Interaction of polyamine gene vectors with RNA leads to the dissociation of plasmid DNA-carrier complexes

Serum-Resistant Ternary DNA Polyplexes for Suicide Gene Therapy of Uterine Leiomyoma

Uterine leiomyoma (UL) is a prevalent benign tumor in women that frequently gives rise to a multitude of reproductive complications. The use of suicide gene therapy has been proposed as a highly promising method for treating UL. To achieve successful gene therapy, it is essential to develop carriers that can efficiently transport nucleic acids into targeted cells and tissues. The instability of polyplexes in blood and other biological fluids is a crucial factor to consider when using non-viral carriers. In this study, we present serum-resistant and cRGD-modified DNA complexes for targeted delivery genes to UL cells. Ternary polyplexes were formed by incorporating cystine-cross-linked polyglutamic acid modified with histidine residues. We employed two techniques in the production of cross-linked polyanionic coating: matrix polymerization and oxidative polycondensation. In this study, we investigated the physicochemical properties of ternary DNA complexes, including the size and zeta-potential of the nanoparticles. Additionally, we evaluated cellular uptake, toxicity levels, transfection efficiency and specificity in vitro. The study involved introducing the HSV-TK gene into primary UL cells as a form of suicide gene therapy modeling. We have effectively employed ternary peptide-based complexes for gene delivery into the UL organtypic model. By implementing in situ suicide gene therapy, the increase in apoptosis genes expression was detected, providing conclusive evidence of apoptosis occurring in the transfected UL tissues. The results of the study strongly suggest that the developed ternary polyplexes show potential as a valuable tool in the implementation of suicide gene therapy for UL.

Combination of Backbone Rigidity and Richness in Aryl Structures Enables Direct Membrane Translocation of Polymer Scaffolds for Efficient Gene Delivery

The development of cell-penetrating polymers with endocytosis-independent cell uptake pathways has emerged as a prominent strategy to enhance the transfection efficiency. Inspired by the rigid α-helical structure that endows polypeptides with cell-penetrating ability, we propose that a rigid backbone can facilitate the corresponding polymer vector's performance in gene delivery by bypassing the difficult endosomal escape process. Meanwhile, the installation of aromatic domains, as a way to promote gene transfection efficiency, is employed through the construction of a poly(benzyl ether) (PBE)-based scaffold in this work. We demonstrate that the direct membrane translocation capability of the synthesized PBE contributes to its enhanced transfection performance and excellent biocompatibility profile, rendering the imidazolium-functionalized PBE scaffold with higher activity and biocompatibility. Molecular details of the PBE-lipid interaction are also revealed in molecular dynamics simulations, indicating the important roles of individual structural elements on the polymeric scaffold in the membrane penetration process.

The Past, Present, and Future of Non-Viral CAR T Cells

Adoptive transfer of chimeric antigen receptor (CAR) T lymphocytes is a powerful technology that has revolutionized the way we conceive immunotherapy. The impressive clinical results of complete and prolonged response in refractory and relapsed diseases have shifted the landscape of treatment for hematological malignancies, particularly those of lymphoid origin, and opens up new possibilities for the treatment of solid neoplasms. However, the widening use of cell therapy is hampered by the accessibility to viral vectors that are commonly used for T cell transfection. In the era of messenger RNA (mRNA) vaccines and CRISPR/Cas (clustered regularly interspaced short palindromic repeat-CRISPR-associated) precise genome editing, novel and virus-free methods for T cell engineering are emerging as a more versatile, flexible, and sustainable alternative for next-generation CAR T cell manufacturing. Here, we discuss how the use of non-viral vectors can address some of the limitations of the viral methods of gene transfer and allow us to deliver genetic information in a stable, effective and straightforward manner. In particular, we address the main transposon systems such as Sleeping Beauty (SB) and piggyBac (PB), the utilization of mRNA, and innovative approaches of nanotechnology like Lipid-based and Polymer-based DNA nanocarriers and nanovectors. We also describe the most relevant preclinical data that have recently led to the use of non-viral gene therapy in emerging clinical trials, and the related safety and efficacy aspects. We will also provide practical considerations for future trials to enable successful and safe cell therapy with non-viral methods for CAR T cell generation.

Magnetic Nanoparticles as a Component of Peptide-Based DNA Delivery System for Suicide Gene Therapy of Uterine Leiomyoma

Suicidegene therapy is considered a promising approach for the treatment of uterine leiomyoma (UL), a benign tumor in women characterized by precise localization. In this study, we investigate the efficiency of αvβ3 integrin-targeted arginine-rich peptide carrier R6p-cRGD electrostatically bound to magnetic nanoparticles (MNPs) for targeted DNA delivery into the UL cells. The physico–chemical and cytotoxic properties, transfection efficiency, and specificity of R6p-cRGD/DNA/MNPs polyplexes were evaluated. The addition of MNPs resulted in a decrease in the time needed for successful transfection with simultaneous increase in efficiency. We revealed a therapeutic effect on primary UL cells after delivery of plasmid encoding the herpes simplex virus type 1 (HSV-1) thymidine kinase gene. Treatment with ganciclovir resulted in 20% efficiency of suicide gene therapy in UL cells transfected with the pPTK-1 plasmid. Based on these results, we conclude that the use of cationic peptide carriers with MNPs can be promising for the development of modular non-viral carriers for suicide gene delivery to UL cells.

mRNA Therapeutic Modalities Design, Formulation and Manufacturing under Pharma 4.0 Principles

In the quest for a formidable weapon against the SARS-CoV-2 pandemic, mRNA therapeutics have stolen the spotlight. mRNA vaccines are a prime example of the benefits of mRNA approaches towards a broad array of clinical entities and druggable targets. Amongst these benefits is the rapid cycle "from design to production" of an mRNA product compared to their peptide counterparts, the mutability of the production line should another target be chosen, the side-stepping of safety issues posed by DNA therapeutics being permanently integrated into the transfected cell's genome and the controlled precision over the translated peptides. Furthermore, mRNA applications are versatile: apart from vaccines it can be used as a replacement therapy, even to create chimeric antigen receptor T-cells or reprogram somatic cells. Still, the sudden global demand for mRNA has highlighted the shortcomings in its industrial production as well as its formulation, efficacy and applicability. Continuous, smart mRNA manufacturing 4.0 technologies have been recently proposed to address such challenges. In this work, we examine the lab and upscaled production of mRNA therapeutics, the mRNA modifications proposed that increase its efficacy and lower its immunogenicity, the vectors available for delivery and the stability considerations concerning long-term storage.

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.

Development of iRGD-Modified Peptide Carriers for Suicide Gene Therapy of Uterine Leiomyoma

Uterine leiomyoma (UL) is one of the most common benign tumors in women that often leads to many reproductive complications. Suicide genetherapy was suggested as a promising approach for UL treatment. In the present study, we describe iRGD ligand-conjugated cysteine-rich peptide carrier RGD1-R6 for targeted DNA delivery to αvβ3 integrin-expressing primary UL cells. The physico-chemical properties, cytotoxicity, transfection efficiency and specificity of DNA/RGD1-R6 polyplexes were investigated. TheHSV-1thymidine kinase encoding plasmid delivery to PANC-1pancreatic carcinoma cells and primary UL cells resulted in significant suicide gene therapy effects. Subsequent ganciclovir treatment decreased cells proliferative activity, induced of apoptosis and promoted cells death.The obtained results allow us to concludethatthe developed RGD1-R6 carrier can be considered a promising candidate for suicide gene therapy of uterine leiomyoma.

Inhalation delivery technology for genome-editing of respiratory diseases

The clustered regulatory interspaced short palindromic repeats (CRISPR)/CRISPR-associated protein 9 (CRISPR/Cas9) system has significant therapeutic potentials for lung congenital diseases such as cystic fibrosis, as well as other pulmonary disorders like lung cancer and obstructive diseases. Local administration of CRISPR/Cas9 therapeutics through inhalation can achieve high drug concentration and minimise systemic exposure. While the field is advancing with better understanding on the biological functions achieved by CRISPR/Cas9 systems, the lack of progress in inhalation formulation and delivery of the molecule may impede their clinical translation efficiently. This forward-looking review discussed the current status of formulations and delivery for inhalation of relevant biologics such as genes (plasmids and mRNAs) and proteins, emphasising on their design strategies and preparation methods. By adapting and optimising formulation strategies used for genes and proteins, we envisage that development of inhalable CRISPR/Cas9 liquid or powder formulations for inhalation administration can potentially be fast-tracked in near future.

Duplex of Polyamidoamine Dendrimer/Custom-Designed Nuclear-Localization Sequence Peptide for Enhanced Gene Delivery

Background: Dendrimers are an attractive alternative to viral vectors due to the low cost of production, larger genetic insert-carrying capacity, and added control over immune- and genotoxic complications through versatile functionalization. However, their transfection rates pale in comparison to their viral counterparts, resulting in widespread research efforts in the attempt to improve transfection efficiency. Materials and Methods: In this work, we designed a synthetic diblock nuclear-localization sequence peptide (NLS) (DDDDDDVKRKKKP) and complexed it with polyamidoamine (PAMAM) dendrimer G4 to form a duplex for gene delivery. We conducted transmission electron microscopy, gel mobility shift assay, and intracellular trafficking studies. We also assessed its transfection efficiency for the delivery of a green fluorescent protein-encoding plasmid (pGFP) to NIH3T3 cells. Results: PAMAM dendrimer G4, NLS, and plasmid DNA can form a stable three-part polyplex and gain enhanced entry into the nucleus. We found transfection efficiency, in large part, depends on the ratio of G4:NLS:plasmid. The triplex prepared at the ratio of 1:60:1 for G4:NLS:pGFP has been shown to be more significantly efficient in transfecting cells than the control group (G4/pGFP, 0.5:1). Conclusions: This new diblock NLS peptide can facilely complex with dendrimers to improve dendrimer-based gene transfection. It can also complex with other polycationic polymers to produce more potent nonviral duplex gene delivery vehicles.

Controlling complexation/decomplexation and sizes of polymer-based electrostatic pDNA polyplexes is one of the key factors in effective transfection

The delivery of plasmid DNA (pDNA) using polycations has been investigated for several decades; however, obstacles that limit efficient gene delivery still hinder the clinical application of gene therapy. One of the major limiting factors is controlling pDNA binding affinity with polymers to control the complexation and decomplexation of polyplexes. To address this challenge, polycations of α-poly(_L-lysine) (APL) and ε-poly(_L-lysine) (EPL) were used to prepare variable complexation/decomplexation polyplexes with binding affinities ranging from too tight to too loose and sizes ranging from small to large. APL-EPL/ATP-pDNA polyplexes were also prepared to compare the effects of endosomolytic ATP on complexation/decomplexation and the sizes of polyplexes. The results showed that smaller and tighter polyplexes delivered more pDNA into the cells and into the nucleus than the larger and looser polyplexes. Larger polyplexes exhibited slower cytosolic transport and consequently less nuclear delivery of pDNA than smaller polyplexes. Tighter polyplexes exhibited poor pDNA release in the nucleus, leading to no improvement in transfection efficiency. Thus, polyplexes should maintain a balance between complexation and decomplexation and should have optimal sizes for effective cellular uptake, cytosolic transport, nuclear import, and gene expression. Understanding the effects of complexation/decomplexation and size is important when designing effective polymer-based electrostatic gene carriers.

Synthetic Approaches for Nucleic Acid Delivery: Choosing the Right Carriers

The discovery of the genetic roots of various human diseases has motivated the exploration of different exogenous nucleic acids as therapeutic agents to treat these genetic disorders (inherited or acquired). However, the physicochemical properties of nucleic acids render them liable to degradation and also restrict their cellular entrance and gene translation/inhibition at the correct cellular location. Therefore, gene condensation/protection and guided intracellular trafficking are necessary for exogenous nucleic acids to function inside cells. Diversified cationic formulation materials, including natural and synthetic lipids, polymers, and proteins/peptides, have been developed to facilitate the intracellular transportation of exogenous nucleic acids. The chemical properties of different formulation materials determine their special features for nucleic acid delivery, so understanding the property-function correlation of the formulation materials will inspire the development of next-generation gene delivery carriers. Therefore, in this review, we focus on the chemical properties of different types of formulation materials and discuss how these formulation materials function as protectors and cellular pathfinders for nucleic acids, bringing them to their destination by overcoming different cellular barriers.

Precise tuning of disulphide crosslinking in mRNA polyplex micelles for optimising extracellular and intracellular nuclease tolerability

The major issues in messenger (m)RNA delivery are rapid mRNA degradation in the extracellular and intracellular spaces, which decreases the efficiency and duration for protein expression from mRNA. Stabilization of mRNA carriers using environment-responsive crosslinkings has promises to overcome these issues. Herein, we fine-tuned the structure of disulphide crosslinkings, which are selectively cleaved in the intracellular reductive environment, using the mRNA-loaded polyplex micelles (PMs) prepared from poly(ethylene glycol)-poly(L-lysine) (PEG-PLys) block copolymers, particularly by focussing on cationic charge density after the crosslinking. Primary amino groups in PLys segment were partially thiolated in two ways: One is to introduce 3-mercaptopropionyl (MP) groups via amide linkage, resulting in the decreased cationic charge density [PEG-PLys(MP)], and the other is the conversion of amino groups to 1-amidine-3-mercaptopropyl (AMP) groups with preserving cationic charge density [PEG-PLys(AMP)]. Compared to non-crosslinked and PEG-PLys(MP) PMs, PEG-PLys(AMP) PM attained tighter mRNA packaging in the PM core, thereby improving mRNA nuclease tolerability in serum and intracellular spaces, and providing enhanced protein expression in cultured cells at the optimal crosslinking density. These findings highlight the importance of cationic charge preservation in installing crosslinking moieties, providing a rationale for mRNA carrier design in the molecular level.

Induced packaging of mRNA into polyplex micelles by regulated hybridization with a small number of cholesteryl RNA oligonucleotides directed enhanced in vivo transfection

There has been a progressive interest in the molecular design of polymers and lipids as synthetic carriers for targeting therapeutic mRNA in vivo with the ability to circumvent nuclease attack for treating intractable diseases. Herein, we developed a simple approach to attain one order of magnitude higher nuclease tolerability of mRNA through the formation of polyplex micelles (PMs) by combining ω-cholesteryl (ω-Chol)-poly (ethylene-glycol) (PEG)-polycation block copolymers with mRNA pre-hybridized with cholesterol (Chol)-tethered RNA oligonucleotides (Chol (+)-OligoRNA). Even one or a few short Chol (+)-OligoRNA anchors harboring along the 46-fold longer mRNA strand was sufficient to induce tight mRNA packaging in the PM core, as evidenced by Förster resonance energy transfer (FRET) measurement as well as by a longitudinal relaxation time (T₁) measurement using NMR. These results suggest that Chol (+)-OligoRNA on mRNA strand serves as a node to attract ω-Chol moiety of the block copolymers to tighten the mRNA packaging in the PM core. These mRNA loaded PMs showed high tolerability against nuclease attack, and exerted appreciable protein translational activity in cultured cells without any inflammatory responses, achieved by shortening of the length of hybridizing Chol (+)-OligoRNAs to 17 nucleotides. Finally, the Chol (+)-OligoRNA-stabilized PM revealed efficient mRNA introduction into the mouse lungs via intratracheal administration, demonstrating in vivo utility of this formulation.

A new developing class of gene delivery: messenger RNA-based therapeutics

Gene therapy has long been held as having the potential to become a front line treatment for various genetic disorders. However, the direct delivery of nucleic acids to correct a genetic disorder has numerous limitations owing to the inability of naked nucleic acids (DNA and RNA) to traverse the cell membrane. Recently, messenger RNA (mRNA) based delivery has become a more attractive alternative to DNA due to the relatively easier transfection process, higher efficiency and safety profile. As with all gene therapies, the central challenge that remains is the efficient delivery of nucleic acids intracellularly. This review presents the recent progress in mRNA delivery, focusing on comparing the advantages and limitations of non-viral based delivery vectors.

Arginine-rich cross-linking peptides with different SV40 nuclear localization signal content as vectors for intranuclear DNA delivery

The major barriers for intracellular DNA transportation by cationic polymers are their toxicity, poor endosomal escape and inefficient nuclear uptake. Therefore, we designed novel modular peptide-based carriers modified with SV40 nuclear localization signal (NLS). Core peptide consists of arginine, histidine and cysteine residues for DNA condensation, endosomal escape promotion and interpeptide cross-linking, respectively. We investigated three polyplexes with different NLS content (10 mol%, 50 mol% and 90 mol% of SV40 NLS) as vectors for intranuclear DNA delivery. All carriers tested were able to condense DNA, to protect it from DNAase I and were not toxic to the cells. We observed that cell cycle arrest by hydroxyurea did not affect transfection efficacy of NLS-modified carriers which we confirmed using quantitative confocal microscopy analysis. Overall, peptide carrier modified with 90 mol% of SV40 NLS provided efficient transfection and nuclear uptake in non-dividing cells. Thus, incorporation of NLS into arginine-rich cross-linking peptides is an adequate approach to the development of efficient intranuclear gene delivery vehicles.

Tumor-targeted delivery of siRNA using fatty acyl-CGKRK peptide conjugates

Tumor-targeted carriers provide efficient delivery of chemotherapeutic agents to tumor tissue. CGKRK is one of the well-known tumor targeting peptides with significant specificity for angiogenic blood vessels and tumor cells. Here, we designed fatty acyl conjugated CGKRK peptides, based on the hypothesis that hydrophobically-modified CGKRK peptide could enhance cellular permeation and delivery of siRNA targeted to tumor cells for effective silencing of selected proteins. We synthesized six fatty acyl-peptide conjugates, using a diverse chain of saturated and unsaturated fatty acids to study the efficiency of this approach. At peptide:siRNA weight/weight ratio of 10:1 (N/P ≈ 13.6), almost all the peptides showed complete binding with siRNA, and at a w/w ratio of 20:1 (N/P ≈ 27.3), complete protection of siRNA from early enzymatic degradation was observed. Conjugated peptides and peptide/siRNA complexes did not show significant cytotoxicity in selected cell lines. The oleic acid-conjugated peptide showed the highest efficiency in siRNA uptake and silencing of kinesin spindle protein at peptide:siRNA w/w ratio of 80:1 (N/P ≈ 109). The siRNA internalization into non-tumorigenic kidney cells was negligible with all fatty acyl-peptide conjugates. These results indicate that conjugation of fatty acids to CGKRK could create an efficient delivery system for siRNA silencing specifically in tumor cells.

Tempo-spatial Activation of Sequential Quadruple Stimuli for High Gene Expression of Polymeric Gene Nanocomplexes

The clinical application of intracellular gene delivery via nanosized carriers is hindered by intracellular multistep barriers that limit high levels of gene expression. To solve these issues, four different intracellular or external stimuli that can efficiently activate a gene carrier, a gene, or a photosensitizer (pheophorbide A [PhA]) were assessed in this study. The designed nanosized polymeric gene complexes were composed of PhA-loaded thiol-degradable polycation (PhA@RPC) and cytomegalovirus (CMV) promoter-equipped pDNA. After cellular internalization of the resulting PhA@RPC/pDNA complexes, the complexes escaped endosomal sequestration, owing to the endosomal pH-induced endosomolytic activity of RPC in PhA@RPC. Subsequently, intracellular thiol-mediated polycation degradation triggered the release of PhA and pDNA from the complexes. Late exposure to light (for example, 12 h post-treatment) activated the released PhA and resulted in the production of reactive oxygen species (ROS). Intracellular ROS successively activated NF-κB, which then reactivated the CMV promoter in the pDNA. These sequential, stimuli-responsive chemical and biological reactions resulted in high gene expression. In particular, the time-point of light exposure was very significant to tune efficient gene expression as well as negligible cytotoxicity: early light treatment induced photochemical internalization but high cytotoxicity, whereas late light treatment influenced the reactivation of silent pDNA via PhA-generated ROS and activation of NF-κB. In conclusion, the quadruple triggers, such as pH, thiol, light, and ROS, successively influenced a gene carrier (RPC), a photosensitizer, and a genetic therapeutic, and the tempo-spatial activation of the designed quadruple stimuli-activatable nanosized gene complexes could be potential in gene delivery applications.

CXCR4-targeted modular peptide carriers for efficient anti-VEGF siRNA delivery

The application of small interfering RNA (siRNA) for specific gene inhibition is a promising strategy in gene therapy treatments. The efficient cellular delivery of therapeutic siRNA is a critical step in RNA interference (RNAi) application. Highly efficient siRNA carriers should be developed for specific cellular uptake, stable RNA-complexes formation and intracellular RNA release. To study these features, we evaluated modular peptide carriers bearing CXCR4 targeting ligand for their ability to condense siRNA, facilitate endosomal escape and VEGFA gene silencing in CXCR4-expressing endothelial and glioblastoma cells. Peptide carriers were shown to condense and protect siRNA from RNAse degradation. Various N/P ratios were used for physicochemical characterization to optimize siRNA/peptide complexes for in vitro studies. On average, cytotoxicity of siRNA-polyplexes depended on cell type and was not higher than that of PEI/siRNA complexes. VEGFA gene knockdown was significantly improved with CXCR4-targeted carriers in contrast to nontargeted peptides. siRNA delivery by means of ligandconjugated carriers resulted in 2.5-3-fold decrease of VEGF expression in glioblastoma cells and in 1.5-2-fold decrease of VEGF expression in endothelial cells. Delivery of siRNA/peptide complexes resulted in 2-6- fold decrease in VEGF protein yield and in significant inhibition of endothelial cells migration. The study shows that implication of peptide carriers modified with CXCR4 ligand is a promising approach to develop targeted siRNA delivery system into CXCR4-expressing cancer and endothelial cells.

Nebulisation of IVT mRNA Complexes for Intrapulmonary Administration

During the last years the potential role of in vitro transcribed (IVT) mRNA as a vehicle to deliver genetic information has come into focus. IVT mRNA could be used for anti-cancer therapies, vaccination purposes, generation of pluripotent stem cells and also for genome engineering or protein replacement. However, the administration of IVT mRNA into the target organ is still challenging. The lung with its large surface area is not only of interest for delivery of genetic information for treatment of e.g. for cystic fibrosis or alpha-1-antitrypsin deficiency, but also for vaccination purposes. Administration of IVT mRNA to the lung can be performed by direct intratracheal instillation or by aerosol inhalation/nebulisation. The latter approach shows a non-invasive tool, although it is not known, if IVT mRNA is resistant during the process of nebulisation. Therefore, we investigated the transfection efficiency of non-nebulised and nebulised IVT mRNA polyplexes and lipoplexes in human bronchial epithelial cells (16HBE). A slight reduction in transfection efficiency was observed for lipoplexes (Lipofectamine 2000) in the nebulised part compared to the non-nebulised which can be overcome by increasing the amount of Lipofectamine. However, Lipofectamine was more than three times more efficient in transfecting 16HBE than DMRIE and linear PEI performed almost 10 times better than its branched derivative. By contrast, the nebulisation process did not affect the cationic polymer complexes. Furthermore, aerosolisation of IVT mRNA complexes did neither affect the protein duration nor the toxicity of the cationic complexes. Taken together, these data show that aerosolisation of cationic IVT mRNA complexes constitute a potentially powerful means to transfect cells in the lung with the purpose of protein replacement for genetic diseases such as cystic fibrosis or alpha-1-antitrypsin deficiency or for infectious disease vaccines, while bringing along the advantages of IVT mRNA as compared to pDNA as transfection agent.

Modified mRNA as an alternative to plasmid DNA (pDNA) for transcript replacement and vaccination therapy

Introduction: Current gene therapy involves replacement of defective gene by delivery of healthy genetic material to precede normal function. Virus-mediated gene delivery is the most successful and efficient method for gene therapy, but it has been challenged due to serious safety concerns. Conversely, gene delivery using plasmid DNA (pDNA) is considered safer, but its transfection efficiency is much lower than virus-mediated gene transfer. Recently, mRNA has been suggested as an alternative option to avoid undesired insertion of delivered DNA sequences with higher transfection efficiency and stability.

Area covered: In this review, we summarize the currently available strategies of mRNA modification to increase the therapeutic efficacy; we also highlight the recent improvements of mRNA delivery for in vivo applications of gene therapy.

Expert opinion: The use of mRNA-based gene transfer could indeed be a promising new strategy for gene therapy. Notable advantages include no risk of integration into the genomic DNA, adjustable gene expression and easier modulation of the immune system. By reducing or utilizing the immunogenic properties, mRNA offers a promising tool for gene/or transcript replacement.

Label-free dendrimer-like silica nanohybrids for traceable and controlled gene delivery

To create advanced functional nanocarriers for achieving excellent gene delivery performance, fluorescence label-free hybridized dendrimer-like silica nanocarriers (HPSNs-AC-PEI) were developed by using the endosomal pH and cytoplasmic glutathione (GSH) responsive autofluorescent acetaldehyde-modified-cystine (AC) to link non-toxic low molecular weight branched polyethyleneimine (PEI) onto amino-functionalized dendrimer-like silica nanoparticles with hierarchical pores (HPSNs-NH2). The specific microstructure of this hybridized nanocarrier makes it not only show low cytotoxicity and high gene loading capability, but also display high gene transfection efficiency. The cleavage of disulfide bonds caused by GSH facilitates plasmid DNA (pDNA) release. Moreover, the pH and GSH controlled gene delivery profile can be real-time tracked using the autofluorescence of HPSNs-AC-PEI.

Nucleic acid vaccines: prospects for non-viral delivery of mRNA vaccines

INTRODUCTION

Nucleic acid-based vaccines are being developed as a means to combine the positive attributes of both live-attenuated and subunit vaccines. Viral vectors and plasmid DNA vaccines have been extensively evaluated in human clinical trials and have been shown to be safe and immunogenic, although none have been licensed for human use. More recently, mRNA-based vaccine alternatives have emerged and might offer certain advantages over their DNA-based counterparts.

AREAS COVERED

This review describes the two main categories of mRNA vaccines: conventional non-amplifying and self-amplifying mRNA. It summarizes the initial clinical proof-of-concept studies and outlines the preclinical testing of the next wave of innovations for the technology. Finally, this review highlights the versatile functionality of the mRNA molecule and introduces opportunities for future improvements in vaccine design.

EXPERT OPINION

The prospects for mRNA vaccines are very promising. Like other types of nucleic acid vaccines, mRNA vaccines have the potential to combine the positive attributes of live attenuated vaccines while obviating many potential safety limitations. Although data from initial clinical trials appear encouraging, mRNA vaccines are far from a commercial product. These initial approaches have spurred innovations in vector design, non-viral delivery, large-scale production and purification of mRNA to quickly move the technology forward. Some improvements have already been tested in preclinical models for both prophylactic and therapeutic vaccine targets and have demonstrated their ability to elicit potent and broad immune responses, including functional antibodies, type 1 T helper cells-type T cell responses and cytotoxic T cells. Though the initial barriers for this nucleic acid vaccine approach seem to be overcome, in our opinion, the future and continued success of this approach lies in a more extensive evaluation of the many non-viral delivery systems described in the literature and gaining a better understanding of the mechanism of action to allow rational design of next generation technologies.

RNA: the new revolution in nucleic acid vaccines

Nucleic acid vaccines have the potential to address issues of safety and effectiveness sometimes associated with vaccines based on live attenuated viruses and recombinant viral vectors. In addition, methods to manufacture nucleic acid vaccines are suitable as generic platforms and for rapid response, both of which will be very important for addressing newly emerging pathogens in a timely fashion. Plasmid DNA is the more widely studied form of nucleic acid vaccine and proof of principle in humans has been demonstrated, although no licensed human products have yet emerged. The RNA vaccine approach, based on mRNA and engineered RNA replicons derived from certain RNA viruses, is gaining increased attention and several vaccines are under investigation for infectious diseases, cancer and allergy. Human clinical trials are underway and the prospects for success are bright.

Oligobenzylethylenimine enriches linear polyethylenimine with a pH-sensitive membrane-disruptive property and leads to enhanced gene delivery activity

We report here the synthesis of a diblock linear polymer of oligo(benzylethylenimine)-b-polyethylenimine (OBzEI-PEI) and investigate its gene delivery properties. The linear copolymer OBzEI-PEI was prepared in a straightforward manner by acidic hydrolysis of a diblock polyoxazoline, which had been made by sequential polymerization of 4-benzyl-2-ethyl-2-oxazoline followed by 2-ethyl-2-oxazoline. pH titration and DNA complexation profiles of the new polymer are similar to regular linear PEIs, but with higher gene transfection efficiencies in various cell lines despite a decreased cellular uptake of plasmid DNA. Further experiments suggest that the OBzEI tail complements the intrinsic proton-sponge endosomolytic activities of PEI with an acid pH-sensitive membrane-perturbing activity.

Bioreducible polymers as a determining factor for polyplex decomplexation rate and transfection

Polyplex formation (complexation) and gene release from the polyplexes (decomplexation) are major events in polymeric gene delivery; however, the effect of the decomplexation rate on transfection has been rarely investigated. This study employed mixed polymers of poly((L)-lysine) (PLL: MW ~7.4 kDa) and reducible PLL (RPLL) (MW ~6.7 kDa) to design decomplexation rate-controllable PLL(100-x)RPLL(x)/pDNA complexes (PRL(x) polyplexes). The transfection efficiency of a model gene (luciferase) in MCF7 and HEK293 cell lines increased with increasing x (RPLL content) in the PRL(x) polyplexes until peaking at x = 2.5 and 10, respectively, after which point transfection efficiency declined rapidly. In MCF7 cells, PRL(2.5) polyplex produced 3 or 223 times higher gene expression than PLL or RPLL polyplexes, respectively. Similarly, the transfection efficiency of PRL(10) polyplex-transfected HEK293 cells was 3.8 or 67 times higher than that of PLL or RPLL polyplexes, respectively. The transfection results were not apparently related to the particle size, surface charge, complexation/compactness, cellular uptake, or cytotoxicity of the tested polyplexes. However, the decomplexation rate varied by RPLL content in the polyplexes, which in turn influenced the gene transfection. The nuclear localization of pDNA delivered by PRL(x) polyplexes showed a similar trend to their transfection efficiencies. This study suggests that an optimum decomplexation rate may result in high nuclear localization of pDNA and transfection. Understanding in decomplexation and intracellular localization of pDNA may help develop more effective polyplexes.

Developing mRNA-vaccine technologies

mRNA vaccines combine desirable immunological properties with an outstanding safety profile and the unmet flexibility of genetic vaccines. Based on in situ protein expression, mRNA vaccines are capable of inducing a balanced immune response comprising both cellular and humoral immunity while not subject to MHC haplotype restriction. In addition, mRNA is an intrinsically safe vector as it is a minimal and only transient carrier of information that does not interact with the genome. Because any protein can be expressed from mRNA without the need to adjust the production process, mRNA vaccines also offer maximum flexibility with respect to development. Taken together, mRNA presents a promising vector that may well become the basis of a game-changing vaccine technology platform. Here, we outline the current knowledge regarding different aspects that should be considered when developing an mRNA-based vaccine technology.

Structure, dynamics, and energetics of siRNA-cationic vector complexation: a molecular dynamics study

The design and synthesis of safe and efficient nonviral vectors for gene delivery has attracted significant attention in recent years. Previous experiments have revealed that the charge density of a polycation (the carrier) plays a crucial role in complexation and the release of the gene from the complex in the cytosol. In this work, we adopt an atomistic molecular dynamics simulation approach to study the complexation of short strand duplex RNA with six cationic carrier systems of varying charge and surface topology. The simulations reveal detailed molecular-level pictures of the structures and dynamics of the RNA-polycation complexes. Estimates for the binding free energy indicate that electrostatic contributions are dominant followed by van der Waals interactions. The binding free energy between the 8(+)polymers and the RNA is found to be larger than that of the 4(+)polymers, in general agreement with previously published data. Because reliable binding free energies provide an effective index of the ability of the polycationic carrier to bind the nucleic acid and also carry implications for the process of gene release within the cytosol, these novel simulations have the potential to provide us with a much better understanding of key mechanistic aspects of gene-polycation complexation and thereby advance the rational design of nonviral gene delivery systems.

A reducible polycationic gene vector derived from thiolated low molecular weight branched polyethyleneimine linked by 2-iminothiolane

To improve transfection efficiency and reduce the cytotoxicity of polymeric gene vectors, reducible polycations (RPC) were synthesized from low molecular weight (MW) branched polyethyleneimine (bPEI) via thiolation and oxidation. RPC (RPC-bPEI(0.8 kDa)) possessed MW of 5 kDa-80 kDa, and 50%-70% of the original proton buffering capacity of bPEI(0.8 kDa) was preserved in the final product. The cytotoxicity of RPC-bPEI(0.8 kDa) was 8-19 times less than that of the gold standard of polymeric transfection reagents, bPEI(25 kDa). Although bPEI(0.8 kDa) exhibited poor gene condensing capacities (∼2 μm at a weight ratio (WR) of 40), RPC-bPEI(0.8 kDa) effectively condensed plasmid DNA (pDNA) at a WR of 2. Moreover, RPC-bPEI(0.8 kDa)/pDNA (WR ≥2) formed 100-200 nm-sized particles with positively charged surfaces (20-35 mV). In addition, the results of the present study indicated that thiol/polyanions triggered the release of pDNA from RPC-bPEI(0.8 kDa)/pDNA via the fragmentation of RPC-bPEI(0.8 kDa) and ion-exchange. With negligible polyplex-mediated cytotoxicity, the transfection efficiencies of RPC-bPEI(0.8 kDa)/pDNA were approximately 1200-1500-fold greater than that of bPEI(0.8 kDa)/pDNA and were equivalent or superior (∼7-fold) to that of bPEI(25 kDa)/pDNA. Interestingly, the distribution of high MW RPC-bPEI(0.8 kDa)/pDNA in the nucleus of the cell was higher than that of low MW RPC-bPEI(0.8 kDa)/pDNA. Thus, the results of the present study suggest that RPC-bPEI(0.8 kDa) has the potential to effectively deliver genetic materials with lower levels of toxicity.

The effect of CpG motifs on gene expression and clearance kinetics of aerosol administered polyethylenimine (PEI)-plasmid DNA complexes in the lung

Presence of CpG motifs within pDNA is widely reported to influence transgene expression as well as inflammatory response to nonviral gene vector complexes. Here, we analyzed gene expression kinetics and lung clearance after aerosol delivery of polyethylenimine (PEI) complexes with two different plasmid vectors: a first generation plasmid, pCMVLuc, and a plasmid with depleted CpG motifs, pCpG-free-Luc. After aerosol delivery, equal nanogram amounts of PEI-pDNA complexes were deposited in murine lungs. Luciferase expression observed at day one post administration of PEI-pCpG-free-Luc complexes was 60-fold higher than for PEI-pCMVLuc complexes and decreased 16-fold at day 7 post application. In contrast, luciferase expression from PEI-pCMVLuc particles remained at levels comparable to day 1 post application. In agreement with these observations, PEI-pCpG-free-Luc complexes were cleared from the lungs at rates 6-fold faster than those observed for PEI-pCMVLuc particles. A more detailed analysis of pDNA distribution within bronchoalveolar lavage fluid (BALF), BALF cells and lung tissue showed 660-fold higher amounts of pCpG-free-Luc in BALF cells compared to pCMVLuc, whereas the amount of pCpG-free-Luc in lung tissue was 15-fold lower compared to pCMVLuc 1h after administration. Our results demonstrate that complexes of PEI with CpG-motif-free DNA are taken up more extensively by BALF cells, while the clearance of pCMVLuc from the lung tissue is significantly slower than for the CpG-free plasmid. Administration of PEI-pCpG-free-Luc caused transient decrease in number of resident lung cells, while their activation was more pronounced with PEI-pCMVLuc particles. Our results demonstrate that the level of transgene expression is increased with CpG-motif-free pDNA but the longevity of expression correlates with pDNA clearance pattern depending on the presence of CpG motifs within the plasmid.

Balancing protection and release of DNA: tools to address a bottleneck of non-viral gene delivery

Engineering polymeric gene-delivery vectors to release an intact DNA payload at the optimal time and subcellular compartment remains a formidable challenge. An ideal vector would provide total protection of complexed DNA from degradation prior to releasing it efficiently near or within the nucleus of a target cell. While optimization of polymer properties, such as molecular weight and charge density, has proved largely inadequate in addressing this challenge, applying polymeric carriers that respond to temperature, light, pH and redox environment to trigger a switch from a tight, protective complex to a more relaxed interaction favouring release at the appropriate time and place has shown promise. Currently, a paucity of gene carriers able to satisfy the contrary requirements of adequate DNA protection and efficient release contributes to the slow progression of non-viral gene therapy towards clinical translation. This review highlights the promising carrier designs that may achieve an optimal balance of DNA protection and release. It also discusses the imaging techniques and three-dimensional in vitro models that can help study these two barriers in the non-viral gene transfer process. Ultimately, efficacious non-viral gene therapy will depend on the combination of intelligent material design, innovative imaging techniques and sophisticated in vitro model systems to facilitate the rational design of polymeric gene-delivery vectors.

Delivery of nucleic acids via disulfide-based carrier systems

Nucleic acids are not only expected to assume a pivotal position as "drugs" in the treatment of genetic and acquired diseases, but could also act as molecular cues to control the microenvironment during tissue regeneration. Despite this promise, the efficient delivery of nucleic acids to their side of action is still the major hurdle. One among many prerequisites for a successful carrier system for nucleic acids is high stability in the extracellular environment, accompanied by an efficient release of the cargo in the intracellular compartment. A promising strategy to create such an interactive delivery system is to exploit the redox gradient between the extra- and intracellular compartments. In this review, emphasis is placed on the biological rationale for the synthesis of redox sensitive, disulfide-based carrier systems, as well as the extra- and intracellular processing of macromolecules containing disulfide bonds. Moreover, the basic synthetic approaches for introducing disulfide bonds into carrier molecules, together with examples that demonstrate the benefit of disulfides at the individual stages of nucleic acid delivery, will be presented.

Combinatorial evaluation of cations, pH-sensitive and hydrophobic moieties for polymeric vector design

Three combinatorial libraries of polymeric vectors were evaluated to investigate the functional roles of molecular weight (MW), cations, pH-sensitive moieties, and hydrophobic derivitization in polymer-mediated gene delivery. Four cationic and pH-sensitive moieties (imidazole, primary, secondary, and tertiary amino) and three hydrophobic residues (C4 butyl, C6 hexyl, and C8 octyl) were assessed in single and serially incremented, binary combinations. Three MWs were evaluated-10, 30, and 50 kDa. The highest levels of transfection, comparable to branched PEI (25 kDa), were achieved by 30 kDa and 50 kDa formulations containing primary amino and imidazole groups. Primary amino groups offered superior charge-neutralizing and size-condensing capacity, while imidazole groups appeared to bind with DNA via nonelectrostatically mediated interactions to produce stable polyplexes that were resistant to premature dissociation. Eight of the 10 highest-transfecting polymers possessed IC(50) values greater than the maximum concentration of free polymers exposed to cells (200 microg/ml). The results herein have identified highly efficient polymeric formulations with superb toxicity profiles and have revealed the functional roles that the investigated pendant groups play in the transfection process. The reported polymeric system offers a versatile and robust platform upon which future structure-function studies may be based to create safer and more efficient polymeric vectors.

Polyethylenimine-mediated gene delivery to the lung and therapeutic applications

Nonviral gene delivery is now considered a promising alternative to viral vectors. Among nonviral gene delivery agents, polyethylenimine (PEI) has emerged as a potent candidate for gene delivery to the lung. PEI has some advantages over other polycations in that it combines strong DNA compaction capacity with an intrinsic endosomolytic activity. However, intracellular (mainly the nuclear membrane) and extracellular obstacles still hamper its efficiency in vitro and in vivo, depending on the route of administration and the type of PEI. Nuclear delivery has been increased by adding nuclear localization signals. To overcome nonspecific interactions with biological fluids, extracellular matrix components and nontarget cells, strategies have been developed to protect polyplexes from these interactions and to increase target specificity and gene expression. When gene delivery into airway epithelial cells of the conducting airways is necessary, aerosolization of complexes seems to be better suited to guarantee higher transgene expression in the airway epithelial cells with lower toxicity than observed with either intratracheal or intravenous administration. Aerosolization, indeed, is useful to target the alveolar epithelium and pulmonary endothelium. Proof-of-principle that PEI-mediated gene delivery has therapeutic application to some genetic and acquired lung disease is presented, using as genetic material either plasmidic DNA or small-interfering RNA, although optimization of formulation and delivery protocols and limitation of toxicity need further studies.

Addition of "charge-shifting" side chains to linear poly(ethyleneimine) enhances cell transfection efficiency

We reported recently that the addition of ester-functionalized, "charge-shifting" side chains to linear poly(ethyleneimine) (LPEI) can be used to design polyamines that promote both self-assembly and self-disassembly with DNA in aqueous environments. This investigation sought to characterize the influence of charge-shifting side chains on the ability of LPEI to mediate cell transfection and understand the extent to which increases (or decreases) in levels of transfection could be understood in terms of time-dependent changes in the net charges of these polymers. We report that the addition of "charge-shifting" side chains to LPEI leads to significant increases in levels of LPEI-mediated transfection. In particular, polymer 1e, functionalized with 20 mol % ester-functionalized side chains, mediates levels of transgene expression in vitro up to 8-fold higher than LPEI. Experiments using an amide-functionalized analog of polymer 1e demonstrated that the esters in polymer 1e play an important role in promoting increased levels of transfection. These results, in combination with the results of additional gel electrophoresis experiments, provide support for the view that increases in transfection result from time-dependent changes in the net charge of polymer 1e and the disruption of ionic interactions in polyplexes. Additional support for this view is provided by the results of confocal microscopy experiments and measurements of fluorescence resonance energy transfer, which suggest that polymer 1e promotes the disruption of polyplexes in intracellular environments effectively. The approach reported here provides a means of addressing one important "late-stage" obstacle to polyplex-mediated transfection (polyplex unpackaging). If integrated successfully with methods that have been developed to address other important barriers to transfection, this general approach could lead to the development of multifunctional polyplexes that mimic more effectively the range of functions of viruses as agents for the delivery of DNA.

Dynamics of photoinduced endosomal release of polyplexes

Endosomal escape is a well-known bottleneck for successful delivery of macromolecular drugs and genes. Photochemical disruption of endosomal membranes is an approach to overcome this bottleneck. In this study, we used the photosensitizer disulphonated meso-tetraphenylporphine with sulfonate groups on adjacent phenyl rings (TPPS(2a)) to investigate photoinduced endosomal release in living cells with high resolution fluorescence wide-field microscopy in real time. We studied the release dynamics of 10 kDa dextran and polyplexes consisting of DNA condensed with the cationic polymers linear polyethyleneimine (LPEI), poly-(L)-lysine (PLL) or poly-(D)-lysine (PDL). By means of dual-color microscopy and the use of double-labeled polyplexes DNA and polymer were imaged simultaneously. We show that the characteristics of the cationic polymer significantly influence the release behavior of the polyplexes. The release of dextran occurred within 100 ms. For LPEI/DNA particles, LPEI quickly spread throughout the cytosol similar to dextran, whereas DNA was released slowly (within 4 s) and remained immobile thereafter. In case of PLL particles, both DNA and polymer showed quick release. PDL particles remained condensed upon photosensitizer activation. In addition, we demonstrate that TPPS(2a) has biological side effects. Besides stop of microtubule dynamics in the dark, the movement of endosomes ceased after photosensitizer activation.

Gene therapy progress and prospects: synthetic polymer-based systems

Low efficiency, significant toxicity, polymer polydispersity and poorly understood delivery mechanisms have initially plagued the field of polymer-based gene therapy. Numerous strategies have helped to improve polyplexes, including the development of biodegradable polymers with reduced toxicity, incorporation of cell targeting, surface shielding and additional transport domains for effective and specific delivery, or improved chemistry for syntheses of polymers with uniform size and topology. Combined biooptical imaging and bioinformatics, providing insights into transfer bottlenecks, have helped to design improved polyplexes. Bioresponsive multifunctional polymers adapt in a dynamic manner to delivery barriers for efficient transfer of pDNA or siRNA to the target site.