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Our laboratory recently developed [ 11 C]PS13 as a PET radioligand to selectively measure cyclooxygenase-1 (COX-1). The cyclooxygenase enzyme family converts arachidonic acid into prostaglandins and thromboxanes, which mediate inflammation. The total brain uptake of [ 11 C]PS13, which is composed of both specific binding and background uptake, can be accurately quantified with gold standard methods of compartmental modeling. This study sought to quantify the specific binding of [ 11 C]PS13 to COX-1 in healthy human brain using scans performed with arterial input function at baseline and after blockade by the COX-1-selective inhibitor ketoprofen. Methods: Eight healthy volunteers underwent two 90-min [ 11 C]PS13 PET scans with radiometabolite-corrected arterial input function, at baseline and about 2 h after oral administration of ketoprofen (75 mg). Results: Two-tissue compartment modeling effectively identified the total uptake of radioactivity in the brain (as distribution volume), showing the highest densities in the hippocampus, the occipital cortex, and the banks of the central sulcus. All brain regions exhibited displaceable and specific binding, and thus none could be used as a reference region. Ketoprofen blocked approximately 84% of the binding sites on COX-1 in the whole brain. After full occupancy was extrapolated, the average whole-brain values of [ 11 C]PS13 were 1.6 ± 0.8 mL·cm -3 for specific uptake, 1.7 ± 0.6 mL·cm -3 for background uptake, and 1.1 ± 0.5 for the specific-to-background ratio. The hippocampus had the highest specific-to-background ratio value of 2.7 ± 0.9. Conclusion: [ 11 C]PS13 exhibited high specific binding to COX-1 in the human brain, but its quantification requires arterial blood sampling., (© 2024 by the Society of Nuclear Medicine and Molecular Imaging.)

The cyclic adenosine monophosphate (cAMP) cascade is thought to play an important role in regulating alcohol-dependent behaviors, with potentially opposite effects following acute versus chronic administration. Phosphodiesterase 4 (PDE4) is the primary brain enzyme that metabolizes cAMP, thereby terminating its signal. Radioligand binding to PDE4 serves as an indirect biomarker of cAMP activity, as cAMP-protein kinase A (PKA)-mediated phosphorylation of PDE4 increases its affinity for radioligand binding ~10-fold. Of the four PDE4 subtypes, PDE4B polymorphisms are known to be strongly associated with alcohol and substance use disorders. This study imaged rats with the PDE4B-preferring positron emission tomography (PET) radioligand [ 18 F]PF-06445974 following acute and chronic ethanol administration, aiming to explore the potential of PDE4B PET imaging for future human studies. Compared to the control group treated with saline, acute alcohol administration (i.p. ethanol 0.5 g/kg) significantly increased whole brain uptake of [ 18 F]PF-06445974 as early as 30 minutes post-exposure. This effect persisted at 2 hours, peaked at 4 hours, and diminished at 6 hours and 24 hours post-exposure. In contrast, in a rat model of alcohol dependence, [ 18 F]PF-06445974 brain uptake was significantly reduced at 5 hours post-exposure and was normalized by 3 days. This reduction may reflect long-term adaptation to repeated alcohol-induced activation of cAMP signaling with chronic exposure. Taken together, the results suggest that PET imaging of PDE4B in individuals with alcohol use disorder (AUD) should be considered in conjunction with ongoing trials of PDE4 inhibitors to treat alcohol withdrawal and reduce alcohol consumption., (© 2024. This is a U.S. Government work and not under copyright protection in the US; foreign copyright protection may apply.)

Positron emission tomography (PET) reporter systems are a valuable means of estimating the level of expression of a transgene in vivo. For example, the safety and efficacy of gene therapy approaches for the treatment of neurological and neuropsychiatric disorders could be enhanced via the monitoring of exogenous gene expression levels in the brain. The present study evaluated the ability of a newly developed PET reporter system [ 18 F]fluoroestradiol ([ 18 F]FES) and the estrogen receptor-based PET reporter ChRERα, to monitor expression levels of a small hairpin RNA (shRNA) designed to suppress choline acetyltransferase (ChAT) expression in rhesus monkey brain. The ChRERα gene and shRNA were expressed from the same transcript via lentivirus injected into monkey striatum. In two monkeys that received injections of viral vector, [ 18 F]FES binding increased by 70% and 86% at the target sites compared with pre-injection, demonstrating that ChRERα expression could be visualized in vivo with PET imaging. Post-mortem immunohistochemistry confirmed that ChAT expression was significantly suppressed in regions in which [ 18 F]FES uptake was increased. The consistency between PET imaging and immunohistochemical results suggests that [ 18 F]FES and ChRERα can serve as a PET reporter system in rhesus monkey brain for in vivo evaluation of the expression of potential therapeutic agents, such as shRNAs., Competing Interests: Declaration of interests M.M. has received research funding from AstraZeneca, Kriya (Redpin) Therapeutics, Dompé farmaceutici, and Attune Neurosciences, and is named as an inventor on a patent describing novel DREADD ligands (WO2019/157083) and a U.S. provisional patent application describing novel fluorinated mu opioid receptor agonists (63/452,879)., (Published by Elsevier Inc.)

Phosphodiesterase-4D (PDE4D) has emerged as a significant target for treating neuropsychiatric disorders, but no PET radioligand currently exists for robustly quantifying human brain PDE4D to assist biomedical research and drug discovery. A prior candidate PDE4D PET radioligand, namely [ 11 C]T1650, failed in humans because of poor time stability of brain PDE4D-specific signal (indexed by total volume of distribution), likely due to radiometabolites accumulating in brain. Its nitro group was considered to be a source of the brain radiometabolites. Methods: We selected 5 high-affinity and selective PDE4D inhibitors, absent of a nitro group, from our prior structure-activity relationship study for evaluation as PET radioligands. Results: All 5 radioligands were labeled with 11 C (half-time, 20.4 min) in useful yields and with high molar activity. All displayed sizable PDE4D-specific signals in rhesus monkey brain. Notably, [ 11 C]JMJ-81 and [ 11 C]JMJ-129 exhibited excellent time stability of signal (total volume of distribution). Furthermore, as an example, [ 11 C]JMJ-81 was found to be free of radiometabolites in ex vivo monkey brain, affirming that this radioligand can provide robust quantification of brain PDE4D with PET. Conclusion: Given their high similarity in structures and metabolic profiles, both [ 11 C]JMJ-81 and [ 11 C]JMJ-129 warrant further evaluation in human subjects. [ 11 C]JMJ-129 shows a higher PDE4D specific-to-nonspecific binding ratio and will be the first to be evaluated., (© 2024 by the Society of Nuclear Medicine and Molecular Imaging.)

Chemogenetic tools are designed to control neuronal signaling. These tools have the potential to contribute to the understanding of neuropsychiatric disorders and to the development of new treatments. One such chemogenetic technology comprises modified Pharmacologically Selective Actuator Modules (PSAMs) paired with Pharmacologically Selective Effector Molecules (PSEMs). PSAMs are receptors with ligand-binding domains that have been modified to interact only with a specific small-molecule agonist, designated a PSEM. PSAM 4 is a triple mutant PSAM derived from the α7 nicotinic receptor (α7 L131G,Q139L,Y217F ). Although having no constitutive activity as a ligand-gated ion channel, PSAM 4 has been coupled to the serotonin 5-HT 3 receptor (5-HT 3 R) and to the glycine receptor (GlyR). Treatment with the partner PSEM to activate PSAM 4 -5-HT 3 or PSAM 4 -GlyR, causes neuronal activation or silencing, respectively. A suitably designed radioligand may enable selective visualization of the expression and location of PSAMs with positron emission tomography (PET). Here, we evaluated uPSEM792, an ultrapotent PSEM for PSAM 4 -GlyR, as a possible lead for PET radioligand development. We labeled uPSEM792 with the positron-emitter, carbon-11 (t 1/2  = 20.4 min), in high radiochemical yield by treating a protected precursor with [ 11 C]iodomethane followed by base deprotection. PET experiments with [ 11 C]uPSEM792 in rodents and in a monkey transduced with PSAM 4 -GlyR showed low peak radioactivity uptake in brain. This low uptake was probably due to high polarity of the radioligand, as evidenced by physicochemical measurements, and to the vulnerability of the radioligand to efflux transport at the blood-brain barrier. These findings can inform the design of a more effective PSAM 4 based PET radioligand, based on the uPSEM792 chemotype., (© 2024. This is a U.S. Government work and not under copyright protection in the US; foreign copyright protection may apply.)

Purpose: [ 18 F]SF51 was previously found to have high binding affinity and selectivity for 18 kDa translocator protein (TSPO) in mouse brain. This study sought to assess the ability of [ 18 F]SF51 to quantify TSPO in rhesus 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 (V T /f P ) using a two-tissue compartment model. Receptor occupancy and nondisplaceable uptake were determined via Lassen plot. Binding potential (BP ND ) was calculated as the ratio of specific binding to nondisplaceable uptake. Time stability of V T 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 [ 18 F]SF51 injection, the concentration of brain radioactivity peaked at 2.0 standardized uptake value (SUV) at ~ 10 min and declined to 30% of the peak at 180 min. V T /f P at baseline was generally high (203 ± 15 mL· cm -3 ) and decreased by ~ 90% after blockade with PK11195. BP ND of the whole brain was 7.6 ± 4.3. V T values reached levels similar to terminal 180-min values by 100 min 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 [ 18 F]SF51 is an excellent radioligand that can quantify TSPO with a good ratio of specific to nondisplaceable uptake and has minimal radiometabolite accumulation in brain. Collectively, the results suggest that [ 18 F]SF51 warrants further evaluation in humans., (© 2023. This is a U.S. Government work and not under copyright protection in the US; foreign copyright protection may apply.)

Introduction: We recently reported 11 C-NR2B-SMe ([S-methyl- 11 C](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 11 C-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 11 C-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. 11 C-NR2B-Me and its enantiomers were prepared by Pd-mediated treatment of boronic ester precursors with 11 C-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. 18 F-FTC146 and enantiomers of 11 C-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. 11 C-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 11 C-(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 11 C-NR2B-SMe enantiomers, except that 11 C-NR2B-Me enantiomers showed faster reversibility of binding. When 18 F-FTC146 was used as a radioligand, FTC146 and BD1407 showed strong pre-blocking effects whereas GluN2B ligands showed only weak blocking effects., Conclusion: 11 C-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., (© 2023. This is a U.S. Government work and not under copyright protection in the US; foreign copyright protection may apply.)

[ 11 C] CPPC has been advocated as a radioligand for colony-stimulating factor 1 receptor (CSF1R) with the potential for imaging neuroinflammation in human subjects with positron emission tomography (PET). This study sought to prepare fluoro analogs of CPPC with higher affinity to provide the potential for labeling with longer-lived fluorine-18 ( t 1/2 = 109.8 min) and for delivery of higher CSF1R-specific PET signal in vivo . Seven fluorine-containing analogs of CPPC were prepared and four were found to have high inhibitory potency (IC 50 in low to sub-nM range) and selectivity at CSF1R comparable with CPPC itself. One of these, a 4-fluoromethyl analog ( Psa374 ), was investigated more deeply by labeling with carbon-11 ( t 1/2 = 20.4 min) for PET studies in mouse and monkey. [ 11 C] Psa374 showed high peak uptake in monkey brain but not in mouse brain. Pharmacological challenges revealed no CSF1R-specific binding in either species at baseline. [ 11 C] CPPC also failed to show specific binding at baseline. Moreover, both [ 11 C] Psa374 and [ 11 C] CPPC showed brain efflux transporter substrate behavior in both species in vivo , although Psa374 did not show liability toward human efflux transporters in vitro . Further development of [ 11 C] Psa374 in non-human primate models of neuroinflammation with demonstration of CSF1R-specific binding would be required to warrant the fluorine-18 labeling of Psa374 with a view to possible application in human subjects., Competing Interests: The authors declare no competing financial interest., (Not subject to U.S. Copyright. Published 2023 by American Chemical Society.)

Both cyclooxygenase-1 (COX-1) and cyclooxygenase-2 (COX-2) convert arachidonic acid to prostaglandin H 2 , which has proinflammatory effects. The recently developed PET radioligand 11 C-PS13 has excellent in vivo selectivity for COX-1 over COX-2 in nonhuman primates. This study sought to evaluate the selectivity of 11 C-PS13 binding to COX-1 in humans and assess the utility of 11 C-PS13 to measure the in vivo potency of nonsteroidal antiinflammatory drugs. Methods: Baseline 11 C-PS13 whole-body PET scans were obtained for 26 healthy volunteers, followed by blocked scans with ketoprofen ( n = 8), celecoxib ( n = 8), or aspirin ( n = 8). Ketoprofen is a highly potent and selective COX-1 inhibitor, celecoxib is a preferential COX-2 inhibitor, and aspirin is a selective COX-1 inhibitor with a distinct mechanism that irreversibly inhibits substrate binding. Because blood cells, including platelets and white blood cells, also contain COX-1, 11 C-PS13 uptake inhibition from blood cells was measured in vitro and ex vivo (i.e., using blood obtained during PET scanning). Results: High 11 C-PS13 uptake was observed in major organs with high COX-1 density, including the spleen, lungs, kidneys, and gastrointestinal tract. Ketoprofen (1-75 mg orally) blocked uptake in these organs far more effectively than did celecoxib (100-400 mg orally). On the basis of the plasma concentration to inhibit 50% of the maximum radioligand binding in the spleen (in vivo IC 50 ), ketoprofen (<0.24 μM) was more than 10-fold more potent than celecoxib (>2.5 μM) as a COX-1 inhibitor, consistent with the in vitro potencies of these drugs for inhibiting COX-1. Blockade of 11 C-PS13 uptake from blood cells acquired during the PET scans mirrored that in organs of the body. Aspirin (972-1,950 mg orally) blocked such a small percentage of uptake that its in vivo IC 50 could not be determined. Conclusion: 11 C-PS13 selectively binds to COX-1 in humans and can measure the in vivo potency of nonsteroidal antiinflammatory drugs that competitively inhibit arachidonic acid binding to COX-1. These in vivo studies, which reflect the net effect of drug absorption and metabolism in all organs of the body, demonstrated that ketoprofen had unexpectedly high potency, that celecoxib substantially inhibited COX-1, and that aspirin acetylation of COX-1 did not block binding of the representative nonsteroidal inhibitor 11 C-PS13., (© 2023 by the Society of Nuclear Medicine and Molecular Imaging.)

Phosphodiesterase-4 (PDE4), which metabolizes the second messenger cyclic adenosine monophosphate (cAMP), has 4 isozymes: PDE4A, PDE4B, PDE4C, and PDE4D. PDE4B and PDE4D have the highest expression in the brain and may play a role in the pathophysiology and treatment of depression and dementia. This study evaluated the properties of the newly developed PDE4B-selective radioligand 18 F-PF-06445974 in the brains of rodents, monkeys, and humans. Methods: Three monkeys and 5 healthy human volunteers underwent PET scans after intravenous injection of 18 F-PF-06445974. Brain uptake was quantified as total distribution volume ( V T ) using the standard 2-tissue-compartment model and serial concentrations of parent radioligand in arterial plasma. Results: 18 F-PF-06445974 readily distributed throughout monkey and human brain and had the highest binding in the thalamus. The value of V T was well identified by a 2-tissue-compartment model but increased by 10% during the terminal portions (40 and 60 min) of the monkey and human scans, respectively, consistent with radiometabolite accumulation in the brain. The average human V T values for the whole brain were 9.5 ± 2.4 mL ⋅ cm -3 Radiochromatographic analyses in knockout mice showed that 2 efflux transporters-permeability glycoprotein (P-gp) and breast cancer resistance protein (BCRP)-completely cleared the problematic radiometabolite but also partially cleared the parent radioligand from the brain. In vitro studies with the human transporters suggest that the parent radioligand was a partial substrate for BCRP and, to a lesser extent, for P-gp. Conclusion: 18 F-PF-06445974 quantified PDE4B in the human brain with reasonable, but not complete, success. The gold standard compartmental method of analyzing brain and plasma data successfully identified the regional densities of PDE4B, which were widespread and highest in the thalamus, as expected. Because the radiometabolite-induced error was only about 10%, the radioligand is, in the opinion of the authors, suitable to extend to clinical studies., (© 2022 by the Society of Nuclear Medicine and Molecular Imaging.)

Because of its excellent ratio of specific to nondisplaceable uptake, the radioligand 11 C-ER176 can successfully image 18-kDa translocator protein (TSPO), a biomarker of inflammation, in the human brain and accurately quantify target density in homozygous low-affinity binders. Our laboratory sought to develop an 18 F-labeled TSPO PET radioligand based on ER176 with the potential for broader distribution. This study used generic 11 C labeling and in vivo performance in the monkey brain to select the most promising among 6 fluorine-containing analogs of ER176 for subsequent labeling with longer-lived 18 F. Methods: Six fluorine-containing analogs of ER176-3 fluoro and 3 trifluoromethyl isomers-were synthesized and labeled by 11 C methylation at the secondary amide group of the respective N -desmethyl precursor. PET imaging of the monkey brain was performed at baseline and after blockade by N -butan-2-yl-1-(2-chlorophenyl)- N -methylisoquinoline-3-carboxamide (PK11195). Uptake was quantified using radiometabolite-corrected arterial input function. The 6 candidate radioligands were ranked for performance on the basis of 2 in vivo criteria: the ratio of specific to nondisplaceable uptake (i.e., nondisplaceable binding potential [ BP ND ]) and the time stability of total distribution volume ( V T ), an indirect measure of lack of radiometabolite accumulation in the brain. Results: Total TSPO binding was quantified as V T corrected for plasma free fraction ( V T / f P ) using Logan graphical analysis for all 6 radioligands. V T / f P was generally high at baseline (222 ± 178 mL·cm -3 ) and decreased by 70%-90% after preblocking with PK11195. BP ND calculated using the Lassen plot was 9.6 ± 3.8; the o -fluoro radioligand exhibited the highest BP ND (12.1), followed by the m -trifluoromethyl (11.7) and m -fluoro (8.1) radioligands. For all 6 radioligands, V T reached 90% of the terminal 120-min values by 70 min and remained relatively stable thereafter, with excellent identifiability (SEs < 5%), suggesting that no significant radiometabolites accumulated in the brain. Conclusion: All 6 radioligands had good BP ND and good time stability of V T Among them, the o -fluoro, m -trifluoromethyl, and m -fluoro compounds were the 3 best candidates for development as radioligands with an 18 F label., (© 2022 by the Society of Nuclear Medicine and Molecular Imaging.)

The continuous rise in opioid overdoses in the United States is predominantly driven by very potent synthetic opioids, mostly fentanyl and its derivatives (fentanyls). Although naloxone (NLX) has been shown to effectively reverse overdoses by conventional opioids, there may be a need for higher or repeated doses of NLX to revert overdoses from highly potent fentanyls. Here, we used positron emission tomography (PET) to assess NLX's dose-dependence on both its rate of displacement of [ 11 C]carfentanil ([ 11 C]CFN) binding and its duration of mu opioid receptor (MOR) occupancy in the male rat brain. We showed that clinically relevant doses of intravenously (IV) administered NLX (0.035 mg/kg, Human Equivalent Dose (HED) 0.4 mg; 0.17 mg/kg, HED 2 mg) rapidly displaced the specific binding of [ 11 C]CFN in the thalamus in a dose-dependent manner. Brain MOR occupancy by IV NLX was greater than 90% at 5 min after NLX administration for both doses, but at 27.3 min after 0.035 mg/kg dose and at 85 min after 0.17 mg/kg NLX, only 50% occupancy remained. This indicates that the duration of NLX occupancy at MORs is short-lived. Overall, these results show that clinically relevant doses of IV NLX can promptly displace fentanyls at brain MORs, but repeated or higher NLX doses may be required to prevent re-narcotization following overdoses with long-acting fentanyls., (© 2022. The Author(s).)

Translocator protein 18 kDa (TSPO) is a biomarker of neuroinflammation. [ 11 C]ER176 robustly quantifies TSPO in the human brain with positron emission tomography (PET), irrespective of subject genotype. We aimed to develop an ER176 analog with potential for labeling with longer-lived fluorine-18 ( t 1/2 = 109.8 min). New fluoro and trifluoromethyl analogs of ER176 were prepared through a concise synthetic strategy. These ligands showed high TSPO affinity and low human genotype sensitivity. Each ligand was initially labeled by a generic 11 C-methylation procedure, thereby enabling speedy screening in mice. Each radioligand was rapidly taken up and well retained in the mouse brain at baseline after intravenous injection. Preblocking of TSPO showed that high proportions of brain uptake were specifically bound to TSPO at baseline. Overall, the 3-fluoro analog of [ 11 C]ER176 ([ 11 C] 3b ) displayed the most promising imaging properties. Therefore, a method was developed to label 3b with [ 18 F]fluoride ion. [ 18 F] 3b gave similarly promising PET imaging results and deserves evaluation in higher species.

Previous work found that [ 11 C]deschloroclozapine ([ 11 C]DCZ) is superior to [ 11 C]clozapine ([ 11 C]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 (hM 4 Di) was injected into the right amygdala of a male rhesus macaque. [ 11 C]DCZ and [ 11 C]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, [ 11 C]DCZ signal was 19% of that for [ 11 C]CLZ, and background uptake was 10% of that for [ 11 C]CLZ. Despite stronger [ 11 C]CLZ binding, the signal-to-background ratio for [ 11 C]DCZ was almost two-fold greater than for [ 11 C]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 [ 11 C]DCZ was due to its lower background uptake.

Seven-transmembrane receptors signal via G-protein- and β-arrestin-dependent pathways. We describe a peripheral CB 1 R antagonist (MRI-1891) highly biased toward inhibiting CB 1 R-induced β-arrestin-2 (βArr2) recruitment over G-protein activation. In obese wild-type and βArr2-knockout (KO) mice, MRI-1891 treatment reduces food intake and body weight without eliciting anxiety even at a high dose causing partial brain CB 1 R occupancy. By contrast, the unbiased global CB 1 R antagonist rimonabant elicits anxiety in both strains, indicating no βArr2 involvement. Interestingly, obesity-induced muscle insulin resistance is improved by MRI-1891 in wild-type but not in βArr2-KO mice. In C2C12 myoblasts, CB 1 R activation suppresses insulin-induced akt-2 phosphorylation, preventable by MRI-1891, βArr2 knockdown or overexpression of CB 1 R-interacting protein. MRI-1891, but not rimonabant, interacts with nonpolar residues on the N-terminal loop, including F108, and on transmembrane helix-1, including S123, a combination that facilitates βArr2 bias. Thus, CB 1 R promotes muscle insulin resistance via βArr2 signaling, selectively mitigated by a biased CB 1 R antagonist at reduced risk of central nervous system (CNS) side effects., Competing Interests: The authors declare the following competing financial interest(s): G.K., R.C., and M.R.I. are listed as co-inventors on two US Government patents covering (S)-MRI-1891. The remaining authors declare no competing interests., (Not subject to U.S. Copyright. Published 2021 by American Chemical Society.)

Background: Previous studies found that the positron emission tomography (PET) radioligand [ 18 F]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 [ 18 F]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 [ 18 F]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 (IRF 120 ). 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 (V T ) values at both the region and voxel levels were strongly correlated with 2TCM-derived V T (r = 0.96-0.99) and showed little bias (up to - 8%). SA was more weakly correlated to 2TCM-derived V T (r = 0.93-0.98) and was more biased (~ 16%). IRF 120 showed a strong correlation with 2TCM-derived V T (r = 0.96) but generated noisier parametric images. All techniques were comparable to 2TCM in terms of test-retest variability and reliability except IRF 120 , which gave significantly worse results. Noninvasive SUVR values were not correlated with 2TCM-derived V T , and arteriovenous equilibrium was never reached., Conclusions: Compared to SA and IRF, Logan and MA1 are more suitable alternatives to 2TCM for quantifying [ 18 F]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)-1 H -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.

Purpose: This study assessed whether the newly developed PET radioligand [ 11 C]PS13, which has shown excellent in vivo selectivity in previous animal studies, could be used to quantify constitutive levels of cyclooxygenase-1 (COX-1) in healthy human brain., Methods: Brain test-retest scans with concurrent arterial blood samples were obtained in 10 healthy individuals. The one- and unconstrained two-tissue compartment models, as well as the Logan graphical analysis were compared, and test-retest reliability and time-stability of total distribution volume (V T ) were assessed. Correlation analyses were conducted between brain regional V T and COX-1 transcript levels provided in the Allen Human Brain Atlas., Results: In the brain, [ 11 C]PS13 showed highest uptake in the hippocampus and occipital cortex. The pericentral cortex also showed relatively higher uptake compared with adjacent neocortices. The two-tissue compartment model showed the best fit in all the brain regions, and the results from the Logan graphical analysis were consistent with those from the two-tissue compartment model. V T values showed excellent test-retest variability (range 6.0-8.5%) and good reliability (intraclass correlation coefficient range 0.74-0.87). V T values also showed excellent time-stability in all brain regions, confirming that there was no radiometabolite accumulation and that shorter scans were still able to reliably measure V T . Significant correlation was observed between V T and COX-1 transcript levels (r = 0.82, P = 0.007), indicating that [ 11 C]PS13 binding reflects actual COX-1 density in the human brain., Conclusions: These results from the first-in-human evaluation of the ability of [ 11 C]PS13 to image COX-1 in the brain justifies extending the study to disease populations with neuroinflammation., Clinical Trial Registration: NCT03324646 at https://clinicaltrials.gov/ . Registered October 30, 2017. Retrospectively registered.

[ S - methyl - 11 C](±)-7-methoxy-3-(4-(4-(methylthio)phenyl)butyl)-2,3,4,5-tetrahydro-1 H -benzo[ d ]azepin-1-ol ( 11 C-NR2B-SMe) and its enantiomers were synthesized as candidates for imaging the NR2B subunit within the N -methyl-d-aspartate receptor with PET. Methods: Brains were scanned with PET for 90 min after intravenous injection of one of the candidate radioligands into rats. To detect any NR2B-specific binding of radioligand in brain, various preblocking or displacing agents were evaluated for their impact on the PET brain imaging data. Radiometabolites from brain and other tissues were measured ex vivo and in vitro. Results: Each radioligand gave high early whole-brain uptake of radioactivity, followed by a brief fast decline and then a slow final decline. 11 C-( S )-NR2B-SMe was studied extensively. Ex vivo measurements showed that radioactivity in rat brain at 30 min after radioligand injection was virtually unchanged radioligand. Only less lipophilic radiometabolites appeared in plasma. High-affinity NR2B ligands, Ro-25-6981, ifenprodil, and CO101244, showed increasing preblocking of whole-brain radioactivity retention with increasing dose (0.01-3.00 mg/kg, intravenously). Five σ 1 antagonists (FTC146, BD1407, F3, F4, and NE100) and 4 σ 1 agonists ((+)-pentazocine, (±)-PPCC, PRE-084, and (+)-SKF10047) were ineffective preblocking agents, except FTC146 and F4 at a high dose. Two potent σ 1 receptor agonists, TC1 and SA4503, showed dose-dependent preblocking effects in the presence or absence of pharmacologic σ 1 receptor blockade with FTC146. Conclusion: 11 C-( S )-NR2B-SMe has adequate NR2B-specific PET signal in rat brain to warrant further evaluation in higher species. TC1 and SA4503 likely have off-target binding to NR2B in vivo., (© 2020 by the Society of Nuclear Medicine and Molecular Imaging.)

It is a growing concern that outcomes of neuroimaging studies often cannot be replicated. To counteract this, the magnetic resonance (MR) neuroimaging community has promoted acquisition standards and created data sharing platforms, based on a consensus on how to organize and share MR neuroimaging data. Here, we take a similar approach to positron emission tomography (PET) data. To facilitate comparison of findings across studies, we first recommend publication standards for tracer characteristics, image acquisition, image preprocessing, and outcome estimation for PET neuroimaging data. The co-authors of this paper, representing more than 25 PET centers worldwide, voted to classify information as mandatory, recommended, or optional. Second, we describe a framework to facilitate data archiving and data sharing within and across centers. Because of the high cost of PET neuroimaging studies, sample sizes tend to be small and relatively few sites worldwide have the required multidisciplinary expertise to properly conduct and analyze PET studies. Data sharing will make it easier to combine datasets from different centers to achieve larger sample sizes and stronger statistical power to test hypotheses. The combining of datasets from different centers may be enhanced by adoption of a common set of best practices in data acquisition and analysis.

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 (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 total volume of distribution (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 positron emission tomography radioligand for quantifying brain OGA in rodent and monkey., (Copyright © 2020 The Authors, some rights reserved; exclusive licensee American Association for the Advancement of Science. No claim to original U.S. Government Works.)

We aimed to develop radioligands for PET imaging of brain phosphodiesterase subtype 4D (PDE4D), a potential target for developing cognition enhancing or antidepressive 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 sizable 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: Cyclooxygenase-2 (COX-2), which is rapidly upregulated by inflammation, is a key enzyme catalyzing the rate-limiting step in the synthesis of several inflammatory prostanoids. Successful positron emission tomography (PET) radioligand imaging of COX-2 in vivo could be a potentially powerful tool for assessing inflammatory response in the brain and periphery. To date, however, the development of PET radioligands for COX-2 has had limited success., Methods: The novel PET tracer [ 11 C]MC1 was used to examine COX-2 expression [1] in the brains of four rhesus macaques at baseline and after injection of the inflammogen lipopolysaccharide (LPS) into the right putamen, and [2] in the joints of two human participants with rheumatoid arthritis and two healthy individuals. In the primate study, two monkeys had one LPS injection, and two monkeys had a second injection 33 and 44 days, respectively, after the first LPS injection. As a comparator, COX-1 expression was measured using [ 11 C]PS13., Results: COX-2 binding, expressed as the ratio of specific to nondisplaceable uptake (BP ND ) of [ 11 C]MC1, increased on day 1 post-LPS injection; no such increase in COX-1 expression, measured using [ 11 C]PS13, was observed. The day after the second LPS injection, a brain lesion (~ 0.5 cm in diameter) with high COX-2 density and high BP ND (1.8) was observed. Postmortem brain analysis at the gene transcript or protein level confirmed in vivo PET results. An incidental finding in an unrelated monkey found a line of COX-2 positivity along an incision in skull muscle, demonstrating that [ 11 C]MC1 can localize inflammation peripheral to the brain. In patients with rheumatoid arthritis, [ 11 C]MC1 successfully imaged upregulated COX-2 in the arthritic hand and shoulder and apparently in the brain. Uptake was blocked by celecoxib, a COX-2 preferential inhibitor., Conclusions: Taken together, these results indicate that [ 11 C]MC1 can image and quantify COX-2 upregulation in both monkey brain after LPS-induced neuroinflammation and in human peripheral tissue with inflammation., Trial Registration: ClinicalTrials.gov NCT03912428. Registered April 11, 2019.

Background: Previous studies found that [ 18 F]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 [ 18 F]LSN3316612 for human clinical research., Methods: Kinetic evaluation of [ 18 F]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 (V T ) 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 V T was assessed down to a 30-min scan time. An alternative quantification method for [ 18 F]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 V T with excellent identifiability using the two-tissue compartment model. [ 18 F]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 V T on the retest scan in nine of 10 volunteers. V T 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 V T values during test-retest conditions (R 2 = 0.84), exhibiting similar reliability to V T (ICC = 0.68 vs. 0.64). Estimated radiation exposure for [ 18 F]LSN3316612 PET was 20.5 ± 2.1 μSv/MBq, comparable to other 18 F-labeled radioligands for brain imaging., Conclusions: [ 18 F]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 V T under retest conditions requires further investigation.

Translocator protein 18 kDa (TSPO) has been widely imaged as a marker of neuroinflammation using several radioligands, including [ 11 C]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 [ 11 C]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 ( V T ) was found to be 1.2-fold higher in HABs across all brain regions. Additionally, the polymorphism plot estimated nondisplaceable uptake ( V ND ) as 1.40 mL · cm -3 , which generated a specific-to-nondisplaceable ratio ( BP ND ) of 1.6 ± 0.6 in HABs and 1.1 ± 0.6 in MABs. V T increased significantly with age in nearly all regions and was well estimated with radio-HPLC measurements from six arterial samples. However, V T did not correlate with body mass index and was not affected by sex. These results underscore which patient characteristics should be accounted for during [ 11 C]PBR28 studies and suggest ways to perform such studies more easily and with fewer blood samples.

Objectives: Neuroinflammation, implicated in epilepsy, can be imaged in humans with positron emission tomography (PET) ligands for translocator protein 18 kDa (TSPO). Previous studies in patients with temporal lobe epilepsy and mesial temporal sclerosis found increased [11C]PBR28 uptake ipsilateral to seizure foci. Neocortical foci present more difficult localization problems and more variable underlying pathology., Methods: We studied 11 patients with neocortical seizure foci using [11C]PBR28 or [11C] N,N-diethyl-2-(4-methoxyphenyl)-5,7-dimethylpyrazolo[1,5-a]pyrimidine-3-acetamide (DPA) 713, and 31 healthy volunteers. Seizure foci were identified with structural magnetic resonance imaging (MRI) and ictal video-electroencephalography (EEG) monitoring. Six patients had surgical resections; five had focal cortical dysplasia type 2A or B and one microdysgenesis. Brain regions were delineated using FreeSurfer and T1-weighted MRI. We measured brain radioligand uptake (standardized uptake values [SUVs]) in ipsilateral and contralateral regions, to compare calculated asymmetry indices [AIs; 200% *(ipsilateral - contralateral)/(ipsilateral + contralateral)] between epilepsy patients and controls, as well as absolute [11C]PBR28 binding as the ratio of distribution volume to free fraction (V T /f P ) in 9 patients (5 high affinity and 4 medium affinity binders) and 11 age-matched volunteers (5 high-affinity and 6 medium affinity) who had metabolite-corrected arterial input functions measured., Results: Nine of 11 patients had AIs exceeding control mean 95% confidence intervals in at least one region consistent with the seizure focus. Three of the nine had normal MRI. There was a nonsignificant trend for patients to have higher binding than volunteers both ipsilateral and contralateral to the focus in the group that had absolute binding measured., Significance: Our study demonstrates the presence of focal and distributed inflammation in neocortical epilepsy. There may be a role for TSPO PET for evaluation of patients with suspected neocortical seizure foci, particularly when other imaging modalities are unrevealing. However, a complex method, inherent variability, and increased binding in regions outside seizure foci will limit applicability., (Published 2019. This article is a U.S. Government work and is in the public domain in the USA.)

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 radioligands that are partial but not full agonists. To explore how the intrinsic activity of a PET radioligand 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(0)-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 precursor 6 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 radioligands. Low brain uptake is partly ascribed to efflux transporter action as well as unfavorable conformations., (Copyright © 2019 Elsevier Inc. All rights reserved.)

Aims: Static fluorodeoxyglucose (FDG)-positron emission tomographic (PET) imaging plays an important role in the localization of epileptic foci. Dynamic FDG PET allows calculation of kinetic parameters. The aim of this study was to investigate whether kinetic parameters have potential for identifying epileptic foci, and to assess the correlation of parameters asymmetry indexes (ASYM) between dynamic and static FDG PET for understanding the pathophysiology of hypometabolism within intractable epilepsy. Methods: Seventeen patients who had refractory epilepsy correctly localized by static FDG PET with good outcome after foci resection were included. Eight controls were also studied. We performed dynamic and static FDG PET scan before operation. Images of both scans were coregistered to the montreal neurological institute space, regional time activity curves and activity concentration (AC) were obtained by applying the automated anatomical labeling template to the two spatially normalized images, respectively. Kinetic parameters were obtained using a two-tissue non-reversible compartmental model with an image-derived input function. AC from the static scan was used. Side-to-side ASYM of both static AC and kinetic parameters were calculated and analyzed in the hypometabolic epileptogenic regions and non-epileptogenic regions. Results: Higher values of ASYM from both kinetic parameters and static AC were found in the patients compared to the controls from epileptogenic regions. In the non-epileptogenic regions, no ASYM differences were seen between patients and controls for all parameters. In patients, static AC showed larger ASYM than influx ( K 1 ) and efflux ( k 2 ) of capillaries, but there were no statistical differences of ASYM between net metabolic flux ( K i ) or the phosphorylation ( k 3 ) and static AC. ASYM of static AC positively correlated with ASYM of k 3 . Conclusion: Dynamic FDG PET can provide equally effective in detecting the epileptic foci compared to static FDG PET in this small cohort. In addition, compared to capillary influx, the hypometabolism of epileptic foci may be related to reduced glucose phosphorylation.

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., (© 2019 by the Society of Nuclear Medicine and Molecular Imaging.)

This study assessed whether the newly developed PET radioligands 11 C-PS13 and 11 C-MC1 could image constitutive levels of cyclooxygenase (COX)-1 and COX-2, respectively, in rhesus monkeys. Methods: After intravenous injection of either radioligand, 24 whole-body PET scans were performed. To measure enzyme-specific uptake, scans of the 2 radioligands were also performed after administration of a nonradioactive drug preferential for either COX-1 or COX-2. Concurrent venous samples were obtained to measure parent radioligand concentrations. SUVs were calculated from 10 to 90 min. Results: 11 C-PS13 showed specific uptake in most organs, including spleen, gastrointestinal tract, kidneys, and brain, which was blocked by COX-1, but not COX-2, preferential inhibitors. Specific uptake of 11 C-MC1 was not observed in any organ except the ovaries and possibly kidneys. Conclusion: The findings suggest that 11 C-PS13 has adequate signal in monkeys to justify its extension to human subjects. In contrast, 11 C-MC1 is unlikely to show significant signal in healthy humans, though it may be able to do so in inflammatory conditions., (© 2018 by the Society of Nuclear Medicine and Molecular Imaging.)

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- 11 C-methylation with [ 11 C]iodomethane and PS2 by O- 18 F-fluoroalkylation with [ 2 H 2 , 18 F]fluorobromomethane. Here, we evaluated these PET radioligands in monkey. All three radioligands gave moderately high uptake in brain, although [ 2 H 2 , 18 F]PS2 also showed undesirable radioactivity uptake in skull. [ 11 C]PS13 was selected for further evaluation, mainly based on more favorable brain kinetics than [ 11 C]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, V T , was well identified from serial brain scans and from the concentrations of parent radioligand in arterial plasma. In addition, V T values were stable within 80 min, suggesting that brain uptake was not contaminated by radiometabolites. [ 11 C]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.

Selective high-affinity antagonists for the dopamine D₃ receptor (D₃R) are sought for treating substance use disorders. Positron emission tomography (PET) with an effective D₃R radioligand could be a useful tool for the development of such therapeutics by elucidating pharmacological specificity and target engagement in vivo. Currently, a D₃R-selective radioligand does not exist. The D₃R ligand, N -(4-(4-(3-chloro-2-methoxyphenyl)piperazin-1-yl)butyl)-1 H -indole-2-carboxamide (BAK4-51, 1 ), has attractive properties for PET radioligand development, including full antagonist activity, very high D₃R affinity, D₃R selectivity, and moderate lipophilicity. We labeled 1 with the positron-emitter carbon-11 ( t 1/2 = 20.4 min) in the methoxy group for evaluation as a radioligand in animals with PET. However, [ 11 C] 1 was found to be an avid substrate for brain efflux transporters and lacked D₃R-specific signal in rodent and monkey brain in vivo.

Neuroinflammation appears to contribute to neurotoxicity observed with heavy alcohol consumption. To assess whether chronic alcohol results in neuroinflammation we used PET and [ 11 C]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 [ 11 C]PBR28 binding. [ 11 C]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 [ 11 C]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 [ 11 C]PBR28 binding in combined groups, due to a correlation in AUD participants. In rodents, we observed no differences in brain [ 11 C]PBR28 uptake between alcohol-dependent and -nondependent rats. These findings, which are consistent with two previous [ 11 C]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.

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 (IC 50 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 (t 1/2 =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 Pd 0 -mediated [ 11 C]carbon monoxide insertion on iodo precursors. Measured logD 7.4 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., (© 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

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 11 C-PBR28 to measure translocator protein 18 kDa. PCA patients showed greater 11 C-PBR28 binding than controls in occipital, posterior parietal, and temporal regions. In contrast, amnestic AD patients showed greater 11 C-PBR28 binding in inferior and medial temporal cortex. Increased 11 C-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., (Published by Elsevier Inc.)

Obesity is associated with physical inactivity, which exacerbates the health consequences of weight gain. However, the mechanisms that mediate this association are unknown. We hypothesized that deficits in dopamine signaling contribute to physical inactivity in obesity. To investigate this, we quantified multiple aspects of dopamine signaling in lean and obese mice. We found that D2-type receptor (D2R) binding in the striatum, but not D1-type receptor binding or dopamine levels, was reduced in obese mice. Genetically removing D2Rs from striatal medium spiny neurons was sufficient to reduce motor activity in lean mice, whereas restoring G i signaling in these neurons increased activity in obese mice. Surprisingly, although mice with low D2Rs were less active, they were not more vulnerable to diet-induced weight gain than control mice. We conclude that deficits in striatal D2R signaling contribute to physical inactivity in obesity, but inactivity is more a consequence than a cause of obesity., (Published by Elsevier Inc.)

Prazosin, a potent and selective α 1 -adrenoceptor antagonist, displaces 25% of 11 C-CUMI-101 ([O-methyl- 11 C]2-(4-(4-(2-methoxyphenyl)piperazin-1-yl)butyl)-4-methyl-1,2,4-triazine-3,5(2H,4H)dione) binding in monkey cerebellum. We sought to estimate the percentage contamination of 11 C-CUMI-101 binding to α 1 -adrenoceptors in human cerebellum under in vivo conditions. In vitro receptor-binding techniques were used to measure α 1 -adrenoceptor density and the affinity of CUMI-101 for these receptors in human, monkey, and rat cerebellum., Methods: Binding potential (maximum number of binding sites × affinity [(1/dissociation constant]) was determined using in vitro homogenate binding assays in human, monkey, and rat cerebellum. 3 H-prazosin was used to determine the maximum number of binding sites, as well as the dissociation constant of 3 H-prazosin and the inhibition constant of CUMI-101., Results: α 1 -adrenoceptor density and the affinity of CUMI-101 for these receptors were similar across species. Cerebellar binding potentials were 3.7 for humans, 2.3 for monkeys, and 3.4 for rats., Conclusion: Reasoning by analogy, 25% of 11 C-CUMI-101 uptake in human cerebellum reflects binding to α 1 -adrenoceptors, suggesting that the cerebellum is of limited usefulness as a reference tissue for quantification in human studies., (© 2016 by the Society of Nuclear Medicine and Molecular Imaging, Inc.)

Efflux transporters at the blood-brain barrier can decrease the entry of drugs and increase the removal of those molecules able to bypass the transporter. We previously hypothesized that (18)F-FCWAY, a radioligand for the serotonin 5-HT1A receptor, is a weak substrate for permeability glycoprotein (P-gp) based on its very early peak and rapid washout from human brain. To determine whether (18)F-FCWAY is a substrate for P-gp, breast cancer resistance protein (BCRP), and multidrug resistance protein (MRP1) - the three most prevalent efflux transporters at the blood-brain barrier - we performed three sets of experiments. In vitro, we conducted fluorescence-activated cell sorting (FACS) flow cytometry studies in cells over-expressing P-gp, BCRP, and MRP1 treated with inhibitors specific to each transporter and with FCWAY. Ex vivo, we measured (18)F-FCWAY concentration in plasma and brain homogenate of transporter knockout mice using γ-counter and radio-HPLC. In vivo, we conducted positron emission tomography (PET) studies to assess changes in humans who received (18)F-FCWAY during an infusion of tariquidar (2-4mg/kg iv), a potent and selective P-gp inhibitor. In vitro studies showed that FCWAY allowed fluorescent substrates to get into the cell by competitive inhibition of all three transporters at the cell membrane. Ex vivo measurements in knockout mice indicate that (18)F-FCWAY is a substrate only for P-gp and not BCRP. In vivo, tariquidar increased (18)F-FCWAY brain uptake in seven of eight subjects by 60-100% compared to each person's baseline. Tariquidar did not increase brain uptake via some peripheral mechanism, given that it did not significantly alter concentrations in plasma of the parent radioligand (18)F-FCWAY or its brain-penetrant radiometabolite (18)F-FC. These results show that (18)F-FCWAY is a weak substrate for efflux transport at the blood-brain barrier; some radioligand can enter brain, but its removal is hastened by P-gp. Although (18)F-FCWAY is not ideal for measuring 5-HT1A receptors, it demonstrates that weak substrate radioligands can be useful for measuring both increased and decreased function of efflux transporters, which is not possible with currently available radioligands such as (11)C-loperamide and (11)C-verapamil that are avid substrates for transporters., (Published by Elsevier Inc.)

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., (Published by Elsevier Inc.)

Liver fibrosis, a consequence of chronic liver injury and a way station to cirrhosis and hepatocellular carcinoma, lacks effective treatment. Endocannabinoids acting via cannabinoid-1 receptors (CB 1 R) induce profibrotic gene expression and promote pathologies that predispose to liver fibrosis. CB 1 R antagonists produce opposite effects, but their therapeutic development was halted due to neuropsychiatric side effects. Inducible nitric oxide synthase (iNOS) also promotes liver fibrosis and its underlying pathologies, but iNOS inhibitors tested to date showed limited therapeutic efficacy in inflammatory diseases. Here, we introduce a peripherally restricted, orally bioavailable CB 1 R antagonist, which accumulates in liver to release an iNOS inhibitory leaving group. In mouse models of fibrosis induced by CCl 4 or bile duct ligation, the hybrid CB 1 R/iNOS antagonist surpassed the antifibrotic efficacy of the CB 1 R antagonist rimonabant or the iNOS inhibitor 1400W, without inducing anxiety-like behaviors or CB 1 R occupancy in the CNS. The hybrid inhibitor also targeted CB 1 R-independent, iNOS-mediated profibrotic pathways, including increased PDGF, Nlrp3/Asc3, and integrin αvβ6 signaling, as judged by its ability to inhibit these pathways in cnr1 -/- but not in nos2 -/- mice. Additionally, it was able to slow fibrosis progression and to attenuate established fibrosis. Thus, dual-target peripheral CB 1 R/iNOS antagonists have therapeutic potential in liver fibrosis.

Unlabelled: A recent study from our laboratory found that (18)F-FIMX is an excellent PET radioligand for quantifying metabotropic glutamate receptor 1 (mGluR1) in monkey brain. This study evaluated the ability of (18)F-FIMX to quantify mGluR1 in humans. A second goal was to use the relative density of mGluR1 gene transcripts in brain regions to estimate specific uptake and nondisplaceable uptake (VND) in each brain region., Methods: After injection of 189 ± 3 MBq of (18)F-FIMX, 12 healthy volunteers underwent a dynamic PET scan over 120 min. For 6 volunteers, images were acquired until 210 min. A metabolite-corrected arterial input function was measured from the radial artery. Four other subjects underwent whole-body scanning to estimate radiation exposure., Results: (18)F-FIMX uptake into the human brain was high (SUV = 4-6 in the cerebellum), peaked at about 10 min, and washed out rapidly. An unconstrained 2-tissue-compartment model fitted the data well, and distribution volume (VT) (mL⋅cm(-3)) values ranged from 1.5 in the caudate to 11 in the cerebellum. A 120-min scan provided stable VT values in all regions except the cerebellum, for which an acquisition time of at least 170 min was necessary. VT values in brain regions correlated well with mGluR1 transcript density, and the correlation suggested that VND of (18)F-FIMX was quite low (0.5 mL⋅cm(-3)). This measure of VND in humans was similar to that from a receptor blocking study in monkeys, after correcting for differences in plasma protein binding. Similar to other (18)F-labeled ligands, the effective dose was about 23 μSv/MBq., Conclusion: (18)F-FIMX can quantify mGluR1 in the human brain with a 120- to 170-min scan. Correlation of brain uptake with the relative density of mGluR1 transcript allows specific receptor binding of a radioligand to be quantified without injecting pharmacologic doses of a blocking agent., (© 2016 by the Society of Nuclear Medicine and Molecular Imaging, Inc.)

Introduction: Metabotropic glutamate subtype receptor 1 (mGluR1) is implicated in several neuropsychiatric disorders and is a target for drug development. [(18)F]FIMX ([(18)F]4-fluoro--N-methyl-N--(4-(6-(methylamino)pyrimidin-4-yl)thiazol-2-yl)benzamide) is an effective radioligand for imaging brain mGluR1 with PET. A similarly effective radioligand with a shorter half-life would usefully allow PET studies of mGluR1 at baseline and after pharmacological or other challenge on the same day. Here we describe the preparation of [(11)C]FIMX for evaluation in monkey with PET., Methods: [(11)C]FIMX was prepared via Pd-promoted carbonylation of 1-fluoro-4-iodobenzene with [(11)C]carbon monoxide, aminolysis of the [(11)C]acyl-palladium complex with the requisite Boc-protected amine, and deprotection with HCl in THF. PET scans of [(11)C]FIMX injected into a monkey were performed at baseline and after preblock of mGluR1 with measurement of the arterial input function., Results: The radiosynthesis required 42 min and gave [(11)C]FIMX in about 5% overall decay-corrected radiochemical yield and with a specific activity of about 100 GBq/μmol. PET in rhesus monkey at baseline showed that radioactivity peaked high in receptor-rich cerebellum and much lower in receptor-poor occipital cortex. Radioactivity in cerebellum declined to 32% of peak at 85 min. VT at baseline appeared stable in all brain regions after 60 min. Under mGluR1 pre-blocked condition, radioactivity uptake in all regions declined more rapidly to a low level. Receptor pre-block reduced VT from 13.0 to 1.5 in cerebellum and from 2.9 to 1.4 in occipital cortex., Conclusion: [(11)C]FIMX is an effective radioligand for imaging mGluR1 in monkey with PET., (Published by Elsevier Inc.)

Importance: Neuroinflammation may play a role in epilepsy. Translocator protein 18 kDa (TSPO), a biomarker of neuroinflammation, is overexpressed on activated microglia and reactive astrocytes. A preliminary positron emission tomographic (PET) imaging study using carbon 11 ([11C])-labeled PBR28 in patients with temporal lobe epilepsy (TLE) found increased TSPO ipsilateral to seizure foci. Full quantitation of TSPO in vivo is needed to detect widespread inflammation in the epileptic brain., Objectives: To determine whether patients with TLE have widespread TSPO overexpression using [11C]PBR28 PET imaging, and to replicate relative ipsilateral TSPO increases in patients with TLE using [11C]PBR28 and another TSPO radioligand, [11C]DPA-713., Design, Setting, and Participants: In a cohort study from March 2009 through September 2013 at the Clinical Epilepsy Section of the National Institute of Neurological Disorders and Stroke, participants underwent brain PET and a subset had concurrent arterial sampling. Twenty-three patients with TLE and 11 age-matched controls were scanned with [11C]PBR28, and 8 patients and 7 controls were scanned with [11C]DPA-713. Patients with TLE had unilateral temporal seizure foci based on ictal electroencephalography and structural magnetic resonance imaging. Participants with homozygous low-affinity TSPO binding were excluded., Main Outcomes and Measures: The [11C]PBR28 distribution volume (VT) corrected for free fraction (fP) was measured in patients with TLE and controls using FreeSurfer software and T1-weighted magnetic resonance imaging for anatomical localization of bilateral temporal and extratemporal regions. Side-to-side asymmetry in patients with TLE was calculated as the ratio of ipsilateral to contralateral [11C]PBR28 and [11C]DPA-713 standardized uptake values from temporal regions., Results: The [11C]PBR28 VT to fp ratio was higher in patients with TLE than in controls for all ipsilateral temporal regions (27%-42%; P < .05) and in contralateral hippocampus, amygdala, and temporal pole (approximately 30%-32%; P < .05). Individually, 12 patients, 10 with mesial temporal sclerosis, had asymmetrically increased hippocampal [11C]PBR28 uptake exceeding the 95% confidence interval of the controls. Binding of [11C]PBR28 was increased significantly in thalamus. Relative [11C]PBR28 and [11C]DPA-713 uptakes were higher ipsilateral than contralateral to seizure foci in patients with TLE ([11C]PBR28: 2%-6%; [11C]DPA-713: 4%-9%). Asymmetry of [11C]DPA-713 was greater than that of [11C]PBR28 (F = 29.4; P = .001)., Conclusions and Relevance: Binding of TSPO is increased both ipsilateral and contralateral to seizure foci in patients with TLE, suggesting ongoing inflammation. Anti-inflammatory therapy may play a role in treating drug-resistant epilepsy., Competing Interests: Disclosures: None reported.

Unlabelled: Alzheimer disease (AD) is associated with an increase in the brain of the 18-kDa translocator protein (TSPO), which is overexpressed in activated microglia and reactive astrocytes. Measuring the density of TSPO with PET typically requires absolute quantitation with arterial blood sampling, because a reference region devoid of TSPO does not exist in the brain. We sought to determine whether a simple ratio method could substitute for absolute quantitation of binding with (11)C-PBR28, a second-generation radioligand for TSPO., Methods: (11)C-PBR28 PET imaging was performed in 21 healthy controls, 11 individuals with mild cognitive impairment, and 25 AD patients. Group differences in (11)C-PBR28 binding were compared using 2 methods. The first was the gold standard method of calculating total distribution volume (V(T)), using the 2-tissue-compartment model with the arterial input function, corrected for plasma-free fraction of radiotracer (f(P)). The second method used a ratio of brain uptake in target regions to that in cerebellum-that is, standardized uptake value ratio (SUVR)., Results: Using absolute quantitation, we confirmed that TSPO binding (V(T)/f(P)) was greater in AD patients than in healthy controls in expected temporoparietal regions and was not significantly different among the 3 groups in the cerebellum. When the cerebellum was used as a pseudo-reference region, the SUVR method detected greater binding in AD patients than controls in the same regions as absolute quantification and in 1 additional region, suggesting SUVR may have greater sensitivity. Coefficients of variation of SUVR measurements were about two-thirds lower than those of absolute quantification, and the resulting statistical significance was much higher for SUVR when comparing AD and healthy controls (e.g., P < 0.0005 for SUVR vs. P = 0.023 for VT/fP in combined middle and inferior temporal cortex)., Conclusion: To measure TSPO density in AD patients and control subjects, a simple ratio method SUVR can substitute for, and may even be more sensitive than, absolute quantitation. The SUVR method is expected to improve subject tolerability by allowing shorter scanning time and not requiring arterial catheterization. In addition, this ratio method allows smaller sample sizes for comparable statistical significance because of the relatively low variability of the ratio values., (© 2015 by the Society of Nuclear Medicine and Molecular Imaging, Inc.)

Objective: 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., Method: 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., Results: 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., Conclusions: 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.

Image-derived input function (IDIF) obtained by manually drawing carotid arteries (manual-IDIF) can be reliably used in [(11)C](R)-rolipram positron emission tomography (PET) scans. However, manual-IDIF is time consuming and subject to inter- and intra-operator variability. To overcome this limitation, we developed a fully automated technique for deriving IDIF with a supervised clustering algorithm (SVCA). To validate this technique, 25 healthy controls and 26 patients with moderate to severe major depressive disorder (MDD) underwent T1-weighted brain magnetic resonance imaging (MRI) and a 90-minute [(11)C](R)-rolipram PET scan. For each subject, metabolite-corrected input function was measured from the radial artery. SVCA templates were obtained from 10 additional healthy subjects who underwent the same MRI and PET procedures. Cluster-IDIF was obtained as follows: 1) template mask images were created for carotid and surrounding tissue; 2) parametric image of weights for blood were created using SVCA; 3) mask images to the individual PET image were inversely normalized; 4) carotid and surrounding tissue time activity curves (TACs) were obtained from weighted and unweighted averages of each voxel activity in each mask, respectively; 5) partial volume effects and radiometabolites were corrected using individual arterial data at four points. Logan-distribution volume (V T/f P) values obtained by cluster-IDIF were similar to reference results obtained using arterial data, as well as those obtained using manual-IDIF; 39 of 51 subjects had a V T/f P error of <5%, and only one had error >10%. With automatic voxel selection, cluster-IDIF curves were less noisy than manual-IDIF and free of operator-related variability. Cluster-IDIF showed widespread decrease of about 20% [(11)C](R)-rolipram binding in the MDD group. Taken together, the results suggest that cluster-IDIF is a good alternative to full arterial input function for estimating Logan-V T/f P in [(11)C](R)-rolipram PET clinical scans. This technique enables fully automated extraction of IDIF and can be applied to other radiotracers with similar kinetics.

Unlabelled: The PET radioligand (11)C-CUMI-101 was previously suggested as a putative agonist radioligand for the serotonin 1A (5-hydroxytryptamine 1A [5-HT1A]) receptor in recombinant cells expressing human 5-HT1A receptor. However, a recent study showed that CUMI-101 behaved as a potent 5-HT1A receptor antagonist in rat brain. CUMI-101 also has moderate affinity (Ki = 6.75 nM) for α1 adrenoceptors measured in vitro. The current study examined the functional properties and selectivity of CUMI-101, both in vitro and in vivo., Methods: The functional assay was performed using (35)S-GTPγS (GTP is guanosine triphosphate) in primate brains. The cross-reactivity of CUMI-101 with α1 adrenoceptors was performed using in vitro radioligand binding studies in rat, monkey, and human brains as well as in vivo PET imaging in mouse, rat, and monkey brains., Results: CUMI-101 did not stimulate (35)S-GTPγS binding in primate brain, in contrast to 8-OH-DPAT, a potent 5-HT1A receptor agonist. Instead, CUMI-101 behaved as a potent 5-HT1A receptor antagonist by dose-dependently inhibiting 8-OH-DPAT-stimulated (35)S-GTPγS binding. Both in vitro and in vivo studies showed that CUMI-101 had significant α1 adrenoceptor cross-reactivity. On average, across all 3 species examined, cross-reactivity was highest in the thalamus (>45%) and lowest in the neocortex and cerebellum (<10%). PET imaging further confirmed that only preblocking with WAY-100635 plus prazosin decreased (11)C-CUMI-101 brain uptake to that of self-block., Conclusion: CUMI-101 behaves as a 5-HT1A receptor antagonist in primate brain, with significant, regional-dependent α1 adrenoceptor cross-reactivity, limiting its potential use as a PET radioligand in humans.

Background: A recent study from our laboratory demonstrated that 11C-LY2428703, a new positron emission tomographic radioligand for metabotropic glutamate receptor 1 (mGluR1), has promising in vitro properties and excellent in vivo performance for imaging rat brain. The present study evaluated 11C-LY2428703 for imaging mGluR1 in monkey and human brains., Methods: Rhesus monkeys were imaged at baseline and after administration of an mGluR1 blocking agent to calculate nonspecific binding, as well as after the administration of permeability glycoprotein (P-gp) and breast cancer resistance protein (BCRP) blockers to assess whether 11C-LY2428703 is a substrate for efflux transporters at the blood-brain barrier. Human imaging was performed at baseline in three healthy volunteers, and arterial input function was measured., Results: Overall brain uptake was low in monkeys, though slightly higher in the cerebellum, where mGluR1s are concentrated. However, the uptake was not clearly displaceable in the scans after mGluR1 blockade. Brain penetration of the ligand did not increase after P-gp and BCRP blockade. Brain uptake was similarly low in all human subjects (mean VT with a two-tissue compartment model, 0.093 ± 0.012 mL/cm3) and for all regions, including the cerebellum., Conclusions: Despite promising in vitro and in vivo results in rodents, 11C-LY2428703 was unsuitable for imaging mGluR1s in monkey or human brain because of low brain uptake, which was likely caused by high binding to plasma proteins.