Robust Quantification of Phosphodiesterase-4D in Monkey Brain with PET and 11C-Labeled Radioligands That Avoid Radiometabolite Contamination.

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 [11C]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 11C (half-time, 20.4 min) in useful yields and with high molar activity. All displayed sizable PDE4D-specific signals in rhesus monkey brain. Notably, [11C]JMJ-81 and [11C]JMJ-129 exhibited excellent time stability of signal (total volume of distribution). Furthermore, as an example, [11C]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 [11C]JMJ-81 and [11C]JMJ-129 warrant further evaluation in human subjects. [11C]JMJ-129 shows a higher PDE4D specific-to-nonspecific binding ratio and will be the first to be evaluated.

Synthesis and preclinical evaluation of [11C]uPSEM792 for PSAM4-GlyR based chemogenetics.

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. PSAM4 is a triple mutant PSAM derived from the α7 nicotinic receptor (α7L131G,Q139L,Y217F). Although having no constitutive activity as a ligand-gated ion channel, PSAM4 has been coupled to the serotonin 5-HT3 receptor (5-HT3R) and to the glycine receptor (GlyR). Treatment with the partner PSEM to activate PSAM4-5-HT3 or PSAM4-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 PSAM4-GlyR, as a possible lead for PET radioligand development. We labeled uPSEM792 with the positron-emitter, carbon-11 (t1/2 = 20.4 min), in high radiochemical yield by treating a protected precursor with [11C]iodomethane followed by base deprotection. PET experiments with [11C]uPSEM792 in rodents and in a monkey transduced with PSAM4-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 PSAM4 based PET radioligand, based on the uPSEM792 chemotype.

In vivo evaluation of a novel 18F-labeled PET radioligand for translocator protein 18 kDa (TSPO) in monkey brain.

PURPOSE: [18F]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 [18F]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 (VT/fP) using a two-tissue compartment model. Receptor occupancy and nondisplaceable uptake were determined via Lassen plot. Binding potential (BPND) was calculated as the ratio of specific binding to nondisplaceable uptake. Time stability of VT was used as an indirect probe to detect radiometabolite accumulation in the brain. In vivo and ex vivo experiments were performed in mice to determine the distribution of the radioligand. RESULTS: After [18F]SF51 injection, the concentration of brain radioactivity peaked at 2.0 standardized uptake value (SUV) at ~ 10 min and declined to 30% of the peak at 180 min. VT/fP at baseline was generally high (203 ± 15 mL· cm-3) and decreased by ~ 90% after blockade with PK11195. BPND of the whole brain was 7.6 ± 4.3. VT values reached levels similar to terminal 180-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 [18F]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 [18F]SF51 warrants further evaluation in humans.

Candidate 3-benzazepine-1-ol type GluN2B receptor radioligands (11C-NR2B-Me enantiomers) have high binding in cerebellum but not to σ1 receptors.

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

Discovery of a High-Affinity Fluoromethyl Analog of [11C]5-Cyano-N-(4-(4-methylpiperazin-1-yl)-2-(piperidin-1-yl)phenyl)furan-2-carboxamide ([11C]CPPC) and Their Comparison in Mouse and Monkey as Colony-Stimulating Factor 1 Receptor Positron Emission Tomography Radioligands.

[11C]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 (IC50 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. [11C]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. [11C]CPPC also failed to show specific binding at baseline. Moreover, both [11C]Psa374 and [11C]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 [11C]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.

Whole-Body PET Imaging in Humans Shows That 11C-PS13 Is Selective for Cyclooxygenase-1 and Can Measure the In Vivo Potency of Nonsteroidal Antiinflammatory Drugs.

Both cyclooxygenase-1 (COX-1) and cyclooxygenase-2 (COX-2) convert arachidonic acid to prostaglandin H2, which has proinflammatory effects. The recently developed PET radioligand 11C-PS13 has excellent in vivo selectivity for COX-1 over COX-2 in nonhuman primates. This study sought to evaluate the selectivity of 11C-PS13 binding to COX-1 in humans and assess the utility of 11C-PS13 to measure the in vivo potency of nonsteroidal antiinflammatory drugs. Methods: Baseline 11C-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, 11C-PS13 uptake inhibition from blood cells was measured in vitro and ex vivo (i.e., using blood obtained during PET scanning). Results: High 11C-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 11C-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: 11C-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 11C-PS13.

First-in-Human Evaluation of 18F-PF-06445974, a PET Radioligand That Preferentially Labels Phosphodiesterase-4B.

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 18F-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 18F-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: 18F-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: 18F-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.

In Vivo Evaluation of 6 Analogs of 11C-ER176 as Candidate 18F-Labeled Radioligands for 18-kDa Translocator Protein.

Because of its excellent ratio of specific to nondisplaceable uptake, the radioligand 11C-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 18F-labeled TSPO PET radioligand based on ER176 with the potential for broader distribution. This study used generic 11C 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 18F. Methods: Six fluorine-containing analogs of ER176-3 fluoro and 3 trifluoromethyl isomers-were synthesized and labeled by 11C 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 [BPND]) and the time stability of total distribution volume (VT), an indirect measure of lack of radiometabolite accumulation in the brain. Results: Total TSPO binding was quantified as VT corrected for plasma free fraction (VT/fP) using Logan graphical analysis for all 6 radioligands. VT/fP was generally high at baseline (222 ± 178 mL·cm-3) and decreased by 70%-90% after preblocking with PK11195. BPND calculated using the Lassen plot was 9.6 ± 3.8; the o-fluoro radioligand exhibited the highest BPND (12.1), followed by the m-trifluoromethyl (11.7) and m-fluoro (8.1) radioligands. For all 6 radioligands, VT 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 BPND and good time stability of VT Among them, the o-fluoro, m-trifluoromethyl, and m-fluoro compounds were the 3 best candidates for development as radioligands with an 18F label.

Repurposing [11C]MC1 for PET Imaging of Cyclooxygenase-2 in Colorectal Cancer Xenograft Mouse Models.

PURPOSE: Cyclooxygenase-2 (COX-2) is a target for inflammation and colorectal cancer (CRC). This study evaluated the COX-2 neuro-PET radiopharmaceutical, [11C]MC1, in CRC xenograft mice. PROCEDURES: [11C]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 [11C]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 [11C]MC1 accounted for >90 % of tumor radioactivity, with <10 % in plasma, at 40 min post-injection of the radiotracer. CONCLUSIONS: [11C]MC1 is a promising PET imaging agent for COX-2 in CRC and translation for cancer research should be considered.

Synthesis and Screening in Mice of Fluorine-Containing PET Radioligands for TSPO: Discovery of a Promising 18F-Labeled Ligand.

Translocator protein 18 kDa (TSPO) is a biomarker of neuroinflammation. [11C]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 (t1/2 = 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 11C-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 [11C]ER176 ([11C]3b) displayed the most promising imaging properties. Therefore, a method was developed to label 3b with [18F]fluoride ion. [18F]3b gave similarly promising PET imaging results and deserves evaluation in higher species.

[11C]deschloroclozapine is an improved PET radioligand for quantifying a human muscarinic DREADD expressed in monkey brain.

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

Region- and voxel-based quantification in human brain of [18F]LSN3316612, a radioligand for O-GlcNAcase.

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

Synthesis of [18F]PS13 and Evaluation as a PET Radioligand for Cyclooxygenase-1 in Monkey.

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

Repurposing 11C-PS13 for PET Imaging of Cyclooxygenase-1 in Ovarian Cancer Xenograft Mouse Models.

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 11C-labeled 1,5-bis(4-methoxyphenyl)-3-(2,2,2-trifluoroethoxy)-1H-1,2,4-triazole (11C-PS13), a COX-1 PET neuroimaging radiopharmaceutical, in OvCa xenograft mouse models. Methods:11C-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 11C-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 11C-PS13 accounted for more than 80% of radioactivity in the tumor, with less than 20% in plasma, at 40 min after injection. Conclusion:11C-PS13 shows promise for PET imaging of COX-1 in OvCa, and rapid translation for clinical cancer research should be considered.

Development of a non-radiometric method for measuring the arterial input function of a 11C-labeled PET radiotracer.

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

PET measurement of cyclooxygenase-2 using a novel radioligand: upregulation in primate neuroinflammation and first-in-human study.

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 [11C]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 [11C]PS13. RESULTS: COX-2 binding, expressed as the ratio of specific to nondisplaceable uptake (BPND) of [11C]MC1, increased on day 1 post-LPS injection; no such increase in COX-1 expression, measured using [11C]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 BPND (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 [11C]MC1 can localize inflammation peripheral to the brain. In patients with rheumatoid arthritis, [11C]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 [11C]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.

First-in-human evaluation of [11C]PS13, a novel PET radioligand, to quantify cyclooxygenase-1 in the brain.

PURPOSE: This study assessed whether the newly developed PET radioligand [11C]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 (VT) were assessed. Correlation analyses were conducted between brain regional VT and COX-1 transcript levels provided in the Allen Human Brain Atlas. RESULTS: In the brain, [11C]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. VT values showed excellent test-retest variability (range 6.0-8.5%) and good reliability (intraclass correlation coefficient range 0.74-0.87). VT 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 VT. Significant correlation was observed between VT and COX-1 transcript levels (r = 0.82, P = 0.007), indicating that [11C]PS13 binding reflects actual COX-1 density in the human brain. CONCLUSIONS: These results from the first-in-human evaluation of the ability of [11C]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.

PET ligands [18F]LSN3316612 and [11C]LSN3316612 quantify O-linked-ß-N-acetyl-glucosamine hydrolase in the brain.

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

PET Imaging of Phosphodiesterase-4 Identifies Affected Dysplastic Bone in McCune-Albright Syndrome, a Genetic Mosaic Disorder.

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, Gsα. 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 Gsα in the affected tissues. The Gsα 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 11C-(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:11C-(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:11C-(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 11C-(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 11C-(R)-rolipram to indirectly measure increased cAMP pathway activation in human disease.

Discovery, Radiolabeling, and Evaluation of Subtype-Selective Inhibitors for Positron Emission Tomography Imaging of Brain Phosphodiesterase-4D.

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 (t1/2 = 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, [11C]T1660 and [11C]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. [11C]T1650 was progressed to evaluation in humans with PET, but the output measure of brain enzyme density (VT) 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 VT. 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.