A Gas Phase Route to [18F]fluoroform with Limited Molar Activity Dilution

[18F]Trifluoroiodomethane - Enabling Photoredox-mediated Radical [18F]Trifluoromethylation for Positron Emission Tomography

The development of new tracers for positron emission tomography (PET) is highly dependent on the available synthetic tools for their radiosynthesis. Herein, we present the radiosynthesis and application of [18F]trifluoroiodomethane - the first reagent for broad scope radical [18F]trifluoromethylation chemistry in high molar activity. CF2 18FI can be prepared from [18F]fluoroform with 67±5 % AY and >99 % RCP. Its synthetic utility is demonstrated by the radiosynthesis of previously unprecedented 18F-labeled α-trifluoromethyl ketones and 18F-labeled trifluoromethyl sulfides, important motifs that are present in a range of bioactive compounds. Both protocols are Ru- and photo-mediated and proceed under mild reaction conditions. They show good functional group tolerance evidenced by the respective reaction scopes and make use of easily obtainable starting materials. The products can be isolated in 8.3-11.1 GBq/μmol (starting from ca. 5 GBq [18F]fluoride). The applicability to PET tracer synthesis is shown by the radiolabeling of bioactive compounds, such as derivatives of probenecid and febuxostat. In a broader context, this work opens the door to the utilization of radical [18F]trifluoromethylation chemistry for the radiolabeling of PET tracers in high molar activity.

Synthesis of 18F-labelled aryl trifluoromethyl ketones with improved molar activity

A method for the radiosynthesis of 18F-labelled aryl trifluoromethyl ketones starting from widely available Weinreb amides using [18F]fluoroform is presented. The method uses potassium hexamethyldisilazane as base and delivers products in high molar activity (up to 24 GBq μmol-1) and excellent radiochemical conversions. The applicability for PET tracer synthesis is demonstrated by the radiosynthesis of ten (hetero)aryl trifluoromethylketones, bearing electron-withdrawing and -donating substituents including a derivative of bioactive probenecid.

Copper(I)-free syntheses of [11C/18F]trifluoromethyl ketones from alkyl or aryl esters and [11C/18F]fluoroform

Herein, we report a copper(I)-free method for labeling the trifluoroacetyl group with positron-emitting carbon-11 (t1/2 = 20.4 min) or fluorine-18 (t1/2 = 109.8 min) as part of our exploration of radiolabeled fluoroforms to access new radiolabeled chemotypes of interest for tracer development. Treatment of alkyl esters and aryl esters, containing electron-donating or electron-withdrawing groups, with [11C/18F]fluoroform in the presence of strong base, gave [11C/18F]trifluoromethyl ketones as novel radiolabeling synthons in moderate to high yields within 15 minutes.

Highlight selection of radiochemistry and radiopharmacy developments by editorial board

Background

The Editorial Board of EJNMMI Radiopharmacy and Chemistry releases a biyearly highlight commentary to update the readership on trends in the field of radiopharmaceutical development.

Results

This commentary of highlights has resulted in 23 different topics selected by each member of the Editorial Board addressing a variety of aspects ranging from novel radiochemistry to first in man application of novel radiopharmaceuticals and also a contribution in relation to MRI-agents is included.

Conclusion

Trends in (radio)chemistry and radiopharmacy are highlighted demonstrating the progress in the research field being the scope of EJNMMI Radiopharmacy and Chemistry.

Synthesis and Screening in Mice of Fluorine-Containing PET Radioligands for TSPO: Discovery of a Promising 18F-Labeled Ligand

Translocator protein 18 kDa (TSPO) is a biomarker of neuroinflammation. [11C]ER176 robustly quantifies TSPO in the human brain with positron emission tomography (PET), irrespective of subject genotype. We aimed to develop an ER176 analog with potential for labeling with longer-lived fluorine-18 (t1/2 = 109.8 min). New fluoro and trifluoromethyl analogs of ER176 were prepared through a concise synthetic strategy. These ligands showed high TSPO affinity and low human genotype sensitivity. Each ligand was initially labeled by a generic 11C-methylation procedure, thereby enabling speedy screening in mice. Each radioligand was rapidly taken up and well retained in the mouse brain at baseline after intravenous injection. Preblocking of TSPO showed that high proportions of brain uptake were specifically bound to TSPO at baseline. Overall, the 3-fluoro analog of [11C]ER176 ([11C]3b) displayed the most promising imaging properties. Therefore, a method was developed to label 3b with [18F]fluoride ion. [18F]3b gave similarly promising PET imaging results and deserves evaluation in higher species.

Advances in [18F]Trifluoromethylation Chemistry for PET Imaging

Positron emission tomography (PET) is a preclinical and clinical imaging technique extensively used to study and visualize biological and physiological processes in vivo. Fluorine-18 (¹⁸F) is the most frequently used positron emitter for PET imaging due to its convenient 109.8 min half-life, high yield production on small biomedical cyclotrons, and well-established radiofluorination chemistry. The presence of fluorine atoms in many drugs opens new possibilities for developing radioligands labelled with fluorine-18. The trifluoromethyl group (CF₃) represents a versatile structural motif in medicinal and pharmaceutical chemistry to design and synthesize drug molecules with favourable pharmacological properties. This fact also makes CF₃ groups an exciting synthesis target from a PET tracer discovery perspective. Early attempts to synthesize [¹⁸F]CF₃-containing radiotracers were mainly hampered by low radiochemical yields and additional challenges such as low radiochemical purity and molar activity. However, recent innovations in [¹⁸F]trifluoromethylation chemistry have significantly expanded the chemical toolbox to synthesize fluorine-18-labelled radiotracers. This review presents the development of significant [¹⁸F]trifluoromethylation chemistry strategies to apply [¹⁸F]CF₃-containing radiotracers in preclinical and clinical PET imaging studies. The continuous growth of PET as a crucial functional imaging technique in biomedical and clinical research and the increasing number of CF₃-containing drugs will be the primary drivers for developing novel [¹⁸F]trifluoromethylation chemistry strategies in the future.

Evaluating N-difluoromethyltriazolium triflate as a precursor for the synthesis of high molar activity [18 F]fluoroform

The trifluoromethyl group is a prominent motif in biologically active compounds and therefore of great interest for the labeling with the positron emitter fluorine-18 for positron emission tomography (PET) imaging. Multiple labeling strategies have been explored in the past; however, most of them suffer from low molar activity due to precursor degradation. In this study, the potential of 1-(difluoromethyl)-3-methyl-4-phenyl-1H-1,2,3-triazol-3-ium triflate as precursor for the synthesis of the [18 F]trifluoromethylation building block [18 F]fluoroform with high molar activity was investigated. The triazolium precursor was reacted under various conditions with [18 F]fluoride, providing [18 F]fluoroform with radiochemical yields (RCY) and molar activities (Am ) comparable and even superior with already existing methods. Highest molar activities (Am  = 153 ± 14 GBq/μmol, dc, EOS) were observed for the automated procedure on the Neptis® perform module. Due to its easy handling and good RCY and Am in the [18 F]fluoroform synthesis, the triazolium precursor is a valuable alternative to already known precursors.

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.

Highlight selection of radiochemistry and radiopharmacy developments by editorial board

BACKGROUND

The Editorial Board of EJNMMI Radiopharmacy and Chemistry releases a biyearly highlight commentary to update the readership on trends in the field of radiopharmaceutical development.

RESULTS

This commentary of highlights has resulted in 21 different topics selected by each member of the Editorial Board addressing a variety of aspects ranging from novel radiochemistry to first in man application of novel radiopharmaceuticals. Also the first contribution in relation to MRI-agents is included.

CONCLUSIONS

Trends in (radio)chemistry and radiopharmacy are highlighted demonstrating the progress in the research field being the scope of EJNMMI Radiopharmacy and Chemistry.

Mechanistic study of nucleophilic fluorination for the synthesis of fluorine-18 labeled fluoroform with high molar activity from N-difluoromethyltriazolium triflate

The synthesis of fluorine-18 labeled fluoroform with high molar activity has grown in importance for the development of fluorine-18 labeled aryl-CF₃ radiopharmaceuticals that are useful as diagnostic radiotracers for the powerful technique of positron emission tomography (PET). We designed a strategy of synthesizing fluorine-18 labeled fluoroform from N1-difluoromethyl-N3-methyltriazolium triflate (1) via S_N2 fluorination without stable fluorine isotope scrambling. Fluoroform was generated at rt in 10 min by fluorination of the triazolium precursor with TBAF (6 equiv.). We propose three routes (a), (b), and (c) for this fluorination. Quantum chemical calculations have been carried out to elucidate the mechanism of experimentally observed nucleophilic attack of fluoride at difluoromethyl group via route (a), not N3-methyl via route (b). ¹H and ¹⁹F NMR studies using deuterium source have been performed to examine the competition between S_N2 fluorination (route (a)) and the formation of difluorocarbene (route (c)). The observed superiority of S_N2 pathway to formation of difluorocarbene in the reaction of the precursor using CsF in (CD₃CN/(CD₃)₃COD (17.8 : 1)) gives the possibility of preparing the fluorine-18 labeled fluoroform in high molar activity.

Route a: desired S_N2 reaction of fluoride to form fluoroform with high molar activity; route b: side reaction to form methyl fluoride; route c: side reaction to form difluorocarbene to give fluoroform with lower molar activity.

Syntheses of [11C]2- and [11C]3-trifluoromethyl-4-aminopyridine: potential PET radioligands for demyelinating diseases

Trifluoromethyl groups are of great interest in PET radiopharmaceuticals. Radiolabelled 4-aminopyridine (4AP) derivatives have been proposed for imaging demyelinating diseases. Here, we describe methods for producing 11C-trifluoromethylated derivatives of 4AP and present early imaging results with [11C]3-trifluoromethyl-4AP in a rhesus macaque. This study shows the utility of [11C]CuCF3 for labelling pyridines and provides initial evidence for the potential use of [11C]3-trifluoromethyl-4AP as a PET radioligand.

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.

Rapid Syntheses of [11C]Arylvinyltrifluoromethanes through Treatment of (E)-Arylvinyl(phenyl)iodonium Tosylates with [11C]Trifluoromethylcopper(I)

We report a method for labeling arylvinyltrifluoromethanes with carbon-11 (t_1/2 = 20.4 min) as representatives of a new radiolabeled chemotype that has potential for developing radiotracers for biomedical imaging with positron emission tomography. Treatment of (E)-arylvinyl(phenyl)iodonium tosylates (1a-1k) with [¹¹C[CuCF₃ gave the corresponding [¹¹C]arylvinyltrifluoromethanes ([¹¹C]2a-[¹¹C]2k) in high radiochemical yields (90-97%) under rapid (2 min) and mild (60 °C) conditions.

Copper-Mediated Late-stage Radiofluorination: Five Years of Impact on Pre-clinical and Clinical PET Imaging

PURPOSE

Copper-mediated radiofluorination (CMRF) is emerging as the method of choice for the formation of aromatic C-18F bonds. This minireview examines proof-of-concept, pre-clinical, and in-human imaging studies of new and established imaging agents containing aromatic C-18F bonds synthesized with CMRF. An exhaustive discussion of CMRF methods is not provided, although key developments that have enabled or improved upon the syntheses of fluorine-18 imaging agents are discussed.

METHODS

A comprehensive literature search from April 2014 onwards of the Web of Science and PubMed library databases was performed to find reports that utilize CMRF for the synthesis of fluorine-18 radiopharmaceuticals, and these represent the primary body of research discussed in this minireview. Select conference proceedings, previous reports describing alternative methods for the synthesis of imaging agents, and preceding fluorine-19 methodologies have also been included for discussion.

CONCLUSIONS

CMRF has significantly expanded the chemical space that is accessible to fluorine-18 radiolabeling with production methods that can meet the regulatory requirements for use in Nuclear Medicine. Furthermore, it has enabled novel and improved syntheses of radiopharmaceuticals and facilitated subsequent PET imaging studies. The rapid adoption of CMRF will undoubtedly continue to simplify the production of imaging agents and inspire the development of new radiofluorination methodologies.

Guest Edited Collection: Radioisotopes and radiochemistry in health science

Radioisotopes can be produced artificially from stable nuclei through the interaction with particles or highly energetic photons. In combination with modern detection and counting techniques, radioisotopes and radiochemical methods uniquely contribute to the health sciences. This Collection showcases salient aspects of medical radioisotope science ranging from the production, recovery and purification of radioisotopes to the methods used to attach them to biomolecules. The Collection also presents studies that highlight the importance of radiochemistry in the assessment of environmental radioactivity.