Stabilization of polyplexes via polymer crosslinking for efficient siRNA delivery

Recent advances in nanotechnology approaches for non-viral gene therapy

Gene therapy has shown great potential in the treatment of many diseases by downregulating the expression of certain genes. The development of gene vectors as a vehicle for gene therapy has greatly facilitated the widespread clinical application of nucleic acid materials (DNA, mRNA, siRNA, and miRNA). Currently, both viral and non-viral vectors are used as delivery systems of nucleic acid materials for gene therapy. However, viral vector-based gene therapy has several limitations, including immunogenicity and carcinogenesis caused by the exogenous viral vectors. To address these issues, non-viral nanocarrier-based gene therapy has been explored for superior performance with enhanced gene stability, high treatment efficiency, improved tumor-targeting, and better biocompatibility. In this review, we discuss various non-viral vector-mediated gene therapy approaches using multifunctional biodegradable or non-biodegradable nanocarriers, including polymer-based nanoparticles, lipid-based nanoparticles, carbon nanotubes, gold nanoparticles (AuNPs), quantum dots (QDs), silica nanoparticles, metal-based nanoparticles and two-dimensional nanocarriers. Various strategies to construct non-viral nanocarriers based on their delivery efficiency of targeted genes will be introduced. Subsequently, we discuss the cellular uptake pathways of non-viral nanocarriers. In addition, multifunctional gene therapy based on non-viral nanocarriers is summarized, in which the gene therapy can be combined with other treatments, such as photothermal therapy (PTT), photodynamic therapy (PDT), immunotherapy and chemotherapy. We also provide a comprehensive discussion of the biological toxicity and safety of non-viral vector-based gene therapy. Finally, the present limitations and challenges of non-viral nanocarriers for gene therapy in future clinical research are discussed, to promote wider clinical applications of non-viral vector-based gene therapy.

Covalent Strategies for Targeting Messenger and Non-Coding RNAs: An Updated Review on siRNA, miRNA and antimiR Conjugates

Oligonucleotide-based therapy has become an alternative to classical approaches in the search of novel therapeutics involving gene-related diseases. Several mechanisms have been described in which demonstrate the pivotal role of oligonucleotide for modulating gene expression. Antisense oligonucleotides (ASOs) and more recently siRNAs and miRNAs have made important contributions either in reducing aberrant protein levels by sequence-specific targeting messenger RNAs (mRNAs) or restoring the anomalous levels of non-coding RNAs (ncRNAs) that are involved in a good number of diseases including cancer. In addition to formulation approaches which have contributed to accelerate the presence of ASOs, siRNAs and miRNAs in clinical trials; the covalent linkage between non-viral vectors and nucleic acids has also added value and opened new perspectives to the development of promising nucleic acid-based therapeutics. This review article is mainly focused on the strategies carried out for covalently modifying siRNA and miRNA molecules. Examples involving cell-penetrating peptides (CPPs), carbohydrates, polymers, lipids and aptamers are discussed for the synthesis of siRNA conjugates whereas in the case of miRNA-based drugs, this review article makes special emphasis in using antagomiRs, locked nucleic acids (LNAs), peptide nucleic acids (PNAs) as well as nanoparticles. The biomedical applications of siRNA and miRNA conjugates are also discussed.

EGFR Targeting and Shielding of pDNA Lipopolyplexes via Bivalent Attachment of a Sequence-Defined PEG Agent

For successful nonviral gene delivery, cationic polymers are promising DNA carrier, which need to comprise several functionalities. The current work focuses on the postincorporation of epidermal growth factor receptor (EGFR) targeted PEGylation agents onto lipopolyplexes for pDNA delivery. T-shaped lipo-oligomers are previously found to be effective sequence-defined carriers for pDNA and siRNA. Here, the bis-oleoyl-oligoaminoethanamide 454 containing tyrosine trimer-cysteine ends is applied for complex formation with pDNA coding for luciferase or sodium iodide symporter (NIS). In a second step, the lipopolyplexes are modified via disulfide formation with sequence-defined monovalent or bivalent PEGylation agents containing one or two 3-nitro-2-pyridinesulfenyl (NPys)-activated cysteines, respectively. For targeting, the polyethylene glycol (PEG) agents comprise the EGFR targeting peptide GE11. In comparison of all transfection complexes, 454 lipopolyplexes modified with the bidentate PEG-GE11 agent show the best, EGFR-dependent uptake as well as luciferase and NIS gene expression into receptor-positive tumor cells.

Development of switchable polymers to address the dilemma of stability and cargo release in polycationic nucleic acid carriers

Cationic polymer gene delivery vehicles that effectively resist premature serum degradation often have difficulty releasing their nucleic acid cargoes. In this work, we report a pH-sensitive polymer (SP), poly(oligo(ethylene glycol) monomethyl ether methacrylate)-co-poly(2-(dimethylamino)ethyl methacrylate)-block- poly(propargyl methacrylate-graft-propyl-(4-methoxy-benzylidene)-amine) (p(PMA-PMBA)-b-(p(OEGMA-DMAEMA)), for successful in vitro and in vivo gene transfer. In the physiological condition, the hydrophobization of p(OEGMA-DMAEMA) polycations by p(PMA-PMBA) significantly enhanced the stability of its polyplexes counterpart. In endosomes, the polymer undergoes an acid-triggered hydrophilic transition through the cleavage of benzoic imines, thus allowing the vector to quickly release nucleic acid cargo due to the loss of hydrophobic functionalization. Compared to a pH-insensitive polymer (IP), SP exhibited more significant luciferase plasmid delivery efficiency with HeLa cells in vitro and with in vivo intraventricular brain injections. Therefore, the polymer designed here is a good solution to address the dilemma of stability and cargo release in gene delivery, and may have broad potential applications in therapeutic agent delivery.

Aptamer-based liposomes improve specific drug loading and release

Aptamer technology has shown much promise in cancer therapeutics for its targeting abilities. However, its potential to improve drug loading and release from nanocarriers has not been thoroughly explored. In this study, we employed drug-binding aptamers to actively load drugs into liposomes. We designed a series of DNA aptamer sequences specific to doxorubicin, displaying multiple binding sites and various binding affinities. The binding ability of aptamers was preserved when incorporated into cationic liposomes, binding up to 15equivalents of doxorubicin per aptamer, therefore drawing the drug into liposomes. Optimization of the charge and drug/aptamer ratios resulted in ≥80% encapsulation efficiency of doxorubicin, ten times higher than classical passively-encapsulating liposomal formulations and similar to a pH-gradient active loading strategy. In addition, kinetic release profiles and cytotoxicity assay on HeLa cells demonstrated that the release and therapeutic efficacy of liposomal doxorubicin could be controlled by the aptamer's structure. Our results suggest that the aptamer exhibiting a specific intermediate affinity is the best suited to achieve high drug loading while maintaining efficient drug release and therapeutic activity. This strategy was successfully applied to tobramycin, a hydrophilic drug suffering from low encapsulation into liposomes, where its loading was improved six-fold using aptamers. Overall, we demonstrate that aptamers could act, in addition to their targeting properties, as multifunctional excipients for liposomal formulations.

History of Polymeric Gene Delivery Systems

As an option for genetic disease treatment and an alternative for traditional cancer chemotherapy, gene therapy achieves significant attention. Nucleic acid delivery, however, remains a main challenge in human gene therapy. Polymer-based delivery systems offer a safer and promising route for therapeutic gene delivery. Over the past five decades, various cationic polymers have been optimized for increasingly effective nucleic acid transfer. This resulted in a chemical evolution of cationic polymers from the first-generation polycations towards bioinspired multifunctional sequence-defined polymers and nanocomposites. With the increasing of knowledge in molecular biological processes and rapid progress of macromolecular chemistry, further improvement of polymeric nucleic acid delivery systems will provide effective tool for gene-based therapy in the near future.

EGF receptor targeted lipo-oligocation polyplexes for antitumoral siRNA and miRNA delivery

Antitumoral siRNA and miRNA delivery was demonstrated by epidermal growth factor receptor (EGFR) targeted oligoaminoamide polyplexes. For this purpose, the T-shaped lipo-oligomer 454 was used to complex RNA into a core polyplex, which was subsequently functionalized with the targeting peptide ligand GE11 via a polyethylene glycol (PEG) linker. To this end, free cysteines on the surface of 454 polyplex were coupled with a maleimide-PEG-GE11 reagent (Mal-GE11). Resulting particles with sizes of 120-150 nm showed receptor-mediated uptake into EGFR-positive T24 bladder cancer cells, MDA-MB 231 breast cancer cells and Huh7 liver cancer cells. Furthermore, these formulations led to ligand-dependent gene silencing. RNA interference (RNAi) triggered antitumoral effects were observed for two different therapeutic RNAs, a miRNA-200c mimic or EG5 siRNA. Using polyplexes modified with a ratio of 0.8 molar equivalents of Mal-GE11, treatment of T24 or MDA-MB 231 cancer cells with miR-200c led to the expected decreased proliferation and migration, changes in cell cycle and enhanced sensitivity towards doxorubicin. Delivery of EG5 siRNA into Huh7 cells resulted in antitumoral activity with G2/M arrest, triggered by loss of mitotic spindle separation and formation of mono-astral spindles. These findings demonstrate the potential of GE11 ligand-containing RNAi polyplexes for cancer treatment.

Intracellular Delivery of Nanobodies for Imaging of Target Proteins in Live Cells

PURPOSE

Cytosolic delivery of nanobodies for molecular target binding and fluorescent labeling in living cells.

METHODS

Fluorescently labeled nanobodies were formulated with sixteen different sequence-defined oligoaminoamides. The delivery of formulated anti-GFP nanobodies into different target protein-containing HeLa cell lines was investigated by flow cytometry and fluorescence microscopy. Nanoparticle formation was analyzed by fluorescence correlation spectroscopy.

RESULTS

The initial oligomer screen identified two cationizable four-arm structured oligomers (734, 735) which mediate intracellular nanobody delivery in a receptor-independent (734) or folate receptor facilitated (735) process. The presence of disulfide-forming cysteines in the oligomers was found critical for the formation of stable protein nanoparticles of around 20 nm diameter. Delivery of labeled GFP nanobodies or lamin nanobodies to their cellular targets was demonstrated by fluorescence microscopy including time lapse studies.

CONCLUSION

Two sequence-defined oligoaminoamides with or without folate for receptor targeting were identified as effective carriers for intracellular nanobody delivery, as exemplified by GFP or lamin binding in living cells. Due to the conserved nanobody core structure, the methods should be applicable for a broad range of nanobodies directed to different intracellular targets.

Polymers for siRNA Delivery: A Critical Assessment of Current Technology Prospects for Clinical Application

The number of polymer-based vectors for siRNA delivery in clinical trials lags behind other delivery strategies; however, the molecular architectures and chemical compositions available to polymers make them attractive candidates for further exploration. Polymer vectors are extensively investigated in academic laboratories worldwide with fundamental progress having recently been made in the areas of high-throughput screening, synthetic methods, cellular internalization, endosomal escape and computational prediction and analysis. This review assesses recent advances within the field and highlights relevant developments from within the complementary fields of nanotechnology and protein chemistry with the intent to propose future work that addresses key gaps within the current body of knowledge, potentially advancing the development of the next generation of polymeric vectors.

Targeted siRNA Delivery Using a Lipo-Oligoaminoamide Nanocore with an Influenza Peptide and Transferrin Shell

Developing RNA-interference-based therapeutic approaches with efficient and targeted cytosolic delivery of small interfering RNA (siRNA) is remaining a critical challenge since two decades. Herein, a multifunctional transferrin receptor (TfR)-targeted siRNA delivery system (Tf&INF7) is designed based on siRNA complexes formed with the cationic lipo-oligoamino amide 454, sequentially surface-modified with polyethylene glycol-linked transferrin (Tf) for receptor targeting and the endosomolytic peptide INF7 for efficient cytosolic release of the siRNA. Effective Tf&INF7 polyplex internalization and target gene silencing are demonstrated for the TfR overexpressing tumor cell lines (K562, D145, and N2a). Treatment with antitumoral EG5 siRNA results in a block of tumor cell growth and triggered apoptosis. Tf-modified polyplexes are far more effective than the corresponding albumin- (Alb) or nonmodified 454 polyplexes. Competition experiments with excess of Tf demonstrate TfR target specificity. As alternative to the ligand Tf, an anti-murine TfR antibody is incorporated into the polyplexes for specific targeting and gene silencing in the murine N2a cell line. In vivo distribution studies not only demonstrate an enhanced tumor residence of siRNA in N2a tumor-bearing mice with the Tf&INF7 as compared to the 454 polyplex group but also a reduced siRNA nanoparticle stability limiting the in vivo performance.

pH-sensitive nanomedicine based on PEGylated nanodiamond for enhanced tumor therapy

pH-sensitive nanomedicine based on PEGylated nanodiamond with excellent dispersity, a slow and sustained drug release capability for enhanced tumor therapy.

Low-Molecular-Weight Polyethyleneimine Grafted Polythiophene for Efficient siRNA Delivery

Owing to its hydrophilicity, negative charge, small size, and labile degradation by endogenous nucleases, small interfering RNA (siRNA) delivery must be achieved by a carrier system. In this study, cationic copolymers composed of low-molecular-weight polyethylenimine and polythiophenes were synthesized and evaluated as novel self-tracking siRNA delivery vectors. The concept underlying the design of these copolymers is that hydrophobicity and rigidity of polythiophenes should enhance the transport of siRNA across the cell membrane and endosomal membrane. A gel retardation assay showed that the nanosized complexes formed between the copolymers and siRNA were stable even at a molar ratio of 1 : 2. The high cellular uptake (>80%) and localization of the copolymer vectors inside the cells were easily analyzed by tracking the fluorescence of polythiophene using fluorescent microscopy and cytometry. An in vitro luciferase knockdown (KD) assay in A549-luc cells demonstrated that the siRNA complexes with more hydrophobic copolymers achieved a higher KD efficiency of 52.8% without notable cytotoxicity, indicating protein-specific KD activity rather than solely the cytotoxicity of the materials. Our polythiophene copolymers should serve as novel, efficient, low cell toxicity, and label-free siRNA delivery systems.

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.

Recent advances in nanoparticle-mediated siRNA delivery

Inhibiting specific gene expression by short interfering RNA (siRNA) offers a new therapeutic strategy to tackle many diseases, including cancer, metabolic disorders, and viral infections, at the molecular level. The macromolecular and polar nature of siRNA hinders its cellular access to exert its effect. Nanoparticulate delivery systems can promote efficient intracellular delivery. Despite showing promise in many preclinical studies and potential in some clinical trials, siRNA has poor delivery efficiency, which continues to demand innovations, from carrier design to formulation, in order to overcome transport barriers. Previous findings for optimal plasmid DNA delivery cannot be generalized to siRNA delivery owing to significant discrepancy in size and subtle differences in chain flexibility between the two types of nucleic acids. In this review, we highlight the recent advances in improving the stability of siRNA nanoparticles, understanding their intracellular trafficking and release mechanisms, and applying judiciously the promising formulations to disease models.

Polymers for nucleic acid transfer-an overview

For the last five decades cationic polymers have been used for nucleic acids transfection. Our understanding of polymer-nucleic acid interactions and their rational use in delivery has continuously increased. The great improvements in macromolecular chemistry and the recognition of distinct biological extra- and intracellular delivery hurdles triggered several breakthrough developments, including the discovery of natural and synthetic polycations for compaction of nucleic acids into stable nanoparticles termed polyplexes; the incorporation of targeting ligands and surface-shielding of polyplexes to enable receptor-mediated gene delivery into defined target tissues; and strongly improved intracellular transfer efficacy by better endosomal escape of vesicle-trapped polyplexes into the cytosol. These experiences triggered the development of second-generation polymers with more dynamic properties, such as endosomal pH-responsive release mechanisms, or biodegradable units for improved biocompatibility and intracellular release of the nucleic acid pay load. Despite a better biological understanding, significant challenges such as efficient nuclear delivery and persistence of gene expression persist. The therapeutic perspectives widened from pDNA-based gene therapy to application of novel therapeutic nucleic acids including mRNA, siRNA, and microRNA. The finding that different therapeutic pay loads require different tailor-made carriers complicates preclinical developments. Convincing evidence of medical efficacy still remains to be demonstrated. Bioinspired multifunctional polyplexes resembling "synthetic viruses" appear as attractive opportunity, but provide additional challenges: how to identify optimum combinations of functional delivery units, and how to prepare such polyplexes reproducibly in precise form? Design of sequence-defined polymers, screening of combinatorial polymer and polyplex libraries are tools for further chemical evolution of polyplexes.

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.