Small nucleic acid drugs are short in length and have fewer than 30nt bases. They mainly act on intracellular mRNA through the principle of base complementary pairing to regulate protein expression so as to achieve a therapeutic effect. It can be further divided into single-stranded antisense oligodeoxynucleotides (ASOs) and double-stranded nucleotides siRNA.

RNAi (RNA interference) technology refers to the highly conservative phenomenon induced by double-stranded RNA (dsRNA) and the efficient and specific degradation of homologous mRNA in the process of evolution. Because RNAi technology can specifically silence the expression of specific genes, it has been widely used in the field of gene therapy to explore gene function, viral diseases, and malignant tumors.

The mechanism of RNAi is that long-stranded double RNA is cut into siRNA and binds to proteins to form siRNA-induced interference complexes (RISC). RISC binds to specific mRNA to degrade mRNA and finally silences the expression of specific genes. Viral ribonucleic acid is composed of four basic elements: A, G, C, and T. The core point of RNAi technology is to replicate a small strand of nucleic acid that happens to complement it, thus silencing viral genes.

RNAi is more efficient than ASOs. At present, RNA that has the function of RNAi is mainly divided into three categories: siRNA (small interfering RNA), microRNA (miRNA), and PIWI-interacting RNA (piRNA). Among them, siRNA drugs have become the most concerning technology because of their good curative effect and technical breakthrough.

SiRNA can mediate the silencing effect of target mRNA many times in vivo. SiRNA is a small, double-stranded RNA with a length of 21-25 nt, which is complementary to the target gene. It was first discovered in plants by British scientist Hamilton in 1999. In 2001, Elbashir and other scientists successfully synthesized siRNA and found that it could induce specific silencing after being transferred into HELA cells. It has been found that siRNA is a kind of post-transcriptional gene silencing (PTGS) that degrades mRNA, which leads to targeted gene silencing by specifically inducing target mRNA degradation. The mechanism is mainly divided into several stages:

First, exogenous or endogenous double-stranded RNA (dsRNA) is cleaved by RNase III (e.g., Dicer) into an active siRNA structure of about 21-25 nt.

Then, with the help of RNA binding protein TRBP, Dicer will load double-stranded siRNA into Argonaute (AGO2) protein to form a complex RNA-induced silencing complex (RISC). Then, the siRNA will untie the double strand, and when the antisense strand binds to the target mRNA, the AGO2 will specifically degrade the target mRNA, thus inhibiting its translation.

Finally, the excised target mRNA is released, the RISC is recycled, and the same loading boot chain is used for several more rounds of slicing. In addition, siRNA can re-form dsRNA under the action of RNA polymerase, thus participating again in the above process.

RNA has unique advantages as a proprietary medicine. RNAi-based silencing techniques can be used to design treatments for a variety of diseases, mainly those caused by one or more genes, such as genetic defects, viral diseases, autoimmune diseases, and cancer. In view of its unique action mechanism advantages, siRNA has great development potential and is one of the most popular fields of small nucleic acid development at present.

High specificity. The binding of siRNA to its target is highly selective, using about the full length of mRNA to identify the target sequence and mediate its cleavage. Sequences with a difference of only one nucleotide can be distinguished, and this high binding specificity makes siRNA a suitable tool for disease treatment.

The selection of targets is not restricted. SiRNA can silence the expression of almost any gene in the genome, so it has extensive therapeutic potential and is expected to target the target of “unavailable medicine” in the existing treatment.

The treatment efficiency is high. Because a single siRNA guide chain can be recycled in multiple rounds of mRNA cutting, higher therapeutic efficiency can be achieved under the correct trigger conditions.