Opinion THE STORY

SO FAR… AND THE PATH AHEAD

ACHIEVEMENTS AND OBJECTIVES OF mRNA VACCINES

One of the many lessons learned during the COVID-19 pandemic was that our immune system was still the ultimate way to combat disease. Building on the success of the Moderna and Pfizer-BioNTech vaccines, more than 155 mRNA-based vaccines are in the clinical pipeline today, with market value expected to surpass $2 billion by 2026. The question remains: what makes mRNA-based medicine so compelling, and what are the challenges in vaccine development?

prompting the body to produce antibodies. mRNA vaccines launched less than a year after the sequencing of the SARSCoV-2 virus, helping to save lives and accelerate a return to normality.

mRNA AS AN ACTIVE VACCINE INGREDIENT

The main takeaway from all these examples is that mRNA-vaccines harbour several advantages that deem them feasible, safe, and efficient. This could come across as rather surprising, given that there was a shift in the 1990s toward the development of DNA vaccines because of the higher stability of DNA when compared to mRNA. Yet, mRNA vaccines exhibited advantages over DNA vaccines, which compensated for this deficiency and is why mRNA vaccine development is significantly more common than DNA vaccine development. Unlike DNA vaccines, which must pass through the cytoplasm and into the nucleus to be expressed, mRNA vaccines are targeted for cytoplasmic delivery and break down shortly after triggering an immune response. This circumvents the risk of integration into the host genome. In addition, mRNA is suitable for cell-free production through a DNA template and in vitro transcription system, eliminating the need for cell cultures.

CHALLENGES: mRNA STABILITY, CELLULAR UPTAKE AND STORAGE

Despite their rising profile, mRNA vaccines come with challenges that have only been partially addressed so far. In particular, the transient nature of mRNA is a double-edged sword. While it can prevent mRNA from overstimulating the immune system, it also makes mRNA unstable and susceptible to rapid degradation by RNases before even reaching the target. Several approaches have been implemented to tackle this pitfall. Synthetic 5’ cap analogs can be integrated into mRNA to shield it from exonucleases and immune sensors while mediating binding to the eukaryotic initiation factor during translation. Sequence optimisation is another strategy to modulate the immunogenicity of the mRNA vaccine. The addition of untranslated sequences before and after the gene, codon optimisation of the gene sequence, the addition of nucleic base analogs, and the length of a poly A tail can all affect the stability and level of expression of the mRNA. Together, these factors offer powerful options to modulate the immune response to be specific to the antigen while limiting non-specific side effects.

Another major hurdle in mRNA vaccines is successful intracellular delivery. The possibility of degradation by endonucleases and exonucleases aside, highly polar mRNA cannot pass the nonpolar cell membrane. This required researchers to invent delivery systems for maximum protection and cellular uptake. To that end, lipid nanoparticles (LNPs) have become the primary delivery vehicle due to their ease of scaleup, biocompatibility, cellular uptake via endocytosis, and pH-controlled mRNA release inside the cells. However, they require additional analytical tools to ensure that mRNA integrity is retained inside the LNP. Furthermore, the type of LNP must be taken into account, as some studies revealed that they stimulated pro-inflammatory cytokine secretion. More research is crucial to uncover the delivery mechanism and safety profiles of LNPs.

The monitoring of mRNA integrity and stability continues even post-manufacturing. Moderna and Pfizer store their vaccines between -20°C and -70°C to prevent breakdown by hydrolysis; however, the LNPs could form aggregates at these temperatures, impairing their function. To overcome these challenges, LNP-mRNA vaccines can be stored in sucrose solutions to keep LNPs apart and stable. Furthermore, freeze-drying allows them to be stored at slightly higher temperatures in refrigerators to prevent aggregation.

FUTURE STEPS IN mRNA VACCINE OPTIMISATION

With optimisation strategies for safety, stability, and targeted delivery, the potential of mRNA vaccines can be augmented. That said, many challenges still exist. For example, can we better optimise delivery and dosages, particularly for those who may be immunocompromised? Can we formulate or stabilise mRNA vaccines to eliminate the need for cold chain storage that prevents access in areas of the world with limited biomedical infrastructure? Also, how can researchers predict posttranslational modifications of the target antigen, which are the key players evoking immunogenicity? In fact, research suggests that the spike protein, which is responsible for viral binding and entry to host cells, is heavily glycosylated. Therefore, considering various post-translational modifications during in vitro transcription can yield stronger immunogenic reactions and enhance efficacy, opening the door for a new era of next-generation vaccines designed to improve human health.