Influence of Disulfide Density and Molecular Weight on Disulfide Cross-Linked Polyethylenimine as Gene Vectors

Bioreducible polymer-mediated delivery of oncolytic adenovirus can attenuate antiviral immune response and concurrently enhance induction of antitumor immune response to effectively prevent metastasis

Oncolytic virotherapy is highly promising and novel treatment modality for cancer. Several clinical trials with oncolytic viruses have illustrated that the potent antitumor efficacy of these viruses may rely on...

Polymeric siRNA delivery targeting integrin-β1 could reduce interactions of leukemic cells with bone marrow microenvironment

Uncontrolled proliferation of the myeloid cells due to BCR-ABL fusion has been successfully treated with tyrosine kinase inhibitors (TKIs), which improved the survival rate of Chronic Myeloid Leukemia (CML) patients. However, due to interactions of CML cells with bone marrow microenvironment, sub-populations of CML cells could become resistant to TKI treatment. Since integrins are major cell surface molecules involved in such interactions, the potential of silencing integrin-β1 on CML cell line K562 cells was explored using short interfering RNA (siRNA) delivered through lipid-modified polyethyleneimine (PEI) polymers. Reduction of integrin-β1 in K562 cells decreased cell adhesion towards human bone marrow stromal cells and to fibronectin, a major extracellular matrix protein for which integrin-β1 is a primary receptor. Interaction of K562 cells with fibronectin decreased the sensitivity of the cells to BCR-ABL siRNA treatment, but a combinational treatment with integrin-β1 and BCR-ABL siRNAs significantly reduced colony forming ability of the cells. Moreover, integrin-β1 silencing enhanced the detachment of K562 cells from hBMSC samples (2 out of 4 samples), which could make them more susceptible to TKIs. Therefore, the polymeric-siRNA delivery targeting integrin-β1 could be beneficial to reduce interactions with bone marrow microenvironment, aiding in the response of CML cells to therapeutic treatment.

Progress on ocular siRNA gene-silencing therapy and drug delivery systems

Age-related macular degeneration (AMD) and glaucoma are global ocular diseases with high blindness rate. RNA interference (RNAi) is being increasingly used in the treatment of these disorders with siRNA drugs, bevasiranib, AGN211745 and PF-04523655 for AMD, and SYL040012 and QPI-1007 for glaucoma. Administration routes and vectors of gene drugs affect their therapeutic effect. Compared with the non-viral vectors, viral vectors have limited payload capacity and potential immunogenicity. This review summarizes the progress of the ocular siRNA gene-silencing therapy by focusing on siRNA drugs for AMD and glaucoma already used in clinical research, the main routes of drug delivery and the non-viral vectors for siRNA drugs.

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.

Thiol redox-sensitive cationic polymers for dual delivery of drug and gene

Recently greater emphasis has been given to combination therapy for generating synergistic effects of treating cancer. Recent studies on thiol-sensitive nanocarriers for the delivery of drug or gene have shown promising results. In this review, we will examine the rationale and advantage in using nanocarriers for the combined delivery of different anticancer drugs and biologics. Here, we also discuss the role of nanocarriers, particularly redox-sensitive polymers in evading or inhibiting the efflux pump in cancer and how they modulate the sensitivity of cancer cells. The review aims to provide a good understanding of the new pattern of cancer treatment and key concerns for designing nanomedicine of synergistic combinations for cancer therapy.

Barriers to inhaled gene therapy of obstructive lung diseases: A review

Knowledge of genetic origins of obstructive lung diseases has made inhaled gene therapy an attractive alternative to the current standards of care that are limited to managing disease symptoms. Initial lung gene therapy clinical trials occurred in the early 1990s following the discovery of the genetic defect responsible for cystic fibrosis (CF), a monogenic disorder. However, despite over two decades of intensive effort, gene therapy has yet to help patients with CF or any other obstructive lung disease. The slow progress is due in part to poor understanding of the biological barriers to inhaled gene therapy. Encouragingly, clinical trials have shown that inhaled gene therapy with various viral vectors and non-viral gene vectors is well tolerated by patients, and continued research has provided valuable lessons and resources that may lead to future success of this therapeutic strategy. In this review, we first introduce representative obstructive lung diseases and examine limitations of currently available therapeutic options. We then review key components for successful execution of inhaled gene therapy, including gene delivery systems, primary physiological barriers and strategies to overcome them, and advances in preclinical disease models with which the most promising systems may be identified for human clinical trials.

Bioreducible polyethylenimine nanoparticles for the efficient delivery of nucleic acids

Electrostatically crosslinked bioreducible nanoparticles of polyethylenimine (DP NPs) have been prepared and evaluated for their cytotoxicity and capability to transport nucleic acids inside the cells.

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.

Stimuli-responsive cross-linked micelles for on-demand drug delivery against cancers

Stimuli-responsive cross-linked micelles (SCMs) represent an ideal nanocarrier system for drug delivery against cancers. SCMs exhibit superior structural stability compared to their non-cross-linked counterpart. Therefore, these nanocarriers are able to minimize the premature drug release during blood circulation. The introduction of environmentally sensitive cross-linkers or assembly units makes SCMs responsive to single or multiple stimuli present in tumor local microenvironment or exogenously applied stimuli. In these instances, the payload drug is released almost exclusively in cancerous tissue or cancer cells upon accumulation via enhanced permeability and retention effect or receptor mediated endocytosis. In this review, we highlight recent advances in the development of SCMs for cancer therapy. We also introduce the latest biophysical techniques, such as electron paramagnetic resonance (EPR) spectroscopy and fluorescence resonance energy transfer (FRET), for the characterization of the interactions between SCMs and blood proteins.

Recent developments in nucleic acid delivery with polyethylenimines

Polyethylenimines (PEIs) have proven to be highly efficient and versatile agents for nucleic acid delivery in vitro and in vivo. Despite the low biodegradability of these polymers, they have been used in several clinical trials and the results suggest that the nucleic acid/PEI complexes have a good safety profile. The high transfection efficiency of PEIs probably relies on the fact that these polymers possess a stock of amines that can undergo protonation during the acidification of endosomes. This buffering capacity likely enhances endosomal escape of the polyplexes through the "proton sponge" effect. PEIs have also attracted great interest because the presence of many amino groups allow for easy chemical modifications or conjugation of targeting moieties and hydrophilic polymers. In the present chapter, we summarize and discuss the mechanism of PEI-mediated transfection, as well as the recent developments in PEI-mediated DNA, antisense oligonucleotide, and siRNA delivery.

Synthesis of bioreducible polycations with controlled topologies

Bioreducible polycations, which possess disulfide linkages in the backbone, have appeared as promising gene delivery carriers due to their high stability in extracellular physiological condition and bioreduction-triggered release of genetic materials, as well as reduced cytotoxicity because intracellular cytosol is a reducing environment containing high level of reducing molecules such as glutathione. Here, we describe the syntheses of bioreducible polycations, and the methods for control over their topology are also presented.

Development of a reduction-sensitive diselenide-conjugated oligoethylenimine nanoparticulate system as a gene carrier

BACKGROUND

The reduction-sensitive cationic polymer is a promising nonviral carrier for gene delivery. Until now, disulfide bonds have been the only golden standard for its design. The aim of this research was to develop a novel reduction-responsive cationic polymer as a gene carrier.

METHODS

Polycationic carriers were synthesized by addition of branched oligoethylenimine 800 Da (OEI(800)) via an active ester containing diselenide bonds. Disulfide bonds cross-linked with OEI(800)-SS(x) and monoselenide bonds linked with OEI(800)-Se(x) were synthesized and compared. Their molecular weights and degradation properties were determined using gel permeation chromatography. Changes in particle size, morphology, and DNA binding were investigated by dynamic light scattering, transmission electron microscopy, and electrophoresis assay in a reduction environment. Cytotoxicity and transfection in vitro were evaluated in a murine melanoma cell line (B16F10) and a human cervical epithelial carcinoma cell line (HeLa), while intracellular degradation and dissociation with DNA were studied by confocal laser scanning microscopy with FITC-labeled OEI(800) derivatives and Cy5-labeled DNA.

RESULTS

Diselenide-conjugated OEI(800) (OEI(800)-SeSe(x)) polymer carriers of high molecular weight were successfully synthesized. After compacting with DNA, the OEI(800)-SeSe(x) polymers formed nanoparticles with an average size of 140 nm at an adequate C/P ratio. OEI(800)-SeSe(x) showed reduction-responsive degradation properties similar to those of the OEI(800)-SS(x) via gel permeation chromatography, dynamic light scattering, and transmission electron microscopy. OEI(800)-SeSe(x) showed much lower cytotoxicity than PEI(25k), and significantly higher transfection efficiency than OEI(800) in both B16F10 and HeLa cells. Transfection of luciferase in the OEI(800)-SeSe(x) group was comparable with that of standard PEI(25k) and traditional reduction-sensitive polymer OEI(800)-SS(x) groups. Furthermore, intracellular degradation of OEI(800)-SeSe(x) and dissociation with DNA were also confirmed by confocal laser scanning microscopy.

CONCLUSION

The OEI(800)-SeSe(x) obtained was able to bind plasmid DNA efficiently to yield nanosized particles and had reduction sensitivity which is as efficient as that for OEI(800)-SS(x). In vitro experiments confirmed its low cytotoxicity and high transfection ability. Diselenide bonds can be used as effective and novel reduction-sensitive linkages for gene delivery.

Bioreducible polymers for gene silencing and delivery

Polymeric gene delivery vectors show great potential for the construction of the ideal gene delivery system. These systems harness their ability to incorporate versatile functional traits to overcome most impediments encountered in gene delivery: from the initial complexation to their target-specific release of the therapeutic nucleic acids at the cytosol. Among the numerous multifunctional polymers that have been designed and evaluated as gene delivery vectors, polymers with redox-sensitive (or bioreducible) functional domains have gained great attention in terms of their structural and functional traits. The redox environment plays a pivotal role in sustaining cellular homeostasis and natural redox potential gradients exist between extra- and intracellular space and between the exterior and interior of subcellular organelles. In some cases, researchers have designed the polymeric delivery vectors to exploit these gradients. For example, researchers have taken advantage of the high redox potential gradient between oxidizing extracellular space and the reducing environment of cytosolic compartments by integrating disulfide bonds into the polymer structure. Such polymers retain their cargo in the extracellular space but selectively release the therapeutic nucleic acids in the reducing space within the cytosol. Furthermore, bioreducible polymers form stable complex with nucleic acids, and researchers can fabricate these structures to impart several important features such as site-, timing-, and duration period-specific gene expression. Additionally, the introduction of disulfide bonds within these polymers promotes their biodegradability and limits their cytotoxicity. Many approaches have demonstrated the versatility of bioreducible gene delivery, but the underlying biological rationale of these systems remains poorly understood. The process of disulfide reduction depends on multiple variables in the cellular redox environment. Therefore, the quest to unravel various issues such as the site and time of disulfide bond reduction during the cellular uptake and trafficking have stimulated a number of interesting studies which have employed disulfide compounds with a variety of reducible linkers. Such studies help researchers understand not only how modifications made to disulfides can alter their thiol-disulfide exchange characteristics but also to decipher the effect of the induced changes on the dynamics of the redox environment. This Account discusses current research trends and recent progress in the disulfide chemistry enabling novel and versatile designs of reducible polymeric gene delivery systems. We present strategies for the introduction of disulfide bonds into polymers. These representative examples and their respective outcomes elaborate the benefit and efficiency of disulfides at the individual stages of gene delivery.

DEGRADABLE POLYETHYLENIMINE DERIVATIVES AS GENE CARRIERS

Gene therapy is a treatment for inborn and acquired diseases, although the development of safe and effective gene delivery system is a great challenge to make a gene therapy a success. Viral vectors have been used in a majority of clinics because of their high transfection efficiency in vitro and in vivo. However, their use has been limited because of several drawbacks, such as induction of immune response, recombination of wild-type viruses, limitation in the size of inserted gene, and difficulty in large-scale production. Nonviral vectors have been widely proposed safe alternatives to viral vectors because they have low immunogenicity, flexibility in the size of gene to be delivered, cell targetibility, and easy scalability of production, although they have low transfection efficiency compared to viral vectors. Among nonviral vectors, polyethylenimine (PEI) has been widely used as a standard gene carriers due to its high pH-buffering capacity for endosomal escape although high-molecular-weight PEI is too toxic owing to non-degradability. Recently, many types of degradable PEI have been studied due to high transfection efficiency with lower cytotoxicity. This review explains recent progress on the development of degradable PEIs as nonviral vectors. The present paper summarizes the transfection efficiency of DNA or silencing efficiency of small interfering RNA (siRNA) based on the kinds of degradable linkage between low PEI and crosslinkers. Degradable linkages, such as ester, disulfide, imines, carbamate, amide and ketal in the degradable PEIs are covered.

Effect of thiol pendant conjugates on plasmid DNA binding, release, and stability of polymeric delivery vectors

Polymers have attracted much attention as potential gene delivery vectors due to their chemical and structural versatility. However, several challenges associated with polymeric carriers, including low transfection efficiencies, insufficient cargo release, and high cytotoxicity levels have prevented clinical implementation. Strong electrostatic interactions between polymeric carriers and DNA cargo can prohibit complete cargo release within the cell. As a result, cargo DNA never reaches the cell's nucleus where gene expression takes place. In addition, highly charged cationic polymers have been correlated with high cytotoxicity levels, making them unsuitable carriers in vivo. Using poly(allylamine) (PAA) as a model, we investigated how pH-sensitive disulfide cross-linked polymer networks can improve the delivery potential of cationic polymer carriers. To accomplish this, we conjugated thiol-terminated pendant chains onto the primary amines of PAA using 2-iminothiolane, developing three new polymer vectors with 5, 13, or 20% thiol modification. Unmodified PAA and thiol-conjugated polymers were tested for their ability to bind and release plasmid DNA, their capacity to protect genetic cargo from enzymatic degradation, and their potential for endolysosomal escape. Our results demonstrate that polymer-plasmid complexes (polyplexes) formed by the 13% thiolated polymer demonstrate the greatest delivery potential. At high N/P ratios, all thiolated polymers (but not unmodified counterparts) were able to resist decomplexation in the presence of heparin, a negatively charged polysaccharide used to mimic in vivo polyplex-protein interactions. Further, all thiolated polymers exhibited higher buffering capacities than unmodified PAA and, therefore, have a greater potential for endolysosomal escape. However, 5 and 20% thiolated polymers exhibited poor DNA binding-release kinetics, making them unsuitable carriers for gene delivery. The 13% thiolated polymers, on the other hand, displayed high DNA binding efficiency and pH-sensitive release.

Low molecular weight polyethylenimine-graft-Tween 85 for effective gene delivery: synthesis and in vitro characteristics

The development of safe and efficient gene delivery systems is still a challenge for successful gene therapy. In this work, low molecular weight polyethylenimine (PEI 2K) was modified by Tween 85, which bears three oleate chains. Tween 85 modified PEI 2K (TP) could condense DNA efficiently, and TP/DNA complexes (TPCs) showed high resistance to salt-induced aggregation and enzymatic degradation. In addition, TP did not show the obvious cytotoxicity. The introduction of Tween 85 led to a significant increase in the cellular uptake of complexes with higher transfection efficiency, which was strongly inhibited by the addition of free Tween 85 in MCF-7/ADR cells, but not in MCF-7 cells. These results indicated that TP could be a potentially safe and effective copolymer for gene delivery, and TPCs could be taken up mainly by Tween 85-mediated endocytosis in MCF-7/ADR cells.

Diblock copolymers with tunable pH transitions for gene delivery

A series of diblock copolymers containing an endosomal-releasing segment composed of diethylaminoethyl methacrylate (DEAEMA) and butyl methacrylate (BMA) were synthesized via reversible addition-fragmentation chain transfer (RAFT) polymerization. The materials were designed to condense plasmid DNA (pDNA) through electrostatic interactions with a cationic poly(N,N-dimethylaminoethyl methacrylate) (DMAEMA) first block. The pDMAEMA was employed as a macro chain transfer agent (macroCTA) for the synthesis of a series in which the relative feed ratios of DEAEMA and BMA were systematically varied from 20% to 70% BMA. The resultant diblock copolymers exhibited low polydispersity (PDI ≤ 1.06) with similar molecular weights (M(n) = 19.3-23.1 kDa). Dynamic light scattering (DLS) measurements in combination with (1)H NMR D(2)O studies demonstrated that the free copolymers assemble into core-shell micelles at physiological pH. Reduction of the solution pH to values representative of endosomal/lysosomal compartments induced an increase in the net cationic charge of the core through protonation of the DEAEMA residues. This protonation promotes micelle destabilization and exposure of the hydrophobic BMA residues that destabilize biological membranes. The pH value at which this micelle-to-unimer transition occurred was dependent on the hydrophobic content of the copolymer, with higher BMA-containing copolymer compositions exhibiting pH-induced transitions to the membrane-destabilizing state at successively lower pH values. The ability of the diblock copolymers to deliver pDNA was subsequently investigated using a GFP expression vector in two monocyte cell lines. High levels of DNA transfection were observed for the copolymer compositions exhibiting the sharpest pH transitions and membrane destabilizing activities, demonstrating the importance of tuning the endosomal-releasing segment composition.

Poly(β-amino amine) cross-linked PEIs as highly efficient gene vectors

To increase the release of DNA into the cytoplasm and further improve transgene expression of nucleic acid novel polymeric gene carriers were prepared which would be biodegradable under the reducing conditions in the cytoplasm. Disulfide-containing poly(β-amino amine)s were first synthesized and then used to cross-link low molecular weight polyethyleneimine (1800 Da) through Michael addition to obtain SS-PBAA-PEIs as the final gene carriers. The physicochemical characteristics of SS-PBAA-PEI/DNA complexes were characterized. In vitro transfection mediated by the SS-PBAA-PEIs under serum conditions was carried out. Cell uptake of the gene delivery systems was observed by confocal laser scanning microscopy. The results of the physicochemical characterisation demonstrated that the SS-PBAA-PEIs could efficiently condense DNA. In vitro transfection under serum conditions showed that SS-PBAA-PEIs had comparable or even higher transfection efficiencies than 25 kDa PEI. And SS-PBAA-PEIs showed much lower cytotoxicity compared with 25 kDa PEI. In summary, the SS-PBAA-PEIs possess great potential as non-viral gene vectors and exhibit high transfection efficiency under serum conditions.

Synthesis of a new potential biodegradable disulfide containing poly(ethylene imine)-poly(ethylene glycol) copolymer cross-linked with click cluster for gene delivery

Poly(ethylene glycol)-grafted-polyethylenimine (PEG-PEI) are promising non-viral gene delivery systems. Herein, we aimed to synthesize a biodegradable disulfide containing PEGylated PEI to attempt to reduce its cytotoxicity and enhance the gene transfer activity. Using click chemistry, low Mw PEI (br. 2 kDa) and short chain length PEG (tetraethylene glycol, TEG) were cross-linked to a high Mw PEG-PEI copolymer (∼ 22 kDa). The chemical structure of the copolymer was characterized using (1)H NMR and GPC. The degradation behavior was investigated under in vitro conditions in the presence of 1,4-dithiothreitol (DTT). The gel retardation assay, dynamic light scattering and atomic force microscopy showed good DNA condensation ability by forming polyplexes with small particle size and positive zeta potential. In particular, MTT assay indicated that this PEG-PEI polymer is about 22-fold less toxic than PEI 25k and only 2-fold more toxic than PEI 2k in L929 cell line. After coupling of small PEG chains and cross-linking by disulfide bridges, the transfection efficiency is increased approximately 6-fold in comparison to PEI 2k and still reaches approximately 17% of PEI 25k. Hence, this click cluster cross-linked disulfide containing PEG-PEI copolymer could be an attractive cationic polymer for non-viral gene delivery.

Low molecular weight polyethylenimine conjugated gold nanoparticles as efficient gene vectors

Gold nanoparticles (GNPs) conjugated with low molecular weight polyethylenimine (PEI 800 Da) were synthesized, and their characteristics as gene transfection vectors were investigated. The polyethylenimine conjugated GNPs (GNP-PEI800s) can retard plasmid DNA completely at N/P ratios above 4 in electrophoresis on agarose gel, and they also render effective protection of DNA from attack by DNase. TEM imaging revealed that GNP-PEI800s with higher PEI grafting density resulted in more compact and smaller complexes with plasmid DNA, compared to those obtained with lower grafting density ones. These complexes showed high efficiency in gene delivery in monkey kidney cells in vitro. In the absence of serum, GNP-PEI800s can transfect pGL-3 to COS-7 cells 3 to 4 orders more efficient than unmodified PEI800, reaching almost the same magnitude of PEI 25 kDa. More importantly, in contrast to the dramatically lowered efficiency of high molecular weight PEIs such as PEI 25 kDa in the presence of serum, the efficiency of GNP-PEI800s can be retained or even enhanced in serum-containing media. GNP-PEI800 1.3 exhibited transfection efficiency exceeding 60-fold that of PEI 25 kDa in 10% serum medium. All GNP-PEI800s exhibit mild cytotoxicity in comparison with that of PEI 25 kDa.

Chemically programmed polymers for targeted DNA and siRNA transfection

Plasmid DNA and siRNA have a large potential for use as therapeutic nucleic acids in medicine. The way to the target cell and its proper compartment is full of obstacles. Polymeric carriers help to overcome the encountered barriers. Cationic polymers can interact with the nucleic acid in a nondamaging way but still require optimization with regard to transfer efficiency and biocompatibility. Aiming at virus-like features, as viruses are the most efficient natural gene carriers, the design of bioresponsive polymers shows promising results regarding DNA and siRNA delivery. By specific chemical modifications dynamic structures are created, programmed to respond towards changing demands on the delivery pathway by cleavage of labile bonds or conformational changes, thus enhancing biocompatible gene delivery.

Polyethyleneimine coating enhances the cellular uptake of mesoporous silica nanoparticles and allows safe delivery of siRNA and DNA constructs

Surface-functionalized mesoporous silica nanoparticles (MSNP) can be used as an efficient and safe carrier for bioactive molecules. In order to make the MSNP a more efficient delivery system, we modified the surface of the particles by a functional group that enhances cellular uptake and allows nucleic acid delivery in addition to traditional drug delivery. Noncovalent attachment of polyethyleneimine (PEI) polymers to the surface not only increases MSNP cellular uptake but also generates a cationic surface to which DNA and siRNA constructs could be attached. While efficient for intracellular delivery of these nucleic acids, the 25 kD PEI polymer unfortunately changes the safety profile of the MSNP that is otherwise very safe. By experimenting with several different polymer molecular weights, it was possible to retain high cellular uptake and transfection efficiency while reducing or even eliminating cationic MSNP cytotoxicity. The particles coated with the 10 kD PEI polymer were particularly efficient for transducing HEPA-1 cells with a siRNA construct that was capable of knocking down GFP expression. Similarly, transfection of a GFP plasmid induced effective expression of the fluorescent protein in >70% cells in the population. These outcomes were quantitatively assessed by confocal microscopy and flow cytometry. We also demonstrated that the enhanced cellular uptake of the nontoxic cationic MSNP enhances the delivery of the hydrophobic anticancer drug, paclitaxel, to pancreatic cancer cells. In summary, we demonstrate that, by a careful selection of PEI size, it is possible to construct cationic MSNP that are capable of nucleotide and enhanced drug delivery with minimal or no cytotoxicity. This novel use of a cationic MSNP extends its therapeutic use potential.