A Comprehensive Review of Small Interfering RNAs (siRNAs): Mechanism, Therapeutic Targets, and Delivery Strategies for Cancer Therapy

Small interfering RNA (siRNA) delivery by nanocarriers has been identified as a promising strategy in the study and treatment of cancer. Short nucleotide sequences are synthesized exogenously to create siRNA, which triggers RNA interference (RNAi) in cells and silences target gene expression in a sequence-specific way. As a nucleic acid-based medicine that has gained popularity recently, siRNA exhibits novel potential for the treatment of cancer. However, there are still many obstacles to overcome before clinical siRNA delivery devices can be developed. In this review, we discuss prospective targets for siRNA drug design, explain siRNA drug properties and benefits, and give an overview of the current clinical siRNA therapeutics for the treatment of cancer. Additionally, we introduce the siRNA chemical modifications and delivery systems that are clinically sophisticated and classify bioresponsive materials for siRNA release in a methodical manner. This review will serve as a reference for researchers in developing more precise and efficient targeted delivery systems, promoting ongoing advances in clinical applications.

Advancement of cell-penetrating peptides in combating triple-negative breast cancer

Extensive research efforts have been made and are still ongoing in the search for an ideal anti-cancer therapy. Almost all chemotherapeutics require a carrier or vehicle, a drug delivery system that can transport the drug specifically to the targeted cancer cells, sparing normal cells. Cell-penetrating peptides (CPPs) provide an effective and efficient pathway for the intra-cellular transportation of various bioactive molecules in several biomedical therapies. They are now well-recognized as facilitators of intracellular cargo delivery and have excellent potential for targeted anti-cancer therapy. In this review, we explain CPPs, recent progress in the development of new CPPs, and their utilization to transport cargoes such as imaging agents, chemotherapeutics, and short-interfering RNAs (siRNA) into tumor cells, contributing to the advancement of novel tumor-specific delivery systems.

Tumor microenvironment responsive nanocarriers for gene therapy

Stimuli responsive nanocarriers are important non-viral gene carriers for gene therapy. We discuss the stimulus conditions and then highlight various stimuli responsive nanocarriers in the tumor microenvironment for cancer gene therapy. We hope that this review will inspire readers to develop more effective stimuli responsive nanocarriers for delivering genes.

Macrophage membrane- and cRGD-functionalized thermosensitive liposomes combined with CPP to realize precise siRNA delivery into tumor cells

Despite the success of small interfering RNAs (siRNAs) in clinical settings, their fast clearance and poor delivery efficiency to target cells still hinder their therapeutic effect. Herein, a new treatment system was constructed by combining thermosensitive liposomes with the macrophage membrane, tumor-targeting cyclic Arg-Gly-Asp peptide, a cell-penetrating peptide, and thermotherapy. The constructed system was found to be thermosensitive and stable; the proteins were inherited from the macrophage membrane. This new system combined with thermotherapy displayed the least uptake by macrophages, the greatest uptake by HepG2 cells, the most obvious HepG2 cell apoptosis, and the strongest inhibition of Bcl-2 mRNA and Bcl-2 protein in HepG2 cells. Moreover, 24 h after system administration in tumor-bearing mice, the most prominent distribution of siRNA was observed in tumors, while almost no siRNA was found in other organs. The strongest inhibition of Bcl-2 mRNA, Bcl-2 protein, and tumors was found in mice that had received the proposed system. In summary, when using the constructed system both in vitro and in mice, less uptake by the reticuloendothelial system, greater accumulation in tumor cells, and improved therapeutic efficacy were observed. Therefore, this new system can deliver siRNA selectively and efficiently, and it is a promising therapeutic candidate for precise tumor-targeted therapy.

Small Molecule Modifications Significantly Increase the Transfection Efficiency of Low-Molecular Polymer

Gene vectors with high biocompatibility and transfection efficiency are critical for successful gene therapy. PEI 25K (Polyethyleneimine 25K) is a common polymeric gene vector that has been employed as a positive control material in gene transfection studies due to its good performance in endosome escape. PEI 25K's indegradability and abundance of positive charges, on the other hand, cause toxicity in cells, limiting its use. In this study, we developed the PEI-ER non-viral vector by adding an endoplasmic reticulum (ER) targeting ligand to low molecular weight PEI 1.8K. These small molecule modifications dramatically improved PEI transfection efficiency while barely interfering with compatibility. PEI-ER/DNA complexes were discovered to enter the cell via caveolin-mediated endocytosis, avoiding destruction in the endosome. We believe that this little chemical alteration is a simple solution to enhance the efficacy of cationic polymer vectors in gene transport, and it has a lot of medicinal applications.

Co-delivery of EGFR and BRD4 siRNA by cell-penetrating peptides-modified redox-responsive complex in triple negative breast cancer cells

AIMS

Triple negative breast cancer (TNBC) has drawn more and more attention due to its high mitotic indices, high metastatic rate and poor prognosis. Gene therapy, especially RNA interference (RNAi), has become a promising targeted therapy. However, improvement of transfection efficiency and discovery of target genes are major problems for the delivery of small interfering RNAs (siRNA).

MATERIALS AND METHODS

In the present study, we developed GALA- and CREKA-modified PEG-SS-PEI to deliver siRNAs targeting on EGFR and BRD4 for TNBC therapy. The PEG-SS-PEI/siRNA complexes were prepared by electrostatic interaction and characterized by dynamic light scattering (DLS) and transmission electron microscope (TEM). The release characteristic, stability, cellular uptake and intracellular localization of the complexes were also studied. The effect of the complexes on cell viability was measured in MDA-MB-231 and HUVEC cells. The in vitro anti-tumor activities of the complexes were analyzed by Transwell invasion assay and wound healing assay. The gene silencing effect was evaluated by quantitative real time-polymerase chain reaction (qRT-PCR) and western blot.

KEY FINDINGS

The results revealed that the GALA- and CREKA-modified PEG-SS-PEI/siRNA complexes showed excellent transfection efficiency with redox-sensitive release profile and good biological compatibility. The complexes protected siRNA from the degradation of RNA enzymes. The complexes significantly inhibited the proliferation, invasion and migration of MDA-MB-231 cells via the synergistic inhibition of EGFR/PI3K/Akt and BRD4/c-Myc pathways.

SIGNIFICANCE

Taken together, co-delivery of siEGFR and siBRD4 by GALA-PEG-SS-PEI and CREKA-PEG-SS-PEI may provide a more effective strategy for the treatment of TNBC.

Recent advances in the redox-responsive drug delivery nanoplatforms: A chemical structure and physical property perspective

Poor water solubility, off-target toxicity, and small therapeutic window are among major obstacles for the development of drug products. Redox-responsive drug delivery nanoplatforms not only overcome the delivery and pharmacokinetic pitfalls observed in conventional drug delivery, but also leverage the site-specific delivery properties. Cleavable diselenide and disulfide bonds in the presence of elevated reactive oxygen species (ROS) and glutathione concentration are among widely used stimuli-responsive bonds to design nanocarriers. This review covers a wide range of redox-responsive chemical structures and their properties for designing nanoparticles aiming controlled loading, delivery, and release of hydrophobic anticancer drugs at tumor site.

In vitro and in vivo evaluations on nanoparticle and phospholipid hybrid nanoparticles with absorption enhancers for oral insulin delivery

Oral delivery of peptide and proteins is challenging due to their poor physical and chemical stability which usually results in inadequate therapeutic efficacy. Nanoparticles encapsulating insulin was developed by the ionic gelation technique using sulfobutyl ether-β-cyclodextrin as an anionic linker. Phospholipid hybrid nanoparticles were formulated by utilizing ionic gelation and thin-film hydration methods using D-α-Tocopheryl polyethylene glycol 1000 succinate, sodium deoxycholate separately and in combination to take the advantage of liposomes and nanoparticles also various absorption enhancement mechanisms. All formulations were characterized and tested for in vitro gastrointestinal stability, in vitro drug release, and cytotoxicity. On the other hand, in vivo effects of developed formulations on reducing blood glucose levels were monitored for 8 hours. Phospholipid hybrid nanoparticles including D-α-Tocopheryl polyethylene glycol 1000 succinate and sodium deoxycholate in combination with 548.7 nm particle size, 0.332 polydispersity index, 22.0 mV zeta potential, and 61.9% encapsulation efficiency, exhibited desired gastrointestinal stability and insulin release in vitro. In addition, the formulation proved its safety with cytotoxicity studies on L929 cells. The subjected phospholipid hybrid nanoparticle formulation was found to be the most effective formulation by reducing and maintaining blood glucose levels with avoiding fluctuations.

Histidine-Based Reduction-Sensitive Star-Polymer Inclusion Complex as a Potential DNA Carrier: Biophysical Studies Using Time-Resolved Fluorescence as an Important Tool

An ideal DNA carrier is one that is capable of effectively condensing DNA into complexes of optimum size and shape, preventing premature decomplexation in the bloodstream and efficiently releasing the DNA into affected cells. In this context, we have developed a novel β-cyclodextrin (β-CD)-based four-arm star-shaped polymer inclusion complex (IC) with arms made of a poly(l-histidine)-based cationic polymer. The polymer was well characterized by gel permeation chromatography, NMR, and matrix-assisted laser desorption ionization time-of-flight mass spectrometry. We have also investigated its DNA complexation and release properties. Bisadamantane containing a disulfide bond was synthesized that linked two such poly(l-histidine)-containing β-CD units via guest-host interactions to prepare the presented IC. Besides using the conventional steady-state fluorescence spectroscopy, the ability of this IC to condense DNA to form polyplexes and their release behavior have been established by using the time-resolved fluorescence spectroscopy technique. Thiazole orange (TO) was used for the first time as a DNA-intercalating dye in the time-resolved fluorescence spectroscopic study. The superior DNA-condensing ability of the IC as compared to that of the precursor two-arm β-CD and linear poly(l-histidine) of a comparable molecular weight, as confirmed by dynamic light scattering, zeta potential, atomic force microscopy, and gel electrophoresis studies, could be attributed to a higher charge density. The IC-DNA polyplexes were found to be stable in a medium similar to an extracellular fluid but could efficiently release DNA in the presence of 10 mM glutathione, a concentration prevalent in the intracellular fluid of cancer cells. Hence, here, we have successfully demonstrated the synthesis of a novel biocompatible star-shaped IC with the potential to carry and release DNA in cancer cells and also established the feasibility of using the time-resolved fluorescence spectroscopic technique to study the complexation behavior of the polycation and DNA using TO as a DNA-intercalating dye.

Triton X-100-Modified Adenosine Triphosphate-Responsive siRNA Delivery Agent for Antitumor Therapy

Modified polyethyleneimine (PEI) has been widely used as siRNA delivery agents. Here, a new Triton X-100-modified low-molecular-weight PEI siRNA delivery agent is developed together with the coupling of 4-carboxyphenylboronic acid (PBA) and dopamine grafted vitamin E (VEDA). Triton X-100, a nonionic detergent, greatly improves the cellular uptake of siRNA as well as the siRNA escape from endosome/lysosome because of its high transmembrane ability. In addition, the boronate bond between PBA and VEDA of the transfection agent can be triggered to release its entrapped siRNA because of the high level of adenosine triphosphate (ATP) in cancer cells. The transfection agent is successfully applied to deliver siRNAs targeting endogenous genes of epidermal growth factor receptor (EGFR) and kinesin-5 (Eg5) to cancer cells, showing good results on Eg5 and EGFR silencing ability and inhibition of cancer cell migration. Further in vivo study indicates that the Triton X-100-modified transfection agent is also efficient to deliver siRNA to cancer cells and shows significant tumor growth inhibition on mice tumor models. These results indicate that the Triton X-100-modified ATP-responsive transfection agent is a promising gene delivery vector for target gene silencing in vitro and in vivo.

Smart Nanocarriers for the Delivery of Nucleic Acid-Based Therapeutics: A Comprehensive Review

Nucleic acid-based therapies are promising therapeutics for the treatment of several systemic disorders, and they offer an exciting opportunity to address emerging biological challenges. The scope of nucleic acid-based therapeutics in the treatment of multiple disease states including cancers has been widened by recent progress in Ribonucleic acids (RNA) biology. However, cascades of systemic and intracellular barriers, including rapid degradation, renal clearance, and poor cellular uptake, hinder the clinical effectiveness of nucleic acid-based therapies. These barriers can be circumvented by utilizing advanced smart nanocarriers that efficiently deliver and release the encapsulated nucleic acids into the target tissues. This review describes the current status of clinical trials on nucleic acid-based therapeutics and highlights representative examples that provide an overview on the current and emerging trends in nucleic acid-based therapies. A better understanding of the design of advanced nanocarriers is essential to promote the translation of therapeutic nucleic acids into a clinical reality.

Stimuli-Responsive Polysaccharide Enveloped Liposome for Targeting and Penetrating Delivery of survivin-shRNA into Breast Tumor

Silencing the inhibitor of apoptosis (IAP) by RNAi is a promising method for tumor therapy. One of the major challenges lies in how to sequentially overcome the system barriers in the course of the tumor targeting delivery, especially in the tumor accumulation and penetration. Herein we developed a novel stimuli-responsive polysaccharide enveloped liposome carrier, which was constructed by layer-by-layer depositing redox-sensitive amphiphilic chitosan (CS) and hyaluronic acid (HA) onto the liposome and then loading IAP inhibitor survivin-shRNA gene and permeation promoter hyaluronidase (HAase) sequentially. The as-prepared HA/HAase/CS/liposome/shRNA (HCLR) nanocarrier was verified to be stable in blood circulation due to the negative charged HA shield. The tumor targeting recognition and the enhanced tumor accumulation of HCLR were visualized by fluorescence resonance energy transfer (FRET) and in vivo fluorescence biodistribution. The deshielding of HA and the protonizing of CS in slightly acidic tumor extracellular pH environment (pH_e, 6.8-6.5) were demonstrated by ζ potential change from -23.1 to 29.9 mV. The pH_e-responsive HAase release was confirmed in the tumor extracellular mimicking environments, and the intratumoral biodistribution showed that the tumor penetration of HCLR was improved. The cell uptake of HCLR in pH_e environment was significantly enhanced compared with that in physiological pH environment. The increased shRNA release of HCLR was approved in 10 mM glutathione (GSH) and tumor cells. Surprisingly, HCLR suppressed the tumor growth markedly through survivin silencing and meanwhile maintained low toxicity to mice. This study indicates that the novel polysaccharide enveloped HCLR is promising in clinical translation, thanks to the stimuli-triggered tumor accumulation, tumor penetration, cell uptake, and intracellular gene release.

Cytotoxicity of Novel Redox Sensitive PEG2000-S-S-PTX Micelles against Drug-Resistant Ovarian and Breast Cancer Cells

PURPOSE

Since the last decade, it is established that nonspecific delivery of chemotherapeutics fails to effectively treat cancer due to systemic cytotoxicity, poor biodistribution at tumor site and most importantly the development of drug resistance (MDR). Stimuli-sensitive drug delivery systems gained significant attention in recent years for effective tumor therapy and reversal of MDR. The aim of this study was developing a redox sensitive micellar prodrug system, by taking the advantage of the significant difference in GSH levels between extracellular and intracellular environments, but more importantly in healthy and tumor tissues.

METHODS

Redox sensitive PEG₂₀₀₀-S-S-PTX micelles were developed for intracellular paclitaxel delivery and characterized in vitro. In vitro release studies were carried out and followed by cytotoxicity studies in chemo-resistant ovarian and breast cancer cells in various reducing environments for different time periods to confirm their potential.

RESULTS

PEG₂₀₀₀-S-S-PTX, was synthesized and characterized as a redox sensitive micellar prodrug system. The reduction sensitivity and in vitro PTX release properties were confirmed in reducing environments comparatively with physiological conditions. Cytotoxicity studies suggested that ovarian (SK-OV-3) cells could be better candidates for treatment with redox-sensitive drug delivery systems than breast (MCF-7) cancer cells.

CONCLUSIONS

The results of this study highlights the importance of personalized therapy since no fits-for-all system can be developed for different cancer with significantly different metabolic activities. Graphical Abstract Schematic representation of self-assembly of reduction-sensitive PEG2000-S-S-PTX micelles and GSH dependent release of PTX.

A highly GSH-sensitive SN-38 prodrug with an "OFF-to-ON" fluorescence switch as a bifunctional anticancer agent

SN-38 (7-ethyl-10-hydroxy-camptothecin) is an active metabolite of irinotecan (CPT-11) and the most potent camptothecin analogue. In this study, 2,4-dinitrobenzene sulfonyl (DNS) was covalently conjugated as a GSH-sensitive trigger to 10'-OH of SN-38 to yield a GSH-sensitive prodrug, denoted as DNS-SN38, with virtually quenched fluorescence due to donor-excited photo-induced electron transfer (d-PeT). By investigating DNS-SN38's activation properties upon fluorescence restoration and cytotoxic potency against ovarian cancer cell lines (A2780 and m-Cherry + OCSC1-F2), its potential applicability as a useful chemotherapeutic agent was demonstrated.

Non-Covalent Associates of siRNAs and AuNPs Enveloped with Lipid Layer and Doped with Amphiphilic Peptide for Efficient siRNA Delivery

Elaboration of non-viral vehicles for delivery of therapeutic nucleic acids, in particular siRNA, into a cell is an actively growing field. Gold nanoparticles (AuNPs) occupy a noticeable place in these studies, and various nanoconstructions containing AuNPs are reported. We aimed our work to the rational design of AuNPs-based siRNA delivery vehicle with enhanced transfection efficiency. We optimized the obtaining of non-covalent siRNAs-AuNPs cores: ionic strength, temperature and reaction time were determined. Formation of cores was confirmed using gel electrophoresis. Stable associates were prepared, and then enveloped into a lipid layer composed of phosphatidylcholine, phosphatidylethanolamine and novel pH-sensitive lipidoid. The constructions were modified with [Str-(RL)₄G-NH₂] peptide (the resulting construction). All intermediate and resulting nanoconstructions were analyzed by dynamic light scattering (DLS) and transmission electron microscopy (TEM) to control their physico-chemical properties. To examine the biological effect of the delivery vehicle, green fluorescent protein (GFP)-expressing human embryonic kidney (HEK) Phoenix cells were incubated with the resulting construction containing anti-GFP siRNA, with the siRNA effect being studied by flow cytometry and confocal microscopy. Transfection of the cells with the resulting construction reduced the GFP fluorescence as efficiently as Lipofectamin 3000. Thus, siRNA vehicle based on non-covalently bound siRNA-AuNP core and enveloped into a lipid layer provides efficient delivery of siRNA into a cell followed by specific gene silencing.