siRNA, with a molecular weight of about 13 kDa, recruits the RNA-induced silencing complex (RISC) to mRNA through base pairing, thereby inhibiting protein translation (Fig. 1). The mRNA is targeted for cleavage through the catalysis of the RISC protein Ago2, a member of the Argonaute family. In addition, other Ago proteins (Ago1, Ago3, and Ago4) catalyze endonuclease-mediated degradation of non-specific mRNA by locating the bound mRNA in processing bodies (P-bodies).
Limitations of siRNA as a proprietary drug
- Naked siRNA is easily degraded by nucleases in blood, and its relatively high molecular weight, negative charge, and hydrophilicity make it difficult to penetrate cell membranes.
- siRNA tends to accumulate in the kidney and be excreted in urine or is captured by the reticuloendothelial system (RES), thus cannot effectively bind to target sequences.
- Naked siRNA can cause phagocytosis in the body’s immune system in a length-dependent manner, and siRNA can also activate Toll-like receptors to trigger immune responses.
- Whether siRNA can successfully escape from endosomes affects its delivery efficiency in vivo.
- siRNA off-target effect.
Chemical Modification of siRNA
- Ribose modification
- Phosphorothioate (PS) modification
- Base modification
- N- & C-terminal modification
Delivery of siRNA
siRNA itself does not possess targeting ability. Meanwhile, unmodified naked siRNA presents a low transfection efficiency in vivo and cannot achieve a high silencing efficiency. Hence, efficient delivery systems help siRNA better reach its target with improved bioavailability in vivo. Liposomes are a kind of spherical carrier composed of a hydrophilic nucleus and hydrophobic phospholipid bilayer. It has good biofilm property and containment and can promote cell uptake and endosomal escape while avoiding nuclease degradation. The positive charge carried by cationic liposomes can neutralize the negative charge carried by siRNA to form a tightly structured complex that greatly extends the cycle time and facilitates the delivery of siRNA. Cell-penetrating peptide (CPP) is a short cationic peptide (<30 amino acids) consisting of arginine and lysine residues, which can deliver a variety of bioactive substances of different sizes and properties to cells through cell membranes. CPP and siRNA form non-covalent compounds through covalent bonding or charge interaction, which can enhance the stability and biological activity of siRNA, promote intracellular releases, and reduce immunogenicity.- Dynamic Polyconjugates (DPC)
- GalNAc-siRNA
Clinical indications for siRNA drugs
siRNA drugs are most widely used in oncology and rare diseases. Currently, several siRNA drugs for rare diseases have been approved for marketing, such as:- Patisiran (Hereditary trans thyroxine amyloidosis, ATTR)
- Exondys51 (Duchenne muscular dystrophy, DMD)
- Givosiran (Acute intermittent porphyria, AIP)
- Inclisiran (Hyperlipidemia)
- Tivanisiran (Dry eye syndrome, DES)
- QPI-1007 (Optic atrophy)
- SYL040012 (Glaucoma)
- QPI-1002 (Severe kidney disease)