Strained Cyclic Disulfides Enable Cellular Uptake by Reacting with the Transferrin Receptor

Peptide dendrimers transfecting CRISPR/Cas9 plasmid DNA: optimization and mechanism

Gene editing by CRISPR/Cas9 offers great therapeutic opportunities but requires delivering large plasmid DNA (pDNA) into cells, a task for which transfection reagents are better suited than viral vectors. Here we performed a structure-activity relationship study of Z22, a d-enantiomeric, arginine containing, lipidated peptide dendrimer developed for pDNA transfection of a CRISPR/Cas9 plasmid co-expressing GFP. While all dendrimer analogs tested bound pDNA strongly and internalized their cargo into cells, d-chirality proved essential for transfection by avoiding proteolysis of the dendrimer structure required for endosome escape and possibly crossing of the nuclear envelope. Furthermore, a cysteine residue at the core of Z22 proved non-essential and was removed to yield the more active analog Z34. This dendrimer shows >83% GFP transfection efficiency in HEK cells with no detrimental effect on cell viability and promotes functional CRISPR/Cas9 mediated gene editing. It is accessible by solid-phase peptide synthesis and therefore attractive for further development.

Intracellular Delivery of Antisense Oligonucleotides by Tri-Branched Cyclic Disulfide Units

Therapeutic oligonucleotides, such as antisense DNA, show promise in treating previously untreatable diseases. However, their applications are still hindered by the poor membrane permeability of naked oligonucleotides. Therefore, it is necessary to develop efficient methods for intracellular oligonucleotide delivery. Previously, our group successfully developed disulfide-based Membrane Permeable Oligonucleotides (MPON), which achieved enhanced cellular uptake and gene silencing effects through an endocytosis-free uptake mechanism. Herein, we report a new molecular design for the next generation of MPON, called trimer MPON. The trimer MPON consists of a tri-branched backbone, three α-lipoic acid units, and a spacer linker between the oligonucleotides and tri-branched cyclic disulfide unit. We describe the design, synthesis, and functional evaluation of the trimer MPON, offering new insights into the molecular design for efficient oligonucleotide delivery.

Inclusive Pattern Generation Protocols to Decode Thiol-Mediated Uptake

Thiol-mediated uptake (TMU) is an intriguing enigma in current chemistry and biology. While the appearance of cell-penetrating activity upon attachment of cascade exchangers (CAXs) has been observed by many and is increasingly being used in practice, the molecular basis of TMU is essentially unknown. The objective of this study was to develop a general protocol to decode the dynamic covalent networks that presumably account for TMU. Uptake inhibition patterns obtained from the removal of exchange partners by either protein knockdown or alternative inhibitors are aligned with original patterns generated by CAX transporters and inhibitors and patterns from alternative functions (here cell motility). These inclusive TMU patterns reveal that the four most significant CAXs known today enter cells along three almost orthogonal pathways. Epidithiodiketopiperazines (ETP) exchange preferably with integrins and protein disulfide isomerases (PDIs), benzopolysulfanes (BPS) with different PDIs, presumably PDIA3, and asparagusic acid (AspA), and antisense oligonucleotide phosphorothioates (OPS) exchange with the transferrin receptor and can be activated by the removal of PDIs with their respective inhibitors. These findings provide a solid basis to understand and use TMU to enable and prevent entry into cells.

Synthesis and evaluation of antisense oligonucleotides prodrug with G-quadruplex assembly and lysosome escape capabilities for oncotherapy

The applications of antisense oligonucleotides (ASOs) in rare or common diseases treatment have garnered great attention in recent years. Nevertheless, challenges associated with stability and bioavailability still persist, hampering the efficiency of ASOs. This work presents an ASO prodrug with parallel G-quadruplex assembly and lysosome escape capabilities for oncotherapy. Our findings revealed that the end-assembled quadruplex structure effectively shielded the ASO from enzymatic degradation. Meanwhile, the conjugation of maleimide within the quadruplex enhanced cellular uptake, potentially offering an alternative cell entry mechanism that circumvents lysosome involvement. Notably, an optimized molecule, Mal2-G4-ASO, exhibited remarkable therapeutic effects both in vitro and in vivo. This work presents a promising avenue for enhancing the activity of nucleic acid drugs in oncotherapy and potentially other disease contexts.

Light-controllable cell-membrane disturbance for intracellular delivery

Highly polar and charged molecules, such as oligonucleotides, face significant barriers in crossing the cell membrane to access the cytoplasm. To address this problem, we developed a light-triggered twistable tetraphenylethene (TPE) derivative, TPE-C-N, to facilitate the intracellular delivery of charged molecules through an endocytosis-independent pathway. The central double bond of TPE in TPE-C-N is planar in the ground state but becomes twisted in the excited state. Under light irradiation, this planar-to-twisted structural change induces continuous cell membrane disturbances. Such disturbance does not lead to permanent damage to the cell membrane. TPE-C-N significantly enhanced the intracellular delivery of negatively charged molecules under light irradiation when endocytosis was inhibited through low-temperature treatment, confirming the endocytosis-independent nature of this delivery method. We have successfully demonstrated that the TPE-C-N-mediated light-controllable method can efficiently promote the intracellular delivery of charged molecules, such as peptides and oligonucleotides, with molecular weights ranging from 1000 to 5000 Da.

AcGlcAs: A Novel P53-Targeting Arsenical with Potent Cellular Uptake and Cancer Cell Selectivity

Arsenic trioxide (ATO) targets PML/RARα and leads to miraculous success in treating acute promyelocytic leukemia. Notably, ATO also targets p53, the most frequently mutated protein in cancers, through a similar binding mechanism. However, p53-targeting ATO trials are challenging due to the poor cellular uptake and cancer selectivity of ATO. Here, we analyzed the structure-activity relationship of arsenicals and rationally developed a novel arsenical (designated AcGlcAs) by conjugating arsenic to sulfur atoms and tetraacetyl-β-d-thioglucose. AcGlcAs exhibited remarkable cellular uptake through a thiol-mediated pathway (maximally 127-fold higher than ATO), thereby potently targeting PML/RARα and mutant p53. Among the 55 tested cell lines, AcGlcAs preferentially killed cancer lines rather than normal lines. In preclinical studies, AcGlcAs significantly extended the survival of mice bearing a xenograft tumor with p53 mutation while showing high plasma stability and oral bioavailability. Thus, AcGlcAs is a potential clinical candidate for precisely treating numerous p53-mutated cancers.

May 1,2-Dithiolane-4-carboxylic Acid and Its Derivatives Serve as a Specific Thioredoxin Reductase 1 Inhibitor?

Thioredoxin reductase is an essential enzyme that plays a crucial role in maintaining cellular redox homeostasis by catalyzing the reduction of thioredoxin, which is involved in several vital cellular processes. The overexpression of TrxR is often associated with cancer development. A series of 1,2-dithiolane-4-carboxylic acid analogs were obtained to verify the selectivity of 1,2-dithiolane moiety toward TrxR. Asparagusic acid analogs and their bioisoters remain inactive toward TrxR, which proves the inability of the 1,2-dithiolane moiety to serve as a pharmacophore during the interaction with TrxR. It was found that the Michael acceptor functionality-containing analogs exhibit higher inhibitory effects against TrxR compared to other compounds of the series. The most potent representatives exhibited micromolar TrxR1 inhibition activity (IC50 varied from 5.3 to 186.0 μM) and were further examined with in vitro cell-based assays to assess the cytotoxic effects on various cancer cell lines and cell death mechanisms.

Asparagusic Acid - A Unique Approach toward Effective Cellular Uptake of Therapeutics: Application, Biological Targets, and Chemical Properties

The synthetic approaches towards unique asparagusic acid and its analogues as well as its chemical use, the breadth of its biological properties and their relevant applications have been explored. The significance of the 1,2-dithiolane ring tension in dithiol-mediated uptake and its use for the intracellular transport of molecular cargoes is discussed alongside some of the challenges that arise from the fast thiolate-disulfide interchange. The short overview with the indication of the available literature on natural 1,2-dithiolanes synthesis and biological activities is also included. The general review structure is based on the time-line perspective of the application of asparagusic acid moiety as well as its primitive derivatives (4-amino-1,2-dithiolane-4-carboxylic acid and 4-methyl-1,2-dithiolane-4-carboxilic acid) used in clinics/cosmetics, focusing on the recent research in this area and including international patents applications.

Fluorescent Probes for Mammalian Thioredoxin Reductase: Mechanistic Analysis, Construction Strategies, and Future Perspectives

The modulation of numerous signaling pathways is orchestrated by redox regulation of cellular environments. Maintaining dynamic redox homeostasis is of utmost importance for human health, given the common occurrence of altered redox status in various pathological conditions. The cardinal component of the thioredoxin system, mammalian thioredoxin reductase (TrxR) plays a vital role in supporting various physiological functions; however, its malfunction, disrupting redox balance, is intimately associated with the pathogenesis of multiple diseases. Accordingly, the dynamic monitoring of TrxR of live organisms represents a powerful direction to facilitate the comprehensive understanding and exploration of the profound significance of redox biology in cellular processes. A number of classic assays have been developed for the determination of TrxR activity in biological samples, yet their application is constrained when exploring the real-time dynamics of TrxR activity in live organisms. Fluorescent probes offer several advantages for in situ imaging and the quantification of biological targets, such as non-destructiveness, real-time analysis, and high spatiotemporal resolution. These benefits facilitate the transition from a poise to a flux understanding of cellular targets, further advancing scientific studies in related fields. This review aims to introduce the progress in the development and application of TrxR fluorescent probes in the past years, and it mainly focuses on analyzing their reaction mechanisms, construction strategies, and potential drawbacks. Finally, this study discusses the critical challenges and issues encountered during the development of selective TrxR probes and proposes future directions for their advancement. We anticipate the comprehensive analysis of the present TrxR probes will offer some glitters of enlightenment, and we also expect that this review may shed light on the design and development of novel TrxR probes.

Recent Progress of Rational Modified Nanocarriers for Cytosolic Protein Delivery

Therapeutic proteins garnered significant attention in the field of disease treatment. In comparison to small molecule drugs, protein therapies offer distinct advantages, including high potency, specificity, low toxicity, and reduced carcinogenicity, even at minimal concentrations. However, the full potential of protein therapy is limited by inherent challenges such as large molecular size, delicate tertiary structure, and poor membrane penetration, resulting in inefficient intracellular delivery into target cells. To address these challenges and enhance the clinical applications of protein therapies, various protein-loaded nanocarriers with tailored modifications were developed, including liposomes, exosomes, polymeric nanoparticles, and nanomotors. Despite these advancements, many of these strategies encounter significant issues such as entrapment within endosomes, leading to low therapeutic efficiency. In this review, we extensively discussed diverse strategies for the rational design of nanocarriers, aiming to overcome these limitations. Additionally, we presented a forward-looking viewpoint on the innovative generation of delivery systems specifically tailored for protein-based therapies. Our intention was to offer theoretical and technical support for the development and enhancement of nanocarriers capable of facilitating cytosolic protein delivery.

Inhibition of Cell Motility by Cell-Penetrating Dynamic Covalent Cascade Exchangers: Integrins Participate in Thiol-Mediated Uptake

Integrins are cell surface proteins responsible for cell motility. Inspired by the rich disulfide exchange chemistry of integrins, we show here the inhibition of cell migration by cascade exchangers (CAXs), which also enable and inhibit cell penetration by thiol-mediated uptake. Fast-moving CAXs such as reversible Michael acceptor dimers, dithiabismepanes, and bioinspired epidithiodiketopiperazines are best, much better than Ellman's reagent. The implication that integrins participate in thiol-mediated uptake is confirmed by reduced uptake in integrin-knockdown cells. Although thiol-mediated uptake is increasingly emerging as a unifying pathway to bring matter into cells, its molecular basis is essentially unknown. These results identify the integrin superfamily as experimentally validated general cellular partners in the dynamic covalent exchange cascades that are likely to account for thiol-mediated uptake. The patterns identified testify to the complexity of the dynamic covalent networks involved. This work also provides chemistry tools to explore cell motility and expands the drug discovery potential of CAXs from antiviral toward antithrombotic and antitumor perspectives.

Divergent Proteome Reactivity Influences Arm-Selective Activation of Pharmacological Endoplasmic Reticulum Proteostasis Regulators

Pharmacological activation of the activating transcription factor 6 (ATF6) arm of the Unfolded Protein Response (UPR) has proven useful for ameliorating proteostasis deficiencies in a variety of etiologically diverse diseases. Previous high-throughput screening efforts identified the small molecule AA147 as a potent and selective ATF6 activating compound that operates through a mechanism involving metabolic activation of its 2-amino- p -cresol substructure affording a quinone methide, which then covalently modifies a subset of ER protein disulfide isomerases (PDIs). Intriguingly, another compound identified in this screen, AA132, also contains a 2-amino- p -cresol moiety; however, this compound showed less transcriptional selectivity, instead globally activating all three arms of the UPR. Here, we show that AA132 activates global UPR signaling through a mechanism analogous to that of AA147, involving metabolic activation and covalent PDI modification. Chemoproteomic-enabled analyses show that AA132 covalently modifies PDIs to a greater extent than AA147. Paradoxically, activated AA132 reacts slower with PDIs, indicating it is less reactive than activated AA147. This suggests that the higher labeling of PDIs observed with activated AA132 can be attributed to its lower reactivity, which allows this activated compound to persist longer in the cellular environment prior to quenching by endogenous nucleophiles. Collectively, these results suggest that AA132 globally activates the UPR through increased engagement of ER PDIs. Consistent with this, reducing the cellular concentration of AA132 decreases PDI modifications and allows for selective ATF6 activation. Our results highlight the relationship between metabolically activatable-electrophile stability, ER proteome reactivity, and the transcriptional response observed with the enaminone chemotype of ER proteostasis regulators, enabling continued development of next-generation ATF6 activating compounds.

Thiol-Disulfide Exchange as a Route for Endosomal Escape of Polymeric Nanoparticles

Endosomal entrapment has remained the major bottleneck for cytosolic delivery of nanoparticle-based delivery systems. Uncovering fundamentally new pathways for endosomal escape is therefore highly sought. Herein, we report that disulfide bonds can enhance endosomal escape through contacts with cellular exofacial thiols, in addition to facilitating cellular uptake. Our results are supported through comparative analysis of polymeric nanogels with variable accessibility to disulfide bonds by placing these functionalities at the core or the shell of the nanogels. The findings here inform future chemical design of delivery vehicles.

Controllable Disulfide Exchange Polymerization of Polyguanidine for Effective Biomedical Applications by Thiol-Mediated Uptake

New preparation methods of vectors are the key to developing the next generation of biomacromolecule delivery systems. In this study, a controllable disulfide exchange polymerization was established to obtain low-toxicity and efficient bioreducible polyguanidines (mPEG₂₂₅ -b-PSS_n , n=13, 26, 39, 75, 105) by regulating the concentration of activated nucleophiles and reaction time under mild reaction conditions. The relationship between the degrees of polymerization and biocompatibility was studied to identify the optimal polyguanidine mPEG₂₂₅ -b-PSS₂₆ . Such polyguanidine exhibited good in vitro performance in delivering different functional nucleic acids. The impressive therapeutic effects of mPEG₂₂₅ -b-PSS₂₆ were further verified in the 4T1 tumor-bearing mice as well as the mice with full-thickness skin defects. Controllable disulfide exchange polymerization provides an attractive strategy for the construction of new biomacromolecule delivery systems.

Pnictogen-Centered Cascade Exchangers for Thiol-Mediated Uptake: As(III)-, Sb(III)-, and Bi(III)-Expanded Cyclic Disulfides as Inhibitors of Cytosolic Delivery and Viral Entry

Dynamic covalent exchange cascades with cellular thiols are of interest to deliver substrates to the cytosol and to inhibit the entry of viruses. The best transporters and inhibitors known today are cyclic cascade exchangers (CAXs), producing a new exchanger with every exchange, mostly cyclic oligochalcogenides, particularly disulfides. The objective of this study was to expand the dynamic covalent chalcogen exchange cascades in thiol-mediated uptake by inserting pnictogen relays. A family of pnictogen-expanded cyclic disulfides covering As(III), Sb(III), and Bi(III) is introduced. Their ability to inhibit thiol-mediated cytosolic delivery is explored with fluorescently labeled CAXs as transporters. The promise of inhibiting viral entry is assessed with SARS-CoV-2 lentiviral vectors. Oxygen-bridged seven-membered 1,3,2-dithiabismepane rings are identified as privileged scaffolds. The same holds for six-membered 1,3,2-dithiarsinane rings made from asparagusic acid and para-aminophenylarsine oxide, which are inactive or toxic when used alone. These chemically complementary Bi(III) and As(III) cascade exchangers inhibit both thiol-mediated cytosolic delivery and SARS-CoV-2 lentivector uptake at concentrations of 10 μM or lower. Crystal structures, computational models, and exchange kinetics support that lentivector entry inhibition of the contracted dithiarsinane and the expanded dithiabismepane rings coincides with exchange cascades that occur without the release of the pnictogen relay and benefit from noncovalent pnictogen bonds. The identified leads open perspectives regarding drug delivery as well as unorthodox approaches toward dynamic covalent inhibition of cellular entry.

Cyclic Thiosulfonates for Thiol-Mediated Uptake: Cascade Exchangers, Transporters, Inhibitors

Thiol-mediated uptake is emerging as a powerful method to penetrate cells. Cyclic oligochalcogenides (COCs) have been identified as privileged scaffolds to enable and inhibit thiol-mediated uptake because they can act as dynamic covalent cascade exchangers, i.e., every exchange produces a new, covalently tethered exchanger. In this study, our focus is on the essentially unexplored COCs of higher oxidation levels. Quantitative characterization of the underlying dynamic covalent exchange cascades reveals that the initial ring opening of cyclic thiosulfonates (CTOs) proceeds at a high speed even at a low pH. The released sulfinates exchange with disulfides in aprotic but much less in protic environments. Hydrophobic domains were thus introduced to direct CTOs into hydrophobic pockets to enhance their reactivity. Equipped with such directing groups, fluorescently labeled CTOs entered the cytosol of living cells more efficiently than the popular asparagusic acid. Added as competitive agents, CTOs inhibit the uptake of various COC transporters and SARS-CoV-2 lentivectors. Orthogonal trends found with different transporters support the existence of multiple cellular partners to account for the diverse expressions of thiol-mediated uptake. Dominant self-inhibition and high activity of dimers imply selective and synergistic exchange in hydrophobic pockets as distinguishing characteristics of thiol-mediated uptake with CTOs. The best CTO dimers with hydrophobic directing groups inhibit the cellular entry of SARS-CoV-2 lentivectors with an IC₅₀ significantly lower than the previous best CTO, below the 10 μM threshold and better than ebselen. Taken together, these results identify CTOs as an intriguing motif for use in cytosolic delivery, as inhibitors of lentivector entry, and for the evolution of dynamic covalent networks in the broadest sense, with reactivity-based selectivity of cascade exchange emerging as a distinguishing characteristic that deserves further attention.

Cyclic 5-membered disulfides are not selective substrates of thioredoxin reductase, but are opened nonspecifically

The cyclic five-membered disulfide 1,2-dithiolane has been widely used in chemical biology and in redox probes. Contradictory reports have described it either as nonspecifically reduced in cells, or else as a highly specific substrate for thioredoxin reductase (TrxR). Here we show that 1,2-dithiolane probes, such as "TRFS" probes, are nonspecifically reduced by thiol reductants and redox-active proteins, and their cellular performance is barely affected by TrxR inhibition or knockout. Therefore, results of cellular imaging or inhibitor screening using 1,2-dithiolanes should not be interpreted as reflecting TrxR activity, and previous studies may need re-evaluation. To understand 1,2-dithiolanes' complex behaviour, probe localisation, environment-dependent fluorescence, reduction-independent ring-opening polymerisation, and thiol-dependent cellular uptake must all be considered; particular caution is needed when co-applying thiophilic inhibitors. We present a general approach controlling against assay misinterpretation with reducible probes, to ensure future TrxR-targeted designs are robustly evaluated for selectivity, and to better orient future research.

Selective cellular probes for mammalian thioredoxin reductase TrxR1: Rational design of RX1, a modular 1,2-thiaselenane redox probe

Quantifying the activity of key cellular redox players is crucial for understanding physiological homeostasis, and for targeting their perturbed states in pathologies including cancer and inflammatory diseases. However, cellularly-selective probes for oxidoreductase turnover are sorely lacking. We rationally developed the first probes that selectively target the mammalian selenoprotein thioredoxin reductase (TrxR), using a cyclic selenenylsulfide oriented to harness TrxR's unique selenolthiol chemistry while resisting the cellular monothiol background. Lead probe RX1 had excellent TrxR1-selective performance in cells, cross-validated by knockout, selenium starvation, knock-in, and chemical inhibitors. Its background-free fluorogenicity enabled us to perform the first quantitative high-throughput live cell screen for TrxR1 inhibitors, which indicated that tempered S_NAr electrophiles may be more selective TrxR drugs than the classical electrophiles used hitherto. The RX1 design thus sets the stage for in vivo imaging of the activity of this key oxidoreductase in health and disease, and can also drive TrxR1-inhibitor drug design.

α-Vinyl azide–cysteine click coupling reaction enabled bioorthogonal peptide/protein modification

α-Alkyl and α-aryl vinyl azides were found to be able to couple with cysteine-derived alkyl thiols chemoselectively under mild conditions, providing the corresponding β-ketosulfides with simultaneous extrusion of N2 and ammonia.

Dithiolane quartets: thiol-mediated uptake enables cytosolic delivery in deep tissue

The cytosolic delivery of various substrates in 3D multicellular spheroids by thiol-mediated uptake is reported. This is important because most orthodox systems, including polycationic cell-penetrating peptides, fail to deliver efficiently into deep tissue. The grand principles of supramolecular chemistry, that is the pH dependence of dynamic covalent disulfide exchange with known thiols on the transferrin receptor, are proposed to account for transcytosis into deep tissue, while the known but elusive exchange cascades along the same or other partners assure cytosolic delivery in kinetic competition. For quantitative detection in the cytosol, the 2D chloroalkane penetration assay (CAPA) is translated to 3D deep tissue. The targeted delivery of quantum dots, otherwise already troublesome in 2D culture, and the controlled release of mechanophores are realized to exemplify the power of thiol-mediated uptake into spheroids. As transporters, dithiolane quartets on streptavidin templates are introduced as modular motifs. Built from two amino acids only, the varied stereochemistry and peptide sequence are shown to cover maximal functional space with minimal structural change, i.e., constitutional isomers. Reviving a classic in peptide chemistry, this templated assembly of β quartets promises to expand streptavidin biotechnology in new directions, while the discovery of general cytosolic delivery in deep tissue as an intrinsic advantage further enhances the significance and usefulness of thiol-mediated uptake.

Transformable Helical Self-Assembly for Cancerous Golgi Apparatus Disruption

Golgi apparatus is a major subcellular organelle responsible for drug resistance. Golgi apparatus-targeted nanomechanical disruption provides an attractive approach for killing cancer cells by multimodal mechanism and avoiding drug resistance. Inspired by the poisonous twisted fibrils in Alzheimer's brain tissue and enhanced rigidity of helical structure in nature, we designed transformable peptide C₆RVRRF₄KY that can self-assemble into nontoxic nanoparticles in aqueous medium but transformed into left-handed helical fibrils (L-HFs) after targeting and furin cleavage in the Golgi apparatus of cancer cells. The L-HFs can mechanically disrupt the Golgi apparatus membrane, resulting in inhibition of cytokine secretion, collapse of the cellular structure, and eventually death of cancer cells. Repeated stimulation of the cancers by the precursors causes no acquired drug resistance, showing that mechanical disruption of subcellular organelle is an excellent strategy for cancer therapy without drug resistance. This nanomechanical disruption concept should also be applicable to multidrug-resistant bacteria and viruses.

Antisense Oligonucleotide Modified with Disulfide Units Induces Efficient Exon Skipping in mdx Myotubes through Enhanced Membrane Permeability and Nucleus Internalization

We have found that antisense oligonucleotides and siRNA molecules modified with repeat structures of disulfide units can be directly introduced into the cytoplasm and exhibit a suppressive effect on gene expression. In this study, we analyzed the mechanism of cellular uptake of these membrane-permeable oligonucleotides (MPONs). Time-course analysis by confocal microscopy showed that the uptake of MPONs from the plasma membrane to the cytoplasm reached 50 % of the total uptake in about 5 min. In addition, analysis of the plasma membrane proteins to which MPONs bind, identified several proteins, including voltage-dependent anion channel. Next, we analyzed the behavior of MPONs in the cell and found them to be abundant in the nucleus as early as 24 h after addition with the amount increasing further after 48 and 72 h. The amount of MPONs was 2.5-fold higher than that of unmodified oligonucleotides in the nucleus after 72 h. We also designed antisense oligonucleotides and evaluated the effect of MPONs on mRNA exon skipping using DMD model cells; MPONs caused exon skipping with 69 % efficiency after 72 h, which was three times higher than the rate of the control. In summary, the high capacity for intracytoplasmic and nuclear translocation of MPONs is expected to be useful for therapeutic strategies targeting exon skipping.

Oligonucleotide Phosphorothioates Enter Cells by Thiol-Mediated Uptake

Oligonucleotide phosphorothioates (OPS) are DNA or RNA mimics where one phosphate oxygen is replaced by a sulfur atom. They have been shown to enter mammalian cells much more efficiently than non-modified DNA. Thus, solving one of the key challenges with oligonucleotide technology, OPS became very useful in practice, with several FDA-approved drugs on the market or in late clinical trials. However, the mechanism accounting for this facile cellular uptake is unknown. Here, we show that OPS enter cells by thiol-mediated uptake. The transient adaptive network produced by dynamic covalent pseudo-disulfide exchange is characterized in action. Inhibitors with nanomolar efficiency are provided, together with activators that reduce endosomal capture for efficient delivery of OPS into the cytosol, the site of action.

Delivery nanoplatforms based on dynamic covalent chemistry

As a paramount factor to restrict the potential action of drugs and biologics, nanoplatforms based on dynamic covalent chemistry have been demonstrated as promising candidates to fulfill the full requirements during the whole delivery process by the virtue of their remarkable features such as adaptiveness, stimuli-responsiveness, specificity, reversibility and feasibility. This contribution summarizes the latest progress in dynamic covalent bond-based nanoplatforms with improved delivery efficiency and therapeutic performance. In addition, major challenges and perspectives in this field are also discussed. We expect that this feature article will provide a valuable and systematic reference for the further development of dynamic covalent bond-based nanoplatforms.

Synthesis of natural products containing highly strained trans-fused bicyclo[3.3.0]octane: historical overview and future prospects

Owing to high strain energy, molecules with trans-fused bicyclo[3.3.0]octane ring systems are very difficult to synthesize, and there are very few approaches to access them. Recently, a number of natural products with such ring systems have been made by the synthetic community. However, there has been no review in this field before. This review provides a systematic and comprehensive discussion on the synthesis of natural products containing trans-fused bicyclo[3.3.0]octanes and the historical context of this work. The prospects for future research in this field are also discussed. Covering the literature before 2021, this review aims to offer a helpful reference for total synthesis of highly strained natural products containing trans-fused bicyclo[3.3.0]octane ring systems.

Thiol-Mediated Uptake

This Perspective focuses on thiol-mediated uptake, that is, the entry of substrates into cells enabled by oligochalcogenides or mimics, often disulfides, and inhibited by thiol-reactive agents. A short chronology from the initial observations in 1990 until today is followed by a summary of cell-penetrating poly(disulfide)s (CPDs) and cyclic oligochalcogenides (COCs) as privileged scaffolds in thiol-mediated uptake and inhibitors of thiol-mediated uptake as potential antivirals. In the spirit of a Perspective, the main part brings together topics that possibly could help to explain how thiol-mediated uptake really works. Extreme sulfur chemistry mostly related to COCs and their mimics, cyclic disulfides, thiosulfinates/-onates, diselenolanes, benzopolysulfanes, but also arsenics and Michael acceptors, is viewed in the context of acidity, ring tension, exchange cascades, adaptive networks, exchange affinity columns, molecular walkers, ring-opening polymerizations, and templated polymerizations. Micellar pores (or lipid ion channels) are considered, from cell-penetrating peptides and natural antibiotics to voltage sensors, and a concise gallery of membrane proteins, as possible targets of thiol-mediated uptake, is provided, including CLIC1, a thiol-reactive chloride channel; TMEM16F, a Ca-activated scramblase; EGFR, the epithelial growth factor receptor; and protein-disulfide isomerase, known from HIV entry or the transferrin receptor, a top hit in proteomics and recently identified in the cellular entry of SARS-CoV-2.

Rational Design of Poly(disulfide)s as a Universal Platform for Delivery of CRISPR-Cas9 Machineries toward Therapeutic Genome Editing

We synthesized a series of poly(disulfide)s by ring-opening polymerization and demonstrated that the copolymerization of monomer 1 containing diethylenetriamine moieties and monomer 2 containing guanidyl ligands could generate an efficient delivery platform for different forms of CRISPR-Cas9-based genome editors, including plasmid, mRNA, and protein. The excellent delivery performance of designed poly(disulfide)s stems from their delicate molecular structures to interact with genome-editing biomacromolecules, unique delivery pathways to mediate the cellular uptake of CRISPR-Cas9 cargoes, and strong ability to escape the endosome. The degradation of poly(disulfide)s by intracellular glutathione not only promotes the timely release of CRISPR-Cas9 machineries into the cytosol but also minimizes the cytotoxicity that nondegradable polymeric carriers often encounter. These merits collectively account for the excellent ability of poly(disulfide)s to mediate different forms of CRISPR-Cas9 for their efficient genome-editing activities in vitro and in vivo.

Specific and nondisruptive interaction of guanidium-functionalized gold nanoparticles with neutral phospholipid bilayers

Understanding and controlling the interaction between nanoparticles and biological entities is fundamental to the development of nanomedicine applications. In particular, the possibility to realize nanoparticles capable of directly targeting neutral lipid membranes would be advantageous to numerous applications aiming at delivering nanoparticles and their cargos into cells and biological vesicles. Here, we use experimental and computational methodologies to analyze the interaction between liposomes and gold nanoparticles (AuNPs) featuring cationic headgroups in their protecting monolayer. We find that in contrast to nanoparticles decorated with other positively charged headgroups, guanidinium-coated AuNPs can bind to neutral phosphatidylcholine liposomes, inducing nondisruptive membrane permeabilization. Atomistic molecular simulations reveal that this ability is due to the multivalent H-bonding interaction between the phosphate residues of the liposome's phospholipids and the guanidinium groups. Our results demonstrate that the peculiar properties of arginine magic, an effect responsible for the membranotropic properties of some naturally occurring peptides, are also displayed by guanidinium-bearing functionalized AuNPs.

Selective, Modular Probes for Thioredoxins Enabled by Rational Tuning of a Unique Disulfide Structure Motif

Specialized cellular networks of oxidoreductases coordinate the dithiol/disulfide-exchange reactions that control metabolism, protein regulation, and redox homeostasis. For probes to be selective for redox enzymes and effector proteins (nM to μM concentrations), they must also be able to resist non-specific triggering by the ca. 50 mM background of non-catalytic cellular monothiols. However, no such selective reduction-sensing systems have yet been established. Here, we used rational structural design to independently vary thermodynamic and kinetic aspects of disulfide stability, creating a series of unusual disulfide reduction trigger units designed for stability to monothiols. We integrated the motifs into modular series of fluorogenic probes that release and activate an arbitrary chemical cargo upon reduction, and compared their performance to that of the literature-known disulfides. The probes were comprehensively screened for biological stability and selectivity against a range of redox effector proteins and enzymes. This design process delivered the first disulfide probes with excellent stability to monothiols yet high selectivity for the key redox-active protein effector, thioredoxin. We anticipate that further applications of these novel disulfide triggers will deliver unique probes targeting cellular thioredoxins. We also anticipate that further tuning following this design paradigm will enable redox probes for other important dithiol-manifold redox proteins, that will be useful in revealing the hitherto hidden dynamics of endogenous cellular redox systems.

Charge-Conversion Strategies for Nucleic Acid Delivery

Nucleic acids are now considered as one of the most potent therapeutic modalities, as their roles go beyond storing genetic information and chemical energy or as signal transducer. Attenuation or expression of desired genes through nucleic acids have profound implications in gene therapy, gene editing and even in vaccine development for immunomodulation. Although nucleic acid therapeutics bring in overwhelming possibilities towards the development of molecular medicines, there are significant loopholes in designing and effective translation of these drugs into the clinic. One of the major pitfalls lies in the traditional design concepts for nucleic acid drug carriers, viz. cationic charge induced cytotoxicity in delivery pathway. Targeting this bottleneck, several pioneering research efforts have been devoted to design innovative carriers through charge-conversion approaches, whereby built-in functionalities convert from cationic to neutral or anionic, or even from anionic to cationic enabling the carrier to overcome several critical barriers for therapeutics delivery, such as serum deactivation, instability in circulation, low transfection and poor endosomal escape. This review will critically analyze various molecular designs of charge-converting nanocarriers in a classified approach for the successful delivery of nucleic acids. Accompanied by the narrative on recent clinical nucleic acid candidates, the review concludes with a discussion on the pitfalls and scope of these interesting approaches.

From Conception to Development: Investigating PROTACs Features for Improved Cell Permeability and Successful Protein Degradation

Proteolysis Targeting Chimeras (PROTACs) are heterobifunctional degraders that specifically eliminate targeted proteins by hijacking the ubiquitin-proteasome system (UPS). This modality has emerged as an orthogonal approach to the use of small-molecule inhibitors for knocking down classic targets and disease-related proteins classified, until now, as "undruggable." In early 2019, the first targeted protein degraders reached the clinic, drawing attention to PROTACs as one of the most appealing technology in the drug discovery landscape. Despite these promising results, PROTACs are often affected by poor cellular permeability due to their high molecular weight (MW) and large exposed polar surface area (PSA). Herein, we report a comprehensive record of PROTAC design, pharmacology and thermodynamic challenges and solutions, as well as some of the available strategies to enhance cellular uptake, including suggestions of promising biological tools for the in vitro evaluation of PROTACs permeability toward successful protein degradation.

Disulfide Bridging Strategies in Viral and Nonviral Platforms for Nucleic Acid Delivery

Self-assembled nanostructures that are sensitive to environmental stimuli are promising nanomaterials for drug delivery. In this class, disulfide-containing redox-sensitive strategies have gained enormous attention because of their wide applicability and simplicity of nanoparticle design. In the context of nucleic acid delivery, numerous disulfide-based materials have been designed by relying on covalent or noncovalent interactions. In this review, we highlight major advances in the design of disulfide-containing materials for nucleic acid encapsulation, including covalent nucleic acid conjugates, viral vectors or virus-like particles, dendrimers, peptides, polymers, lipids, hydrogels, inorganic nanoparticles, and nucleic acid nanostructures. Our discussion will focus on the context of the design of materials and their impact on addressing the current shortcomings in the intracellular delivery of nucleic acids.

The role of reversible and irreversible covalent chemistry in targeted protein degradation

Proteolysis-targeting chimeras (PROTACs) that degrade disease-causing proteins by hijacking the endogenous ubiquitin-proteasome system have emerged as an exciting and transformative technology in both chemical biology and drug discovery. Currently, the majority of PROTACs use reversible non-covalent ligands for both the target protein of interest (POI) and E3 ligase. In this review, we explore the burgeoning role of reversible and irreversible covalent chemistry in targeted protein degradation. We highlight the key advantages of targeted covalent inhibitors, whether as the target POI or E3 ligase ligand, such as their ability to enhance the selectivity of PROTACs, enable access to more of the "undruggable" proteome and expand the repertoire of recruited E3 ligases.

Synergistic Interplay of Covalent and Non-Covalent Interactions in Reactive Polymer Nanoassembly Facilitates Intracellular Delivery of Antibodies

The primary impediments in developing large antibodies as drugs against intracellular targets involve their low transfection efficiency and suitable reversible encapsulation strategies for intracellular delivery with retention of biological activity. To address this, we outline an electrostatics-enhanced covalent self-assembly strategy to generate polymer-protein/antibody nanoassemblies. Through structure-activity studies, we down-select the best performing self-immolative pentafluorophenyl containing activated carbonate polymer for bioconjugation. With the help of an electrostatics-aided covalent self-assembly approach, we demonstrate efficient encapsulation of medium to large proteins (HRP, 44 kDa and β-gal, 465 kDa) and antibodies (ca. 150 kDa). The designed polymeric nanoassemblies are shown to successfully traffic functional antibodies (anti-NPC and anti-pAkt) to cytosol to elicit their bioactivity towards binding intracellular protein epitopes and inducing apoptosis.

Inhibitors of thiol-mediated uptake

Ellman's reagent has caused substantial confusion and concern as a probe for thiol-mediated uptake because it is the only established inhibitor available but works neither efficiently nor reliably. Here we use fluorescent cyclic oligochalcogenides that enter cells by thiol-mediated uptake to systematically screen for more potent inhibitors, including epidithiodiketopiperazines, benzopolysulfanes, disulfide-bridged γ-turned peptides, heteroaromatic sulfones and cyclic thiosulfonates, thiosulfinates and disulfides. With nanomolar activity, the best inhibitors identified are more than 5000 times better than Ellman's reagent. Different activities found with different reporters reveal thiol-mediated uptake as a complex multitarget process. Preliminary results on the inhibition of the cellular uptake of pseudo-lentivectors expressing SARS-CoV-2 spike protein do not exclude potential of efficient inhibitors of thiol-mediated uptake for the development of new antivirals.

Thiol-reactive inhibitors for the cellular entry of cyclic oligochalcogenide (COC) transporters and SARS-CoV-2 spike pseudo-lentivirus are reported.

Oligomers of Cyclic Oligochalcogenides for Enhanced Cellular Uptake

Monomeric cyclic oligochalcogenides (COCs) are emerging as attractive transporters to deliver substrates of interest into the cytosol through thiol-mediated uptake. The objective of this study was to explore COC oligomers. We report a systematic evaluation of monomers, dimers, and trimers of asparagusic, lipoic, and diselenolipoic acid as well as their supramolecular monomers, dimers, trimers, and tetramers. COC dimers were more than twice as active as the monomers on both the covalent and noncovalent levels, whereas COC trimers were not much better than dimers. These trends might suggest that thiol-mediated uptake of COCs is synergistic over both short and long distances, that is, it involves more than two COCs and more than one membrane protein, although other interpretations cannot be excluded at this level of complexity. These results thus provide attractive perspectives for structural evolution as well as imminent use in practice. Moreover, they validate automated HC-CAPA as an invaluable method to collect comprehensive data on cytosolic delivery within a reasonable time at a level of confidence that is otherwise inconceivable.

Enhancing intracellular accumulation and target engagement of PROTACs with reversible covalent chemistry

Current efforts in the proteolysis targeting chimera (PROTAC) field mostly focus on choosing an appropriate E3 ligase for the target protein, improving the binding affinities towards the target protein and the E3 ligase, and optimizing the PROTAC linker. However, due to the large molecular weights of PROTACs, their cellular uptake remains an issue. Through comparing how different warhead chemistry, reversible noncovalent (RNC), reversible covalent (RC), and irreversible covalent (IRC) binders, affects the degradation of Bruton's Tyrosine Kinase (BTK), we serendipitously discover that cyano-acrylamide-based reversible covalent chemistry can significantly enhance the intracellular accumulation and target engagement of PROTACs and develop RC-1 as a reversible covalent BTK PROTAC with a high target occupancy as its corresponding kinase inhibitor and effectiveness as a dual functional inhibitor and degrader, a different mechanism-of-action for PROTACs. Importantly, this reversible covalent strategy is generalizable to improve other PROTACs, opening a path to enhance PROTAC efficacy.

Synthesis of functional 1,2-dithiolanes from 1,3-bis-tert-butyl thioethers

We report the one-step synthesis of diversely substituted functional 1,2-dithiolanes by reacting readily accessible 1,3-bis-tert-butyl thioethers with bromine. The reaction proceeds to completion within minutes under mild conditions, presumably via a sulfonium-mediated ring closure. Using X-ray crystallography and UV-vis spectroscopy, we demonstrate how substituent size and ring substitution pattern can affect the geometry and photophysical properties of 1,2-dithiolanes.

Grafting multi-maleimides on antisense oligonucleotide to enhance its cellular uptake and gene silencing capability

A multitude of maleimides are grafted onto the backbone of a phosphorothioate antisense oligonucleotide (ASO) to generate the construct of maleimide-grafted ASO (Mal-g-ASO). Through click conjugation with cell membrane thiols that triggers endocytosis-independent cellular internalization, Mal-g-ASO exhibited enhanced cellular uptake efficiency, resulting in a remarkable improvement of ASO-based gene silencing.

Cell-Penetrating Streptavidin: A General Tool for Bifunctional Delivery with Spatiotemporal Control, Mediated by Transport Systems Such as Adaptive Benzopolysulfane Networks

In this report, cell-penetrating streptavidin (CPS) is introduced to exploit the full power of streptavidin-biotin biotechnology in cellular uptake. For this purpose, transporters, here cyclic oligochalcogenides (COCs), are covalently attached to lysines of wild-type streptavidin. This leaves all four biotin binding sites free for at least bifunctional delivery. To maximize the standards of the quantitative evaluation of cytosolic delivery, the recent chloroalkane penetration assay (CAPA) is coupled with automated high content (HC) imaging, a technique that combines the advantages of fluorescence microscopy and flow cytometry. According to the resulting HC-CAPA, cytosolic delivery of CPS equipped with four benzopolysulfanes was the best among all tested CPSs, also better than the much smaller TAT peptide, the original cell-penetrating peptide from HIV. HaloTag-GFP fusion proteins expressed on mitochondria were successfully targeted using CPS carrying two different biotinylated ligands, HaloTag substrates or anti-GFP nanobodies, interfaced with peptide nucleic acids, flipper force probes, or fluorescent substrates. The delivered substrates could be released from CPS into the cytosol through desthiobiotin-biotin exchange. These results validate CPS as a general tool which enables unrestricted use of streptavidin-biotin biotechnology in cellular uptake.

Combination of chemotherapy and oxidative stress to enhance cancer cell apoptosis

Cancer cells are vulnerable to reactive oxygen species (ROS) due to their abnormal redox environment. Accordingly, combination of chemotherapy and oxidative stress has gained increasing interest for the treatment of cancer. We report a novel seleno-prodrug of gemcitabine (Gem), Se-Gem, and evaluated its activation and biological effects in cancer cells. Se-Gem was prepared by introducing a 1,2-diselenolane (a five-membered cyclic diselenide) moiety into the parent drug Gem via a carbamate linker. Se-Gem is preferably activated by glutathione (GSH) and displays a remarkably higher potency than Gem (up to a 6-fold increase) to a panel of cancer cell lines. The activation of Se-Gem by GSH releases Gem and a seleno-intermediate nearly quantitatively. Unlike the most ignored side products in prodrug activation, the seleno-intermediate further catalyzes a conversion of GSH and oxygen to GSSG (oxidized GSH) and ROS via redox cycling reactions. Thus Se-Gem may be considered as a suicide agent to deplete GSH and works by a combination of chemotherapy and oxidative stress. This is the first case that employs a cyclic diselenide in prodrug design, and the success of Se-Gem as well as its well-defined action mechanism demonstrates that the 1,2-diselenolane moiety may serve as a general scaffold to advance constructing novel therapeutic molecules with improved potency via a combination of chemotherapy and oxidative stress.

Dynamic Covalent Polymers for Biomedical Applications

The rapid development of supramolecular polymer chemistry and constitutional dynamic chemistry over the last decades has made tremendous impact on the emergence of dynamic covalent polymers. These materials are formed through reversible covalent bonds, endowing them with adaptive and responsive features that have resulted in high interest throughout the community. Owing to their intriguing properties, such as self-healing, shape-memory effects, recyclability, degradability, stimuli-responsiveness, etc., the materials have found multiple uses in a wide range of areas. Of special interest is their increasing use for biomedical applications, and many examples have been demonstrated in recent years. These materials have thus been used for the recognition and sensing of biologically active compounds, for the modulation of enzyme activity, for gene delivery, and as materials for cell culture, delivery, and wound-dressing. In this review, some of these endeavors are discussed, highlighting the many advantages and unique properties of dynamic covalent polymers for use in biology and biomedicine.