Gene silencing efficiency of siRNA-PEG conjugates: effect of PEGylation site and PEG molecular weight

A cathepsin B/GSH dual-responsive fluorinated peptide for effective siRNA delivery to cancer cells

Small interfering RNA (siRNA) can be exploited to silence specific genes associated with cancer development, and successful siRNA therapy is highly dependent on the efficiency of the siRNA delivery vector. Herein, a well-designed novel redox- and enzyme-responsive fluorinated polyarginine (PFC-PR) was developed to be used as an anti-cancer siRNA carrier. The multiple guanidine groups could provide positive charges and bind with siRNA efficiently, and further fluorination modification enhanced the interaction with siRNA, resulting in a more stable PFC-PR/siRNA nanocomplex, improving serum tolerance, and promoting cellular uptake and endosome escape. Meanwhile, the PFC-PR was responsive to overexpressed cathepsin B and high levels of glutathione in cancer cells, conferring its ability to enhance siRNA release within cancer cells and making it cancer-targeting. Consequently, PFC-PR showed good biocompatibility and high gene silencing efficiency, which could inhibit cancer cell growth when delivered the siRNA targeting vascular endothelial growth factor, suggesting that it can be potentially used for anti-cancer gene therapy applications.

PK-modifying anchors significantly alter clearance kinetics, tissue distribution, and efficacy of therapeutics siRNAs

Effective systemic delivery of small interfering RNAs (siRNAs) to tissues other than liver remains a challenge. siRNAs are small (∼15 kDa) and therefore rapidly cleared by the kidneys, resulting in limited blood residence times and tissue exposure. Current strategies to improve the unfavorable pharmacokinetic (PK) properties of siRNAs rely on enhancing binding to serum proteins through extensive phosphorothioate modifications or by conjugation of targeting ligands. Here, we describe an alternative strategy for enhancing blood and tissue PK based on dynamic modulation of the overall size of the siRNA. We engineered a high-affinity universal oligonucleotide anchor conjugated to a high-molecular-weight moiety, which binds to the 3' end of the guide strand of an asymmetric siRNA. Data showed a strong correlation between the size of the PK-modifying anchor and clearance kinetics. Large 40-kDa PK-modifying anchors reduced renal clearance by ∼23-fold and improved tissue exposure area under the curve (AUC) by ∼26-fold, resulting in increased extrahepatic tissue retention (∼3- to 5-fold). Furthermore, PK-modifying oligonucleotide anchors allowed for straightforward and versatile modulation of blood residence times and biodistribution of a panel of chemically distinct ligands. The effects were more pronounced for conjugates with low lipophilicity (e.g., N-Acetylgalactosamine [GalNAc]), where significant improvement in uptake by hepatocytes and dose-dependent silencing in the liver was observed.

Translational Applications of Hydrogels

Advances in hydrogel technology have unlocked unique and valuable capabilities that are being applied to a diverse set of translational applications. Hydrogels perform functions relevant to a range of biomedical purposes-they can deliver drugs or cells, regenerate hard and soft tissues, adhere to wet tissues, prevent bleeding, provide contrast during imaging, protect tissues or organs during radiotherapy, and improve the biocompatibility of medical implants. These capabilities make hydrogels useful for many distinct and pressing diseases and medical conditions and even for less conventional areas such as environmental engineering. In this review, we cover the major capabilities of hydrogels, with a focus on the novel benefits of injectable hydrogels, and how they relate to translational applications in medicine and the environment. We pay close attention to how the development of contemporary hydrogels requires extensive interdisciplinary collaboration to accomplish highly specific and complex biological tasks that range from cancer immunotherapy to tissue engineering to vaccination. We complement our discussion of preclinical and clinical development of hydrogels with mechanical design considerations needed for scaling injectable hydrogel technologies for clinical application. We anticipate that readers will gain a more complete picture of the expansive possibilities for hydrogels to make practical and impactful differences across numerous fields and biomedical applications.

Deliver the promise: RNAs as a new class of molecular entities for therapy and vaccination

The concepts of developing RNAs as new molecular entities for therapies have arisen again and again since the discoveries of antisense RNAs, direct RNA-protein interactions, functional noncoding RNAs, and RNA-directed gene editing. The feasibility was demonstrated with the development and utilization of synthetic RNA agents to selectively control target gene expression, modulate protein functions or alter the genome to manage diseases. Rather, RNAs are labile to degradation and cannot cross cell membrane barriers, making it hard to develop RNA medications. With the development of viable RNA technologies, such as chemistry and pharmaceutics, eight antisense oligonucleotides (ASOs) (fomivirsen, mipomersen, eteplirsen, nusinersen, inotersen, golodirsen, viltolarsen and casimersen), one aptamer (pegaptanib), and three small interfering RNAs (siRNAs) (patisiran, givosiran and lumasiran) have been approved by the United States Food and Drug Administration (FDA) for therapies, and two mRNA vaccines (BNT162b2 and mRNA-1273) under Emergency Use Authorization for the prevention of COVID-19. Therefore, RNAs have become a great addition to small molecules, proteins/antibodies, and cell-based modalities to improve the public health. In this article, we first summarize the general characteristics of therapeutic RNA agents, including chemistry, common delivery strategies, mechanisms of actions, and safety. By overviewing individual RNA medications and vaccines approved by the FDA and some agents under development, we illustrate the unique compositions and pharmacological actions of RNA products. A new era of RNA research and development will likely lead to commercialization of more RNA agents for medical use, expanding the range of therapeutic targets and increasing the diversity of molecular modalities.

A Trojan-Horse Strategy by In Situ Piggybacking onto Endogenous Albumin for Tumor-Specific Neutralization of Oncogenic MicroRNA

MicroRNAs (miRNAs), a recently discovered class of noncoding RNAs, play pivotal roles in regulating fundamental biological processes by suppressing the expression of target genes. Aberrant miRNA expression is commonly correlated with human diseases, including cancers. Anti-miRNA oligonucleotides provide an innovative therapeutic strategy for silencing disease-associated miRNAs. However, the clinical application of anti-miRNA therapy has been limited by formulation challenges and physiological delivery barriers. Here, to provide the safe and effective tumor-targeted delivery of anti-miRNAs, we designed carrier-free maleimide-functionalized anti-miRNAs (MI-Anti-miRNAs) that enable "piggybacking" onto albumin in vivo. These functionalized MI-Anti-miRNAs covalently bind to cysteine-34 of endogenous albumin within minutes. In addition to resulting in a markedly extended blood circulation lifetime, this strategy allows MI-Anti-miRNAs to "hitchhike" to the tumor site. Importantly, in situ-generated albumin-Anti-miRNAs are capable of intracellularly internalizing highly negatively charged anti-miRNA molecules and knocking down target miRNAs. In particular, MI-Anti-miRNAs that targeted miRNA-21, which is involved in tumor initiation, progression, invasion, and metastasis in several types of cancer, successfully repressed miRNA-21 activity, resulting in a superior antitumor activity in both solid and metastatic tumor models without causing systemic toxicity. This endogenous albumin-piggybacking approach using MI-Anti-miRNAs provides a simple and broadly applicable platform strategy for the systemic delivery of anti-miRNA therapeutics.

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.

Tumor microenvironment as the "regulator" and "target" for gene therapy

In this review, we focus on strategies for designing functional nano gene carriers, as well as choosing therapeutic genes targeting the tumor microenvironment. Gene mutations have a great impact on the occurrence of cancer. Thus, gene therapy plays a major role in cancer therapy and has the potential to cure cancer. Well-designed gene therapy largely relies on effective gene carriers, which can be divided into viral carriers and non-viral carriers. A gene carrier delivers functional genes to their intracellular target and avoids nucleic acids being degraded by nucleases in the serum. Most conventional cancer gene therapies only target cancer cells and do not appear to be sufficintly efficient to pass clinical trials. Accumulating evidence has shown that extending the therapeutic strategies to the tumor microenvironment, rather than the tumor cell itself, can allow more options for achieving robust anti-cancer efficiency. In addition, unusual features between tumor microenvironment and normal tissues, such as a lower pH, higher glutathione and reactive oxygen species concentrations, and overexpression of some enzymes, facilitate the design of smart stimuli-responsive gene carriers regulated by the tumor microenvironment. These carriers interact with nucleic acids and then form stable nanoparticles under physiological conditions. By regulation of the tumor microenvironment, stimuli-responsive gene carriers are able to change their properties and achieve high gene delivery efficiency. Considering the tumor microenvironment as the "regulator" and "target" when designing gene carriers and choosing therapeutic genes shows significant benefit with respect to improving the accuracy and efficiency of cancer gene therapy.

The Functional Role of Prostate Cancer Metastasis-related Micro-RNAs

The mortality of patients with hormone-resistant prostate cancer can be ascribed to a large degree to metastasis to distant organs, predominantly to the bones. In this review, we discuss the contribution of micro-RNAs (miRs) to the metastatic process of prostate cancer. The criteria for selection of miRs for this review were the availability of preclinical in vivo metastasis-related data in conjunction with prognostic clinical data. Depending on their function in the metastatic process, the corresponding miRs are up- or down-regulated in prostate cancer tissues when compared to matching normal tissues. Up-regulated miRs preferentially target suppressors of cytokine signaling or tumor suppressor-related genes and metastasis-inhibitory transcription factors. Down-regulated miRs promote epithelial-mesenchymal transition or mesenchymal-epithelial transition and diverse pro-metastatic signaling pathways. Some of the discussed miRs exert their function by simultaneously targeting epigenetic pathways as well as cell-cycle-related, anti-apoptotic and signaling-promoting targets. Finally, we discuss potential therapeutic options for the treatment of prostate cancer-related metastases by substitution or inhibition of miRs.

MicroRNAs as Potential Targets for Therapeutic Intervention With Metastasis of Non-small Cell Lung Cancer

The death toll of non-small cell lung cancer (NSCLC) patients is primarily due to metastases, which are poorly amenable to therapeutic intervention. In this review we focus on miRs associated with metastasis of NSCLC as potential new targets for anti-metastatic therapy. We discuss miRs validated as therapeutic targets by in vitro data, identification of target(s) and pathway(s) and in vivo efficacy data in at least one clinically-relevant metastasis-related model. A few of the discussed miRs correlate with the clinical status of NSCLC patients. Using miRs as therapeutic agents has the advantage that targeting a single miR can potentially interfere with several metastatic pathways. Depending on their mode of action, the corresponding miRs can be up- or down-regulated compared to normal matching tissues. Here, we describe therapeutic approaches for reconstitution therapy and miR inhibition, general principles of anti-metastatic therapy as well as current technical pitfalls.

PEGylation of therapeutic oligonucletides: From linear to highly branched PEG architectures

PEGylation, the attachment of poly(ethylene glycol) (PEG), has been adopted to improve the pharmacokinetic properties of oligonucleotide therapeutics for nearly 30 years. Prior efforts mainly focused on the investigation of linear or slightly branched PEG having different molecular weights, terminal functional groups, and possible oligonucleotide sites for functionalization. Recent studies on highly branched PEG (including brush, star, and micellar structures) indicate superior properties in several areas including cellular uptake, gene regulation efficacy, reduction of side effects, and biodistribution. This review focuses on comparing the effects of PEG architecture on the physiochemical and biological properties of the PEGylated oligonucleotide.

CpG-PEG Conjugates and their Immune Modulating Effects after Systemic Administration

PURPOSE

Synthetic oligodeoxynucleotides (ODN) containing unmethylated CpG motifs were found to be able to target cells that express Toll-like receptor 9 to modulate innate and adaptive immune reactions. But their in vivo application in immunotherapy against cancer has not been successful. We attempted in this study to examine polyethylene-glycol (PEG) conjugated CpG ODNs and investigated their mechanism of immune modulation in anti-cancer therapy.

METHODS

CpG-PEG conjugates with different PEG lengths were synthesized. In vitro activity as well as in vivo pharmacokinetics and pharmacodynamics properties were evaluated.

RESULTS

CpG-PEG20Ks were found to be able to persist longer in circulation and activate various downstream effector cells. After intravenous injection, they resulted in higher levels of IL-12p70 in the circulation and lower M-MDSC infiltrates in the tumor microenvironment. Such activities were different from those of CpG ODNs without PEGylation, suggesting different PK-PD profiles systemically and locally.

CONCLUSIONS

Our data support the development of CpG-PEGs as a new therapeutic agent that can be systemically administered to modulate immune responses and the microenvironment in tumor tissues.

The Role of micro RNAs in Breast Cancer Metastasis: Preclinical Validation and Potential Therapeutic Targets

Despite the approval of several molecular therapies in the last years, breast cancer-associated death ranks as the second highest in women. This is due to metastatic disease, which represents a challenge for treatment. A better understanding of the molecular mechanisms of metastasis is, therefore, of paramount importance. In this review we summarize the role of micro RNAs (miRs) involved in metastasis of breast cancer. We present an overview on metastasis-promoting, -suppressing and context-dependent miRs with both activities. We have categorized the corresponding miRs according to their target classes, interaction with stromal cells or exosomes. The pathways affected by individual miRs are outlined in regard to in vitro properties, activity in metastasis-related in vivo models and clinical significance. Current approaches that may be suitable for therapeutic inhibition or restauration of miR activity are outlined. Finally, we discuss the delivery bottlenecks which present as a major challenge in nucleic acid (miR)-based therapies.

Enhanced Intracellular Delivery of siRNA by Controlling ATP-Responsivity of Phenylboronic Acid-Functionalized Polyion Complex Micelles

Intracellular delivery of small interfering RNA (siRNA) is a long-standing challenge in oligonucleotide therapeutics. Herein, adenosine triphosphate (ATP)-responsive polyion complex micelles assembled from poly(ethylene glycol)-block-poly(l-lysine) (PEG-PLys) bearing 4-carboxy-3-fluorophenylboronic acid (FPBA) moiety in the PLys side chains (FPBA micelle) for the delivery of cholesterol-modified siRNA (Chol-siRNA) are described. The pK_a of FPBA moiety is 7.2 and, therefore, it exists in equilibrium between negatively charged tetravalent and noncharged hydrophobic trivalent forms in physiological pH conditions. Each form cooperatively stabilizes the micelle in distinct modes, that is, a covalent ester-linkage between charged boronate and ribose functionality at 3' ends of Chol-siRNA and a hydrophobic interaction between noncharged boronic acid and Chol-siRNA. When exposed to ATP at a concentration associated with the intracellular environment, the Chol-siRNA/boronate linkage is readily cleaved to facilitate the release of Chol-siRNA into cytoplasm. In order to further optimize this switching capability, the effect of FPBA modification rate is studied for the resulting ATP-responsive behavior of the micelles. As a result, the range of 23-35% in the modification rate is found suitable to maximize the gene silencing efficiency, demonstrating the potential of the FPBA-modified micelles as ATP-responsive smart siRNA carrier systems.

Cyclodextrin-siRNA conjugates as versatile gene silencing agents

Functional siRNAs (luciferase and PLK1) have been conjugated to β-cyclodextrin and the ability of the conjugates to retain gene knockdown activity has been assessed by delivery to cancer cell lines using various formulations. Initially two formulations used complexation with polycations, namely Lipofectamine 2000 and an amphiphilic polycationic cyclodextrin. Gene knockdown results for human glioblastoma cells (U87) and prostate cancer cells (PC3, DU145) showed that conjugation to the cyclodextrin did not reduce gene silencing by the RNA. A third mode of delivery involved formation of targeted nanoparticles in which the conjugate was first complexed with adamantyl-PEG-ligands (targeting ligand RVG peptide or dianisamide) by adamantyl inclusion in the cyclodextrin cavities of the conjugates, followed by charge neutralisation with the cationic polymer chitosan. Enhanced knockdown was achieved by these ligand-targeted formulations. In summary, while this study illustrated the gene silencing efficacy of a simple cyclodextrin-siRNA conjugate it is envisaged that future studies will explore the use of conjugates with a modified cyclodextrin which would be self-delivering. Detailed data such as stability, lysosomal escape etc. will then be reported for each conjugate, since this will be appropriate for conjugates which are intended to exploit, rather than merely demonstrate, the concept. The present paper was intended to demonstrate the viability and generality of this novel concept.

Reductive nanocomplex encapsulation of cRGD-siRNA conjugates for enhanced targeting to cancer cells

In this study, through covalent conjugation and lipid material entrapment, a combined modification strategy was established for effective delivery of small interfering RNA (siRNA). Single strands of siRNA targeting to BRAF^V600E gene (siMB3) conjugated with cRGD peptide at 3'-terminus or 5'-terminus via cleavable disulfide bond was synthesized and then annealed with corresponding strands to obtain single and bis-cRGD-siRNA conjugates. A cationic lipid material (CLD) developed by our laboratory was mixed with the conjugates to generate nanocomplexes; their uniformity and electrical property were revealed by particle size and zeta potential measurement. Compared with CLD/siBraf, CLD/cRGD-siBraf achieved higher cell uptake and more excellent tumor-targeting ability, especially 21 (sense-5'/antisense-3″-cRGD-congjugate) nanocomplex. Moreover, they can regulate multiple pathways to varying degree and reduce acidification of endosome. Compared with the gene silencing of different conjugates, single or bis-cRGD-conjugated siRNA showed little differences except 22 (5/5) which cRGD was conjugated at 5'-terminus of antisense strand and sense strand. However bis-cRGD conjugate 21 nanocomplex exhibited better specific target gene silencing at multiple time points. Furthermore, the serum stabilities of the bis-cRGD conjugates were higher than those of the single-cRGD conjugates. In conclusion, all these data suggested that CLD/bis-conjugates, especially CLD/21, can be an effective system for delivery of siRNA to target BRAF^V600E gene for therapy of melanoma.

High-Yield Synthesis of Monomeric LMWP(CPP)-siRNA Covalent Conjugate for Effective Cytosolic Delivery of siRNA

Because of the unparalleled efficiency and universal utility in treating a variety of disease types, siRNA agents have evolved as the future drug-of-choice. Yet, the inability of the polyanionic siRNA macromolecules to cross the cell membrane remains as the bottleneck of possible clinical applications. With the cell penetrating peptides (CPP) being discovered lately, the most effective tactic to achieve the highest intracellular siRNA delivery deems to be by covalently conjugating the drug to a CPP; for instance, the arginine-rich Tat or low molecular weight protamine (LMWP) peptides. However, construction of such a chemical conjugate has been referred by scientists in this field as the “Holy Grail” challenge due to self-assembly of the cationic CPP and anionic siRNA into insoluble aggregates that are deprived of the biological functions of both compounds. Based on the dynamic motion of PEG, we present herein a concise coupling strategy that is capable of permitting a high-yield synthesis of the cell-permeable, cytosol-dissociable LMWP-siRNA covalent conjugates. Cell culture assessment demonstrates that this chemical conjugate yields by far the most effective intracellular siRNA delivery and its corresponded gene-silencing activities. This work may offer a breakthrough advance towards realizing the clinical potential of all siRNA therapeutics and, presumably, most anionic macromolecular drugs such as anti-sense oligonucleotides, gene compounds, DNA vectors and proteins where conjugation with the CPP encounters with problems of aggregation and precipitation. To this end, the impact of this coupling technique is significant, far-reaching and wide-spread.

Current preclinical small interfering RNA (siRNA)-based conjugate systems for RNA therapeutics

Recent promising clinical results of RNA therapeutics have drawn big attention of academia and industries to RNA therapeutics and their carrier systems. To improve their feasibility in clinics, systemic evaluations of currently available carrier systems under clinical trials and preclinical studies are needed. In this review, we focus on recent noticeable preclinical studies and clinical results regarding siRNA-based conjugates for clinical translations. Advantages and drawbacks of siRNA-based conjugates are discussed, compared to particle-based delivery systems. Then, representative siRNA-based conjugates with aptamers, peptides, carbohydrates, lipids, polymers, and nanostructured materials are introduced. To improve feasibility of siRNA conjugates in preclinical studies, several considerations for the rational design of siRNA conjugates in terms of cleavability, immune responses, multivalent conjugations, and mechanism of action are also presented. Lastly, we discuss lessons from previous preclinical and clinical studies related to siRNA conjugates and perspectives of their clinical applications.

Oligonucleotide conjugates - Candidates for gene silencing therapeutics

The potential therapeutic and diagnostic applications of oligonucleotides (ONs) have attracted great attention in recent years. The capability of ONs to selectively inhibit target genes through antisense and RNA interference mechanisms, without causing un-intended sideeffects has led them to be investigated for various biomedical applications, especially for the treatment of viral diseases and cancer. In recent years, many researchers have focused on enhancing the stability and target specificity of ONs by encapsulating/complexing them with polymers or lipid chains to formulate nanoparticles/nanocomplexes/micelles. Also, chemical modification of nucleic acids has emerged as an alternative to impart stability to ONs against nucleases and other degrading enzymes and proteins found in blood. In addition to chemically modifying the nucleic acids directly, another strategy that has emerged, involves conjugating polymers/peptide/aptamers/antibodies/proteins, preferably to the sense strand (3'end) of siRNAs. Conjugation to the siRNA not only enhances the stability and targeting specificity of the siRNA, but also allows for the development of self-administering siRNA formulations, with a much smaller size than what is usually observed for nanoparticle (∼200nm). This review concentrates mainly on approaches and studies involving ON-conjugates for biomedical applications.

Anisamide-targeted gold nanoparticles for siRNA delivery in prostate cancer - synthesis, physicochemical characterisation and in vitro evaluation

Metastatic prostate cancer is a leading cause of cancer-related death in men and current chemotherapies are largely inadequate in terms of efficacy and toxicity. Hence improved treatments are required. The application of siRNA as a cancer therapeutic holds great promise. However, translation of siRNA into the clinic is dependent on the availability of an effective delivery system. Gold nanoparticles (AuNPs) are known to be effective and non-toxic siRNA delivery agents. In this study, a stable gold nanosphere coated with poly(ethylenimine) (PEI) was prepared to yield PEI capped AuNPs (Au-PEI). The PEI was further conjugated with the targeting ligand anisamide (AA, is known to bind to the sigma receptor overexpressed on the surface of prostate cancer cells) to produce an anisamide-targeted nanoparticle (Au-PEI-AA). The resulting untargeted and targeted nanoparticles (Au-PEI and Au-PEI-AA respectively) were positively charged and efficiently complexed siRNA. Au-PEI-AA mediated siRNA uptake into PC3 prostate cancer cells via binding to the sigma receptor. In addition, the Au-PEI-AA·siRNA complexes resulted in highly efficient knockdown of the RelA gene (∼70%) when cells were transfected in serum-free medium. In contrast, no knockdown was observed in the presence of serum, suggesting that adsorption of serum proteins inhibits the binding of the anisamide moiety to the sigma receptor. This study provides (for the first time) proof of principle that anisamide-labelled gold nanoparticles can target the sigma receptor. Further optimisation of the formulation to increase serum stability will enhance its potential to treat prostate cancer.

Therapeutic oligonucleotides with polyethylene glycol modifications

In the field of oligonucleotide drugs, the attachment of PEG is a well-established strategy to prevent enzymatic degradation and avoid renal elimination. Pegaptanib and other oligonucleotides in clinical development utilize the attachment of linear or branched high molecular weight PEG chains for increase of accumulation and duration of the effect after local or systemic application. The length of PEG chains is decisive for the pharmacokinetic and pharmacodynamic effects. Longer chains increase circulation times, but generally decrease gene-silencing efficiencies for antisense and siRNA agents and binding affinities for aptamers. Shorter chains are less efficient in preventing renal filtration, but have also less impact on the gene-silencing machinery and binding kinetics.

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.

Cationic amphiphilic macromolecule (CAM)-lipid complexes for efficient siRNA gene silencing

The accumulated evidence has shown that lipids and polymers each have distinct advantages as carriers for siRNA delivery. Composite materials comprising both lipids and polymers may present improved properties that combine the advantage of each. Cationic amphiphilic macromolecules (CAMs) containing a hydrophobic alkylated mucic acid segment and a hydrophilic poly(ethylene glycol) (PEG) tail were non-covalently complexed with two lipids, 1,2-dioleoyl-sn-glycero-3-phosphoethanolamine (DOPE) and 1,2-dioleoyl-3-trimethylammonium-propane (DOTAP), to serve as a siRNA delivery vehicle. By varying the weight ratio of CAM to lipid, cationic complexes with varying compositions were obtained in aqueous media and their properties evaluated. CAM-lipid complex sizes were relatively independent of composition, ranging from 100 to 200nm, and zeta potentials varied from 10 to 30mV. Transmission electron microscopy confirmed the spherical morphology of the complexes. The optimal N/P ratio was 50 as determined by electrophoretic mobility shift assay. The ability to achieve gene silencing was evaluated by anti-luciferase siRNA delivery to a U87-luciferase cell line. Several weight ratios of CAM-lipid complexes were found to have similar delivery efficiency compared to the gold standard, Lipofectamine. Isothermal titration calorimetry revealed that siRNA binds more tightly at pH=7.4 than pH=5 to CAM-lipid (1:10 w/w). Further intracellular trafficking studies monitored the siRNA escape from the endosomes at 24h following transfection of cells. The findings in the paper indicate that CAM-lipid complexes can serve as a novel and efficient siRNA delivery vehicle.

Chemically defined polyethylene glycol siRNA conjugates with enhanced gene silencing effect

The therapeutic application of siRNA suffers from poor bioavailability caused by rapid degradation and elimination. The covalent attachment of PEG is a universal concept to increase molecular size and enhance the pharmacokinetic properties of biomacromolecules. We devised a facile approach for attachment of PEG molecules with a defined molecular weight, and successful purification of the resulting conjugates. We directly conjugated structurally defined PEG chains with twelve ethylene glycol units to the 3'-terminal hydroxyl group of both sense and antisense strands via an aminoalkyl linker. The conjugates were easily purified by HPLC and successful PEGylation and molecule integrity were confirmed by ESI-MS. The evaluation of in vitro gene knockdown of two different targets in MCF-7 breast cancer cells showed stable pharmacologic activity when combined with a standard transfection reagent. Sense strand PEGylation even increased the silencing potency of a CRCX4-siRNA which had modest activity in its wild-type form. The results indicate that PEG chains at the 3'-terminus of both strands of siRNA are well tolerated by the RNAi effector. The attachment of short, chemically defined PEG chains is a feasible approach to improve the pharmacokinetic properties of siRNA, and can be combined with other targeted and untargeted delivery vehicles.

Enzymatic formation of PEGylated oligonucleotides

Gene therapy, siRNA, and therapeutic aptamers attract great interest owing to their versatility to treat a wide range of diseases and their potential high selectivity. Unfortunately, oligonucleotide-based therapeutics suffer rapid degradation by nucleases, scarce cell internalization, and fast kidney clearance. To address these limitations, the covalent attachment by mild chemical reactions of an activated polyethylene glycol (PEG) is widely used to obtain PEGylated nucleic acids showing a more favorable pharmacokinetic profile. We describe here a method for the enzymatic formation of PEGylated nucleic acids employing T4 DNA ligase: the ligation protocol was set up and optimized allowing the complete achievement of PEGylated oligonucleotides amenable to further enzymatic reactions. The feasibility of this approach for bioconjugation was demonstrated employing a set of PEG-donors and oligonucleotide acceptors, differing in the chemical link between PEG and the oligonucleotide donor, and in the length, sequence, and structure of the oligonucleotides employed. The ligase reaction allowed us to obtain double-stranded as well as single-stranded oligonucleotides, thus demonstrating the applicability of the method to a variety of substrates suitable for diagnostic and therapeutic applications.

Nanoparticles for gene delivery

Nanocarriers are a new type of nonviral gene carriers, many of which have demonstrated a broad range of pharmacological and biological properties, such as being biodegradable in the body, stimulus-responsive towards the surrounding environment, and an ability to specifically targeting certain disease sites. By summarizing some main types of nanocarriers, this Concept considers the current status and possible future directions of the potential clinical applications of multifunctional nanocarriers, with primary attention on the combination of such properties as biodegradability, targetability, transfection ability, and stimuli sensitivity.

Cationic and PEGylated Amphiphilic Cyclodextrins: Co-Formulation Opportunities for Neuronal Sirna Delivery

Optimising non-viral vectors for neuronal siRNA delivery presents a significant challenge. Here, we investigate a co-formulation, consisting of two amphiphilic cyclodextrins (CDs), one cationic and the other PEGylated, which were blended together for siRNA delivery to a neuronal cell culture model. Co-formulated CD-siRNA complexes were characterised in terms of size, charge and morphology. Stability in salt and serum was also examined. Uptake was determined by flow cytometry and toxicity was measured by MTT assay. Knockdown of a luciferase reporter gene was used as a measure of gene silencing efficiency. Incorporation of a PEGylated CD in the formulation had significant effects on the physical and biological properties of CD.siRNA complexes. Co-formulated complexes exhibited a lower surface charge and greater stability in a high salt environment. However, the inclusion of the PEGylated CD also dramatically reduced gene silencing efficiency due to its effects on neuronal uptake. The co-formulation strategy for cationic and PEGylated CDs improved the stability of the CD.siRNA delivery systems, although knockdown efficiency was impaired. Future work will focus on the addition of targeting ligands to the co-formulated complexes to restore transfection capabilities.

Reducible hyaluronic acid-siRNA conjugate for target specific gene silencing

Despite wide applications of polymer-drug conjugates, there are only a few polymer-siRNA conjugates like poly(ethylene glycol) conjugated siRNA. In this work, reducible hyaluronic acid (HA)-siRNA conjugate was successfully developed for target specific systemic delivery of siRNA to the liver. The conjugation of siRNA to HA made it possible to form a compact nanocomplex of siRNA with relatively nontoxic linear polyethyleneimine (LPEI). After characterization of HA-siRNA conjugate by size exclusion chromatography (SEC) and gel electrophoresis, its complex formation with LPEI was investigated with a particle analyzer. The HA-siRNA/LPEI complex had a mean particle size of ca. 250 nm and a negative or neutral surface charge at physiological condition. The reducible HA-siRNA/LPEI complex showed a higher in vitro gene silencing efficiency than noncleavable HA-siRNA/LPEI complex. Furthermore, after systemic delivery, apolipoprotein B (ApoB) specific HA-siApoB/LPEI complex was target specifically delivered to the liver, which resulted in statistically significant reduction of ApoB mRNA expression in a dose dependent manner. The HA-siRNA conjugate can be effectively applied as a model system to the treatment of liver diseases using various siRNAs and relatively nontoxic polycations.

Optimized siRNA-PEG conjugates for extended blood circulation and reduced urine excretion in mice

Some of the main concerns with in vivo application of naked small interfering RNA are rapid degradation and urinary excretion resulting in a short plasma half-life. In this study we investigated how conjugation of polyethylene glycol (PEG) with variable chain length affects siRNA pharmacokinetics and biodistribution.

The PEG chains were conjugated to chemically stabilized siRNA at the 5' terminal end of the passenger strand using click chemistry. The siRNA conjugate remained functionally active and showed significantly prolonged circulation in the blood stream after intravenous injection. siRNA conjugated with 20kDa PEG (PEG20k-siRNA) was most persistent, approximately 50% PEG20k-siRNA remained 1h post-injection, while the uncoupled siRNA was rapidly removed >90% at 15min. In vivo fluorescent imaging of the living animal showed increased concentration of siRNA in peripheral tissue and delayed urine excretion when coupled to PEG 20k. Biodistribution studies by northern blotting revealed equal distribution of conjugated siRNA in liver, kidney, spleen and lung without significant degradation 24 h post-injection. Our study demonstrates that PEG conjugated siRNA can be applied as a delivery system to improve siRNA bioavailability in vivo and may potentially increase the efficiency of siRNA in therapeutic applications.

A highly entangled polymeric nanoconstruct assembled by siRNA and its reduction-triggered siRNA release for gene silencing

A nanoconstruct (NC) is developed from a biocompatible natural polymer and siRNA conjugates to deliver small interfering RNA (siRNA) target-specifically without cationic condensation reagents. This study reports a novel siRNA-mediated cross-linked NC produced by hybridizing two complementary single-stranded siRNAs that are conjugated to the polymer dextran via a disulfide linkage. The reducible disulfide bond between the siRNA and polymer allow siRNA release from the NC in the reducible cytoplasmic region after the NC enters the cell. In addition, when the NC contains the prostate-carcinoma-binding peptide aptamer DUP-1, it can selectively deliver siRNA into prostate cancer cells of the PC-3 lines; thus, the newly formulated NC has reduced the cytotoxicity and improved the efficacy of target-specific siRNA delivery. Moreover, this new concept of NCs using biocompatible siRNA and a neutral polymer may provide insightful knowledge for future directions for designing NCs for stimuli-responsive and advanced target-specific siRNA delivery.

Enhancement of gene silencing effect and membrane permeability by Peptide-conjugated 27-nucleotide small interfering RNA

Two different sizes of siRNAs, of which one type was 21-nucleotide (nt) siRNA containing 2-nt dangling ends and the other type was 27-nt siRNA with blunt ends, were conjugated with a nuclear export signal peptide of HIV-1 Rev at the 5'-sense end. Processing by Dicer enzyme, cell membrane permeability, and RNAi efficiency of the peptide-conjugated siRNAs were examined. Dicer cleaved the peptide-conjugated 27-nt siRNA leading to the release of 21-nt siRNA, whereas the peptide-conjugated 21-nt siRNA was not cleaved. High membrane permeability and cytoplasmic localization was found in the conjugates. Moreover, the peptide-conjugated 27-nt siRNA showed increased potency of RNAi in comparison with the nonmodified 21-nt and 27-nt siRNAs, whereas the peptide-conjugated 21-nt siRNA showed decreased RNAi efficacy. This potent RNAi efficacy is probably owing to acceleration of RISC through recognition by Dicer, as well as to the improvement of cell membrane permeability and intracellular accumulation.

Bioreducible polymers for gene silencing and delivery

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

Small-interfering RNA (siRNA)-based functional micro- and nanostructures for efficient and selective gene silencing

Because of RNA's ability to encode structure and functional information, researchers have fabricated diverse geometric structures from this polymer at the micro- and nanoscale. With their tunable structures, rigidity, and biocompatibility, novel two-dimensional and three-dimensional RNA structures can serve as a fundamental platform for biomedical applications, including engineered tissues, biosensors, and drug delivery vehicles. The discovery of the potential of small-interfering RNA (siRNA) has underscored the applications of RNA-based micro- and nanostructures in medicine. Small-interfering RNA (siRNA), synthetic double-stranded RNA consisting of approximately 21 base pairs, suppresses problematic target genes in a sequence-specific manner via inherent RNA interference (RNAi) processing. As a result, siRNA offers a potential strategy for treatment of many human diseases. However, due to inefficient delivery to cells and off-target effects, the clinical application of therapeutic siRNA has been very challenging. To address these issues, researchers have studied a variety of nanocarrier systems for siRNA delivery. In this Account, we describe several strategies for efficient siRNA delivery and selective gene silencing. We took advantage of facile chemical conjugation and complementary hybridization to design novel siRNA-based micro- and nanostructures. Using chemical crosslinkers and hydrophobic/hydrophilic polymers at the end of siRNA, we produced various RNA-based structures, including siRNA block copolymers, micelles, linear siRNA homopolymers, and microhydrogels. Because of their increased charge density and flexibility compared with conventional siRNA, these micro- and nanostructures can form polyelectrolyte complexes with poorly charged and biocompatible cationic carriers that are both more condensed and more homogenous than the complexes formed in other carrier systems. In addition, the fabricated siRNA-based structures are linked by cleavable disulfide bonds for facile generation of original siRNA in the cytosol and for target-specific gene silencing. These newly developed siRNA-based structures greatly enhance intracellular uptake and gene silencing both in vitro and in vivo, making them promising biomaterials for siRNA therapeutics.

Palmitic acid-conjugated 21-nucleotide siRNA enhances gene-silencing activity

Short interfering RNA (siRNA) technology is a powerful tool for suppressing gene expression in mammalian cells. In this study, we focused on the development of siRNAs conjugated with palmitic acid at the 5'-end of the sense strand (C16-siRNAs) using our novel synthesis strategy in order to improve the potency of siRNA. The C16-siRNAs exhibited enhanced nuclease stability. In addition, they showed potent gene-silencing efficacy against exogenous Renilla luciferase in HeLa cells compared with a nonmodified siRNA in the presence of Lipofectamine 2000. The C16-siRNAs also had a more potent inhibitory effect on Renilla luciferase activity than the other siRNA conjugated with lipids at the 5'-end and the 3'-end by palmitoyl conjugation. For further improvement, the gene silencing potency of the C16-siRNAs against the endogenous vascular endothelial growth factor (VEGF) gene in HeLa cells was investigated. In this investigation, the siRNAs were prepared not only with the normal RNA sequence but also coupled with an inverted thymidine (idT) at the 3'-ends of both the sense and antisense strands (siRNA-idT), including palmitic acid conjugations at the 5'-end of the sense strand, to improve stability. The C16-siRNA including idT modifications exhibited a significantly greater inhibitory effect on the VEGF gene in the presence of Lipofectamine 2000. It is noteworthy that C16-siRNA-idT demonstrated long-term gene-silencing efficacy of up to 5 days. Interestingly, the C16-siRNAs, including that with idT modifications, exhibited strong RNAi potency in the absence of any transfection reagents, although only at high concentrations. Both the C16-siRNAs and C16-siRNA-idT induced a high level of membrane permeability in HeLa cells. Our developed C16-siRNAs, particularly C16-siRNA-idT, are thus among the promising candidates for a new generation of modified siRNAs that can solve the many problems associated with siRNA technology.

Innovative strategy for treatment of lung cancer: targeted nanotechnology-based inhalation co-delivery of anticancer drugs and siRNA

A tumor targeted mesoporous silica nanoparticles (MSN)-based drug delivery system (DDS) was developed for inhalation treatment of lung cancer. The system was capable of effectively delivering inside cancer cells anticancer drugs (doxorubicin and cisplatin) combined with two types of siRNA targeted to MRP1 and BCL2 mRNA for suppression of pump and nonpump cellular resistance in non-small cell lung carcinoma, respectively. Targeting of MSN to cancer cells was achieved by the conjugation of LHRH peptide on the surface of MSN via poly(ethylene glycol) spacer. The delivered anticancer drugs and siRNA preserved their specific activity leading to the cell death induction and inhibition of targeted mRNA. Suppression of cellular resistance by siRNA effectively delivered inside cancer cells and substantially enhanced the cytotoxicity of anticancer drugs. Local delivery of MSN by inhalation led to the preferential accumulation of nanoparticles in the mouse lungs, prevented the escape of MSN into the systemic circulation, and limited their accumulation in other organs. The experimental data confirm that the developed DDS satisfies the major prerequisites for effective treatment of non-small cell lung carcinoma. Therefore, the proposed cancer-targeted MSN-based system for complex delivery of drugs and siRNA has high potential in the effective treatment of lung cancer.

Stabilization and photochemical regulation of antisense agents through PEGylation

Oligonucleotides are effective tools for the regulation of gene expression in cell culture and model organisms, most importantly through antisense mechanisms. Due to the inherent instability of DNA antisense agents, various modifications have been introduced to increase the efficacy of oligonucleotides, including phosphorothioate DNA, locked nucleic acids, peptide nucleic acids, and others. Here, we present antisense agent stabilization through conjugation of a poly(ethylene glycol) (PEG) group to a DNA oligonucleotide. By employing a photocleavable linker between the PEG group and the antisense agent, we were able to achieve light-induced deactivation of antisense activity. The bioconjugated PEG group provides stability to the DNA antisense agent without affecting its native function of silencing gene expression via RNase H-catalyzed mRNA degradation. Once irradiated with UV light of 365 nm, the PEG group is cleaved from the antisense agent leaving the DNA unprotected and open for degradation by endogenous nucleases, thereby restoring gene expression. By using a photocleavable PEG group (PhotoPEG), antisense activity can be regulated with high spatial and temporal resolution, paving the way for precise regulation of gene expression in biological systems.

Commercially Supplied Amine-Modified siRNAs May Require Ultrafiltration prior to Conjugation with Amine-Reactive Compounds

Conjugation of siRNA to macromolecules such as serum albumin has multiple potential benefits, including enhanced extravasation via albumin-mediated transcytosis across endothelial cells and reduced renal clearance. In attempting to conjugate siRNA to albumin, we used commercially sourced amine-modified siRNA and reacted it with the heterobifunctional linker succinimidyl 4-[N-maleimidomethyl]cyclohexane-1-carboxylate (SMCC) to introduce a maleimide group suitable for conjugation to the thiol group of the surface-exposed cysteine residue (Cys 34) within albumin. We found the conjugation of the SMCC-treated siRNA to bovine serum albumin (BSA) to be very inefficient and investigated the cause of the low yield of conjugate. Ultrafiltration with phosphate-buffered saline prior to activation with SMCC dramatically increased the yield of siRNA-albumin conjugate (~15-fold). Communication with the commercial supplier revealed that ammonium acetate buffer was used in a desalting step as part of the siRNA purification process prior to supply, likely resulting in ammonium counterions to the siRNA polyanion, which would interfere with conjugation by consuming the SMCC. After ultrafiltration, a greatly reduced amount of SMCC could be used to affect conjugation, without significant reduction in yield. These data indicate that amine-modified siRNA sourced commercially may require ultrafiltration or dialysis prior to use in conjugation reactions.