RNA Therapeutics: Limited or Limitless?

Written by Charlotte Pugsley, edited by Caroline Babisz and Matthew McCann.

Ribonucleic acid, RNA, is a molecule that the public has heard more about over the last two years than ever before. This is thanks to the outbreak of the Covid-19 pandemic, and the seemingly rapid development of the Moderna and BioNTech/Pfizer mRNA vaccines. However, for many scientists and biotechnology companies, the age of RNA-based technologies has been a long time coming, with the first successful injection of mRNA in a living organism harking back to 1990 (1).

Recap: What is RNA?

RNA has several forms, all of which have specific roles in our cells. Messenger-RNA (mRNA) acts as it says on the tin – a messenger. Its main role is to carry instructions which contain the ‘code’ required for proteins to be synthesised. Transcription, occurring in the nucleus, is the process by which this code is passed onto the mRNA. The mRNA then leaves the nucleus and moves into the cytoplasm, where it delivers the genetic code. This code is then translated and synthesised into specific proteins. Other forms of RNA exist and are touched upon later in the article.

‘Undruggable’ Proteins to Bioethics

Manipulating the presence, absence, amount, or function of proteins is critical to tackling diseases. Typically, drug research focuses on small molecules that bind to proteins to alter their function within the body. However only 10 – 14% of proteins have feasible binding sites known as ‘druggable’ targets. This is why gene therapies have become an area of interest, as DNA and RNA act prior to protein synthesis rather than after it, making the seemingly ‘undruggable’ proteins ‘druggable’(2). DNA therapeutics are also a hot topic thanks to the advent of genome editing with CRISPR-Cas9 (3). Genome editing enables accurate disease modelling, however there is a general feeling of apprehension towards implanting foreign DNA into our cells. There are concerns about the unwanted permanent integration of DNA, and the bioethics of altering the genome (4).

The development of RNA therapeutics alleviates these concerns, as RNA works within the cell cytoplasm without having to cross the nuclear membrane meaning RNA won't integrate in our cellular make-up. In some cases, such as chronic diseases, a long-term, systemic therapeutic effect might be required but more often, we want a short-lived effect that RNA can provide.

Four Camps

RNA therapeutics can be split into four camps: antisense RNA, RNA interference (RNAi), RNA aptamers and messenger-RNA.

1. Antisense RNA uses single-stranded RNA that is a complementary match to the target (i.e. disease-associated) mRNA. Binding to the mRNA causes its degradation or blocking of translation. Thus, protein synthesis, and subsequently disease, is prevented (5).
2. RNAi involves the delivery of double-stranded RNA (dsRNA). These dsRNAs disrupt the synthesis of disease-associated proteins by degrading mRNA using RNAi machinery that is already present in cells. (An aside: There is potential for RNAi to be not only used in humans, but also by farmers as a species-specific bio-insecticide, targeting only pests and leaving other species unharmed (6).)
3. Delivery of mRNA induces the synthesis of new proteins within the cytoplasm. In cases where a disease has compromised wild-type protein synthesis, the mRNA could encode proteins to replace or restore those missing or ineffective. In other cases, it could encode the instructions to synthesise antigens to a virus, such as in the case of the Covid-19 vaccines.
4. RNA aptamers are designed to target disease-associated proteins directly. Similar to the action of small drug molecules, aptamers bind to proteins and alter their function.

When Engineering meets Biology meets Chemistry

Unfortunately, RNA therapeutics all suffer from a challenge of stability. RNA is inherently susceptible to degradation by enzymes in the environment and in the body; single-stranded RNA is particularly vulnerable (2). Much research is ongoing to understand how we can more effectively deliver RNA. In the BioNTech/Pfizer and Moderna vaccines, lipids are used to encapsulate the mRNA, forming nanoparticle complexes that can safely deliver the mRNA into cells.  Many research groups are working on polymers to protect RNA molecules, as polymers are versatile and easy to chemically modify.

However, in a fascinating mix of engineering, biology and chemistry, researchers from MIT have recently published a study detailing devices that can deliver mRNA to the stomach lining. The novel capsules are called ‘Self-Orienting Millimetre-Scale Actuators’, or SOMAs for short, and are described as ‘tortoise’-like in their ability to correctly orient themselves to lie flat on the surface of the stomach. Inside these devices is a needle that will inject mRNA nanoparticles into the stomach lining, with the mRNA further protected with poly(beta-amino ester) polymers. In testing the SOMA mRNA devices, the research groups of Langer and Traverso were able to deliver up to 150 mg of mRNA at a time, a much higher dose than is given in Covid-19 mRNA vaccines (30 – 100 mg). This technology therefore both halts the need for painful injections and allows higher levels of dosing (7).

‘Breath-taking’ Investment

As well as within academic research, RNA therapeutics are booming in the investor landscape. In recent news, the German company, Ethris, raised €23.3 million in a Series B funding round at the start of February 2022, led by Laureus Capital. Ethris, founded in 2009, will use the funding to develop their mRNA therapies. Uniquely, Ethris designs mRNA therapies that are inhalable, specifically for use for respiratory viral infections and genetic conditions (8).

The era of RNA therapeutics is dawning, bringing with it new and innovative drugs, vaccines and therapies that shouldn’t suffer the same pitfalls of those before them. However, as with any novel biotechnology, RNA therapeutics come with their own set of challenges. Research must focus on further developing safe and efficient treatments, maximising potency whilst limiting side effects. Since the publicity of the mRNA vaccine for Covid-19, public opinion of RNA therapeutics has been mixed, but the door has opened to these treatments with a surge of RNA-based start-ups having sprung up in response. Hopefully these start-ups will continue to grow as public perception slowly improves, promising an exciting future of innovative therapies with a big impact on our healthcare.

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