Advantages
mRNA has a unique position in gene therapy and protein therapy, combining the advantages of both therapies while circumventing the challenges faced by both. For example, the difficulty of producing multimeric proteins in a bioreactor is a challenge that can be solved by self-assembly in the patient after a single mRNA or multiple mRNAs encode different subunits of the protein.
The application of mRNA in a CMV vaccine candidate and in multiple oncology fields demonstrates the flexibility of this technology. In the current situation, the world is facing the threat of COVID-19 as new strains emerge one after another. These strains have different sensitivity to the approved COVID-19 vaccine. With the continuous optimization and improvement of technology in this field, it is possible to develop mRNA drugs for infectious diseases and non-tumor indications in the future.
The transient expression of encoded proteins makes mRNA therapy an ideal way to express a single or a few proteins, such as infectious disease vaccines. The characteristics of repeatable administration, adjustable dose and optional administration interval make this technique as flexible as classical drug therapy, making it a choice that can adapt to the individual needs of patients and allay patients’ doubts about this new technology. From the point of view of safety, the results of two clinical trials (mRNA-1273 and mRNA-1647) showed that the adverse reactions after the second vaccination were more serious than those after the first vaccination, but this is not a common phenomenon. Some patients received more than eight doses of the BNT111 vaccine and no serious adverse reactions were observed. The inhaled mRNA drug MRT5005 was taken up to 5 times a week, during which no safety deterioration was observed.
Through the study of recombinant protein, the potential of mRNA therapy will be further expanded. For example, mRNA can encode a fusion protein of the Fc domain and therapeutic domain, which can effectively prolong the half-life of proteins in vivo. Even more exciting is the use of mRNA in cell therapy. The transient effect of mRNA gene editing can avoid the adverse reactions caused by permanent expression. In addition, antibodies, antibody fragments, or other protein-binding motifs expressed in cells can be expressed in specific organelles (such as nuclei) to give full play to the function of encoded proteins.
At present, mRNA only acts on the immune system, and its application scenarios include infectious disease vaccines and tumor vaccines. The approval and marketing of COVID-19 mRNA vaccines provide an unprecedented opportunity for the feasibility verification of mRNA technology, but at present, no new drug has been approved for tumor vaccine. The results of BNT111 show that combining sufficient protein expression with an immune activation pathway can solve some of the difficulties encountered by previous protein vaccines.
Potentials
Because RNA can activate the immune system through TLR and RIG-1 signaling pathways, the immune stimulation of RNA is of great significance to mRNA drugs. CV8102, for example, does not have the ability to encode but acts as an immune adjuvant. The disadvantage of this immunostimulatory effect is that it may cause problems with the safety and tolerance of some mRNA drugs. There is growing evidence that the most common side effects of mRNA drugs are certain inflammatory reactions. For example, local reactions of pain, redness and swelling may occur during intramuscular or subcutaneous injection; febrile syndrome or influenza-like reactions may occur during intramuscular injection or inhalation of drugs. These symptoms can be treated with anti-inflammatory drugs. In clinical trials of intravenous injection of mRNA encoding chikungunya virus mAb, pre-treatment with steroids in subjects who received the highest dose reduced the incidence of adverse reactions, but this treatment reduced the expression level of the encoded protein.
These data provide some clues for further study of the development direction of repeated administration of mRNA in the future. For example, are steroids effective and necessary to alleviate mRNA-induced inflammation? If the answer is yes, does this mean at the expense of the expression level of the encoded protein? Does the decrease in side effects and the level of coding proteins mean that a certain degree of inflammation is actually a prerequisite for adequate expression of encoded proteins? If the answer to the above questions is yes, then in clinical practice, the degree of inflammation must be balanced with the level of protein expression, and there should be no adverse reactions that hinder repeated administration.
It is not easy to answer these questions, because most mRNA drugs are not injected directly with naked mRNA, but are wrapped in LNP or PNP to improve the tolerance of the preparation. In subsequent clinical trials, the use of blank LNP without mRNA in the control group was helpful to answer the effect of mRNA and LNP on the tolerance of the preparation. However, this control also has a defect, that is, when empty LNP does not bind to negatively charged mRNA, its physical and chemical properties are different from those of LNP loaded with mRNA, and can not be called a strict control group.
Although transient inflammation is acceptable in life-threatening applications such as single vaccination and tumor treatment, for indications that require long-term treatment, especially intravenous administration, it is important to choose lipids and other excipients that are well-tolerated and safe. The value of animal data in this regard is limited because it is reported that the concentration of cytokines triggered by BNT111 in humans is more than 1000 times lower than in mice, while no febrile response in human experiments was observed in MRT5005’s animal experiments.
In addition to immediate tolerance, the long-term problems caused by lipid accumulation also need to be considered. If the half-life of the protein encoded by mRNA is short, it is necessary to shorten the administration interval to maintain the protein expression necessary for clinical efficacy, which may lead to the accumulation of lipids in the target tissue and non-target tissue, which brings health risks. The prescription process of mRNA, together with mRNA biology, will become the focus of future research and development.
The route of administration of mRNA is also very imaginable, such as intramuscular administration, intradermal administration, subcutaneous administration, lymph node injection, intratumoral injection, intravenous injection and inhalation administration and so on. After further development, intranasal vaccines, eye drops or nasal drops, skin ointments, anal suppository, bladder infusion solution, intrathecal infusion or Ommaya sac are expected in the future. In the future, the treatment of mRNA will depend directly on the progress of nano-drug delivery technology. There is growing evidence that LNP and PNP can target tissues (such as the liver, endothelium, lungs, bones and multiple tissues of the immune system). Through the improvement of delivery materials and the modification of other functional materials to improve the targeting and delivery ability of the delivery system, the application scene of mRNA will be further broadened.
The two EUA-approved COVID-19 vaccines now highlight one of the major advantages of mRNA, which is the rapid productivity of clinical trial drugs. The preparation process initially developed for the individualized neoantigen vaccines allowed clinical trials of the mRNA vaccine candidates to commence within weeks of the publication of COVID-19 sequences, demonstrating their rapid response capability.
However, the use of the COVID-19 vaccine also highlights another problem with the technology: reliance on cold chain storage and transportation. mRNA needs to be stored and transported at -80 ℃, which is not available in all pharmacies and clinical trial bases. For patients who give drugs at home, the storage conditions at -20 ℃ may be difficult to meet. Therefore, improving the stability of mRNA drugs will be the next focus of its prescription process research.