Site-Specific Deoxyfluorination of Small Peptides with [18 F]Fluoride

Ru-catalyzed activation of free phenols in a one-step Suzuki-Miyaura cross-coupling under mechanochemical conditions

Activation of phenols by a Ru-catalyst allows for the resulting η5-phenoxo complex to selectively react with a variety of nucleophiles under mechanochemical conditions. Conversion of phenolic hydroxy groups without derivatization is important for late-stage modifications of pharmaceuticals and in the context of lignin-material processing. We present a one-step, Ru-catalyzed cross-coupling of phenols with boronic acids, aryl trialkoxysilanes and potassium benzoyltrifluoroborates under mechano-chemical conditions. The protocol accepts a wide scope of starting materials and allows for gram-scale synthesis in excellent yields. The developed approach constitutes a very interesting and waste-limiting alternative to the known methods.

Molecular Imaging of Alzheimer's Disease-Related Sigma-1 Receptor in the Brain a Novel Ru-Mediated Aromatic 18F-deoxyfluorination Probe

Sigma-1 receptor (σ1R) is an intracellular protein implicated in a spectrum of neurodegenerative conditions, notably Alzheimer's disease (AD). Positron emission tomography (PET) imaging of brain σ1R could provide a powerful tool for better understanding the underlying pathomechanism of σ1R in AD. In this study, we successfully developed a 18F-labeled σ1R radiotracer [18F]CNY-05 via an innovative ruthenium (Ru)-mediated 18F-deoxyfluorination method. [18F]CNY-05 exhibited preferable brain uptake, high specific binding, and slightly reversible pharmacokinetics within the PET scanning time window. PET imaging of [18F]CNY-05 in nonhuman primates (NHP) indicated brain permeability, metabolic stability, and safety. Moreover, autoradiography and PET studies of [18F]CNY-05 in the AD mouse model found a significantly decreased brain uptake compared to that in wild-type mice. Collectively, we have provided a novel 18F-radiolabeled σ1R PET probe, which enables visualizing brain σ1R in health and neurological diseases.

Recent Progress in Peptide-Based Molecular Probes for Disease Bioimaging

Recent strides in molecular pathology have unveiled distinctive alterations at the molecular level throughout the onset and progression of diseases. Enhancing the in vivo visualization of these biomarkers is crucial for advancing disease classification, staging, and treatment strategies. Peptide-based molecular probes (PMPs) have emerged as versatile tools due to their exceptional ability to discern these molecular changes with unparalleled specificity and precision. In this Perspective, we first summarize the methodologies for crafting innovative functional peptides, emphasizing recent advancements in both peptide library technologies and computer-assisted peptide design approaches. Furthermore, we offer an overview of the latest advances in PMPs within the realm of biological imaging, showcasing their varied applications in diagnostic and therapeutic modalities. We also briefly address current challenges and potential future directions in this dynamic field.

Light-Driven Radiochemistry with Fluorine-18, Carbon-11 and Zirconium-89

This review discusses recent advances in light-driven radiochemistry for three key isotopes: fluorine-18, carbon-11, and zirconium-89, and their applications in positron emission tomography (PET). In the case of fluorine-18, the predominant approach involves the use of cyclotron-produced [18F]fluoride or reagents derived thereof. Light serves to activate either the substrate or the fluorine-18 labeled reagent. Advancements in carbon-11 photo-mediated radiochemistry have been leveraged for the radiolabeling of small molecules, achieving various transformations, including 11C-methylation, 11C-carboxylation, 11C-carbonylation, and 11C-cyanation. Contrastingly, zirconium-89 photo-mediated radiochemistry differs from fluorine-18 and carbon-11 approaches. In these cases, light facilitates a postlabeling click reaction, which has proven valuable for the labeling of large biomolecules such as monoclonal antibodies (mAbs). New technological developments, such as the incorporation of photoreactors in commercial radiosynthesizers, illustrate the commitment the field is making in embracing photochemistry. Taken together, these advances in photo-mediated radiochemistry enable radiochemists to apply new retrosynthetic strategies in accessing novel PET radiotracers.

Quaternary ammonium fluorides and difluorosilicates as nucleophilic fluorination reagents

TBAT (tetrabutylammonium difluorotriphenylsilicate) is an excellent homogeneous nucleophilic fluorination reagent, but a high excess of the reagent was reported to be essential. We hence optimized the reaction conditions and compared its nucleophilic fluorination reactivity with that of other common commercial nucleophilic fluorination reagents, such as anhydrous TBAF and TASF (tris(dimethylamino)sulfonium difluorotrimethylsilicate). As the substrates, we employed a standard set of primary and secondary octyl substrates under identical conditions. To eliminate the possibility of hydrogen fluoride elimination in the above reagents, we prepared four quaternary ammonium fluorides lacking β-elimination possibility in the hydrocarbon chain, transformed them to the corresponding difluorotriphenylsilicates, and compared their reactivity with that of the commercial reagents. Furthermore, attempts to isolate analogous tetrabutylammonium difluoromethyldiphenylsilicate or difluorodimethylphenylsilicate failed, as was confirmed by comparison of the published experimental data with computed 19F NMR spectra. Finally, we studied the transition states of decomposition of various tetramethylammonium methylphenyldifluorosilicates by DFT methods and found that their relative energies increase with an increasing number of phenyl groups. The formation of difluorosilicates is a nearly barrierless process.

Deoxyfluorination of phenols for chemoselective 18F-labeling of peptides

The challenge of forming C-18F bonds is often a bottleneck in the development of new 18F-labeled tracer molecules for noninvasive functional imaging studies using positron emission tomography (PET). Nucleophilic aromatic substitution is the most widely employed reaction to functionalize aromatic substrates with the radioactive fluorine-18 but its scope is restricted to arenes containing electron-withdrawing substituents. Furthermore, many protic functional groups are incompatible with basic fluoride anions. Peptide substrates, which are highly desirable targets for PET molecular imaging, are particularly challenging to label with fluorine-18 because they are densely functionalized and sensitive to high temperatures and basic conditions. To expand the utility of nucleophilic aromatic substitution with fluorine-18, we describe two complementary procedures for the radiodeoxyfluorination of bench-stable and easy-to-access phenols that ensure rapid access to densely functionalized electron-rich and electron-poor 18F-aryl fluorides. The first procedure details the synthesis of an 18F-synthon and its subsequent ligation to the cysteine residue of Arg-Gly-Asp-Cys in 10.5 h from commercially available starting materials (189-min radiosynthesis). The second procedure describes the incorporation of commercially available CpRu(Fmoc-tyrosine)OTf into a fully protected peptide Lys-Met-Glu-(CpRu-Tyr)-Leu via solid-phase peptide synthesis and subsequent ruthenium-mediated uronium deoxyfluorination with fluorine-18 followed by deprotection, accomplished within 7 d (116-min radiosynthesis). Both radiolabeling methods are highly chemoselective and have conveniently been automated using commercially available radiosynthesis equipment so that the procedures described can be employed for the synthesis of peptide-based PET probes for in vivo imaging studies according to as low as reasonably achievable (ALARA) principles.

Fluorinated Protein and Peptide Materials for Biomedical Applications

Fluorination represents one of the most powerful modern design strategies to impart biomacromolecules with unique functionality, empowering them for widespread application in the biomedical realm. However, the properties of fluorinated protein materials remain unpredictable due to the heavy context-dependency of the surrounding atoms influenced by fluorine’s strong electron-withdrawing tendencies. This review aims to discern patterns and elucidate design principles governing the biochemical synthesis and rational installation of fluorine into protein and peptide sequences for diverse biomedical applications. Several case studies are presented to deconvolute the overgeneralized fluorous stabilization effect and critically examine the duplicitous nature of the resultant enhanced chemical and thermostability as it applies to use as biomimetic therapeutics, drug delivery vehicles, and bioimaging modalities.

Hetero-Diels-Alder click reaction of dithioesters for a catalyst-free indirect 18F-radiolabelling of peptides

The HDA reaction of dithioesters was developed as a new click-reaction compatible with the indirect ¹⁸F-labelling of peptides. It involves dithioester-peptides and a radiofluorinated diene as a novel prosthetic group. The method was applied to a PSMA-ligand for the in vivo detection of LNCap tumors in xenografted mice.

Last-step 18F-fluorination of supported 2-(aryl-di-tert-butylsilyl)-N-methyl-imidazole conjugates for applications in positron emission tomography

Aiming for potential applications in positron emission tomography, fully automated productions of ¹⁸F-labelled bioconjugates were achieved using heterogenous precursors obtained by anchoring imidazole-di-tert-butyl-arylsilanes to a polystyrene resin. The reactions were performed using either "batch" or "flow" procedures, avoiding both the time-consuming azeotropic drying and HPLC purifications usually required.

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

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

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

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

Copper-Mediated Radiocyanation of Unprotected Amino Acids and Peptides

This report describes a copper-mediated radiocyanation of aryl halides that is applicable to complex molecules. This transformation tolerates an exceptionally wide range of functional groups, including unprotected amino acids. As such, it enables the site-specific introduction of [¹¹C]CN into peptides at an iodophenylalanine residue. The use of a diamine-ligated copper(I) mediator is crucial for achieving high radiochemical yield under relatively mild conditions, thus limiting racemization and competing side reactions of other amino acid side chains. The reaction has been scaled and automated to deliver radiolabeled peptides, including analogues of adrenocorticotropic hormone 1-27 (ACTH) and nociceptin (NOP). For instance, this Cu-mediated radiocyanation was leveraged to prepare >40 mCi of [¹¹C]cyano-NOP to evaluate biodistribution in a primate using positron emission tomography. This investigation provides preliminary evidence that nociceptin crosses the blood-brain barrier and shows uptake across all brain regions (SUV > 1 at 60 min post injection), consistent with the known distribution of NOP receptors in the rhesus brain.

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.

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.

Positron Emission Tomography Tracer Design of Targeted Synthetic Peptides via 18F-Sydnone Alkyne Cycloaddition

Chemically synthesized, small peptides that bind with high affinity and specificity to CD8-expressing (CD8+) tumor-infiltrating T cells, yet retain the desirable characteristics of small molecules, hold valuable potential for diagnostic molecular imaging of immune response. Here, we report the development of 18F-labeled peptides targeting human CD8α with nanomolar affinity via the strain-promoted sydnone-alkyne cycloaddition with 4-[18F]fluorophenyl sydnone. The 18F-sydnone is produced in one step, in high radiochemical yield, and the peptide labeling proceeds rapidly. A hydrophilic chemical linker results in a tracer with favorable pharmacokinetic properties and improved image contrast, as demonstrated by in vivo PET imaging studies.

Ultrafast Photoclick Reaction for Selective 18F-Positron Emission Tomography Tracer Synthesis in Flow

The development of very fast, clean, and selective methods for indirect labeling in PET tracer synthesis is an ongoing challenge. Here we present the development of an ultrafast photoclick method for the synthesis of short-lived ¹⁸F-PET tracers based on the photocycloaddition reaction of 9,10-phenanthrenequinones with electron-rich alkenes. The respective precursors are synthetically easily accessible and can be functionalized with various target groups. Using a flow photo-microreactor, the photoclick reaction can be performed in 60 s, and clinically relevant tracers for prostate cancer and bacterial infection imaging were prepared to demonstrate practicality of the method.

1,3-Dipolar Cycloaddition between Dehydroalanines and C,N-Cyclic Azomethine Imines: Application to Late-Stage Peptide Modification

A non-catalytic, mild, and easy-to-handle protecting group switched 1,3-dipolar cycloaddition (1,3-DC) between bi- or mono-N-protected Dha and C,N-cyclic azomethine imines, which afford various quaternary amino acids with diverse scaffolds, is disclosed. Specifically, normal-electron-demand 1,3-DC reaction occurs between bi-N-protected Dha and C,N-cyclic azomethine imines, while inverse-electron-demand 1,3-DC reaction occurs between mono-N-protected Dha and C,N-cyclic azomethine imines. Above all, the reactions can be carried out between peptides with Dha residues at the position of interest and C,N-cyclic azomethine imines, both in homogeneous phase and on resins in SPPS. It provides a new toolkit for late-stage peptide modification, labeling, and peptide-drug conjugation. To shed light on the high regioselectivity of the reaction, DFT calculations were carried out, which were qualitatively consistent with the experimental observations.

One-Step Synthesis of [18F]Fluoro-4-(vinylsulfonyl)benzene: A Thiol Reactive Synthon for Selective Radiofluorination of Peptides

Radiolabeled peptide-based molecular imaging probes exploit the advantages of large biologics and small molecules, providing both exquisite selectivity and favorable pharmacokinetic properties. Here, we report an operationally simple and broadly applicable approach for the ¹⁸F-fluorination of unprotected peptides via a new radiosynthon, [¹⁸F]fluoro-4-(vinylsulfonyl)benzene. This reagent demonstrates excellent chemoselectivity at the cysteine residue and rapid ¹⁸F-labeling of a diverse scope of peptides to generate stable thioether constructs.

Ruthenium-Mediated 18F-Fluorination and Preclinical Evaluation of a New CB1 Receptor Imaging Agent [18F]FPATPP

Cannabinoid receptor 1 (CB1R) controls various physiological and pathological conditions, including memory, motivation, and inflammation, and is thus an interesting target for positron emission tomography (PET). Herein, we report a ruthenium-mediated radiolabeling synthesis and preclinical evaluation of a new CB1R specific radiotracer, [¹⁸F]FPATPP. [¹⁸F]FPATPP was produced with 16.7 ± 5.7% decay-corrected radiochemical yield and >95 GBq/μmol molar activity. The tracer showed high stability, low defluorination, and high specific binding to CB1Rs in mouse brain.

Radiohybrid Ligands: A Novel Tracer Concept Exemplified by 18F- or 68Ga-Labeled rhPSMA Inhibitors

When we critically assess the reason for the current dominance of 68Ga-labeled peptides and peptide-like ligands in radiopharmacy and nuclear medicine, we have to conclude that the major advantage of such radiopharmaceuticals is the apparent lack of suitable 18F-labeling technologies with proven clinical relevance. To prepare and to subsequently perform a clinical proof-of-concept study on the general suitability of silicon-fluoride-acceptor (SiFA)-conjugated radiopharmaceuticals, we developed inhibitors of the prostate-specific membrane antigen (PSMA) that are labeled by isotopic exchange (IE). To compensate for the pronounced lipophilicity of the SiFA unit, we used metal chelates, conjugated in close proximity to SiFA. Six different radiohybrid PSMA ligands (rhPSMA ligands) were evaluated and compared with the commonly used 18F-PSMA inhibitors 18F-DCFPyL and 18F-PSMA-1007. Methods: All inhibitors were synthesized by solid-phase peptide synthesis. Human serum albumin binding was measured by affinity high-performance liquid chromatography, whereas the lipophilicity of each tracer was determined by the n-octanol/buffer method. In vitro studies (IC50, internalization) were performed on LNCaP cells. Biodistribution studies were conducted on LNCaP tumor-bearing male CB-17 SCID mice. Results: On the laboratory scale (starting activities, 0.2-9.0 GBq), labeling of 18F-rhPSMA-5 to -10 by IE was completed in < 20 min (radiochemical yields, 58% ± 9%; radiochemical purity, >97%) with molar activities of 12-60 GBq/μmol. All rhPSMAs showed low nanomolar affinity and high internalization by PSMA-expressing cells when compared with the reference radiopharmaceuticals, medium-to-low lipophilicity, and high human serum albumin binding. Biodistribution studies in LNCaP tumor-bearing mice revealed high tumor uptake, sufficiently fast clearance kinetics from blood, low hepatobiliary excretion, fast renal excretion, and very low uptake of 18F activity in bone. Conclusion: The novel 18F-rhPSMA radiopharmaceuticals developed under the radiohybrid concept show equal or better targeting characteristics than the established 18F-PSMA tracers 18F-DCFPyL and 18F-PSMA-1007. The unparalleled simplicity of production, the possibility to produce the identical 68Ga-labeled 19F-68Ga-rhPSMA tracers, and the possibility to extend this concept to true theranostic radiohybrid radiopharmaceuticals, such as F-Lu-rhPSMA, are unique features of these radiopharmaceuticals.

[18F]Atorvastatin: synthesis of a potential molecular imaging tool for the assessment of statin-related mechanisms of action

Background

Statins are lipid-lowering agents that inhibit cholesterol synthesis and are clinically used in the primary and secondary prevention of cardiovascular diseases. However, a considerable group of patients does not respond to statin treatment, and the reason for this is still not completely understood. [¹⁸F]Atorvastatin, the ¹⁸F-labeled version of one of the most widely prescribed statins, may be a useful tool for statin-related research.

Results

[¹⁸F]Atorvastatin was synthesized via an optimized ruthenium-mediated late-stage ¹⁸F-deoxyfluorination. The defluoro-hydroxy precursor was produced via Paal-Knorr pyrrole synthesis and was followed by coordination of the phenol to a ruthenium complex, affording the labeling precursor in approximately 10% overall yield. Optimization and automation of the labeling procedure reliably yielded an injectable solution of [¹⁸F]atorvastatin in 19% ± 6% (d.c.) with a molar activity of 65 ± 32 GBq·μmol⁻¹. Incubation of [¹⁸F]atorvastatin in human serum did not lead to decomposition. Furthermore, we have shown the ability of [¹⁸F]atorvastatin to cross the hepatic cell membrane to the cytosolic and microsomal fractions where HMG-CoA reductase is known to be highly expressed. Blocking assays using rat liver sections confirmed the specific binding to HMG-CoA reductase. Autoradiography on rat aorta stimulated to develop atherosclerotic plaques revealed that [¹⁸F]atorvastatin significantly accumulates in this tissue when compared to the healthy model.

Conclusions

The improved ruthenium-mediated ¹⁸F-deoxyfluorination procedure overcomes previous hurdles such as the addition of salt additives, the drying steps, or the use of different solvent mixtures at different phases of the process, which increases its practical use, and may allow faster translation to clinical settings. Based on tissue uptake evaluations, [¹⁸F]atorvastatin showed the potential to be used as a tool for the understanding of the mechanism of action of statins. Further knowledge of the in vivo biodistribution of [¹⁸F]atorvastatin may help to better understand the origin of off-target effects and potentially allow to distinguish between statin-resistant and non-resistant patients.

Original synthesis of radiolabeling precursors for batch and on resin one-step/late-stage radiofluorination of peptides

Radiolabeling of peptides with fluorine-18 is hurdled by their chemical sensitivity and complicated processes. Original triflyl-pyridine intermediates afforded ammonium precursors that were radiolabeled at low temperature. From that study, a generic tag has been designed to allow a simple one-step/late-stage radiolabelling of peptides. The strategy has been transposed to an automated "on-resin" radiolabelling.

Site-Selective Late-Stage Aromatic [18 F]Fluorination via Aryl Sulfonium Salts

Site-selective functionalization of C-H bonds in small complex molecules is a long-standing challenge in organic chemistry. Herein, we report a broadly applicable and site-selective aromatic C-H dibenzothiophenylation reaction. The conceptual advantage of this transformation is further demonstrated through the two-step C-H [18 F]fluorination of a series of marketed small-molecule drugs.

18F-Trifluoromethanesulfinate Enables Direct C-H 18F-Trifluoromethylation of Native Aromatic Residues in Peptides

18F labeling strategies for unmodified peptides with [18F]fluoride require 18F-labeled prosthetics for bioconjugation more often with cysteine thiols or lysine amines. Here we explore selective radical chemistry to target aromatic residues applying C-H 18F-trifluoromethylation. We report a one-step route to [18F]CF3SO2NH4 from [18F]fluoride and its application to direct [18F]CF3 incorporation at tryptophan or tyrosine residues using unmodified peptides as complex as recombinant human insulin. The fully automated radiosynthesis of octreotide[Trp(2-CF218F)] enables in vivo positron emission tomography imaging.