Synthesis and preliminary evaluation of novel 11C-labeled GluN2B-selective NMDA receptor negative allosteric modulators.

N-methyl-D-aspartate receptors (NMDARs) play critical roles in the physiological function of the mammalian central nervous system (CNS), including learning, memory, and synaptic plasticity, through modulating excitatory neurotransmission. Attributed to etiopathology of various CNS disorders and neurodegenerative diseases, GluN2B is one of the most well-studied subtypes in preclinical and clinical studies on NMDARs. Herein, we report the synthesis and preclinical evaluation of two 11C-labeled GluN2B-selective negative allosteric modulators (NAMs) containing N,N-dimethyl-2-(1H-pyrrolo[3,2-b]pyridin-1-yl)acetamides for positron emission tomography (PET) imaging. Two PET ligands, namely [11C]31 and [11C]37 (also called N2B-1810 and N2B-1903, respectively) were labeled with [11C]CH3I in good radiochemical yields (decay-corrected 28% and 32% relative to starting [11C]CO2, respectively), high radiochemical purity (>99%) and high molar activity (>74 GBq/µmol). In particular, PET ligand [11C]31 demonstrated moderate specific binding to GluN2B subtype by in vitro autoradiography studies. However, because in vivo PET imaging studies showed limited brain uptake of [11C]31 (up to 0.5 SUV), further medicinal chemistry and ADME optimization are necessary for this chemotype attributed to low binding specificity and rapid metabolism in vivo.

Synthesis and pharmacokinetic study of a 11C-labeled cholesterol 24-hydroxylase inhibitor using 'in-loop' [11C]CO2 fixation method.

Cholesterol 24-hydroxylase, also known as CYP46A1 (EC 1.14.13.98), is a monooxygenase and a member of the cytochrome P450 family. CYP46A1 is specifically expressed in the brain where it controls cholesterol elimination by producing 24S-hydroxylcholesterol (24-HC) as the major metabolite. Modulation of CYP46A1 activity may affect Aß deposition and p-tau accumulation by changing 24-HC formation, which thereafter serves as potential therapeutic pathway for Alzheimer's disease. In this work, we showcase the efficient synthesis and preliminary pharmacokinetic evaluation of a novel cholesterol 24-hydroxylase inhibitor 1 for use in positron emission tomography.

A concisely automated synthesis of TSPO radiotracer [18 F]FDPA based on spirocyclic iodonium ylide method and validation for human use.

Fluorine-18 labeled N,N-diethyl-2-(2-(4-(2-fluoroethoxy)phenyl)-5,7-dimethylpyrazolo[1,5-a]pyrimidin-3-yl)acetamide ([18 F]FDPA) is a potent and selective radiotracer for positron-emission tomography (PET) imaging of the translocator protein 18 kDa (TSPO). Our previous in vitro and in vivo evaluations have proven that this tracer is promising for further human translation. Our study addresses the need to streamline the automatic synthesis of this radiotracer to make it more accessible for widespread clinical evaluation and application. Here, we successfully demonstrate a one-step radiolabeling of [18 F]FDPA based on a novel spirocyclic iodonium ylide (SCIDY) precursor using tetra-n-butyl ammonium methanesulfonate (TBAOMs), which has demonstrated the highest radiochemical yields and molar activity from readily available [18 F]fluoride ion. The nucleophilic radiofluorination was completed on a GE TRACERlab FX2 N synthesis module, and the formulated [18 F]FDPA was obtained in nondecay corrected (n.d.c) radiochemical yields of 15.6 ± 4.2%, with molar activities of 529.2 ± 22.5 GBq/µmol (14.3 ± 0.6 Ci/µmol) at the end of synthesis (60 minutes, n = 3) and validated for human use. This methodology facilitates efficient synthesis of [18 F]FDPA in a commercially available synthesis module, which would be broadly applicable for routine production and widespread clinical PET imaging studies.

cGMP production of the radiopharmaceutical [18 F]MK-6240 for PET imaging of human neurofibrillary tangles.

Fluorine-18-labelled 6-(fluoro)-3-(1H-pyrrolo[2,3-c]pyridin-1-yl)isoquinolin-5-amine ([18 F]MK-6240) is a novel potent and selective positron emission tomography (PET) radiopharmaceutical for detecting human neurofibrillary tangles, which are made up of aggregated tau protein. Herein, we report the fully automated 2-step radiosynthesis of [18 F]MK-6240 using a commercially available radiosynthesis module, GE Healthcare TRACERlab FXFN . Nucleophilic fluorination of the 5-diBoc-6-nitro precursor with potassium cryptand [18 F]fluoride (K[18 F]/K222 ) was performed by conventional heating, followed by acid deprotection and semipreparative high-performance liquid chromatography under isocratic conditions. The isolated product was diluted with formulation solution and sterile filtered under Current Good Manufacturing Practices, and quality control procedures were established to validate this radiopharmaceutical for human use. At the end of synthesis, 6.3 to 9.3 GBq (170-250 mCi) of [18 F]MK-6240 was formulated and ready for injection, in an uncorrected radiochemical yield of 7.5% ± 1.9% (relative to starting [18 F]fluoride) with a specific activity of 222 ± 67 GBq/µmol (6.0 ± 1.8 Ci/µmol) at the end of synthesis (90 minutes; n = 3). [18 F]MK-6240 was successfully validated for human PET studies meeting all Food and Drug Administration and United States Pharmacopeia requirements for a PET radiopharmaceutical. The present method can be easily adopted for use with other radiofluorination modules for widespread clinical research use.

Microfluidic radiosynthesis of [18F]FEMPT, a high affinity PET radiotracer for imaging serotonin receptors.

Continuous-flow microfluidics has shown increased applications in radiochemistry over the last decade, particularly for both pre-clinical and clinical production of fluorine-18 labeled radiotracers. The main advantages of microfluidics are the reduction in reaction times and consumption of reagents that often result in increased radiochemical yields and rapid optimization of reaction parameters for 18F-labeling. In this paper, we report on the two-step microfluidic radiosynthesis of the high affinity partial agonist of the serotonin 1A receptor, [18F]FEMPT (pKi = 9. 79; Ki = 0.16 nM) by microfluidic radiochemistry. [18F]FEMPT was obtained in ≈7% isolated radiochemical yield and in >98% radiochemical and chemical purity. The molar activity of the final product was determined to be >148 GBq/µmol (>4 Ci/µmol).

Preclinical PET Neuroimaging of [11C]Bexarotene.

Activation of retinoid X receptors (RXRs) has been proposed as a therapeutic mechanism for the treatment of neurodegeneration, including Alzheimer's and Parkinson's diseases. We previously reported radiolabeling of a Food and Drug Administration-approved RXR agonist, bexarotene, by copper-mediated [(11)C]CO2 fixation and preliminary positron emission tomography (PET) neuroimaging that demonstrated brain permeability in nonhuman primate with regional binding distribution consistent with RXRs. In this study, the brain uptake and saturability of [(11)C]bexarotene were studied in rats and nonhuman primates by PET imaging under baseline and greater target occupancy conditions. [(11)C]Bexarotene displays a high proportion of nonsaturable uptake in the brain and is unsuitable for RXR occupancy measurements in the central nervous system.

Synthesis of 18F-Arenes from Spirocyclic Iodonium(III) Ylides via Continuous-Flow Microfluidics.

Spirocyclic hypervalent iodine(III) ylides have proven to be synthetically versatile precursors for efficient radiolabelling of a diverse range of non-activated (hetero)arenes, highly functionalised small molecules, building blocks and radiopharmaceuticals from [18F]fluoride ion. Herein, we report the implementation of these reactions onto a continuous-flow microfluidic platform, thereby offering an alterative and automated synthetic procedure of a radiopharmaceutical, 3-[18F]fluoro-5-[(pyridin-3-yl)ethynyl]benzonitrile ([18F]FPEB) and a routinely used building block for click-radiochemistry, 4-[18F]fluorobenzyl azide. This new protocol was applied to the synthesis of [18F]FPEB (radiochemical conversion (RCC) = 68 ± 5%) and 4-[18F]fluorobenzyl azide (RCC=68 ± 5%; isolated radiochemical yield = 24±0%). We anticipate that the high throughput microfluidic platform will accelerate the discovery and applications of 18F-labelled building blocks and labelled compounds prepared by iodonium ylide precursors as well as the production of radiotracers for preclinical imaging studies.

Synthesis and Preliminary Evaluation of [11 C]GNE-1023 as a Potent PET Probe for Imaging Leucine-Rich Repeat Kinase†2 (LRRK2) in Parkinson's Disease.

Leucine-rich repeat kinase 2 (LRRK2) is a large protein involved in the pathogenesis of Parkinson's disease (PD). It has been demonstrated that PD is mainly conferred by LRRK2 mutations that bring about increased kinase activity. As a consequence, selective inhibition of LRRK2 may help to recover the normal functions of LRRK2, thereby serving as a promising alternative therapeutic target for PD treatment. The mapping of LRRK2 by positron emission tomography (PET) studies allows a thorough understanding of PD and other LRRK2-related disorders; it also helps to validate and translate novel LRRK2 inhibitors. However, no LRRK2 PET probes have yet been reported in the primary literature. Herein we present a facile synthesis and preliminary evaluation of [11 C]GNE-1023 as a novel potent PET probe for LRRK2 imaging in PD. [11 C]GNE-1023 was synthesized in good radiochemical yield (10 % non-decay-corrected RCY), excellent radiochemical purity (>99 %), and high molar activity (>37 GBq µmol-1 ). Excellent in vitro binding specificity of [11 C]GNE-1023 toward LRRK2 was demonstrated in cross-species studies, including rat and nonhuman primate brain tissues by autoradiography experiments. Subsequent whole-body biodistribution studies indicated limited brain uptake and urinary and hepatobiliary elimination of this radioligand. This study may pave the way for further development of a new generation of LRRK2 PET probes.

Design, Synthesis, and Evaluation of Reversible and Irreversible Monoacylglycerol Lipase Positron Emission Tomography (PET) Tracers Using a "Tail Switching" Strategy on a Piperazinyl Azetidine Skeleton.

Monoacylglycerol lipase (MAGL) is a serine hydrolase that degrades 2-arachidonoylglycerol (2-AG) in the endocannabinoid system (eCB). Selective inhibition of MAGL has emerged as a potential therapeutic approach for the treatment of diverse pathological conditions, including chronic pain, inflammation, cancer, and neurodegeneration. Herein, we disclose a novel array of reversible and irreversible MAGL inhibitors by means of "tail switching" on a piperazinyl azetidine scaffold. We developed a lead irreversible-binding MAGL inhibitor 8 and reversible-binding compounds 17 and 37, which are amenable for radiolabeling with 11C or 18F. [11C]8 ([11C]MAGL-2-11) exhibited high brain uptake and excellent binding specificity in the brain toward MAGL. Reversible radioligands [11C]17 ([11C]PAD) and [18F]37 ([18F]MAGL-4-11) also demonstrated excellent in vivo binding specificity toward MAGL in peripheral organs. This work may pave the way for the development of MAGL-targeted positron emission tomography tracers with tunability in reversible and irreversible binding mechanisms.

Synthesis and preliminary PET imaging of 11C and 18F isotopologues of the ROS1/ALK inhibitor lorlatinib.

Lorlatinib (PF-06463922) is a next-generation small-molecule inhibitor of the orphan receptor tyrosine kinase c-ros oncogene 1 (ROS1), which has a kinase domain that is physiologically related to anaplastic lymphoma kinase (ALK), and is undergoing Phase I/II clinical trial investigations for non-small cell lung cancers. An early goal is to measure the concentrations of this drug in brain tumour lesions of lung cancer patients, as penetration of the blood-brain barrier is important for optimal therapeutic outcomes. Here we prepare both 11C- and 18F-isotopologues of lorlatinib to determine the biodistribution and whole-body dosimetry assessments by positron emission tomography (PET). Non-traditional radiolabelling strategies are employed to enable an automated multistep 11C-labelling process and an iodonium ylide-based radiofluorination. Carbon-11-labelled lorlatinib is routinely prepared with good radiochemical yields and shows reasonable tumour uptake in rodents. PET imaging in non-human primates confirms that this radiotracer has high brain permeability.

Development of [18F]Maleimide-Based Glycogen Synthase Kinase-3ß Ligands for Positron Emission Tomography Imaging.

Dysregulation of glycogen synthase kinase-3ß (GSK-3ß) is implicated in the pathogenesis of neurodegenerative and psychiatric disorders. Thus, development of GSK-3ß radiotracers for positron emission tomography (PET) imaging is of paramount importance, because such a noninvasive imaging technique would allow better understanding of the link between the activity of GSK-3ß and central nervous system disorders in living organisms, and it would enable early detection of the enzyme's aberrant activity. Herein, we report the synthesis and biological evaluation of a series of fluorine-substituted maleimide derivatives that are high-affinity GSK-3ß inhibitors. Radiosynthesis of a potential GSK-3ß tracer [18F]10a is achieved. Preliminary in vivo PET imaging studies in rodents show moderate brain uptake, although no saturable binding was observed in the brain. Further refinement of the lead scaffold to develop potent [18F]-labeled GSK-3 radiotracers for PET imaging of the central nervous system is warranted.

Design and Prototype of an Automated Column-Switching HPLC System for Radiometabolite Analysis.

Column-switching high performance liquid chromatography (HPLC) is extensively used for the critical analysis of radiolabeled ligands and their metabolites in plasma. However, the lack of streamlined apparatus and consequently varying protocols remain as a challenge among positron emission tomography laboratories. We report here the prototype apparatus and implementation of a fully automated and simplified column-switching procedure to allow for the easy and automated determination of radioligands and their metabolites in up to 5 mL of plasma. The system has been used with conventional UV and coincidence radiation detectors, as well as with a single quadrupole mass spectrometer.

Synthesis and Preliminary PET Imaging Studies of a FAAH Radiotracer ([¹¹C]MPPO) Based on α-Ketoheterocyclic Scaffold.

Fatty acid amide hydrolase (FAAH) is one of the principle enzymes for metabolizing endogenous cannabinoid neurotransmitters such as anandamide, and thus regulates endocannabinoid (eCB) signaling. Selective pharmacological blockade of FAAH has emerged as a potential therapy to discern the endogenous functions of anandamide-mediated eCB pathways in anxiety, pain, and addiction. Quantification of FAAH in the living brain by positron emission tomography (PET) would help our understanding of the endocannabinoid system in these conditions. While most FAAH radiotracers operate by an irreversible ("suicide") binding mechanism, a FAAH tracer with reversibility would facilitate quantitative analysis. We have identified and radiolabeled a reversible FAAH inhibitor, 7-(2-[(11)C]methoxyphenyl)-1-(5-(pyridin-2-yl)oxazol-2-yl)heptan-1-one ([(11)C]MPPO) in 13% radiochemical yield (nondecay corrected) with >99% radiochemical purity and 2 Ci/µmol (74 GBq/µmol) specific activity. The tracer showed moderate brain uptake (0.8 SUV) with heterogeneous brain distribution. However, blocking studies with a potent FAAH inhibitor URB597 demonstrated a low to modest specificity to the target. Measurement of lipophilicity, metabolite, and efflux pathway analysis were also performed to study the pharmacokinetic profile of [(11)C]MPPO. In all, we reported an efficient radiolabeling and preliminary evaluation of the first-in-class FAAH inhibitor [(11)C]MPPO with α-ketoheterocyclic scaffold.

Enhanced copper-mediated (18)F-fluorination of aryl boronic esters provides eight radiotracers for PET applications.

[(18)F]FMTEB, [(18)F]FPEB, [(18)F]flumazenil, [(18)F]DAA1106, [(18)F]MFBG, [(18)F]FDOPA, [(18)F]FMT and [(18)F]FDA are prepared from the corresponding arylboronic esters and [(18)F]KF/K222 in the presence of Cu(OTf)2py4. The method was successfully applied using three radiosynthetic platforms, and up to 26 GBq of non-carrier added starting activity of (18)F-fluoride.

Diversity-oriented approach to CF3CHF-, CF3CFBr-, CF3CF2-, (CF3)2CH-, and CF3(SCF3)CH-substituted arenes from 1-(diazo-2,2,2-trifluoroethyl)arenes.

Arenes substituted with perfluoroalkyl groups are attractive targets for drug and agrochemical development. Exploiting the carbenic character of donor/acceptor diazo compounds, a diversity-oriented synthesis of perfluoroalkylated arenes, for late stage fluorofunctionalization, is described. The reaction of 1-(diazo-2,2,2-trifluoroethyl)arenes with HF, F/Br, F2, CF3H, and CF3SH sources give direct access to a variety of perfluoroalkyl-substituted arenes presenting with incremental fluorine content. The value of this approach is also demonstrated for radiochemistry and positron emission tomography with the [(18)F]-labeling of CF3CHF-, CF3CBrF-, and CF3CF2-arenes from [(18)F]fluoride.

Synthesis of [(11)C]Bexarotene by Cu-Mediated [(11)C]Carbon Dioxide Fixation and Preliminary PET Imaging.

Bexarotene (Targretin) is a retinoid X receptor (RXR) agonist that has applications for treatment of T cell lymphoma and proposed mechanisms of action in Alzheimer's disease that have been the subject of recent controversy. Carbon-11 labeled bexarotene ([(11)C-carbonyl]4-[1-(3,5,5,8,8-pentamethyltetralin-2-yl)ethenyl]benzoic acid) was synthesized using a Cu-mediated cross-coupling reaction employing an arylboronate precursor 1 and [(11)C]carbon dioxide under atmospheric pressure in 15 ± 2% uncorrected radiochemical yield (n = 3), based on [(11)C]CO2. Judicious choice of solvents, catalysts, and additives, as well as precursor concentration and purity of [(11)C]CO2, enabled the preparation of this (11)C-labeled carboxylic acid. Formulated [(11)C]bexarotene was isolated (>37 mCi) with >99% radiochemical purity in 32 min. Preliminary positron emission tomography-magnetic resonance imaging revealed rapid brain uptake in nonhuman primate in the first 75 s following intravenous administration of the radiotracer (specific activity >0.3 Ci/µmol at time of injection), followed by slow clearance (Δ = -43%) over 60 min. Modest uptake (SUVmax = 0.8) was observed in whole brain and regions with high RXR expression.

Microfluidic continuous-flow radiosynthesis of [18F]FPEB suitable for human PET imaging.

The synthesis of fluorine-18 labeled 3-fluoro-5-[(pyridin-3-yl)ethynyl] benzonitrile ([18F]FPEB) for imaging metabotropic glutamate receptor subtype type 5 (mGluR5) was achieved with a commercial continuous-flow microfluidics device. This work represents the first positron emission tomography (PET) radiopharmaceutical that is suitable for human use with this technology. We also describe a validated synthesis of [18F]FPEB with a commercial reactor-based system.

Rapid microfluidic flow hydrogenation for reduction or deprotection of 18F-labeled compounds.

We have combined the benefits of both microfluidics and flow hydrogenation to provide facile access to previously underutilized reduction and protecting group chemistries for PET imaging applications. The rapid removal of an O-benzyl protecting group to prepare 2-[(18)F]fluoroquinolin-8-ol and the reduction of a nitro group in the synthesis of 4-[(18)F]fluoroaniline were achieved within 3 minutes.

11CO2 fixation: a renaissance in PET radiochemistry.

Carbon-11 labelled carbon dioxide is the cyclotron-generated feedstock reagent for most positron emission tomography (PET) tracers using this radionuclide. Most carbon-11 labels, however, are installed using derivative reagents generated from [(11)C]CO2. In recent years, [(11)C]CO2 has seen a revival in applications for the direct incorporation of carbon-11 into functional groups such as ureas, carbamates, oxazolidinones, carboxylic acids, esters, and amides. This review summarizes classical [(11)C]CO2 fixation strategies using organometallic reagents and then focuses on newly developed methods that employ strong organic bases to reversibly capture [(11)C]CO2 into solution, thereby enabling highly functionalized labelled compounds to be prepared. Labelled compounds and radiopharmaceuticals that have been translated to the clinic are highlighted.