Current Challenges in Delivery and Cytosolic Translocation of Therapeutic RNAs

Abstract

RNA interference (RNAi) is a fundamental cellular process for the posttranscriptional regulation of gene expression. RNAi can exogenously be modulated by small RNA oligonucleotides, such as microRNAs (miRNAs) and small interfering RNAs (siRNAs), or by antisense oligonucleotides. These small oligonucleotides provided the scientific community with powerful and versatile tools to turn off the expression of genes of interest, and hold out the promise of new therapeutic solutions against a wide range of gene-associated pathologies. However, unmodified nucleic acids are highly instable in biological systems, and their weak interaction with plasma proteins confers an unfavorable pharmacokinetics. In this review, we first provide an overview of the most efficient chemical strategies that, over the past 30 years, have been used to significantly improve the therapeutic potential of oligonucleotides. Oligonucleotides targeting and delivery technologies are then presented, including covalent conjugates between oligonucleotides and targeting ligand, and noncovalent association with lipid or polymer nanoparticles. Finally, we specifically focus on the endosomal escape step, which represents a major stumbling block for the effective use of oligonucleotides as therapeutic agents. The need for approaches to quantitatively measure endosomal escape and cytosolic arrival of biomolecules is discussed in the context of the development of efficient oligonucleotide targeting and delivery vectors.

Keywords: Shiga toxin B-subunit; checkpoint inhibitor; immunotherapy; miRNA; oligonucleotides; siRNA.

FIG. 1.

Modifications of native RNA structure (a) have been developed to improve oligonucleotide stability and their drug-like properties. (b) Phosphorothioate was the first group used as a bioisosteric replacement of the phosphodiester backbone. Many other stable chemical groups, such as phosphorodiamidate morpholino oligonucleotides, peptidic nucleic acids, and locked nucleic acids have since been developed. (c) The majority of modifications involve changes at the 2′-OH position. 2′-O-Me, 2′-F and 2′-O-(2-methoxyethyl) (MOE) represent the most commonly used ribose modifications.

FIG. 2.

Common modalities for oligonucleotide delivery. (a) Protamine–antibody fusion protein complexed with oligoRNAs. (b) Example of polymeric nanoparticles containing siRNA molecules. (c) Schematic illustration of PEGylated lipidic nanoparticles comprising siRNA. (d) Direct conjugation of siRNAs with a targeting ligand. (e) Despite great progress in vector development, most of internalized molecules accumulate in endosomes. Only a small fraction reaches the cytosol, where the RISC machinery is located. The mechanisms of endosomal escape still remain poorly understood. PEG, polyethylene glycol; RISC, RNA-induced silencing complex; siRNA, small interfering RNA.