scott.pritchett

On November 27, 2023, the Japanese Ministry of Health, Labour and Welfare approved ARCT-154, a self-amplifying RNA (saRNA) COVID-19 vaccine developed jointly by CSL and Arcturus Therapeutics, for use in initial and booster vaccinations in adults aged 18 and above. ARCT-154 became the world's first saRNA vaccine to be approved for marketing. This vaccine triggers a highly efficient immune response at extremely low doses, making it a pioneering breakthrough in the next generation of RNA vaccines.

The RNA Therapeutic Revolution

mRNA vaccines are a novel type of vaccine that introduce specific mRNA molecules into cells to instruct them to produce antigen proteins, triggering an immune response. Compared to traditional vaccines, mRNA vaccines offer several advantages, including rapid design and production, lack of live viral components, and a non-integrating mechanism of action. However, mRNA presents some inherent challenges. Within cells, traditional mRNA is prone to degradation, resulting in unsustainable levels of protein expression. When used for long-term disease treatment, large amounts of mRNA may need to be administered to patients, potentially increasing the toxicity and side effects of the treatment. To overcome this challenge, scientists are researching and developing novel modifications and RNA forms to improve mRNA stability and persistence.

What is self-amplifying RNA?

The saRNA vaccine originates from the genome of alphaviruses and comprises a gene responsible for viral RNA replication and another encoding a therapeutic antigen. It is significantly larger than conventional mRNA, containing basic mRNA elements (including 5′cap, 5′UTR, 3′UTR, and polyA tail), along with encoding four non-structural viral proteins (nsP1-4) and a subgenomic promoter derived from the alphavirus genome. In the saRNA sequence, the gene encoding viral structural proteins is replaced by the vaccine antigen gene. The absence of viral structural proteins ensures that saRNA cannot generate infectious virus, thereby ensuring safety. After entering the cytoplasm, saRNA first translates the RNA-dependent RNA polymerase (RDRP), which synthesizes complementary antisense RNA strands using the positive-sense RNA as a template. It then utilizes these antisense RNA strands to replicate multiple copies of positive-sense RNA. This process results in higher and sustained levels of antigen expression compared to traditional mRNA, which is why saRNA vaccines generally require lower doses.

Advantages and disadvantages of saRNA

saRNA represents a significant advancement in vaccine technology. Due to its self-amplifying nature, saRNA vaccines can achieve a significantly lower injection dosage compared to traditional mRNA to elicit an equivalent immune response. For instance, compared to Moderna’s mRNA vaccine at 100μg and BioNTech/Pfizer’s mRNA vaccine at 30μg, the COVID-19 saRNA vaccine ARCT-154 only needs an injection dosage of 5 μg/dose. This benefit becomes particularly advantageous for rapid response to outbreaks, as the lower dosage not only reduces vaccine production costs but also eases some of the strain on storage logistics and distribution, thereby enhancing vaccination coverage potential.

saRNA’s remarkable ability to propagate within host cells also means that saRNA vaccines have the potential to achieve sustained and high-level expression of vaccine antigens without the need for repeated doses. This feature not only enhances the efficiency of antigen production, but also simplifies vaccine administration, reducing the logistical challenges associated with multiple injections.

Also, during replication, saRNA forms double-stranded RNA structures resembling viral RNA genomes, which can be recognized by pattern recognition receptors (PRRs) within the cell, consequently activating innate immune responses. This recognition has the potential to further enhance the immune response elicited by saRNA as a vaccine.

At the same time, saRNA also presents some drawbacks. There is a risk that saRNA vaccines may excessively activate inflammatory responses, potentially leading to adverse reactions or exacerbation of inflammatory conditions. The non-structural proteins from alphaviruses present in saRNA vaccines might interfere with normal cell signaling pathways, potentially disrupting cellular functions, which can lead to cellular exhaustion, ultimately culminating in apoptosis through immune stimulation and the host cell's antiviral response. These limitations must be carefully considered in the development and deployment of saRNA vaccines, with appropriate strategies implemented to mitigate associated risks.

Clinical progress of saRNA vaccines

Currently, there are relatively few companies globally involved in the development of saRNA vaccines and therapeutics. BioNTech is developing saRNA vaccines alongside its conventional mRNA vaccine efforts, with saRNA vaccines targeting the novel coronavirus having progressed to clinical stages. Novartis began saRNA vaccine development in 2012, but in 2015, they sold their saRNA vaccine business to GSK. GSK now has a saRNA vaccine for rabies in Phase I clinical trials, and a saRNA vaccine for COVID-19 has also entered Phase I clinical trials. Gritstone bio announced positive data from a Phase I clinical trial of their second-generation T cell-enhanced saRNA COVID-19 vaccine on January 4, 2022.

JCXH-211, developed by Immorna, is based on saRNA technology platform and encodes human interleukin-12 (IL-12). It is delivered via lipid nanoparticles. In November 2022, Immorna announced that JCXH-211 injection has been approved by the China National Medical Products Administration (NMPA) to initiate Phase I clinical trials. It is intended for the treatment of advanced malignant solid tumors. This marks the world's first saRNA drug encoding human interleukin to enter clinical trials.

ARCT-154 COVID-19, which developed by Arcturus Therapeutics, started clinical trials in August 2021 in Vietnam. On November 27, 2023, Arcturus Therapeutics and CSL jointly announced that the Ministry of Health, Labour and Welfare (MHLW) of Japan has approved the self-amplifying mRNA (sa-mRNA) COVID-19 vaccine ARCT-154 for initial and booster vaccinations in adults aged 18 and older. This markeds the world's first formally- approved saRNA COVID-19 vaccine.

Challenges with saRNA technology

Despite the promising potential of saRNA vaccines, several challenges must be addressed to realize their full benefits. One significant challenge is the complexity and cost of the production process. The molecular weight of the entire saRNA is much larger (~10kb) and more complex compared to traditional mRNA as it includes a significant number of secondary structures. Accordingly, saRNA vaccines require precise manufacturing techniques to ensure stability, purity, and potency, which can increase production costs and pose logistical challenges.

The high immunogenicity of saRNA is a double-edged sword. While as a vaccine, enhanced immune responses are usually beneficial, as a therapeutic agent, stimulated immune reactions can cause unexpected side effects. Moreover, if the innate immune response is excessively activated, it may inhibit mRNA expression, thereby affecting therapeutic efficacy. Therefore, further research and evaluation are needed to assess the potential immune-stimulating reactions of saRNA.

Additionally, the optimization of delivery systems, such as lipid nanoparticles, is essential to enhance the stability and efficacy of saRNA vaccines in vivo. To date, scientists have explored several delivery platforms for saRNA, including lipid nanoparticles (LNPs), polymeric complexes, and cationic nanoemulsions. Among these, LNPs have emerged as the most successful RNA delivery system. However, these delivery platforms, validated in large populations via widespread administration of COVID-19 vaccines, were initially developed and optimized for delivering shorter RNAs, such as siRNA (~20 nucleotides) or conventional mRNA (1000-5000nt), and thus may not be ideally suited for longer saRNA molecules. Further research and innovation may be needed to develop more effective delivery systems tailored for the delivery of long-chain saRNA.

GenScript is proud to be one of the world’s first saRNA suppliers and offers comprehensive saRNA development and preparation solutions. Our team possesses extensive experience in saRNA design and production, aiding researchers in vector design, cloning, in vitro synthesis, LNP encapsulation, as well as in vitro/in vivo functional testing of saRNA. This offers tailored design and modifications with reliable quality to help speed up your innovative research projects.

Future Prospects of saRNA Vaccines

saRNA vaccines are poised to revolutionize global vaccination strategies due to their rapid development, scalability, and adaptability to emerging variants. Their ability to elicit robust immune responses with lower doses holds promise for improving vaccine distribution and coverage, particularly in resource-limited settings. Moreover, saRNA technology extends beyond infectious diseases, offering potential therapeutics for diverse medical conditions like cancer, genetic disorders, and autoimmune diseases. However, more efforts need to be done on stability, immunogenicity and delivery of saRNA to enhance vaccine efficacy and safety. Addressing these challenges will require collaborative efforts between scientists, industry partners, and regulatory agencies to optimize vaccine design, manufacturing processes, and delivery strategies. Despite these hurdles, continued advancements in saRNA vaccine technology hold the promise of revolutionizing vaccine development and offering innovative solutions to global health challenges.

Learn more about GenScript’s Self-Amplifying RNA Synthesis services here.

References:

[1] First self-amplifying mRNA vaccine approved. Nat Biotechnol. 2024;42(1):4.

[2] Gote V, Bolla PK, Kommineni N, et al. A Comprehensive Review of mRNA Vaccines. Int J Mol Sci. 2023;24(3):2700.

[3] Blakney AK, Ip S, Geall AJ. An Update on Self-Amplifying mRNA Vaccine development. Vaccine (Basel). 2021;9(2):97.

[4] Aliahmad P, Miyake-Stoner SJ, Geall AJ, Wang NS. Next generation self-replicating RNA vectors for vaccines and immunotherapies. Cancer Gene Ther. 2023;30(6):785-793.

[5] Vanluchene H, Gillon O, Peynshaert K, et al. Less is more: Self-amplifying mRNA becomes self-killing upon dose escalation in immune-competent retinal cells. Eur J Pharm Biopharm. 2024;196:114204.

[6] Oda Y, Kumagai Y, Kanai M, et al. Persistence of immune responses of a self-amplifying RNA COVID-19 vaccine (ARCT-154) versus BNT162b2. Lancet Infect Dis. 2024;24(4):341-343.