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

Automated radiosynthesis of mGluR5 PET tracer [18F]FPEB from aryl-chloro precursor and validation for clinical application

The radioligand [18F]FPEB, used for PET imaging of the brain's metabotropic glutamate receptor subtype 5 (mGluR5), undergoes a thorough validation process to ensure its safety, efficacy, and quality for clinical use. The process starts by optimizing the synthesis of [18F]FPEB to achieve high radiochemical yield and purity. This study focuses on optimizing the radiolabeling process using an aryl-chloro precursor and validating the GMP production for clinical applications. Fully automated radiolabeling was achieved via one-step nucleophilic substitution reaction. [18F]FPEB was produced and isolated in high radioactivity and radiochemical purity. Throughout the validation process, thorough quality control measures are implemented. Radiopharmaceutical batch release criteria are established, including testing for physical appearance, filter integrity, pH, radiochemical purity, molar activity, radiochemical identity, chemical impurity, structural identity, stability, residual solvent, sterility, and endotoxin levels. In conclusion, the validation of [18F]FPEB involved a comprehensive process of synthesis optimization, quality control, which ensure the safety, efficacy, and quality of [18F]FPEB, enabling its reliable use in clinical PET. Here, we successfully radiolabeled and validated [18F]FPEB using aryl-chloro precursor according to GMP production for clinical application.

Characterization of (R)- and (S)-[18F]OF-NB1 in Rodents as Positron Emission Tomography Probes for Imaging GluN2B Subunit-Containing N-Methyl-d-Aspartate Receptors

The N-methyl-d-aspartate receptor (NMDAR) subtype 2B (GluN1/2B) is implicated in various neuropathologies. Given the lack of a validated radiofluorinated positron emission tomography (PET) probe for the imaging of GluN1/2B receptors, we comprehensively investigated the enantiomers of [18F]OF-NB1 in rodents. Particularly, the (R)- and (S)- enantiomers were evaluated using in silico docking, in vitro autoradiography, in vivo PET imaging, and ex vivo biodistribution studies. A select panel of GluN1/2B antagonists (CP-101,606, CERC-301, and eliprodil) and the off-target sigma-1 receptor ligands (fluspidine and SA4503) were used to determine the specificity and selectivity of the tested enantiomers. Additionally, a nonmetal-mediated radiofluorination strategy was devised that harnesses the potential of diaryliodoniums in the nucleophilic radiofluorination of nonactivated aromatic compounds. Both enantiomers exhibited known GluN1/2B binding patterns; however, the R-enantiomer showed higher GluN1/2B-specific accumulation in rodent autoradiography and higher brain uptake in PET imaging experiments compared to the S-enantiomer. Molecular simulation studies provided further insights with respect to the difference in binding, whereby a reduced ligand-receptor interaction was observed for the S-enantiomer. Nonetheless, both enantiomers showed dose dependency when two different doses (1 and 5 mg/kg) of the GluN1/2B antagonist, CP-101,606, were used in the PET imaging study. Taken together, (R)-[18F]OF-NB1 appears to exhibit the characteristics of a suitable PET probe for imaging of GluN2B-containing NMDARs in clinical studies.

Synthesis and Biological Evaluation of Enantiomerically Pure (R)- and (S)-[18F]OF-NB1 for Imaging the GluN2B Subunit-Containing NMDA Receptors

GluN2B subunit-containing N-methyl-d-aspartate (NMDA) receptors have been implicated in various neurological disorders. Nonetheless, a validated fluorine-18 labeled positron emission tomography (PET) ligand for GluN2B imaging in the living human brain is currently lacking. The aim of this study was to develop a novel synthetic approach that allows an enantiomerically pure radiosynthesis of the previously reported PET radioligands (R)-[18F]OF-NB1 and (S)-[18F]OF-NB1 as well as to assess their in vitro and in vivo performance characteristics for imaging the GluN2B subunit-containing NMDA receptor in rodents. A novel synthetic approach was successfully developed, which allows for the enantiomerically pure radiosynthesis of (R)-[18F]OF-NB1 and (S)-[18F]OF-NB1 and the translation of the probe to the clinic. While both enantiomers were selective over sigma2 receptors in vitro and in vivo, (R)-[18F]OF-NB1 showed superior GluN2B subunit specificity by in vitro autoradiography and higher volumes of distribution in the rodent brain by small animal PET studies.

Preparation of 18F-Labeled Tracers Targeting Fibroblast Activation Protein via Sulfur [18F]Fluoride Exchange Reaction

Early detection and treatment of cancers can significantly increase patient prognosis and enhance the quality of life of affected patients. The emerging significance of the tumor microenvironment (TME) as a new frontier for cancer diagnosis and therapy may be exploited by radiolabeled tracers for diagnostic imaging techniques such as positron emission tomography (PET). Cancer-associated fibroblasts (CAFs) within the TME are identified by biomarkers such as fibroblast activation protein alpha (FAPα), which are expressed on their surfaces. Targeting FAPα using small-molecule 18F-labeled inhibitors (FAPIs) has recently garnered significant attention for non-invasive tumor visualization using PET. Herein, two potent aryl-fluorosulfate-based FAPIs, 12 and 13, were synthetically prepared, and their inhibition potency was determined using a fluorimetric FAP assay to be IC50 9.63 and 4.17 nM, respectively. Radiofluorination was performed via the sulfur [18F]fluoride exchange ([18F]SuFEx) reaction to furnish [18F]12 and [18F]13 in high activity yields (AY) of 39-56% and molar activities (Am) between 20-55 GBq/µmol. In vitro experiments focused on the stability of the radiolabeled FAPIs after incubation with human serum, liver microsomes and liver cytosol. Preliminary PET studies of the radioligands were performed in healthy mice to investigate the in vivo biodistribution and 18F defluorination rate. Fast pharmacokinetics for the FAP-targeting tracers were retained and considerable bone uptake, caused by either 18F defluorination or radioligand accumulation, was observed. In summary, our findings demonstrate the efficiency of [18F]SuFEx as a radiolabeling method as well as its advantages and limitations with respect to PET tracer development.

18F-Labeled brain-penetrant EGFR tyrosine kinase inhibitors for PET imaging of glioblastoma

Significant evidence suggests that the failure of clinically tested epidermal growth factor receptor (EGFR) tyrosine kinase inhibitors (e.g. erlotinib, lapatinib, gefitinib) in glioblastoma (GBM) patients is primarily attributed to insufficient brain penetration, resulting in inadequate exposure to the targeted cells. Molecular imaging tools can facilitate GBM drug development by visualizing drug biodistribution and confirming target expression and localization. To assess brain exposure via PET molecular imaging, we synthesized fluorine-18 isotopologues of two brain-penetrant EGFR tyrosine kinase inhibitors developed specifically for GBM. Adapting our recently reported radiofluorination of N-arylsydnones, we constructed an ortho-disubstituted [18F]fluoroarene as the key intermediate. The radiotracers were produced on an automated synthesis module in 7-8% activity yield with high molar activity. In vivo PET imaging revealed rapid brain uptake in rodents and tumor accumulation in an EGFR-driven orthotopic GBM xenograft model.

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.

Exploring the role of halogen bonding in iodonium ylides: insights into unexpected reactivity and reaction control

Halogen bonding is commonly found with iodine-containing molecules, and it arises when Lewis bases interact with iodine's σ-holes. Halogen bonding and σ-holes have been encountered in numerous monovalent and hypervalent iodine-containing compounds, and in 2022 σ-holes were computationally confirmed and quantified in the iodonium ylide subset of hypervalent iodine compounds. In light of this new discovery, this article provides an overview of the reactions of iodonium ylides in which halogen bonding has been invoked. Herein, we summarize key discoveries and mechanistic proposals from the early iodonium ylide literature that invoked halogen bonding-type mechanisms, as well as recent reports of reactions between iodonium ylides and Lewis basic nucleophiles in which halogen bonding has been specifically invoked. The reactions discussed herein are organized to enable the reader to build an understanding of how halogen bonding might impact yield and chemoselectivity outcomes in reactions of iodonium ylides. Areas of focus include nucleophile σ-hole selectivity, and how ylide structural modifications and intramolecular halogen bonding (e.g., the ortho-effect) can improve ylide stability or solubility, and alter reaction outcomes.

On the Selectivity in the Synthesis of 3-Fluoropiperidines Using BF3-Activated Hypervalent Iodine Reagents

Fluorinated piperidines find wide applications, most notably in the development of novel therapies and agrochemicals. Cyclization of alkenyl N-tosylamides promoted by BF₃-activated aryliodine(III) carboxylates is an attractive strategy to construct 3-fluoropiperidines, but it suffers from selectivity issues arising from competitive oxoaminations and the inability to easily modulate the reactions diastereoselectivity. Herein, we report an itemized optimization of the reaction conditions carried out on both cyclic and acyclic substrates and outline the origins of substrate- and reagent-based stereo-, regio-, and chemoselectivity. Extensive mechanistic studies encompassing multinuclear NMR spectroscopy, deuterium labeling, rearrangements on stereodefined substrates, and careful structural analyses (NMR and X-ray) of the reaction products are performed. This revealed the processes and interactions crucial for achieving controlled preparation of 3-fluoropiperidines using I(III) chemistry and has provided an advanced understanding of the reaction mechanism. In brief, we propose that BF₃-coordinated I(III) reagents attack C═C to produce the corresponding iodiranium(III) ion, which then undergoes diastereodetermining 5-exo-cyclization. Transiently formed pyrrolidines with an exocyclic σ-alkyl-I(III) moiety can further undergo aziridinium ion formation or reductive ligand coupling processes, which dictate not only the final product's ring size but also the chemoselectivity. Importantly, the selectivity of the reaction depends on the nature of the ligand bound to I(III) and the presence of electrolytes such as TBABF₄. Reported findings will facilitate the usage of ArI(III)-dicarboxylates in the reliable construction of fluorinated azaheterocycles.

Copper-catalyzed N-arylation of amines with aryliodonium ylides in water

Copper sulfate catalyzed an efficient, inexpensive, and environment-friendly protocol that has been developed for N-arylation of amines with 1,3-cyclohexadione-derived aryliodonium ylides in water as a green solvent. Aromatic primary amines substituted with electron-donating as well as electron-withdrawing groups on the aryl ring reacted smoothly with iodonium ylides to give the corresponding diarylamines with good to excellent yields. Also, secondary amines underwent N-arylation to deliver tertiary amines with moderate yields.

Radiochemistry for positron emission tomography

Positron emission tomography (PET) constitutes a functional imaging technique that is harnessed to probe biological processes in vivo. PET imaging has been used to diagnose and monitor the progression of diseases, as well as to facilitate drug development efforts at both preclinical and clinical stages. The wide applications and rapid development of PET have ultimately led to an increasing demand for new methods in radiochemistry, with the aim to expand the scope of synthons amenable for radiolabeling. In this work, we provide an overview of commonly used chemical transformations for the syntheses of PET tracers in all aspects of radiochemistry, thereby highlighting recent breakthrough discoveries and contemporary challenges in the field. We discuss the use of biologicals for PET imaging and highlight general examples of successful probe discoveries for molecular imaging with PET - with a particular focus on translational and scalable radiochemistry concepts that have been entered to clinical use.

Synthesis and Biological Evaluation of Enantiomerically Pure (R)- and (S)-[18F]OF-NB1 for Imaging the GluN2B Subunit-Containing NMDA receptors

GluN2B subunit-containing N-methyl-d-aspartate (NMDA) receptors have been implicated in various neurological disorders. Nonetheless, a validated fluorine-18 labeled positron emission tomography (PET) ligand for GluN2B imaging in the living human brain is currently lacking. As part of our PET ligand development program, we have recently reported on the preclinical evaluation of [18F]OF-NB1 - a GluN2B PET ligand with promising attributes for potential clinical translation. However, the further development of [18F]OF-NB1 is currently precluded by major limitations in the radiolabeling procedure. These limitations include the use of highly corrosive reactants and racemization during the radiosynthesis. As such, the aim of this study was to develop a synthetic approach that allows an enantiomerically pure radiosynthesis of (R)-[18F]OF-NB1 and (S)-[18F]OF-NB1, as well as to assess their in vitro and in vivo performance characteristics for imaging the GluN2B subunit-containing NMDA receptor in rodents. A two-step radiosynthesis involving radiofluorination of the boronic acid pinacol ester, followed by coupling to the 3-benzazepine core structure via reductive amination was employed. The new synthetic approach yielded enantiomerically pure (R)-[18F]OF-NB1 and (S)-[18F]OF-NB1, while concurrently circumventing the use of corrosive reactants. In vitro autoradiograms with mouse and rat brain sections revealed a higher selectivity of (R)-[18F]OF-NB1 over (S)-[18F]OFNB1 for GluN2B-rich brain regions. In concert with these observations, blockade studies with commercially available GluN2B antagonist, CP101606, showed a significant signal reduction, which was more pronounced for (R)-[18F]OF-NB1 than for (S)-[18F]OF-NB1. Conversely, blockade experiments with sigma2 ligand, FA10, did not result in a significant reduction of tracer binding for both enantiomers. PET imaging experiments with CD1 mice revealed a higher brain uptake and retention for (R)-[18F]OF-NB1, as assessed by visual inspection and volumes of distribution from Logan graphical analyses. In vivo blocking experiments with sigma2 ligand, FA10, did not result in a significant reduction of the brain signal for both enantiomers, thus corroborating the selectivity over sigma2 receptors. In conclusion, we have developed a novel synthetic approach that is suitable for upscale to human use and allows the enantiomerically pure radiosynthesis of (R)-[18F]OF-NB1 and (S)-[18F]OF-NB1. While both enantiomers were selective over sigma2 receptors in vitro and in vivo, (R)-[18F]OF-NB1 showed superior GluN2B subunit specificity by in vitro autoradiography and higher volumes of distribution in small animal PET studies.

Radiochemical Synthesis of 4-[18F]FluorobenzylAzide and Its Conjugation with EGFR-Specific Aptamers

Central nervous system tumors related to gliomas are of neuroectodermal origin and cover about 30% of all primary brain tumors. Glioma is not susceptible to any therapy and surgical attack remains one of the main approaches to its treatment. Preoperative tumor imaging methods, such as positron emission tomography (PET), are currently used to distinguish malignant tissue to increase the accuracy of glioma removal. However, PET is lacking a specific visualization of cells possessing certain molecular markers. Here, we report an application of aptamers to enhancing specificity in imaging tumor cells bearing the epidermal growth factor receptor (EGFR). Glioblastoma is characterized by increased EGFR expression, as well as mutations of this receptor associated with active division, migration, and adhesion of tumor cells. Since 2021, EGFR has been included into the WHO classification of gliomas as a molecular genetic marker. To obtain conjugates of aptamers GR20 and GOL1-specific to EGFR, a 4-[¹⁸F]fluorobenzylazide radiotracer was used as a synthon. For the production of the synthon, a method of automatic synthesis on an Eckert & Ziegler research module was adapted and modified using spirocyclic iodonium ylide as a precursor. Conjugation of 4-[¹⁸F]fluorobenzylazide and alkyne-modified aptamers was carried out using Cu(I)-catalyzed azide-alkyne cycloaddition (CuAAC) with/without the TBTA ligand. As a result, it was possible to obtain ¹⁸F-labelled conjugates with 97% radiochemical purity for [¹⁸F]FB-GR20 and 98% for [¹⁸F]FB-GOL1. The obtained conjugates can be used for further studies in PET analysis on model animals with grafted glioblastoma.

Isotopic Radiolabeling of Crizotinib with Fluorine-18 for In Vivo Pet Imaging

Crizotinib is a tyrosine kinase inhibitor approved for the treatment of non-small-cell lung cancer, but it is inefficient on brain metastases. Crizotinib is a substrate of the P-glycoprotein, and non-invasive nuclear imaging can be used to assess the brain penetration of crizotinib. Positron emission tomography (PET) imaging using fluorine-18-labeled crizotinib would be a powerful tool for investigating new strategies to enhance the brain distribution of crizotinib. We have synthesized a spirocyclic hypervalent iodine precursor for the isotopic labeling of crizotinib in a 2.4% yield. Because crizotinib is an enantiomerically pure drug, a chiral separation was performed to afford the (R)-precursor. A two-step radiolabeling process was optimized and automated using the racemic precursor to afford [18F](R,S)-crizotinib in 15 ± 2 radiochemical yield and 103 ± 18 GBq/µmol molar activity. The same radiolabeling process was applied to the (R)-precursor to afford [18F](R)-crizotinib with comparable results. As a proof-of-concept, PET was realized in a single non-human primate to demonstrate the feasibility of [18F](R)-crizotinib in in vivo imaging. Whole-body PET highlighted the elimination routes of crizotinib with negligible penetration in the brain (SUVmean = 0.1). This proof-of-concept paves the way for further studies using [18F](R)-crizotinib to enhance its brain penetration depending on the P-glycoprotein function.

Highly efficient radiosynthesis and biological evaluation of [18F]safinamide, a radiolabelled anti-parkinsonian drug for PET imaging

As an add-on drug approved for Parkinson's disease treatment, safinamide has multiple functions, such as selective and reversible monoamine oxidase-B inhibition, voltage-sensitive sodium/potassium channel blockage, and glutamate release inhibition. Meanwhile, safinamide shows tremendous therapeutic potential in the context of other central nervous system diseases (e.g., ischaemic stroke, amyotrophic lateral sclerosis, depression, etc.).  In this work, [18F]safinamide, which is safinamide labelled by the positron-emitting radionuclide [18F]fluorine, was synthesized automatically based on iodonium ylide precursors with high radiochemical yield and high molar activity. Density functional theory was applied to calculate the Gibbs free energy change during iodonium ylide-mediated fluorination and to interpret the effect of tetraethylammonium (TEA+) as the counter cation in these reactions to improve the nucleophilicity of [18F/19F]fluoride. In addition, positron emission tomography studies on Sprague Dawley rats were carried out to determine the imaging characteristics, pharmacokinetics, and metabolism of the [18F]safinamide radiotracer. The results displayed the complete biodistribution of the radiotracer, especially in rat brains, and revealed that [18F]safinamide has moderate brain uptake, rapid and reversible binding kinetics, and good stability.

Positron Emission Tomography (PET) Imaging Tracers for Serotonin Receptors

Serotonin (5-hydroxytryptamine, 5-HT) and 5-HT receptors (5-HTRs) have crucial roles in various neuropsychiatric disorders and neurodegenerative diseases, making them attractive diagnostic and therapeutic targets. Positron emission tomography (PET) is a noninvasive nuclear molecular imaging technique and is an essential tool in clinical diagnosis and drug discovery. In this context, numerous PET ligands have been developed for "visualizing" 5-HTRs in the brain and translated into human use to study disease mechanisms and/or support drug development. Herein, we present a comprehensive repertoire of 5-HTR PET ligands by focusing on their chemotypes and performance in PET imaging studies. Furthermore, this Perspective summarizes recent 5-HTR-focused drug discovery, including biased agonists and allosteric modulators, which would stimulate the development of more potent and subtype-selective 5-HTR PET ligands and thus further our understanding of 5-HTR biology.

Mechanism of the Aryl-F Bond-Forming Step from Bi(V) Fluorides

In this article, we describe a combined experimental and theoretical mechanistic investigation of the C(sp²)–F bond formation from neutral and cationic high-valent organobismuth(V) fluorides, featuring a dianionic bis-aryl sulfoximine ligand. An exhaustive assessment of the substitution pattern in the ligand, the sulfoximine, and the reactive aryl on neutral triarylbismuth(V) difluorides revealed that formation of dimeric structures in solution promotes facile Ar–F bond formation. Noteworthy, theoretical modeling of reductive elimination from neutral bismuth(V) difluorides agrees with the experimentally determined kinetic and thermodynamic parameters. Moreover, the addition of external fluoride sources leads to inactive octahedral anionic Bi(V) trifluoride salts, which decelerate reductive elimination. On the other hand, a parallel analysis for cationic bismuthonium fluorides revealed the crucial role of tetrafluoroborate anion as fluoride source. Both experimental and theoretical analyses conclude that C–F bond formation occurs through a low-energy five-membered transition-state pathway, where the F anion is delivered to a C(sp²) center, from a BF₄ anion, reminiscent of the Balz–Schiemann reaction. The knowledge gathered throughout the investigation permitted a rational assessment of the key parameters of several ligands, identifying the simple sulfone-based ligand family as an improved system for the stoichiometric and catalytic fluorination of arylboronic acid derivatives.

Evidence for C-F Bond Formation through Formal Reductive Elimination from Tellurium(VI)

The fundamental challenge of C-F bond formation by reductive elimination has been met by compounds of select transition metals and fewer main group elements. The work detailed herein expands the list of main group elements known to be capable of reductively eliminating a C-F bond to include tellurium. Surprising and novel modes of both sp² and sp³ C-F bond formation were observed alongside formation of Te^IV cations during two separate attempts to synthesize/characterize fluorinated organotellurium(VI) cations in superacidic media (SbF₅ /SO₂ ClF). Following detailed low-temperature NMR experiments, the mechanisms of the two unique reductive elimination reactions were probed and investigated using density functional theory (DFT) calculations. Ultimately, we found that an "indirect" reductive elimination pathway is likely operative whereby Sb plays a key role in fluoride abstraction and C-F bond formation, as opposed to unimolecular reductive elimination from a discrete Te^VI cation.

An Organometallic Gold(III) Reagent for 18F Labeling of Unprotected Peptides and Sugars in Aqueous Media

The ¹⁸F labeling of unprotected peptides and sugars with a Au(III)-[¹⁸F]fluoroaryl complex is reported. The chemoselective method generates ¹⁸F-labeled S-aryl bioconjugates in an aqueous environment in 15 min with high radiochemical yields and displays excellent functional group tolerance. This approach utilizes an air and moisture stable, robust organometallic Au(III) complex and highlights the versatility of designer organometallic reagents as efficient agents for rapid radiolabeling.

Fluorine-18: Radiochemistry and Target-Specific PET Molecular Probes Design

The positron emission tomography (PET) molecular imaging technology has gained universal value as a critical tool for assessing biological and biochemical processes in living subjects. The favorable chemical, physical, and nuclear characteristics of fluorine-18 (97% β⁺ decay, 109.8 min half-life, 635 keV positron energy) make it an attractive nuclide for labeling and molecular imaging. It stands that 2-[¹⁸F]fluoro-2-deoxy-D-glucose ([¹⁸F]FDG) is the most popular PET tracer. Besides that, a significantly abundant proportion of PET probes in clinical use or under development contain a fluorine or fluoroalkyl substituent group. For the reasons given above, ¹⁸F-labeled radiotracer design has become a hot topic in radiochemistry and radiopharmaceutics. Over the past decades, we have witnessed a rapid growth in ¹⁸F-labeling methods owing to the development of new reagents and catalysts. This review aims to provide an overview of strategies in radiosynthesis of [¹⁸F]fluorine-containing moieties with nucleophilic [¹⁸F]fluorides since 2015.

Cu catalyzed [4 + 2] cycloaddition for the synthesis of highly substituted 3-fluoropyridines

A copper catalyzed annulation-aromatization of benzyl trifluoromethyl ketimines with 3-acryloyloxazolidin-2-ones for the synthesis of 3-fluoropyridines through double C-F bond cleavages has been developed. In this approach, the annulation occurred between the in situ formed dienes from trifluoromethyl ketimines via the first C-F bond cleavage and 3-acryloyloxazolidin-2-ones. Then the aromatization afforded 3-fluoropyridines in moderate yields through the second C-F bond cleavage. The 3-fluoropyridine products could be further hydrolyzed to multi-substituted 3-pyridinecarboxylic acids.

Isotopic Radiolabeling of the Antiretroviral Drug [18F]Dolutegravir for Pharmacokinetic PET Imaging

Deciphering the drug/virus/host interactions at infected cell reservoirs is a key leading to HIV-1 remission for which positron emission tomography (PET) imaging using radiolabeled antiretroviral (ARV) drugs is a powerful asset. Dolutegravir (DTG) is one of the preferred therapeutic options to treat HIV and can be isotopically labeled with fluorine-18. [18F]DTG was synthesized via a three-step approach of radiofluorination/nitrile reduction/peptide coupling with optimization for each step. Radiofluorination was performed on 2-fluoro-4-nitrobenzonitrile in 90% conversion followed by nitrile reduction using sodium borohydride and aqueous nickel(II) chloride with 72% conversion. Final peptide coupling reaction followed by HPLC purification and formulation afforded ready-to-inject [18F]DTG in 5.1 ± 0.8% (n = 10) decay-corrected radiochemical yield within 95 min. The whole process was automatized using a TRACERlab® FX NPro module, and quality control performed by analytical HPLC showed that [18F]DTG was suitable for in vivo injection with >99% chemical and radiochemical purity and a molar activity of 83 ± 18 GBq/µmol (n = 10). Whole-body distribution of [18F]DTG was performed by PET imaging on a healthy macaque and highlighted the elimination routes of the tracer. This study demonstrated the feasibility of in vivo [18F]DTG PET imaging and paved the way to explore drug/virus/tissues interactions in animals and humans.

Recent Advances in Synthetic Methodologies to Form C-18F Bonds

Positron emission tomography (PET) is an important technique for the early diagnosis of disease. Due to the specific physical and chemical properties of Fluorine-18, this important isotope is widely used in PET for labelling and molecular imaging, and its introduction into medicine molecules could produce PET tracers. Developing with the development of organic synthetic methodologies, the introduction of Fluorine-18 into drug molecules efficiently and rapidly under mild conditions, and the formation of C-¹⁸F chemical bonds, has become one of the leading topics in both organic synthetic chemistry and radiochemistry. In this mini-review, we review a series of recent advances in the organic synthesis of C-¹⁸F bonds (2015–2021), including non-catalytic radiofluorinations via good leaving functional groups, transition metal-catalyzed radiofluorinations, and photo- or electro-catalytic synthetic radiofluorinations. As a result of the remarkable advancements in this field, organic synthetic methods for forming C-¹⁸F bonds are expected to continue growing.

Discovery of a highly specific 18F-labeled PET ligand for phosphodiesterase 10A enabled by novel spirocyclic iodonium ylide radiofluorination

As a member of cyclic nucleotide phosphodiesterase (PDE) enzyme family, PDE10A is in charge of the degradation of cyclic adenosine (cAMP) and guanosine monophosphates (cGMP). While PDE10A is primarily expressed in the medium spiny neurons of the striatum, it has been implicated in a variety of neurological disorders. Indeed, inhibition of PDE10A has proven to be of potential use for the treatment of central nervous system (CNS) pathologies caused by dysfunction of the basal ganglia–of which the striatum constitutes the largest component. A PDE10A-targeted positron emission tomography (PET) radioligand would enable a better assessment of the pathophysiologic role of PDE10A, as well as confirm the relationship between target occupancy and administrated dose of a given drug candidate, thus accelerating the development of effective PDE10A inhibitors. In this study, we designed and synthesized a novel ¹⁸F-aryl PDE10A PET radioligand, codenamed [¹⁸F]P10A-1910 ([¹⁸F]9), in high radiochemical yield and molar activity via spirocyclic iodonium ylide-mediated radiofluorination. [¹⁸F]9 possessed good in vitro binding affinity (IC₅₀ = 2.1 nmol/L) and selectivity towards PDE10A. Further, [¹⁸F]9 exhibited reasonable lipophilicity (logD = 3.50) and brain permeability (P_app > 10 × 10⁻⁶ cm/s in MDCK-MDR1 cells). PET imaging studies of [¹⁸F]9 revealed high striatal uptake and excellent in vivo specificity with reversible tracer kinetics. Preclinical studies in rodents revealed an improved plasma and brain stability of [¹⁸F]9 when compared to the current reference standard for PDE10A-targeted PET, [¹⁸F]MNI659. Further, dose–response experiments with a series of escalating doses of PDE10A inhibitor 1 in rhesus monkey brains confirmed the utility of [¹⁸F]9 for evaluating target occupancy in vivo in higher species. In conclusion, our results indicated that [¹⁸F]9 is a promising PDE10A PET radioligand for clinical translation.

Arene radiofluorination enabled by photoredox-mediated halide interconversion

Positron emission tomography (PET) is a powerful imaging technology that can visualize and measure metabolic processes in vivo and/or obtain unique information about drug candidates. The identification of new and improved molecular probes plays a critical role in PET, but its progress is somewhat limited due to the lack of efficient and simple labelling methods to modify biologically active small molecules and/or drugs. Current methods to radiofluorinate unactivated arenes are still relatively limited, especially in a simple and site-selective way. Here we disclose a method for constructing C-¹⁸F bonds through direct halide/¹⁸F conversion in electron-rich halo(hetero)arenes. [¹⁸F]F^- is introduced into a broad spectrum of readily available aryl halide precursors in a site-selective manner under mild photoredox conditions. Notably, our direct ¹⁹F/¹⁸F exchange method enables rapid PET probe diversification through the preparation and evaluation of an [¹⁸F]-labelled O-methyl tyrosine library. This strategy also results in the high-yielding synthesis of the widely used PET agent L-[¹⁸F]FDOPA from a readily available L-FDOPA analogue.

Recent progress in the application of iodonium ylides in organic synthesis

This review summarizes the recent advances in the synthetic application of iodonium ylides covering 2017 to 2022.

211At and 125I-labeling of (hetero)aryliodonium ylides: astatine wins again

Despite the growing interest in radioiodine and 211 At-labeled radiopharmaceuticals, the search for radiolabeling reactions has been somewhat neglected, resulting in a limited number of available radiosynthetic strategies. Herein we report a comparative study of nucleophilic 125 I and 211 At-labeling of aryliodonium ylides. Whereas radioiodination efficiency was low, 211 At-labeling performed efficiently on a broad scope of precursors. The most activated aryliodonium ylides led rapidly to quantitative reactions at room temperature in acetonitrile. For deactivated precursors, heating up to 90°C in glyme and addition of 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) as radical scavenger appeared essential to avoid precursor degradation and to achieve high radiochemical yields and molar activity. The approach was applied successfully to the preparation of 4-[ 211 At]astatophenylalanine (4-APA), an amino acid derivative increasingly studied as radiotherapeutic drug for cancers. This validated aryliodonium ylides as a valuable tool for nucleophilic 211 At-labeling and will complement the short but now growing list of available astatination reactions.

A Swine Hind Limb Ischemia Model Useful for Testing Peripheral Artery Disease Therapeutics

Currently, there is no large animal model of sustained limb ischemia suitable for testing novel angiogenic therapeutics for peripheral artery disease (PAD) such as drugs, genes, materials, or cells. We created a large animal model suitable for efficacy assessment of these therapies by testing 3 swine hind limb ischemia (HLI) variations and quantifying vascular perfusion, muscle histology, and limb function. Ligation of the ipsilateral external and bilateral internal iliac arteries produced sustained gait dysfunction compared to isolated external iliac or unilateral external and internal iliac artery ligations. Hyperemia-dependent muscle perfusion deficits, depressed limb blood pressure, arteriogenesis, muscle atrophy, and microscopic myopathy were quantifiable in ischemic limbs 6 weeks post-ligation. Porcine mesenchymal stromal cells (MSCs) engineered to express a reporter gene were visualized post-administration via positron emission tomography (PET) in vivo. These results establish a preclinical platform enabling better optimization of PAD therapies, including cellular therapeutics, increasing bench-to-bedside translational success. A preclinical platform for porcine studies of peripheral artery disease therapies including (1) a hind limb ischemia model and (2) non-invasive MSC viability and retention assessment via PET.

Cu/base co-catalyzed [3+3] cycloaddition for the synthesis of highly functionalized 4-fluoropyridines

An efficient, straightforward and general method for the de novo synthesis of highly functionalized 4-fluoropyridines was developed via a cooperative copper- and base-catalyzed [3+3] cycloaddition of active methylene isocyanides with difluorocyclopropenes. The resulting 4-fluoropyridines can be readily diversified by various nucleophiles.

Cardiac Sympathetic Positron Emission Tomography Imaging with Meta-[18F]Fluorobenzylguanidine is Sensitive to Uptake-1 in Rats

Dysfunction of the cardiac sympathetic nervous system contributes to the development of cardiovascular diseases including ischemia, heart failure, and arrhythmias. Molecular imaging probes such as meta-[¹²³I]iodobenzylguanidine have demonstrated the utility of assessing neuronal integrity by targeting norepinephrine transporter (NET, uptake-1). However, current radiotracers can report only on innervation due to suboptimal kinetics and lack sensitivity to NET in rodents, precluding mechanistic studies in these species. The objective of this work was to characterize myocardial sympathetic neuronal uptake mechanisms and kinetics of the positron emission tomography (PET) radiotracer meta-[¹⁸F]fluorobenzylguanidine ([¹⁸F]mFBG) in rats. Automated synthesis using spirocyclic iodonium(III) ylide radiofluorination produces [¹⁸F]mFBG in 24 ± 1% isolated radiochemical yield and 30-95 GBq/μmol molar activity. PET imaging in healthy rats delineated the left ventricle, with monoexponential washout kinetics (k_mono = 0.027 ± 0.0026 min^-1, A_mono = 3.08 ± 0.33 SUV). Ex vivo biodistribution studies revealed tracer retention in the myocardium, while pharmacological treatment with selective NET inhibitor desipramine, nonselective neuronal and extraneuronal uptake-2 inhibitor phenoxybenzamine, and neuronal ablation with neurotoxin 6-hydroxydopamine reduced myocardial retention by 33, 76, and 36%, respectively. Clearance of [¹⁸F]mFBG from the myocardium was unaffected by treatment with uptake-1 and uptake-2 inhibitors following peak myocardial activity. These results suggest that myocardial distribution of [¹⁸F]mFBG in rats is dependent on both NET and extraneuronal transporters and that limited reuptake to the myocardium occurs. [¹⁸F]mFBG may therefore prove useful for imaging intraneuronal dysfunction in small animals.

Radiofluorination of non-activated aromatic prosthetic groups for synthesis and evaluation of fluorine-18 labelled ghrelin(1-8) analogues

The growth hormone secretagogue receptor 1a (GHSR) is differentially expressed in various disease states compared to healthy tissues and thus is a target for molecular imaging. The endogenous ligand for the GHSR is ghrelin, a 28 amino acid peptide with a unique octanoyl group on the serine-3 residue. A recently reported ghrelin analogue revealed the successful use of fluorine-containing, polycyclic aromatic groups in place of the octanoyl side chain, thereby providing potential access to new ¹⁸F-PET imaging probes. The peptide [Inp¹,Dpr³(6-FN),1Nal⁴,Thr⁸]ghrelin(1-8) amide (1) showed sub-nanomolar receptor affinity (IC₅₀ = 0.11 nM) toward the GHSR making it the strongest affinity ghrelin analogue reported to date. However, attempts to label such non-activated aromatic groups with fluoride-18 through conventional substitution methods resulted in low radiochemical yields, impractical for use in vivo. Since larger, non-activated aromatic groups appear to be of value for incorporating fluorine into ghrelin(1-8) analogues, an additional peptide bearing a 4'-fluorobiphenyl-4-carboxyl (4'-FBC) group in place of the octanoyl side chain was also of interest. Herein, we describe the radiosynthesis of [Inp¹,Dpr³([¹⁸F]6-FN),1Nal⁴,Thr⁸]ghrelin(1-8) amide ([18F]1) and [Inp¹,Dpr³([¹⁸F]4'-FBC),1Nal⁴,Thr⁸]ghrelin(1-8) amide ([18F]2) using a prosthetic group approach from iodonium ylide precursors as well as initial in vitro and in vivo evaluation of [18F]1 as a potential PET tracer for targeted imaging of the GHSR.

SNAr Radiofluorination with In Situ Generated [18F]Tetramethylammonium Fluoride

This report describes a method for the nucleophilic radiofluorination of (hetero)aryl chlorides, (hetero)aryl triflates, and nitroarenes using a combination of [¹⁸F]KF·K_2.2.2 and Me₄NHCO₃ for the in situ formation of a strongly nucleophilic fluorinating reagent (proposed to be [¹⁸F]Me₄NF). This method is applied to 24 substrates bearing diverse functional groups, and it generates [¹⁸F](hetero)aryl fluoride products in good to excellent radiochemical yields in the presence of ambient air/moisture. The reaction is applied to the preparation of ¹⁸F-labeled HQ-415 for potential (pre)clinical use.

18F-Fluorination: Challenge and Opportunity for Organic Chemists

18F-fluorination is an important and growing field in organic synthesis that has attracted many chemists in the recent past. Here we present our own, biased perspective with a focus on our own chemistry that evaluates recent advances in the field and provides our opinion on the challenges for the development of new chemistry, so that it may have an impact on imaging. We hope that the manuscript will provide a useful guide to chemists to develop reliable and robust reaction chemistry suitable for radiofluorination to have a real impact on human health.

Closing the gap between 19F and 18F chemistry

Positron emission tomography (PET) has become an invaluable tool for drug discovery and diagnosis. The positron-emitting radionuclide fluorine-18 is frequently used in PET radiopharmaceuticals due to its advantageous characteristics; hence, methods streamlining access to 18F-labelled radiotracers can make a direct impact in medicine. For many years, access to 18F-labelled radiotracers was limited by the paucity of methodologies available, and the poor diversity of precursors amenable to 18F-incorporation. During the last two decades, 18F-radiochemistry has progressed at a fast pace with the appearance of numerous methodologies for late-stage 18F-incorporation onto complex molecules from a range of readily available precursors including those that do not require pre-functionalisation. Key to these advances is the inclusion of new activation modes to facilitate 18F-incorporation. Specifically, new advances in late-stage 19F-fluorination under transition metal catalysis, photoredox catalysis, and organocatalysis combined with the availability of novel 18F-labelled fluorination reagents have enabled the invention of novel processes for 18F-incorporation onto complex (bio)molecules. This review describes these major breakthroughs with a focus on methodologies for C-18F bond formation. This reinvigorated interest in 18F-radiochemistry that we have witnessed in recent years has made a direct impact on 19F-chemistry with many laboratories refocusing their efforts on the development of methods using nucleophilic fluoride instead of fluorination reagents derived from molecular fluorine gas.

Automated Synthesis and Initial Evaluation of (4'-Amino-5',8'-difluoro-1'H-spiro[piperidine-4,2'-quinazolin]-1-yl)(4-[18F]fluorophenyl)methanone for PET/MR Imaging of Inducible Nitric Oxide Synthase

Background

Inducible nitric oxide synthase (iNOS) plays a crucial role in neuroinflammation, especially microglial activity, and may potentially represent a useful biomarker of neuroinflammation. In this study, we carefully defined a strategic plan to develop iNOS-targeted molecular PET imaging using (4′-amino-5′,8′-difluoro-1′H-spiro[piperidine-4,2′-quinazolin]-1-yl)(4-fluorophenyl)methanone ([¹⁸F]FBAT) as a tracer in a mouse model of lipopolysaccharide- (LPS-) induced brain inflammation.

Methods

An in vitro model, murine microglial BV2 cell line, was used to assess the uptake of [¹⁸F]FBAT in response to iNOS induction at the cellular level. In vivo whole-body dynamic PET/MR imaging was acquired in LPS-treated (5 mg/kg) and control mice. Standard uptake value (SUV), total volume of distribution (V_t), and area under the curve (AUC) based on the [¹⁸F]FBAT PET signals were determined. The expression of iNOS was confirmed by immunohistochemistry (IHC) of brain tissues.

Results

At the end of synthesis, the yield of [¹⁸F]FBAT was 2.2–3.1% (EOS), radiochemical purity was >99%, and molar radioactivity was 125–137 GBq/μmol. In vitro, [¹⁸F]FBAT rapidly and progressively accumulated in murine microglial BV2 cells exposed to LPS; however, [¹⁸F]FBAT accumulation was inhibited by aminoguanidine, a selective iNOS inhibitor. In vivo biodistribution studies of [¹⁸F]FBAT showed a significant increase in the liver and kidney on LPS-treated mice. At 3 h postinjection of LPS, in vivo, the [¹⁸F]FBAT accumulation ratios at 30 min post intravenous (i.v.) radiotracer injection for the whole brain, cortex, cerebellum, and brainstem were 2.16 ± 0.18, 1.53 ± 0.25, 1.41 ± 0.21, and 1.90 ± 0.12, respectively, compared to those of mice not injected with LPS. The mean area under the curve (AUC_0-30min), total volume of distribution (V_t, mL/cm³), and K_i (influx rate) of [¹⁸F]FBAT were 1.9 ± 0.21- and 1.4 ± 0.22-fold higher in the 3 h LPS group, respectively, than in the control group. In the pharmacokinetic two-compartment model, the whole brain K_i of [¹⁸F]FBAT was significantly higher in mice injected with LPS compared to the control group. Aminoguanidine, selective iNOS inhibitor, pretreatment significantly reduced the AUC_0-30min and V_t values in LPS-induced mice. Quantitative analysis of immunohistochemically stained brain sections confirmed iNOS was preferentially upregulated in the cerebellum and cortex of mice injected with LPS.

Conclusion

An automated robotic method was established for radiosynthesis of [¹⁸F]FBAT, and the preliminary in vitro and in vivo results demonstrated the feasibility of detecting iNOS activity/expression in LPS-treated neuroinflammation by noninvasive imaging with [¹⁸F]FBAT PET/MRI.

Transition-metal-free nucleophilic 211At-astatination of spirocyclic aryliodonium ylides

The transition-metal-free 211At-astatination of spirocyclic aryliodonium ylides via a nucleophilic aromatic substitution reaction is described. This method enables the preparation of 211At-radiolabeled compounds derived from multi-functionalized molecules and heteroarenes in good to excellent radiochemical yields.

Fluorine-18 labeled aldehydes as prosthetic groups for oxime coupling with a FVIIa protein

New ¹⁸ F-labeled nonvolatile aldehyde prosthetic groups derived from [¹⁸ F]F-Py-TFP and spirocyclic iodonium (III)ylide precursors for late stage ¹⁸ F-labeling were developed. These precursors were characterized, ¹⁸ F-labeled, and compared in reactivity for oxime coupling. Oxime coupling was performed on an amino-oxy modified inhibited factor VII (FVIIai-ONH₂ ) in low concentration to prove the applicability of the proposed method.

Sequential Ir/Cu-Mediated Method for the Meta-Selective C-H Radiofluorination of (Hetero)Arenes

This article describes a sequential Ir/Cu-mediated process for the meta-selective C-H radiofluorination of (hetero)arene substrates. In the first step, Ir-catalyzed C(sp²)-H borylation affords (hetero)aryl pinacolboronate (BPin) esters. The intermediate organoboronates are then directly subjected to copper-mediated radiofluorination with [¹⁸F]tetrabutylammonium fluoride to afford fluorine-18 labeled (hetero)arenes in high radiochemical yield and radiochemical purity. This entire process is performed on a benchtop without Schlenk or glovebox techniques and circumvents the need to isolate (hetero)aryl boronate esters. The reaction was automated on a TracerLab FX_FN module with 1,3-dimethoxybenzene and a meta-tyrosine derivative. The products, [¹⁸F]1-fluoro-3,5-dimethoxybenzene and an ¹⁸F-labeled meta-tyrosine derivative, were obtained in 37 ± 5% isolated radiochemical yield and >99% radiochemical purity and 25% isolated radiochemical yield and 99% radiochemical purity, and 0.52 Ci/μmol (19.24 GBq/μmol) molar activity (A_m), respectively.

Advances in the automated synthesis of 6-[18F]Fluoro-L-DOPA

The neurotracer 6-[18F] FDOPA has been, for many years, a powerful tool in PET imaging of neuropsychiatric diseases, movement disorders and brain malignancies. More recently, it also demonstrated good results in the diagnosis of other malignancies such as neuroendocrine tumours, pheochromocytoma or pancreatic adenocarcinoma.The multiple clinical applications of this tracer fostered a very strong interest in the development of new and improved methods for its radiosynthesis. The no-carrier-added nucleophilic 18F-fluorination process has gained increasing attention, in recent years, due to the high molar activities obtained, when compared with the other methods although the radiochemical yield remains low (17-30%). This led to the development of several nucleophilic synthetic processes in order to obtain the product with molar activity, radiochemical yield and enantiomeric purity suitable for human PET studies.Automation of the synthetic processes is crucial for routine clinical use and compliance with GMP requirements. Nevertheless, the complexity of the synthesis makes the production challenging, increasing the chance of failure in routine production. Thus, for large-scale clinical application and wider use of this radiopharmaceutical, progress in the automation of this complex radiosynthesis is of critical importance.This review summarizes the most recent developments of 6-[18F]FDOPA radiosynthesis and discusses the key issues regarding its automation for routine clinical use.

Sulfur [18F]Fluoride Exchange Click Chemistry Enabled Ultrafast Late-Stage Radiosynthesis

The lack of efficient [¹⁸F]fluorination processes and target-specific organofluorine chemotypes remains the major challenge of fluorine-18 positron emission tomography (PET). We report here an ultrafast isotopic exchange method for the radiosynthesis of novel PET agent aryl [¹⁸F]fluorosulfate enabled by the emerging sulfur fluoride exchange (SuFEx) click chemistry. The method has been applied to the fully automated ¹⁸F-radiolabeling of 25 structurally and functionally diverse aryl fluorosulfates with excellent radiochemical yield (83-100%, median 98%) and high molar activity (280 GBq μmol^-1) at room temperature in 30 s. The purification of radiotracers requires no time-consuming HPLC but rather a simple cartridge filtration. We further demonstrate the imaging application of a rationally designed poly(ADP-ribose) polymerase 1 (PARP1)-targeting aryl [¹⁸F]fluorosulfate by probing subcutaneous tumors in vivo.

Positron Emission Tomography Imaging of the Endocannabinoid System: Opportunities and Challenges in Radiotracer Development

The endocannabinoid system (ECS) is involved in a wide range of biological functions and comprises cannabinoid receptors and enzymes responsible for endocannabinoid synthesis and degradation. Over the past 2 decades, significant advances toward developing drugs and positron emission tomography (PET) tracers targeting different components of the ECS have been made. Herein, we summarized the recent development of PET tracers for imaging cannabinoid receptors 1 (CB1R) and 2 (CB2R) as well as the key enzymes monoacylglycerol lipase (MAGL) and fatty acid amide hydrolase (FAAH), particularly focusing on PET neuroimaging applications. State-of-the-art PET tracers for the ECS will be reviewed including their chemical design, pharmacological properties, radiolabeling, as well as preclinical and human PET imaging. In addition, this review addresses the current challenges for ECS PET biomarker development and highlights the important role of PET ligands to study disease pathophysiology as well as to facilitate drug discovery.

One-Step, Rapid, 18F-19F Isotopic Exchange Radiolabeling of Difluoro-dioxaborinins: Substituent Effect on Stability and In Vivo Applications

The β-diketone moiety is commonly present in many anticancer drugs, antibiotics, and natural products. We describe a general method for radiolabeling β-diketone-bearing molecules with fluoride-18. Radiolabeling was carried out via ¹⁸F-¹⁹F isotopic exchange on nonradioactive difluoro-dioxaborinins, which were generated by minimally modifying the β-diketone as a difluoroborate. Radiochemistry was one-step, rapid (<10 min), and high-yielding (>80%) and proceeded at room temperature to accommodate the half-life of F-18 (t_1/2 = 110 min). High molar activities (7.4 Ci/μmol) were achieved with relatively low starting activities (16.4 mCi). It was found that substituents affected both the solvolytic stability and fluorescence properties of difluoro-dioxaborinins. An F-18 radiolabeled difluoro-dioxaborinin probe that was simultaneously fluorescent showed sufficient stability for in vivo positron emission tomography (PET)/fluorescence imaging in mice, rabbits, and patients. These findings will guide the design of probes with specific PET/fluorescence properties; the development of new PET/fluorescence dual-modality reporters; and accurate in vivo tracking of β-diketone molecules.