Oligonucleotides have been gaining considerable attention as promising and effective candidate therapeutics against various diseases. This special issue is aimed at providing a better understanding of the recent progress in the development of oligonucleotide-based therapeutics to encourage further research and innovation in this field to achieve these advancements. Several Japanese scientists have been invited to contribute to this issue by describing their recent findings, overviews, insights, or commentaries on rational designing of therapeutic oligonucleotide molecules and their novel delivery technologies, especially nanocarrier systems.

In recent decades, nucleic acid therapeutics have been attracting much attention because of their potential as treatments for various diseases that require high target specificity. Nevertheless, the number of approved nucleic acid drugs is very limited due to substantially poor bioavailability, thereby generating the need for nucleic acid delivery carriers. A variety of delivery formulations have been reported so far. This review particularly describes polymer-based delivery carriers for systemic administration, highlighting the potential of biological environment-responsive chemistry for overcoming the biological barriers to nucleic acid delivery.

Gene therapy is promising for the treatment of many diseases including cancers and genetic diseases. From the viewpoint of safety, ultrasound (US)-mediated gene delivery with nano/ microbubbles was recently developed as a novel non-viral vector system. US-mediated gene delivery using nano/microbubbles are able to produce transient changes in the permeability of the cell membrane after US-induced cavitation while reducing cellular damage and enables the tissue-specific or the site-specific intracellular delivery of gene both in vitro and in vivo. We have recently developed novel lipid nanobubbles (Lipid Bubbles). These nanobubbles can also be used to enhance the efficacy of the US-mediated genes (plasmid DNA, siRNA, and miRNA etc.) delivery. In this review, we describe US-mediated delivery systems combined with nano/microbubbles and discuss their feasibility as non-viral vector systems.

Therapeutic oligonucleotides are promising technologies. Nevertheless, improvement of their efficacy is an important issue. Introducing this drug delivery system (DDS) makes for a great enhancement for delivery of oligonucleotides to targeted tissue or cells. The strategy of DDS for therapeutic oligonucleotides is divided into four categories, A) single piece of oligonucleotide, B) oligonucleotide-ligand conjugate, C) oligonucleotide-polymer conjugate, and D) nanoparticle. In this review we will describe those basic concepts, especially for the technology of conjugating ligand. In addition, we developed a new technology, heteroduplex oligonucleotide (HDO), binding ligand-molecule to antisense oligonucleotide indirectly. We also outline α-tocopherol (a natural isomer of vitamin E) conjugated HDO.

Phosphorothioate modification is one of the most widely investigated and promising chemical modifications in oligonucleotide (ON) based therapeutics. Structurally similar 5'-thio or phosphorothiolate-modified nucleotides, in which a 5'-bridging oxygen is replaced with a sulfur atom, are gaining importance for ON-based research. Several reports have been published describing the synthesis of 5'-thio-modified ONs but no detailed in vitro and in vivo data are available. Here, we report the synthesis of 5'-thio-modified 2'-deoxy-5-methylcytidine. 5'-Thio-modified thymidine and 2'-deoxy-5-methylcytidine were incorporated into target ONs, then we evaluated their binding affinity, nuclease stability, RNase H mediated scission, stability in blood serum, and in vitro and in vivo activity. This is the first report showing the influence of 5'-thio-modified antisense ONs in in vitro and in vivo experiments.

Nucleic acids are biologically active materials, and chemically modified nucleic acids are now being used in nucleic acid drugs. DNA, one of the two types of nucleic acids, can also be used as a material to construct DNA-based delivery systems, such as DNA hydrogel, for therapeutic compounds. Use of chemically modified nucleic acids would greatly increase the therapeutic potency of such DNA-based delivery systems. However, attention should be paid to the differences in the physicochemical properties of natural and chemically modified nucleic acids. Another, more important concern for chemically modified nucleic acids is the high cost of their synthesis. Reducing the cost of synthesizing oligonucleotides, and especially ones with chemically modified nucleic acids, is crucial to the expanded use of both nucleic acid drugs and DNA-based delivery systems.

Nucleic acid-based therapeutics including antisense and siRNA oligonucleotides has been expected as an innovative treatment for intractable diseases. Oral drug delivery is the most patient-friendly route of administration but developing an effective delivery system for oligonucleotides remains a major challenge. In this commentary, we discuss the potential benefits of the colorectal route as another platform for the development of oral oligonucleotide therapeutics. The importance of the targeting or the availability of oligonucleotides in targeted tissue is highlighted in contrast to systemic availability, while the liver-targeted enteral siRNA delivery technology that we recently developed is introduced.