lower specific activities (and high receptor occupancies) of the 11C radioligands. The genomic plot indicated ∼60% specific binding in cerebellum, which makes it inappropriate as a reference region. Whole-body scans performed in healthy subjects demonstrated a low radiation burden typical for 11C-ligands. Thus, the evidence suggests that [11C]FPEB is superior to [11C]SP203. If prepared in higher specific activity, [11C]FPEB would presumably be as effective as [18F]FPEB for quantifying mGluR5 in human brain.

This longitudinal study sought to determine whether the 18 kDa translocator protein (TSPO), a marker of neuroinflammation, increases over time in Alzheimer's disease. Positron emission tomography imaging with the TSPO radioligand 11 C-PBR28 was performed at baseline and after a median follow-up of 2.7 years in 14 amyloid-positive patients and 8 amyloid-negative controls. Patients had a greater increase in TSPO binding than controls in inferior parietal lobule, precuneus, occipital cortex, hippocampus, entorhinal cortex, and combined middle and inferior temporal cortex. TSPO binding in temporoparietal regions increased from 3.9% to 6.3% per annum in patients, but ranged from −0.5% to 1% per annum in controls. The change in TSPO binding correlated with cognitive worsening on clinical dementia rating scale-sum of boxes and reduced cortical volume. The annual rate of increased TSPO binding in temporoparietal regions was about 5-fold higher in patients with clinical progression (n = 9) compared with those who did not progress (n = 5). TSPO may serve as a biomarker of Alzheimer's progression and response to anti-inflammatory therapies.

Treatment of (diacetoxyiodo)arenes (1a-1u) with cyclotron-produced [(18)F]fluoride ion rapidly affords no-carrier-added [(18)F]fluoroarenes (2a-2u) in useful yields and constitutes a new method for converting substituted iodoarenes into substituted [(18)F]fluoroarenes in just two steps.

Introduction Rhodamine-123 is a known substrate for the efflux transporter, P-glycoprotein (P-gp). We wished to assess whether rhodamine-123 might serve as a useful substrate for developing probes for imaging efflux transporters in vivo with positron emission tomography (PET). For this purpose, we aimed to label rhodamine-123 with carbon-11 (t1/2 = 20.4 min) and to study its biodistribution in rodents.

Nociceptin/orphanin FQ peptide (NOP) receptor is a new class of opioid receptor that may play a pathophysiologic role in anxiety and drug abuse and is a potential therapeutic target in these disorders. We previously developed a high-affinity PET ligand, 11C-NOP-1A, which yielded promising results in monkey brain. Here, we assessed the ability of 11C-NOP-1A to quantify NOP receptors in human brain and estimated its radiation safety profile. Methods: After intravenous injection of 11C-NOP-1A, 7 healthy subjects underwent brain PET for 2 h and serial sampling of radial arterial blood to measure parent radioligand concentrations. Distribution volume (VT; a measure of receptor density) was determined by compartmental (1- and 2-tissue) and noncompartmental (Logan analysis and Ichise9s bilinear analysis [MA1]) methods. A separate group of 9 healthy subjects underwent whole-body PET to estimate whole-body radiation exposure (effective dose). Results: After 11C-NOP-1A injection, the peak concentration of radioactivity in brain was high (∼5–7 standardized uptake values), occurred early (∼10 min), and then washed out quickly. The unconstrained 2-tissue-compartment model gave excellent VT identifiability (∼1.1% SE) and fitted the data better than a 1-tissue-compartment model. Regional VT values (mL·cm−3) ranged from 10.1 in temporal cortex to 5.6 in cerebellum. VT was well identified in the initial 70 min of imaging and remained stable for the remaining 50 min, suggesting that brain radioactivity was most likely parent radioligand, as supported by the fact that all plasma radiometabolites of 11C-NOP-1A were less lipophilic than the parent radioligand. Voxel-based MA1 VT values correlated well with results from the 2-tissue-compartment model, showing that parametric methods can be used to compare populations. Whole-body scans showed radioactivity in brain and in peripheral organs expressing NOP receptors, such as heart, pancreas, and spleen. 11C-NOP-1A was significantly metabolized and excreted via the hepatobiliary route. Gallbladder had the highest radiation exposure (21 μSv/MBq), and the effective dose was 4.3 μSv/MBq. Conclusion:11C-NOP-1A is a promising radioligand that reliably quantifies NOP receptors in human brain. The effective dose in humans is low and similar to that of other 11C-labeled radioligands, allowing multiple scans in 1 subject.

Animal studies and clinical observations suggest that epilepsy is associated with inflammation. Translocator protein (TSPO) (18 kDa), a marker of inflammation, is increased in vitro in surgical samples from patients with temporal lobe epilepsy. TSPO can be measured in the living human brain with PET and the novel radioligand 11C-PBR28. In this study, we sought to determine whether in vivo expression of TSPO is increased ipsilateral to the seizure focus in patients with temporal lobe epilepsy. Methods: Sixteen patients with unilateral temporal lobe epilepsy and 30 healthy subjects were studied with 11 CPBR28 PET and MRI. Uptake of radioactivity after injection of 11 C-PBR28 was measured from regions of interest drawn bilaterally onto MR images. Brain uptake from ipsilateral and contralateral hemispheres was compared using a paired-samples t test. Results: We found that brain uptake was higher ipsilateral to the seizure focus in the hippocampus, parahippocampal gyrus, amygdala, fusiform gyrus, and choroid plexus but not in other brain regions. This asymmetry was more pronounced in patients with hippocampal sclerosis than in those without. Conclusion: We found increased uptake of radioactivity after injection of 11C-PBR28 ipsilateral to the seizure focus in patients with temporal lobe epilepsy, suggesting increased expression of TSPO. Studies in larger samples are required to confirm this finding and determine the clinical utility of imaging TSPO in temporal lobe epilepsy.

Chronic cannabis (marijuana, hashish) smoking can result in dependence. Rodent studies show reversible downregulation of brain cannabinoid CB(1) (cannabinoid receptor type 1) receptors after chronic exposure to cannabis. However, whether downregulation occurs in humans who chronically smoke cannabis is unknown. Here we show, using positron emission tomography imaging, reversible and regionally selective downregulation of brain cannabinoid CB(1) receptors in human subjects who chronically smoke cannabis. Downregulation correlated with years of cannabis smoking and was selective to cortical brain regions. After ∼4 weeks of continuously monitored abstinence from cannabis on a secure research unit, CB(1) receptor density returned to normal levels. This is the first direct demonstration of cortical cannabinoid CB(1) receptor downregulation as a neuroadaptation that may promote cannabis dependence in human brain.