Delivery of siRNA therapeutics: barriers and carriers

Abstract

RNA interference is a naturally occurring endogenous regulatory process where short double-stranded RNA causes sequence-specific posttranscriptional gene silencing. Small interference RNA (siRNA) represents a promising therapeutic strategy. Clinical evaluations of siRNA therapeutics in locoregional treatment settings began in 2004. Systemic siRNA therapy is hampered by the barriers for siRNA to reach their intended targets in the cytoplasm and to exert their gene silencing activity. The three goals of this review were to provide an overview of (a) the barriers to siRNA delivery, from the perspectives of physicochemical properties of siRNA, pharmacokinetics and biodistribution, and intracellular trafficking; (b) the non-viral siRNA carriers including cell-penetrating peptides, polymers, dendrimers, siRNA bioconjugates, and lipid-based siRNA carriers; and (c) the current status of the clinical trials of siRNA therapeutics.

Fig. 1

siRNA-mediated gene silencing. In the cytoplasm, long dsRNAs are cleaved by the endoribonuclease Dicer into short dsRNA duplexes or siRNA. siRNAs are loaded onto and activate RISC, which contains Ago-2. Ago-2 cleaves and releases one strand, resulting in an activated form of RISC with a single-strand RNA molecule that directs the specificity of the target mRNA recognition through complementary base pairing. Ago-2 then cleaves the target mRNA, resulting in mRNA degradation and gene silencing

Fig. 2

Barriers to systemic siRNA delivery. Barriers to distribution to the target organs include the degradation of siRNA and carriers, protein absorption (opsonization) and phagocytosis by mononuclear phagocyte system, and entrapment in reticuloendothelial system (RES). Barriers to extravasation and penetration in extracellular matrix are dependent on the physiological structure of the target tissue (see text). Barriers to cellular internalization are dependent on the surface properties of siRNA and carriers (e.g., charge, pegylation, and specific binding antigen). The major barriers for delivering siRNA to its site of action are the endosomal entrapment and the lysosomal degradation of siRNA and carriers

Fig. 3

Examples of siRNA delivery carriers. a siRNA bioconjugates: siRNA is conjugated with selected molecules through chemical cross-linking or disulfide bonds. b Aptamer–siRNA chimeras: the aptamer portion mediates the specific binding to the target cell and the siRNA portion mediates the gene silencing. c Dendrimer: the example shows the three-generation layer-by-layer nanostructure with interior encapsulation of siRNA. d Surface-modified nanoparticulate siRNA carriers: the surface of a nanoparticulate can be modified with pegylation to obtain stealth properties and/or attached to a targeting ligand to attain specific delivery to cells expressing the ligand receptors