Nucleophilic18F-Fluorination of Heteroaromatic Iodonium Salts with No-Carrier-Added [18F]Fluoride

Metal-free and atom-efficient protocol for diarylation of selenocyanate by diaryliodonium salts

We developed an atom- and reaction mass efficient strategy for the preparation of diarylselenides using iodonium salts as reactants. The developed approach allows the obtaining of diarylselenides from the corresponding trimethoxyphenyl-substituted iodonium salts via a two-step one-pot reaction sequence. The proposed metal-free methodology is based on the involvement of both iodonium aryl groups for diarylation.

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.

Surfactants Accelerate Isotope Exchange-Based 18F-Fluorination in Water

Radiochemical yields (RCYs) of isotope exchange-based ¹⁸F-fluorination of non-carbon-centered substrates in water are rationally enhanced by adding surfactants, which increases both the rate constant k and local reactant concentrations. Among 12 surfactants, the cationic surfactant cetrimonium bromide (CTAB) and two nonionic surfactants (Tween 20 and Tween 80) were selected for their superior catalytic effects, namely, electrostatic effects or solubilization effects. For a model substrate, bis(4-methoxyphenyl)phosphinic fluoride, the ¹⁸F-fluorination rate constant (k) increased up to 7-fold, while its saturation concentration rose up to 15-fold due to micelle formation, encapsulating 70-94% of the substrate. With 30.0 mmol/L CTAB, the required ¹⁸F-labeling temperature of a typical organofluorosilicon prosthesis ([¹⁸F]SiFA) decreased from 95 °C to room temperature, achieving an RCY of 22%. For an E[c(RGDyK)]₂-derived peptide tracer with an organofluorophosphine prosthesis, the RCY in water at 90 °C achieved 25%, correspondingly increasing the molar activity (A_m). After high-performance liquid chromatography (HPLC) or solid-phase purification, the residual selected surfactant concentrations in the tracer injections were well below the FDA DII (Inactive Ingredient Database) limits or the LD50 in mice.

Ligand-free Ullmann-type arylation of oxazolidinones by diaryliodonium salts

The arylation of azaheterocycles can be considered as one of the most important processes for the preparation of various biologically active compounds. In the present work, we describe a method for the copper-catalyzed N-arylation of hindered oxazolidinones using diaryliodonium salts. The method succeeds in good to excellent yields for the arylation of 4-alkyloxazolidinones, including sterically hindered isopropyl- and tert-butyl-substituted. The efficiency of the method was demonstrated for a wide range of diaryliodonium salts - symmetric and unsymmetric as well as ortho-substituted derivatives. The developed approach will provide an important contribution in the development and preparation of novel drugs and bioactive molecules containing oxazolidinone moieties.

Utilizing PET and MALDI Imaging for Discovery of a Targeted Probe for Brain Endocannabinoid α/β-Hydrolase Domain 6 (ABHD6)

Multimodal imaging provides rich biological information, which can be exploited to study drug activity, disease associated phenotypes, and pharmacological responses. Here we show discovery and validation of a new probe targeting the endocannabinoid α/β-hydrolase domain 6 (ABHD6) enzyme by utilizing positron emission tomography (PET) and matrix-assisted laser desorption/ionization (MALDI) imaging. [¹⁸F]JZP-MA-11 as the first PET ligand for in vivo imaging of the ABHD6 is reported and specific uptake in ABHD6-rich peripheral tissues and major brain regions was demonstrated using PET. A proof-of-concept study in nonhuman primate confirmed brain uptake. In vivo pharmacological response upon ABHD6 inhibition was observed by MALDI imaging. These synergistic imaging efforts used to identify biological information cannot be obtained by a single imaging modality and hold promise for improving the understanding of ABHD6-mediated endocannabinoid metabolism in peripheral and central nervous system disorders.

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.

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.

Iodolium salts as halogen-bond donor catalysts in the Nazarov cyclization: the molecular oxygen enigma

Nazarov cyclizations of activated precurosrs are achieved under iodolium catalysis, provided that oxygen is present for catalyst activation and turnover.

Diaryliodonium(III) Salts in One-Pot Double Functionalization of C–IIII and ortho C–H Bonds

Since the 1950s, diaryliodonium(III) salts have been demonstrated to participate in various arylation reactions, forming aryl–heteroatom and aryl–carbon bonds. Incorporating the arylation step into sequential transformations would provide access to...

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.

Direct Cu-mediated aromatic 18F-labeling of highly reactive tetrazines for pretargeted bioorthogonal PET imaging

Pretargeted imaging can be used to visualize and quantify slow-accumulating targeting vectors with short-lived radionuclides such as fluorine-18 - the most popular clinically applied Positron Emission Tomography (PET) radionuclide. Pretargeting results in higher target-to-background ratios compared to conventional imaging approaches using long-lived radionuclides. Currently, the tetrazine ligation is the most popular bioorthogonal reaction for pretargeted imaging, but a direct 18F-labeling strategy for highly reactive tetrazines, which would be highly beneficial if not essential for clinical translation, has thus far not been reported. In this work, a simple, scalable and reliable direct 18F-labeling procedure has been developed. We initially studied the applicability of different leaving groups and labeling methods to develop this procedure. The copper-mediated 18F-labeling exploiting stannane precursors showed the most promising results. This approach was then successfully applied to a set of tetrazines, including highly reactive H-tetrazines, suitable for pretargeted PET imaging. The labeling succeeded in radiochemical yields (RCYs) of up to approx. 25%. The new procedure was then applied to develop a pretargeting tetrazine-based imaging agent. The tracer was synthesized in a satisfactory RCY of ca. 10%, with a molar activity of 134 ± 22 GBq μmol-1 and a radiochemical purity of >99%. Further evaluation showed that the tracer displayed favorable characteristics (target-to-background ratios and clearance) that may qualify it for future clinical translation.

Multifunctional Crown-5-calix[4]arene-based Phase-Transfer Catalysts for Aromatic 18F-Fluorination

Methylated bis-triethylene glycolic crown-5-calix[4]arene (M-BTC5A) as a phase-transfer catalyst showed the best performance among other analogues and even conventional Kryptofix 222 in the nucleophilic aromatic ¹⁸F-fluorination of diaryliodonium tosylate precursors owing to (i) the efficient release of reactive "naked" [¹⁸F]fluoride, (ii) the high stabilization of the precursor in the reaction, and, presumably, (iii) the ease of access between the precursor and the K¹⁸F/M-BTC5A complex facilitated by π-π interactions. [¹⁸F]Flumazenil was produced in high radiochemical yield using M-BTC5A.

Direct Radiofluorination of Arene C-H Bonds via Photoredox Catalysis Using a Peroxide as the Terminal Oxidant

Herein, we describe an organic photoredox system for direct arene C-H radiofluorination, using a peroxide oxidizing agent and LEDs as the light source. In conjunction with an optimized photocatalyst and a microtubing reactor, this system is applicable to a range of electron-rich aromatics and heteroaromatics. We also demonstrate the feasibility of C-H radiofluorination without an azeotropic drying step, which greatly simplifies the workflow of the labeling process.

Fluorinations of unsymmetrical diaryliodonium salts containing ortho-sidearms; influence of sidearm on selectivity

Activated aromatics were reacted with two different fluoroidoane reagents 1 and 2 in the presence of triflic acid to prepare only the para-substituted diaryliodonium salts. With fluoroiodane 1 the unsymmetrical diaryliodonium salts contained an ortho-propan-2-ol sidearm, whereas the alcohol sidearm was eliminated to form an ortho-styrene sidearm in the reaction with fluoroiodane 2. Only the diaryliodonium salts containing a styrene sidearm were fluorinated successfully to deliver para-fluorinated aromatics in good yields.

Diaryliodoniums Salts as Coupling Partners for Transition-Metal Catalyzed C- and N-Arylation of Heteroarenes

Owing to the pioneering works performed on the metal-catalyzed sp2 C–H arylation of indole and pyrrole by Sanford and Gaunt, N– and C-arylation involving diaryliodonium salts offers an attractive complementary strategy for the late-stage diversification of heteroarenes. The main feature of this expanding methodology is the selective incorporation of structural diversity into complex molecules which usually have several C–H bonds and/or N–H bonds with high tolerance to functional groups and under mild conditions. This review summarizes the main recent achievements reported in transition-metal-catalyzed N– and/or C–H arylation of heteroarenes using acyclic diaryliodonium salts as coupling partners.

Harnessing Ionic Interactions and Hydrogen Bonding for Nucleophilic Fluorination

We review recent works for nucleophilic fluorination of organic compounds in which the Coulombic interactions between ionic species and/or hydrogen bonding affect the outcome of the reaction. S_N2 fluorination of aliphatic compounds promoted by ionic liquids is first discussed, focusing on the mechanistic features for reaction using alkali metal fluorides. The influence of the interplay of ionic liquid cation, anion, nucleophile and counter-cation is treated in detail. The role of ionic liquid as bifunctional (both electrophilic and nucleophilic) activator is envisaged. We also review the S_NAr fluorination of diaryliodonium salts from the same perspective. Nucleophilic fluorination of guanidine-containing of diaryliodonium salts, which are capable of forming hydrogen bonds with the nucleophile, is exemplified as an excellent case where ionic interactions and hydrogen bonding significantly affect the efficiency of reaction. The origin of experimental observation for the strong dependence of fluorination yields on the positions of -Boc protection is understood in terms of the location of the nucleophile with respect to the reaction center, being either close to far from it. Recent advances in the synthesis of [¹⁸F]F-dopa are also cited in relation to S_NAr fluorination of diaryliodonium salts. Discussions are made with a focus on tailor-making promoters and solvent engineering based on ionic interactions and hydrogen bonding.

Fast and efficient copper-mediated 18F-fluorination of arylstannanes, aryl boronic acids, and aryl boronic esters without azeotropic drying

Background

Copper-mediated radiofluorination is a straightforward method to produce a variety of [¹⁸F]fluoroarenes and [¹⁸F]fluoroheteroarenes. To minimize the number of steps in the production of ¹⁸F-labelled radiopharmaceuticals, we have developed a short and efficient azeotropic drying-free ¹⁸F-labelling method using copper-mediated fluorination. Our goal was to improve the copper-mediated method to achieve wide substrate scope with good radiochemical yields with short synthesis time.

Results

Solid phase extraction with Cu (OTf)₂ in dimethylacetamide is a suitable activation method for [¹⁸F]fluoride. Elution efficiency with Cu (OTf)₂ is up to 79% and radiochemical yield (RCY) of a variety of model molecules in the crude reaction mixture has reached over 90%. Clinically relevant molecules, norepinephrine transporter tracer [¹⁸F]NS12137 and monoamine transporter tracer [¹⁸F]CFT were produced with 16.5% RCY in 98 min and 5.3% RCY in 64 min, respectively.

Conclusions

Cu (OTf)₂ is a suitable elution agent for releasing [¹⁸F]fluoride from an anion exchange cartridge. The method is fast and efficient and the Cu-complex is customizable after the release of [¹⁸F]fluoride. Alterations in the [¹⁸F]fluoride elution techniques did not have a negative effect on the subsequent labelling reactions. We anticipate this improved [¹⁸F]fluoride elution technique to supplant the traditional azeotropic drying of [¹⁸F]fluoride in the long run and to concurrently enable the variations of the copper-complex.

A Novel 18F-Labeled Radioligand for Positron Emission Tomography Imaging of 11β-Hydroxysteroid Dehydrogenase (11β-HSD1): Synthesis and Preliminary Evaluation in Nonhuman Primates

11β-Hydroxysteroid dehydrogenase type 1 (11β-HSD1) catalyzes the conversion of cortisone to cortisol and controls a key pathway in the regulation of stress. Studies have implicated 11β-HSD1 in metabolic diseases including type 2 diabetes and obesity, as well as stress-related disorders and neurodegenerative diseases, such as depression and Alzheimer's disease (AD). We have previously developed [¹¹C]AS2471907 as a PET radiotracer to image 11β-HSD1 in the brain of nonhuman primates and humans. However, the radiosynthesis of [¹¹C]AS2471907 was unreliable and low-yielding. Here, we report the development of the ¹⁸F-labeled version [¹⁸F]AS2471907, including the synthesis of two iodonium ylide precursors and the optimization of ¹⁸F-radiosynthesis. Preliminary PET experiments, composed of a baseline scan of [¹⁸F]AS2471907 and a blocking scan with the reversible 11β-HSD1 inhibitor ASP3662 (0.3 mg/kg), was also conducted in a rhesus monkey to verify the pharmacokinetics of [¹⁸F]AS2471907 and its specific binding in the brain. The iodonium ylide precursors were prepared in a seven-step synthetic route with an optimized overall yield of ∼2%. [¹⁸F]AS2471907 was synthesized in good radiochemical purity, with the ortho regioisomer of iodonium ylide providing greater radiochemical yield as compared with the para regioisomer. In monkey brain, [¹⁸F]AS2471907 displayed high uptake and heterogeneous distribution, while administration of the 11β-HSD1 inhibitor ASP3662 significantly reduced radiotracer uptake, thus demonstrating the binding specificity of [¹⁸F]AS2471907. Given the longer half-life of F-18 and feasibility for central production and distribution, [¹⁸F]AS2471907 holds great promise to be a valuable PET radiotracer to image 11β-HSD1 in the brain.

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.

Chemistry for Positron Emission Tomography: Recent Advances in 11 C-, 18 F-, 13 N-, and 15 O-Labeling Reactions

Positron emission tomography (PET) is a molecular imaging technology that provides quantitative information about function and metabolism in biological processes in vivo for disease diagnosis and therapy assessment. The broad application and rapid advances of PET has led to an increased demand for new radiochemical methods to synthesize highly specific molecules bearing positron-emitting radionuclides. This Review provides an overview of commonly used labeling reactions through examples of clinically relevant PET tracers and highlights the most recent developments and breakthroughs over the past decade, with a focus on 11 C, 18 F, 13 N, and 15 O.

An azeotropic drying-free approach for copper-mediated radiofluorination without addition of base

Copper-mediated radiofluorination provides a quick and versatile approach for ¹⁸ F-labeling of arenes and heteroarenes. However, this method is known to be base sensitive, which has been a barrier for preparative scale radiosynthesis. In this report, we provide an approach for copper-mediated radiofluorination without azeotropic drying or adding a base. [¹⁸ F]Fluoride trapped on a PS-HCO₃ Sep-Pak was quantitatively eluted with a solution of 4-dimethylaminopyridinium trifluoromethanesulfonate (DMAP·OTf) in anhydrous N,N-dimethylformamide (DMF). The eluted solution was directly used for copper-mediated radiofluorination. Twelve boronic ester substrates were tested, yielding fluorinated products in 27% to 83% radiochemical yield based on HPLC analysis. This approach was successfully applied to the radiosynthesis of [¹⁸ F]flumazenil, a well-known positron emission tomography (PET) tracer for imaging central benzodiazepine receptors, with a radiochemical yield of 47%. This highly efficient protocol significantly augments the powerful copper-mediated radiofluorination approach.

Synthesis and Derivatization of 1,1-[18 F]Difluorinated Alkenes

A general method for the synthesis of 1,1-[¹⁸ F]difluorinated alkenes from [¹⁸ F]fluoride is reported. This transformation is highly regioselective giving the desired ¹⁸ F-fluoroalkenes with radiochemical purities of up to 77 % within 20 minutes and a molar activity (A_m ) of 1 GBq μmol^-1 . The transformations are operationally simple to perform and were readily translated onto a commercial automated synthesis unit. The resultant 1,1-[¹⁸ F]difluorinated alkene motif is prevalent in numerous drug molecules, and this is the first general method to synthesize this motif with fluorine-18. ¹⁸ F-fluorinated alkenes are excellent building blocks and participate in a number of post-labeling transformations to access a range of ¹⁸ F-perfluorinated functional groups that have never before been radiolabeled with non-carrier-added [¹⁸ F]fluoride. This method considerably expands the range of ¹⁸ F-motifs accessible to radiochemists.

Catalyst-Free Aromatic Radiofluorination via Oxidized Iodoarene Precursors

Oxidized iodoarenes (OIAs), prepared via mCPBA-mediated oxidation, have been demonstrated as versatile precursors for the synthesis of [¹⁸F]fluoroarenes in the absence of catalysts. OIAs have been identified as intermediates in single-pot syntheses of iodonium salts and ylides but have never been recognized as radiofluorination precursors. Here, the isolated OIAs were used without any catalysts to produce functionalized [¹⁸F]fluoroarenes, regardless of the electronic nature of the arenes. This method was also applied to the production of radiolabeling synthons for use as aromatic ¹⁸F-labeled building blocks.

Hypervalent aryliodine compounds as precursors for radiofluorination

Over the last 2 decades or so, hypervalent iodine compounds, such as diaryliodonium salts and aryliodonium ylides, have emerged as useful precursors for labeling homoarenes and heteroarenes with no-carrier-added cyclotron-produced [¹⁸ F]fluoride ion (t_1/2  = 109.8 min). They permit rapid and effective radiofluorination at electron-rich as well as electron-deficient aryl rings, and often with unrestricted choice of ring position. Consequently, hypervalent aryliodine compounds have found special utility as precursors to various small-molecule ¹⁸ F-labeling synthons and to many radiotracers for biomedical imaging with positron emission tomography. This review summarizes this advance in radiofluorination chemistry, with emphasis on precursor synthesis, radiofluorination mechanism, method scope, and method application.

Site-selective 18F fluorination of unactivated C-H bonds mediated by a manganese porphyrin

A direct aliphatic C–H ¹⁸F labeling method using [¹⁸F]fluoride ion at inaccessible and unreactive sites is reported.

The first direct C–H ¹⁸F fluorination reaction of unactivated aliphatic sites using no-carrier-added [¹⁸F]fluoride is reported. Under the influence of a manganese porphyrin/iodosylbenzene system, a variety of unactivated aliphatic C–H bonds can be selectively converted to C–¹⁸F bonds. The mild conditions, broad substrate scope and generally inaccessible regiochemistry make this radio-fluorination a powerful alternate to established nucleophilic substitution for the preparation of ¹⁸F labeled radio tracers.

Synthesis of [18 F]Fluoroarenes by Nucleophilic Radiofluorination of N-Arylsydnones

A practical method for radiofluorination of anilines with [18 F]fluoride via N-arylsydnone intermediates is described. These precursors are stable, easy to handle and facilitate direct and regioselective 18 F-labeling to prepare [18 F]fluoroarenes. The value of this methodology is further highlighted by successful application to prepare an 18 F-labeled neuropeptide.

Bifunctional aryliodonium salts for highly efficient radioiodination and astatination of antibodies

In this report we describe the development of an alternative approach to arylstannane chemistry for radiolabeling antibodies with radioiodine or astatine based on aryliodonium salts precursors. Bifunctional aryliodonium salts were designed and tested for the synthesis of ¹²⁵I and ²¹¹At labeled prosthetic groups for bioconjugation. The nature of the electron rich aryl group was varied and its impact on the regioselectivity of radiohalogenation was evaluated. Unexpectedly, whereas the 2-thienyl group provided the best regioselectivity towards the radioiodination of the aryl moiety of interest (98:2), it was less selective for astatination (87:13); the anisyl group providing the best regioselectivity of astatination (94:6). Under optimized conditions, both radioiodination and astatination could be performed very efficiently in mild conditions (radiochemical yields>85%). The ionic nature of the precursors was exploited to develop an efficient purification approach: the HPLC step that is usually necessary in conventionnal approaches to optimize removal of organotin toxic precursors and side products was replaced by a filtration through a silica cartridge with a significantly reduced loss of radiolabeled product. The purified radioiodinated and astatinated prosthetic groups were then conjugated efficiently to an anti-CD138 monoclonal antibody (75-80% conjugation yield). By using this novel and simple radiohalogenation procedure, higher overall radiochemical yields of astatination were obtained in comparison with the use of an arylstannane precursor and procedures of the litterature for labeling the same antibody. Overall, due to their simplicity of use and high robustness, these new precursors should simplify the labeling of proteins of interest with iodine and astatine radioisotopes for imaging and therapeutic applications.

[18F]Fluoro-Hydroxyphenethylguanidines: Efficient Synthesis and Comparison of Two Structural Isomers as Radiotracers of Cardiac Sympathetic Innervation

Fluorine-18 labeled phenethylguanidines are currently under development in our laboratory as radiotracers for quantifying regional cardiac sympathetic nerve density using PET imaging techniques. In this study, we report an efficient synthesis of ¹⁸F-hydroxyphenethylguanidines consisting of nucleophilic aromatic [¹⁸F]fluorination of a protected diaryliodonium salt precursor followed by a single deprotection step to afford the desired radiolabeled compound. This approach has been shown to reliably produce 4-[¹⁸F]fluoro-m-hydroxyphenethylguanidine ([¹⁸F]4F-MHPG, [¹⁸F]1) and its structural isomer 3-[¹⁸F]fluoro-p-hydroxyphenethylguanidine ([¹⁸F]3F-PHPG, [¹⁸F]2) with good radiochemical yields. Preclinical evaluations of [¹⁸F]2 in nonhuman primates were performed to compare its imaging properties, metabolism, and myocardial kinetics with those obtained previously with [¹⁸F]1. The results of these studies have demonstrated that [¹⁸F]2 exhibits imaging properties comparable to those of [¹⁸F]1. Myocardial tracer kinetic analysis of each tracer provides quantitative metrics of cardiac sympathetic nerve density. Based on these findings, first-in-human PET studies with [¹⁸F]1 and [¹⁸F]2 are currently in progress to assess their ability to accurately measure regional cardiac sympathetic denervation in patients with heart disease, with the ultimate goal of selecting a lead compound for further clinical development.