Sulfur [18F]Fluoride Exchange Click Chemistry Enabled Ultrafast Late-Stage Radiosynthesis

Synthesis and Anticancer Activity Evaluation of Novel Carborane-Containing Isoflavonoid Analogues

Isoflavonoids represent a privileged structure derived from natural products with diverse bioactivities. Carborane has been utilized as a three-dimensional mimetic of phenyl rings in medicinal chemistry. Herein, we replaced the phenyl group of isoflavonoids with carborane and prepared a series of carborane-containing isoflavonoid analogues. Compounds 1d, 1g, and 1m showed significantly enhanced antiproliferative activities on a broad scope of cancer cell lines. Further studies indicated that both 1d and 1m inhibited JAK/STAT5, PI3K/AKT, and p38 MAPK pathways, leading to G1 cell cycle phase arrest. Additionally, both compounds reduced the expression of P-glycoprotein (P-gp), a key mediator in multidrug resistance, and reversed the resistance of chemotherapeutic agents in multidrug-resistant cells in vitro. The biodistribution of compounds 1d and 1m was evaluated through ICP-mass and positron emission tomography imaging studies. Taken together, these results suggested promising pharmaceutical properties for the carborane-containing isoflavonoid analogues.

Advances in sulfonyl exchange chemical biology: expanding druggable target space

Targeted covalent inhibitors possess advantages over reversible binding drugs, that include higher potency, enhanced selectivity and prolonged pharmacodynamic duration. The standard paradigm for covalent inhibitor discovery relies on the use of α,β-unsaturated carbonyl electrophiles to engage the nucleophilic cysteine thiol, but due to its rarity in binding sites, the amino acid is often not available for targeting. 10 years ago we highlighted the emerging potential of sulfonyl fluoride chemical probes that were initially found to serendipitously modify residues beyond cysteine, including tyrosine, lysine, histidine, serine and threonine. Since then, the rational application of sulfonyl fluorides and related sulfonyl exchange warheads to site-specifically target diverse amino acid residues in proteins using small molecules, oligonucleotides, peptides and proteins, has made considerable progress, which has significantly advanced covalent therapeutic discovery. Additionally, sulfonyl exchange chemistry has recently shown utility in the labeling of RNA and carbohydrates, further expanding the biomolecular diversity of addressable targets. This Perspective provides not only a timely update regarding this exciting area of research, thus serving as a useful resource to scientists working in the field, but areas of challenge and opportunity are highlighted that may stimulate new research at the chemistry-biology interface.

Organocatalytic SuFEx click reactions of SO2F2

An organocatalytic method for the SuFEx click reaction of gaseous SO2F2 is described. Different organic bases such as DBU, TBD, triethylamine and Hünig's base can efficiently catalyze the SuFEx of SO2F2 with various phenols to produce aryl fluorosulfates in 61-97% yields. Under the same conditions, pyridone, pyrazolone and amines can also react with SO2F2 to afford the corresponding heteroaryl fluorosulfates or sulfamoyl fluorides in good yields. In this process, molecular sieves absorb the acidic HF efficiently, avoiding the use of stoichiometric amounts of organosilicon reagents and excess bases.

Quinazoline-2,4(1 H,3 H)-dione Scaffold for development of a novel PARP-targeting PET probe for tumor imaging

PURPOSE

Overexpression of Poly (ADP-ribose) polymerase (PARP) is associated with many diseases such as oncological diseases. Several PARP-targeting radiotracers have been developed to detect tumor in recent years. Two 18F labelled probes based on Olaparib and Rucaparib molecular scaffolds have been evaluated in clinical trials, but their slow hepatic clearance hinders their tumor imaging performance. Although a number of positron emission tomography (PET) probes with lower liver uptake have been designed, the tumor to background ratios remains to be low. Therefore, we designed a probe with low lipid-water partition coefficient to solve this problem.

METHODS

A pyridine-containing quinazoline-2,4(1 H,3 H)-dione PARP-targeting group was rationally designed and used to conjugate with the chelator 2,2',2'',2'''-(1,4,7,10-tetraazacyclododecane-1,4,7,10-tetrayl)tetraacetic acid (DOTA) to prepare the lead compound named as SMIC-2001 for radiolabeling. In vitro experiments, the lipid-water partition coefficient, stability, binding affinity, and cellular uptake of [68Ga]Ga-SMIC-2001 were determined. In vivo experiments, the U87MG xenograft models were used to evaluate its tumor imaging properties.

RESULTS

[68Ga]Ga-SMIC-2001 showed a low Log D7.4 (-3.82 ± 0.06) and high affinity for PARP-1 (48.13 nM). In vivo study revealed that it exhibited a high tumor-to-background contrast in the U87MG xenograft models and mainly renal clearance. And the ratios of tumor to main organs were high except for the kidney (e.g. tumor to liver ratio reached 2.20 ± 0.51) at 60 min p.i.

CONCLUSION

In summary, pyridine-containing quinazoline-2,4(1 H,3 H)-dione is a novel PARP-targeting molecular scaffold for imaging probe development, and [68Ga]Ga-SMIC-2001 is a highly promising PET probe capable of imaging tumors with PARP overexpression.

Innovative Peptide Bioconjugation Chemistry with Radionuclides: Beyond Classical Click Chemistry

Background: The incorporation of radionuclides into peptides and larger biomolecules requires efficient and sometimes biorthogonal reaction conditions, to which click chemistry provides a convenient approach. Methods: Traditionally, click-based radiolabeling techniques have focused on classical click chemistry, such as copper(I)-catalyzed alkyne-azide [3+2] cycloaddition (CuAAC), strain-promoted azide-alkyne [3+2] cycloaddition (SPAAC), traceless Staudinger ligation, and inverse electron demand Diels-Alder (IEDDA). Results: However, newly emerging click-based radiolabeling techniques, including tyrosine-click, sulfo-click, sulfur(VI) fluoride exchange (SuFEx), thiol-ene click, azo coupling, hydrazone formations, oxime formations, and RIKEN click offer valuable alternatives to classical click chemistry. Conclusions: This review will discuss the applications of these techniques in peptide radiochemistry.

Synthesis and Reactivity of Masked Organic Sulfates

Nature offers a variety of structurally unique, sulfated endobiotics including sulfated glycosaminoglycans, sulfated tyrosine peptides, sulfated steroids/bile acids/catecholamines. Sulfated molecules display a large number of biological activities including antithrombotic, antimicrobial, anticancer, anti-inflammatory, and others, which arise from modulation of intracellular signaling and enhanced in vivo retention of certain hormones. These characteristics position sulfated molecules very favorably as drug-like agents. However, few have reached the clinic. Major hurdles exist in realizing sulfated molecules as drugs. This state-of-the-art has been transformed through recent works on the development of sulfate masking technologies for both alkyl (sulfated carbohydrates, sulfated steroids) and aryl (sTyr-bearing peptides/proteins, sulfated flavonoids) sulfates. This review compiles the literature on different strategies implemented for different types of sulfate groups. Starting from early efforts in protection of sulfate groups to the design of newer SuFEx, trichloroethyl, and gem-dimethyl-based protection technologies, this review presents the evolution and application of concepts in realizing highly diverse, sulfated molecules as candidate drugs and/or prodrugs. Overall, the newer strategies for sulfate masking and demasking are likely to greatly enhance the design and development of sulfated molecules as non-toxic drugs of the future.

Synthesis and preliminary evaluation of novel PET probes for GSK-3 imaging

Non-invasive imaging of GSK-3 expression in the brain will help to understand the role of GSK-3 in disease pathology and progression. Herein, we report the radiosynthesis and evaluation of two novel isonicotinamide based 18F labeled PET probes, [18F]2 and [18F]6 for noninvasive imaging of GSK3. Among the developed PET probes, the in vitro blood-brain permeability coefficient of 2 (38 ± 20 × 10-6 cm/s, n = 3) was found to be better than 6 (8.75 ± 3.90 × 10-6 cm/s, n = 5). The reference compounds 2 and 6 showed nanomolar affinity towards GSK-3α and GSK-3β. PET probe [18F]2 showed higher stability (100%) in mouse and human serums compared to [18F]6 (67.01 ± 4.93%, n = 3) in mouse serum and 66.20 ± 6.38%, n = 3) in human serum at 120 min post incubation. The in vivo imaging and blocking studies were performed in wild-type mice only with [18F]2 due to its observed stability. [18F]2 showed a SUV of 0.92 ± 0.28 (n = 6) in mice brain as early as 5 min post-injection followed by gradual clearance over time.

"AquaF" Building Blocks for Water-Compatible SN2 18F-Fluorination of Small-Molecule Radiotracers

Despite the widespread use of hydrophilic building blocks to incorporate 18F and improve tracer pharmacokinetics, achieving effective leaving group-mediated nucleophilic 18F-fluorination in water (excluding 18F/19F-exchange) remains a formidable challenge. Here, we present a water-compatible SN2 leaving group-mediated 18F-fluorination method employing preconjugated "AquaF" (phosphonamidic fluorides) building blocks. Among 19 compact tetracoordinated pentavalent P(V)-F candidates, the "AquaF" building blocks exhibit superior water solubility, sufficient capacity for 18F-fluorination in water, and excellent in vivo metabolic properties. Two nitropyridinol leaving groups, identified from a pool of leaving group candidates that further enhance the precursor water solubility, enable 18F-fluorination in water with a 10-2 M-1 s-1 level reaction rate constant (surpassing the 18F/19F-exchange) at room temperature. With the exergonic concerted SN2 18F-fluorination mechanism confirmed, this 18F-fluorination method achieves ∼90% radiochemical conversions and reaches a molar activity of 175 ± 40 GBq/μmol (using 12.2 GBq initial activity) in saline for 12 "AquaF"-modified proof-of-concept functional substrates and small-molecule 18F-tracers. [18F]AquaF-Flurpiridaz demonstrates significantly improved radiochemical yield and molar activity compared to 18F-Flurpiridaz, alongside enhanced cardiac uptake and heart/liver ratio in targeted myocardial perfusion imaging, providing a comprehensive illustration of "AquaF" building blocks-assisted water-compatible SN2 18F-fluorination of small-molecule radiotracers.

Covalent targeted radioligands potentiate radionuclide therapy

Targeted radionuclide therapy, in which radiopharmaceuticals deliver potent radionuclides to tumours for localized irradiation, has addressed unmet clinical needs and improved outcomes for patients with cancer1-4. A therapeutic radiopharmaceutical must achieve both sustainable tumour targeting and fast clearance from healthy tissue, which remains a major challenge5,6. A targeted ligation strategy that selectively fixes the radiopharmaceutical to the target protein in the tumour would be an ideal solution. Here we installed a sulfur (VI) fluoride exchange (SuFEx) chemistry-based linker on radiopharmaceuticals to prevent excessively fast tumour clearance. When the engineered radiopharmaceutical binds to the tumour-specific protein, the system undergoes a binding-to-ligation transition and readily conjugates to the tyrosine residues through the 'click' SuFEx reaction. The application of this strategy to a fibroblast activation protein (FAP) inhibitor (FAPI) triggered more than 80% covalent binding to the protein and almost no dissociation for six days. In mice, SuFEx-engineered FAPI showed 257% greater tumour uptake than did the original FAPI, and increased tumour retention by 13-fold. The uptake in healthy tissues was rapidly cleared. In a pilot imaging study, this strategy identified more tumour lesions in patients with cancer than did other methods. SuFEx-engineered FAPI also successfully achieved targeted β- and α-radionuclide therapy, causing nearly complete tumour regression in mice. Another SuFEx-engineered radioligand that targets prostate-specific membrane antigen (PSMA) also showed enhanced therapeutic efficacy. Considering the broad scope of proteins that can potentially be ligated to SuFEx warheads, it might be possible to adapt this strategy to other cancer targets.

Design, synthesis and evaluation of novel prostate-specific membrane antigen-targeted aryl [18F]fluorosulfate PET tracers

The expression of prostate-specific membrane antigen (PSMA) in prostate cancer is 100-1000 times higher than that in normal tissues, and it has shown great advantages in the diagnosis and treatment of prostate cancer. The combination of PSMA and PET imaging technology based on the principle of metabolic imaging can achieve high sensitivity and high specificity for diagnosis. Due to its suitable half-life (109 min) and good positron abundance (97%), as well as its cyclotron accelerated generation, 18F has the potential to be commercialize, which has attracted much attention. In this article, we synthesized a series of fluorosulfate PET tracers targeting PSMA. All four analogues have shown high affinity to PSMA (IC50 = 1.85-5.15 nM). After the radioisotope exchange labeling, [18F]L9 and [18F]L10 have PSMA specific cellular uptake (0.65 ± 0.04% AD and 1.19 ± 0.03% AD) and effectively accumulated in 22Rv1 xenograft mice model. This study demonstrates that PSMA-1007-based PSMA-targeted aryl [18F]fluorosulfate novel tracers have the potential for PET imaging in tumor tissues.

Iterative SuFEx approach for sequence-regulated oligosulfates and its extension to periodic copolymers

The synthesis of sequence-regulated oligosulfates has not yet been established due to the difficulties in precise reactivity control. In this work, we report an example of a multi-directional divergent iterative method to furnish oligosulfates based on a chain homologation approach, in which the fluorosulfate unit is regenerated. The oligosulfate sequences are determined by high resolution mass spectrometry of the hydrolyzed fragments, and polysulfate periodic copolymers are synthesized by using oligomeric bisfluorosulfates in a bi-directional fashion. The synthetic utility of this iterative ligation is demonstrated by preparing crosslinked network polymers as synthetic adhesive materials.

N-Fluorosulfonyl Guanidine: An Entry to N-Guanyl Sulfamides and Sulfamates

Here, we present the straightforward synthesis of N-fluorosulfonyl guanidine (1) from two industrial feedstocks, guanidine hydrochloride and sulfuryl fluoride (SO2F2), using SuFEx chemistry. Compound 1 exhibits excellent stability under ambient conditions and displays unique SuFEx reactivity toward amines and phenols to generate N-guanyl sulfamides and sulfamates that have rarely been accessed. Notably, water serves as an effective solvent in this process. Our protocol provides a reliable pathway for the synthesis and investigation of these novel guanidine-containing molecules.

Bioinspired Selenium-Nitrogen Exchange (SeNEx) Click Chemistry Suitable for Nanomole-Scale Medicinal Chemistry and Bioconjugation

Click chemistry is a powerful molecular assembly strategy for rapid functional discovery. The development of click reactions with new connecting linkage is of great importance for expanding the click chemistry toolbox. We report the first selenium-nitrogen exchange (SeNEx) click reaction between benzoselenazolones and terminal alkynes (Se-N to Se-C), which is inspired by the biochemical SeNEx between Ebselen and cysteine (Cys) residue (Se-N to Se-S). The formed selenoalkyne connection is readily elaborated, thus endowing this chemistry with multidimensional molecular diversity. Besides, this reaction is modular, predictable, and high-yielding, features fast kinetics (k2≥14.43 M-1 s-1), excellent functional group compatibility, and works well at miniaturization (nanomole-scale), opening up many interesting opportunities for organo-Se synthesis and bioconjugation, as exemplified by sequential click chemistry (coupled with ruthenium-catalyzed azide-alkyne cycloaddition (RuAAC) and sulfur-fluoride exchange (SuFEx)), selenomacrocycle synthesis, nanomole-scale synthesis of Se-containing natural product library and DNA-encoded library (DEL), late-stage peptide modification and ligation, and multiple functionalization of proteins. These results indicated that SeNEx is a useful strategy for new click chemistry developments, and the established SeNEx chemistry will serve as a transformative platform in multidisciplinary fields such as synthetic chemistry, material science, chemical biology, medical chemistry, and drug discovery.

Speciation and kinetics of fluoride transfer from tetra-n-butylammonium difluorotriphenylsilicate ('TBAT')

Tetra-n-butylammonium difluorotriphenylsilicate (TBAT) is a conveniently handled anhydrous fluoride source, commonly used as a surrogate for tetra-n-butylammonium fluoride (TBAF). While prior studies indicate that TBAT reacts rapidly with fluoride acceptors, little is known about the mechanism(s) of fluoride transfer. We report on the interrogation of the kinetics of three processes in which fluoride is transferred from TBAT, in THF and in MeCN, using a variety of NMR methods, including chemical exchange saturation transfer, magnetisation transfer, diffusion analysis, and 1D NOESY. These studies reveal ion-pairing between the tetra-n-butylammonium and difluorotriphenylsilicate moieties, and a very low but detectable degree of fluoride dissociation, which then undergoes further equilibria and/or induces decomposition, depending on the conditions. Degenerate exchange between TBAT and fluorotriphenylsilane (FTPS) is very rapid in THF, inherently increases in rate over time, and is profoundly sensitive to the presence of water. Addition of 2,6-di-tert-butylpyridine and 3 Å molecular sieves stabilises the exchange rate, and both dissociative and direct fluoride transfer are shown to proceed in parallel under these conditions. Degenerate exchange between TBAT and 2-naphthalenyl fluorosulfate (ARSF) is not detected at the NMR timescale in THF, and is slow in MeCN. For the latter, the exchange is near-fully inhibited by exogenous FTPS, indicating a predominantly dissociative character to this exchange process. Fluorination of benzyl bromide (BzBr) with TBAT in MeCN-d3 exhibits moderate progressive autoinhibition, and the initial rate of the reaction is supressed by the presence of exogenous FTPS. Overall, TBAT can act as a genuine surrogate for TBAF, as well as a reservoir for rapidly-reversible release of traces of it, with the relative contribution of the pathways depending, inter alia, on the identity of the fluoride acceptor, the solvent, and the concentration of endogenous or exogenous FTPS.

Desmethyl SuFEx-IT: SO2F2-Free Synthesis and Evaluation as a Fluorosulfurylating Agent

Access to SuFExable compounds was remarkably simplified by introduction of the solid FO2S-donor SuFEx-IT. However, the published process for preparation of this reagent relies on the use of sulfuryl fluoride (SO2F2), which is difficult to obtain and highly toxic. Herein, we disclose a simple protocol for SO2F2-free, hectogram-scale preparation of the analogous desmethyl SuFEx-IT from inexpensive starting materials. The reagent was prepared in a high (85%) total yield and without chromatographic purification steps. In addition, we demonstrate the utility of desmethyl SuFEx-IT by successful preparation of a series of fluorosulfates and sulfamoyl fluorides in high to excellent yields. As such, our work recognizes desmethyl SuFEx-IT as a valuable alternative to common FO2S-donors and enables cost-efficient access to substrates for SuFEx click chemistry.

One-Pot Three-Component Synthesis of Indolyl-4H-chromene-3-sulfonyl Fluoride: A Class of Important Pharmacophore

A one-pot, sequential three-component reaction between salicylaldehyde, indole, and 2-bromoprop-2-ene-1-sulfonyl fluoride (BPESF) has been demonstrated for the synthesis of sulfonyl fluoride substituted 4H-chromene derivatives in moderate to excellent yields (45%-94%). This one-pot sequential method features easily available starting materials, wide substrate scope, mild conditions, and great efficiency.

Design of Novel 18F-Labeled Amino Acid Tracers Using Sulfur 18F-Fluoride Exchange Click Chemistry

[18F]Gln-OSO2F, [18F]Arg-OSO2F, and [18F]FSY-OSO2F were designed by introducing sulfonyl 18F-fluoride onto glutamine, arginine, and tyrosine, respectively. [18F]FSY-OSO2F can be prepared directly by sulfur 18F-fluoride exchange, while [18F]Gln-OSO2F and [18F]Arg-OSO2F require a two-step labeling method. Those tracers retain their typical transport characteristics for unmodified amino acids. Both PET imaging and biodistribution confirmed that [18F]FSY-OSO2F visualized MCF-7 and 22Rv1 subcutaneous tumors with high contrast, and its tumor-to-muscle ratio was better than that of [18F]FET. However, [18F]Gln-OSO2F and [18F]Arg-OSO2F poorly image MCF-7 subcutaneous tumors, possibly due to differences in the types and amounts of transporters expressed in tumors. All three tracers can visualize the U87MG glioma. According to our biological evaluation, none of the tracers evaluated in this study exhibited obvious defluorination, and subtle structural changes led to different imaging characteristics, indicating that the application of sulfur 18F-fluoride exchange click chemistry in the design of radioactive sulfonyl fluoride amino acids is feasible and offers significant advantages.

Mask and Release Strategy-Enabled Diversity-Oriented Synthesis for DNA-Encoded Library

An ideal DNA-encoded library (DEL) selection requires the library to consist of diverse core skeletons and cover chemical space as much as possible. However, the lack of efficient on-DNA synthetic approaches toward core skeletons has greatly restricted the diversity of DEL. To mitigate this issue, this work disclosed a "Mask & Release" strategy to streamline the challenging on-DNA core skeleton synthesis. N-phenoxyacetamide is used as a masked phenol and versatile directing group to mediate diversified DNA-compatible C-H functionalization, introducing the 1st-dimensional diversity at a defined site, and simultaneously releasing the phenol functionality, which can facilitate the introduction of the 2nd diversity. This work not only provides a set of efficient syntheses toward DNA-conjugated drug-like core skeletons such as ortho-alkenyl/sulfiliminyl/cyclopropyl phenol, benzofuran, dihydrobenzofuran but also provides a paradigm for on-DNA core skeleton synthetic method development.

Evaluation of F-537-Tetrazine in a model for brain pretargeting imaging. Comparison to N-(3-[18F] fluoro-5-(1,2,4,5-tetrazin-3-yl)benzyl)propan-1-amine

Brain pretargeted nuclear imaging for the diagnosis of various neurodegenerative diseases is a quickly developing field. The tetrazine ligation is currently the most explored approach to achieve this goal due to its remarkable properties. In this work, we evaluated the performance of F-537-Tetrazine, previously developed by Biogen, and N-(3-[18F]fluoro-5-(1,2,4,5-tetrazin-3-yl)benzyl)propan-1-amine, previously developed in our group, thereby allowing for the direct comparison of these two imaging probes. The evaluation included synthesis, radiolabeling and a comparison of the physicochemical properties of the compounds. Furthermore, their performance was evaluated by in vitro and in vivo pretargeting models. This study indicated that N-(3-[18F] fluoro-5-(1,2,4,5-tetrazin-3-yl)benzyl)propan-1-amine might be more suited for brain pretargeted imaging.

CycloSiFA: The Next Generation of Silicon-Based Fluoride Acceptors for Positron Emission Tomography (PET)

The ring-opening Si-fluorination of a variety of azasilole derivatives cyclo-1-(iPr2 Si)-4-X-C6 H3 -2-CH2 NR (4: R=2,6-iPr2 C6 H3 , X=H; 4 a: R=2,4,6-Me3 C6 H2 , X=H; 9: R=2,6-iPr2 C6 H3 , X=tBuMe2 SiO; 10: R=2,6-iPr2 C6 H3 , X=OH; 13: R=2,6-iPr2 C6 H3 , X=HCCCH2 O; 22: R=2,6-iPr2 C6 H3 , X=tBuMe2 SiCH2 O) with different 19 F-fluoride sources was studied, optimized and the experience gained was used in a translational approach to create a straightforward 18 F-labelling protocol for the azasilole derivatives [18 F]6 and [18 F]14. The latter constitutes a potential clickable CycloSiFA prosthetic group which might be used in PET tracer development using Cu-catalysed triazole formation. Based on our findings, CycloSiFA has the potential to become a new entry into non-canonical labelling methodologies for radioactive PET tracer development.

Preparation of 18F-Labeled Tracers Targeting Fibroblast Activation Protein via Sulfur [18F]Fluoride Exchange Reaction

Early detection and treatment of cancers can significantly increase patient prognosis and enhance the quality of life of affected patients. The emerging significance of the tumor microenvironment (TME) as a new frontier for cancer diagnosis and therapy may be exploited by radiolabeled tracers for diagnostic imaging techniques such as positron emission tomography (PET). Cancer-associated fibroblasts (CAFs) within the TME are identified by biomarkers such as fibroblast activation protein alpha (FAPα), which are expressed on their surfaces. Targeting FAPα using small-molecule 18F-labeled inhibitors (FAPIs) has recently garnered significant attention for non-invasive tumor visualization using PET. Herein, two potent aryl-fluorosulfate-based FAPIs, 12 and 13, were synthetically prepared, and their inhibition potency was determined using a fluorimetric FAP assay to be IC50 9.63 and 4.17 nM, respectively. Radiofluorination was performed via the sulfur [18F]fluoride exchange ([18F]SuFEx) reaction to furnish [18F]12 and [18F]13 in high activity yields (AY) of 39-56% and molar activities (Am) between 20-55 GBq/µmol. In vitro experiments focused on the stability of the radiolabeled FAPIs after incubation with human serum, liver microsomes and liver cytosol. Preliminary PET studies of the radioligands were performed in healthy mice to investigate the in vivo biodistribution and 18F defluorination rate. Fast pharmacokinetics for the FAP-targeting tracers were retained and considerable bone uptake, caused by either 18F defluorination or radioligand accumulation, was observed. In summary, our findings demonstrate the efficiency of [18F]SuFEx as a radiolabeling method as well as its advantages and limitations with respect to PET tracer development.

Photoredox-catalyzed stereo- and regioselective vicinal fluorosulfonyl-borylation of unsaturated hydrocarbons

There has been considerable research on sulfur(vi) fluoride exchange (SuFEx) chemistry, which is considered to be a next-generation click reaction, and relies on the unique balance between reactivity and stability inherent in high valent organosulfur. The synthetic versatility of the bifunctional handles containing the fluorosulfonyl group presents great synthetic value and opportunity for drug discovery. However, the direct photoredox-catalyzed fluorosulfonyl-borylation process remains unexplored and challenging due to its system incompatibility and limited synthetic strategies. Herein, we developed a sequential photocatalytic radical difunctionalization strategy for the highly efficient stereoselective synthesis of vicinal fluorosulfonyl borides (VFSBs) with an integrated redox-active SO2F radical reagent. The VFSBs acted as orthogonal synthons, and were subjected to a range of convenient transformations via the cleavage of the C-B and S(vi)-F bonds, including halogenation, Suzuki coupling, hydrogenation, and the SuFEX click reaction, which demonstrated the great potential of the VFSB moieties for use in skeleton linkage and drug modification.

Click Chemistry and Radiochemistry: An Update

The term "click chemistry" describes a class of organic transformations that were developed to make chemical synthesis simpler and easier, in essence allowing chemists to combine molecular subunits as if they were puzzle pieces. Over the last 25 years, the click chemistry toolbox has swelled from the canonical copper-catalyzed azide-alkyne cycloaddition to encompass an array of ligations, including bioorthogonal variants, such as the strain-promoted azide-alkyne cycloaddition and the inverse electron-demand Diels-Alder reaction. Without question, the rise of click chemistry has impacted all areas of chemical and biological science. Yet the unique traits of radiopharmaceutical chemistry have made it particularly fertile ground for this technology. In this update, we seek to provide a comprehensive guide to recent developments at the intersection of click chemistry and radiopharmaceutical chemistry and to illuminate several exciting trends in the field, including the use of emergent click transformations in radiosynthesis, the clinical translation of novel probes synthesized using click chemistry, and the advent of click-based in vivo pretargeting.

Orthogonal Olefination with Organogermanes

Reported herein is a fully orthogonal olefination, which involves the site- and E-selective coupling of aryl germanes with alkenes, tolerating otherwise widely employed coupling handles such as aromatic (pseudo)halogens (C-I, C-Br, C-Cl, C-F, C-OTf, C-OSO2 F), silanes and boronic acid derivatives as well as alternative functionalities. This unprecedented [Ge]-based oxidative Heck coupling proceeds at room temperature with high speed (10 min to 2 hours) and operational simplicity owing to its base-free and air-tolerant features.

Recent Advances of S-18F Radiochemistry for Positron Emission Tomography

The click chemistry of sulfur(VI) fluoride exchange (SuFEx) has facilitated the widespread application of sulfur-fluoride compounds such as sulfonyl fluorides, fluorosulfates, and sulfamoyl fluorides in various fields, especially in the development of 18F ligands for PET (positron emission tomography) imaging. In recent years, the prominent progress of sulfur-[18F]fluoride compounds has been achieved through the combination of 18F and sulfur-fluoride chemistry. These compounds serve as potential 18F tracers, 18F synthons, and reagents for 18F-fluorination, thereby complementing the range of 18F ligands, typically C-18F structures, used in PET studies. This review aims to provide an overview of S-18F labeling reactions through examples of relevant 18F compounds and highlight the advancements and breakthroughs achieved in the past decade.

Good practices for the automated production of 18F-SiFA radiopharmaceuticals

BACKGROUND

The positron emitting isotope fluorine-18 (18F) possesses almost ideal physicochemical properties for the development of radiotracers for diagnostic molecular imaging employing positron emission tomography (PET). 18F in its nucleophilic anionic 18F- form is usually prepared by bombarding an enriched 18O water target with protons of various energies between 5 and 20 MeV depending on the technical specifications of the cyclotron. Large thick-target yields between 5 and 14 GBq/µA can be obtained, enough to prepare large batches of radiotracers capable to serve a considerable contingent of patients (50 + per clinical batch). The overall yield of the radiotracer however depends on the efficiency of the 18F labeling chemistry. The Silicon Fluoride Acceptor chemistry (SiFA) has introduced a convenient and highly efficient way to provide clinical peptide-based 18F-radiotracers in a kit-like procedure matching the convenience of 99mTc radiopharmaceuticals.

MAIN BODY

A radiotracer's clinical success primarily hinges on whether its synthesis can be automated. Due to its simplicity, the SiFA chemistry, which is based on isotopic exchange (18F for 19F), does not only work in a manual setup but has been proven to be automatable, yielding large batches of 18F-radiotracers of high molar activity (Am). The production of SiFA radiotracer can be centralized and the radiopharmaceutical be distributed via the "satellite" principle, where one production facility economically serves multiple clinical application sites. Clinically validated tracers such as [18F]SiTATE and [18F]Ga-rhPSMA-7/-7.3 have been synthesized in an automated synthesis unit under good manufacturing practice conditions and used in large patient cohorts. Communication of common guidelines and practices is warranted to further the dissemination of SiFA radiopharmaceuticals and to give easy access to this technology.

CONCLUSION

This current review highlights the most recent achievements in SiFA radiopharmaceutical automation geared towards large batch production for clinical application. Best practice advice and guidance towards a facilitated implementation of the SiFA technology into new and already operating PET tracer production facilities is provided. A brief outlook spotlights the future potential of SiFA radiochemistry within the landscape of non-canonical labeling chemistries.

Automated sulfur-[18F]fluoride exchange radiolabelling of a prostate specific membrane antigen (PSMA) targeted ligand using the GE FASTlab™ cassette-based platform

Sulfur-[18F]fluoride exchange radiochemistry is a rapid and convenient method for incorporating fluorine-18 into biologically active molecules. We report a fully automated radiolabelling procedure for the synthesis of a [18F]SO3F-bearing prostate specific membrane antigen (PSMA) targeted ligand ([18F]5) using the GE FASTLab™ cassette-based platform in a 25.0 ± 2.6% radiochemical yield (decay corrected). Uptake in vitro and in vivo correlated with PSMA expression, and the radioligand exhibited favourable biodistribution and pharmacokinetic profiles.

Preparation and Bioevaluation of 18F-Labeled Small-Molecular Radiotracers via Sulfur(VI) Fluoride Exchange Chemistry for Imaging of Programmed Cell Death Protein Ligand 1 Expression in Tumors

Nowadays, one of the most effective methods of tumor immunotherapy is blocking programmed cell death protein 1/programmed cell death protein ligand 1 (PD-1/PD-L1) immune checkpoints. However, there is still a significant challenge in selecting patients to benefit from immune checkpoint therapies. Positron emission tomography (PET), a noninvasive molecular imaging technique, offers a new approach to accurately detect PD-L1 expression and allows for a better prediction of response to PD-1/PD-L1 target immunotherapy. Here, we designed and synthesized a novel group of aryl fluorosulfate-containing small-molecule compounds (LGSu-1, LGSu-2, LGSu-3, and LGSu-4) based on the phenoxymethyl-biphenyl scaffold. After screening by the time-resolved fluorescence resonance energy transfer (TR-FRET) assay, the most potent compound LGSu-1 (half maximal inhibitory concentration (IC50): 15.53 nM) and the low-affinity compound LGSu-2 (IC50: 189.70 nM) as a control were selected for 18F-radiolabeling by sulfur(VI) fluoride exchange chemistry (SuFEx) to use for PET imaging. [18F]LGSu-1 and [18F]LGSu-2 were prepared by a one-step radiofluorination reaction in over 85% radioconversion and nearly 30% radiochemical yield. In B16-F10 melanoma cell assays, [18F]LGSu-1 (5.00 ± 0.06%AD) showed higher cellular uptake than [18F]LGSu-2 (2.55 ± 0.04%AD), in which cell uptake could be significantly blocked by the nonradioactivity LGSu-1. In vivo experiments, micro-PET imaging of B16-F10 tumor-bearing mice and radiographic autoradiography of tumor sections showed that [18F]LGSu-1 was more effectively accumulated in the tumor due to the higher binding affinity with PD-L1. The above experimental results confirmed the potential of the small-molecule probe LGSu-1 as a targeting PD-L1 imaging tracer in tumor tissues.

Sulfur fluoride exchange

Sulfur Fluoride Exchange (SuFEx) is a click reaction par excellence that has revolutionized multiple research fields. In this Primer, we delve into the essential elements of SuFEx operation, catalysis, and SuFExable connective hubs. We also explore the cutting-edge applications of SuFEx in drug development, polymer science, and biochemistry. Additionally, we examine the potential limitations and promising prospects for this versatile click reaction.

SVI-C Linkage for Allylsulfonximines via BF3-Mediated Allylation

A SuFEx linkage reaction between sulfonimidoyl fluoride and allyltrimethylsilane was achieved for the construction of N-modified allylsulfoximines in minutes with BF₃ as a nonmetal difunctional activator enabling the activation of both S-F and C-Si bonds to forge the S-C_allyl (sp³) bond swiftly. Mechanistic studies and DFT calculations indicated that the linkage was initiated with the activation of sulfonimidoyl fluoride and then followed with the transfer of the fluoride anion to the TMS group.

Prudent tactics to sail the boat of PARP inhibitors as therapeutics for diverse malignancies

INTRODUCTION

PARP inhibitors block the DNA-repairing mechanism of PARP and represent a promising class of anti-cancer therapy. The last decade has witnessed FDA approvals of several PARP inhibitors, with some undergoing advanced-stage clinical investigation. Medicinal chemists have invested much effort to expand the structure pool of PARP inhibitors. Issues associated with the use of PARP inhibitors that make their standing disconcerting in the pharmaceutical sector have been addressed via the design of new structural assemblages.

AREA COVERED

In this review, the authors present a detailed account of the medicinal chemistry campaigns conducted in the recent past for the construction of PARP1/PARP2 inhibitors, PARP1 biased inhibitors, and PARP targeting bifunctional inhibitors as well as PARP targeting degraders (PROTACs). Limitations associated with FDA-approved PARP inhibitors and strategies to outwit the limitations are also discussed.

EXPERT OPINION

The PARP inhibitory field has been rejuvenated with numerous tractable entries in the last decade. With numerous magic bullets in hand coupled with unfolded tactics to outwit the notoriety of cancer cells developing resistance toward PARP inhibitors, the dominance of PARP inhibitors as a sagacious option of targeted therapy is highly likely to be witnessed soon.

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.

Click chemistry: a transformative technology in nuclear medicine

The 2022 Nobel Prize in Chemistry was awarded to Professors K. Barry Sharpless, Morten Meldal and Carolyn Bertozzi for their pioneering roles in the advent of click chemistry. Sharpless and Meldal worked to develop the canonical click reaction-the copper-catalyzed azide-alkyne cycloaddition-while Bertozzi opened new frontiers with the creation of the bioorthogonal strain-promoted azide-alkyne cycloaddition. These two reactions have revolutionized chemical and biological science by facilitating selective, high yielding, rapid and clean ligations and by providing unprecedented ways to manipulate living systems. Click chemistry has affected every aspect of chemistry and chemical biology, but few disciplines have been impacted as much as radiopharmaceutical chemistry. The importance of speed and selectivity in radiochemistry make it an almost tailor-made application of click chemistry. In this Perspective, we discuss the ways in which the copper-catalyzed azide-alkyne cycloaddition, the strain-promoted azide-alkyne cycloaddition and a handful of 'next-generation' click reactions have transformed radiopharmaceutical chemistry, both as tools for more efficient radiosyntheses and as linchpins of technologies that have the potential to improve nuclear medicine.

Advances in the construction of diverse SuFEx linkers

Sulfur fluoride exchange (SuFEx), a new generation of click chemistry, was first presented by Sharpless, Dong and co-workers in 2014. Owing to the high stability and yet efficient reactivity of the S^VI-F bond, SuFEx has found widespread applications in organic synthesis, materials science, chemical biology and drug discovery. A diverse collection of SuFEx linkers has emerged, involving gaseous SO₂F₂ and SOF₄ hubs; SOF₄-derived iminosulfur oxydifluorides; O-, N- and C-attached sulfonyl fluorides and sulfonimidoyl fluorides; and novel sulfondiimidoyl fluorides. This review summarizes the progress of these SuFEx connectors, with an emphasis on analysing the advantages and disadvantages of synthetic strategies of these connectors based on the SuFEx concept, and it is expected to be beneficial to researchers to rapidly and correctly understand this field, thus inspiring further development in SuFEx chemistry.

High-Throughput Diversification of Complex Bioactive Molecules by Accelerated Synthesis in Microdroplets

Late-stage diversification of drug molecules is an important strategy in drug discovery that can be facilitated by reaction screening using high-throughput experimentation. Here we present a rapid method for functionalizing bioactive molecules based on accelerated reactions in microdroplets. Reaction mixtures are nebulized at throughputs better than 1 reaction/second and the accelerated reactions occurring in the microdroplets are followed by desorption electrospray ionization mass spectrometry (DESI-MS). Because the accelerated reactions occur on the millisecond timescale, they allow an overall screening throughput of 1 Hz working at the low nanogram scale. Using this approach, an opioid agonist (PZM21) and an antagonist (naloxone) were diversified using three reactions important in medicinal chemistry: sulfur fluoride exchange (SuFEx) click reactions, imine formation reactions, and ene-type click reactions. Some 269 functionalized analogs of naloxone and PZM21 were generated and characterized by tandem mass spectrometry (MS/MS) after screening over 500 reactions.

Radiometallation and photo-triggered release of ready-to-inject radiopharmaceuticals from the solid phase

The efficient, large-scale synthesis of radiometallated radiopharmaceuticals represents an emerging clinical need which, to date, is inherently limited by time consuming, sequential procedures to conduct isotope separation, radiochemical labeling and purification prior to formulation for injection into the patient. In this work, we demonstrate that a solid-phase based, concerted separation and radiosynthesis strategy followed by photochemical release of radiotracer in biocompatible solvents can be employed to prepare ready-to-inject, clinical grade radiopharmaceuticals. Optimization of resin base, resin loading, and radiochemical labeling capacity are demonstrated with ⁶⁷Ga and ⁶⁴Cu radioisotopes using a short model peptide sequence and further validated using two peptide-based radiopharmaceuticals with clinical relevance, targeting the gastrin-releasing peptide and the prostate specific membrane antigen. We also demonstrate that the solid-phase approach enables separation of non-radioactive carrier ions Zn²⁺ and Ni²⁺ present at 10⁵-fold excess over ⁶⁷Ga and ⁶⁴Cu by taking advantage of the superior Ga³⁺ and Cu²⁺ binding affinity of the solid-phase appended, chelator-functionalized peptide. Finally, a proof of concept radiolabeling and subsequent preclinical PET-CT study with the clinically employed positron emitter ⁶⁸Ga successfully exemplifies that Solid Phase Radiometallation Photorelease (SPRP) allows the streamlined preparation of radiometallated radiopharmaceuticals by concerted, selective radiometal ion capture, radiolabeling and photorelease.

Synthesis, Fluorine-18 Radiolabeling, and In Vivo PET Imaging of a Hydrophilic Fluorosulfotetrazine

The development of ¹⁸F-fluorotetrazines, suitable for the radiolabeling of biologics such as proteins and antibodies by IEDDA ligation, represents a major challenge, especially for pre-targeting applications. The hydrophilicity of the tetrazine has clearly become a crucial parameter for the performance of in vivo chemistry. In this study, we present the design, the synthesis, the radiosynthesis, the physicochemical characterization, the in vitro and in vivo stability, as well as the pharmacokinetics and the biodistribution determined by PET imaging in healthy animals of an original hydrophilic ¹⁸F-fluorosulfotetrazine. This tetrazine was prepared and radiolabelled with fluorine-18 according to a three-step procedure, starting from propargylic butanesultone as the precursor. The propargylic sultone was converted into the corresponding propargylic fluorosulfonate by a ring-opening reaction with ^18/19F-fluoride. Propargylic ^18/19F-fluorosulfonate was then subject to a CuACC reaction with an azidotetrazine, followed by oxidation. The overall automated radiosynthesis afforded the ¹⁸F-fluorosulfotetrazine in 29-35% DCY, within 90-95 min. The experimental LogP and LogD_7.4 values of -1.27 ± 0.02 and -1.70 ± 0.02, respectively, confirmed the hydrophilicity of the ¹⁸F-fluorosulfotetrazine. In vitro and in vivo studies displayed a total stability of the ¹⁸F-fluorosulfotetrazine without any traces of metabolization, the absence of non-specific retention in all organs, and the appropriate pharmacokinetics for pre-targeting applications.

Direct 18F-Fluorosulfurylation of Phenols and Amines Using an [18F]FSO2+ Transfer Agent Generated In Situ

We report the direct radiofluorosulfurylation method for the synthesis of ¹⁸F-labeled fluorosulfuryl derivatives from phenols and amines using an [¹⁸F]FSO₂⁺ transfer agent generated in situ. Nucleophilic radiofluorination is achieved even in a hydrous organic medium, obviating the need for azeotropic drying and the use of cryptands. This unprecedented, operationally simple isotopic functionalization facilitates the reliable production of potential radiotracers for positron emission tomography, rendering facile access to SuFEx radiochemistry.

Radiosynthesis of 18F-Labeled Arenesulfonyl Fluorides through Two-Bond Construction with [18F]Fluoride

A novel ¹⁸F labeling strategy was developed to directly construct aryl-SO₂-¹⁸F from arenediazonium tosylates with a SO₂ source and [¹⁸F]fluoride. This approach is compatible with a wide range of substrates and enabled the production of ¹⁸F-labeled drug-like derivatives through late-stage ¹⁸F fluorination, representing a significant advance in the radiosynthesis of ¹⁸F-labeled arenesulfonyl fluorides. A reactive ¹⁸F labeling synthon, bearing a maleimide-based prosthetic group, allowed for the generation of ¹⁸F-labeled temperature-sensitive biomolecules containing cysteine residues via maleimide-cysteine chemistry.

Imidazolium-methylene-trifluoroborate: A novel radioprosthetic group validated with preclinical 18 F-Positron Emission Tomography imaging of Prostate Specific Membrane Antigen in mice

Organotrifluoroborates have gained acceptance as radioprosthetic groups for radiofluorination. Of these, the zwitterionic prosthetic group "AMBF₃ " with a quaternary dimethylammonium ion dominates the trifluoroborate space. Herein, we report on imidazolium-methylene trifluoroborate (ImMBF₃ ) as an alternative radioprosthetic group and report on its properties in the context of a PSMA-targeting EUK ligand that was previously been conjugated to AMBF₃ . The ImMBF₃ is readily synthesized from imidazole and conjugated via CuAAC "click" chemistry to give a structure similar to PSMA-617. ¹⁸ F-labeling proceeded in one step per our previous reports and imaged in LNCaP-xenograft bearing mice. The [¹⁸ F]-PSMA-617-ImMBF₃ tracer proved to be less polar (LogP_7.4  = -2.95 ± 0.03) while showing a significantly lower solvolytic rate (t_1/2  = 8100 min) and slightly higher molar activity (Am) at 174 ± 38 GBq/μmol. Tumor uptake was measured at 13.7 ± 4.8%ID/g and a tumor:muscle ratio of 74.2 ± 35.0, tumor:blood ratio of 21.4 ± 7.0, tumor:kidney ratio of 0.29 ± 0.14, and tumor:bone ratio of 23.5 ± 9.5. In comparison with previously reported PSMA-targeting EUK-AMBF₃ conjugates, we have altered the LogP_7.4 value, tuned the solvolytic half-life of the prosthetic, and increased radiochemical conversion while achieving similar tumor uptake, contrast ratios, and molar activities compared with AMBF₃ bioconjugates.

Sulfur(VI) fluorides as tools in biomolecular and medicinal chemistry

Recent advances in the synthesis of sulfur(VI)-fluorides has enabled incredible growth in their application in biomolecular chemistry. This review aims to serve as a primer highlighting synthetic strategies toward a diversity of S(VI) fluorides and their application in chemical biology, bioconjugation, and medicinal chemistry.

Mechanistic Modeling of Lys745 Sulfonylation in EGFR C797S Reveals Chemical Determinants for Inhibitor Activity and Discriminates Reversible from Irreversible Agents

Targeted covalent inhibitors hold promise for drug discovery, particularly for kinases. Targeting the catalytic lysine of epidermal growth factor receptor (EGFR) has attracted attention as a new strategy to overcome resistance due to the emergence of C797S mutation. Sulfonyl fluoride derivatives able to inhibit EGFR^{L858R/T790M/C797S} by sulfonylation of Lys745 have been reported. However, atomistic details of this process are still poorly understood. Here, we describe the mechanism of inhibition of an innovative class of compounds that covalently engage the catalytic lysine of EGFR, through a sulfur(VI) fluoride exchange (SuFEx) process, with the help of hybrid quantum mechanics/molecular mechanics (QM/MM) and path collective variables (PCVs) approaches. Our simulations identify the chemical determinants accounting for the irreversible activity of agents targeting Lys745 and provide hints for the further optimization of sulfonyl fluoride agents.

An Investigation into the In Vitro Metabolic Stability of Aryl Sulfonyl Fluorides for their Application in Medicinal Chemistry and Radiochemistry

Molecules that feature a sulfonyl fluoride (SO₂F) moiety have been gaining increasing interest due to their unique reactivity and potential applications in synthetic chemistry, medicinal chemistry, and other biological uses. A particular interest is towards ¹⁸F-radiochemistry where sulfonyl fluorides can be used as a method to radiolabel biomolecules or can be used as radiofluoride relay reagents that facilitate radiolabeling of other molecules. The low metabolic stability of sulfonyl fluoride S-F bonds, however, presents an issue and limits the applicability of sulfonyl fluorides. The aim of this work was to increase understanding of what features contribute to the metabolic instability of the S-F bond in model aryl sulfonyl fluorides and identify approaches to increasing sulfonyl fluoride stability for ¹⁸F-radiochemistry and other medicinal, synthetic chemistry and biological applications. To undertake this, 14 model aryl sulfonyl fluorides compounds with varying functional groups and substitution patterns were investigated, and their stabilities were examined in various media, including phosphate-buffered saline and rat serum as a model for biological conditions. The results indicate that both electronic and steric factors affect the stability of the S-F bond, with the 2,4,6-trisubstituted model aryl sulfonyl fluorides examined displaying the highest in vitro metabolic stability.

Isotope Exchange-Based 18F-Labeling Methods

¹⁸F-Labeling methods for the preparation of ¹⁸F-labeled molecular probes can be classified into electrophilic fluorination, nucleophilic fluorination, metal-F coordination, and ¹⁸F/¹⁹F isotope exchange. Isotope exchange-based ¹⁸F-labeling methods demonstrate mild conditions featuring water resistance and facile high-performance liquid chromatography-free purification in direct ¹⁸F-labeling of substrates. This paper systematically reviews isotope exchange-based ¹⁸F-labeling methods sorted by the adjacent atom bonding with F, i.e., carbon and noncarbon atoms (Si, B, P, S, Ga, Fe, etc.). The respective isotope exchange mechanism, radiolabeling condition, radiochemical yield, molar activity, and stability of the ¹⁸F-product are mainly discussed for each isotope exchange-based ¹⁸F-labeling method as well as the cutting-edge application of the corresponding ¹⁸F-labeled molecular probes.

Design, synthesis, and preliminary evaluation of [18F]-aryl flurosulfates PET radiotracers via SuFEx methods for β-amyloid plaques in Alzheimer's disease

[18F]BAY-94-9172, [18F]AV-45, and [18F]GE-067 were FDA approved positron emission tomography (PET) imaging radiotracer of β-amyloid plaques (Aβ) in Alzheimer's disease (AD). However, the radiochemical synthesis requires multi-step reactions and complex procedure. Recently, a protocol for radiochemical synthesis of sulfur fluoride exchange (SuFEx) using ultrafast 19F/18F isotopic exchange had been reported. We developed three pairs of novel 18F-labeled radiotracers by the "SuFEx" method for PET imaging Aβ plaques. The 18F labeling reaction can be completed quickly (30 s) at room temperature and purified using solid-phase extraction (SPE). The radiochemical purity (RCP) of the products was all greater than 95 %. In vitro fluorescent staining using Aβ-transgenesis mice section preliminary verified the affinity of tracers with Aβ. Competitive binding assay displayed high affinity of tracers for towards artificial Aβ1-42 aggregates (Ki values ranging from 3.53 ± 0.39 to 42.0 ± 4.24 nM). In vivo biodistribution and Micro-PET imaging showed that [18F]-Sulfur Fluoride β-Amyloid ([18F]SFA 1-6) could penetration the blood-brain barrier (BBB) in wild-type mice, and [18F]SFA 5-6 had a high initial brain uptake value (3.65 ± 0.9 % and 5.07 ± 0.1 % ID/g, respectively) and a fast washout (Brain uptake2 min/60 min = 4.15 and 4.61, respectively) from the brain. In vitro autoradiography demonstrated the affinity of the [18F]SFA 5-6 to Aβ plaques in AD human brain tissues. Our results suggested that [18F]SFA maybe a potential PET radiotracers for detecting Aβ in Alzheimer's disease.

Aryl Fluorosulfate Based Inhibitors That Covalently Target the SIRT5 Lysine Deacylase

The sirtuin enzymes are a family of lysine deacylases that regulate gene transcription and metabolism. Sirtuin 5 (SIRT5) hydrolyzes malonyl, succinyl, and glutaryl ϵ-N-carboxyacyllysine posttranslational modifications and has recently emerged as a vulnerability in certain cancers. However, chemical probes to illuminate its potential as a pharmacological target have been lacking. Here we report the harnessing of aryl fluorosulfate-based electrophiles as an avenue to furnish covalent inhibitors that target SIRT5. Alkyne-tagged affinity-labeling agents recognize and capture overexpressed SIRT5 in cultured HEK293T cells and can label SIRT5 in the hearts of mice upon intravenous injection of the compound. This work demonstrates the utility of aryl fluorosulfate electrophiles for targeting of SIRT5 and suggests this as a means for the development of potential covalent drug candidates. It is our hope that these results will serve as inspiration for future studies investigating SIRT5 and general sirtuin biology in the mitochondria.

Recent Advances in 18F-Labeled Amino Acids Synthesis and Application

Radiolabeled amino acids are an important class of agents for positron emission tomography imaging that target amino acid transporters in many tumor types. Traditional ¹⁸F-labeled amino acid synthesis strategies are always based on nucleophilic aromatic substitution reactions with multistep radiosynthesis and low radiochemical yields. In recent years, new ¹⁸F-labeling methodologies such as metal-catalyzed radiofluorination and heteroatom (B, P, S, Si, etc.)-¹⁸F bond formation are being effectively used to synthesize radiopharmaceuticals. This review focuses on recent advances in the synthesis, radiolabeling, and application of a series of ¹⁸F-labeled amino acid analogs using new ¹⁸F-labeling strategies.