First-in-Human PET Imaging of [18F]SDM-4MP3: A Cautionary Tale.

[18F]SynVesT-1 is a PET radiopharmaceutical that binds to the synaptic vesicle protein 2A (SV2A) and serves as a biomarker of synaptic density with widespread clinical research applications in psychiatry and neurodegeneration. The initial goal of this study was to concurrently conduct PET imaging studies with [18F]SynVesT-1 at our laboratories. However, the data in the first two human PET studies had anomalous biodistribution despite the injected product meeting all specifications during the prerelease quality control protocols. Further investigation, including imaging in rats as well as proton and carbon 2D-NMR spectroscopic studies, led to the discovery that a derivative of the precursor had been received from the manufacturer. Hence, we report our investigation and the first-in-human study of [18F]SDM-4MP3, a structural variant of [18F]SynVesT-1, which does not have the requisite characteristics as a PET radiopharmaceutical for imaging SV2A in the central nervous system.

Preliminary PET imaging of [11 C]evobrutinib in mouse models of colorectal cancer, SARS-CoV-2, and lung damage: Radiosynthesis via base-aided palladium-NiXantphos-mediated 11 C-carbonylation.

Evobrutinib is a second-generation, highly selective, irreversible Bruton's tyrosine kinase (BTK) inhibitor that has shown efficacy in the autoimmune diseases arthritis and multiple sclerosis. Its development as a positron emission tomography (PET) radiotracer has potential for in vivo imaging of BTK in various disease models including several cancers, severe acute respiratory syndrome-coronavirus-2 (SARS-CoV-2), and lipopolysaccharide (LPS)-induced lung damage. Herein, we report the automated radiosynthesis of [11 C]evobrutinib using a base-aided palladium-NiXantphos-mediated 11 C-carbonylation reaction. [11 C]Evobrutinib was reliably formulated in radiochemical yields of 5.5 ± 1.5% and a molar activity of 34.5 ± 17.3 GBq/µmol (n = 12) with 99% radiochemical purity. Ex vivo autoradiography studies showed high specific binding of [11 C]evobrutinib in HT-29 colorectal cancer mouse xenograft tissues (51.1 ± 7.1%). However, in vivo PET/computed tomography (CT) imaging with [11 C]evobrutinib showed minimal visualization of HT-29 colorectal cancer xenografts and only a slight increase in radioactivity accumulation in the associated time-activity curves. In preliminary PET/CT studies, [11 C]evobrutinib failed to visualize either SARS-CoV-2 pseudovirus infection or LPS-induced injury in mouse models. In conclusion, [11 C]evobrutinib was successfully synthesized by 11 C-carbonylation and based on our preliminary studies does not appear to be a promising BTK-targeted PET radiotracer in the rodent disease models studied herein.

Reactive Palladium-Ligand Complexes for 11C-Carbonylation at Ambient Pressure: A Breakthrough in Carbon-11 Chemistry.

The Pd-Xantphos-mediated 11C-carbonylation protocol (also known as the "Xantphos- method"), due to its simplistic and convenient nature, has facilitated researchers in meeting a longstanding need for preparing 11C-carbonyl-labeled radiopharmaceuticals at ambient pressure for positron emission tomography (PET) imaging and drug discovery. This development could be viewed as a breakthrough in carbon-11 chemistry, as evidenced by the rapid global adoption of the method by the pharmaceutical industry and academic laboratories worldwide. The method has been fully automated for the good manufacturing practice (GMP)-compliant production of novel radiopharmaceuticals for human use, and it has been adapted for "in-loop" reactions and microwave technology; an impressive number of 11C-labeled compounds (>100) have been synthesized. Given the simplicity and efficiency of the method, as well as the abundance of carbonyl groups in bioactive drug molecules, we expect that this methodology will be even more widely adopted in future PET radiopharmaceutical research and drug development.

In Silico Discovery and Subsequent Characterization of Potent 4R-Tauopathy Positron Emission Tomography Radiotracers.

A chemical fingerprint search identified Z3777013540 (1-(5-(6-fluoro-1H-indol-2-yl)pyrimidin-2-yl)piperidin-4-ol; 1) as a potential 4R-tau binding ligand. Binding assays in post-mortem Alzheimer's disease (AD), progressive supranuclear palsy (PSP), and corticobasal degeneration (CBD) brain with [3H]1 provided KD (nM) values in AD = 4.0, PSP = 5.1, and CBD = 4.5. In vivo positron emission tomography (PET) imaging in rats with [18F]1 demonstrated good brain penetration and rapid clearance from normal brain tissues. A subsequent molecular similarity search using 1 as the query revealed an additional promising compound, Z4169252340 (4-(5-(6-fluoro-1H-indol-2-yl)pyrimidin-2-yl)morpholine; 21). Binding assays with [3H]21 provided KD (nM) values in AD = 1.2, PSP = 1.6, and CBD = 1.7 and lower affinities for binding aggregated α-synuclein and amyloid-beta. PET imaging in rats with [18F]21 demonstrated a higher brain penetration than [18F]1 and rapid clearance from normal brain tissues. We anticipate that 1 and 21 will be useful for the identification of other potent novel 4R-tau radiotracers.

Spirocyclic Iodonium Ylides for Fluorine-18 Radiolabeling of Non-Activated Arenes: From Concept to Clinical Research.

Positron emission tomography (PET) is a powerful imaging tool for drug discovery, clinical diagnosis, and monitoring of disease progression. Fluorine-18 is the most common radionuclide used for PET, but advances in radiotracer development have been limited by the historical lack of methodologies and precursors amenable to radiolabeling with fluorine-18. Radiolabeling of electron-rich (hetero)aromatic rings remains a long-standing challenge in the production of PET radiopharmaceuticals. In this personal account, we discuss the history of spirocyclic iodonium ylide precursors, from inception to applications in clinical research, for the incorporation of fluorine-18 into complex non-activated (hetero)aromatic rings.

Strategies for designing novel positron emission tomography (PET) radiotracers to cross the blood-brain barrier.

Positron emission tomography (PET) is a powerful tool for imaging biological processes in the central nervous system (CNS). Designing PET radiotracers capable of crossing the blood-brain barrier (BBB) remains a major challenge. In addition to being brain-penetrant, a quantifiable CNS PET radiotracer must have high target affinity and selectivity, appropriate pharmacokinetics, minimal non-specific binding, negligible radiometabolites in the brain, and generally must be amenable to labeling with carbon-11 (11 C) or fluorine-18 (18 F). This review aims to give an overview of some of the critical physicochemical and biochemical contributors specific for CNS PET radiotracer design and how they can differ from pharmaceutical drug development, including in vitro assays, in silico predictions, and in vivo studies, with examples for how such methods can be implemented to optimize brain uptake of radiotracers based on experiences from our neuroimaging program.

Recent Developments in Carbon-11 Chemistry and Applications for First-In-Human PET Studies.

Positron emission tomography (PET) is a molecular imaging technique that makes use of radiolabelled molecules for in vivo evaluation. Carbon-11 is a frequently used radionuclide for the labelling of small molecule PET tracers and can be incorporated into organic molecules without changing their physicochemical properties. While the short half-life of carbon-11 (11C; t½ = 20.4 min) offers other advantages for imaging including multiple PET scans in the same subject on the same day, its use is limited to facilities that have an on-site cyclotron, and the radiochemical transformations are consequently more restrictive. Many researchers have embraced this challenge by discovering novel carbon-11 radiolabelling methodologies to broaden the synthetic versatility of this radionuclide. This review presents new carbon-11 building blocks and radiochemical transformations as well as PET tracers that have advanced to first-in-human studies over the past five years.

Work incapacity and psychiatric patient care following attempted suicide - a cohort study of 65 097 Swedish twins.

BACKGROUND: Research is scarce on the role of familial factors and previous psychiatric care on the association between suicide attempt and future work incapacity as well as deterioration in mental health. We aimed to investigate the associations between suicide attempt and sickness absence, disability pension and psychiatric patient care and to study the influence of previous psychiatric care and familial factors (genetics and shared environment) on the associations. METHODS: The study included 65 097 twins living in Sweden on 31st of December 2006, aged 19-60 years. The twins were followed 2007-2013 regarding sickness absence, disability pension, inpatient care or specialized outpatient care for a mental diagnosis. Cox regression models were performed for the whole sample, and conditional models for discordant twin pairs. The analyses were also stratified by psychiatric care before 2007. RESULTS: We found that suicide attempt predicted sickness absence, disability pension, and future mental diagnosis among the whole sample. The discordant twin pair analyses showed that the association between suicide attempt and sickness absence or disability pension was influenced by familial factors. Stratified analyses of individuals with or without psychiatric care before 2007 showed that previous psychiatric care had some impact on the associations. CONCLUSIONS: A suicide attempt is a risk factor for work incapacity and psychiatric patient care. Familial factors and previous psychiatric care play a role in the associations between attempting suicide and work incapacity as well as psychiatric patient care. These factors are important when developing measures preventing work incapacity among those with a suicide attempt.

Ring-opening of non-activated aziridines with [11C]CO2 via novel ionic liquids.

Novel ionic liquids based on DBU and DBN halide salts were developed as a catalytic system for ring-opening of non-activated aziridines with [11C]CO2. The ability of ionic liquids to activate aziridines represents a simple methodology for the synthesis of 11C-carbamates and can be extended for CO2-fixation in organic and radiochemistry.

Preliminary Evaluations of [11C]Verubulin: Implications for Microtubule Imaging With PET.

[11C]Verubulin (a.k.a.[11C]MCP-6827), [11C]HD-800 and [11C]colchicine have been developed for imaging microtubules (MTs) with positron emission tomography (PET). The objective of this work was to conduct an in vivo comparison of [11C]verubulin for MT imaging in mouse and rat brain, as well as an in vitro study with this radiotracer in rodent and human Alzheimer's Disease tissue. Our preliminary PET imaging studies of [11C]verubulin in rodents revealed contradictory results between mouse and rat brain uptake under pretreatment conditions. In vitro autoradiography with [11C]verubulin showed an unexpected higher uptake in AD patient tissue compared with healthy controls. We also conducted the first comparative in vivo PET imaging study with [11C]verubulin, [11C]HD-800 and [11C]colchicine in a non-human primate. [11C]Verubulin and [11C]HD-800 require pharmacokinetic modeling and quantification studies to understand the role of how these radiotracers bind to MTs before translation to human use.

Radiosynthesis, In Vitro and In Vivo Evaluation of [18F]CBD-2115 as a First-in-Class Radiotracer for Imaging 4R-Tauopathies.

CBD-2115 was selected from a library of 148 compounds based on a pyridinyl-indole scaffold as a first-in-class 4R-tau radiotracer. In vitro binding assays showed [3H]CBD-2115 had a KD value of 6.9 nM and a nominal Bmax of 500 nM in 4R-tau expressing P301L transgenic mouse tissue. In binding assays with human brain tissue homogenates, [3H]CBD-2115 has a higher affinity (4.9 nM) for progressive supranuclear palsy specific 4R-tau deposits than [3H]flortaucipir (45 nM) or [3H]MK-6240 (>50 nM). [18F]CBD-2115 was reliably synthesized (3-11% radiochemical yield with molar activity of 27-111 GBq/µmol and >97% radiochemical purity). Dynamic PET imaging was conducted in mice, rats, and nonhuman primates, and all species showed initial brain uptake of 0.5-0.65 standardized uptake value with fast clearance from normal tissues. [3H]CBD-2115 could be a useful lead radioligand for further research in 4R-tauopathies, and PET radiotracer development will focus on improving brain uptake and binding affinity.

Development of a 18F-labeled PET radioligand for imaging 5-HT1B receptors: [18F]AZ10419096.

INTRODUCTION: In the last decade PET has been useful in studying and understanding the 5-HT1B receptor. [11C]AZ10419369 and [11C]P943 have been applied as radioligands in these studies. Both use carbon-11 (t1/2 = 20.4 min) as radionuclide, which limits the application to PET centres that have an on-site cyclotron and radiochemistry facilities. In this paper we report the synthesis and initial evaluation of the first fluorine-18 PET radioligand to image 5-HT1B receptors in brain, [18F]AZ10419096. MATERIALS AND METHODS: A boronate-precursor for [18F]AZ10419096 was synthesized from an intermediate provided by AstraZeneca and was labeled with fluorine 18 using Cu-mediated radio-fluorination. [18F]AZ10419096 was used in PET baseline and pretreatment measurements in nonhuman primates. PET data were analyzed using SRTM using the cerebellum as reference region. Blood samples for radio-metabolite analysis were collected during PET measurements. RESULTS: Radio-fluorination gave [18F]AZ10419096 in sufficient amounts and molar activity and with high radiochemical purity to be applied in PET measurements. In a baseline PET measurement [18F]AZ10419096 showed a high brain uptake and regional distribution consistent with reported 5-HT1B receptor densities. In a pretreatment PET measurement, AR-A000002 (2.0 mg/kg) blocked the binding of [18F]AZ10419096 to 5-HT1B receptors in occipital cortex by 80%, thereby demonstrating high specific binding. CONCLUSION: [18F]AZ10419096 is the first fluorine-18 PET radioligand for imaging 5-HT1B receptors in vivo with high specific binding and binding potential. [18F]AZ10419096 is a candidate for further development for use in clinical PET studies.

Potential for imaging the high-affinity state of the 5-HT1B receptor: a comparison of three PET radioligands with differing intrinsic activity.

BACKGROUND: Over the last decade, a few radioligands have been developed for PET imaging of brain 5-HT1B receptors. The 5-HT1B receptor is a G-protein-coupled receptor (GPCR) that exists in two different agonist affinity states. An agonist ligand is expected to be more sensitive towards competition from another agonist, such as endogenous 5-HT, than an antagonist ligand. It is of interest to know whether the intrinsic activity of a PET radioligand for the 5-HT1B receptor impacts on its ability to detect changes in endogenous synaptic 5-HT density. Three high-affinity 11C-labeled 5-HT1B PET radioligands with differing intrinsic activity were applied to PET measurements in cynomolgus monkey to evaluate their sensitivity to be displaced within the brain by endogenous 5-HT. For these experiments, fenfluramine was pre-administered at two different doses (1.0 and 5.0 mg/kg, i.v.) to induce synaptic 5-HT release. RESULTS: A dose-dependent response to fenfluramine was detected for all three radioligands. At the highest dose of fenfluramine (5.0 mg/kg, i.v.), reductions in specific binding in the occipital cortex increased with radioligand agonist efficacy, reaching 61% for [11C]3. The most antagonistic radioligand showed the lowest reduction in specific binding. CONCLUSIONS: Three 5-HT1B PET radioligands were identified with differing intrinsic activity that could be used in imaging high- and low-affinity states of 5-HT1B receptors using PET. From this limited study, radioligand sensitivity to endogenous 5-HT appears to depend on agonist efficacy. More extensive studies are required to substantiate this suggestion.

Synthesis and evaluation of two new candidate high-affinity full agonist PET radioligands for imaging 5-HT1B receptors.

INTRODUCTION: The serotonin 1B receptor subtype is of interest in the pathophysiology and treatment of depression, anxiety, and migraine. Over recent years 5-HT1B receptor binding in human brain has been examined with PET using radioligands that are partial but not full agonists. To explore how the intrinsic activity of a PET radioligand may affect imaging performance, two high-affinity full 5-HT1B receptor agonists (AZ11136118, 4; and AZ11895987, 5) were selected from a large compound library and radiolabeled for PET examination in non-human primates. METHODS: [11C]4 was obtained through Pd(0)-mediated insertion of [11C]carbon monoxide between prepared iodoarene and homochiral amine precursors. [11C]5 was obtained through N-11C-methylation of N-desmethyl precursor 6 with [11C]methyl triflate. [11C]4 and [11C]5 were studied with PET in rhesus or cynomolgus monkey. [11C]4 was studied with PET in mice and rats to measure brain uptake and specific binding. Ex-vivo experiments in rats were performed to identify whether there were radiometabolites in brain. Physiochemical parameters for [11C]4 (pKa, logD and conformational energetics) were evaluated. RESULTS: Both [11C]4 and [11C]5 were successfully produced in high radiochemical purity and in adequate amounts for PET experiments. After intravenous injection of [11C]4, brain radioactivity peaked at a low level (0.2 SUV). Pretreatment with tariquidar, an inhibitor of the brain P-gp efflux transporter, increased brain exposure four-fold whereas pretreatment with a high pharmacological dose of the 5-HT1B antagonist, AR-A000002, had no effect on the binding. Ex-vivo experiments in rats showed no radiometabolites entering brain. [11C]5 also failed to enter monkey brain under baseline conditions. CONCLUSIONS: [11C]4 and [11C]5 show too low brain uptake and specific binding to be useful PET radioligands. Low brain uptake is partly ascribed to efflux transporter action as well as unfavorable conformations.

[11C]AZ10419096 - a full antagonist PET radioligand for imaging brain 5-HT1B receptors.

INTRODUCTION: The serotonergic system is widely present in all regions of the central nervous system (CNS) and plays a key modulatory role in many of its functions. Positron emission tomography (PET) is used to study several serotonin receptors in CNS in vivo. The G-protein coupled receptor 5-HT1B is mostly present in the occipital cortex and in midbrain and is linked to several psychiatric disorders. There is evidence that agonist PET radioligands for neuroreceptors are more sensitive to endogenous neurotransmitters than antagonists. Our previously developed 5-HT1B receptor PET radioligand, [11C]AZ10419369, is now considered a partial agonist. In this work we are aiming to develop a full antagonist PET radioligand for imaging brain 5-HT1B receptors, and evaluate its sensitivity to increased endogenous serotonin concentration. MATERIALS: [11C]AZ10419096 was synthesized by rapid methylation of the prepared corresponding N-desmethyl precursor with [11C]methyl triflate. Five PET measurements were performed in cynomolgus monkeys, consisting of two at baseline, one after treatment of a monkey with a 5-HT1B antagonist, AR-A000002, and two in which fenfluramine was administered during scanning to induce endogenous serotonin release. RESULTS AND DISCUSSION: [11C]AZ10419096 was synthesized in high yield and purity within 30 min, including purification, formulation and sterile filtration. The baseline PET measurements demonstrated [11C]AZ10419096 to have favorable radioligand characteristics, including high specific binding in brain regions that have high 5-HT1B density, such as occipital cortex and globus pallidus, as well as subsequent rapid elimination from brain and a minor abundance of lipophilic radiometabolites in plasma. AR-A00002 completely blocked radioligand receptor-specific binding. Fenfluramine produced a distinct displacement of radioligand consistent with an expected increase of synaptic endogenous serotonin concentration. CONCLUSIONS: [11C]AZ10419096, a full 5-HT1B antagonist PET radioligand, demonstrates high specific binding in monkey brain that is sensitive to competition from a known 5-HT1B antagonist as well as to putatively increased endogenous serotonin levels.