DFT calculations guide the design of dual-responsive molecule 9,10-phenanthrenequinone-sydnone

With the aid of density functional theory (DFT) calculations, we designed and synthesized a dual-responsive molecule 9,10-phenanthrenequinone-sydnone PQ-Syd, enabling tandem orthogonal reactions with furan-2(3H)-one derivatives and bicyclo[6.1.0]nonyne (BCN) for turn-on and fluorescence modulation. The PQ-Syd has been applied for protein labeling, and has potential for rapid screening of cells.

Molecular Matchmakers: Bioconjugation Techniques Enhance Prodrug Potency for Immunotherapy

Cancer patients suffer greatly from the severe off-target side effects of small molecule drugs, chemotherapy, and radiotherapy─therapies that offer little protection following remission. Engineered immunotherapies─including cytokines, immune checkpoint blockade, monoclonal antibodies, and CAR-T cells─provide better targeting and future tumor growth prevention. Still, issues such as ineffective activation, immunogenicity, and off-target effects remain primary concerns. "Prodrug" therapies─classified as therapies administered as inactive and then selectively activated to control the time and area of release─hold significant promise in overcoming these concerns. Bioconjugation techniques (e.g., natural linker conjugation, bioorthogonal reactions, and noncanonical amino acid incorporation) enable the rapid and homogeneous synthesis of prodrugs and offer selective loading of immunotherapeutic agents to carrier molecules and protecting groups to prevent off-target effects after administration. Several prodrug activation mechanisms have been highlighted for cancer therapeutics, including endogenous activation by hypoxic or acidic conditions common in tumors, exogenous activation by targeted bioorthogonal cleavage, or stimuli-responsive light activation, and dual-stimuli activation, which adds specificity by combining these mechanisms. This review will explore modern prodrug conjugation and activation options, focusing on how these strategies can enhance immunotherapy responses and improve patient outcomes. We will also discuss the implications of computational methodology for therapy design and recommend procedures to determine how and where to conjugate carrier systems and "prodrug" groups onto therapeutic agents to enhance the safety and control of these delivery platforms.

Sydnthiones are versatile bioorthogonal hydrogen sulfide donors

Hydrogen sulfide (H2S) is an important endogenous gasotransmitter, but the bioorthogonal reaction triggered H2S donors are still rare. Here we show one type of bioorthogonal H2S donors, sydnthiones (1,2,3-oxadiazol-3-ium-5-thiolate derivatives), which was designed with the aid of density functional theory (DFT) calculations. The reactions between sydnthiones and strained alkynes provide a platform for controllable, tunable and mitochondria-targeted release of H2S. We investigate the reactivity of sydnthiones‒dibenzoazacyclooctyne (DIBAC) reactions and their orthogonality with two other bioorthogonal cycloaddition pairs: tetrazine‒norbornene (Nor) and tetrazine‒monohydroxylated cyclooctyne (MOHO). By taking advantage of these mutually orthogonal reactions, we can realize selective labeling or drug release. Furthermore, we explore the role of H2S, which is released from the sydnthione-DIBAC reaction, on doxorubicin-induced cytotoxicity. The results demonstrate that the viability of H9c2 cells can be significantly improved by pretreating with sydnthione 1b and DIBAC for 6 h prior to exposure to Dox.

Sydnone-based prosthetic groups for radioiodination

The potential of Strained-Promoted Sydnone-Alkyne Cycloaddition (SPSAC) for radioiodination was evaluated with model cyclooctyne-conjugated peptides. Starting with a series of sydnones with varying N3 and C4 substitution, a preliminary kinetic study with non-radioactive iodinated compounds highlighted the superiority of an arylsydnone substituted by a chlorine atom in C4 position. Interestingly, reaction rate up to 11 times higher than using an azide was achieved with the best system. Access to 125I-labelled sydnones was granted with high efficiency from arylboronic acid precursors by copper catalyzed nucleophilic substitution. Application of SPSAC on the model peptide in radiotracer conditions showed the same trend than in non-radioactive kinetic study and complete reactions could be achieved within less than an hour for the best systems. These results are favorable for use in the production of radiopharmaceuticals with heavy halogens and increase the diversity of available bioorthogonal reaction for nuclear imaging and therapy.

Incorporation of Intracellular NanoSIMS Tracers to Oligonucleotide Conjugates via Strain Promoted Sydnone-Alkyne Cycloaddition

Extensive efforts have been dedicated to developing cell-specific targeting ligands that can be conjugated to therapeutic cargo, offering a promising yet still challenging strategy to deliver oligonucleotide therapeutics beyond the liver. Indeed, while the cargo and the ligand are crucial, the third component, the linker, is integral but is often overlooked. Here, we present strain-promoted sydnone-alkyne cycloaddition as a versatile linker chemistry for oligonucleotide synthesis, expanding the choices for bioconjugation of therapeutics while enabling subcellular detection of the linker and payload using nanoscale secondary ion mass spectrometry (NanoSIMS) imaging. This strategy was successfully applied to peptide and lipid ligands and profiled using the well characterized N-acetylgalactosamine (GalNAc) targeting ligand. The linker did not affect the expected activity of the conjugate and was detectable and distinguishable from the labeled cargo. Finally, this work not only offers a practical bioconjugation method but also enables the assessment of the linker's subcellular behavior, facilitating NanoSIMS imaging to monitor the three key components of therapeutic conjugates.

Hetero-Diels-Alder and CuAAC Click Reactions for Fluorine-18 Labeling of Peptides: Automation and Comparative Study of the Two Methods

Radiolabeled peptides are valuable tools for diagnosis or therapies; they are often radiofluorinated using an indirect approach based on an F-18 prosthetic group. Herein, we are reporting our results on the F-18 radiolabeling of three peptides using two different methods based on click reactions. The first one used the well-known CuAAC reaction, and the second one is based on our recently reported hetero-Diels-Alder (HDA) using a dithioesters (thia-Diels-Alder) reaction. Both methods have been automated, and the 18F-peptides were obtained in similar yields and synthesis time (37-39% decay corrected yields by both methods in 120-140 min). However, to obtain similar yields, the CuAAC needs a large amount of copper along with many additives, while the HDA is a catalyst and metal-free reaction necessitating only an appropriate ratio of water/ethanol. The HDA can therefore be considered as a minimalist method offering easy access to fluorine-18 labeled peptides and making it a valuable additional tool for the indirect and site-specific labeling of peptides or biomolecules.

Bioorthogonal Radiolabeling of Azide-Modified Bacteria Using [18F]FB-sulfo-DBCO

Purpose: This study was motivated by the need for better positron emission tomography (PET)-compatible tools to image bacterial infection. Our previous efforts have targeted bacteria-specific metabolism via assimilation of carbon-11 labeled d-amino acids into the bacterial cell wall. Since the chemical determinants of this incorporation are not fully understood, we sought a high-throughput method to label d-amino acid derived structures with fluorine-18. Our strategy employed a chemical biology approach, whereby an azide (-N3) bearing d-amino acid is incorporated into peptidoglycan muropeptides, with subsequent "click" cycloaddition with an 18F-labeled strained cyclooctyne partner. Procedures: A water-soluble, 18F-labeled and dibenzocyclooctyne (DBCO)-derived radiotracer ([18F]FB-sulfo-DBCO) was synthesized. This tracer was incubated with pathogenic bacteria treated with azide-bearing d-amino acids, and incorporated 18F was determined via gamma counting. In vitro uptake in bacteria previously treated with azide-modified d-amino acids was compared to that in cultures treated with amino acid controls. The biodistribution of [18F]FB-sulfo-DBCO was studied in a cohort of healthy mice with implications for future in vivo imaging. Results: The new strain-promoted azide-alkyne cycloaddition (SPAAC) radiotracer [18F]FB-sulfo-DBCO was synthesized with high radiochemical yield and purity via N-succinimidyl 4-[18F]fluorobenzoate ([18F]SFB). Accumulation of [18F]FB-sulfo-DBCO was significantly higher in several bacteria treated with azide-modified d-amino acids than in controls; for example, we observed 7 times greater [18F]FB-sulfo-DBCO ligation in Staphylococcus aureus cultures incubated with 3-azido-d-alanine versus those incubated with d-alanine. Conclusions: The SPAAC radiotracer [18F]FB-sulfo-DBCO was validated in vitro via metabolic labeling of azide-bearing peptidoglycan muropeptides. d-Amino acid-derived PET radiotracers may be more efficiently screened via [18F]FB-sulfo-DBCO 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.

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.

Sydnone: Synthesis, Reactivity and Biological Activities

Sydnones are among the most well-known mesoionic compounds. Since their synthesis in 1935 by Earl and Mecknay, numerous researches have shown that the chemical behavior, physical and biological properties of sydnones make them the most useful compounds in organic chemistry. Sydnones undergo thermal 1,3-dipolar cycloaddition reaction with dipolarophiles (alkynes or alkenes) to give exclusively derivatives containing a pyrazole moiety exhibiting numerous applications, such as pharmaceuticals and agrochemicals. However, the sydnone cycloaddition reaction with alkynes requires harsh conditions, like high temperatures and long reaction times, giving poor regioselectivity to the resulting products. To overcome these constraints, new reactions named CuSAC (Copper- Catalyzed Sydnone-Alkyne Cycloaddition) and SPSAC (Strain-Promoted Sydnone- Alkyne Cycloaddition) have been developed, leading to pyrazoles with interesting constant kinetics.

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.

Fluorination and hydrolytic stability of water-soluble platinum complexes with a borane-bridged diphosphoramidite ligand

The high fluorophilicity of borane-containing ligands offers promise for accessing new metallodrug candidates capable of bifunctional [¹⁸F]-positron emission tomography (PET) imaging, but this requires water soluble and hydrolytically stable ligands that can be fluorinated under mild conditions. Toward this goal, here we report the synthesis and characterization of water-soluble Pt(II) complexes containing a triaminoborane-bridged diphosphoramidite ligand called ^MeOTBDPhos that can be fluorinated using simple fluoride salts. NMR and XRD studies show that (^MeOTBDPhos)PtCl₂ (1) dissolves in water with cooperative H-OH addition across the bridgehead N-B bond to form 1-H2O. The B-OH bond in 1-H2O undergoes rapid displacement with fluoride (<10 min) when treated with CsF in MeCN to form 1-HF. 1-HF can also be prepared in <10 min by addition of KF to 1 in the presence Kryptofix® 222 and (HNEt₃)Cl in MeCN. In addition to using fluoride salts, we show how mononuclear 1 can be fluorinated with HBF₄·Et₂O to form dinuclear [(^MeOTBDPhos-HF)Pt(μ-Cl)]₂(BF₄)₂ (4-HF). Comparative studies show that the B-F bond in 1-HF undergoes hydrolysis as soon as it is dissolved in water or saline, but the B-F bond persists for hours when the pH of the solution is lowered to pH ≤ 2. In contrast to 1-HF, the B-F bond in dinuclear 4-HF persists for days when dissolved in water, which may be attributed to slow, sacrificial release of fluoride from the BF₄^- anion. The results show how cooperative N-B reactivity on the ligand can be leveraged to rapidly fluorinate water-soluble ^MeOTBDPhos complexes under mild conditions and afford suggestions for how to enhance hydrolytic B-F stability, as required for use in biomedical applications.

Biomacromolecule-Assisted Screening for Reaction Discovery and Catalyst Optimization

Reaction discovery and catalyst screening lie at the heart of synthetic organic chemistry. While there are efforts at de novo catalyst design using computation/artificial intelligence, at its core, synthetic chemistry is an experimental science. This review overviews biomacromolecule-assisted screening methods and the follow-on elaboration of chemistry so discovered. All three types of biomacromolecules discussed─enzymes, antibodies, and nucleic acids─have been used as "sensors" to provide a readout on product chirality exploiting their native chirality. Enzymatic sensing methods yield both UV-spectrophotometric and visible, colorimetric readouts. Antibody sensors provide direct fluorescent readout upon analyte binding in some cases or provide for cat-ELISA (Enzyme-Linked ImmunoSorbent Assay)-type readouts. DNA biomacromolecule-assisted screening allows for templation to facilitate reaction discovery, driving bimolecular reactions into a pseudo-unimolecular format. In addition, the ability to use DNA-encoded libraries permits the barcoding of reactants. All three types of biomacromolecule-based screens afford high sensitivity and selectivity. Among the chemical transformations discovered by enzymatic screening methods are the first Ni(0)-mediated asymmetric allylic amination and a new thiocyanopalladation/carbocyclization transformation in which both C-SCN and C-C bonds are fashioned sequentially. Cat-ELISA screening has identified new classes of sydnone-alkyne cycloadditions, and DNA-encoded screening has been exploited to uncover interesting oxidative Pd-mediated amido-alkyne/alkene coupling reactions.

Clickable Albumin Nanoparticles for Pretargeted Drug Delivery toward PD-L1 Overexpressing Tumors in Combination Immunotherapy

We present a simple methodology to design a pretargeted drug delivery system, based on clickable anti-programmed death ligand 1 (anti-PD-L1) antibodies (Abs) and clickable bovine serum albumin (BSA) nanoparticles (NPs). Pretargeted drug delivery is based on the decoupling of a targeting moiety and a drug-delivering vector which can then react in vivo after separate injections. This may be key to achieve active targeting of drug-delivering NPs toward cancerous tissue. In pretargeted approaches, drug-delivering NPs were observed to accumulate in a higher amount in the targeted tissue due to shielding-related enhanced blood circulation and size-related enhanced tissue penetration. In this work, BSA NPs were produced using the solvent precipitation methodology that renders colloidally stable NPs, which were subsequently functionalized with a clickable moiety based on chlorosydnone (Cl-Syd). Those reactive groups are able to specifically react with dibenzocyclooctyne (DBCO) groups in a click-type fashion, reaching second-order reaction rate constants as high as 1.9 M^-1·s^-1, which makes this reaction highly suitable for in vivo applications. The presence of reactive Cl-Syd was demonstrated by reacting the functionalized NPs with a DBCO-modified sulfo-cyanine-5 dye. With this reaction, it was possible to infer the number of reactive moieties per NPs. Finally, and with the aim of demonstrating the suitability of this system to be used in pretargeted strategies, functionalized fluorescent NPs were used to label H358 cells with a clickable anti-PD-L1 Ab, applying the reaction between Cl-Syd and DBCO as corresponding clickable groups. The results of these experiments demonstrate the bio-orthogonality of the system to perform the reaction in vitro, in a period as short as 15 min.

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.

Click and Bio-Orthogonal Reactions with Mesoionic Compounds

Click and bio-orthogonal reactions are dominated by cycloaddition reactions in general and 1,3-dipolar cycloadditions in particular. Among the dipoles routinely used for click chemistry, azides, nitrones, isonitriles, and nitrile oxides are the most popular. This review is focused on the emerging click chemistry that uses mesoionic compounds as dipole partners. Mesoionics are a very old family of molecules, but their use as reactants for click and bio-orthogonal chemistry is quite recent. The facility to derivatize these dipoles and to tune their reactivity toward cycloaddition reactions makes mesoionics an attractive opportunity for future click chemistry development. In addition, some compounds from this family are able to undergo click-and-release reactions, finding interesting applications in cells, as well as in animals. This review covers the synthetic access to main mesoionics, their reaction with dipolarophiles, and recent applications in chemical biology and heterocycle synthesis.

To View Your Biomolecule, Click inside the Cell

The surging development of bioorthogonal chemistry has profoundly transformed chemical biology over the last two decades. Involving chemical partners that specifically react together in highly complex biological fluids, this branch of chemistry now allows researchers to probe biomolecules in their natural habitat through metabolic labelling technologies. Chemical reporter strategies include metabolic glycan labelling, site-specific incorporation of unnatural amino acids in proteins, and post-synthetic labelling of nucleic acids. While a majority of literature reports mark cell-surface exposed targets, implementing bioorthogonal ligations in the interior of cells constitutes a more challenging task. Owing to limiting factors such as membrane permeability of reagents, fluorescence background due to hydrophobic interactions and off-target covalent binding, and suboptimal balance between reactivity and stability of the designed molecular reporters and probes, these strategies need mindful planning to achieve success. In this review, we discuss the hurdles encountered when targeting biomolecules localized in cell organelles and give an easily accessible summary of the strategies at hand for imaging intracellular targets.

Bioorthogonal chemistry

Bioorthogonal chemistry represents a class of high-yielding chemical reactions that proceed rapidly and selectively in biological environments without side reactions towards endogenous functional groups. Rooted in the principles of physical organic chemistry, bioorthogonal reactions are intrinsically selective transformations not commonly found in biology. Key reactions include native chemical ligation and the Staudinger ligation, copper-catalysed azide-alkyne cycloaddition, strain-promoted [3 + 2] reactions, tetrazine ligation, metal-catalysed coupling reactions, oxime and hydrazone ligations as well as photoinducible bioorthogonal reactions. Bioorthogonal chemistry has significant overlap with the broader field of 'click chemistry' - high-yielding reactions that are wide in scope and simple to perform, as recently exemplified by sulfuryl fluoride exchange chemistry. The underlying mechanisms of these transformations and their optimal conditions are described in this Primer, followed by discussion of how bioorthogonal chemistry has become essential to the fields of biomedical imaging, medicinal chemistry, protein synthesis, polymer science, materials science and surface science. The applications of bioorthogonal chemistry are diverse and include genetic code expansion and metabolic engineering, drug target identification, antibody-drug conjugation and drug delivery. This Primer describes standards for reproducibility and data deposition, outlines how current limitations are driving new research directions and discusses new opportunities for applying bioorthogonal chemistry to emerging problems in biology and biomedicine.

Review: PET imaging with macro- and middle-sized molecular probes

Recent progress in radiolabeling of macro- and middle-sized molecular probes has been extending possibilities to use PET molecular imaging for dynamic application to drug development and therapeutic evaluation. Theranostics concept also accelerated the use of macro- and middle-sized molecular probes for sharpening the contrast of proper target recognition even the cellular types/subtypes and proper selection of the patients who should be treated by the same molecules recognition. Here, brief summary of the present status of immuno-PET, and then further development of advanced technologies related to immuno-PET, peptidic PET probes, and nucleic acids PET probes are described.

Bioorthogonal Reactions in Animals

The advent of bioorthogonal chemistry has led to the development of powerful chemical tools that enable increasingly ambitious applications. In particular, these tools have made it possible to achieve what is considered to be the holy grail of many researchers involved in chemical biology: to perform unnatural chemical reactions within living organisms. In this minireview, we present an update of bioorthogonal reactions that have been carried out in animals for various applications. We outline the advances made in the understanding of fundamental biological processes, and the development of innovative imaging and therapeutic strategies using bioorthogonal chemistry.

Bioorthogonal Ligations and Cleavages in Chemical Biology

Bioorthogonal reactions including the bioorthogonal ligations and cleavages have become an active field of research in chemical biology, and they play important roles in chemical modification and functional regulation of biomolecules. This review summarizes the developments and applications of the representative bioorthogonal reactions including the Staudinger reactions, the metal-mediated bioorthogonal reactions, the strain-promoted cycloadditions, the inverse electron demand Diels-Alder reactions, the light-triggered bioorthogonal reactions, and the reactions of chloroquinoxalines and ortho-dithiophenols.

Antibody Pretargeting Based on Bioorthogonal Click Chemistry for Cancer Imaging and Targeted Radionuclide Therapy

Bioorthogonal click chemistry-employing antibody-conjugated trans-cyclooctenes (TCO) and tetrazine (Tz)-based radioligands able to covalently bind in vivo-appeared recently as a potential alternative to circumvent the hematotoxicity induced by radioimmunotherapy of solid tumors. This Review focuses on the recent advances concerning TCO/Tz pretargeting in both cancer imaging and targeted-radionuclide therapy for prospective clinical transfer. We exhaustively identified 25 PubMed publications reporting preclinical imaging and 5 therapy studies with full mAbs as targeting vectors, since its first application in 2010. The fast, safe, modulable, and specific TCO/Tz pretargeting showed high potential as a theranostic tool to get more personalized and precise cancer care. The recent optimizations reported here highlighted a possible first clinical evaluation of IEDDA pretargeting in the coming years.