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

A disulfide molecule-vancomycin nanodrug delivery system efficiently eradicates intracellular bacteria

Intracellular bacteria often lead to chronic and recurrent infections; however, most of the known antibiotics have poor efficacy against intracellular bacteria due to their poor cell membrane penetration efficiency into the cytosol. Here, a thiol-mediated nanodrug delivery system, named Van-DM NPs, was developed to improve vancomycin's penetration efficiency and intracellular antibacterial activities. Van-DM NPs were prepared through self-assembly of vancomycin (Van) and the disulfide molecule (DM) in NaOH buffer solution. On the one hand, the disulfide exchange reaction between Van-DM NPs and the bacterial surface enhances vancomycin accumulation in bacteria, increasing the local concentration of vancomycin. On the other hand, the disulfide exchange reaction between Van-DM NPs and the mammalian cell membrane triggered the translocation of Van-DM NPs across the mammalian cell membrane into the cell cytosol. These dual mechanisms promote antibacterial activities of vancomycin against both extracellular and intracellular bacteria S. aureus. Furthermore, in an intravenous S. aureus infection mouse model, Van-DM NPs exhibited high antibacterial capability and efficiently reduced the bacterial load in liver and spleen, where intracellular bacteria tend to reside. Altogether, the reported Van-DM NPs would be highly promising against intracellular pathogenic infections.

Reversible Stabilization of Nanofiber-Polyplexes through Introducing Cross-Linkages

Non-viral gene delivery systems are typically designed vector systems with contradictory properties, namely sufficient stability before cellular uptake and instability to ensure the release of nucleic acid cargoes in the transcription process after being taken up into cells. We reported previously that poly-(L-lysine) terminally bearing a multi-arm PEG (maPEG-PLL) formed nanofiber-polyplexes that suppressed excessive DNA condensation via steric repulsion among maPEGs and exhibited effective transcriptional capability in PCR amplification experiments and a cell-free gene expression system. In this study, the reversible stabilization of a nanofiber-polyplex without impairing the effective transcriptional capability was investigated by introducing cross-links between the PLL side chains within the polyplex using a cross-linking reagent with disulfide (SS) bonds that can be disrupted under reducing conditions. In the presence of dextran sulfate and/or dithiothreitol, the stability of the polyplex and the reactivity of the pDNA were evaluated using agarose gel electrophoresis and real-time PCR. We succeeded in reversibly stabilizing nanofiber-polyplexes using dithiobis (succinimidyl propionate) (DSP) as the cross-linking reagent. The effect of the reversible stabilization was confirmed in experiments using cultured cells, and the DSP-crosslinked polyplexes exhibited gene expression superior to that of polyethyleneimine polyplexes, which are typical polyplexes.

Advances in cell-penetrating poly(disulfide)s for intracellular delivery of therapeutics

Efficient intracellular delivery is essential for most therapeutic agents; however, existing delivery vectors face a dilemma between efficiency and toxicity, and always encounter the challenge of endolysosomal trapping. The cell-penetrating poly(disulfide) (CPD) is an effective tool for intracellular delivery, as it is taken up through thiol-mediated cellular uptake, thus avoiding endolysosomal entrapment and ensuring efficient cytosolic availability. Upon cellular uptake, CPD undergoes reductive depolymerization by glutathione inside cells and has minimal cytotoxicity. This review summarizes CPD's chemical synthesis approaches, cellular uptake mechanism, and recent advances in the intracellular delivery of proteins, antibodies, nucleic acids, and other nanoparticles. Overall, CPD is a promising candidate carrier for efficient intracellular delivery.

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.

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.

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.

Polymeric Delivery of Therapeutic Nucleic Acids

The advent of genome editing has transformed the therapeutic landscape for several debilitating diseases, and the clinical outlook for gene therapeutics has never been more promising. The therapeutic potential of nucleic acids has been limited by a reliance on engineered viral vectors for delivery. Chemically defined polymers can remediate technological, regulatory, and clinical challenges associated with viral modes of gene delivery. Because of their scalability, versatility, and exquisite tunability, polymers are ideal biomaterial platforms for delivering nucleic acid payloads efficiently while minimizing immune response and cellular toxicity. While polymeric gene delivery has progressed significantly in the past four decades, clinical translation of polymeric vehicles faces several formidable challenges. The aim of our Account is to illustrate diverse concepts in designing polymeric vectors towards meeting therapeutic goals of in vivo and ex vivo gene therapy. Here, we highlight several classes of polymers employed in gene delivery and summarize the recent work on understanding the contributions of chemical and architectural design parameters. We touch upon characterization methods used to visualize and understand events transpiring at the interfaces between polymer, nucleic acids, and the physiological environment. We conclude that interdisciplinary approaches and methodologies motivated by fundamental questions are key to designing high-performing polymeric vehicles for gene therapy.

Inorganic Nanomaterial-Mediated Gene Therapy in Combination with Other Antitumor Treatment Modalities

Cancer is a genetic disease originating from the accumulation of gene mutations in a cellular subpopulation. Although many therapeutic approaches have been developed to treat cancer, recent studies have revealed an irrefutable challenge that tumors evolve defenses against some therapies. Gene therapy may prove to be the ultimate panacea for cancer by correcting the fundamental genetic errors in tumors. The engineering of nanoscale inorganic carriers of cancer therapeutics has shown promising results in the efficacious and safe delivery of nucleic acids to treat oncological diseases in small-animal models. When these nanocarriers are used for co-delivery of gene therapeutics along with auxiliary treatments, the synergistic combination of therapies often leads to an amplified health benefit. In this review, an overview of the inorganic nanomaterials developed for combinatorial therapies of gene and other treatment modalities is presented. First, the main principles of using nucleic acids as therapeutics, inorganic nanocarriers for medical applications and delivery of gene/drug payloads are introduced. Next, the utility of recently developed inorganic nanomaterials in different combinations of gene therapy with each of chemo, immune, hyperthermal, and radio therapy is examined. Finally, current challenges in the clinical translation of inorganic nanomaterial-mediated therapies are presented and outlooks for the field are provided.

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.

Peptide Spiders: Peptide-Polymer Conjugates to Traffic Nucleic Acids

Therapeutic nucleic acids hold great promise for the treatment of genetic diseases, yet the delivery of this highly charged macromolecular drug remains a challenge in the field. Peptides are promising agents to mediate nucleic acid delivery because they can encode a biological function to overcome the trafficking barriers. Electrostatic nanocomplexes of nucleic acid and peptides can achieve effective delivery, but the balance between their stability and biological function must be finely tuned. In this work, we explore two peptide building blocks that have been studied in the literature: targeting ligands and intracellular trafficking peptides. We grafted these peptides on a polyethylene glycol (PEG) backbone with eight sites for substitution to create so-called "peptide spiders". These conjugates achieve stability via the well-known hydrophilic shielding effect of PEG. In addition, the coordination of peptide building blocks into multimers may create new biological properties, such as the well-known phenomena of increased binding avidity with multivalent ligands. In this work, we linked two trafficking peptides to the PEG backbone using either nonreducible or reducible chemistries and investigated the ability of these materials to carry silencing RNAs into mammalian cells. We then investigated these nanomaterials for their pharmacokinetic properties and silencing of undruggable targets in a mouse model of cancer. While reducible linkages were more potent at silencing in vitro, this effect was reversed when applied in the context of living animals. This work offers an insight into peptide-based delivery materials and investigates peptide-polymer linkages.

In Vitro Evaluation of Lipopolyplexes for Gene Transfection: Comparing 2D, 3D and Microdroplet-Enabled Cell Culture

Complexes combining nucleic acids with lipids and polymers (lipopolyplexes) show great promise for gene therapy since they enable compositional, physical and functional versatility to be optimized for therapeutic efficiency. When developing lipopolyplexes for gene delivery, one of the first evaluations performed is an in vitro transfection efficiency experiment. Many different in vitro models can be used, and the effect of the model on the experiment outcome has not been thoroughly studied. The objective of this work was to compare the insights obtained from three different in vitro models, as well as the potential limitations associated with each of them. We have prepared a series of lipopolyplex formulations with three different cationic polymers (poly-l-lysine, bioreducible poly-l-lysine and polyethyleneimine), and assessed their in vitro biological performance in 2D monolayer cell culture, 3D spheroid culture and microdroplet-based single-cell culture. Lipopolyplexes from different polymers presented varying degrees of transfection efficiency in all models. The best-performing formulation in 2D culture was the polyethyleneimine lipopolyplex, while lipoplexes prepared with bioreducible poly-l-lysine were the only ones achieving any transfection in microdroplet-enabled cell culture. None of the prepared formulations achieved significant gene transfection in 3D culture. All of the prepared formulations were well tolerated by cells in 2D culture, while at least one formulation (poly-l-lysine polyplex) delayed 3D spheroid growth. These results highlight the need for selecting the appropriate in vitro model depending on the intended application.

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.

Probing for Thiol-Mediated Uptake into Bacteria

Cellular uptake mediated by cyclic oligochalcogenides (COCs) is emerging as a conceptually innovative method to penetrate mammalian cells. Their mode of action is based on dynamic covalent oligochalcogenide exchange with cellular thiols. To test thiol-mediated uptake in bacteria, five antibiotics have been equipped with up to three different COCs: One diselenolane and two dithiolanes. We found that the COCs do not activate antibiotics in Gram-negative bacteria. In Gram-positive bacteria, the COCs inactivate antibiotics that act in the cytoplasm and reduce the activity of antibiotics that act on the cell surface. These results indicate that thiol-mediated uptake operates in neither of the membranes of bacteria. COCs are likely to exchange with thiols on the inner, maybe also on the outer membrane, but do not move on. Concerning mammalian cells, the absence of a COC-mediated uptake into bacteria observed in this study disfavors trivial mechanisms, such as passive diffusion, and supports the existence of more sophisticated, so far poorly understood uptake pathways.

Quantitative Chemical Delivery of Quantum Dots into the Cytosol of Cells

The ability to image single molecules in living cells has been impaired by the absence of bright, photostable fluorophores. Quantum dots (QDs) offer an attractive solution to this problem due to their exceptional photostability and brightness. Here, we describe in detail a protocol to chemically deliver functionalized QDs into the cytosol of living cells, based on cell-penetrating poly(disulfide)s (CPDs). This protocol is highly efficient and delivers hundreds of QDs per cell after incubation of cells with functionalized QDs at nanomolar concentrations. We also detail a pipeline for automated detection and tracking of diffusive QDs in living cells, which may provide a useful means to study the biophysical properties of the cytosol and their dynamics. Last, we describe a protocol for conjugating streptavidin fusion proteins to QDs, in order to permit the codelivery of QDs with functional proteins of interest into cells. The protocol has been successfully applied to a broad range of different cell types, thus offering a flexible and generalizable means to image single molecules in living cells.

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.

Nuclear delivery of plasmid DNA determines the efficiency of gene expression

As a cationic non-viral gene delivery vector, poly(agmatine/ N, N'-cystamine-bis-acrylamide) (AGM-CBA) showed significantly higher plasmid DNA (pDNA) transfection ability than polyethylenimine (PEI) in NIH/3T3 cells. The transfection expression of AGM-CBA/pDNA polyplexes was found to have a non-linear relationship with AGM-CBA/pDNA weight ratios. To further investigate the mechanism involved in the transfection process of poly(AGM-CBA), we used pGL3-control luciferase reporter gene (pLUC) as a reporter pDNA in this study. The distribution of pLUC in NIH/3T3 cells and nuclei after AGM-CBA/pLUC and PEI/pLUC transfection were determined by quantitative polymerase chain reaction (qPCR) analysis. The intracellular trafficking of the polyplexes was evaluated by cellular uptake and nuclei delivery of pLUC, and the intracellular availability was evaluated by the ratio of transfection expression to the numbers of pLUC delivered in nuclei. It was found that pLUC intracellular trafficking did not have any correlation with the transfection expression, while an excellent correlation was found between the nuclei pLUC availability and transfection expression. These results suggested that the intracellular availability of pLUC in nuclei was the rate-limiting step for pLUC transfection expression. Further optimization of the non-viral gene delivery system can be focused on the improvement of gene intracellular availability.

Recent advances in the development of gene delivery systems

Background

Gene delivery systems are essentially necessary for the gene therapy of human genetic diseases. Gene therapy is the unique way that is able to use the adjustable gene to cure any disease. The gene therapy is one of promising therapies for a number of diseases such as inherited disorders, viral infection and cancers. The useful results of gene delivery systems depend open the adjustable targeting gene delivery systems. Some of successful gene delivery systems have recently reported for the practical application of gene therapy.

Main body

The recent developments of viral gene delivery systems and non-viral gene delivery systems for gene therapy have briefly reviewed. The viral gene delivery systems have discussed for the viral vectors based on DNA, RNA and oncolytic viral vectors. The non-viral gene delivery systems have also treated for the physicochemical approaches such as physical methods and chemical methods. Several kinds of successful gene delivery systems have briefly discussed on the bases of the gene delivery systems such as cationic polymers, poly(L-lysine), polysaccharides, and poly(ethylenimine)s.

Conclusion

The goal of the research for gene delivery system is to develop the clinically relevant vectors such as viral and non-viral vectors that use to combat elusive diseases such as AIDS, cancer, Alzheimer, etc. Next step research will focus on advancing DNA and RNA molecular technologies to become the standard treatment options in the clinical area of biomedical application.

Synthesis of Bioreducible Polycations with Controlled Topologies

Bioreducible polycations, which possess disulfide linkages in the backbone, have emerged as promising nucleic acid delivery carriers due to their high stability in extracellular physiological condition and bioreduction-triggered release of the genetic material. Further benefits of bioreducible polycations include decreased cytotoxicity due to intracellular reducing environment in the cytoplasm that contains high levels of reducing molecules such as glutathione. Here, we describe the synthesis of bioreducible polycations with emphasis on methods to control their topology.

Reversibly Stabilized Polycation Nanoparticles for Combination Treatment of Early- and Late-Stage Metastatic Breast Cancer

Metastatic breast cancer is a major cause of cancer-related female mortality worldwide. The signal transducer and activator of transcription 3 (STAT3) and the chemokine receptor CXCR4 are involved in the metastatic spread of breast cancer. The goal of this study was to develop nanomedicine treatment based on combined inhibition of STAT3 and CXCR4. We synthesized a library of CXCR4-inhibiting polymers with a combination of beneficial features that included PEGylation, fluorination, and bioreducibility to achieve systemic delivery of siRNA to silence STAT3 expression in the tumors. An in vivo structure-activity relationship study in an experimental lung metastasis model revealed superior antimetastatic activity of bioreducible fluorinated polyplexes when compared with nonreducible controls despite similar CXCR4 antagonism and the ability to inhibit in vitro cancer cell invasion. When compared with nonreducible and nonfluorinated polyplexes, improved siRNA delivery was observed with the bioreducible fluorinated polyplexes. The improvement was ascribed to a combination of enhanced physical stability, decreased serum destabilization, and improved intracellular trafficking. Pharmacokinetic analysis showed that fluorination decreased the rate of renal clearance of the polyplexes and contributed to enhanced accumulation in the tumors. Therapeutic efficacy of the polyplexes with STAT3 siRNA was assessed in early stage breast cancer and late-stage metastatic breast cancer with primary tumor resection. Strong inhibition of the primary tumor growth and pronounced antimetastatic effects were observed in both models of metastatic breast cancer. Mechanistic studies revealed multifaceted mechanism of action of the combined STAT3 and CXCR4 inhibition by the developed polyplexes relying both on local and systemic effects.

Modular Synthesis of Bioreducible Gene Vectors through Polyaddition of N,N'-Dimethylcystamine and Diglycidyl Ethers

Bioreducible, cationic linear poly(amino ether)s (PAEs) were designed as promising gene vectors. These polymers were synthesized by the reaction of a disulfide-functional monomer, N,N'-dimethylcystamine (DMC), and several different diglycidyl ethers. The resulting PAEs displayed a substantial buffer capacity (up to 64%) in the endosomal acidification region of pH 7.4⁻5.1. The PAEs condense plasmid DNA into 80⁻200 nm sized polyplexes, and have surface charges ranging from +20 to +40 mV. The polyplexes readily release DNA upon exposure to reducing conditions (2.5 mM DTT) due to the cleavage of the disulfide groups that is present in the main chain of the polymers, as was demonstrated by agarose gel electrophoresis. Upon exposing COS-7 cells to polyplexes that were prepared at polymer/DNA w/w ratios below 48, cell viabilities between 80⁻100% were observed, even under serum-free conditions. These polyplexes show comparable or higher transfection efficiencies (up to 38%) compared to 25 kDa branched polyethylenimine (PEI) polyplexes (12% under serum-free conditions). Moreover, the PAE-based polyplexes yield transfection efficiencies as high as 32% in serum-containing medium, which makes these polymers interesting for gene delivery applications.

Design of PEI-conjugated bio-reducible polymer for efficient gene delivery

The poly(cystaminebis(acrylamide)-diaminohexane) (poly(CBA-DAH)) was designed previously as a bio-reducible efficient gene delivery carrier. However, the high weight ratio required to form the polyplexes between poly(CBA-DAH) with pDNA is still a problem that needs to be addressed. To solve this problem and increase the transfection efficiency, poly(ethylenimine) (PEI, 1.8 kDa) was conjugated to poly(CBA-DAH) via disulfide bond. The PEI conjugated poly(CBA-DAH) (PCDP) can bind with pDNA at a very low weight ratio of 0.5 and above, like PEI 25 kDa, and form the polyplexes with nano-size (102-128 nm) and positive surface charge (27-34 mV). PCDP and PCDP polyplexes had negligible cytotoxicity and indicated similar or better cellular uptake than the comparison groups such as PEI 25 kDa and Lipofectamine® polyplexes. To confirm the transfection efficiency, the plasmid DNA (pDNA) encoded with the luciferase reporter gene (gWiz-Luc) and green fluorescent protein reporter gene (GFP) were used and treated with PCDP into the A549, Huh-7, and Mia PaCa-2 cells. PCDP/pDNA polyplexes showed highest transfection efficiency in all tested cell lines. In the luciferase assay, PCDP polyplexes showed 10.2 times higher gene transfection efficiency than Lipofectamine® polyplexes in mimic in vivo conditions (30% FBS, A549 cells). The VEGF siRNA expressing plasmid (pshVEGF), which is constructed as a therapeutic gene by our previous work, was delivered by PCDP into the cancer cells. The VEGF gene expression of PCDP/pshVEGF polyplexes was dramatically lower than control and the VEGF gene silencing efficiencies of PCDP/pshVEGF (w/w; 10/1) polyplexes were 54% (A549 cells), 77% (Huh-7 cells), and 66% (Mia PaCa-2 cells). In addition, PCDP/pshVEGF had reduced cell viability rates of about 31% (A549 cells), 39% (Huh-7 cells), and 42% (Mia PaCa-2 cells) and showed better results than all comparison groups. In the transfection efficiency and VEGF silencing assay, PCDP polyplexes showed better results than poly(CBA-DAH) at 4-fold lower weight ratio. The data of all experiments demonstrate that the synthesized PCDP could be used for efficient gene delivery and could be widely applied.

Fluorination Enhances Serum Stability of Bioreducible Poly(amido amine) Polyplexes and Enables Efficient Intravenous siRNA Delivery

The use of small interfering RNA (siRNA) in cancer treatment has been limited by the lack of effective systemic delivery methods. Although synthetic polycations have been widely explored in siRNA delivery, polycation/siRNA polyplexes often suffer from insufficient stability in vivo. Here, rationally designed siRNA delivery systems that meet the requirements for systemic siRNA delivery to distant tumors are reported. The hypothesis that modular design of delivery systems based on poly(amido amine)s that combine fluorination for systemic stability with bioreducibility for easy intracellular siRNA release, and PEGylation for improved safety and colloidal stability will overcome problems with contradicting siRNA delivery demands is tested. PEGylated, fluorinated, and bioreducible copolymers (PEG-PCD-F) with different degree of fluorination are thus synthesized. The fluorinated copolymers readily formed polyplexes with siRNA and achieved greatly improved gene silencing efficacy in multiple cell lines in vitro when compared with nonfluorinated controls. The results show fluorination-induced enhancement of stability, cellular uptake, and endosomal escape of the polyplexes, while exhibiting efficient siRNA release in reducing intracellular environment. PEG-PCD-F polyplexes with siRNA against Bcl2 inhibit breast tumor growth following systemic intravenous administration. The results provide strong evidence of successful combination of bioreducibility with fluorination and PEGylation to achieve systemic siRNA polyplex delivery.

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.

Conjugate Polyplexes with Anti-Invasive Properties and Improved siRNA Delivery In Vivo

This study reports on a simple method to prepare siRNA-polycation conjugate polyplexes by in situ thiol-disulfide exchange reaction. The conjugate polyplexes are prepared using thiol-terminated siRNA and a bioreducible branched polycationic inhibitor of the CXCR4 chemokine receptor (rPAMD). The rPAMD-SS-siRNA conjugate polyplexes exhibit improved colloidal stability and resistance against disassembly with heparin, serum, and physiological salt concentrations when compared with control conventional rPAMD/siRNA polyplexes. Coating the polyplexes with human serum albumin masks the positive surface charge and contributes to the enhanced in vitro gene silencing and improved safety in vivo. The conjugate polyplexes display improved in vivo reporter gene silencing following intravenous injection in tumor-bearing mice. Because the conjugate polyplexes retained the ability of rPAMD to inhibit CXCR4 and restrict cancer cell invasion, the developed systems show promise for future combination anti-metastatic siRNA therapies of cancer.

Synthesis and Evaluation of Chloroquine-Containing DMAEMA Copolymers as Efficient Anti-miRNA Delivery Vectors with Improved Endosomal Escape and Antimigratory Activity in Cancer Cells

Chloroquine-containing 2-(dimethylamino)ethyl methacrylate copolymers (PDCs) are synthesized by reversible addition-fragmentation chain-transfer polymerization. Systematic evaluation is performed to test the hypothesis that presence of chloroquine (CQ) in the PDC structure will improve miRNA delivery due to enhanced endosomal escape while simultaneously contribute to anticancer activity of PDC/miRNA polyplexes through inhibition of cancer cell migration. The results show that miRNA delivery efficiency is dependent both on the molecular weight and CQ. The best performing PDC/miRNA polyplexes show effective endosomal escape of miRNA. PDC polyplexes with therapeutic miR-210 show promising anticancer activity in human breast cancer cells. PDC/miRNA polyplexes show excellent ability to inhibit migration of cancer cells. Overall, this study supports the use of PDC as a promising polymeric drug platform for use in combination anti-metastatic and anticancer miRNA therapeutic strategies.

Efficient Delivery of Quantum Dots into the Cytosol of Cells Using Cell-Penetrating Poly(disulfide)s

Quantum dots (QDs) are extremely bright, photostable, nanometer particles broadly used to investigate single molecule dynamics in vitro. However, the use of QDs in vivo to investigate single molecule dynamics is impaired by the absence of an efficient way to chemically deliver them into the cytosol of cells. Indeed, current methods (using cell-penetrating peptides for instance) provide very low yields: QDs stay at the plasma membrane or are trapped in endosomes. Here, we introduce a technology based on cell-penetrating poly(disulfide)s that solves this problem: we deliver about 70 QDs per cell, and 90% appear to freely diffuse in the cytosol. Furthermore, these QDs can be functionalized, carrying GFP or anti-GFP nanobodies for instance. Our technology thus paves the way toward single molecule imaging in cells and living animals, allowing to probe biophysical properties of the cytosol.

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.