Development Base and Future Trends of RNA Drug Research
RNA used to be considered just a molecule that passed information between genes and proteins. In fact, at the beginning of life, RNA is the only living molecule, which can store information and also has the function of an enzyme. It took millions of years of evolution and evolution to produce DNA and protein molecules. In addition to acting as a messenger (mRNA) for protein synthesis, RNA also has a very important regulatory function, such as non-transcribed RNA includes miRNA, siRNA, lncRNA, piwiRNA, etc. Among them, there are more than 400 miRNA molecules alone, regulating at least one-third of human genes. Since the discovery of the DNA double helix structure, scientists began research on the realization of "programmed" pharmaceuticals based on the genetic information contained in nucleic acids and the base pairing law.
The earliest beginning of RNA drug research includes antisense nucleic acid (ASO), small interfering nucleic acid (siRNA), aptamer, aptamer, snRNA, and messenger RNA (mRNA). Recently, RNA molecules contained in gene editing tools such as GRISPR-Cas9 are also classified as RNA drugs. At this stage, RNA drugs can fully cover the three major target types of DNA, RNA and protein.
In 1978, scientists first proposed the concept of antisense nucleic acid (ASO), hoping to block the translation and transcription of DNA or RNA through base pairing. However, beyond expectations, ASO can also recruit RNase enzymes to degrade mRNA, interfere with pre-mRNA splicing, and even indirectly enhance the expression of certain proteins. These functions can be used to design ASO drugs for various targets.
RNA drugs have really attracted the attention of the pharmaceutical industry because of the discovery of Nobel Prize achievement RNAi. The RNAi phenomenon is actually post-transcriptional gene silencing (PTGS) that degrades mRNA. Double-stranded RNA can be cut into multiple small fragments (21 to 23 bp, siRNA) in the cytoplasm by the endonuclease Dicer. siRNA and helicase in vivo combine to form a silencing complex (RNA-induced silencing complex, RISC). RISC induces degradation of mRNA that complementarily binds to the antisense strand in siRNA. The phenomenon of RNAi is highly conservative and has been discovered in many eukaryotes, and co-suppression of genes in plants is also due to interference caused by accidental siRNA generated during artificial transcription of genes. The specificity, efficiency and simplicity of RNAi make siRNA quickly become the most popular gene regulation tool.
Aptamer, which do not take effect through base pairing, but are similar to proteins, relying on their three-dimensional structure to bind to ligands. Theoretically, the aptamer has low immunogenicity, is easy to synthesize, and has a wide range of targets. It can even bind to active cells and can also target intracellular targets.
The potential of RNA drugs is huge, however, to enter human body, researchers still have to break through the firewall designed by nature that has evolved for millions of years: 1) The molecular weight and negative charge of nucleic acid prevent it from freely passing through the biofilm; 2) RNA is easily degraded by RNase in plasma and tissues, quickly cleared by the liver and kidneys and recognized by the immune system; 3) after entering the cell, the "stuck" cannot function in the endosome. So for half a century, the technical obstacle faced by the development of RNA drugs is still drug delivery. There are two ideas to solve the delivery, one is to modify the nucleic acid molecule to make it stable and avoid the recognition of the immune system; the other is to use drug delivery systems, such as lipid nanoparticles (LNP) and GalNAc coupling technology.
RNA drugs have made great progress in recent years. There have been 7 ASOs, 2 siRNAs and 1 Aptamar on the market, and more than 50 are in various stages of clinical practice. The future development direction can be divided into two aspects: technology and business. The focus of technology is how to break through target organs outside the liver, the key of which is to crack the mechanism of endosome escape, while the biggest commercial concern is whether RNA drugs can bring the era of true precision medicine.