The exciting potential of mRNA vaccines

Basic mRNA vaccine pharmacology

Basic mRNA vaccine pharmacology involves the study of two types of RNA used as vaccines: non-replicating mRNA and self-amplifying RNA. Non-replicating mRNA-based vaccines encode the antigen of interest and contain untranslated regions (UTRs) at both ends. Self-amplifying RNAs, on the other hand, encode both the antigen and the viral replication machinery, allowing for intracellular RNA amplification and abundant protein expression.

For successful protein production in mRNA therapeutics, the optimal translation of in vitro transcribed (IVT) mRNA is crucial. Factors such as the length of the poly(A) tail, codon usage, and sequence optimization can influence translation efficiency and accuracy.

Adding an optimal length of poly(A) to mRNA is necessary for efficient translation. This can be achieved by directly incorporating it from the encoding DNA template or by using poly(A) polymerase. Codon usage also plays a role in protein translation. Replacing rare codons with frequently used synonymous codons, which have abundant cognate tRNA in the cytosol, can enhance protein production from mRNA. However, the accuracy of this model has been subject to questioning.

Optimally translated IVT mRNA encoding mRNA

IVT mRNA plays a crucial role in mRNA vaccines as it is designed for optimal translation, ensuring efficient protein production. To achieve this, a 5ʹ cap structure is added, which is essential for efficient protein synthesis. Different versions of 5ʹ caps can be added during or after the transcription process. Furthermore, the poly(A) tail plays a significant regulatory role in mRNA translation and stability.

Sequence optimization is another critical factor that can enhance mRNA levels and protein expression. Increasing the G:C content has been shown to elevate steady-state mRNA levels in vitro and improve protein expression in vivo. Furthermore, modifying the codon composition or introducing modified nucleosides can positively influence protein expression. However, it is important to note that these sequence engineering techniques may impact mRNA secondary structure, translation kinetics, accuracy, protein folding, as well as the expression of alternative reading frames and cryptic T-cell epitopes.

Sequence optimization for protein translation

Sequence optimization plays a crucial role in the development of mRNA vaccines. It involves modifying the mRNA sequence to improve the efficiency of protein translation. By optimizing the sequence, researchers can enhance the expression and stability of therapeutic mRNAs.

However, the immunogenicity of exogenous mRNA is a concern, as it can trigger a response from various innate immune receptors. In some cases, encoding mRNA in the hypothalamus may even elicit a physiological response. Despite initial promising outcomes, the development of mRNA therapeutics has been hindered by concerns regarding mRNA instability, high innate immunogenicity, and inefficient in vivo delivery. As a result, DNA-based and protein-based therapeutic approaches have been preferred in the past.

Modulation of immunogenicity

Modulation of immunogenicity is a crucial aspect of mRNA vaccine development. Researchers aim to design mRNA vaccines that elicit a strong immune response while minimizing adverse reactions. This involves careful selection of antigens and optimization of the mRNA sequence to enhance immunogenicity.

Self-replicating RNA vaccines and adjuvant strategies, such as TriMix, have shown increased immunogenicity and effectiveness. The immunostimulatory properties of mRNA can be further enhanced by including adjuvants. The size of the mRNA-carrier complex and the level of innate immune sensing in targeted cell types can influence the immunogenicity of mRNA vaccines.

Advantages of mRNA vaccines

mRNA vaccines offer several advantages over conventional vaccine approaches. First, they have high potency, meaning they can induce a strong immune response. Second, they have a capacity for rapid development, allowing for quick vaccine production in response to emerging infectious diseases or new strains. Third, mRNA vaccines have the potential for rapid, inexpensive, and scalable manufacturing, mainly due to the high yields of in vitro transcription reactions. Additionally, mRNA vaccines are minimal genetic vectors, avoiding anti-vector immunity, and can be administered repeatedly.

However, recent technological innovations and research investments have made mRNA a promising therapeutic tool in vaccine development and protein replacement therapy. mRNA has several advantages over other vaccine platforms, including safety and efficacy. It is non-infectious and non-integrating, reducing the risk of infection and insertional mutagenesis. mRNA can be regulated in terms of in vivo half-life and immunogenicity through various modifications and delivery methods.

Production of mRNA vaccines

The production of mRNA vaccines involves in vitro transcription (IVT) of the optimized mRNA sequence. This process allows for the rapid and scalable manufacturing of mRNA vaccines. High yields of IVT mRNA can be obtained, making the production process cost-effective.

Making mRNA more stable and highly translatable is achievable through modifications. Efficient in vivo delivery can be achieved by formulating mRNA into carrier molecules. The choice of carrier and the size of the mRNA-carrier complex can also modulate the cytokine profile induced by mRNA delivery.

Current mRNA vaccine approaches (Figure 1)

There are several current mRNA vaccine approaches being explored. These include the development of mRNA vaccines against infectious diseases and various types of cancer. mRNA vaccines have shown promising results in both animal models and humans.

Cancer vaccines

Cancer vaccines are a type of immunotherapy that aim to stimulate the body's immune system to recognize and destroy cancer cells. These vaccines work by introducing specific antigens, which are substances that can stimulate an immune response, into the body. 

The immune system then recognizes these antigens as foreign and mounts an immune response against them, targeting and destroying cancer cells that express these antigens. There are different types of cancer vaccines, including personalized vaccines and predefined shared antigen vaccines. Personalized vaccines are tailored to each patient and are designed to target specific mutations or antigens present in their tumor. These vaccines are created by identifying tumor-specific antigens by sequencing the patient's tumor DNA and predicting which antigens are most likely to elicit an immune response. These antigens are then used to create a vaccine that is specific to that patient's tumor. 

On the other hand, predefined shared antigen vaccines are designed to target antigens that are commonly expressed in certain types of cancer. These vaccines can be used in multiple patients with the same type of cancer and are not personalized to each individual. The antigens used in these vaccines are selected based on their ability to induce an immune response and their potential to be recognized by T cells. 

Despite the promising potential of cancer vaccines, their clinical progress is limited, and skepticism surrounds their effectiveness. While there have been some examples of vaccines that have shown systemic regression of tumors and prolonged survival in small clinical trials, many trials have yielded marginal survival benefits. Challenges such as small trial sizes, resource-intensive approaches, and immune escape of heterogeneous tumors have hindered the field's progress. However, it is important to note that other immunotherapies, such as monoclonal antibodies and chimeric antigen receptor (CAR) T-cell therapies, have also faced challenges and setbacks before eventually achieving success. Therefore, cancer vaccines may also have the potential for eventual success, given their clear rationale and compelling preclinical data. 

To improve the efficacy of cancer vaccines, researchers are exploring various strategies. These include optimizing antigen presentation and immune activation by using adjuvants or agonists of pattern-recognition receptors. Additionally, advancements in sequencing technologies and computational algorithms for epitope prediction allow for the identification of more specific tumor mutagens and the production of personalized neo-epitope vaccines. Neo-epitope vaccines are a type of personalized vaccine that target specific mutations or neo-epitopes present in a patient's tumor. These vaccines exploit the most specific tumor mutagens identified through computational methods and prioritize highly expressed neo-epitopes. They can be given with adjuvants to enhance their immunogenicity. Hence, cancer vaccines hold promise as a potential standard anti-cancer therapy. 

While their progress has been limited, a clear rationale and compelling preclinical data support their further development. Personalized vaccines targeting specific mutations or antigens present in a patient's tumor, as well as predefined shared antigen vaccines targeting commonly expressed antigens, are being explored.

Future of mRNA vaccines

mRNA vaccines have emerged as a promising alternative to traditional vaccine approaches due to their high potency, rapid development capabilities, and potential for low-cost manufacture and safe administration. Recent technological advancements have addressed the challenges of mRNA instability and inefficient in vivo delivery, leading to encouraging results in the development of mRNA vaccine platforms against infectious diseases and various types of cancer. Looking ahead, the future of mRNA vaccines holds great potential for further advancements and widespread therapeutic use. 

Efficient in vivo delivery of mRNA remains a critical area of focus for future development. Researchers are working on improving delivery systems to ensure targeted delivery to specific cells or tissues, thereby enhancing the effectiveness of mRNA vaccines. This includes the development of lipid nanoparticles, viral vectors, and other delivery mechanisms to optimize mRNA delivery and cellular uptake.

The success of mRNA vaccines against infectious diseases and cancer has opened doors to exploring their potential in other areas of medicine. Future research may involve the development of mRNA vaccines for autoimmune disorders, allergies, and chronic diseases. The versatility of mRNA technology allows for the rapid adaptation of vaccine candidates to address various medical conditions.

One exciting prospect for mRNA vaccines is their potential for personalized medicine. The ability to easily modify the genetic sequence of mRNA allows for the development of personalized vaccines tailored to an individual's specific genetic makeup or disease profile. This could revolutionize preventive medicine by enabling targeted immunization strategies.

Combining mRNA vaccines with other treatment modalities, such as immunotherapies or traditional therapies, could lead to synergistic effects and improved clinical outcomes. The unique properties of mRNA vaccines, such as their ability to induce potent immune responses and modulate the expression of specific proteins, make them attractive candidates for combination therapies.

Continued advancements in manufacturing processes will be crucial for the widespread adoption of mRNA vaccines. Efforts are underway to optimize and scale up the production of mRNA vaccines, making them more accessible and cost-effective. This includes refining in vitro transcription reactions and implementing efficient quality control measures.

The regulatory landscape surrounding mRNA vaccines will evolve as the field progresses. Regulatory agencies will need to establish guidelines and frameworks specific to mRNA vaccine development and approval. Ensuring safety, efficacy, and quality control will be essential to gain widespread acceptance and public trust in mRNA vaccines.

Conclusion

mRNA vaccines have shown great potential in revolutionizing the field of medicine, particularly in the areas of personalized medicine and preventive medicine. The ability to easily modify the genetic sequence of mRNA allows for the development of personalized vaccines tailored to an individual's specific genetic makeup or disease profile. Furthermore, the unique properties of mRNA vaccines, such as their ability to induce potent immune responses and modulate the expression of specific proteins, make them attractive candidates for combination therapies. However, there are still challenges to overcome, such as ensuring safety, efficacy, quality control, addressing concerns regarding immunogenicity. Nonetheless, with continued advancements in manufacturing processes and regulatory guidelines, the future of mRNA vaccines holds great promise for further advancements and widespread therapeutic use. Efforts to improve in vivo delivery systems and explore the potential of mRNA vaccines in other areas of medicine, such as autoimmune disorders and chronic diseases, further contribute to the promising outlook for this technology.

Written by Sara Maria Majernikova

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REFERENCES

Lin, M.J., Svensson-Arvelund, J., Lubitz, G.S. et al. Cancer vaccines: the next immunotherapy frontier. Nat Cancer 3, 911–926 (2022). https://doi.org/10.1038/s43018-022-00418-6

Pardi, N., Hogan, M., Porter, F. et al. mRNA vaccines — a new era in vaccinology. Nat Rev Drug Discov 17, 261–279 (2018). DOI: https://doi.org/10.1038/nrd.2017.243