Protein-resistant, reductively dissociable polyplexes for in vivo systemic delivery and tumor-targeting of siRNA

Exploring the Fibrous Nature of Single-Stranded DNA-Collagen Complexes: Nanostructural Observations and Physicochemical Insights

Nucleic acid-collagen complexes (NACCs) are a self-assembled biomimetic fibrillary platform arising from the spontaneous complexation of single-stranded DNA (ssDNA) oligonucleotides and collagen. NACCs merge the extracellular matrix functionality of collagen with the tunable bioactivity of ssDNA as aptamers for broad biomedical applications. We hypothesize that NACCs offer a hierarchical architecture across multiple length scales that significantly varies compared to native collagen. We investigate this using atomic force microscopy and electron microscopy (transmission electron microscopy and cryogenic electron microscopy). Results demonstrate key topographical differences induced by adding ssDNA oligonucleotides to collagen type I. NACCs form a dense network of intertwined collagen fiber bundles in the microscale and nanoscale while retaining their characteristic D-band periodicities (∼67 nm). Additionally, our exploration of thermodynamic parameters governing the interaction indicates an entropically favorable NACC formation driven by ssDNA. Thermal analysis demonstrates the preservation of collagen's triple helical domains and a more stabilized polypeptide structure at higher temperatures than native collagen. These findings offer important insights into our understanding of the ssDNA-induced complexation of collagen toward the further establishment of structure-property relationships in NACCs and their future development into practical biomaterials. They also provide pathways for manipulating and enhancing collagenous matrices' properties without requiring complex chemical modifications or fabrication procedures.

Rationally designed multifunctional nanoparticles as GSH-responsive anticancer drug delivery systems based on host-guest polymers derived from dextran and β-cyclodextrin

Tumor proliferation and metastasis rely on energy provided by mitochondria. The hexokinase inhibitor lonidamine (LND) could suppress the activities in mitochondria, being a potential antitumor drug. However, limited water-solubility of LND may hinder its biomedical applications. Besides, the cancer-killing effect of LND is compromised by the high level of glutathione (GSH) in cancer cells. Therefore, it is urgent to find a proper method to simultaneously deliver LND and deplete GSH as well as monitor GSH level in cancer cells. Herein, a host polymer β-cyclodextrin-polyethylenimine (β-CD-PEI) and a guest polymer dextran-5-dithio-(2-nitrobenzoic acid) (Dextran-SS-TNB) were synthesized and allowed to form LND-loaded GSH-responsive nanoparticles through host-guest inclusion complexation between β-CD and TNB as host and guest molecular moieties, respectively, which functioned as a system for simultaneous delivery of LND and -SS-TNB species into cancer cells. As a result, the delivery system could deplete GSH and elevate reactive oxygen species (ROS) level in cancer cells, further induce LND-based mitochondrial dysfunction and ROS-based immunogenic cell death (ICD), leading to a synergistic and efficient anticancer effect. In addition, -SS-TNB reacted with GSH to release TNB2-, which could be a probe with visible light absorption at 410 nm for monitoring the GSH level in the cells.

Targeted drug delivery strategies for precision medicines

Progress in the field of precision medicine has changed the landscape of cancer therapy. Precision medicine is propelled by technologies that enable molecular profiling, genomic analysis, and optimized drug design to tailor treatments for individual patients. Although precision medicines have resulted in some clinical successes, the use of many potential therapeutics has been hindered by pharmacological issues, including toxicities and drug resistance. Drug delivery materials and approaches have now advanced to a point where they can enable the modulation of a drug's pharmacological parameters without compromising the desired effect on molecular targets. Specifically, they can modulate a drug's pharmacokinetics, stability, absorption, and exposure to tumours and healthy tissues, and facilitate the administration of synergistic drug combinations. This Review highlights recent progress in precision therapeutics and drug delivery, and identifies opportunities for strategies to improve the therapeutic index of cancer drugs, and consequently, clinical outcomes.

Delivery Systems for Nucleic Acids and Proteins: Barriers, Cell Capture Pathways and Nanocarriers

Gene therapy has been used as a potential approach to address the diagnosis and treatment of genetic diseases and inherited disorders. In this line, non-viral systems have been exploited as promising alternatives for delivering therapeutic transgenes and proteins. In this review, we explored how biological barriers are effectively overcome by non-viral systems, usually nanoparticles, to reach an efficient delivery of cargoes. Furthermore, this review contributes to the understanding of several mechanisms of cellular internalization taken by nanoparticles. Because a critical factor for nanoparticles to do this relies on the ability to escape endosomes, researchers have dedicated much effort to address this issue using different nanocarriers. Here, we present an overview of the diversity of nanovehicles explored to reach an efficient and effective delivery of both nucleic acids and proteins. Finally, we introduced recent advances in the development of successful strategies to deliver cargoes.

Improved Nucleic Acid Therapy with Advanced Nanoscale Biotechnology

Due to a series of systemic and intracellular obstacles in nucleic acid (NA) therapy, including fast degradation in blood, renal clearance, poor cellular uptake, and inefficient endosomal escape, NAs may need delivery methods to transport to the cell nucleus or cytosol to be effective. Advanced nanoscale biotechnology-associated strategies, such as controlling the particle size, charge, drug loading, response to environmental signals, or other physical/chemical properties of delivery carriers, have provided great help for the in vivo and in vitro delivery of NA therapeutics. In this review, we introduce the characteristics of different NA modalities and illustrate how advanced nanoscale biotechnology assists NA therapy. The specific features and challenges of various nanocarriers in clinical and preclinical studies are summarized and discussed. With the help of advanced nanoscale biotechnology, some of the major barriers to the development of NA therapy will eventually be overcome in the near future.

Covalently tethering siRNA to hydrogels for localized, controlled release and gene silencing

Small interfering RNA (siRNA) has found many applications in tissue regeneration and disease therapeutics. Effective and localized siRNA delivery remains challenging, reducing its therapeutic potential. Here, we report a strategy to control and prolong siRNA release by directly tethering transfection-capable siRNA to photocrosslinked dextran hydrogels. siRNA release is governed via the hydrolytic degradation of ester and/or disulfide linkages between the siRNA and hydrogels, which is independent of hydrogel degradation rate. The released siRNA is shown to be bioactive by inhibiting protein expression in green fluorescent protein-expressing HeLa cells without the need of a transfection agent. This strategy provides an excellent platform for controlling nucleic acid delivery through covalent bonds with a biomaterial and regulating cellular gene expression, which has promising potential in many biomedical applications.

Dual-targeting and microenvironment-responsive micelles as a gene delivery system to improve the sensitivity of glioma to radiotherapy

Dbait is a small double-stranded DNA molecule that has been utilized as a radiosensitizer to enhance the sensitivity of glioma to radiotherapy (RT). However, there is no effective drug delivery system to effectively overcome the blood-brain barrier (BBB). The aim of this study was to develop a gene delivery system by using the BBB and glioma dual-targeting and microenvironment-responsive micelles (ch-Kn(s-s)R8-An) to deliver Dbait into glioma for RT. Angiopep-2 can target the low-density lipoprotein receptor-related protein-1 (LRP1) that is overexpressed on brain capillary endothelial cells (BCECs) and glioma cells. In particular, due to upregulated matrix metalloproteinase 2 (MMP-2) in the tumor microenvironment, we utilized MMP-2-responsive peptides as the enzymatically degradable linkers to conjugate angiopep-2. The results showed that ch-Kn(s-s)R8-An micelles maintained a reasonable size (80-160 nm) with a moderate distribution and a decreased mean diameter from the cross-linking as well as exhibited low critical micelle concentration (CMC) with positive surface charge, ranging from 15 to 40 mV. The ch-K5(s-s)R8-An/pEGFP showed high gene transfection efficiency in vitro, improved uptake in glioma cells and good biocompatibility in vitro and in vivo. In addition, the combination of ch-K5(s-s)R8-An/Dbait with RT significantly inhibited the growth of U251 cells in vitro. Thus, ch-K5(s-s)R8-An/Dbait may prove to be a promising gene delivery system to target glioma and enhance the efficacy of RT on U251 cells.

The therapeutic effect in gliomas of nanobubbles carrying siRNA combined with ultrasound-targeted destruction

Background

Nanobubbles (NBs) combined with ultrasound-targeted destruction (UTD) have become promising potential carriers for drug or siRNA delivery. Due to their nano-size, NBs could penetrate tumor blood vessels and accumulate in intercellular spaces so that "sonoporation" induced by UTD would act directly on the tumor cells to increase cell membrane permeability.

Methods

Based on the successful the fabrication of NBs, we synthesized NBs carrying siRNA (NBs-siRNA) by using a biotin-streptavidin system. We then utilized ultrasound irradiation (UI)-targeted NBs-siRNA to improve siRNA transfection and achieve the inhibition of glioma growth.

Results

NBs as carriers combined with UI effectively enhanced siRNA transfection and the effect of silencing targeted genes in vitro. Additionally, a better therapeutic effect was shown in the NBs-siRNA with UI group in vivo compared with that of microbubbles (MBs) with UI or NBs-siRNA without UI.

Conclusion

These results indicated that NBs combined with UTD might be an ideal delivery vector for siRNA to achieve the noninvasive treatment of glioma.

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

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

Chemical modifications of nucleic acid drugs and their delivery systems for gene-based therapy

Gene-based therapy is one of essential therapeutic strategies for precision medicine through targeting specific genes in specific cells of target tissues. However, there still exist many problems that need to be solved, such as safety, stability, selectivity, delivery, as well as immunity. Currently, the key challenges of gene-based therapy for clinical potential applications are the safe and effective nucleic acid drugs as well as their safe and efficient gene delivery systems. In this review, we first focus on current nucleic acid drugs and their formulation in clinical trials and on the market, including antisense oligonucleotide, siRNA, aptamer, and plasmid nucleic acid drugs. Subsequently, we summarize different chemical modifications of nucleic acid drugs as well as their delivery systems for gene-based therapeutics in vivo based on nucleic acid chemistry and nanotechnology methods.

Acid-degradable Dextran as an Image Guided siRNA Carrier for COX-2 Downregulation

Purpose: Effective in vivo delivery of siRNA to silence genes is a highly sought-after goal in the treatment of multiple diseases. Cyclooxygenase-2 (COX-2) is a major mediator of inflammation and its effective and specific downregulation has been of major interest to treat conditions ranging from auto-immune diseases to gastric inflammation and cancer. Here we developed a novel and efficient method to produce a multiple imaging reporter labeled cationic dextran nanopolymer with cleavable positive charge groups for COX-2 siRNA delivery.

Methods: Small molecules containing amine groups were conjugated to the dextran scaffold through acetal bonds that were cleaved in weak acid conditions. With multiple imaging reporters located on different regions of the nanopolymer, cleavage of acetal bonds was visualized and quantified by imaging, for the first time, in cancer cells and tumors.

Results: The biocompatibility of dextran and the rapid cleavage and release of amine groups minimized proinflammatory side effects and COX-2 induction observed with other siRNA carriers, to successfully achieve COX-2 downregulation in cancer cells and tumors. Imaging results confirmed that this nanoplex, consisting of the dextran nanopolymer with COX-2 siRNA, accumulated in tumors, and the amine functional groups were rapidly cleaved in cancer cells and tumors. Along with effective downregulation of COX-2, we also demonstrated, for the first time, effective downregulation of its major product prostaglandin E₂ (PGE₂).

Conclusions: We successfully developed an efficient method to produce an acid-degradable dextran nanopolymer containing cleavable amine groups as the siRNA carrier. Because of its biocompatibility, this degradable dextran delivered COX-2 siRNA within tumors and efficiently downregulated COX-2 expression.

Targeted Delivery of siRNA Therapeutics to Malignant Tumors

Over the past 20 years, a diverse group of ligands targeting surface biomarkers or receptors has been identified with several investigated to target siRNA to tumors. Many approaches to developing tumor-homing peptides, RNA and DNA aptamers, and single-chain variable fragment antibodies by using phage display, in vitro evolution, and recombinant antibody methods could not have been imagined by researchers in the 1980s. Despite these many scientific advances, there is no reason to expect that the ligand field will not continue to evolve. From development of ligands based on novel or existing biomarkers to linking ligands to drugs and gene and antisense delivery systems, several fields have coalesced to facilitate ligand-directed siRNA therapeutics. In this review, we discuss the major categories of ligand-targeted siRNA therapeutics for tumors, as well as the different strategies to identify new ligands.

Polymeric Nanoparticle-Mediated Gene Delivery for Lung Cancer Treatment

In recent years, researchers have focused on targeted gene therapy for lung cancer, using nanoparticle carriers to overcome the limitations of conventional treatment methods. The main goal of targeted gene therapy is to develop more efficient therapeutic strategies by improving the bioavailability, stability, and target specificity of gene therapeutics and to reduce off-target effects. Polymer-based nanoparticles, an alternative to lipid and inorganic nanoparticles, efficiently carry nucleic acid therapeutics and are stable in vivo. Receptor-targeted delivery is a promising approach that can limit non-specific gene delivery and can be achieved by modifying the polymer nanoparticle surface with specific receptor ligands or antibodies. This review highlights the recent developments in gene delivery using synthetic and natural polymer-based nucleic acid carriers for lung cancer treatment. Various nanoparticle systems based on polymers and polymer combinations are discussed. Further, examples of targeting ligands or moieties used in targeted, polymer-based gene delivery to lung cancer are reviewed.

Folate-mediated chemotherapy and diagnostics: An updated review and outlook

Folate receptor (FR) is highly expressed in many types of human cancers, and it has been actively studied for developing targeted chemotherapy and diagnostic agents. Tremendous efforts have been made in developing FR-targeted nanomedicines and nanoprobes and translating them into clinical applications. This article provides a concise review on the latest development of folate-mediated nanomedicines and nanoprobes for chemotherapy and diagnostics with an emphasis on in vivo applications. The cellular uptake mechanisms, pharmacokinetics (PK), administration routes and major challenges in FR-targeted nanoparticles are discussed.

Hypoxia-responsive ionizable liposome delivery siRNA for glioma therapy

Here, we report the hypoxia-responsive ionizable liposomes to deliver small interference RNA (siRNA) anticancer drugs, which can selectively enhance cellular uptake of the siRNA under hypoxic and low-pH conditions to cure glioma. For this purpose, malate dehydrogenase lipid molecules were synthesized, which contain nitroimidazole groups that impart hypoxia sensitivity and specificity as hydrophobic tails, and tertiary amines as hydrophilic head groups. These malate dehydrogenase molecules, together with DSPE-PEG2000 and cholesterol, were self-assembled into O'1,O1-(3-(dimethylamino)propane-1,2-diyl) 16-bis(2-(2-methyl-5-nitro-1H-imidazol-1-yl)ethyl) di(hexadecanedioate) liposomes (MLP) to encapsulate siRNA through electrostatic interaction. Our study showed that the MLP could deliver polo-like kinase 1 siRNA (siPLK1) into glioma cells and effectively enhance the cellular uptake of MLP/siPLK1 because of increased positive charges induced by hypoxia and low pH. Moreover, MLP/siPLK1 was shown to be very effective in inhibiting the growth of glioma cells both in vitro and in vivo. Therefore, the MLP is a promising siRNA delivery system for tumor therapy.

Tumour microenvironment-responsive lipoic acid nanoparticles for targeted delivery of docetaxel to lung cancer

In the present study, we developed a novel type of reduction-sensitive nanoparticles (NPs) for docetaxel (DTX) delivery based on cross-linked lipoic acid NPs (LANPs). The physicochemical properties, cellular uptake and in vitro cytotoxicity of DTX loaded LANPs (DTX-LANPs) on A549 cells were investigated. Furthermore, the in vivo distribution and in vivo efficacy of DTX-LANPs was evaluated. The results showed that DTX-LANPs had a particle size of 110 nm and a negative zeta potential of −35 mv with excellent colloidal stability. LANPs efficiently encapsulated DTX with a high drug loading of 4.51% ± 0.49% and showed remarkable reduction-sensitive drug release in vitro. Cellular uptake experiments demonstrated that LANPs significantly increased intracellular DTX uptake by about 10 fold as compared with free DTX. The cytotoxicity of DTX-LANPs showed significantly higher potency in inhibiting A549 cell growth than free DTX, while blank LANPs had a good biocompatibility. In addition, in vivo experiments demonstrated that DTX-LANPs could enhance tumour targeting and anti-tumour efficacy with low systemic toxicity. In conclusion, LANPs may prove to be a potential tumour microenvironment-responsive delivery system for cancer treatment, with the potential for commercialization due to the simple component, controllable synthesis, stability and economy.

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.

Aptamer-functionalized peptide H3CR5C as a novel nanovehicle for codelivery of fasudil and miRNA-195 targeting hepatocellular carcinoma

Liver cancer is the fifth most commonly diagnosed malignancy, of which hepatocellular carcinoma (HCC) represents the dominating histological subtype. Antiangiogenic therapy aimed at vascular endothelial growth factor (VEGF) has shown promising but deficient clinical prospects on account of vasculogenic mimicry, a highly patterned vascular channel distinguished from the endothelium-dependent blood vessel, which may function as blood supply networks occurring in aggressive tumors including HCC. In this study, we used a new cationic peptide, disulfide cross-linked stearylated polyarginine peptide modified with histidine (H3R5), as a reducible vector, cell penetrating peptide-modified aptamer (ST21) with specific binding to HCC cells to conjugate to peptide H3R5 as the targeting probe, miRNA-195 (miR195) as a powerful gene drug to inhibit VEGF, and fasudil to suppress vasculogenic mimicry by blocking ROCK2, all of which were simultaneously encapsulated in the same nanoparticles. Fasudil was loaded by ammonium sulfate-induced transmembrane electrochemical gradient and miR195 was condensed through electrostatic interaction. ST21-H3R5-polyethylene glycol (PEG) exhibited excellent loading capacities for both fasudil and miR195 with adjustable dosing ratios. Western blot analysis showed that (Fasudil)ST21-H3R5-PEGmiR195 had strong silencing activity of ROCK2 and VEGF, as compared with (Fasudil)H3R5-PEGmiR195. In vitro and in vivo experiments confirmed that ST21-modified nanoparticles showed significantly higher cellular uptake and therapeutic efficacy in tumor cells or tumor tissues than the unmodified counterparts. These findings suggest that aptamer-conjugated peptide holds great promise for delivering chemical drugs and gene drugs simultaneously to overcome HCC.

Dextran functionalization enhances nanoparticle-mediated siRNA delivery and silencing

Understanding the endocytosis and intracellular trafficking of short interfering RNA (siRNA) delivery vehicle complexes remains a critical bottleneck in designing siRNA delivery vehicles for highly active RNA interference (RNAi)-based therapeutics. In this study, we show that dextran functionalization of silica nanoparticles enhanced uptake and intracellular delivery of siRNAs in cultured cells. Using pharmacological inhibitors for endocytotic pathways, we determined that our complexes are endocytosed via a previously unreported mechanism for siRNA delivery in which dextran initiates scavenger receptor-mediated endocytosis through a clathrin/caveolin-independent process. Our findings suggest that siRNA delivery efficiency could be enhanced by incorporating dextran into existing delivery platforms to activate scavenger receptor activity across a variety of target cell types.

Degradable Dextran Nanopolymer as a Carrier for Choline Kinase (ChoK) siRNA Cancer Therapy

Although small interfering RNA (siRNA) therapy has proven to be a specific and effective treatment in cells, the delivery of siRNA is a challenge for the applications of siRNA therapy. We present a degradable dextran with amine groups as an siRNA nano-carrier. In our nano-carrier, the amine groups are conjugated to the dextran platform through the acetal bonds, which are acid sensitive. Therefore this siRNA carrier is stable in neutral and basic conditions, while the amine groups can be cleaved and released from dextran platform under weak acid conditions (such as in endosomes). The cleavage and release of amine groups can reduce the toxicity of cationic polymer and enhance the transfection efficiency. We successfully applied this nano-carrier to deliver choline kinase (ChoK) siRNA for ChoK inhibition in cells.

Reduction-responsive cross-linked stearyl peptide for effective delivery of plasmid DNA

Low efficiency and significant toxicity are the main obstacles to successful gene delivery. We have developed a cationic reduction-responsive vector based on a disulfide cross-linked stearylated polyarginine peptide modified with histidine (C-SHR) for DNA delivery. The structure of the C-SHR was characterized, and the in vitro and in vivo transfection efficiency and cytotoxicity of C-SHR/plasmid DNA complexes were examined. Compared with non-cross-linked stearylated polyarginine peptide (SHR), C-SHR increased the intracellular uptake and dissociation behavior of the complexes. In addition, the gene transfection efficiency of C-SHR/plasmid DNA complexes in HEK293 and HeLa cells was improved and was comparable with that of bPEI-25K/plasmid DNA complexes, and the cytotoxicity of C-SHR was significantly less than that of bPEI-25K. Importantly, the in vivo gene transfection efficiency of C-SHR/plasmid DNA complexes was five fold higher than that of SHR/plasmid DNA complexes, suggesting that C-SHR is an efficient non-viral vector for DNA delivery.

Polymeric nanoparticle-based delivery of microRNA-199a-3p inhibits proliferation and growth of osteosarcoma cells

Our prior screening of microRNAs (miRs) identified that miR-199a-3p expression is reduced in osteosarcoma cells, one of the most common types of bone tumor. miR-199a-3p exhibited functions of tumor cell growth inhibition, suggesting the potential application of miR-199a-3p as an anticancer agent. In the study reported here, we designed and developed a lipid-modified dextran-based polymeric nanoparticle platform for encapsulation of miRs, and determined the efficiency and efficacy of delivering miR-199a-3p into osteosarcoma cells. In addition, another potent miR, let-7a, which also displayed tumor suppressive ability, was selected as a candidate miR for evaluation. Fluorescence microscopy studies and real-time polymerase chain reaction results showed that dextran nanoparticles could deliver both miR-199a-3p and let-7a into osteosarcoma cell lines (KHOS and U-2OS) successfully. Western blotting analysis and 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assays demonstrated that dextran nanoparticles loaded with miRs could efficiently downregulate the expression of target proteins and effectively inhibit the growth and proliferation of osteosarcoma cells. These results demonstrate that a lipid-modified dextran-based polymeric nanoparticle platform may be an effective nonviral carrier for potential miR-based anticancer therapeutics.

Going beyond the liver: progress and challenges of targeted delivery of siRNA therapeutics

Therapeutic gene silencing promises significant progress in pharmacotherapy, including considerable expansion of the druggable target space and the possibility for treating orphan diseases. Technological hurdles have complicated the efficient use of therapeutic oligonucleotides, and siRNA agents suffer particularly from insufficient pharmacokinetic properties and poor cellular uptake. Intense development and evolution of delivery systems have resulted in efficient uptake predominantly in liver tissue, in which practically all nanoparticulate and liposomal delivery systems show the highest accumulation. The most efficacious strategies include liposomes and bioconjugations with N-acetylgalactosamine. Both are in early clinical evaluation stages for treatment of liver-associated diseases. Approaches for achieving knockdown in other tissues and tumors have been proven to be more complicated. Selective targeting to tumors may be enabled through careful modulation of physical properties, such as particle size, or by taking advantage of specific targeting ligands. Significant barriers stand between sufficient accumulation in other organs, including endothelial barriers, cellular membranes, and the endosome. The brain, which is shielded by the blood-brain barrier, is of particular interest to facilitate efficient oligonucleotide therapy of neurological diseases. Transcytosis of the blood-brain barrier through receptor-specific docking is investigated to increase accumulation in the central nervous system. In this review, the current clinical status of siRNA therapeutics is summarized, as well as innovative and promising preclinical concepts employing tissue- and tumor-targeted ligands. The requirements and the respective advantages and drawbacks of bioconjugates and ligand-decorated lipid or polymeric particles are discussed.

Functional nanostructures for effective delivery of small interfering RNA therapeutics

Small interfering RNA (siRNA) has proved to be a powerful tool for target-specific gene silencing via RNA interference (RNAi). Its ability to control targeted gene expression gives new hope to gene therapy as a treatment for cancers and genetic diseases. However, siRNA shows poor pharmacological properties, such as low serum stability, off-targeting, and innate immune responses, which present a significant challenge for clinical applications. In addition, siRNA cannot cross the cell membrane for RNAi activity because of its anionic property and stiff structure. Therefore, the development of a safe, stable, and efficient system for the delivery of siRNA therapeutics into the cytoplasm of targeted cells is crucial. Several nanoparticle platforms for siRNA delivery have been developed to overcome the major hurdles facing the therapeutic uses of siRNA. This review covers a broad spectrum of non-viral siRNA delivery systems developed for enhanced cellular uptake and targeted gene silencing in vitro and in vivo and discusses their characteristics and opportunities for clinical applications of therapeutic siRNA.

Recent advances in characterization of nonviral vectors for delivery of nucleic acids: impact on their biological performance

INTRODUCTION

Nucleic acid delivery is a complex process that requires transport across numerous extracellular and intracellular barriers, whose impact is often neglected during optimization studies. As such, the development of nonviral vectors for efficient delivery would benefit from an understanding of how these barriers relate to the physicochemical properties of lipoplexes and polyplexes.

AREAS COVERED

This review focuses on the evaluation of parameters associated with barriers to delivery such as blood and immune cells compatibility which, as a collective, may serve as a useful prescreening tool for the advancement of nonviral vectors in vivo. An outline of the most relevant rationally developed polyplexes and lipoplexes for clinical application is also given.

EXPERT OPINION

The evaluation of scientifically recognized parameters enabled the identification of systemic delivered nonviral vectors' behavior while in blood as one of the key determinants of vectors function and activity both in vitro and in vivo. This multiparametric approach complements the use of in vitro efficacy results alone for prescreening and improves in vitro-in vivo translation by minimizing false negatives. Further, it can aid in the identification of meaningful structure-function-activity relationships, improve the in vitro screening process of nonviral vectors before in vivo use and facilitate the future development of potent and safe nonviral vectors.

Polyspermine imine, a pH responsive polycationic siRNA carrier degradable to endogenous metabolites

Cationic polymers readily degradable in response to cellular environment are especially favored as easy-formulating materials to pack siRNA into a nanoparticle and to release the cargo in the cytoplasm in time. In addition to the efficiency of cytosomal release, the degradation products should best be free of safety concerns, a typical challenge for cationic polymers. To satisfy the two criteria, we report a new cationic polymer, polyspermine imine, named as PSP-Imine, which is formed by condensing two endogenous molecules, spermine and glyoxal, through conjugated π linkage, -N═C-C═N- (Schiff base reaction), a poly linkage structure sufficiently stable under neutral condition but dissociative under the endosomal pH. Cellular assays under a confocal microscope indicated that the polyplex formed of PSP-Imine readily released the loaded siRNA to the cytoplasm after being engulfed in the target cells and efficiently silenced the target genes in different cell lines and xenograft mouse model of human cervical carcinoma, as compared with nondegradable PEI 25 kDa. Cell viability assays confirmed that PSP-Imine showed no visible cytotoxicity within the concentration being tested. The present study suggests that PSP-Imine is an excellent siRNA condensing material for forming the core of a therapeutically feasible synthetic carrier system.

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.

Recent In Vivo Evidences of Particle-Based Delivery of Small-Interfering RNA (siRNA) into Solid Tumors

Small-interfering RNA (siRNA) is both a powerful tool in research and a promising therapeutic platform to modulate expression of disease-related genes. Malignant tumors are attractive disease targets for nucleic acid-based therapies. siRNA directed against oncogenes, and genes driving metastases or angiogenesis have been evaluated in animal models and in some cases, in humans. The outcomes of these studies indicate that drug delivery is a significant limiting factor. This review provides perspectives on in vivo validated nanoparticle-based siRNA delivery systems. Results of recent advances in liposomes and polymeric and inorganic formulations illustrate the need for mutually optimized attributes for performance in systemic circulation, tumor interstitial space, plasma membrane, and endosomes. Physiochemical properties conducive to efficient siRNA delivery are summarized and directions for future research are discussed.

Dendronized nanoconjugates of lysine and folate for treatment of cancer

Poly-L-lysine (PLL) dendrimers are currently being investigated as antiangiogenic agent for therapy of cancer. In this study, we report folate conjugated poly-l-lysine dendrimers (FPLL) as an efficient carrier for model anticancer drug, doxorubicin hydrochloride (Dox); for pH sensitive drug release, selective targeting to cancer cells, anticancer activity and antiangiogenic activity. This nanoconjugate of Dox showed initial rapid in vitro release followed by gradual slow release, and the drug release was found to be pH sensitive with greater release at acidic pH. In the CAM assay and tubule formation assay with HUVEC, Dox-FPLL formulation showed the significant antiangiogenic activity confirming that activity of PLL was not compromised by the presence of Dox and folic acid. The ex vivo investigations with human breast cancer cell lines MCF-7 showed enhanced cytotoxicity of Dox-FPLL with significantly enhanced intracellular uptake (p<0.001). The in vivo therapeutic potential of nanoconjugate was determined in MCF-7 breast cancer xenograft model in tumor-bearing mice. Dox-FPLL increased the concentration of Dox in tumor by 121.5-fold after 24 h in comparison with free Dox formulation. The folate conjugated dendrimeric Dox showed superior anti-tumor activity in tumor xenograft model with significantly prolonged survival determined by Kaplan Meier survival analysis (p<0.001).

Small interfering RNA nunchucks with a hydrophobic linker for efficient intracellular delivery

The high stiffness and low spatial charge density of siRNA limit the effectiveness of the electrostatic condensation of siRNA with cationic polyelectrolytes. Here, a facile method to stabilize nanoscale siRNA/polyelectrolyte complexes by introducing a reductively cleavable alkyl chain to siRNA as a hybrophobic linker of dimeric siRNA conjugates is reported. The increased length of the hydrophobic linker increases the intracellular translocation and gene silencing activity of the dimeric siRNA conjugates when they are complexed with linear polyethylenimine (LPEI). The results suggest that the introduction of a hydrophobic linker in the dimeric siRNA conjugates can facilitate the intracellular delivery of siRNA through effective condensation with cationic polyelectrolytes.