Circling back to RNA vaccines
Although circRNA is often touted as a more stable and long-lasting alternative to linear mRNA, its actual benefits remain largely theoretical and it is unclear whether greater molecular stability will translate into significant clinical gains.
In recent years, mRNA vaccines have evolved from a laboratory concept to a rapid and effective global health tool. Since their success during the COVID-19 pandemic, mRNA vaccines have expanded into cancer immunotherapy, respiratory vaccines, and vaccines for infectious diseases such as HIV and malaria. This expansion has sparked a new wave of interest in next-generation RNA technologies, including circular RNA (circRNA). CircRNAs are covalently closed single-stranded RNA loops that lack 5′ or 3′ ends, making them very stable against enzymatic degradation, unlike their linear RNA counterparts. Researchers are actively developing circRNA-based vaccines to treat various cancers and infectious diseases, but it remains unclear whether they will replace mRNA or even succeed through clinical trials.
Originally thought to be byproducts of mis-splicing, circRNAs have been revealed by high-throughput sequencing to be highly evolutionarily conserved, physiologically functional, and widespread in eukaryotic cells, with a half-life significantly longer than that of linear RNA. Scientists have discovered that they function as scaffolds, binding and regulating microRNAs and proteins, and helping to facilitate protein-protein interactions. Over the past decade, researchers have discovered how to precisely engineer circRNAs to sustainably produce proteins in mammalian cells.1which led to the idea that they could be used as an alternative platform for vaccines – an idea that took off after mRNA vaccines set a new vaccine standard.
But it is in cancer vaccines where circRNAs could have real clinical impact – not because the platform has proven superior, but because the immunological requirements of cancer align exceptionally well with what circRNAs are theoretically capable of delivering. Unlike infectious disease vaccines, which rely primarily on rapid, short-lived bursts of antibody production, effective cancer vaccines depend on sustained antigen presentation to generate robust cytotoxic T cell responses and overcome the immunosuppressive environment of tumors. In this context, the resistance of circRNAs to degradation and the potential for prolonged protein expression become particularly relevant. Expanded antigen availability may enhance T cell priming and enhance immune memory, particularly when combined with checkpoint inhibitors that release the inhibitory brakes of the immune system. This is particularly relevant for “cold” tumors that do not naturally trigger strong immune responses.
Currently, researchers are trying to prove that circRNAs actually provide these benefits in mouse models. Initial studies have shown that circRNA can act as an adjuvant, activate immune responses and inhibit melanoma growth.2. A study published last year showed that vaccines containing small circRNAs had a longer shelf life and produced a stronger immune cellular response in mice than vaccines containing mRNA, larger circRNAs, or even modifications containing mRNA to improve stability. The mice also appeared to tolerate higher doses of the small circRNA vaccine, with all vaccines eliciting dose-dependent responses over 180 days.3. The small circRNA vaccine has also been tested on several types of tumors in mice in combination with other immunotherapies, with promising results. Another group recently presented data that a circRNA vaccine increased tumor-related immune responses more than an mRNA vaccine in mouse models of lung cancer.4but we don’t know how much.
Personalized neoantigen vaccines are another area where circRNA appears particularly attractive compared to personalized mRNA vaccines – so far in mouse models. Neoantigen vaccines are designed around mutations unique to an individual’s tumor, and the goal is to train the immune system to recognize these highly specific “non-self” targets that are very specific to the tumor and less likely to be shared with normal tissues. CircRNA is modular, relatively stable, and can potentially produce sustained expression of antigens from very small amounts of input material, which can simplify formulation to produce highly personalized constructs that could be modified relatively quickly depending on disease progression.
Initial studies showed that a circRNA neoantigen vaccine combined with anti-PD-1 treatment improved CD8 tumor infiltration+ T cells in a mouse colon adenocarcinoma tumor model, reducing immunosuppressive tumor microenvironment and inhibiting tumor growth3. Other studies have looked at hepatocellular carcinoma5 and melanoma models6although there is no direct comparison with mRNA vaccines and sample sizes are small in all cases. For vaccination against infectious diseases, the appeal of circRNA is based on the hypothesis that longer-lasting antigen expression could prolong immune memory compared to the short-lived protein production typical of mRNA vaccines. In principle, the prolonged presence of the antigen could improve antibody maturation and strengthen T- and B-cell memory, potentially resulting in longer-lasting protection or a reduced need for boosters. CircRNA vaccines produce potent antibody and T cell responses against SARS-CoV-2 and its emerging variants in mice and non-human primates7.8. However, the reality is that mRNA vaccines already generate strong and long-lasting immunity in many settings, and it is not yet clear that extending antigen expression necessarily improves outcomes, rather than simply prolonging stimulation. Unlike cancer, where persistent exposure to an antigen can help overcome immune suppression within tumors, vaccination against infectious diseases already works well under a “brief exposure and strong memory” model.
With all this promise, it is somewhat surprising that nothing is currently progressing in clinical trials. Earlier this year, Eli Lilly acquired Orna Therapeutics for $2.4 billion, gaining access to Orna’s circRNA and lipid nanoparticle therapies for autoimmune diseases. Another biotech, Circuna, has a handful of circRNAs for vaccines and immunotherapies looking to reach phase 1 trials in the next two years. Otherwise, the terrain is calm.
There may be several reasons for this. First, the overall funding landscape around mRNA and vaccines has declined significantly over the past year in the United States. The main reason, however, could be that circRNA has not yet been proven to be a better option than linear RNAs, especially compared to stably modified versions or self-amplifying RNAs.9. When you already have a good vaccine, there is little incentive to change it.
It is not yet clear whether circRNAs will become a second wave of RNA-based drugs or whether they will end up being just an alternative for specific, targeted applications. Like mRNA vaccines, circRNAs have distribution issues that need improvement, and have not yet been shown to be able to be produced at scale. The long-lasting nature of circRNA might also have unknown safety trade-offs.
Biology has long cured cancer in mice: only a fraction of therapies effective in mouse tumor models prove safe and effective in humans. CircRNA vaccines hold promise, but whether that promise survives the transition from mice to humans will determine whether they are a footnote or the next era of vaccinology.
