Radical-Mediated Thiol-Ene Strategy: Photoactivation of Thiol-Containing Drugs in Cancer Cells

Smart responsive in situ hydrogel systems applied in bone tissue engineering

The repair of irregular bone tissue suffers severe clinical problems due to the scarcity of an appropriate therapeutic carrier that can match dynamic and complex bone damage. Fortunately, stimuli-responsive in situ hydrogel systems that are triggered by a special microenvironment could be an ideal method of regenerating bone tissue because of the injectability, in situ gelatin, and spatiotemporally tunable drug release. Herein, we introduce the two main stimulus-response approaches, exogenous and endogenous, to forming in situ hydrogels in bone tissue engineering. First, we summarize specific and distinct responses to an extensive range of external stimuli (e.g., ultraviolet, near-infrared, ultrasound, etc.) to form in situ hydrogels created from biocompatible materials modified by various functional groups or hybrid functional nanoparticles. Furthermore, "smart" hydrogels, which respond to endogenous physiological or environmental stimuli (e.g., temperature, pH, enzyme, etc.), can achieve in situ gelation by one injection in vivo without additional intervention. Moreover, the mild chemistry response-mediated in situ hydrogel systems also offer fascinating prospects in bone tissue engineering, such as a Diels-Alder, Michael addition, thiol-Michael addition, and Schiff reactions, etc. The recent developments and challenges of various smart in situ hydrogels and their application to drug administration and bone tissue engineering are discussed in this review. It is anticipated that advanced strategies and innovative ideas of in situ hydrogels will be exploited in the clinical field and increase the quality of life for patients with bone damage.

A novel bioresponsive self-immolative spacer based on aza-quinone methide reactivity for the controlled release of thiols, phenols, amines, sulfonamides or amides

A stimuli-sensitive linker is one of the indispensable components of prodrugs for cancer therapy as it covalently binds the drug and releases it upon external stimulation at the tumour site. Quinone methide elimination has been widely used as the key transformation to release drugs based on their nucleofugacity. The usual approach is to bind the drug to the linker as a carbamate and release it as a free amine after a self-immolative 1,6-elimination. Although this approach is very efficient, it is limited to amines (as carbamates), alcohols or phenols (as carbonates) or other acidic functional groups. We report here a self-immolative spacer capable of directly linking and releasing amines, phenols, thiols, sulfonamides and carboxyamides after a reductive stimulus. The spacer is based on the structure of (5-nitro-2-pyrrolyl)methanol (NPYM-OH), which was used for the direct alkylation of the functional groups mentioned above. The spacer is metabolically stable and has three indispensable sites for bioconjugation: the bioresponsive trigger, the conjugated 1,6 self-immolative system and a third arm suitable for conjugation with a carrier or other modifiers. Release was achieved by selective reduction of the nitro group over Fe/Pd nanoparticles (NPs) in a micellar aqueous environment (H2O/TPGS-750-M), or by NADH mediated nitroreductase activation. A DFT study demonstrates that, during the 1,6 elimination, the transition state formed from 5-aminopyrrole has a lower activation energy compared to other 5-membered heterocycles or p-aminobenzyl derivatives. The NPYM scaffold was validated by late-stage functionalisation of approved drugs such as celecoxib, colchicine, vorinostat or ciprofloxacin. A hypoxia-activated NPYM-based prodrug (HAP) derived from HDAC inhibitor ST7612AA1 was also produced, which was active in cancer cells under hypoxic conditions.

Ultra-rapid Electrophilic Cysteine Arylation

Rapid bond-forming reactions are crucial for efficient bioconjugation. We describe a simple and practical strategy for facilitating ultra-rapid electrophilic cysteine arylation. Using a variety of sulfone-activated pyridinium salts, this uncatalyzed reaction proceeds with exceptionally high rate constants, ranging from 9800 to 320,000 M-1·s-1, in pH 7.0 aqueous buffer at 25 °C. Such reactions allow for stoichiometric bioconjugation of micromolar cysteine within minutes or even seconds. Even though the arylation is extremely fast, the chemistry exhibits excellent selectivity, thus furnishing functionalized peptides and proteins with both high conversion and purity.

Cyanine- and Rhodamine-Derived Alkynes for the Selective Targeting of Cancerous Mitochondria through Radical Thiol-Yne Coupling in Live Cells

Despite their long history and their synthetic potential underlined by various recent advances, radical thiol-yne coupling reactions have so far only rarely been exploited for the functionalization of biomolecules, and no examples yet exist for their application in live cells - although natural thiols show widespread occurrence therein. By taking advantage of the particular cellular conditions of mitochondria in cancer cells, we have demonstrated that radical thiol-yne coupling represents a powerful reaction principle for the selective targeting of these organelles. Within our studies, fluorescently labeled reactive alkyne probes were investigated, for which the fluorescent moiety was chosen to enable both mitochondria accumulation as well as highly sensitive detection. After preliminary studies under cell-free conditions, the most promising alkyne-dye conjugates were evaluated in various cellular experiments comprising analysis by flow cytometry and microscopy. All in all, these results pave the way for improved future therapeutic strategies relying on live-cell compatibility and selectivity among cellular compartments.

The thiol-sulfoxonium ylide photo-click reaction for bioconjugation

Visible-light-mediated methods were heavily studied as a useful tool for cysteine-selective bio-conjugation; however, many current methods suffer from bio-incompatible reaction conditions and slow kinetics. To address these challenges, herein, we report a transition metal-free thiol-sulfoxonium ylide photo-click reaction that enables bioconjugation under bio-compatible conditions. The reaction is highly cysteine-selective and generally finished within minutes with naturally occurring riboflavin derivatives as organic photocatalysts. The catalysts and substrates are readily accessible and bench stable and have satisfactory water solubility. As a proof-of-concept study, the reaction was smoothly applied in chemo-proteomic analysis, which provides efficient tools to explore the druggable content of the human proteome.

NIR-II Light Leveraged Dual Drug Synthesis for Orthotopic Combination Therapy

Pd-catalyzed bioorthogonal bond cleavage reactions are widely used and frequently reported. It is circumscribed by low reaction efficiency, which may encumber the therapeutic outcome when applied to physiological environments. Herein, an NIR-II light promoted integrated catalyst (CuS@PDA/Pd) (PDA - polydopamine) is designed to accelerate the reaction efficiency and achieve a dual bioorthogonal reaction for combination therapy. As NIR-II light can penetrate deeply into tissue, the Pd-mediated cleavage reaction can be promoted both in vitro and in vivo by the photothermal properties of CuS, beneficial to orthotopic 4T1 tumor treatment. In addition, CuS also catalyzes the synthesis of active resveratrol analogs by the CuAAC reaction. These simultaneously produced anticancer agents result in enhanced antitumor cytotoxicity in comparison to the single treatments. This is a fascinating study to devise an integrated catalyst boosted by NIR-II light for dual bioorthogonal catalysis, which may provide the impetus for efficient bioorthogonal combination therapy in vivo.

Application of "Click" Chemistry in Biomedical Hydrogels

Since "click" chemistry was first reported in 2001, it has remained a popular research topic in the field of chemistry due to its high yield without byproducts, fast reaction rate, simple reaction, and biocompatibility. It has achieved good applications in various fields, especially for the preparation of hydrogels. The development of biomedicine presents new challenges and opportunities for hydrogels, and "click" chemistry provides a library of chemical tools for the preparation of various innovative hydrogels, including cell culture, 3D bioprinting, and drug release. This article summarizes several common "click" reactions, including copper-catalyzed azide-alkyne cycloaddition reactions, strain-promoted azide-alkyne cycloaddition (SPAAC) reaction, thiol-ene reaction, the Diels-Alder reaction, and the inverse electron demand Diels-Alder (IEDDA) reaction. We introduce the "click" reaction in the nucleic acid field to expand the concept of "click" chemistry. This article focuses on the application of "click" chemistry for preparing various types of biomedical hydrogels and highlights the advantages of "click" reactions for cross-linking to obtain hydrogels. This review also discusses applications of "click" chemistry outside the field of hydrogels, such as drug synthesis, targeted delivery, and surface modification, hydrogels have great application potential in these fields in the future and hopefully inspire other applications of hydrogels.

Consecutive Alkylation, "Click", and "Clip" Reactions for the Traceless Methionine-Based Conjugation and Release of Methionine-Containing Peptides

"Click" reactions have revolutionized research in many areas of science. However, a disadvantage of the high stability of the Click product is that identifying simple treatments for cleanly dissociating the latter under the same guiding principles, i.e., a "Clip" reaction, remains a challenge. This study demonstrates that electron-deficient alkynes, conveniently installed on methionine residues, can participate in well-known Click (nucleophilic thiol-allene addition) and subsequent Clip reactions (radical thiol-ene addition). To illustrate this concept, a variety of bioconjugates (peptide-peptide; peptide-fluorophore; peptide-polymer; and peptide-protein) were prepared. Interestingly, the Clip reaction of these bioconjugates releases the original peptides concurrent with regeneration of their unmodified methionine residue, in minutes. Moreover, the conjugates demonstrate substantial stability toward endogenous levels of reactive species in bacteria, illustrating the potential for this chemistry in the biosciences. The reaction conditions employed in the Click and Clip steps are compatible with the preservation of the integrity of biomolecules/fluorophores and involve readily accessible reagents and the natural functional groups on peptides/proteins.

Flipping the Switch: Innovations in Inducible Probes for Protein Profiling

Over the past two decades, activity-based probes have enabled a range of discoveries, including the characterization of new enzymes and drug targets. However, their suitability in some labeling experiments can be limited by nonspecific reactivity, poor membrane permeability, or high toxicity. One method for overcoming these issues is through the development of “inducible” activity-based probes. These probes are added to samples in an unreactive state and require in situ transformation to their active form before labeling can occur. In this Review, we discuss a variety of approaches to inducible activity-based probe design, different means of probe activation, and the advancements that have resulted from these applications. Additionally, we highlight recent developments which may provide opportunities for future inducible activity-based probe innovations.

Structural optimization of reversible dibromomaleimide peptide stapling

Methods to constrain peptides in a bioactive α-helical conformation for inhibition of protein-protein interactions represent an ongoing area of investigation in chemical biology. Recently, the first example of a reversible "stapling" methodology was described which exploits native cysteine or homocysteine residues spaced at the i and i + 4 positions in a peptide sequence together with the thiol selective reactivity of dibromomaleimides (a previous study). This manuscript reports on the optimization of the maleimide based constraint, focusing on the kinetics of macrocyclization and the extent to which helicity is promoted with different thiol containing amino acids. The study identified an optimal stapling combination of X 1 = L-Cys and X 5 = L-hCys in the context of the model peptide Ac-X1AAAX5-NH2, which should prove useful in implementing the dibromomaleimide stapling strategy in peptidomimetic ligand discovery programmes.

Photoactivatable Odorants for Chemosensory Research

The chemosensory system of any animal relies on a vast array of detectors tuned to distinct chemical cues. Odorant receptors and the ion channels of the TRP family are all uniquely expressed in olfactory tissues in a species-specific manner. Great effort has been made to characterize the molecular and pharmacological properties of these proteins. Nevertheless, most of the natural ligands are highly hydrophobic molecules that are not amenable to controlled delivery. We sought to develop photoreleasable, biologically inactive odorants that could be delivered to the target receptor or ion channel and effectively activated by a short light pulse. Chemically distinct ligands eugenol, benzaldehyde, 2-phenethylamine, ethanethiol, butane-1-thiol, and 2,2-dimethylethane-1-thiol were modified by covalently attaching the photoremovable protecting group (8-cyano-7-hydroxyquinolin-2-yl)methyl (CyHQ). The CyHQ derivatives were shown to release the active odorant upon illumination with 365 and 405 nm light. We characterized their bioactivity by measuring activation of recombinant TRPV1 and TRPA1 ion channels expressed in HEK 293 cells and the electroolfactogram (EOG) response from intact mouse olfactory epithelium (OE). Illumination with 405 nm light was sufficient to robustly activate TRP channels within milliseconds of the light pulse. Photoactivation of channels was superior to activation by conventional bath application of the ligands. Photolysis of the CyHQ-protected odorants efficiently activated an EOG response in a dose-dependent manner with kinetics similar to that evoked by the vaporized odorant amyl acetate (AAc). We conclude that CyHQ-based, photoreleasable odorants can be successfully implemented in chemosensory research.

Labeling and Natural Post-Translational Modification of Peptides and Proteins via Chemoselective Pd-Catalyzed Prenylation of Cysteine

The prenylation of peptides and proteins is an important post-translational modification observed in vivo. We report that the Pd-catalyzed Tsuji-Trost allylation with a Pd/BIPHEPHOS catalyst system allows the allylation of Cys-containing peptides and proteins with complete chemoselectivity and high n/i regioselectivity. In contrast to recently established methods, which use non-native connections, the Pd-catalyzed prenylation produces the natural n-prenylthioether bond. In addition, a variety of biophysical probes such as affinity handles and fluorescent tags can be introduced into Cys-containing peptides and proteins. Furthermore, peptides containing two cysteine residues can be stapled or cyclized using homobifunctional allylic carbonate reagents.

A Dual Killing Strategy: Photocatalytic Generation of Singlet Oxygen with Concomitant PtIV Prodrug Activation

A ruthenium-based mitochondrial-targeting photosensitiser that undergoes efficient cell uptake, enables the rapid catalytic conversion of Pt^IV prodrugs into their active Pt^II counterparts, and drives the generation of singlet oxygen was designed. This dual mode of action drives two orthogonal cancer-cell killing mechanisms with temporal and spatial control. The designed photosensitiser was shown to elicit cell death of a panel of cancer cell lines including those showing oxaliplatin-resistance.

Visible-Light-Mediated Organocatalyzed Thiol-Ene Reaction Initiated by a Proton-Coupled Electron Transfer

A convenient method for performing a thiol-ene reaction is described. The reaction is performed under blue-light irradiation and catalyzed by photoactive Lewis basic molecules such as acridine orange or naphthalene-fused N-acylbenzimidazole. It is believed that the process is initiated by a proton-coupled electron transfer process within the complex between the thiol and the Lewis basic catalyst.

Radical-Mediated Thiol-Ene Strategy: Photoactivation of Thiol-Containing Drugs in Cancer Cells

Photoactivated drugs provide an opportunity to improve efficacy alongside reducing side-effects in the treatment of severe diseases such as cancer. Described herein is a photoactivation decaging method of isobutylene-caged thiols through a UV-initiated thiol-ene reaction. The method was demonstrated with an isobutylene-caged cysteine, cyclic disulfide-peptide, and thiol-containing drug, all of which were rapidly and efficiently released under mild UV irradiation in the presence of thiol sources and a photoinitiator. Importantly, it is shown that the activity of histone deacetylase inhibitor largazole can be switched off when stapled, but selectively switched on within cancer cells when irradiated with non-phototoxic light.