THE STORY BEHIND ARCT-15: THE FIRST SELF-AMPLIFYING RNA VACCINE

 

Till today, COVID-19 vaccines created by Moderna and the Pfizer-BioNTech partnership are widely used for vaccination. They use mRNA to provide instructions to cells, prompting them to produce copies of a specific protein present on the surface of SARS-CoV-2 virus particles, which is known as the spike protein.  This process encourages the body to generate antibodies that specifically recognize and target this protein, while also initiating various other immune responses. An essential element of COVID-19 mRNA vaccines included lipid nanoparticles (LNPs), which encase the modified RNA and facilitate its entry into cells. 

This led to a breaking thought and was considered a “Vaccine revolution”, and to address it biochemist Katalin Karikó and immunologist Drew Weissman were awarded the Nobel Prize in Physiology or Medicine this year for discoveries that enabled the development of mRNA vaccines against COVID-19. 

The initial authorization of mRNA vaccines marked a milestone in the fight against SARS-CoV-2, showcasing their effectiveness in preventing severe illness and fatalities. Nevertheless, drawbacks have been acknowledged, including the necessity for cold storage, limited longevity of immunity, and challenges in distributing them to low-resource areas.

Inspired by the studies done prior, researchers have come up with advancements in the COVID-19 vaccine where the RNA self amplifiers known as ‘self-amplifying’ RNA (saRNA) vaccine. The improvised part is that now a small amount of dosage is only needed for this vaccination. The exciting news is this RNA-based vaccine for COVID-19 is approved. 

In clinical trials, the recently approved ARCT-154 vaccine, created by Arcturus Therapeutics in San Diego, CA, in collaboration with CSL, a biotech company based in Melbourne, Australia, elicited elevated levels of virus-fighting antibodies compared to a typical mRNA COVID-19 vaccine, with these antibodies remaining in circulation for an extended period.

Considering the RNA-based COVID-19 shot contains mainly the genetic instructions for a viral protein, surrounded by regulatory sequences, the cell produces protein - antibodies until the instruction is present. 

On the other hand, saRNA injections take an additional step by incorporating the necessary genes for replicating and synthesising the RNA that encodes the antigen. This essentially creates a biological mechanism within cells to produce the vaccine, akin to a printing press for manufacturing the vaccine internally. Due to its resemblance to a virus, saRNA interacts uniquely with the immune system, potentially offering advantages in various disease scenarios. For instance, in preventing infections, its self-amplifying properties allow for the utilisation of smaller vaccine doses.

" ARCT-154 necessitates only one-tenth to one-sixth of the vaccine quantity per individual compared to other RNA-based COVID-19 booster shots "

The aim of creating the self-amplifying RNA (saRNA)-based vaccine is to improve vaccine effectiveness and safety profile. The saRNA expresses membrane-anchored receptor binding domain (RBD) of SARS-CoV-2 S protein (S-RBD) and has shown that a minimal dose of the vaccine induces a robust range of immune responses. 

To increase the immune response of the saRNA vaccine, 5-methylcytidine (5mC) is incorporated into the vaccine, leading to reduced adverse effects while maintaining strong immune responses. In a study, the mechanism of this strong immune response (innate and acquired) is explained as they prolong the expression of antigen, inducing type-I interferon (IFN-I), the major driver of vaccine reactogenicity.

Additionally, the study found that the production of IFN-I, typically associated with adverse effects, was reduced in peripheral blood mononuclear cells (PBMCs) but not in macrophages and dendritic cells. Interestingly, plasmacytoid dendritic cells (pDCs) were identified as the primary source of IFN-I production in PBMCs, and this production was attenuated with the incorporation of 5mC into saRNA.Importantly, the 5mC-incorporating saRNA vaccine candidate induced robust IgG responses against the target antigen, S-RBD, in mice, indicating its potential for clinical use.

However, saRNA also had its barrier, the efficiency of the saRNA got decreased due to the earl interferon response that trigger upon cell entry, leading to saRNA degradation and translational inhibition. So mRNAs were altered with modified nucleotides (modNTPs), such as N1-methyl pseudouridine (N1mΨ), but they resulted in reduced interferon response and enhanced expression levels. However, eventually, researchers identified multiple modNTPs that, when incorporated with saRNA showed 100% substitution conferred immune evasion and enhanced expression potency. This breakthrough significantly expands the potential of self-amplifying RNA, allowing access to cell types previously inaccessible and opening up possibilities for utilizing saRNA technology in areas beyond vaccines, like cell therapy and protein replacement.

In conclusion, the development of mRNA vaccines, particularly those for COVID-19, represented a groundbreaking advancement in vaccine technology. The contributions of scientists like Katalin Karikó and Drew Weissman, recognized with the Nobel Prize in Physiology or Medicine, paved the way for the successful deployment of mRNA-based vaccines. However, challenges such as cold storage requirements and limited access in low-resource regions prompted further innovation, leading to the emergence of self-amplifying RNA (saRNA) vaccines. The approval of saRNA-based vaccines like ARCT-154 offers promising prospects for enhanced vaccine efficacy and reduced dosage requirements. Incorporating advancements like 5-methylcytidine (5mC) and modified nucleotides (modNTPs) has further improved the safety and effectiveness of saRNA vaccines. This breakthrough not only expands the potential of RNA-based vaccines but also opens doors for applications in diverse fields such as cell therapy and protein replacement, heralding a new era in biotechnology.

REFERENCE

Dolgin E. Self-copying RNA vaccine wins first full approval: What’s next? Nature. 2023 Dec 6;624(7991):236–7. doi:10.1038/d41586-023-03859-w 

Voigt EA, Gerhardt A, Hanson D, Jennewein MF, Battisti P, Reed S, et al. A self-amplifying RNA vaccine against COVID-19 with long-term room-temperature stability. npj Vaccines. 2022 Nov 2;7(1). doi:10.1038/s41541-022-00549-y 

Oda Y, Kumagai Y, Kanai M, Iwama Y, Okura I, Minamida T, et al. Booster dose of self-amplifying SARS-COV-2 RNA vaccine vs mrna vaccine: A phase 3 comparison of ARCT-154 with Comirnaty®. 2023 Jul 13; doi:10.1101/2023.07.13.23292597 

Komori M, Morey AL, Quiñones-Molina AA, Fofana J, Romero L, Peters E, et al. Incorporation of 5 methylcytidine alleviates innate immune response to self-amplifying RNA vaccine. 2023 Nov 1; doi:10.1101/2023.11.01.565056

McGee JE, Kirsch JR, Kenney D, Chavez E, Shih T-Y, Douam F, et al. Complete substitution with modified nucleotides suppresses the early interferon response and increases the potency of self-amplifying RNA. 2023 Sept 17; doi:10.1101/2023.09.15.557994 

IMAGE CREDITS

Nature, https://www.nature.com/articles/s41434-020-00204-y/figures/1

John Hopkins, https://images.app.goo.gl/YmDwtjfcAVwhty8c6