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

Automated production of 11C-labeled carboxylic acids and esters via "in-loop" 11C-carbonylation using GE FX synthesis modules

An in-loop 11C-carbonylation process for the radiosynthesis of 11C-carboxylic acids and esters from halide precursors has been developed. The reaction proceeds at room temperature under mild conditions and enables 11C-carbonylation of both electron deficient and electron rich (hetero)aromatic halides to provide 11C-carboxylic acids and esters in good to excellent radiochemical yields, high radiochemical purity, and excellent molar activity. The process has been fully automated using commercial radiochemistry synthesis modules, and application to clinical production is demonstrated via validated cGMP radiosyntheses of [11C]bexarotene and [11C]acetoacetic acid.

Multicenter Experience with Good Manufacturing Practice Production of [11C]PiB for Amyloid Positron Emission Tomography Imaging

Alzheimer's disease (AD) is a neurodegenerative disorder with increasing global prevalence and accounts for over half of all dementia cases. Early diagnosis is paramount for not only the management of the disease, but also for the development of new AD treatments. The current golden standard for diagnosis is performed by positron emission tomography (PET) scans with the tracer [11C]Pittsburg Compound B ([11C]PiB), which targets amyloid beta protein (Aβ) that builds up as plaques in the brain of AD patients. The increasing demand for AD diagnostics is in turn expected to drive an increase in [11C]PiB-PET scans and the setup of new [11C]PiB production lines at PET centers globally. Here, we present the [11C]PiB production setups, experiences, and use from four Danish PET facilities and discuss the challenges and potential pitfalls of [11C]PiB production. We report on the [11C]PiB production performed with the 6-OH-BTA-0 precursor dissolved in either dry acetone or 2-butanone and by using either [11C]CO2 or [11C]CH4 as 11C- precursors on three different commercial synthesis modules: TracerLab FX C Pro, ScanSys, or TracerMaker. It was found that the [11C]CO2 method gives the highest radioactive yield (1.5 to 3.2 GBq vs. 0.8 ± 0.3 GBq), while the highest molar activity (98.0 ± 61.4 GBq/μmol vs. 21.2 to 95.6 GBq/μmol) was achieved using [11C]CH4. [11C]PiB production with [11C]CO2 on a TracerLab FX C Pro offered the most desirable results, with the highest yield of 3.17 ± 1.20 GBq and good molar activity of 95.6 ± 44.2 GBq/μmol. Moreover, all reported methods produced [11C]PiB in quantities suitable for clinical applications, thus providing a foundation for other PET facilities seeking to establish their own [11C]PiB production.

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.

Core-Labeling (Radio) Synthesis of Phenols

We report a method that enables the fast incorporation of carbon isotopes into the ipso carbon of phenols. Our approach relies on the synthesis of a 1,5-dibromo-1,4-pentadiene precursor, which upon lithium-halogen exchange followed by treatment with carbonate esters results in a formal [5 + 1] cyclization to form the phenol product. Using this strategy, we have prepared 12 1-13C-labeled phenols, show proof-of-concept for the labeling of phenols with carbon-14, and demonstrate phenol synthesis directly from cyclotron-produced [11C]CO2.

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.

Recent Developments in Carbon-11 Chemistry and Applications for First-In-Human PET Studies

Positron emission tomography (PET) is a molecular imaging technique that makes use of radiolabelled molecules for in vivo evaluation. Carbon-11 is a frequently used radionuclide for the labelling of small molecule PET tracers and can be incorporated into organic molecules without changing their physicochemical properties. While the short half-life of carbon-11 (¹¹C; t_½ = 20.4 min) offers other advantages for imaging including multiple PET scans in the same subject on the same day, its use is limited to facilities that have an on-site cyclotron, and the radiochemical transformations are consequently more restrictive. Many researchers have embraced this challenge by discovering novel carbon-11 radiolabelling methodologies to broaden the synthetic versatility of this radionuclide. This review presents new carbon-11 building blocks and radiochemical transformations as well as PET tracers that have advanced to first-in-human studies over the past five years.

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.

Ring-opening of non-activated aziridines with [11C]CO2 via novel ionic liquids

Novel ionic liquids based on DBU and DBN halide salts were developed as a catalytic system for ring-opening of non-activated aziridines with [¹¹C]CO₂. The ability of ionic liquids to activate aziridines represents a simple methodology for the synthesis of ¹¹C-carbamates and can be extended for CO₂-fixation in organic and radiochemistry.

Novel ionic liquids based on DBU and DBN halide salts were developed as a catalytic system for ring-opening of non-activated aziridines with [¹¹C]CO₂.

Non-invasive radionuclide imaging of trace metal trafficking in health and disease: "PET metallomics"

Several specific metallic elements must be present in the human body to maintain health and function. Maintaining the correct quantity (from trace to bulk) and location at the cell and tissue level is essential. The study of the biological role of metals has become known as metallomics. While quantities of metals in cells and tissues can be readily measured in biopsy and autopsy samples by destructive analytical techniques, their trafficking and its role in health and disease are poorly understood. Molecular imaging with radionuclides - positron emission tomography (PET) and single photon emission computed tomography (SPECT) - is emerging as a means to non-invasively study the acute trafficking of essential metals between organs, non-invasively and in real time, in health and disease. PET scanners are increasingly widely available in hospitals, and methods for producing radionuclides of some of the key essential metals are developing fast. This review summarises recent developments in radionuclide imaging technology that permit such investigations, describes the radiological and physicochemical properties of key radioisotopes of essential trace metals and useful analogues, and introduces current and potential future applications in preclinical and clinical investigations to study the biology of essential trace metals in health and disease.

Radiopharmaceuticals for PET and SPECT Imaging: A Literature Review over the Last Decade

Positron emission tomography (PET) uses radioactive tracers and enables the functional imaging of several metabolic processes, blood flow measurements, regional chemical composition, and/or chemical absorption. Depending on the targeted processes within the living organism, different tracers are used for various medical conditions, such as cancer, particular brain pathologies, cardiac events, and bone lesions, where the most commonly used tracers are radiolabeled with 18F (e.g., [¹⁸F]-FDG and NA [¹⁸F]). Oxygen-15 isotope is mostly involved in blood flow measurements, whereas a wide array of ¹¹C-based compounds have also been developed for neuronal disorders according to the affected neuroreceptors, prostate cancer, and lung carcinomas. In contrast, the single-photon emission computed tomography (SPECT) technique uses gamma-emitting radioisotopes and can be used to diagnose strokes, seizures, bone illnesses, and infections by gauging the blood flow and radio distribution within tissues and organs. The radioisotopes typically used in SPECT imaging are iodine-123, technetium-99m, xenon-133, thallium-201, and indium-111. This systematic review article aims to clarify and disseminate the available scientific literature focused on PET/SPECT radiotracers and to provide an overview of the conducted research within the past decade, with an additional focus on the novel radiopharmaceuticals developed for medical imaging.

Synthesis of carbon-11 radiolabelled transition metal complexes using 11C-dithiocarbamates

A novel radiolabelling method exploiting ¹¹C-dithiocarbamate ligands has been used to generate ¹¹C-labelled Au(I), Au(III), Pd(II) and Pt(II) complexes in high radiochemical yields (71-99%). Labelled complexes were prepared in a rapid one-pot procedure via the substitution reaction of ¹¹C-dithiocarbamate ligands with appropriate transition metal chloride precursors.

PET Molecular Imaging in Drug Development: The Imaging and Chemistry Perspective

Positron emission tomography with selective radioligands advances the drug discovery and development process by revealing information about target engagement, proof of mechanism, pharmacokinetic and pharmacodynamic profiles. Positron emission tomography (PET) is an essential and highly significant tool to study therapeutic drug development, dose regimen, and the drug plasma concentrations of new drug candidates. Selective radioligands bring up target-specific information in several disease states including cancer, cardiovascular, and neurological conditions by quantifying various rates of biological processes with PET, which are associated with its physiological changes in living subjects, thus it reveals disease progression and also advances the clinical investigation. This study explores the major roles, applications, and advances of PET molecular imaging in drug discovery and development process with a wide range of radiochemistry as well as clinical outcomes of positron-emitting carbon-11 and fluorine-18 radiotracers.

A Direct Fixation of CO2 for Isotopic Labelling of Hydantoins Using Iodine-Phosphine Charge Transfer Complexes

Herein, we report a method for the isotopic labelling of hydantoins directly from CO2 by means of trimethyl-λ5-phosphine diiodide mediated carbonyl insertion. The method is suitable for 13C-labelling of diverse...

[11 C]Carboxylated Tetrazines for Facile Labeling of Trans-Cyclooctene-Functionalized PeptoBrushes

Functionalization of macromolecules (antibodies, polymers, nanoparticles) with click-reactive groups greatly enhances the versatility of their potential applications. Click chemistry based on tetrazine - trans-cyclooctene (TCO) ligation is especially promising and is already widely applied for pretargeted imaging and therapy. Indirect radiolabeling of TCO-functionalized macromolecules with substoichiometric amounts of radioactive tetrazines is a convenient way to monitor the fate of those macromolecules by means of positron emission tomography (PET) imaging after their administration into the test subject. In this work, the preparation is reported of TCO-containing graft copolymers, namely PeptoBrushes (polyglutamic acid-graft-polysarcosine), novel [¹¹ C]carboxylated tetrazines, and their combined use in radiolabeling the polymer by inverse electron demand Diels Alder reaction, to investigate it is potential for an application in pretarget imaging or injectable brachytherapy. The procedure for [¹¹ C]tetrazine production is easy and scalable, while indirect TCO-PeptoBrushes labeling with these [¹¹ C]tetrazines is mild, fast, and quantitative. This strategy allows facile ¹¹ C-labeling of diverse TCO-functionalized macromolecules, so that their localization and distribution shortly after injection can be assessed by PET.

Radiolabeling with [11C]HCN for Positron emission tomography

Hydrogen cyanide (HCN) is a versatile synthon for generating carbon‑carbon and carbon-heteroatom bonds. Unlike other one-carbon synthons (i.e., CO, CO2), HCN can function as a nucleophile (as in potassium cyanide, KCN) and an electrophile (as in cyanogen bromide, (CN)Br). The incorporation of the CN motif into organic molecules generates nitriles, hydantoins and (thio)cyanates, which can be converted to carboxylic acids, aldehydes, amides and amines. Such versatile chemistry is particularly attractive in PET radiochemistry where diverse bioactive small molecules incorporating carbon-11 in different positions need to be produced. The first examples of making [11C]HCN for radiolabeling date back to the 1960s. During the ensuing decades, [11C]cyanide labeling was popular for producing biologically important molecules including 11C-labeled α-amino acids, sugars and neurotransmitters. [11C]cyanation is now reemerging in many PET centers due to its versatility for making novel tracers. Here, we summarize the chemistry of [11C]HCN, review the methods to make [11C]HCN past and present, describe methods for labeling different types of molecules with [11C]HCN, and provide an overview of the reactions available to convert nitriles into other functional groups. Finally, we discuss some of the challenges and opportunities in [11C]HCN labeling such as developing more robust methods to produce [11C]HCN and developing rapid and selective methods to convert nitriles into other functional groups in complex molecules.

Structure-Activity-Relationship-Aided Design and Synthesis of xCT Antiporter Inhibitors

The xCT antiporter is a cell membrane protein involved in active counter‐transportation of glutamate (outflux) with cystine (influx) over the human cell membrane. This feature makes the xCT antiporter a crucial element of the biosynthesis of the vital free radical scavenger glutathione. The prodrug sulfasalazine, a medication for the treatment of ulcerative colitis, was previously proven to inhibit the xCT antiporter. Starting from sulfasalazine, a molecular scaffold jumping followed by SAR‐assisted design and synthesis provided a series of styryl hydroxy‐benzoic acid analogues that were biologically tested in vitro for their ability to decrease intracellular glutathione levels using four different cancer cell lines: A172 (glioma), A375 (melanoma), U87 (glioma) and MCF7 (breast carcinoma). Depletion of glutathione levels varied among the compounds as well as among the cell lines. Flow cytometry using propidium iodide and the annexin V marker demonstrated minimal toxicity in normal human astrocytes for a promising candidate molecule (E)‐5‐(2‐([1,1′‐biphenyl]‐4‐yl)vinyl)‐2‐hydroxybenzoic acid.

Fast Carbon Isotope Exchange of Carboxylic Acids Enabled by Organic Photoredox Catalysis

Carbazole/cyanobenzene photocatalysts promote the direct isotopic carboxylate exchange of C(sp³) acids with labeled CO₂. Substrates that are not compatible with transition-metal-catalyzed degradation-reconstruction approaches or prone to thermally induced reversible decarboxylation undergo isotopic incorporation at room temperature in short reaction times. The radiolabeling of drug molecules and precursors with [¹¹C]CO₂ is demonstrated.

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.

Cross-coupling of [11C]methyllithium for 11C-labelled PET tracer synthesis

The cross-coupling of aryl bromides with [¹¹C]CH₃Li for the labelling of a variety of tracers for positron emission tomography (PET) is presented. The radiolabelled products were obtained in excellent yields, at rt and after short reaction times (3-5 min) compatible with the half-life of ¹¹C (20.4 min). The automation of the protocol on a synthesis module is investigated, representing an important step towards a fast method for the synthesis of ¹¹C-labelled compounds for PET imaging.

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.

An Exposition of 11C and 18F Radiotracers Synthesis for PET Imaging

The development of new radiolabeled Positron emission tomography tracers has been extensively utilized to access the increasing diversity in the research process and to facilitate the development in research methodology, clinical usage of drug discovery and patient care. Recent advances in radiochemistry, as well as the latest techniques in automated radio-synthesizer, have encouraged and challenged the radiochemists to produce the routinely developed radiotracers. Various radionuclides like 18F, 11C, 15O, 13N 99mTc, 131I, 124I and 64Cu are used for incorporating into different chemical scaffolds; among them, 18F and 11C tagged radiotracers are mostly explored such as 11C-Methionine, 11C-Choline, 18F-FDG, 18F-FLT, and 18F-FES. This review is focused on the development of radiochemistry routes to synthesize different radiotracers of 11C and 18F for clinical studies.

Fully Automated Radiosynthesis of [11C]Guanidines for Cardiac PET Imaging

Radiolabeled guanidines such as meta-iodobenzylguanidine (MIBG) find utility in nuclear medicine as both diagnostic imaging agents and radiotherapeutics and, over the years, numerous methods for incorporating radionuclides into guanidines have been developed. In connection with a project developing new positron emission tomography (PET) radiotracers for cardiac sympathetic nerve density, we had cause to prepare [¹¹C]3F-PHPOG. However, it quickly became apparent that radiolabeling of guanidine scaffolds with carbon-11 has remained challenging, and historical methods lack compatibility with modern automated radiochemistry synthesis platforms and current Good Manufacturing Practice (cGMP) requirements. To address this challenge, we report a new automated method for radiolabeling guanidines with carbon-11. The method was used to prepare a series of [¹¹C]guanidines in good radiochemical yield (8-76% by radio-HPLC) and was found to have broad substrate scope and tolerance of unprotected OH and NH functional groups. The method was used to synthesize [¹¹C]3F-PHPOG for preclinical imaging, and suitability of the radiotracer for preclinical use was demonstrated through preliminary cardiac PET in New Zealand white rabbits which revealed good cardiac uptake and expected retention in the heart.

Rapid one-pot radiosynthesis of [carbonyl-11C]formamides from primary amines and [11C]CO2

BACKGROUND

Formamides are common motifs of biologically-active compounds (e.g. formylated peptides) and are frequently employed as intermediates to yield a number of other functional groups. A rapid, simple and reliable route to [carbonyl-11C]formamides would enable access to this important class of compounds as in vivo PET imaging agents.

RESULTS

A novel radiolabelling strategy for the synthesis of carbon-11 radiolabelled formamides ([11C]formamides) is presented. The reaction proceeded with the conversion of a primary amine to the corresponding [11C]isocyanate using cyclotron-produced [11C]CO2, a phosphazene base (2-tert-butylimino-2-diethylamino-1,3-dimethylperhydro-1,3,2-diazaphosphorine, BEMP) and phosphoryl chloride (POCl3). The [11C]isocyanate was subsequently reduced to [11C]formamide using sodium borohydride (NaBH4). [11C]Benzyl formamide was obtained with a radiochemical yield (RCY) of 80% in 15 min from end of cyclotron target bombardment and with an activity yield of 12%. This novel method was applied to the radiolabeling of aromatic and aliphatic formamides and the chemotactic amino acid [11C]formyl methionine (RCY = 48%).

CONCLUSIONS

This study demonstrates the feasibility of 11C-formylation of primary amines with the primary synthon [11C]CO2. The reactivity is proportional to the nucleophilicity of the precursor amine. This novel method can be used for the production of biomolecules containing a radiolabelled formyl group.

Rapid and Efficient Synthesis of [11C]Trifluoromethylarenes from Primary Aromatic Amines and [11C]CuCF3

Prior studies have shown that trifluoromethylarenes can be labeled in high molar activities (A m > 200 GBq/μmol) with positron-emitting carbon-11 (t 1/2 = 20.4 min) by the reaction of the copper(I) derivative of [11C]fluoroform [11C]CuCF3, with several types of precursors, such as aryl iodides, arylboronic acids, and aryldiazonium salts. Nonetheless, these precursors can be challenging to synthesize, and in the case of diazonium salts, are unstable. Methods that reduce challenges in precursor preparation for the synthesis of [11C]trifluoromethylarenes are desirable to enhance possibilities for developing biologically relevant 11C-labeled compounds as radiotracers for biomedical imaging with positron emission tomography (PET). Here, we explored the production of no-carrier-added [11C]trifluoromethylarenes from commercially available primary aromatic amines through reactions of [11C]CuCF3 with diazonium salts that were generated in situ. Moderate to high isolated decay-corrected radiochemical yields (RCY) (32-84%) were obtained rapidly (within 2 min) for many para-substituted and meta-substituted primary aromatic amines bearing a halo, methoxy, thiomethyl, hydroxy, nitro, nitrile, carboxyl, ethylcarboxy, or trifluoromethyl substituent. Null to low RCYs (0-13%) were observed only for ortho bromo-, nitro-, or nitrile-substituted precursors. This new radiosynthetic method usefully expands options for producing PET radiotracers bearing a [11C]trifluoromethyl group, especially from aryl amine precursors.

[11C]phosgene: Synthesis and application for development of PET radiotracers

Carbon-11-labeled phosgene ([¹¹C]phosgene, [¹¹C]COCl₂) is a useful labeling agent that connects two heteroatoms by inserting [¹¹C]carbonyl (¹¹C=O) function in carbamates, ureas, and carbonates, which are components of biologically important heterocyclic compounds and functional groups in drugs as a linker of fragments with in vivo stability. Development of ¹¹C-labeled PET tracers has been performed using [¹¹C]phosgene as a labeling agent. However, [¹¹C]phosgene has not been frequently used for ¹¹C-labeling because preparation of [¹¹C]phosgene required dedicated synthesis apparatus (not commercially available) and had problems in reproducibility and reliability. In our laboratory, an improved method for synthesizing [¹¹C]phosgene using a carbon tetrachloride detection tube kit in environmental air analysis and the automated synthesis system for preparing [¹¹C]phosgene have been developed in 2009. This apparatus has been used for routine synthesis of ¹¹C-labeled tracers 1-4 times/week. Using [¹¹C]phosgene we have developed and produced many PET radiotracers containing [¹¹C]urea and [¹¹C]carbamate moieties. In this review, we report the performance of our method for preparing [¹¹C]phosgene, including automated synthesis apparatus developed in house, and the application of [¹¹C]phosgene for development and production of ¹¹C-labeled PET tracers.

Copper(I)-Mediated 11C-Carboxylation of (Hetero)arylstannanes

A novel copper-mediated carboxylation strategy of aryl- and heteroaryl-stannanes is described. The method serves as a mild (i.e., 1 atm) carboxylation method using stable carbon dioxide and is transferable as a radiosynthetic approach for carbon-11-labeled aromatic and heteroaromatic carboxylic acids using sub-stoichiometric quantities of [¹¹C]CO₂. The methodology was applied to the radiosynthesis of the retinoid X receptor agonist, [¹¹C]bexarotene, with a decay-corrected radiochemical yield of 32 ± 5% and molar activity of 38 ± 23 GBq/μmol (n = 3).

A general 11C-labeling approach enabled by fluoride-mediated desilylation of organosilanes

Carbon-11 (¹¹C) is one of the most ideal positron emitters for labeling bioactive molecules for molecular imaging studies. The lack of convenient and fast incorporation methods to introduce ¹¹C into organic molecules often hampers the use of this radioisotope. Here, a fluoride-mediated desilylation (FMDS) ¹¹C-labeling approach is reported. This method relies on thermodynamically favored Si-F bond formation to generate a carbanion, therefore enabling the highly efficient and speedy incorporation of [¹¹C]CO₂ and [¹¹C]CH₃I into molecules with diversified structures. It provides facile and rapid access to ¹¹C-labeled compounds with carbon-11 attached at various hybridized carbons as well as oxygen, sulfur and nitrogen atoms with broad functional group tolerance. The exemplified syntheses of several biologically and clinically important radiotracers illustrates the potentials of this methodology.

Convenient and fast methods to introduce ¹¹C into organic molecules are of great help for molecular imaging studies. Here, the authors developed an efficient incorporation of [¹¹C]CO₂ and [¹¹C]CH₃I into molecules via a fluoride-mediated desilylation process.

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.

Fast carbonylation reaction from CO2 using plasma gas/liquid microreactors for radiolabeling applications

The major challenge for ¹¹C-radiolabelling is the short half-life time of ¹¹C (t_1/2 = 20.4 min) – in this study, a novel efficient process combining microfluidics and plasma is proposed for fast carbonylation reactions from CO₂.

[PET imaging for better understanding of normal and pathological neurotransmission]

Positron emission tomography imaging is still an expanding field of preclinical and clinical investigations exploring the brain and its normal and pathological functions. In addition to technological improvements in PET scanners, the availability of suitable radiotracers for unexplored pharmacological targets is a key factor in this expansion. Many radiotracers (or radiopharmaceuticals, when administered to humans) have been developed by multidisciplinary teams to visualize and quantify a growing numbers of brain receptors, transporters, enzymes and other targets. The development of new PET radiotracers still represents an exciting challenge, given the large number of neurochemical functions that remain to be explored. In this article, we review the development context of the first preclinical radiotracers and their passage to humans. The main current contributions of PET radiotracers are described in terms of imaging neuronal metabolism, quantification of receptors and transporters, neurodegenerative and neuroinflammatory imaging. The different approaches to functional imaging of neurotransmission are also discussed. Finally, the contributions of PET imaging to the research and development of new brain drugs are described.

[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.

Design and Challenges of Radiopharmaceuticals

This review describes general concepts with regard to radiopharmaceuticals for diagnostic or therapeutic applications that help to understand the specific challenges encountered during the design, (radio)synthesis, in vitro and in vivo evaluation and clinical translation of novel radiopharmaceuticals. The design of a radiopharmaceutical requires upfront decisions with regard to combining a suitable vector molecule with an appropriate radionuclide, considering the type and location of the molecular target, the desired application, and the time constraints imposed by the relatively short half-life of radionuclides. Well-designed in vitro and in vivo experiments allow nonclinical validation of radiotracers. Ultimately, in combination with a limited toxicology package, the radiotracer becomes a radiopharmaceutical for clinical evaluation, produced in compliance with regulatory requirements for medicines for intravenous (IV) injection.

Synthesis and preliminary radiopharmacological characterisation of an 11 C-labelled azadipeptide nitrile as potential PET tracer for imaging of cysteine cathepsins

An O-methyltyrosine-containing azadipeptide nitrile was synthesised and investigated for its inhibitory activity towards cathepsins L, S, K, and B. Labelling with carbon-11 was accomplished by reaction of the corresponding phenolic precursor with [¹¹ C]methyl iodide starting from cyclotron-produced [¹¹ C]methane. Radiopharmacological evaluation of the resulting radiotracer in a mouse xenograft model derived from a mammary tumour cell line by small animal PET imaging indicates tumour targeting with complex pharmacokinetics. Radiotracer uptake in the tumour region was considerably lower under treatment with the nonradioactive reference compound and the epoxide-based irreversible cysteine cathepsin inhibitor E64. The in vivo behaviour observed for this radiotracer largely confirms that of the corresponding ¹⁸ F-fluoroethylated analogue and suggests the limited suitability of azadipeptide nitriles for the imaging of tumour-associated cysteine cathepsins despite target-mediated uptake is evidenced.

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.

New trends and applications in carboxylation for isotope chemistry

Carboxylations are an important method for the incorporation of isotopically labeled ¹⁴CO₂ into molecules. This manuscript will review labeled carboxylations since 2010 and will present a perspective on the potential of recent unlabeled methodology for labeled carboxylations. The perspective portion of the manuscript is broken into 3 major sections based on product type, arylcarboxylic acids, benzylcarboxylic acids, and alkyl carboxylic acids, and each of those sections is further subdivided by substrate.

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.

Boron reagents for divergent radiochemistry

This review discusses boron reagents as precursors for divergent radiolabelling with a focus on carbon-11, fluorine-18 and iodine-123, -125, -131.