Bioresponsive polymer-based nucleic acid carriers

Stimuli-Responsive Nanotechnology for RNA Delivery

Ribonucleic acid (RNA) drugs have shown promising therapeutic effects for various diseases in clinical and preclinical studies, owing to their capability to regulate the expression of genes of interest or control protein synthesis. Different strategies, such as chemical modification, ligand conjugation, and nanotechnology, have contributed to the successful clinical translation of RNA medicine, including small interfering RNA (siRNA) for gene silencing and messenger RNA (mRNA) for vaccine development. Among these, nanotechnology can protect RNAs from enzymatic degradation, increase cellular uptake and cytosolic transportation, prolong systemic circulation, and improve tissue/cell targeting. Here, a focused overview of stimuli-responsive nanotechnologies for RNA delivery, which have shown unique benefits in promoting RNA bioactivity and cell/organ selectivity, is provided. Many tissue/cell-specific microenvironmental features, such as pH, enzyme, hypoxia, and redox, are utilized in designing internal stimuli-responsive RNA nanoparticles (NPs). In addition, external stimuli, such as light, magnetic field, and ultrasound, have also been used for controlling RNA release and transportation. This review summarizes a wide range of stimuli-responsive NP systems for RNA delivery, which may facilitate the development of next-generation RNA medicines.

Systematic Combination of Oligonucleotides and Synthetic Polymers for Advanced Therapeutic Applications

The potential of oligonucleotides is exceptional in therapeutics because of their high safety, potency, and specificity compared to conventional therapeutic agents. However, many obstacles, such as low in vivo stability and poor cellular uptake, have hampered their clinical success. Use of polymeric carriers can be an effective approach for overcoming the biological barriers and thereby maximizing the therapeutic efficacy of the oligonucleotides due to the availability of highly tunable synthesis and functional modification of various polymers. As loaded in the polymeric carriers, the therapeutic oligonucleotides, such as antisense oligonucleotides, small interfering RNAs, microRNAs, and even messenger RNAs, become nuclease-resistant by bypassing renal filtration and can be efficiently internalized into disease cells. In this review, we introduced a variety of systematic combinations between the therapeutic oligonucleotides and the synthetic polymers, including the uses of highly functionalized polymers responding to a wide range of endogenous and exogenous stimuli for spatiotemporal control of oligonucleotide release. We also presented intriguing characteristics of oligonucleotides suitable for targeted therapy and immunotherapy, which can be fully supported by versatile polymeric carriers.

Spatiotemporal control of CRISPR/Cas9 gene editing

The clustered regularly interspaced short palindromic repeats (CRISPR)/associated protein 9 (CRISPR/Cas9) gene editing technology, as a revolutionary breakthrough in genetic engineering, offers a promising platform to improve the treatment of various genetic and infectious diseases because of its simple design and powerful ability to edit different loci simultaneously. However, failure to conduct precise gene editing in specific tissues or cells within a certain time may result in undesirable consequences, such as serious off-target effects, representing a critical challenge for the clinical translation of the technology. Recently, some emerging strategies using genetic regulation, chemical and physical strategies to regulate the activity of CRISPR/Cas9 have shown promising results in the improvement of spatiotemporal controllability. Herein, in this review, we first summarize the latest progress of these advanced strategies involving cell-specific promoters, small-molecule activation and inhibition, bioresponsive delivery carriers, and optical/thermal/ultrasonic/magnetic activation. Next, we highlight the advantages and disadvantages of various strategies and discuss their obstacles and limitations in clinical translation. Finally, we propose viewpoints on directions that can be explored to further improve the spatiotemporal operability of CRISPR/Cas9.

Viral Mimicry as a Design Template for Nucleic Acid Nanocarriers

Therapeutic nucleic acids hold immense potential in combating undruggable, gene-based diseases owing to their high programmability and relative ease of synthesis. While the delivery of this class of therapeutics has successfully entered the clinical setting, extrahepatic targeting, endosomal escape efficiency, and subcellular localization. On the other hand, viruses serve as natural carriers of nucleic acids and have acquired a plethora of structures and mechanisms that confer remarkable transfection efficiency. Thus, understanding the structure and mechanism of viruses can guide the design of synthetic nucleic acid vectors. This review revisits relevant structural and mechanistic features of viruses as design considerations for efficient nucleic acid delivery systems. This article explores how viral ligand display and a metastable structure are central to the molecular mechanisms of attachment, entry, and viral genome release. For comparison, accounted for are details on the design and intracellular fate of existing nucleic acid carriers and nanostructures that share similar and essential features to viruses. The review, thus, highlights unifying themes of viruses and nucleic acid delivery systems such as genome protection, target specificity, and controlled release. Sophisticated viral mechanisms that are yet to be exploited in oligonucleotide delivery are also identified as they could further the development of next-generation nonviral nucleic acid vectors.

Development of a nanoparticle that releases nucleic acids in response to a mitochondrial environment

The delivery of nucleic acids targeting mutant mtDNA represent a potential strategy for addressing a variety of mitochondria-related diseases. We previously developed a MITO-Porter, a nano carrier that is capable of delivering nanoparticles of nucleic acids to mitochondria of human cells. Here, we report on an investigation of a series of nanoparticles formed with various poly cationic peptides that can release nucleic acids in response to a mitochondrial environment. A significant relationship was found between the number of and the location of arginine and histidine residues in the peptide sequence and the release of nucleic acids in a mitochondrial environment.

Bioresponsive polyplexes - chemically programmed for nucleic acid delivery

INTRODUCTION

The whole delivery process of nucleic acids is very challenging. Appropriate carrier systems are needed, which show extracellular stability and intracellular disassembly. Viruses have developed various strategies to meet these requirements, as they are optimized by biological evolution to transfer genetic information into host cells. Taking viruses as models, smart synthetic carriers can be designed, mimicking the efficient delivery process of viral infection. These 'synthetic viruses' are pre-programmed and respond to little differences in their microenvironment, caused by either exogenous or endogenous stimuli.

AREAS COVERED

This review deals with polymer-based, bioresponsive nanosystems (polyplexes) for the delivery of nucleic acids. Strategies utilizing pH-responsiveness, redox-responsiveness as well as sensitivity towards enzymes will be described more in detail. Systems, which respond to other endogenous triggers (i.e. reactive oxygen species, adenosine triphosphate, hypoxia), will be briefly illustrated. Moreover, some examples for combined bioresponsiveness will be presented.

EXPERT OPINION

Bioresponsive polyplexes are a smart way to facilitate programmed, timely delivery of nucleic acids to desired, specific sites. Nevertheless, further optimization is necessary to improve the still moderate transfection efficiency and specificity - also in regard to medical translation. For this purpose, precise carrier structures are desirable and stability issues of bioresponsive systems must be considered.

Hybrid nanoparticles based on chlorin e6-conjugated hyaluronic acid/poly(l-histidine) copolymer for theranostic application to tumors

The aim of this study is to synthesize multifunctional hybrid nanoparticles composed of hyaluronic acid (HA) and poly(l-histidine) (PHS) with a disulfide linkage and chlorin e6 (HAPHSce6ss) for diagnostic and therapeutic application against breast tumor cells. The reductive end of HA was conjugated with cystamine to make a disulfide linkage (HA-cystamine). PHS was conjugated with Ce6 with the aid of carbodiimide chemistry (PHS-ce6). Then, HA-cystamine was conjugated with the carboxyl group of Ce6 to make an HAPHSce6ss copolymer. Nanoparticles of HAPHSce6ss copolymer have small particle sizes of less than 100 nm and their diameters increased with acidic pH, indicating that HAPHSce6ss nanoparticles have pH-sensitivity. Furthermore, ce6 was activated in the acidic environment and had redox-status in a fluorescence study. In a cell culture study, the nanoparticles were specifically targeted at the CD44 receptor of MDA-MB231 cells while CD44-negative MCF7 cells had no CD44-specificity. The nanoparticles exhibited an enhanced association with cells and were more fluorescent at acidic pH or in the presence of GSH. They inhibited the growth of tumor cells in a CD44 receptor specific or pH-sensitive manner. In an in vivo animal tumor xenograft study using mice, HAPHSce6ss nanoparticles predominantly targeted an MDA-MB231 tumor rather than an MCF7 tumor and effectively inhibited tumor growth. HAPHSce6ss nanoparticles have CD44 specificity, pH/redox dual sensitivity and a fluorescence diagnostic function against tumor cells. We suggest that HAPHSce6ss nanoparticles are a promising candidate for theranostic application to tumors.

Advances in Stimulus-Responsive Polymeric Materials for Systemic Delivery of Nucleic Acids

Polymeric materials that respond to a variety of endogenous and external stimuli are actively developed to overcome the main barriers to successful systemic delivery of therapeutic nucleic acids. Here, an overview of viable stimuli that are proved to improve systemic delivery of nucleic acids is provided. The main focus is placed on nucleic acid delivery systems (NADS) based on polymers that respond to pathological or physiological changes in pH, redox state, enzyme levels, hypoxia, and reactive oxygen species levels. Additional discussion is focused on NADS suitable for applications that use external stimuli, such as light, ultrasound, and local hyperthermia.

Functional polymer-based siRNA delivery carrier that recognizes site-specific biosignals

Responsive molecular designs to specific biosignals in microenvironments endow site-specific functionalities with associated polymers. Thus, the construction of small interfering RNA (siRNA) carriers with functional polymers enables smart programs that are triggered by sequential biosignals in a pathway to the targeted cytosol for effective gene silencing. In this review, we explain rational strategies for the design of functional polymers with responsiveness to biosignals and describe the examples of smart carriers for siRNA delivery.

Cross-linked polymers with fluorinated bridges for efficient gene delivery

A new strategy for the construction of fluorinated cationic polymers for gene delivery was introduced.

Evolving lessons on nanomaterial-coated viral vectors for local and systemic gene therapy

Viral vectors are promising gene carriers for cancer therapy. However, virus-mediated gene therapies have demonstrated insufficient therapeutic efficacy in clinical trials due to rapid dissemination to nontarget tissues and to the immunogenicity of viral vectors, resulting in poor retention at the disease locus and induction of adverse inflammatory responses in patients. Further, the limited tropism of viral vectors prevents efficient gene delivery to target tissues. In this regard, modification of the viral surface with nanomaterials is a promising strategy to augment vector accumulation at the target tissue, circumvent the host immune response, and avoid nonspecific interactions with the reticuloendothelial system or serum complement. In the present review, we discuss various chemical modification strategies to enhance the therapeutic efficacy of viral vectors delivered either locally or systemically. We conclude by highlighting the salient features of various nanomaterial-coated viral vectors and their prospects and directions for future research.

Temperature-dependent drug release from DPPC:C12H25-PNIPAM-COOH liposomes: control of the drug loading/release by modulation of the nanocarriers' components

Novel polymer-modified thermosensitive liposomes were developed for the delivery of indomethacin in order to control its release profile. When attached to 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) liposomes, the end functionalized C12H25-poly(N-isopropylacrylamide)-COOH (C12H25-PNIPAM-COOH) polymer was membrane-disruptive in a temperature-dependent manner. The interest for this polymer is driven by its famous lower critical solution temperature (LCST) behavior, where heating an aqueous solution of PNIPAM above 32°C induces nanophase separation and polymer chain aggregation. The physicochemical/structural behavior of these polymer-modified thermosensitive liposomes was found to depend on the PNIPAM:lipid molar ratio and the composition of the polymeric guest. The incorporation of PNIPAM has caused alterations in the thermotropic behavior of DPPC liposomes, as the differential scanning calorimetry (DSC) experiments revealed. The drug loading and the release were found to be strongly dependent on the thermotropic characteristics of the PNIPAM grafted DPPC liposomes. Namely, the in vitro release is immediate at 37°C (>LCST) ("burst" effect), while the prepared mixed nanocarriers did not release the encapsulated bioactive substance at <32°C (<LCST). Thus, the thermosensitivity and the drug loading/release properties of the prepared formulations can be modulated by varying the ratio of DPPC/PNIPAM components, as well as the molecular characteristics of the polymeric guest.