Advances in Chemical Modification of siRNA

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).

Ribose modification

Modification at the 2' position of the glycosyl group. The most widely used ribose modifications include 2'-O-methylation (2'-OME), 2'-Fluro modification (2'-F), and 2'-O-methoxyethyl (2'-MOE), which can improve the nuclease resistance and thermal stability of the structure. At the same time, it can also promote the binding of siRNA with complementary mRNA and enhance the targeting efficiency of siRNA.

Phosphorothioate (PS) modification

The phosphodiester bond connecting the phosphate backbone of RNA is the chemical bond of nuclease action. PS modification of the vulnerable phosphorus atom can improve the nuclease resistance, pharmacokinetic properties, and serum stability of the modified nuclease, but high PS modification can lead to serious toxicities.

Base modification

Base modification can enhance the interaction between bases. The most commonly used base modification refers to introducing bromine or iodine at the 5' position of uracils, such as 5-bromouracil and 5-iodouracil.

N- & C-terminal modification

Terminal modifications include polyethylene glycol (PEG) modifications and cholesterol modifications, which are usually introduced into the 5' or 3' end of the sense sequence. PEG modification can increase particle size, shield negative charge, and reduce cytotoxicity, thus avoiding nuclease degradation and renal clearance of siRNA in vivo as well as reducing toxicity in vivo. Cholesterol-modified siRNA can prolong circulation time in vivo, enhance membrane permeability of siRNA, and promote cellular uptake.