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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.)

Abstract: Ossification is uncommon, generally asymptomatic, and often incidentally identified in imaging studies. We report on a 54-year-old man who participated as a healthy volunteer in a clinical trial using PET imaging to investigate neuroinflammation. An incidental ossified lesion in the anterior falx cerebri was revealed by MRI. CT scan showed a small hypodense center in the lesion, probably corresponding to bone marrow. The PET scans using 18 F-SF51, 11 C-PS13, and 11 C-MC1 showed increased uptake within this lesion, which was probably related to bone marrow activity within the ossification., Competing Interests: Conflicts of interest and sources of funding: This study was funded by the Intramural Research Program of the National Institute of Mental Health, National Institutes of Health (projects ZIAMH002852 and ZIAMH002793). The authors have no potential conflicts of interest to disclose, financial, or otherwise., (Copyright © 2024 Wolters Kluwer Health, Inc. All rights reserved.)

A 2-phenyl-3-difluoromethoxy-pyridinyl moiety features in potent phosphodiesterase 4D inhibitors that are considered to be candidate radiotracers for positron emission tomography if they are labeled with fluorine-18. Fluorine-18 could be installed as desired at the 3'-phenyl position with acridinium-mediated photoredox radiodeoxyfluorination in homologues bearing variously substituted 3'-aryloxy groups. However, a distal 3-difluoromethoxide (-OCHF 2 ) group strongly competes as a leaving group, especially when an electron-deficient aryloxy group is present at position 3'. A yield of up to 50% may occur without observable 19 F for 18 F exchange.

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.)

Ligand-based virtual screening (LBVS) has rarely been tested as a method for discovering new structural scaffolds for PET radioligand development. This study used LBVS to discover potential chemotype leads for developing radioligands for PET imaging of tauopathies. ZINC12, a free database of over 12 million commercially available compounds, was searched to discover novel scaffolds based on similarities to four query compounds. Thirteen high-ranking hits were purchased and assayed for their ability to compete against three tritiated radioligands at their distinct binding sites in Alzheimer's disease brain tissue. Three hits were 2-substituted 6-methoxy naphthalenes. Synthetic elaboration of this new chemotype yielded three new ligands ( 25 , 26 , and 28 ) with high affinity for the [ 3 H] 6 (flortaucipur) neurofibrillary tangle binding site. Compound 28 showed remarkably high affinity ( K i , 7 nM) and other desirable properties for a candidate PET radioligand, including low topological polar surface area, moderate computed log  D , and amenability for labeling with carbon-11. LBVS appears to be uniquely valuable for discovering new chemotypes for candidate PET radioligands.

The 2,2,2-trifluoroethoxy group increasingly features in drugs and potential tracers for biomedical imaging with positron emission tomography (PET). Herein, we describe a rapid and transition metal-free conversion of fluoroform with paraformaldehyde into highly reactive potassium 2,2,2-trifluoroethoxide (CF 3 CH 2 OK) and demonstrate robust applications of this synthon in one-pot, two-stage 2,2,2-trifluoroethoxylations of both aromatic and aliphatic precursors. Moreover, we show that these transformations translate easily to fluoroform that has been labeled with either carbon-11 (t 1/2  = 20.4 min) or fluorine-18 (t 1/2  = 109.8 min), so allowing the appendage of complex molecules with a no-carrier-added 11 C- or 18 F- 2,2,2-trifluoroethoxy group. This provides scope to create candidate PET tracers with radioactive and metabolically stable 2,2,2-trifluoroethoxy moieties. We also exemplify syntheses of isotopologues of potassium 2,2,2-trifluoroethoxide and show their utility for stable isotopic labeling which can be of further benefit for drug discovery and development., (© 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.)

Herein, we report a copper(I)-free method for labeling the trifluoroacetyl group with positron-emitting carbon-11 ( t 1/2 = 20.4 min) or fluorine-18 ( t 1/2 = 109.8 min) as part of our exploration of radiolabeled fluoroforms to access new radiolabeled chemotypes of interest for tracer development. Treatment of alkyl esters and aryl esters, containing electron-donating or electron-withdrawing groups, with [ 11 C/ 18 F]fluoroform in the presence of strong base, gave [ 11 C/ 18 F]trifluoromethyl ketones as novel radiolabeling synthons in moderate to high yields within 15 minutes.

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.)

The short-lived positron-emitter carbon-11 ( t 1/2 = 20.4 min; β + , 99.8%) is prominent for labeling tracers for use in biomedical research with positron emission tomography (PET). Carbon-11 is produced for this purpose with a cyclotron, nowadays almost exclusively by the 14 N(p,α) 11 C nuclear reaction, either on nitrogen containing a low concentration of oxygen (0.1-0.5%) or hydrogen (~5%) to produce [ 11 C]carbon dioxide or [ 11 C]methane, respectively. These primary radioactive products can be produced in high yields and with high molar activities. However, only [ 11 C]carbon dioxide has some utility for directly labeling PET tracers. Primary products are required to be converted rapidly and efficiently into secondary labeling synthons to provide versatile radiochemistry for labeling diverse tracer chemotypes at molecular positions of choice. This review surveys known gas phase transformations of carbon-11 and summarizes the important roles that many of these transformations now play for producing a broad range of labeling synthons in carbon-11 chemistry.

Aryliodonium precursors are widely applied for copper-free labeling of positron emission tomography (PET) tracers with fluorine-18. We assessed 18 F-fluoroarene regioisomer formation in examples of these labeling methods. Aryliodonium ylides derived from Meldrum's acid bearing para electron-donating groups react with [ 18 F]fluoride in acetonitrile to produce regioisomeric 18 F-fluoroarenes via a competing aryne pathway. Regioisomer formation is decreased or absent in DMF. Analytically checking for the absence of the 18 F-regioisomer from any particular PET tracer radiosynthesis using these or similar methods is recommended.

Positron emission tomography (PET) can be used as a noninvasive method to longitudinally monitor and quantify the expression of proteins in the brain in vivo. It can be used to monitor changes in biomarkers of mental health disorders, and to assess therapeutic interventions such as stem cell and molecular genetic therapies. The utility of PET monitoring depends on the availability of a radiotracer with good central nervous system (CNS) penetration and high selectivity for the target protein. This review evaluates existing methods for the visualization of reporter proteins and/or protein function using PET imaging, focusing on engineered systems, and discusses possible approaches for future success in the development of high-sensitivity and high-specificity PET reporter systems for the brain., Competing Interests: Declaration of interests M.M. has received research funding from AstraZeneca, Redpin Therapeutics, Dompé Farmaceutici, and Attune Neurosciences, and is named as an inventor on a patent describing novel DREADD ligands (WO2019/157083). T.M. is named as an inventor on a patent for the use of DCZ as a PET reporter probe (WO2019/245047). The other coauthors declare no conflicts of interest., (Published by Elsevier Ltd.)

The tyrosine kinase, colony-stimulating factor 1 receptor (CSF1R), has attracted attention as a potential biomarker of neuroinflammation for imaging studies with positron emission tomography (PET), especially because of its location on microglia and its role in microglia proliferation. The development of an effective radiotracer for specifically imaging and quantifying brain CSF1R is highly challenging. Here we review the progress that has been made on PET tracer development and discuss issues that have arisen and which remain to be addressed and resolved., Competing Interests: The authors declare no competing financial interest., (Not subject to U.S. Copyright. Published 2023 by American Chemical Society.)

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.)

Efficient methods for labeling aryl trifluoromethyl groups to provide novel radiotracers for use in biomedical research with positron emission tomography (PET) are keenly sought. We report a broad-scope method for labeling trifluoromethylarenes with either carbon-11 (t 1/2 =20.4 min) or fluorine-18 (t 1/2 =109.8 min) from readily accessible aryl(mesityl)iodonium salts. In this method, the aryl(mesityl)iodonium salt is treated rapidly with no-carrier-added [ 11 C]CuCF 3 or [ 18 F]CuCF 3 . The mesityl group acts as a spectator allowing radiolabeled trifluoromethylarenes to be obtained with very high chemoselectivity. Radiochemical yields from aryl(mesityl)iodonium salts bearing either electron-donating or electron-withdrawing groups at meta- or para- position are good to excellent (67-96 %). Ortho-substituted and otherwise sterically hindered trifluoromethylarenes still give good yields (15-34 %). Substituted heteroaryl(mesityl)iodonium salts are also viable substrates. The broad scope of this method was further exemplified by labeling a previously inaccessible target, [ 11 C]p-trifluoromethylphenyl boronic acid, as a potentially useful labeling synthon. In addition, fluoxetine, leflunomide, and 3-trifluoromethyl-4-aminopyridine, as examples of small drug-like molecules and candidate PET radioligands, were successfully labeled in high yields (69-81 %)., (© 2023 Wiley-VCH GmbH.)

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.)

A positron emission tomography (PET) radioligand for imaging phosphodiesterase 4D (PDE4D) would benefit drug discovery and the investigation of neuropsychiatric disorders. The most promising radioligand to date, namely, [ 11 C]T1650, has shown unstable quantification in humans. Structural elaboration of [ 11 C]T1650 was therefore deemed necessary. High target affinity in the low nM range is usually required for successful PET radioligands. In our PDE4D PET radioligand development, we formulated and optimized an empirical equation (log[IC 50 (nM)] = P1 + P2 + P3 + P4) that well described the relationship between binding affinity and empirically derived values (P1-P4) for the individual fragments in four subregions commonly composing each inhibitor ( R 2 = 0.988, n = 62). This equation was used to predict compounds that would have high inhibitory potency. Fourteen new compounds were obtained with IC 50 of 0.3-10 nM. Finally, eight compounds were judged to be worthy of future radiolabeling and evaluation as PDE4D PET radioligands.

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.)

Positron emission tomography (PET) uses many tracers labeled with fluorine-18 ( t 1/2 = 109.8 min; β + 97%) for quantitative imaging of biochemical and physiological processes in animal and human subjects. In PET methodology, the radioactivity in a dose of an 18 F-labeled tracer to be administered to a living subject is measured with a calibrated ionization chamber. This type of detector measures the radioactivity of a sample relative to those of certified amounts of longer-lived surrogate isotopes that are recommended for detector calibration. No alternative means for corroborating widely varying fluorine-18 radioactivity measurements from calibrated ionization chambers has been available. Here, we describe an independent nonradiometric method for this purpose. In this method, highly sensitive liquid chromatography-tandem mass spectrometry (LC-MS/MS) is used to quantify the relative masses of the radioactive isotopologue ([ 18 F] 1 ) and the accompanying nonradioactive counterpart (carrier 1 ) in an 18 F-labeled tracer preparation to give the mole ratio of [ 18 F] 1 . High-performance liquid chromatography (HPLC) with a mass-calibrated absorbance detection is used alongside to provide a separate measurement of the aggregate mass of all isotopologues. The radioactivity of the radiotracer is then derived in becquerels (Bq) from these two measurements, plus Avogadro's number and the decay constant of fluorine-18. For the chosen example [ 18 F]LSN3316612, the radioactivity values determined nonradiometrically and with a selected ionization chamber were in fair agreement. In addition, LC-MS/MS alone was found to provide an accurate measure of the half-life of fluorine-18., Competing Interests: The authors declare no competing financial interest., (Not subject to U.S. Copyright. Published 2022 by American Chemical Society.)

The translocator protein 18kDa (TSPO) is expressed in the outer mitochondrial membrane and is implicated in several functions, including cholesterol transport and steroidogenesis. Under normal physiological conditions, TSPO is present in very low concentrations in the human brain but is markedly upregulated in response to brain injury and inflammation. This upregulation is strongly associated with activated microglia. Therefore, TSPO is particularly suited for assessing active gliosis associated with brain lesions following injury or disease. For over three decades, TSPO has been studied as a biomarker. Numerous radioligands for positron emission tomography (PET) that target TSPO have been developed for imaging inflammatory progression in the brain. Although [ 11 C]PK11195, the prototypical first-generation PET radioligand, is still widely used for in vivo studies, mainly now as its single more potent R-enantiomer, it has severe limitations, including low sensitivity and poor amenability to quantification. Second-generation radioligands are characterized by higher TSPO specific signals but suffer from other drawbacks, such as sensitivity to the TSPO single nucleotide polymorphism (SNP) rs6971. Therefore, their applications in human studies have the burden of needing to genotype subjects. Consequently, recent efforts are focused on developing improved radioligands that combine the optimal features of the second generation with the ability to overcome the differences in binding affinities across the population. This review presents essential principles in the design and development of TSPO PET ligands and discusses prominent examples among the main chemotypes., (Copyright© Bentham Science Publishers; For any queries, please email at epub@benthamscience.net.)

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 NMDA receptor GluN2B subunit is a target of interest in neuropsychiatric disorders but to date there is no selective radiotracer available to quantify its availability in vivo . Here we report direct comparisons in non-human primates of three GluN2B-targeting radioligands: (R) -[ 11 C]NR2B-Me, (R) -[ 18 F]OF-Me-NB1, and (S) -[ 18 F]OF-NB1. Plasma free fraction, metabolism, tissue distribution and kinetics, and quantitative kinetic modeling methods and parameters were evaluated in two adult rhesus macaques. Free fraction in plasma was <2% for (R) -[ 11 C]NR2B-Me and (R) -[ 18 F]OF-Me-NB1 and higher for (S) -[ 18 F]OF-NB1 (15%). All radiotracers showed good brain uptake and distribution throughout grey matter, with substantial (>68%) blockade across the brain by the GluN2B-targeting drug Co-101,244 (0.25 mg/kg), including in the cerebellum. Time-activity curves were well-fitted by the one-tissue compartment model, with volume of distribution values of 20-40 mL/cm 3 for (R) -[ 11 C]NR2B-Me, 8-16 mL/cm 3 for (R) -[ 18 F]OF-Me-NB1, and 15-35 mL/cm 3 for (S) -[ 18 F]OF-NB1. Estimates of regional non-displaceable binding potential were in the range of 2-3 for (R) -[ 11 C]NR2B-Me and (S) -[ 18 F]-OF-NB1, and 0.5-1 for (R) -[ 18 F]OF-Me-NB1. Altogether, each radiotracer showed an acceptable profile for quantitative imaging of GluN2B. (S) -[ 18 F]OF-NB1 has particularly promising imaging characteristics for potential translation into humans. However, the source of unexpected displaceable binding in the cerebellum for each of these compounds requires further investigation.

Purpose: Cyclooxygenase-2 (COX-2) is a target for inflammation and colorectal cancer (CRC). This study evaluated the COX-2 neuro-PET radiopharmaceutical, [ 11 C]MC1, in CRC xenograft mice., Procedures: [ 11 C]MC1 was evaluated in ICRscid mice with HT-29 and HCT-116 CRC xenografts, with high and low COX-2 expression, respectively, by immunohistochemistry, cellular uptake, dynamic PET/MR imaging, ex vivo biodistribution, and radiometabolite analysis., Results: HT-29 xenografts were well visualized with [ 11 C]MC1 using PET/MR. Time-activity curves revealed steady tumor radioactivity accumulation in HT-29 xenografts that plateaued from 40 to 60 min (3.07 ± 0.65 %ID/g) and was significantly reduced by pre-treatment with MC1 or celecoxib (1.62 ± 0.29 and 1.18 ± 0.21 %ID/g, respectively, p = 0.045 and p = 0.005). Radiometabolite analysis showed that [ 11 C]MC1 accounted for >90 % of tumor radioactivity, with <10 % in plasma, at 40 min post-injection of the radiotracer., Conclusions: [ 11 C]MC1 is a promising PET imaging agent for COX-2 in CRC and translation for cancer research should be considered., (© 2021. World Molecular Imaging Society.)

The most frequently studied target of neuroinflammation using PET is 18-kDa translocator protein, but its limitations have spurred the molecular imaging community to find more promising targets. This article reviews the development of PET radioligands for cyclooxygenase (COX) subtypes 1 and 2, enzymes that catalyze the production of inflammatory prostanoids in the periphery and brain. Although both isozymes produce the same precursor compound, prostaglandin H 2 , they have distinct functions based on their differential cellular localization in the periphery and brain. For example, COX-1 is located primarily in microglia, a resident inflammatory cell in the brain whose role in producing inflammatory cytokines is well documented. In contrast, COX-2 is located primarily in neurons and can be markedly upregulated by inflammatory and excitatory stimuli, but its functions are poorly understood. This article reviews these 2 isozymes as biomarkers of neuroinflammation, as well as the radioligands that have recently been developed to image them in animals and humans. To place this work into context, the properties of COX-1 and COX-2 are compared with 18-kDa translocator protein, with special consideration of their application in Alzheimer disease as a representative neurodegenerative disorder., (© 2022 by the Society of Nuclear Medicine and Molecular Imaging.)

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.

Radiotracers labeled with carbon-11 (t 1/2 = 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 [ 11 C]ER176. This radiotracer is produced by rapid conversion of cyclotron-produced [ 11 C]carbon dioxide into [ 11 C]iodomethane, which is then used to treat N-desmethyl-ER176 in the presence of base ( t BuOK) at room temperature for 5 min. [ 11 C]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., (© 2021. This is a U.S. government work and not under copyright protection in the U.S.; foreign copyright protection may apply.)

Effective methods are needed for labelling acyclic ureas with carbon-11 (t 1/2 =20.4 min) as potential radiotracers for biomedical imaging with positron emission tomography (PET). Herein, we describe the rapid and high-yield syntheses of unsymmetrical acyclic [ 11 C]ureas under mild conditions (room temperature and within 7 min) using no-carrier-added [ 11 C]carbonyl difluoride with aliphatic and aryl amines. This methodology is compatible with diverse functionality (e. g., hydroxy, carboxyl, amino, amido, or pyridyl) in the substrate amines. The labelling process proceeds through putative [ 11 C]carbamoyl fluorides and for primary amines through isolable [ 11 C]isocyanate intermediates. Unsymmetrical [ 11 C]ureas are produced with negligible amounts of unwanted symmetrical [ 11 C]urea byproducts. Moreover, the overall labelling method tolerates trace water and the generally moderate to excellent yields show good reproducibility. [ 11 C]Carbonyl difluoride shows exceptional promise for application to the synthesis of acyclic [ 11 C]ureas as new radiotracers for biomedical imaging with PET., (© 2021 Wiley-VCH GmbH.)

Cyclooxygenase-1 (COX-1), a biomarker for neuroinflammation, is implicated in the progression and prognosis of ovarian cancer (OvCa). This study considered the repurposing of 11 C-labeled 1,5-bis(4-methoxyphenyl)-3-(2,2,2-trifluoroethoxy)-1 H -1,2,4-triazole ( 11 C-PS13), a COX-1 PET neuroimaging radiopharmaceutical, in OvCa xenograft mouse models. Methods: 11 C-PS13 was evaluated in ICRscid mice with subcutaneous or intraperitoneal human OVCAR-3 OvCa xenografts by dynamic PET/MRI, ex vivo biodistribution, and radiometabolite analysis of plasma and tumor. Results: OVCAR-3 xenografts were well visualized with 11 C-PS13 in xenograft mouse models. Time-activity curves revealed a steady accumulation of tumor radioactivity that plateaued from 40 to 60 min and was significantly reduced by pretreatment with ketoprofen (3.56 ± 0.81 and 1.30 ± 0.18 percentage injected dose/g without and with pretreatment, respectively, P = 0.01). Radiometabolite analysis showed that intact 11 C-PS13 accounted for more than 80% of radioactivity in the tumor, with less than 20% in plasma, at 40 min after injection. Conclusion: 11 C-PS13 shows promise for PET imaging of COX-1 in OvCa, and rapid translation for clinical cancer research should be considered., (© 2021 by the Society of Nuclear Medicine and Molecular Imaging.)

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.

McCune-Albright syndrome (MAS) is a mosaic disorder arising from gain-of-function mutations in the GNAS gene, which encodes the 3',5'-cyclic adenosine monophosphate (cAMP) pathway-associated G-protein, G s α. Clinical manifestations of MAS in a given individual, including fibrous dysplasia, are determined by the timing and location of the GNAS mutation during embryogenesis, the tissues involved, and the role of G s α in the affected tissues. The G s α mutation results in dysregulation of the cAMP signaling cascade, leading to upregulation of phosphodiesterase type 4 (PDE4), which catalyzes the hydrolysis of cAMP. Increased cAMP levels have been found in vitro in both animal models of fibrous dysplasia and in cultured cells from individuals with MAS but not in humans with fibrous dysplasia. PET imaging of PDE4 with 11 C-( R )-rolipram has been used successfully to study the in vivo activity of the cAMP cascade. To date, it remains unknown whether fibrous dysplasia and other symptoms of MAS, including neuropsychiatric impairments, are associated with increased PDE4 activity in humans. Methods: 11 C-( R )-rolipram whole-body and brain PET scans were performed on 6 individuals with MAS (3 for brain scans and 6 for whole-body scans) and 9 healthy controls (7 for brain scans and 6 for whole-body scans). Results: 11 C-( R )-rolipram binding correlated with known locations of fibrous dysplasia in the periphery of individuals with MAS; no uptake was observed in the bones of healthy controls. In peripheral organs and the brain, no difference in 11 C-( R )-rolipram uptake was noted between participants with MAS and healthy controls. Conclusion: This study is the first to find evidence for increased cAMP activity in areas of fibrous dysplasia in vivo. No differences in brain uptake between MAS participants and controls were detected-a finding that could be due to several reasons, including the limited anatomic resolution of PET. Nevertheless, the results confirm the usefulness of PET scans with 11 C-( R )-rolipram to indirectly measure increased cAMP pathway activation in human disease., (© 2020 by the Society of Nuclear Medicine and Molecular Imaging.)

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 (t 1/2  = 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 11 C-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 (A m ; 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 [ 11 C]PBR28 as a representative tracer. The method uses liquid chromatography-tandem mass spectrometry for measuring radiotracer A m and then the carrier in plasma sampled regularly over the course of a PET experiment. A m 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.