Revisiting the Radiosynthesis of [18F]FPEB and Preliminary PET Imaging in a Mouse Model of Alzheimer's Disease.

[18F]FPEB is a positron emission tomography (PET) radiopharmaceutical used for imaging the abundance and distribution of mGluR5 in the central nervous system (CNS). Efficient radiolabeling of the aromatic ring of [18F]FPEB has been an ongoing challenge. Herein, five metal-free precursors for the radiofluorination of [18F]FPEB were compared, namely, a chloro-, nitro-, sulfonium salt, and two spirocyclic iodonium ylide (SCIDY) precursors bearing a cyclopentyl (SPI5) and a new adamantyl (SPIAd) auxiliary. The chloro- and nitro-precursors resulted in a low radiochemical yield (<10% RCY), whereas both SCIDY precursors and the sulfonium salt precursor produced [18F]FPEB in the highest RCYs of 25% and 36%, respectively. Preliminary PET/CT imaging studies with [18F]FPEB were conducted in a transgenic model of Alzheimer's Disease (AD) using B6C3-Tg(APPswe,PSEN1dE9)85Dbo/J (APP/PS1) mice, and data were compared with age-matched wild-type (WT) B6C3F1/J control mice. In APP/PS1 mice, whole brain distribution at 5 min post-injection showed a slightly higher uptake (SUV = 4.8 ± 0.4) than in age-matched controls (SUV = 4.0 ± 0.2). Further studies to explore mGluR5 as an early biomarker for AD are underway.

"In-loop" 18 F-fluorination: A proof-of-concept study.

There is a great demand to develop more cost-efficient and robust manufacturing processes for fluorine-18 (18 F) labelled compounds and radiopharmaceuticals. Herein, we present to our knowledge the first radiofluorination "in-loop," where [18 F]triflyl fluoride was used as the labelling agent. Initial development of the "in-loop" [18 F]fluorination method was optimized by reacting [18 F]triflyl fluoride with 1,4-dinitrobenzene to form [18 F]1-fluoro-4-nitrobenzene. This methodology was then applied for the syntheses of two well-known radiopharmaceuticals, namely, [18 F]T807 for imaging of tau protein and [18 F]FEPPA for imaging the translocator protein 18 KDa. Both radiotracers were synthesized and formulated using an automated radiosynthesis module with nondecay corrected radiochemical yields of 27% and 29% (relative [18 F]F- ), respectively. The overall syntheses times for [18 F]T807 and [18 F]FEPPA were 65 and 55 minutes, respectively. In these cases, our "in-loop" radiofluorination methodology enabled us to obtain equal or superior yields compared with conventional reactions in a vial. The radiochemical purities were more than 99%, and the molar activities were more than 350 GBq/µmol at the end-of-synthesis for both radiotracers. This novel method is simple, efficient, and allows for a reliable production of radiofluorinated compounds and radiopharmaceuticals.

Facile 18F labeling of non-activated arenes via a spirocyclic iodonium(III) ylide method and its application in the synthesis of the mGluR5 PET radiopharmaceutical [18F]FPEB.

Non-activated (electron-rich and/or sterically hindered) arenes are prevalent chemical scaffolds in pharmaceuticals and positron emission tomography (PET) diagnostics. Despite substantial efforts to develop a general method to introduce 18F into these moieties for molecular imaging by PET, there is an urgent and unmet need for novel radiofluorination strategies that result in sufficiently labeled tracers to enable human imaging. Herein, we describe an efficient method that relies on spirocyclic iodonium ylide (SCIDY) precursors for one-step and regioselective radiofluorination, as well as proof-of-concept translation to the radiosynthesis of a clinically useful PET tracer, 3-[18F]fluoro-5-[(pyridin-3-yl)ethynyl] benzonitrile ([18F]FPEB). The protocol begins with the preparation of a SCIDY precursor for FPEB, followed by radiosynthesis of [18F]FPEB, by either manual operation or an automated synthesis module. [18F]FPEB can be obtained in quantities >7.4 GBq (200 mCi), ready for injection (20 ± 5%, non-decay corrected), and has excellent chemical and radiochemical purity (>98%) as well as high molar activity (666 ± 51.8 GBq/µmol; 18 ± 1.4 Ci/µmol). The total time for the synthesis and purification of the corresponding labeling SCIDY precursor is 10 h. The subsequent radionuclide production, experimental setup, 18F labeling, and formulation of a product that is ready for injection require 2 h.

Recent applications of a single quadrupole mass spectrometer in 11C, 18F and radiometal chemistry.

Mass spectrometry (MS) has longstanding applications in radiochemistry laboratories, stemming from carbon-dating. However, research on the development of radiotracers for molecular imaging with either positron emission tomography (PET) or single photon emission computed tomography has yet to take full advantage of MS. This inertia has been attributed to the relatively low concentrations of radiopharmaceutical formulations and lack of access to the required MS equipment due to the high costs for purchase and maintenance of specialized MS systems. To date, single quadrupole (SQ)-MS coupled to liquid chromatography (LC) systems is the main form of MS that has been used in radiochemistry laboratories. These LC/MS systems are primarily used for assessing the chemical purity of radiolabeling precursor or standard molecules but also have applications in the determination of metabolites. Herein, we highlight personal experiences using a compact SQ-MS in our PET radiochemistry laboratories, to monitor the small amounts of carrier observed in most radiotracer preparations, even at high molar activities. The use of a SQ-MS in the observation of the low mass associated with non-radioactive species which are formed along with the radiotracer from the trace amounts of carrier found is demonstrated. Herein, we describe a pre-concentration system to detect dilute radiopharmaceutical formulations and metabolite analyses by SQ-MS. Selected examples where SQ-MS was critical for optimization of radiochemical reactions and for unequivocal characterization of radiotracers are showcased. We also illustrate examples where SQ-MS can be applied in identification of radiometal complexes and development of a new purification methodology for Pd-catalyzed radiofluorination reactions, shedding light on the identity of metal complexes present in the labelling solution.

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.

Metal-free 18F-labeling of aryl-CF2H via nucleophilic radiofluorination and oxidative C-H activation.

A metal-free and selective method to form [18F]aryl-CF2H through nucleophilic radiofluorination of benzyl (pseudo)halides and oxidative C-H activation of benzylic C-H bonds has been developed. The method is operationally simple and tolerates a variety of electron-neutral/deficient arenes and heteroarenes.

PET neuroimaging studies of [(18)F]CABS13 in a double transgenic mouse model of Alzheimer's disease and nonhuman primates.

Fluorine-18 labeled 2-fluoro-8-hydroxyquinoline ([(18)F]CABS13) is a promising positron emission tomography (PET) radiopharmaceutical based on a metal chelator developed to probe the "metal hypothesis of Alzheimer's disease". Herein, a practical radiosynthesis of [(18)F]CABS13 was achieved by radiofluorination followed by deprotection of an O-benzyloxymethyl group. Automated production and formulation of [(18)F]CABS13 resulted in 19 ± 5% uncorrected radiochemical yield, relative to starting [(18)F]fluoride, with ≥95% chemical and radiochemical purities, and high specific activity (>2.5 Ci/µmol) within 80 min. Temporal PET neuroimaging studies were carried out in female transgenic B6C3-Tg(APPswe,PSEN 1dE9)85Dbo/J (APP/PS1) and age-matched wild-type (WT) B6C3F1/J control mice at 3, 7, and 10 months of age. [(18)F]CABS13 showed an overall higher uptake and retention of radioactivity in the central nervous system of APP/PS1 mice versus WT mice with increasing age. However, PET/magnetic resonance imaging in normal nonhuman primates revealed that the tracer had low uptake in the brain and rapid formation of a hydrophilic radiometabolite. Identification of more metabolically stable (18)F-hydroxyquinolines that can be readily accessed by the radiochemical strategy presented herein is underway.

Iodonium ylide-mediated radiofluorination of 18F-FPEB and validation for human use.

UNLABELLED: Translation of new methodologies for labeling nonactivated aromatic molecules with (18)F remains a challenge. Here, we report a one-step, regioselective, metal-free (18)F-labeling method that uses a hypervalent iodonium(III) ylide precursor, to prepare the radiopharmaceutical (18)F-3-fluoro-5-[(pyridin-3-yl)ethynyl]benzonitrile ((18)F-FPEB). METHODS: Automated radiosynthesis of (18)F-FPEB was achieved by reaction of the ylide precursor (4 mg) with (18)F-Et4NF in dimethylformamide at 80°C for 5 min and formulated for injection within 1 h. RESULTS: (18)F-FPEB was synthesized in 20% ± 5% (n = 3) uncorrected radiochemical yields relative to (18)F-fluoride, with specific activities of 666 ± 51.8 GBq (18 ± 1.4 Ci)/µmol at the end of synthesis and was validated for human use. CONCLUSION: Radiofluorination of iodonium (III) ylides proved to be an efficient radiosynthetic strategy for synthesis of (18)F-labeled radiopharmaceuticals.

Spirocyclic hypervalent iodine(III)-mediated radiofluorination of non-activated and hindered aromatics.

Fluorine-18 (t½=109.7 min) is the most commonly used isotope to prepare radiopharmaceuticals for molecular imaging by positron emission tomography (PET). Nucleophilic aromatic substitution reactions of suitably activated (electron-deficient) aromatic substrates with no-carrier-added [(18)F]fluoride ion are routinely carried out in the synthesis of radiotracers in high specific activities. Despite extensive efforts to develop a general (18)F-labelling technique for non-activated arenes there is an urgent and unmet need to achieve this goal. Here we describe an effective solution that relies on the chemistry of spirocyclic hypervalent iodine(III) complexes, which serve as precursors for rapid, one-step regioselective radiofluorination with [(18)F]fluoride. This methodology proves to be efficient for radiolabelling a diverse range of non-activated functionalized arenes and heteroarenes, including arene substrates bearing electron-donating groups, bulky ortho functionalities, benzylic substituents and meta-substituted electron-withdrawing groups. Polyfunctional molecules and a range of previously elusive (18)F-labelled building blocks, compounds and radiopharmaceuticals are synthesized.

Alternative approaches for PET radiotracer development in Alzheimer's disease: imaging beyond plaque.

Alzheimer's disease (AD) and related dementias show increasing clinical prevalence, yet our understanding of the etiology and pathobiology of disease-related neurodegeneration remains limited. In this regard, noninvasive imaging with radiotracers for positron emission tomography (PET) presents a unique tool for quantifying spatial and temporal changes in characteristic biological markers of brain disease and for assessing potential drug efficacy. PET radiotracers targeting different protein markers are being developed to address questions pertaining to the molecular and/or genetic heterogeneity of AD and related dementias. For example, radiotracers including [(11) C]-PiB and [(18) F]-AV-45 (Florbetapir) are being used to measure the density of Aß-plaques in AD patients and to interrogate the biological mechanisms of disease initiation and progression. Our focus is on the development of novel PET imaging agents, targeting proteins beyond Aß-plaques, which can be used to investigate the broader mechanism of AD pathogenesis. Here, we present the chemical basis of various radiotracers which show promise in preclinical or clinical studies for use in evaluating the phenotypic or biochemical characteristics of AD. Radiotracers for PET imaging neuroinflammation, metal ion association with Aß-plaques, tau protein, cholinergic and cannabinoid receptors, and enzymes including glycogen-synthase kinase-3ß and monoamine oxidase B amongst others, and their connection to AD are highlighted.

Catalytic transamidation reactions compatible with tertiary amide metathesis under ambient conditions.

The carbon-nitrogen bond of carboxamides is extremely stable under most conditions. The present study reveals that simple zirconium- and hafnium-amido complexes are highly efficient catalysts for equilibrium-controlled transamidation reactions between secondary amines and tertiary amides. In a number of cases, transamidation proceeds rapidly at room temperature. We find that these new catalysts are sufficiently active to promote the metathesis of tertiary amides, which arises from successive transamidation cycles. The catalytic activities we observe are unprecedented and represent a substantial step toward a long-range goal of conducting equilibrium-controlled reactions with carboxamides.