Cleavage of disulfide bonds in endocytosed macromolecules. A processing not associated with lysosomes or endosomes

Enhancing the cytotoxicity of immunotoxins by facilitating their dissociation from target receptors under the reducing conditions of the endocytic pathway

Immunotoxins (ITs) are recombinant chimeric proteins that combine a protein toxin with a targeting moiety to facilitate the selective delivery of the toxin to cancer cells. Here, we present a novel strategy to enhance the cytosolic access of ITs by promoting their dissociation from target receptors under the reducing conditions of the endocytic pathway. We engineered monobodySS, a human fibronectin type III domain-based monobody with disulfide bond (SS)-containing paratopes, targeting receptors such as EGFR, EpCAM, Her2, and FAP. MonobodySS exhibited SS-dependent target receptor binding with a significant reduction in binding under reducing conditions. We then created monobodySS-based ITs carrying a 25 kDa fragment of Pseudomonas exotoxin A (PE25), termed monobodySS-PE25. These ITs showed dose-dependent cytotoxicity against target receptor-expressing cancer cells and a wider therapeutic window due to higher efficacy at lower doses compared to controls with SS reduction inhibited. ERSS/28-PE25, with a KD of 28 nM for EGFR, demonstrated superior tumor-killing potency compared to ER/21-PE25, which lacks an SS bond, at equivalent and lower doses. In vivo, ERSS/28-PE25 outperformed ER/21-PE25 in suppressing tumor growth in EGFR-overexpressing xenograft mouse models. This study presents a strategy for developing solid tumor-targeting ITs using SS-containing paratopes to enhance cytosolic delivery and antitumor efficacy.

IFI30 as a key regulator of PDL1 immunotherapy prognosis in breast cancer

BACKGROUND

IFI30 is a lysosomal thiol reductase involved in antigen presentation and immune regulation in various cancers, including breast cancer. Despite its known involvement, the precise mechanism, function, and relationship with the PD-L1 axis and immune response remain unclear.

METHODS

We conducted an extensive investigation into IFI30 mRNA expression in breast cancer utilizing data from The Cancer Genome Atlas (TCGA) and Genotype-Tissue Expression (GTEx) databases. Furthermore, we characterized IFI30 mRNA expression across various cell types using publicly available single-cell RNA sequencing datasets, and assessed protein expression through immunohistochemistry using an in-house breast cancer tissue microarray. Functional experiments were performed to elucidate the effects of IFI30 overexpression on PD-L1 expression and inhibitory efficacy in both macrophages and breast tumor cells.

RESULTS

Our study unveiled a marked upregulation of IFI30 expression in breast cancer tissues compared to their normal counterparts, with notable associations identified with tumor stage and prognosis. Additionally, IFI30 expression demonstrated significant correlations with various immune-related signaling pathways, encompassing peptide antigen binding, cytokine binding, and MHC class II presentation. Notably, breast cancer samples exhibiting high IFI30 expression in tumor cells displayed high PD-L1 expression on corresponding cells, alongside a diminished ratio of CD8 + T cell infiltration within the tumor microenvironment. Furthermore, ectopic knockdown of IFI30 in both tumor cells and macrophages resulted in a reduction of PD-L1 expression, while conversely, overexpression of IFI30 led to an increase in PD-L1 expression.

CONCLUSIONS

This study offers new insights into the involvement of IFI30 in breast cancer, elucidating its interplay with the PD-L1 axis and immune response dynamics. Our findings suggest that modulation of the IFI30-PD-L1 axis could serve as a promising strategy for regulating T cells infiltration in breast cancer thus treating breast cancer.

Design, Synthesis, and Targeted Delivery of an Immune Stimulant that Selectively Reactivates Exhausted CAR T Cells

Although chimeric antigen receptor (CAR) T cells have demonstrated significant promise in suppressing hematopoietic cancers, their applications in treating solid tumors have been limited by onset of CAR T cell exhaustion that accompanies continuous CAR T cell exposure to tumor antigen. To address this limitation, we have exploited the abilities of recently designed universal CARs to bind fluorescein and internalize a fluorescein-TLR7 agonist conjugate by CAR-mediated endocytosis. We demonstrate here that anti-fluorescein CAR-mediated uptake of a fluorescein-TLR7-3 conjugate can reactivate exhausted CAR T cells, leading to dramatic reduction in T cell exhaustion markers (PD-1⁺ Tim-3⁺ ) and shrinkage of otherwise resistant tumors without inducing systemic activation of the immune system. We conclude that CAR T cell exhaustion can be reversed by administration of a CAR-targeted TLR7 agonist, thereby enabling the CAR T cells to successfully treat solid tumors without incurring the systemic toxicity that commonly accompanies administration of nontargeted TLR7 agonists.

Visible Light and Glutathione Dually Responsive Delivery of a Polymer-Conjugated Temozolomide Intermediate for Glioblastoma Chemotherapy

Temozolomide (TMZ) is a prodrug of 5-(3-methyltriazene-1-yl)imidazole-4-carboxamide (MTIC, short-lived) and used as a first-line therapy drug for glioblastoma multiforme (GBM). However, little progress has been made in regulating the kinetics of TMZ to MTIC degradation to improve the therapeutic effect, particularly in the case of TMZ-resistant GBM. In this work, we introduced a strategy to cage MTIC by N-acylation of the triazene moiety to boost the MTIC stability, designed a diblock copolymer-based MTIC prodrug installed with a disulfide linkage, and achieved self-assembled polymer micelles without the concern of MTIC leakage under physiological conditions. Polymer micelles could be induced to disassemble by stimuli factors such as glutathione (GSH) and visible light irradiation through thiol/sulfide exchange and homolytic sulfide scission mechanisms, which contributed to MTIC release in GSH-dependent and GSH-independent pathways. The in vitro results demonstrated that microenvironment-responsive polymeric micelles benefited the suppression of both TMZ-sensitive and TMZ-resistant GBM cells. The chemistry of polymer-MTIC prodrug provided a new option for TMZ-based glioma treatment.

Vitamin B12 Derivatives Suitably Tailored for the Synthesis of Photolabile Conjugates

Vitamin B₁₂ conjugates are broadly studied in biological sciences. As light offers spatiotemporal control, we decided to develop a method for the preparation of vitamin B₁₂ conjugates that release tethered molecules upon exposure to light. Herein, we report vitamin B₁₂ derivatives possessing a photolabile linker suitable for conjugation with amines, azides, and alkynes. The potential applications of such conjugates are broad and include the delivery of drugs, labels, and imaging agents to their place of action and spatiotemporal release.

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.

Development of HER2-Specific Aptamer-Drug Conjugate for Breast Cancer Therapy

In this study, HER2 RNA aptamers were conjugated to mertansine (DM1) and the anti-cancer effectiveness of the conjugate was evaluated in HER2-overexpressing breast cancer models. The conjugate of HER2 aptamer and anticancer drug DM1 (aptamer-drug conjugate, ApDC) was prepared and analyzed using HPLC and mass spectrometry. The cell-binding affinity and cytotoxicity of the conjugate were determined using confocal microscopy and WST-1 assay. The in vivo anti-tumoral efficacy of ApDC was also evaluated in mice carrying BT-474 breast tumors overexpressing HER2. The synthesized HER2-specific RNA aptamers were able to specifically and efficiently bind to HER-positive BT-474 breast cancer cells, but not to HER2-negative MDA-MB-231 breast cancer cells. Also, the HER2-specific ApDC showed strong toxicity to the target cells, BT-474, but not to MDA-MB-231 cells. According to the in vivo analyses drawn from the mouse xenografts of BT-747 tumor, the ApDC was able to more effectively inhibit the tumor growth. Compared to the control group, the mice treated with the ApDC showed a significant reduction of tumor growth. Besides, any significant body weight losses or hepatic toxicities were monitored in the ApDC-treated mice. This research suggests the HER2 aptamer-DM1 conjugate as a target-specific anti-cancer modality and provides experimental evidence supporting its enhanced effectiveness for HER2-overexpressing target tumors. This type of aptamer-conjugated anticancer drug would be utilized as a platform structure for the development of versatile targeted high-performance anticancer drugs by adopting the easy deformability and high affinity of aptamers.

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.

Automated high-content imaging for cellular uptake, from the Schmuck cation to the latest cyclic oligochalcogenides

Recent progress with chemistry tools to deliver into living cells has seen a shift of attention from counterion-mediated uptake of cell-penetrating peptides (CPPs) and their mimics, particularly the Schmuck cation, toward thiol-mediated uptake with cell-penetrating poly(disulfide)s (CPDs) and cyclic oligochalcogenides (COCs), here exemplified by asparagusic acid. A persistent challenge in this evolution is the simultaneous and quantitative detection of cytosolic delivery and cytotoxicity in a high-throughput format. Here, we show that the combination of the HaloTag-based chloroalkane penetration assay (CAPA) with automated high-content (HC) microscopy can satisfy this need. The automated imaging of thousands of cells per condition in multiwell plates allows us to obtain quantitative data on not only the fluorescence intensity but also on the localization in a very short time. Quantitative and statistically relevant results can be obtained from dose-response curves of the targeted delivery to selected cells and the cytotoxicity in the same experiment, even with poorly optimized cellular systems.

Biological Effect Evaluation of Glutathione-Responsive Cyclodextrin-Based Nanosponges: 2D and 3D Studies

This study aims to evaluate the bioeffects of glutathione-responsive β-cyclodextrin-based nanosponges (GSH-NSs) on two- (2D) and three-dimensional (3D) cell cultures. The bioeffects of two types of GSH-NS formulations, with low (GSH-NS B) and high (GSH-NS D) disulfide-bond content, were evaluated on 2D colorectal (HCT116 and HT-29) and prostatic (DU-145 and PC3) cancer cell cultures. In particular, the cellular uptake of GSH-NS was evaluated, as their effects on cell growth, mitochondrial activity, membrane integrity, cell cycle distribution, mRNA expression, and reactive oxygen species production. The effect of GSH-NSs on cell growth was also evaluated on multicellular spheroids (MCS) and a comparison of the GSH-NS cell growth inhibitory activity, in terms of inhibition concentration (IC)₅₀ values, was performed between 2D and 3D cell cultures. A significant decrease in 2D cell growth was observed at high GSH-NS concentrations, with the formulation with a low disulfide-bond content, GSH-NS B, being more cytotoxic than the formulation with a high disulfide-bond content, GSH-NS D. The cell growth decrease induced by GSH-NS was owing to G₁ cell cycle arrest. Moreover, a significant down-regulation of mRNA expression of the cyclin genes CDK1, CDK2, and CDK4 and up-regulation of mRNA expression of the cyclin inhibitor genes CDKN1A and CDKN2A were observed. On the other hand, a significant decrease in MCS growth was also observed at high GSH-NS concentrations, but not influenced by the nanosponge disulfide-bond content, with the MCS IC₅₀ values being significantly higher than those obtained on 2D cell cultures. GSH-NSs are suitable nanocarries as they provoke limited cellular effects, as cell cycle arrest only occurred at concentrations significantly higher than those used for drug delivery.

Disulfide bridge as a linker in nucleic acids' bioconjugation. Part II: A summary of practical applications

Disulfide conjugation invariably remains a key tool in research on nucleic acids. This versatile and cost-effective method plays a crucial role in structural studies of DNA and RNA as well as their interactions with other macromolecules in a variety of biological systems. In this article we review applications of disulfide-bridged conjugates of oligonucleotides with other (bio)molecules such as peptides, proteins etc. and present key findings obtained with their help.

Development of a Cysteine-Conjugatable Disulfide FRET Probe: Influence of Charge on Linker Cleavage and Payload Trafficking for an Anti-HER2 Antibody Conjugate

Disulfide-linked bioconjugates allow the delivery of pharmacologically active or other cargo to specific tissues in a redox-sensitive fashion. However, an understanding of the kinetics, subcellular distribution, and mechanism of disulfide cleavage in such bioconjugates is generally lacking. Here, we report a modular disulfide-linked TAMRA-BODIPY based FRET probe that can be readily synthesized, modified, and conjugated to a cysteine-containing biomolecule to enable real-time monitoring of disulfide cleavage during receptor-mediated endocytosis in cells. We demonstrate the utility of this probe to study disulfide reduction during HER2 receptor-mediated uptake of a Cys-engineered anti-HER2 THIOMAB antibody. We found that introduction of positive, but not negative, charges in the probe improved retention of the BODIPY catabolite. This permitted the observation of significant disulfide cleavage in endosomes or lysosomes on par with proteolytic cleavage of a similarly charged valine-citrulline peptide-based probe. In general, the FRET probe we describe should enable real-time cellular monitoring of disulfide cleavage in other targeted delivery systems for mechanistic or diagnostic applications. Furthermore, modifications to the released BODIPY moiety permit evaluation of physicochemical properties that govern lysosomal egress or retention, which may have implications for the development of next-generation antibody-drug conjugates.

Improving cellular uptake of therapeutic entities through interaction with components of cell membrane

Efficient cellular delivery of biologically active molecules is one of the key factors that affect the discovery and development of novel drugs. The plasma membrane is the first barrier that prevents direct translocation of chemic entities, and thus obstructs their efficient intracellular delivery. Generally, hydrophilic small molecule drugs are poor permeability that reduce bioavailability and thus limit the clinic application. The cellular uptake of macromolecules and drug carriers is very inefficient without external assistance. Therefore, it is desirable to develop potent delivery systems for achieving effective intracellular delivery of chemic entities. Apart from of the types of delivery strategies, the composition of the cell membrane is critical for delivery efficiency due to the fact that cellular uptake is affected by the interaction between the chemical entity and the plasma membrane. In this review, we aimed to develop a profound understanding of the interactions between delivery systems and components of the plasma membrane. For the purpose, we attempt to present a broad overview of what delivery systems can be used to enhance the intracellular delivery of poorly permeable chemic entities, and how various delivery strategies are applied according to the components of plasma membrane.

Responsive Antibody Conjugates Enable Quantitative Determination of Intracellular Bond Degradation Rate

Degradable crosslinkers that respond to intracellular biological stimuli are a critical component of many drug delivery systems. With numerous stimuli-responsive drug delivery systems in development, it is important to quantitatively study their intracellular processing. Herein we report a framework for quantifying the rate of intracellular bond degradation in the endocytic pathway. Toward this end, we devised and synthesized a reduction-sensitive FRET-based crosslinker that can be readily conjugated to a variety of targeting ligands. This crosslinker was conjugated to trastuzumab, a humanized monoclonal antibody against the HER2 receptor. We developed a model based on mass-action kinetics to describe the intracellular processing of this conjugate. The kinetic model was developed in conjunction with live-cell experiments to extract the rate constant for intracellular disulfide bond degradation. This framework may be applied to other endocytosis pathways, bond types, and cell types to quantify this fundamental degradation rate parameter.

Cell-Penetrating Dynamic-Covalent Benzopolysulfane Networks

Cyclic oligochalcogenides (COCs) are emerging as promising systems to penetrate cells. Clearly better than and different to the reported diselenolanes and epidithiodiketopiperazines, we introduce the benzopolysulfanes (BPS), which show efficient delivery, insensitivity to inhibitors of endocytosis, and compatibility with substrates as large as proteins. This high activity coincides with high reactivity, selectively toward thiols, exceeding exchange rates of disulfides under tension. The result is a dynamic-covalent network of extreme sulfur species, including cyclic oligomers, from dimers to heptamers, with up to nineteen sulfurs in the ring. Selection from this unfolding adaptive network then yields the reactivities and selectivities needed to access new uptake pathways. Contrary to other COCs, BPS show high retention on thiol affinity columns. The identification of new modes of cell penetration is important because they promise new solutions to challenges in delivery and beyond.

DNA nanodevices map enzymatic activity in organelles

Cellular reporters of enzyme activity are based on either fluorescent proteins or small molecules. Such reporters provide information corresponding to wherever inside cells the enzyme is maximally active and preclude minor populations present in subcellular compartments. Here we describe a chemical imaging strategy to selectively interrogate minor, subcellular pools of enzymatic activity. This new technology confines the detection chemistry to a designated organelle, enabling imaging of enzymatic cleavage exclusively within the organelle. We have thus quantitatively mapped disulfide reduction exclusively in endosomes in Caenorhabditis elegans and identified that exchange is mediated by minor populations of the enzymes PDI-3 and TRX-1 resident in endosomes. Impeding intra-endosomal disulfide reduction by knocking down TRX-1 protects nematodes from infection by Corynebacterium diphtheriae, revealing the importance of this minor pool of endosomal TRX-1. TRX-1 also mediates endosomal disulfide reduction in human cells. A range of enzymatic cleavage reactions in organelles are amenable to analysis by this new reporter strategy.

pH-sensitive polymer micelles provide selective and potentiated lytic capacity to venom peptides for effective intracellular delivery

Endocytosed biomacromolecule delivery systems must escape the endosomal trafficking pathway in order for their cargo to exert effects in other cellular compartments. Although endosomal release is well-recognized as one of the greatest barriers to efficacy of biologic drugs with intracellular targets, most drug carriers have relied on cationic materials that passively induce endosomal swelling and membrane rupture with low efficiency. To address the endosome release challenge, our lab has developed a diblock copolymer system for nucleic acid delivery that selectively displays a potent membrane-lytic peptide (melittin) in response to the pH drop during the endosomal maturation. To further optimize this system, we evaluated a panel of peptides with reported lytic activity in comparison to melittin. Nineteen different lytic peptides were synthesized and their membrane-lytic properties at both neutral and acidic pH characterized using a red blood cell hemolysis assay. The top five performing peptides were then conjugated to our pH-sensitive diblock copolymer via disulfide linkers and used to deliver a variety of nucleic acids to cultured mammalian cells as well as in vivo to the mouse brain. We demonstrate that the sharp pH-transition of VIPER compensates for potential advantages from pH-sensitive peptides, such that polymer-peptide conjugates with poorly selective but highly lytic peptides achieve safe and effective transfection both in vitro and in vivo. In addition, peptides that require release from polymer backbones for lysis were less effective in the VIPER system, likely due to limited endosomal reducing power of target cells. Finally, we show that certain peptides are potentiated in lytic ability by polymer conjugation and that these peptide-polymer constructs are most effective in vivo.

Host functions used by hepatitis B virus to complete its life cycle: Implications for developing host-targeting agents to treat chronic hepatitis B

Similar to other mammalian viruses, the life cycle of hepatitis B virus (HBV) is heavily dependent upon and regulated by cellular (host) functions. These cellular functions can be generally placed in to two categories: (a) intrinsic host restriction factors and innate defenses, which must be evaded or repressed by the virus; and (b) gene products that provide functions necessary for the virus to complete its life cycle. Some of these functions may apply to all viruses, but some may be specific to HBV. In certain cases, the virus may depend upon the host function much more than does the host itself. Knowing which host functions regulate the different steps of a virus' life cycle, can lead to new antiviral targets and help in developing novel treatment strategies, in addition to improving a fundamental understanding of viral pathogenesis. Therefore, in this review we will discuss known host factors which influence key steps of HBV life cycle, and further elucidate therapeutic interventions targeting host-HBV interactions.

Enhanced uptake in 2D- and 3D- lung cancer cell models of redox responsive PEGylated nanoparticles with sensitivity to reducing extra- and intracellular environments

In the treatment of lung cancer, there is an urgent need of innovative medicines to optimize pharmacological responses of conventional chemotherapeutics while attenuating side effects. Here, we have exploited some relatively unexplored subtle differences in reduction potential, associated with cancer cell microenvironments in addition to the well-known changes in intracellular redox environment. We report the synthesis and application of novel redox-responsive PLGA (poly(lactic-co-glycolic acid)) -PEG (polyethylene glycol) nanoparticles (RR-NPs) programmed to change surface properties when entering tumor microenvironments, thus enhancing cell internalization of the particles and their drug cargo. The new co-polymers, in which PEG and PLGA were linked by 'anchiomeric effector' dithiylethanoate esters, were synthesized by a combination of ring-opening polymerization and Michael addition reactions and employed to prepare NPs. Non redox-responsive nanoparticles (nRR-NPs) based on related PLGA-PEG copolymers were also prepared as comparators. Spherical NPs of around 120 nm diameter with a low polydispersity index and negative zeta potential as well as good drug loading of docetaxel were obtained. The NPs showed prolonged stability in relevant simulated biological fluids and a high ability to penetrate an artificial mucus layer due to the presence of the external PEG coating. Stability, FRET and drug release studies in conditions simulating intracellular reductive environments demonstrated a fast disassembly of the external shell of the NPs, thus triggering on-demand drug release. FACS measurements and confocal microscopy showed increased and faster uptake of RR-NPs in both 2D- and 3D- cell culture models of lung cancer compared to nRR-NPs. In particular, the 'designed-in' reductive instability of RR-NPs in conditioned cell media, the fast PEG release in the extracellular compartment, as well as a diminution of uptake rate in control experiments where extracellular thiols were neutralized, suggested a partial extracellular release of the PEG fringe that promoted rapid internalization of the residual NPs into cells. Taken together, these results provide further evidence of the effectiveness of PEGylated reducible nanocarriers to permeate mucus layer barriers, and establish a new means to enhance cancer cell uptake of drug carriers by extra-and intra-cellular cleavage of protein- and cell-shielding hydrophilic blocks.

Diselenolane-mediated cellular uptake

Selenophilicity, minimized dihedral angles, acidic selenols, multitarget hopping: cytosolic delivery with 1,2-diselenolanes outperforms 1,2-dithiolanes, by far.

The emerging power of thiol-mediated uptake with strained disulfides called for a move from sulfur to selenium. We report that according to results with fluorescent model substrates, cellular uptake with 1,2-diselenolanes exceeds uptake with 1,2-dithiolanes and epidithiodiketopiperazines with regard to efficiency as well as intracellular localization. The diselenide analog of lipoic acid performs best. This 1,2-diselenolane delivers fluorophores efficiently to the cytosol of HeLa Kyoto cells, without detectable endosomal capture as with 1,2-dithiolanes or dominant escape into the nucleus as with epidithiodiketopiperazines. Diselenolane-mediated cytosolic delivery is non-toxic (MTT assay), sensitive to temperature but insensitive to inhibitors of endocytosis (chlorpromazine, methyl-β-cyclodextrin, wortmannin, cytochalasin B) and conventional thiol-mediated uptake (Ellman's reagent), and to serum. Selenophilicity, the extreme CSeSeC dihedral angle of 0° and the high but different acidity of primary and secondary selenols might all contribute to uptake. Thiol-exchange affinity chromatography is introduced as operational mimic of thiol-mediated uptake that provides, in combination with rate enhancement of DTT oxidation, direct experimental evidence for existence and nature of the involved selenosulfides.

Disulphide crosslinked star block copolypeptide hydrogels: influence of block sequence order on hydrogel properties

Strong redox responsive hydrogels with mechanical properties depending on the positioning of oligo(cysteine) within the star polypeptides were obtained.

Epidithiodiketopiperazines: Strain-Promoted Thiol-Mediated Cellular Uptake at the Highest Tension

The disulfide dihedral angle in epidithiodiketopiperazines (ETPs) is near 0°. Application of this highest possible ring tension to strain-promoted thiol-mediated uptake results in efficient delivery to the cytosol and nucleus. Compared to the previous best asparagusic acid (AspA), ring-opening disulfide exchange with ETPs occurs more efficiently even with nonactivated thiols, and the resulting thiols exchange rapidly with nonactivated disulfides. ETP-mediated cellular uptake is more than 20 times more efficient compared to AspA, occurs without endosomal capture, depends on temperature, and is "unstoppable" by inhibitors of endocytosis and conventional thiol-mediated uptake, including siRNA against the transferrin receptor. These results suggest that ETP-mediated uptake not only maximizes delivery to the cytosol and nucleus but also opens the door to a new multitarget hopping mode of action.

Highly Constrained Bicyclic Scaffolds for the Discovery of Protease-Stable Peptides via mRNA Display

Highly constrained peptides such as the knotted peptide natural products are promising medicinal agents because of their impressive biostability and potent activity. Yet, libraries of highly constrained peptides are challenging to prepare. Here, we present a method which utilizes two robust, orthogonal chemical steps to create highly constrained bicyclic peptide libraries. This technology was optimized to be compatible with in vitro selections by mRNA display. We performed side-by-side monocyclic and bicyclic selections against a model protein (streptavidin). Both selections resulted in peptides with mid-nanomolar affinity, and the bicyclic selection yielded a peptide with remarkable protease resistance.

A Redox-Activatable Fluorescent Sensor for the High-Throughput Quantification of Cytosolic Delivery of Macromolecules

Efficient delivery of biomacromolecules (e.g., proteins, nucleic acids) into cell cytosol remains a critical challenge for the development of macromolecular therapeutics or diagnostics. To date, most common approaches to assess cytosolic delivery rely on fluorescent labeling of macromolecules with an "always on" reporter and subcellular imaging of endolysosomal escape by confocal microscopy. This strategy is limited by poor signal-to-noise ratio and only offers low throughput, qualitative information. Herein we describe a quantitative redox-activatable sensor (qRAS) for the real-time monitoring of cytosolic delivery of macromolecules. qRAS-labeled macromolecules are silent (off) inside the intact endocytic organelles, but can be turned on by redox activation after endolysosomal disruption and delivery into the cytosol, thereby greatly improving the detection accuracy. In addition to confocal microscopy, this quantitative sensing technology allowed for a high-throughput screening of a panel of polymer carriers toward efficient cytosolic delivery of model proteins on a plate reader. The simple and versatile qRAS design offers a useful tool for the investigation of new strategies for endolysosomal escape of biomacromolecules to facilitate the development of macromolecular therapeutics for a variety of disease indications.

Targeted toxins

Objective. Current modalities used in the treatment of cancer often cause unacceptable damage to normal tissue. Toxins targeted toward tumor cells by antibodies or growth factors have the potential to selectively kill tumor cells while leaving normal tissue intact. The purpose of this review is to provide background information on targeted toxins and current clinical studies for this new class of anti-cancer compounds.

Data sources. A MEDLINE search was conducted using the term “immunotoxins.” Relevant articles were also obtained by the systematic examination of article references.

Data synthesis. The toxins Pseudomonas exotoxin, diphtheria toxin, and ricin toxin are often used as targeted toxins. Deletion or mutation of the binding domains of these toxins decreased binding of the toxins to normal tissues. Antibodies or growth factors can be used as targeting moiety, and the resulting agents are called immunotoxins or fusion proteins, respectively. DNA technology and chemical modifications of the toxin as well as the antibody moiety led to smaller and less immunogenic targeted toxins. Smaller targeted toxins are less toxic and penetrate further into the tumor. The summary of several targeted toxins elicited during clinical trials in this review makes it clear that several targeted toxins are potential agents for the treatment of various cancers, although some problems still need to be overcome. These problems include toxicity, immunogenicity, cross-reactivity of the targeted toxin with life-sustaining tissue, heterogenicity of tumor cells, and limited tumor penetration.

Cellular uptake: lessons from supramolecular organic chemistry

The objective of this Feature Article is to reflect on the importance of established and emerging principles of supramolecular organic chemistry to address one of the most persistent problems in life sciences. The main topic is dynamic covalent chemistry on cell surfaces, particularly disulfide exchange for thiol-mediated uptake. Examples of boronate and hydrazone exchange are added for contrast, comparison and completion. Of equal importance are the discussions of proximity effects in polyions and counterion hopping, and more recent highlights on ring tension and ion pair-π interactions. These lessons from supramolecular organic chemistry apply to cell-penetrating peptides, particularly the origin of "arginine magic" and the "pyrenebutyrate trick," and the currently emerging complementary "disulfide magic" with cell-penetrating poly(disulfide)s. They further extend to the voltage gating of neuronal potassium channels, gene transfection, and the delivery of siRNA. The collected examples illustrate that the input from conceptually innovative chemistry is essential to address the true challenges in biology beyond incremental progress and random screening.

Versatile nuclear localization signal-based oligopeptide as a gene vector

To develop a versatile nuclear-targeted gene vector, nuclear localization signal (NLS) oligopeptides combining cysteine (C), histidine (H), and stearic acid (STR) were investigated in this study. The original SV40 sequence (SV40: Pro-Lys-Lys-Lys-Arg-Lys-Val) was selected as the NLS sequence, and physical characterizations of various NLS-based oligopeptides (CSV40C, STR-CSV40C, and STR-CH2SV40H2C), including mean diameter, zeta-potential, complex condensation, and decondensation, were evaluated. In addition, cellular and nuclear uptake of plasmid DNA (pDNA) and gene expression in COS7 and dendritic cells (JAWS II) were determined. As a result, C and STR enhanced formation of a smaller and more stable nano-complex with pDNA based on ionic interactions, the disulfide linkage and hydrophobic interactions. STR-CSV40C and STR-CH2SV40H2C had significantly higher cellular uptake ability and transfection efficiency than SV40 and CSV40C. In particular, STR-CH2SV40H2C had higher nuclear uptake and gene expression efficiency than STR-CSV40C. Furthermore, STR-CH2SV40H2C could deliver pDNA to the nuclei and had high gene expression efficiency in dendritic cells. Our results indicate that STR-CH2SV40H2C is a promising gene delivery system in non- or slow-dividing cells.

Vitamin B12 Suitably Tailored for Disulfide-Based Conjugation

Vitamin B12 has been proposed to be a natural vector for the in vivo delivery of biologically active compounds. Most synthetic methodologies leading to vitamin B12 conjugates involve functionalization at the 5' position via either carbamate-based linkages or using copper(I)-catalyzed azide-alkyne cycloaddition (CuAAC), resulting in stable conjugates that are not cleaved within the cell. We have developed a novel vitamin B12 derivative suitably tailored for disulfide-based conjugation that can undergo cleavage in the presence of glutathione, the most abundant thiol in mammalian cells. This active compound is simple to prepare and allows for the direct disulfide-based attachment of therapeutic cargos.

Ethynyl benziodoxolones: functional terminators for cell-penetrating poly(disulfide)s

Hypervalent iodine terminators are introduced to secure synthetic access to doubly-labeled cell-penetrating poly(disulfide)s.

Enzymatic synthesis of poly(ω-pentadecalactone-co-butylene-co-3,3′-dithiodipropionate) copolyesters and self-assembly of the PEGylated copolymer micelles as redox-responsive nanocarriers for doxorubicin delivery

The PEG-polyester copolymers bearing disulfide groups were synthesized to serve as redox-responsive anticancer drug carriers with an enhanced efficacy.

Multicomponent polymeric nanoparticles enhancing intracellular drug release in cancer cells

Three kinds of amphiphilic copolymer, that is, poly(ε-caprolactone)-SS-poly(ethylene glycol) (PCL-SS-PEG), poly(ε-caprolactone)-polyethylenimine (PCL-PEI), and poly(ε-caprolactone)-polyethylenimine-folate (PCL-PEI-Fol) were synthesized and self-assembled into surface engineered hybrid nanoparticles (NPs). Morphological studies elucidated the stable, spherical, and uniform sandwich structure of the NPs. PCL-PEI and PCL-SS-PEG segments have introduced pH and reduction responsive characteristics in these NPs, while PCL-PEI-FA copolymers could provide specific targeting capability to cancer cells. The stimuli responsive capabilities of these NPs were carried out. Negative-to-positive charge reversible property, in response to the pH change, was investigated by zeta potential and nuclear magnetic resonance (NMR) measurements. The structure cleavage, due to redox gradient, was studied by dynamic light scattering (DLS) and transmission electron microscopy (TEM). These NPs showed controlled degradation, better drug release, less toxicity, and effective uptake in MCF-7 breast cancer cells. These multifunctional NPs showed promising potential in the treatment of cancer.

Layer-by-layer films with bioreducible and nonbioreducible polycations for sequential DNA release

Layer-by-layer (LbL) films containing cationic polyelectrolytes and anionic bioactive molecules such as DNA are promising biomaterials for controlled and localized gene delivery for a number of biomedical applications including cancer DNA vaccine delivery. Bioreducible LbL films made of disulfide-containing poly(amido amine)s (PAAs) and plasmid DNA can be degraded by redox-active membrane proteins through the thiol-disulfide exchange reaction to release DNA exclusively into the extracellular microenvironment adjacent to the film. In order to better understand the film degradation mechanism and nature of the released species, the bioreducible film degradation is studied by atomic force microscopy, fluorescence, and dynamic light scattering in solutions containing a reducing agent. The PAA/DNA LbL film undergoes fast bulk degradation with micrometer-sized pieces breaking off from the substrate. This bulk degradation behavior is arrested by periodic insertions of a nonbioreducible poly(ethylenimine) (PEI) layer. The LbL films containing PAA/DNA and PEI/DNA bilayers display sequential film disassembly and are capable of continuously releasing DNA nanoparticles over a prolonged time. Insertion of the PEI layer enables the bioreducible LbL films to transfect human embryonic kidney 293 cells. The data conclude that the PEI layer is effective as a barrier layer against interlayer diffusion during LbL film assembly and more importantly during film disassembly. Without the barrier layer, the high mobility of cleaved PAA fragments is responsible for bulk degradation of bioreducible LbL films, which may prevent their ultimate gene-delivery applications. This work establishes a direct link among film internal structure, disassembly mechanism, and transfection efficiency. It provides a simple method to design bioreducible LbL films for sequential and long-time DNA release.

Redox-responsive magnetic nanoparticle for targeted convection-enhanced delivery of O6-benzylguanine to brain tumors

Resistance to temozolomide (TMZ) based chemotherapy in glioblastoma multiforme (GBM) has been attributed to the upregulation of the DNA repair protein O(6)-methylguanine-DNA methyltransferase (MGMT). Inhibition of MGMT using O(6)-benzylguanine (BG) has shown promise in these patients, but its clinical use is hindered by poor pharmacokinetics that leads to unacceptable toxicity. To improve BG biodistribution and efficacy, we developed superparamagnetic iron oxide nanoparticles (NP) for targeted convection-enhanced delivery (CED) of BG to GBM. The nanoparticles (NPCP-BG-CTX) consist of a magnetic core coated with a redox-responsive, cross-linked, biocompatible chitosan-PEG copolymer surface coating (NPCP). NPCP was modified through covalent attachment of BG and tumor targeting peptide chlorotoxin (CTX). Controlled, localized BG release was achieved under reductive intracellular conditions and NPCP-BG-CTX demonstrated proper trafficking of BG in human GBM cells in vitro. NPCP-BG-CTX treated cells showed a significant reduction in MGMT activity and the potentiation of TMZ toxicity. In vivo, CED of NPCP-BG-CTX produced an excellent volume of distribution (Vd) within the brain of mice bearing orthotopic human primary GBM xenografts. Significantly, concurrent treatment with NPCP-BG-CTX and TMZ showed a 3-fold increase in median overall survival in comparison to NPCP-CTX/TMZ treated and untreated animals. Furthermore, NPCP-BG-CTX mitigated the myelosuppression observed with free BG in wild-type mice when administered concurrently with TMZ. The combination of favorable physicochemical properties, tumor cell specific BG delivery, controlled BG release, and improved in vivo efficacy demonstrates the great potential of these NPs as a treatment option that could lead to improved clinical outcomes.

Redox-sensitive materials for drug delivery: targeting the correct intracellular environment, tuning release rates, and appropriate predictive systems

SIGNIFICANCE

The development of responsive drug delivery systems (DDS) holds great promise as a tool for improving the pharmacokinetic properties of drug compounds. Redox-sensitive systems are particularly attractive given the rich variety of redox gradients present in vivo. These gradients, where the circulation is generally considered oxidizing and the cellular environment is substantially more reducing, provide attractive options for targeted, specific cargo delivery.

RECENT ADVANCES

Experimental evidence suggests that a "one size fits all" redox gradient does not exist. Rather, there are subtle differences in redox potential within a cell, while the chemical nature of reducing agents in these microenvironments varies. Recent works have demonstrated an ability to modulate the degradation rate of redox-susceptible groups and, hence, provide new tools to engineer precision-targeted DDS.

CRITICAL ISSUES

Modern synthetic and macromolecular chemistry provides access to a wide range of redox-susceptible architectures. However, in order to utilize these in real applications, the actual chemical nature of the redox-susceptible group, the sub-cellular location being targeted, and the redox microenvironment being encountered should be considered in detail. This is critical to avoid the over-simplification possible when using non-biological reducing agents, which may provide inaccurate kinetic information, and to ensure these materials can be advanced beyond simple "on/off" systems. Furthermore, a strong case can be made for the use of biorelevant reducing agents such as glutathione when demonstrating a materials redox response.

FUTURE DIRECTIONS

A further understanding of the complexities of the extra- and intracellular microenvironments would greatly assist with the design and application of DDS.

Bioreducible polycations as shuttles for therapeutic nucleic acid and protein transfection

SIGNIFICANCE

Nucleic acids such as gene-encoding DNAs, gene-silencing small interfering RNAs, or recombinant proteins addressing intracellular molecular targets present a major new therapeutic modality, provided efficient solutions for intracellular delivery can be found. The different physiological redox environments inside and outside the cell can be utilized for optimizing the involved transport processes.

RECENT ADVANCES

Intracellular delivery of nucleic acids or proteins requires dynamic carriers that discriminate between different cellular locations. Bioreducible cationic polymers can package their therapeutic cargo stably in the extracellular environment, but sense the reducing intracellular cytosolic environment. Based on disulfide cleavage, carriers are degraded into biocompatible fragments and release the cargo in functional form. Disulfide linkages between oligocations, between the carrier and the cargo, or spatial caging of complexed cargo by disulfides have been pursued, with polymers or precise sequence-defined peptides and oligomers.

CRITICAL ISSUES

A quantitative knowledge of the bioreductive capacities within different biological compartments and the involved cellular reduction processes would be greatly helpful for improved carriers with disulfides cleaved within the right compartment at the right time.

FUTURE DIRECTIONS

Novel designs of multifunctional nanocarriers will incorporate macromolecular disulfide entry mechanisms previously optimized by natural evolution of toxins and viruses. In addition to extracellular stabilization and intracellular disassembly, tuned disulfides will contribute to deshielding at the cell surface, or translocation from intracellular compartments to the cytosol.

Bioreducible polycations in nucleic acid delivery: past, present, and future trends

Polycations that are degradable by reduction of disulfide bonds are developed for applications in delivery of nucleic acids. This Feature Article surveys methods of synthesis of bioreducible polycations and discusses current understanding of the mechanism of action of bioreducible polyplexes. Emphasis is placed on the relationship between the biological redox environment and toxicity, trafficking, transfection activity, and in vivo behavior of bioreducible polycations and polyplexes.