Since the first successful application of messenger ribonucleic acid (mRNA) as a vaccine agent in clinical studies, many advances have been made in the field of mRNA therapeutic technology after nearly 30 years of development. Vaccines are essential tools for the prevention, control and/or eradication of infectious diseases and are an essential part of global public health programmes.
The development and approval of an effective coronavirus disease 2019 (COVID-19) vaccine is an important milestone during the current pandemic. To combat the disease caused by a previously unknown pathogen, severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), a large number of vaccine development projects have been launched simultaneously. According to the World Health Organization's Vaccine Tracker Report, as of January 14, 2022, there are 333 vaccine candidates in development, of which 139 have entered the clinical stage.
Compared with traditional vaccines, which are relatively slow and laborious to develop, mRNA-based vaccines are characterized by rapid design and rapid scale-up, with the advantages of high potency and low cost. Although mRNA vaccines have been studied in clinical trials for other diseases, such as cancer, their far-reaching potential was not recognized until the COVID-19 pandemic - mRNA vaccines are the first to enter clinical trials and accelerate regulatory access approved vaccines. This achievement would not have been possible without the extensive research and technological advancements of the past 30 years. This essay highlights some of the most important breakthroughs that ultimately led to the development and approval of a COVID-19 mRNA vaccine.
With the discovery of mRNA, research in the field proliferated and resulted in several scientific breakthroughs that culminated in the development of RNA vaccines. Previously, detection of mRNA properties has been hampered by minimal cellular uptake of naked mRNA. However, the development of protective lipid formulations has made subsequent work on mRNA less complicated. Protective lipid formulations were first used successfully in a 1978 study. This study introduced rabbit reticulocyte 9S mRNA into mouse lymphocytes, resulting in globulin synthesis. In the same year, protein expression following liposomal mRNA transport was also induced in human cells. Later, the efficiency of transfection was further improved, and synthetic cationic lipids were fused into liposomes for mRNA delivery.
The identification of deoxyribonucleic acid (DNA) -dependent RNA polymerases is a key step for in vitro mRNA transcription (IVT) using DNA templates. IVT, first published in 1984, enables the desired amount of transcription of functional mrnas selected from templates. It was not until 1993 that mRNA was first used as a vaccine, using lipid delivery in preclinical trials to induce specific immune responses against encoded disease-causing antigens. Two decades later, mRNA vaccines against infectious diseases were being investigated in phase I clinical trials, or proof-of-concept. Because of the potential toxicity of liposomes in clinical applications, the successful approval of the first small interfering RNA-lipid nanoparticles (LNP) therapeutics and mRNA COVID-19 vaccines resulted only from their delivery using ionized lipid-containing LNP. Its delivery efficiency was significantly higher in hepatocytes after intravenous injection (i.v.) or muscle cells after intramuscular injection (i.m.). In addition, LNPs have recently been found to function as powerful adjuvants, which further demonstrates their beneficial role in vaccine application.
In the development of mRNA as a drug, the lack of stability and innate immune activation have been important issues for many years. Uridin-containing mrnas stimulate innate immune responses and have defined functions as adjuvants in vaccines. However, the addition of modified nucleosides to mRNA can significantly improve the biostability and translation ability of mRNA, while reducing the innate immune response. To further improve mRNA quality, IVT mRNA can be purified by cellulose, high performance liquid chromatography (HPLC), fast protein liquid chromatography (FPLC), oligomer (dT) purification or tangential flow filtration (TFF). Since details of the available mRNA vaccines have not been disclosed, they can only be speculated.
Combined with LNP, modified mRNA is the basis of the current independent mRNA vaccine platform. This platform proved to be the most successful mRNA vaccine platform due to its ability to generate the best immune response. The optimal immune response is derived from the balance between LNP adjuvant function and modified mRNA to improve efficacy and safety. Despite the multiple advantages offered by the mrNA-LNP platform, there is still room for improvement and we are likely to see further iterations of this technology. Recently, limitations in the prediction of clinical outcomes based on data from mouse models have been identified. This limitation comes from the downstream effect of mRNA liposomes stimulating Toll-like receptor (TLR) 7/8, which induces systemic inflammation.
It has been reported that humans secrete proinflammatory interleukin (IL) -1b, whereas mice control inflammation by inducing upregulation of IL-1 receptor antagonists. Another challenge in this area is that lipid components themselves may activate immune responses, which may vary depending on the composition and type of preparation. A study comparing LNP and liposomes found that the difference was due to cytokine induction, suggesting that the ionizable lipids of LNP preparations may be responsible for the difference. With this in mind, future studies of downstream effects of immunity after mRNA application need to be cautious, which may affect mRNA safety, and the type and composition of mRNA preparations need to be carefully selected according to the disease.
In addition to linear mRNA, other mRNA vaccine platforms have also received attention. circRNAs were first discovered in 1976 and were later detected in human cells. While it was initially thought not to be a translation template, later reports disproved this hypothesis, prompting further research. Compared with linear RNAs, circRNAs may be a promising research direction due to the enhanced biological stability of circRNAs due to the lack of ends that prevent degradation. Another way to lower dose levels is to use mRNA platforms that are capable of self-amplification. Subsequent studies of viral biology utilized the insertion of an RNA-dependent RNA polymerase (RdRp) sequence next to the antigen-encoding sequence, resulting in cytoplasmic amplification of the antigen of interest. With intracellular RNA replication of the antigen of interest, saRNA can achieve high antigen yields at low mRNA doses.
The COVID-19 pandemic has led to the fastest rate of vaccine development in history, and while the therapeutic potential of mRNA has been studied for decades before, it has not yet been able to be marketed as a medical treatment, and the time is now ripe to exploit its unique properties compared to traditional vaccine approaches. We aim to give an overview of approved COVID19 mRNA vaccines and vaccine candidates, as well as their different molecular approaches in this rapidly evolving field.
With a suitable target antigen sequence, mRNA vaccine technology can be designed and produced rapidly because it does not involve pathogens and does not require a specific cell culture process or fermentation to produce the vaccine. In contrast, these challenges make the research, development, and production process for traditional vaccines more complex and lengthy.
The first two COVID-19 vaccines to receive accelerated approval and meet the required safety and efficacy criteria were mRNA vaccines. mRNA vaccine success in the COVID-19 vaccine race demonstrates the potential of the technology for a wide variety of future applications, including other infectious diseases, cancer therapies, and protein replacement therapies, as reflected in the current pipeline of developers.
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