Palladium-Mediated [11C]Carbonylation at Atmospheric Pressure: A General Method Using Xantphos as Supporting Ligand

Validation of a good manufacturing practice procedure for the production of [11C]AZD4747, a CNS penetrant KRASG12c inhibitor

AZD4747 is a KRASG12C inhibitor recently shown to cross the non-human primate blood-brain barrier efficiently. In the current study, a GMP-compliant production of [11C]AZD4747 was developed to enable PET studies in human subjects. The validated procedure afforded [11C]AZD4747 as an injectable solution in good radioactivity yield (1656 ± 532 MBq), excellent radiochemical purity (100%), and a molar activity of 77 ± 13 GBq/μmol at the end of the synthesis, which took 46 ± 1 min from the end of the bombardment. Quality control on the final product was performed satisfactorily and met all acceptance criteria.

11C-Fixation Techniques

This protocol describes the application of cyclotron-generated [11C]CO2 fixation reactions for direct 11C-carboxylation reactions and [11C]CO for 11C-carbonylations. Herein we describe one-pot methods wherein the radioactive gas is first trapped in a reaction mixture at room temperature and atmospheric pressure prior to the radiolabeling reactions. Such procedures are widely applicable to numerous small molecules to form 11C-labeled carboxylic acids, amides, esters, ketones, oxazolidinones, carbamates, and ureas. The steps for 11C-fixation techniques described herein are tailored for a commercial automated synthesis unit and are readily adapted for routine radiopharmaceutical production.

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

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

Development of a Novel [11C]CO-Labeled Positron Emission Tomography Radioligand [11C]BIO-1819578 for the Detection of O-GlcNAcase Enzyme Activity

Imaging O-GlcNAcase OGA by positron emission tomography (PET) could provide information on the pathophysiological pathway of neurodegenerative diseases and important information on drug-target engagement and be helpful in dose selection of therapeutic drugs. Our aim was to develop an efficient synthetic method for labeling BIO-1819578 with carbon-11 using ¹¹CO for evaluation of its potential to measure levels of OGA enzyme in non-human primate (NHP) brain using PET. Radiolabeling was achieved in one-pot via a carbon-11 carbonylation reaction using [¹¹C]CO. The detailed regional brain distribution of [¹¹C]BIO-1819578 binding was evaluated using PET measurements in NHPs. Brain radioactivity was measured for 93 min using a high-resolution PET system, and radiometabolites were measured in monkey plasma using gradient radio HPLC. Radiolabeling of [¹¹C]BIO-1819578 was successfully accomplished, and the product was found to be stable at 1 h after formulation. [¹¹C]BIO-1819578 was characterized in the cynomolgus monkey brain where a high brain uptake was found (7 SUV at 4 min). A pronounced pretreatment effect was found, indicating specific binding to OGA enzyme. Radiolabeling of [¹¹C]BIO-1819578 with [¹¹C]CO was successfully accomplished. [¹¹C]BIO-1819578 binds specifically to OGA enzyme. The results suggest that [¹¹C]BIO-1819578 is a potential radioligand for imaging and for measuring target engagement of OGA in the human brain.

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.

Strategies for the Production of [11C]LY2795050 for Clinical Use

This report describes a comparison of four different routes for the clinical-scale radiosynthesis of the κ-opioid receptor antagonist [11C]LY2795050. Palladium-mediated radiocyanation and radiocarbonylation of an aryl iodide precursor as well as copper-mediated radiocyanation of an aryl iodide and an aryl boronate ester have been investigated. Full automation of all four methods is reported, each of which provides [11C]LY2795050 in sufficient radiochemical yield, molar activity, and radiochemical purity for clinical use. The advantages and disadvantages of each radiosynthesis method are compared and contrasted.

Phosphine-Free Aminocarbonylation Using Pd/DBU Catalyst: Carbonylative Coupling of Aryl Iodides and Amines

An improved carbonylation method allowing amide bond formation between aryl iodides and aromatic amines is presented. In contrast to usual methods based on Pd catalysis, this method does not require a phosphine ligand. The catalyst system simply employs bis(dibenzylideneacetone)palladium (0.5 mol %) and DBU (10 mol %). The method was applied to the synthesis of various aromatic amides from aryl iodides and amines, and was scaled to gram order synthesis under as low as 1 atm of carbon monoxide.

Palladium-Mediated Synthesis of [Carbonyl-11C]acyl Amidines from Aryl Iodides and Aryl Bromides and Their One-Pot Cyclization to 11C-Labeled Oxadiazoles

Positron emission tomography (PET) is a highly valuable imaging technique with many clinical applications. The possibility to study physiological and biochemical processes in vivo also makes PET an important tool in drug discovery. Of importance is the possibility of labelling the compound of interest with a positron-emitting radionuclide, such as carbon-11. Carbonylation reactions with [¹¹C]carbon monoxide ([¹¹C]CO) has been used to label a number of molecules containing a carbonyl derivative, such as amides and esters, with carbon-11. Presented herein is the palladium-mediated carbonylative synthesis of [carbonyl-¹¹C]acyl amidines and their subsequent cyclization to ¹¹C-labeled 1,2,4-oxadiazoles. Starting from amidines, [¹¹C]CO, and either aryl iodides or aryl bromides, [carbonyl-¹¹C]acyl amidines were synthesized and isolated in good to very good radiochemical yields (RCY). The ¹¹C-labeled 1,2,4-oxadiazoles were synthesized without the isolation of the intermediate [carbonyl-¹¹C]acyl amidines and isolated in useful RCYs, including the NF-E2-related factor 2 activator DDO-7263. 3-Phenyl-5-(4-tolyl)-1,2,4-(5-¹¹C)oxadiazole was synthesized and isolated with a clinically relevant molar activity. The broadened substrate scope, together with the good RCY and high A_m, demonstrates the utility of this method for the incorporation of carbon-11 into acyl amidines and 1,2,4-oxadiazoles, structural motifs of pharmacological interest.

Radiosynthesis, Preclinical, and Clinical Positron Emission Tomography Studies of Carbon-11 Labeled Endogenous and Natural Exogenous Compounds

The presence of positron emission tomography (PET) centers at most major hospitals worldwide, along with the improvement of PET scanner sensitivity and the introduction of total body PET systems, has increased the interest in the PET tracer development using the short-lived radionuclides carbon-11. In the last few decades, methodological improvements and fully automated modules have allowed the development of carbon-11 tracers for clinical use. Radiolabeling natural compounds with carbon-11 by substituting one of the backbone carbons with the radionuclide has provided important information on the biochemistry of the authentic compounds and increased the understanding of their in vivo behavior in healthy and diseased states. The number of endogenous and natural compounds essential for human life is staggering, ranging from simple alcohols to vitamins and peptides. This review collates all the carbon-11 radiolabeled endogenous and natural exogenous compounds synthesised to date, including essential information on their radiochemistry methodologies and preclinical and clinical studies in healthy subjects.

One-Pot Synthesis of 11 C-Labelled Primary Benzamides via Intermediate [11 C]Aroyl Dimethylaminopyridinium Salts

Electrophilic ¹¹C‐labelled aroyl dimethylaminopyridinium salts, obtained by carbonylative cross‐coupling of aryl halides with [¹¹C]carbon monoxide, were prepared for the first time and shown to be valuable intermediates in the synthesis of primary [¹¹C]benzamides. The methodology furnished a set of benzamide model compounds, including the two poly (ADP‐ribose) polymerase (PARP) inhibitors niraparib and veliparib, in moderate to excellent radiochemical yields. In addition to providing a convenient and practical route to primary [¹¹C]benzamides, the current method paves the way for future application of [¹¹C]aroyl dimethylaminopyridinium halide salts in positron emission tomography (PET) tracer synthesis.

Carbon-11: Radiochemistry and Target-Based PET Molecular Imaging Applications in Oncology, Cardiology, and Neurology

The positron emission tomography (PET) molecular imaging technique has gained its universal value as a remarkable tool for medical diagnosis and biomedical research. Carbon-11 is one of the promising radiotracers that can report target-specific information related to its pharmacology and physiology to understand the disease status. Currently, many of the available carbon-11 (t_1/2 = 20.4 min) PET radiotracers are heterocyclic derivatives that have been synthesized using carbon-11 inserted different functional groups obtained from primary and secondary carbon-11 precursors. A spectrum of carbon-11 PET radiotracers has been developed against many of the upregulated and emerging targets for the diagnosis, prognosis, prediction, and therapy in the fields of oncology, cardiology, and neurology. This review focuses on the carbon-11 radiochemistry and various target-specific PET molecular imaging agents used in tumor, heart, brain, and neuroinflammatory disease imaging along with its associated pathology.

Synthesis and [*C]CO-labelling of (C,N) gem-dimethylbenzylamine-palladium complexes for potential applications in positron emission tomography

Various aryl-palladium complexes were synthesised from gem-dimethylbenzylamine derivatives by C-H activation under extremely mild conditions. Interestingly, these highly stable structures reacted with [13C]carbon monoxide to produce the desired labelled lactams in 29% to 51% yields over the C-H activation/carbonylation steps. As representative examples, a non-natural amino acid and an estradiol-based conjugate were prepared and labelled in model experiments with [13C]CO in homogeneous or heterogeneous conditions. Especially, the latter was radiolabelled with [11C]CO using a convenient procedure from the resin-supported palladium complex precursor. Thus, these results strongly suggest that cyclometallated palladium complexes obtained from gem-dimethylbenzylamine moieties are promising precursors for the practical synthesis of new [11C]tracers for Positron Emission Tomography.

A case study of Pd⋯Pd intramolecular interaction in a benzothiazole based palladacycle; catalytic activity toward amide synthesis via an isocyanide insertion pathway

An acetate bridged benzothiazole palladacycle containing a metallophilic intramolecular Pd⋯Pd interaction was anchored on SBA-15 to form a catalyst for amide synthesis.

Radiosynthesis of a Bruton's tyrosine kinase inhibitor, [11 C]Tolebrutinib, via palladium-NiXantphos-mediated carbonylation

Bruton's tyrosine kinase (BTK) is a key component in the B-cell receptor signaling pathway and is consequently a target for in vivo imaging of B-cell malignancies as well as in multiple sclerosis (MS) with positron emission tomography (PET). A recent Phase 2b study with Sanofi's BTK inhibitor, Tolebrutinib (also known as [a.k.a.] SAR442168, PRN2246, or BTK'168) showed significantly reduced disease activity associated with MS. Herein, we report the radiosynthesis of [¹¹ C]Tolebrutinib ([¹¹ C]5) as a potential PET imaging agent for BTK. The N-[¹¹ C]acrylamide moiety of [¹¹ C]5 was labeled by ¹¹ C-carbonylation starting from [¹¹ C]CO, iodoethylene, and the secondary amine precursor via a novel palladium-NiXantphos-mediated carbonylation protocol, and the synthesis was fully automated using a commercial carbon-11 synthesis platform (TracerMaker™, Scansys Laboratorieteknik). [¹¹ C]5 was obtained in a decay-corrected radiochemical yield of 37 ± 2% (n = 5, relative to starting [¹¹ C]CO activity) in >99% radiochemical purity, with an average molar activity of 45 GBq/μmol (1200 mCi/μmol). We envision that this methodology will be generally applicable for the syntheses of labeled N-acrylamides.

PET ligands [18F]LSN3316612 and [11C]LSN3316612 quantify O-linked-β-N-acetyl-glucosamine hydrolase in the brain

We aimed to develop effective radioligands for quantifying brain O-linked-β-N-acetyl-glucosamine (O-GlcNAc) hydrolase (OGA) using positron emission tomography in living subjects as tools for evaluating drug target engagement. Posttranslational modifications of tau, a biomarker of Alzheimer's disease, by O-GlcNAc through the enzyme pair OGA and O-GlcNAc transferase (OGT) are inversely related to the amounts of its insoluble hyperphosphorylated form. Increase in tau O-GlcNAcylation by OGA inhibition is believed to reduce tau aggregation. LSN3316612, a highly selective and potent OGA ligand [half-maximal inhibitory concentration (IC50) = 1.9 nM], emerged as a lead ligand after in silico analysis and in vitro evaluations. [3H]LSN3316612 imaged and quantified OGA in postmortem brains of rat, monkey, and human. The presence of fluorine and carbonyl functionality in LSN3316612 enabled labeling with positron-emitting fluorine-18 or carbon-11. Both [18F]LSN3316612 and [11C]LSN3316612 bound reversibly to OGA in vivo, and such binding was blocked by pharmacological doses of thiamet G, an OGA inhibitor of different chemotype, in monkeys. [18F]LSN3316612 entered healthy human brain avidly (~4 SUV) without radiodefluorination or adverse effect from other radiometabolites, as evidenced by stable brain total volume of distribution (VT) values by 110 min of scanning. Overall, [18F]LSN3316612 is preferred over [11C]LSN3316612 for future human studies, whereas either may be an effective positron emission tomography radioligand for quantifying brain OGA in rodent and monkey.

Radiolabeling of [11C]FPS-ZM1, a receptor for advanced glycation end products-targeting positron emission tomography radiotracer, using a [11C]CO2-to-[11C]CO chemical conversion

Aim: The receptor for advanced glycation end products (RAGE) is a viable target for early Alzheimer's disease (AD) diagnosis using positron emission tomography (PET) as RAGE overexpression precedes Aβ plaque formation. The development of a carbon-11 analog of FPS-ZM1 (N-benzyl-4-chloro-N-cyclohexylbenzamide, [¹¹C]FPS-ZM1), possessing nanomolar affinity for RAGE, may enable the imaging of RAGE for early AD detection. Methodology & results: Herein we report an optimized [¹¹C]CO₂-to-[¹¹C]CO chemical conversion for the synthesis of [¹¹C]FPS-ZM1 and in vitro brain autoradiography. The [¹¹C]CO₂-to-[¹¹C]CO conversion via ¹¹C-silanecarboxylate derivatives was achieved with a 57% yield within 30 s from end of [¹¹C]CO₂ delivery. [¹¹C]FPS-ZM1 was obtained with a decay-corrected isolated radiochemical yield of 9.5%. Conclusion: [¹¹C]FPS-ZM1 distribution in brain tissues of wild-type versus transgenic AD model mice showed no statistically significant difference and high nondisplaceable binding.

The development of 11C-carbonylation chemistry: A systematic view

The prospects for using carbon-11 labelled compounds in molecular imaging has improved with the development of diverse synthesis methods, including ¹¹C-carbonylations and refined techniques to handle [¹¹C]carbon monoxide at a nanomole scale. Facilitating biological research and molecular imaging was the driving force when [¹¹C]carbon monoxide was used in the first in vivo application with carbon-11 in human (1945) and when [¹¹C]carbon monoxide was used for the first time as a chemical reagent in the synthesis of [¹¹C]phosgene (1978). This review examines a rich plethora of labelled compounds synthesized from [¹¹C]carbon monoxide, their chemistry and use in molecular imaging. While the strong development of the ¹¹C-carbonylation chemistry has expanded the carbon-11 domain considerably, it could be argued that the number of ¹¹C-carbonyl compounds entering biological investigations should be higher. The reason for this may partly be the lack of commercially available synthesis instruments designed for ¹¹C-carbonylations. But as this review shows, novel and greatly simplified methods to handle [¹¹C]carbon monoxide have been developed. The next important challenge is to make full use of these technologies and synthesis methods in PET research. When there is a PET-tracer that meets a more general need, the incentive to implement ¹¹C-carbonylation protocols will increase.

"In-loop" carbonylation-A simplified method for carbon-11 labelling of drugs and radioligands

Transition‐metal mediated carbonylation with ¹¹C‐labelled carbon monoxide ([¹¹C]CO) is a versatile method for introducing ¹¹C (t_1/2 = 20.3 min) into drugs and radioligands for subsequent use in positron emission tomography (PET). The aim of the current study was to perform the ¹¹C‐carbonylation reaction on the interior surface of a stainless‐steel loop used for high performance liquid chromatography (HPLC). In the experimental setup, cyclotron produced ¹¹C‐labelled carbon dioxide ([¹¹C]CO₂) was converted to [¹¹C]CO by reduction over heated Molybdenum and swept into an HPLC loop pre‐charged with the appropriate reaction mixture. Following a 5 min reaction, the radiochemical purity (RCP) and the trapping efficiency (TE) of the reaction mixture was determined. After optimization, [¹¹C]N‐Benzylbenzamide was obtained in quantitative radiochemical yield (RCY) following a 5 min reaction at room temperature. The methodology was further applied to label [¹¹C]benzoic acid (RCP≥99%, TE>91%), [¹¹C]methyl benzoate (RCP≥99%, TE>93%) and [¹¹C]phthalide (RCP≥99%, TE>88%). A set of pharmaceuticals was finally radiolabelled using non‐optimized conditions. Excellent yields were obtained for the histamine‐3 receptor radioligand [¹¹C]AZ13198083, the oncology drug [¹¹C]olaparib and the dopamine D2 receptor radioligand [¹¹C]raclopride, whereas a moderate yield was observed for the high‐affinity dopamine D2 receptor radioligand [¹¹C]FLB457. The presented “in‐loop” process proved efficient for diverse ¹¹C‐carbonylations, providing [¹¹C]amides, [¹¹C]esters and [¹¹C]carboxylic acids in moderate to excellent RCYs. Based on the advantages associated with performing the radiolabelling step as an integrated part of the purification system, this methodology may become a valuable addition to the toolbox of methodologies used for ¹¹C‐carbonylation of drugs and radioligands for PET.

[11C]Carbon monoxide: advances in production and application to PET radiotracer development over the past 15 years

[¹¹C]Carbon monoxide is an appealing synthon for introducing carbon-11 at a carbonyl position (C=O) in a wide variety of chemotypes (e.g., amides, ketones, acids, esters, and ureas). The prevalence of the carbonyl group in drug molecules and the present-day broad versatility of carbonylation reactions have led to an upsurge in the production of this synthon and in its application to PET radiotracer development. This review focuses on the major advances of the past 15 years.

Synthesis of Substituted Benzaldehydes via a Two-Step, One-Pot Reduction/Cross-Coupling Procedure

The synthesis of functionalized (benz)aldehydes, via a two-step, one-pot procedure, is presented. The method employs a stable aluminum hemiaminal as a tetrahedral intermediate, protecting a latent aldehyde, making it suitable for subsequent cross-coupling with (strong nucleophilic) organometallic reagents, leading to a variety of alkyl and aryl substituted benzaldehydes. This very fast methodology also facilitates the effective synthesis of a ¹¹C radiolabeled aldehyde. Aluminum–ate complexes enable transmetalation of alkyl fragments onto palladium and subsequent cross-coupling.

Efficient synthesis of carbon-11 labelled acylsulfonamides using [11C]CO carbonylation chemistry

Herein, a novel method for carbon-11 labeling of acyl sulfonamides by a one-step insertive [11C]CO carbonylative cross-coupling reaction between aryl halides and sulfonamides is presented. Various model compounds as well as drug molecules LY573636 (tasisulam) and ABT-199 were obtained in excellent yields. This method provides a valuable and widely applicable contribution to the continuously expanding radiochemical toolbox for PET research.

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.

Bioconjugated arylpalladium complexes on solid supports for a convenient last-step synthesis of 11C-labelled tracers for positron emission tomography

Bioconjugated arylpalladium complexes anchored onto polystyrene beads provided [¹¹C]CO-labelled compounds with excellent radiochemical purities after a simple filtration.

A [11 C] CO dispensing system for rapid screening of carbonylation reactions

[¹¹ C] CO is a highly versatile synthon that allows for labeling at carbonyl positions of many molecules by means of transition metal-mediated carbonylation reactions. The intrinsic complexity of carbonylation reactions often requires tedious screening of reaction conditions for obtaining satisfying yields. Herein, a [¹¹ C] CO dispending system for performing multiple reactions with a single batch of cyclotron-produced [¹¹ C]CO₂ is described. This semiautomated setup allows for more rapid and efficient screening of reactions and reaction conditions compared with the traditional "one beam for one reaction" strategy.

Development of [ Carbonyl-11C]AZ13198083, a Novel Histamine Type-3 Receptor Radioligand with Favorable Kinetics

The histamine subtype-3 receptor (H₃R) is implicated in a range of central nervous system disorders, and several radioligands have been developed for H₃R positron emission tomography imaging. However, a limitation of currently used PET radioligands for H₃R is the slow binding kinetics in high density brain regions. To address this, we herein report the development of three novel candidate H₃R radioligands, namely, [ carbonyl-¹¹C]AZ13153556 ([ carbonyl-¹¹C]4), [ carbonyl-¹¹C]AZD5213([ carbonyl-¹¹C]5), and [ carbonyl-¹¹C]AZ13198083 ([ carbonyl-¹¹C]6), and their subsequent preclinical evaluation in nonhuman primates (NHP). Radioligands [ carbonyl-¹¹C]4-6 were produced and isolated in high radioactivity (>1000 MBq), radiochemical purity (>99%), and moderate molar activity (19-28 GBq/μmol at time of injection) using a palladium-mediated ¹¹C-aminocarbonylation protocol. All three radioligands showed high brain permeability as well as a regional brain radioactivity distribution in accordance with H₃R expression (striatum > cortex > cerebellum). [ Carbonyl-¹¹C]6 displayed the most favorable in vivo kinetics and brain uptake, with an early peak in the striatal time-activity curve followed by a progressive washout from the brain. The specificity and on-target kinetics of [ carbonyl-¹¹C]6 were next investigated in pretreatment and displacement studies. After pretreatment or displacement with 5 (0.1 mg/kg), a uniformly low distribution of radioactivity across the NHP brain was observed. Collectively, this work demonstrates that [ carbonyl-¹¹C]6 is a promising candidate for H₃R imaging in human subjects.

Recent progress in [11 C]carbon dioxide ([11 C]CO2 ) and [11 C]carbon monoxide ([11 C]CO) chemistry

[11 C]Carbon dioxide ([11 C]CO2 ) and [11 C]carbon monoxide ([11 C]CO) are 2 attractive precursors for labelling the carbonyl position (C═O) in a vast range of functionalised molecules (eg, ureas, amides, and carboxylic acids). The development of radiosynthetic methods to produce functionalised 11 C-labelled compounds is required to enhance the radiotracers available for positron emission tomography, molecular, and medical imaging applications. Following a brief summary of secondary 11 C-precursor production and uses, the review focuses on recent progress with direct 11 C-carboxylation routes with [11 C]CO2 and 11 C-carbonylation with [11 C]CO. Novel approaches to generate [11 C]CO using CO-releasing molecules (CO-RMs), such as silacarboxylic acids and disilanes, applied to radiochemistry are described and compared with standard [11 C]CO production methods. These innovative [11 C]CO synthesis strategies represent efficient and reliable [11 C]CO production processes, enabling the widespread use of [11 C]CO chemistry within the wider radiochemistry community.

[Carboxyl-11 C]Labelling of Four High-Affinity cPLA2α Inhibitors and Their Evaluation as Radioligands in Mice by Positron Emission Tomography

Cytosolic phospholipase A2α (cPLA2α) may play a critical role in neuropsychiatric and neurodegenerative disorders associated with oxidative stress and neuroinflammation. An effective PET radioligand for imaging cPLA2α in living brain might prove useful for biomedical research, especially on neuroinflammation. We selected four high-affinity (IC₅₀ 2.1-12 nm) indole-5-carboxylic acid-based inhibitors of cPLA2α, namely 3-isobutyryl-1-(2-oxo-3-(4-phenoxyphenoxy)propyl)-1H-indole-5-carboxylic acid (1); 3-acetyl-1-(2-oxo-3-(4-(4-(trifluoromethyl)phenoxy)phenoxy)propyl)-1H-indole-5-carboxylic acid (2); 3-(3-methyl-1,2,4-oxadiazol-5-yl)-1-(2-oxo-3-(4-phenoxyphenoxy)propyl)-1H-indole-5-carboxylic acid (3); and 3-(3-methyl-1,2,4-oxadiazol-5-yl)-1-(3-(4-octylphenoxy)-2-oxopropyl)-1H-indole-5-carboxylic acid (4), for labelling in carboxyl position with carbon-11 (t_1/2 =20.4 min) to provide candidate PET radioligands for imaging brain cPLA2α. Compounds [¹¹ C]1-4 were obtained for intravenous injection in adequate overall yields (1.1-5.5 %) from cyclotron-produced [¹¹ C]carbon dioxide and with moderate molar activities (70-141 GBq μmol^-1 ) through the use of Pd⁰ -mediated [¹¹ C]carbon monoxide insertion on iodo precursors. Measured logD_7.4 values were within a narrow moderate range (1.9-2.4). After intravenous injection of [¹¹ C]1-4 in mice, radioactivity uptakes in brain peaked at low values (≤0.8 SUV) and decreased by about 90 % over 15 min. Pretreatments of the mice with high doses of the corresponding non-radioactive ligands did not alter brain time-activity curves. Brain uptakes of radioactivity after administration of [¹¹ C]1 to wild-type and P-gp/BCRP dual knock-out mice were similar (peak 0.4 vs. 0.5 SUV), indicating that [¹¹ C]1 and others in this structural class, are not substrates for efflux transporters.

Pd-catalysed carbonylative annulation of salicylaldehydes with benzyl chlorides using N-formylsaccharin as a CO surrogate

A highly efficient synthesis of 3-arylcoumarins by Pd-catalysed carbonylative cyclisation of salicylaldehydes with benzyl chlorides using N-formylsaccharin as a CO source is developed.

Pd(0)-Mediated 11C-Carbonylation of Aryl(mesityl)iodonium Salts as a Route to [11C]Arylcarboxylic Acids and Derivatives

Pd(0)-mediated ¹¹C-carbonylation of aryl(mesityl)iodonium salts followed by suitable quench provides a rapid room-temperature two-pot procedure for labeling arylcarboxylic acids and amide derivatives with the short-lived positron emitter carbon-11 (t_1/2 = 20.4 min) in generally good to high yields (up to 71%). High product ring selectivity (≥13) was achieved when using mesityl as a spectator group in the diaryliodonium salt precursors. This process has potential for preparing new radiotracers for molecular imaging with positron emission tomography.

Synthesis of 11C-Labelled Ureas by Palladium(II)-Mediated Oxidative Carbonylation

Positron emission tomography is an imaging technique with applications in clinical settings as well as in basic research for the study of biological processes. A PET tracer, a biologically active molecule where a positron-emitting radioisotope such as carbon-11 has been incorporated, is used for the studies. Development of robust methods for incorporation of the radioisotope is therefore of the utmost importance. The urea functional group is present in many biologically active compounds and is thus an attractive target for incorporation of carbon-11 in the form of [¹¹C]carbon monoxide. Starting with amines and [¹¹C]carbon monoxide, both symmetrical and unsymmetrical ¹¹C-labelled ureas were synthesised via a palladium(II)-mediated oxidative carbonylation and obtained in decay-corrected radiochemical yields up to 65%. The added advantage of using [¹¹C]carbon monoxide was shown by the molar activity obtained for an inhibitor of soluble epoxide hydrolase (247 GBq/μmol–319 GBq/μmol). DFT calculations were found to support a reaction mechanism proceeding through an ¹¹C-labelled isocyanate intermediate.

Instantaneous Conversion of [11 C]CO2 to [11 C]CO via Fluoride-Activated Disilane Species

The development of a fast and novel methodology to generate carbon-11 carbon monoxide ([11 C]CO) from cyclotron-produced carbon-11 carbon dioxide ([11 C]CO2 ) mediated by a fluoride-activated disilane species is described. This methodology allows up to 74 % conversion of [11 C]CO2 to [11 C]CO using commercially available reagents, readily available laboratory equipment and mild reaction conditions (room temperature). As proof of utility, radiochemically pure [carbonyl-11 C]N-benzylbenzamide was successfully synthesized from produced [11 C]CO in up to 74 % radiochemical yield (RCY) and >99 % radiochemical purity (RCP) in ≤10 min from end of [11 C]CO2 delivery.

Electrochemical [11C]CO2 to [11C]CO conversion for PET imaging

The development of a novel electrochemical methodology to generate carbon-11 carbon monoxide ([11C]CO) from cyclotron-produced carbon-11 carbon dioxide ([11C]CO2) using Ni(cyclam) and Zn(cyclen) complexes is described. This methodology allows up to 10% yields of [11C]CO from [11C]CO2. Produced [11C]CO was subsequently converted to [11C]N-benzylbenzamide under mild conditions with a radiochemical purity (RCP) of >98%.

New methodologies for the preparation of carbon-11 labeled radiopharmaceuticals

PURPOSE

This short review aims to cover the more recent and promising developments of carbon-11 (11C) labeling radiochemistry and its utility in the production of novel radiopharmaceuticals, with special emphasis on methods that have the greatest potential to be translated for clinical positron emission tomography (PET) imaging.

METHODS

A survey of the literature was undertaken to identify articles focusing on methodological development in 11C chemistry and their use within novel radiopharmaceutical preparation. However, since 11C-labeling chemistry is such a narrow field of research, no systematic literature search was therefore feasible. The survey was further restricted to a specific timeframe (2000-2016) and articles in English.

RESULTS

From the literature, it is clear that the majority of 11C-labeled radiopharmaceuticals prepared for clinical PET studies have been radiolabeled using the standard heteroatom methylation reaction. However, a number of methodologies have been developed in recent years, both from a technical and chemical point of view. Amongst these, two protocols may have the greatest potential to be widely adapted for the preparation of 11C-radiopharmaceuticals in a clinical setting. First, a novel method for the direct formation of 11C-labeled carbonyl groups, where organic bases are utilized as [11C]carbon dioxide-fixation agents. The second method of clinical importance is a low-pressure 11C-carbonylation technique that utilizes solvable xenon gas to effectively transfer and react [11C]carbon monoxide in a sealed reaction vessel. Both methods appear to be general and provide simple paths to 11C-labeled products.

CONCLUSION

Radiochemistry is the foundation of PET imaging which relies on the administration of a radiopharmaceutical. The demand for new radiopharmaceuticals for clinical PET imaging is increasing, and 11C-radiopharmaceuticals are especially important within clinical research and drug development. This review gives a comprehensive overview of the most noteworthy 11C-labeling methods with clinical relevance to the field of PET radiochemistry.

Palladium-mediated11C-carbonylations using aryl halides and cyanamide

A rapid, efficient and high-yielding synthesis of¹¹C-cyanobenzamides, including novel analogs of various drug molecules, is described.