Introduction to Nucleic Acid Drugs

Nucleic acid is the general name of DNA and RNA, in which DNA is the basis for storing, copying, and transmitting genetic information, while RNA plays an important role in protein synthesis. They widely exist in animal and plant cells as well as microorganisms. Generally speaking, the drugs with DNA as carrier or operation object are called gene drugs, and the drugs with RNA as carrier or target are called nucleic acid drugs that usually bind directly to the pathogenic target RNA to cure the disease at the molecular level.

Development Process

In 1992, researchers at the American Scripps Institute successfully relieved the symptoms of diabetes insipidus by injecting vasopressin mRNA into the brain of rats, showing the potential of mRNA in the treatment of diseases for the first time. However, due to some problems such as easy degradation and uncontrollable immunogenicity, mRNA was not recognized by pharmaceutical companies and the market in the early stage. With the breakthrough of siRNA drug delivery systems—LNP and GalNAc one after another, mRNA has developed rapidly using this technology, gradually occupying one of the dominant fields in nucleic acid drugs, which outperfoms in the epidemic of COVID-19.

  • Mechanism of action: the use of human humoral immune characteristics

When the human body contacts the pathogen for the first time, it takes a long time to produce immune response and antibodies that are often small and not specific, and in the meantime, the human body also produces memory cells. When exposed to the same pathogen, memory cells are immediately activated to produce a large number of specific antibodies.

mRNA vaccine integrates a variety of viral antigens into a single mRNA to produce a complex multi-antigen vaccine which is difficult to achieve by traditional technology. Vaccination of mRNA vaccine in vivo will induce the production of antibodies that can activate the human immune response more accurately and quickly. In the second contact, the body’s immune system will quickly produce more antibodies to resist the invasion of pathogens based on the original memory.

  • Classification: non-replicating mRNA vaccine and self-amplifying mRNA vaccine

1) A complete segment of mRNA transcribed by non-replicating mRNA vaccine in vitro. It benefits from a simple structure, a short sequence, and exclusively encoding the target antigen. The downside is that the in vivo half-life is short, antigen expression is poor, and a greater dose is required to elicit an effective immune response.

2) Self-amplified mRNA vaccine (SAM mRNA) is genetically engineered mRNA. It can accomplish self-amplification in the body because it has genes that can duplicate RNA, and a modest amount can induce an effective immune response.

  • Advantages: R&D of accurate treatment with short, fast production and broad prospects
  1. Rapid process development, simpler manufacturing, vaccines that keep pace with the mutation rate of the virus

Once the sequence encoding the antigen is obtained, the corresponding vaccine can be synthesized using the existing technology. In comparison to other techniques, the research and development of an mRNA vaccine requires just a change in the antigen sequence on a proven technology platform.

mRNA vaccine is produced by transcription in vitro. Compared with traditional protein fermentation, its unique production process saves the procedures of cell culture, antigen extraction, and purification, shortening the timeline

The above characteristics bestow mRNA vaccine a great advantage dealing with sudden infectious diseases, which is the only vaccine that can keep up with the mutation rate of the virus.

  1. Vaccine with broad application prospect with equal emphasis on anti-infectious disease and anti-tumor

When an mRNA vaccine enters the body, it can express functional proteins to treat tumor diseases caused by gene defects or protein abnormalities, and it can also express viruses or tumor antigen proteins, which can be used as vaccines in infectious diseases.

After being injected into the body, the anti-tumor mRNA vaccine can use the human body’s own protein synthesis mechanism to synthesize a specific antigen protein, and then stimulate the body to produce an immune response to the antigenic protein, thus attacking tumor cells. mRNA tumor vaccine, on the other hand, can use the entire immune system of patients to generate a greater and more specific immune response, unlike PD-1 and CAR-T.

To prevent influenza viruses, for example, it is required to target the conserved regions of their effector proteins; thus, mRNA vaccines that encode conserved regions of influenza virus effector proteins are thought to be the most efficient strategy.

  1. Accurate and individualized treatment plan

mRNA vaccine can be tailored to different tumor cells of each patient. A variety of tumor cell pathogens are integrated into a single mRNA molecule to produce a complex multi-pathogen vaccine. mRNA tumor vaccine can help the immune system recognize and kill tumor cells more accurately and efficiently.

  1. Safer than gene therapy (DNA)

After the mRNA was transfected into the target cells, the antigen was expressed by translation that occurs in the cytoplasm and not the nucleus. Compared with gene therapy, mRNA has a much lower rate of integration into the genome.

  • Core barriers: drug design, delivery system, production process
  1. mRNA sequence optimization and structural modification

When mRNA vaccines enter the body, they will be recognized as foreign substances by the immune system, and thereby trigger an immune response. This immune response can cause the vaccine to be destroyed by the immune system before it begins to work, so the vaccine can be effective only by overcoming immunogenicity.

In 2010, Dr. Katalin Karik ó, Senior Vice President of BioNTech, found that the addition of pseudouridine to the raw material for the synthesis of mRNA could significantly reduce the ability of mRNA to stimulate the immune system, but the technology has not yet achieved the goal of supporting continuous administration in large doses. Therefore, only by continuously optimizing and developing mRNA sequences, can we not only overcome immunogenicity, but also maintain the integrity of mRNA structure, so that the vaccine can be expressed more stably and efficiently in vivo.

The mRNA sequence consists of 5′- end cap, 5′-and 3′-end non-coding region (UTR), open coding region (ORF), and Ploy (A) tail. Different optimization strategies for different regions can significantly improve the physical and chemical properties of mRNA vaccine, the most important of which is to make a tradeoff between drug stability and translation efficiency.

2. Delivery system

Delivery system guarantees the targeted and stable efficacy of mRNA vaccine. Companies have developed LNPs (lipid nanoparticles), LPXs (cationic liposomes), and LPPs (polymer nano-carrier liposomes) based on different techniques.

Among many technologies, LNP is the most mainstream delivery system at present. According to the statistics of Rubik’s Cube, more than 30 of the over 40 mRNA vaccine projects in the clinical use LNP technology. And as early as 2018, Patisiran, the first siRNA drug approved applied this technology. In this COVID-19 epidemic, it was once again favored by Moderna, BioNTech, and many other companies and applied to the development of COVID-19 vaccine.

The core technology of LNP is ionizable cationic liposome whose polarity can change with the change in pH. In low pH, it carries positive charge and combines with negatively charged mRNA molecules to form a complex to preserve stability and not be degraded. In neutral pH, it can protect the integrity of LNP structure and reduce side effects.

Although various carriers have been developed, the delivery technology of mRNA still has a lot of room for improvement. For example, LNPs still have limitations such as allergic reaction, easy oxidation and degradation, and poor preparation reproducibility.

3. Commercial production

Owing the COVID-19 vaccine, mRNA vaccine quickly came out of the laboratory and ushered in commercialization. Difficulties in commercial production of mRNA are as follows:

Most of the mRNA vaccine adopt LNP, and the lack of upstream raw material suppliers leads to the shortage of raw materials for vaccine production. Lipid nanoparticle (LNP) is a minority chemical used in gene therapy, and there are not many companies producing it. Affected by the epidemic, the demand for mRNA vaccine has soared, resulting in a shortage of raw materials, especially cationic liposomes.

At present, large-scale production is lack of commercial equipment, and the core technology and existing equipment are in the hands of small groups of companies. Take Pfizer’s COVID-19 vaccine as an example, in order to solve the problem of large-scale production of the vaccine, it uses the impact jet mixing method, which allows the various components of LNP to be fully mixed with mRNA molecules. The company that makes the impingement jet mixer is Knauer of Germany, but the product has been removed from the official website. The specific structure and parameters of the impact jet mixer are the absolute trade secrets of all manufacturers. Giants such as Moderna and BioNTech basically cooperate with these suppliers. It will take some time for other companies to break through this bottleneck.