Ternary Polymeric Nanoparticles for Oral siRNA Delivery

Oral delivery of RNAi for cancer therapy

Cancer is a major health concern worldwide and is still in a continuous surge of seeking for effective treatments. Since the discovery of RNAi and their mechanism of action, it has shown promises in targeted therapy for various diseases including cancer. The ability of RNAi to selectively silence the carcinogenic gene makes them ideal as cancer therapeutics. Oral delivery is the ideal route of administration of drug administration because of its patients' compliance and convenience. However, orally administered RNAi, for instance, siRNA, must cross various extracellular and intracellular biological barriers before it reaches the site of action. It is very challenging and important to keep the siRNA stable until they reach to the targeted site. Harsh pH, thick mucus layer, and nuclease enzyme prevent siRNA to diffuse through the intestinal wall and thereby induce a therapeutic effect. After entering the cell, siRNA is subjected to lysosomal degradation. Over the years, various approaches have been taken into consideration to overcome these challenges for oral RNAi delivery. Therefore, understanding the challenges and recent development is crucial to offer a novel and advanced approach for oral RNAi delivery. Herein, we have summarized the delivery strategies for oral delivery RNAi and recent advancement towards the preclinical stages.

Oral Delivery of Nucleic Acid Therapies for Local and Systemic Action

Nucleic acid (NA) therapy has gained importance over the past decade due to its high degree of selectivity and minimal toxic effects over conventional drugs. Currently, intravenous (IV) or intramuscular (IM) formulations constitute majority of the marketed formulations containing nucleic acids. However, oral administration is traditionally preferred due to ease of administration as well as higher patient compliance. To leverage the benefits of oral delivery for NA therapy, the NA of interest must be delivered to the target site avoiding all degrading and inhibiting factors during its transition through the gastrointestinal tract. The oral route presents myriad of challenges to NA delivery, making formulation development challenging. Researchers in the last few decades have formulated various delivery systems to overcome such challenges and several reviews summarize and discuss these strategies in detail. However, there is a need to differentiate between the approaches based on target so that in future, delivery strategies can be developed according to the goal of the study and for efficient delivery to the desired site. The goal of this review is to summarize the mechanisms for target specific delivery, list and discuss the formulation strategies used for oral delivery of NA therapies and delineate the similarities and differences between local and systemic targeting oral delivery systems and current challenges.

Slowing down lipolysis significantly enhances the oral absorption of intact solid lipid nanoparticles

Only a limited amount of orally administered lipid nanoparticles are absorbed as intact particles due to lipolysis by lipases in the gastrointestinal tract. It is hypothesized that by counteracting lipolysis, more particles will survive gastrointestinal digestion and be absorbed as intact particles. In this study, incorporation of a lipase inhibitor orlistat (OLST), as well as polyethylene glycol (PEG) coating, is employed to slow down the lipolysis using solid lipid nanoparticles (SLNs) as model particles. To explore the in vivo behaviors of the particles, near-infrared fluorescent probes with absolute aggregation-caused quenching (ACQ) properties are used to label and track the unmodified, PEG-coated and OLST-loaded SLNs. The in vitro lipolysis study indicates very fast first-order degradation of unmodified SLNs and significantly decreased degradation of OLST-SLNs. Live imaging reveals the same trend of slowed-down lipolysis in vivo which correlates well with the in vitro lipolysis. The scanning of ex vivo gastrointestinal segments confirms the considerably prolonged residence time of OLST-SLNs, mirroring the significantly decreased lipolysis rate. The observation of fluorescence in the blood, though very weak, and in the liver speaks of the oral absorption of intact SLNs. The substantially higher hepatic levels of OLST-SLNs than unmodified SLNs should be attributed to the significantly enhanced survival rate because both particles exhibit similar cellular recognition as well as similar physicochemical properties except for the survival rate. Similarly, slowing down lipolysis also contributes to the significantly enhanced cumulative lymphatic transport of OLST-SLNs (7.56% vs. 1.27% for the unmodified SLNs). The PEG coating slows down the lipolysis rate as well but not to the degree as done by OLST. As a result, the gastrointestinal residence time of PEG-SLNs has been moderately prolonged and the hepatic levels moderately increased. The weakened cellular recognition of PEG-SLNs implies that the enhanced oral absorption is solely ascribed to the slowed-down lipolysis and enhanced mucus penetration. In conclusion, the oral absorption of intact SLNs can be significantly enhanced by slowing down lipolysis, especially by OLST, showing potential as carriers for the oral delivery of labile biomacromolecules.

Nanomedicines: current status and future perspectives in aspect of drug delivery and pharmacokinetics

Nanomedicines have evolved into various forms including dendrimers, nanocrystals, emulsions, liposomes, solid lipid nanoparticles, micelles, and polymeric nanoparticles since their first launch in the market. Widely highlighted benefits of nanomedicines over conventional medicines include superior efficacy, safety, physicochemical properties, and pharmacokinetic/pharmacodynamic profiles of pharmaceutical ingredients. Especially, various kinetic characteristics of nanomedicines in body are further influenced by their formulations. This review provides an updated understanding of nanomedicines with respect to delivery and pharmacokinetics. It describes the process and advantages of the nanomedicines approved by FDA and EMA. New FDA and EMA guidelines will also be discussed. Based on the analysis of recent guidelines and approved nanomedicines, key issues in the future development of nanomedicines will be addressed.

Anionic Polymer and Quantum Dot Excipients to Facilitate siRNA Release and Self-Reporting of Disassembly in Stimuli-Responsive Nanocarrier Formulations

The incorporation of anionic excipients into polyplexes is a promising strategy for modulating siRNA binding versus release and integrating diagnostic capabilities; however, specific design criteria and structure-function relationships are needed to facilitate the development of nanocarrier-based theranostics. Herein, we incorporated poly(acrylic acid) (PAA) and quantum dot (QD) excipients into photolabile siRNA polyplexes to increase gene silencing efficiencies by up to 100% and enable self-reporting of nanocarrier disassembly. Our systematic approach identified the functional relationships between gene silencing and key parameters such as excipient loading fractions and molecular weights that facilitated the establishment of design rules for optimization of nanocarrier efficacy. For example, we found that PAA molecular weights ∼10-20× greater than that of the coencapsulated siRNA exhibited the most efficient release and silencing. Furthermore, siRNA release assays and RNAi modeling allowed us to generate a PAA "heat map" that predicted gene silencing a priori as a function of PAA molecular weight and loading fraction. QDs further promoted selective siRNA release and provided visual as well as Förster resonance energy transfer (FRET)-based monitoring of the dynamic changes in nanostructure in situ. Moreover, even with the addition of anionic components, our formulations exhibited substantially improved stability and shelf life relative to typical formulations, with complete stability after a week of storage and full activity in the presence of serum. Taken together, this study enabled synergistic improvements in siRNA release and diagnostic capabilities, along with the development of mechanistic insights that are critical for advancing the translation of nucleic acid theranostics into the clinic.

Enzyme- and pH-Responsive Microencapsulated Nanogels for Oral Delivery of siRNA to Induce TNF-α Knockdown in the Intestine

Inflammatory bowel diseases (IBD) manifest from excessive intestinal inflammation. Local delivery of siRNA that targets these inflammatory cytokines would provide a novel treatment approach. Microencapsulated nanogels are designed and validated as platforms for oral delivery of siRNA targeting TNF-α, a common clinical target of IBD treatments. The preferred platform was designed to (i) protect siRNA-loaded nanogels from the harsh acidic environment of the upper GI tract and (ii) enzymatically degrade and release the nanogels once the carrier has reached the intestinal region. This platform consists of microgels composed of poly(methacrylic acid-co-N-vinyl-2-pyrrolidone) (P[MAA-co-NVP]) cross-linked with a trypsin-degradable peptide linker. The P(MAA-co-NVP) backbone is designed to collapse around and protect encapsulated nanogel from degradation at the low pH levels seen in the stomach (pH 2-4). At pH levels of 6-7.5, as typically observed in the intestine, the P(MAA-co-NVP) matrix swells, potentially facilitating diffusion of intestinal fluid and degradation of the matrix by intestinal enzymes such as trypsin, thus "freeing" the therapeutic nanogels for delivery and cellular uptake within the intestine. TNF-α siRNA-loaded nanogels released from this platform were capable of inducing potent knockdown of secreted TNF-α levels in murine macrophages, further validating the potential for this approach to be used for the treatment of IBD.

Advanced targeted therapies in cancer: Drug nanocarriers, the future of chemotherapy

Cancer is the second worldwide cause of death, exceeded only by cardiovascular diseases. It is characterized by uncontrolled cell proliferation and an absence of cell death that, except for hematological cancers, generates an abnormal cell mass or tumor. This primary tumor grows thanks to new vascularization and, in time, acquires metastatic potential and spreads to other body sites, which causes metastasis and finally death. Cancer is caused by damage or mutations in the genetic material of the cells due to environmental or inherited factors. While surgery and radiotherapy are the primary treatment used for local and non-metastatic cancers, anti-cancer drugs (chemotherapy, hormone and biological therapies) are the choice currently used in metastatic cancers. Chemotherapy is based on the inhibition of the division of rapidly growing cells, which is a characteristic of the cancerous cells, but unfortunately, it also affects normal cells with fast proliferation rates, such as the hair follicles, bone marrow and gastrointestinal tract cells, generating the characteristic side effects of chemotherapy. The indiscriminate destruction of normal cells, the toxicity of conventional chemotherapeutic drugs, as well as the development of multidrug resistance, support the need to find new effective targeted treatments based on the changes in the molecular biology of the tumor cells. These novel targeted therapies, of increasing interest as evidenced by FDA-approved targeted cancer drugs in recent years, block biologic transduction pathways and/or specific cancer proteins to induce the death of cancer cells by means of apoptosis and stimulation of the immune system, or specifically deliver chemotherapeutic agents to cancer cells, minimizing the undesirable side effects. Although targeted therapies can be achieved directly by altering specific cell signaling by means of monoclonal antibodies or small molecules inhibitors, this review focuses on indirect targeted approaches that mainly deliver chemotherapeutic agents to molecular targets overexpressed on the surface of tumor cells. In particular, we offer a detailed description of different cytotoxic drug carriers, such as liposomes, carbon nanotubes, dendrimers, polymeric micelles, polymeric conjugates and polymeric nanoparticles, in passive and active targeted cancer therapy, by enhancing the permeability and retention or by the functionalization of the surface of the carriers, respectively, emphasizing those that have received FDA approval or are part of the most important clinical studies up to date. These drug carriers not only transport the chemotherapeutic agents to tumors, avoiding normal tissues and reducing toxicity in the rest of the body, but also protect cytotoxic drugs from degradation, increase the half-life, payload and solubility of cytotoxic agents and reduce renal clearance. Despite the many advantages of all the anticancer drug carriers analyzed, only a few of them have reached the FDA approval, in particular, two polymer-protein conjugates, five liposomal formulations and one polymeric nanoparticle are available in the market, in contrast to the sixteen FDA approval of monoclonal antibodies. However, there are numerous clinical trials in progress of polymer-protein and polymer-drug conjugates, liposomal formulations, including immunoliposomes, polymeric micelles and polymeric nanoparticles. Regarding carbon nanotubes or dendrimers, there are no FDA approvals or clinical trials in process up to date due to their unresolved toxicity. Moreover, we analyze in detail the more promising and advanced preclinical studies of the particular case of polymeric nanoparticles as carriers of different cytotoxic agents to active and passive tumor targeting published in the last 5 years, since they have a huge potential in cancer therapy, being one of the most widely studied nano-platforms in this field in the last years. The interest that these formulations have recently achieved is stressed by the fact that 90% of the papers based on cancer therapeutics with polymeric nanoparticles have been published in the last 6 years (PubMed search).

Is nanotechnology a boon for oral drug delivery?

The oral route for drug delivery is regarded as the optimal route for achieving therapeutic benefits owing to increased patient compliance. Despite phenomenal advances in injectable, transdermal, nasal and other routes of administration, the reality is that oral drug delivery remains well ahead of the pack as the preferred delivery route. Nanocarriers can overcome the major challenges associated with this route of administration: mainly poor solubility, stability and biocompatibility of drugs. This review focuses on the potential of various polymeric drug delivery systems in oral administration, their pharmacokinetics, in vitro and in vivo models, toxicity and regulatory aspects.

Nanodrugs: pharmacokinetics and safety

To date, various nanodrug systems have been developed for different routes of administration, which include dendrimers, nanocrystals, emulsions, liposomes, solid lipid nanoparticles, micelles, and polymeric nanoparticles. Nanodrug systems have been employed to improve the efficacy, safety, physicochemical properties, and pharmacokinetic/pharmacodynamic profile of pharmaceutical substances. In particular, functionalized nanodrug systems can offer enhanced bioavailability of orally taken drugs, prolonged half-life of injected drugs (by reducing immunogenicity), and targeted delivery to specific tissues. Thus, nanodrug systems might lower the frequency of administration while providing maximized pharmacological effects and minimized systemic side effects, possibly leading to better therapeutic compliance and clinical outcomes. In spite of these attractive pharmacokinetic advantages, recent attention has been drawn to the toxic potential of nanodrugs since they often exhibit in vitro and in vivo cytotoxicity, oxidative stress, inflammation, and genotoxicity. A better understanding of the pharmacokinetic and safety characteristics of nanodrugs and the limitations of each delivery option is necessary for the further development of efficacious nanodrugs with high therapeutic potential and a wide safety margin. This review highlights the recent progress in nanodrug system development, with a focus on the pharmacokinetic advantages and safety challenges.

Chitosan-based siRNA delivery systems

Recently, chitosan has attracted significant attention in the formulation of small interfering RNA (siRNA). Because of its cationic nature, chitosan can easily complex siRNA, thus readily forming nanoparticles. Moreover, chitosan is biocompatible and biodegradable, which make it a good candidate for siRNA delivery in vivo. However, chitosan requires further development to achieve high efficiency. This review will describe the major barriers that impair the efficiency of the chitosan-based siRNA delivery systems, including the stability of the delivery system in biological fluids and endosomal escape. Several solutions to counteract these barriers have been developed and will be discussed. The parameters to consider for designing powerful delivery systems will be described, particularly the possibilities for grafting targeting ligands. Finally, optimized systems that allow in vivo therapeutic applications for both local and systemic delivery will be reviewed. This review will present recent improvements in chitosan-based siRNA delivery systems that overcome many of these system's previous pitfalls and pave the way to a new generation of siRNA delivery systems.

New aspects of gene-silencing for the treatment of cardiovascular diseases

Coronary heart disease (CHD), mainly caused by atherosclerosis, represents the single leading cause of death in industrialized countries. Besides the classical interventional therapies new applications for treatment of vascular wall pathologies are appearing on the horizon. RNA interference (RNAi) represents a novel therapeutic strategy due to sequence-specific gene-silencing through the use of small interfering RNA (siRNA). The modulation of gene expression by short RNAs provides a powerful tool to theoretically silence any disease-related or disease-promoting gene of interest. In this review we outline the RNAi mechanisms, the currently used delivery systems and their possible applications to the cardiovascular system. Especially, the optimization of the targeting and transfection procedures could enhance the efficiency of siRNA delivery drastically and might open the way to clinical applicability. The new findings of the last years may show the techniques to new innovative therapies and could probably play an important role in treating CHD in the future.