Heralding a New Era of RNA Therapeutics
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Recent advances in mRNA technology, accelerated by the COVID-19 pandemic, mark the beginning of a therapeutic revolution in medicine.
Messenger RNA, or mRNA, was first discovered in 1961. More than half a century later, the world’s first mRNA vaccines against COVID-19 gained emergency approval in several countries – just 11 months after the genome sequence of SARS-CoV-2 was released.
“The mRNA acts as a stepping stone between the DNA and the proteins that do the actions in our cells,” describes Anna Blakney, assistant professor at the University of British Columbia, Vancouver. “It carries a piece of genetic code to the cell’s protein-making machinery.”
Traditional vaccines use viral protein fragments or weakened viruses to train our immune system. However, the crucial ingredient in the BNT162b2 (BioNTech/Pfizer) and mRNA-1273 (Moderna) COVID-19 vaccines was instead mRNA coding for the spike protein that allows the coronavirus to enter our cells.
“Cells around the injection site take up the mRNA and begin to make the viral protein, displaying it on their surface,” explains John Cooke, chair of the department of cardiovascular sciences and medical director of the Center for RNA Therapeutics at the Houston Methodist Research Institute. “Our immune system recognizes it as abnormal and begins to mount a response against it.”
Billions of people worldwide have now received a COVID-19 mRNA vaccine. But the potential applications of this game-changing technology extend far beyond preventative vaccines – promising a new wave of RNA-based therapeutics to help tackle a wide range of illnesses.
Researchers have been investigating mRNA vaccines for several decades – making important discoveries that underpinned the rapid deployment of the technology to combat the COVID-19 pandemic. Previously, the safe and efficient delivery of fragile mRNA molecules into cells had proved a major obstacle to the success of the approach. The solution to this problem came from the development of protective tiny bubbles of fat – lipid nanoparticles, or LNPs, as a delivery system.
“This was a major breakthrough,” enthuses Blakney. “It meant that we didn’t have to deliver grams of mRNA because the process was so inefficient.”
The discovery of modified building blocks in our RNA – which help it to evade immune detection was also important. Both the BNT162b2 and mRNA-1273 COVID-19 vaccines incorporate pseudouridine, helping to enhance the stability of the synthetic mRNA and shelter it from the immune system.
While the approval of two COVID-19 mRNA vaccines within days of each other marked a major milestone in the field, there are ongoing challenges that still need to be overcome. Not least, both vaccines rely on a costly series of temperature-controlled shipments and storage – creating barriers around their distribution, especially to remote communities that lack reliable electricity or refrigeration.
“When the pandemic hit, we just hadn’t reached the point of making sure these vaccines were shelf-stable for a long time,” says Blakney. “It just wasn’t a priority because we just needed to get them to work first.”
As well as developing mRNA vaccine formulations with less stringent temperature requirements, another huge priority is finding ways to lower the dose to reduce the risk of unwanted side effects, including arm soreness or flu-like symptoms.
“In the context of a pandemic, feeling ill for a short period is a compromise most people are willing to take,” states Blakney. “But they may not want to take that risk for more routine injections.”
Blakney’s team is working on the next generation of RNA vaccines that aim to achieve that goal – using self-amplifying RNA (saRNA) that can make copies of itself once it’s inside the cell. In their experiments, they have shown it may be possible to use around a hundred times lower dose of saRNA compared with standard mRNA.
A major advantage of the mRNA vaccine platform is its flexibility – the same formulation can be used each time, adding only one unique component: an mRNA sequence for the required protein.
“It takes only around two days to design a new mRNA vaccine, as long as you know which protein you need to target,” says Blakney.
Scientists are now intensifying efforts to develop mRNA vaccines for various other infectious diseases – from seasonal-flu to HIV and malaria. But the possibilities extend much further than preventative vaccines.
“RNA therapeutics are going to revolutionize medicine because there are so many opportunities to ameliorate disease,” predicts Cooke. “You can make mRNA encoding any protein in the body – and that provides a lot of potential for new therapies.”
The seemingly endless possibilities include using mRNA to replace faulty or deficient proteins – or to help the immune system to fight cancer. There is even the tantalizing opportunity to apply mRNA-based approaches to produce therapeutic antibodies inside the body – circumventing the need to manufacture and deliver these treatments in protein format.
But the short lifespan of synthetic mRNA in the body is currently a barrier to unlocking the full potential of the technology. Once inside a cell, the linear molecules are prone to degradation by enzymes.
“The RNA will last for a couple of hours – and the protein it codes for may last for a couple of days, but then it’s gone,” says Cooke. “While that’s long enough for a vaccine, it’s a problem if you want to have a longer-lasting effect.”
One potential solution could be the use of circular RNAs (circRNAs), which are unusually stable as their lack of open ends prevents enzymatic degradation.
“In our program, we’ve found that circular RNA can last for days,” says Cooke.
Targeting RNA-based therapeutics to specific places in the body is also proving a major hurdle.
“If you’re injecting mRNA in lipid nanoparticle formulations intramuscularly or subcutaneously, that’s where the RNA reaches,” explains Cooke. “If it’s delivered intravenously, it mainly goes to the liver and stays there – which is great if you’re treating a liver disease, but not if you’re trying to reach other organs.”
Researchers are exploring a variety of different methods to target the RNA to different organs. For example, Cooke’s team is developing ways to deliver RNA-based therapies into the heart, such as directly into the pericardium or into the venous system of the heart through the jugular vein.
Targeting mRNA to specific immune cells could help improve a type of cancer immunotherapy – called chimeric antigen receptor (CAR) T-cell therapy. The treatment involves removing the patient’s T cells from their blood and modifying them by adding the gene for a receptor that helps them to recognize cancer cells better. The engineered CAR T-cells are then given back to the patient – boosting the ability of the immune system to fight their disease.
“It’s a very complex process,” says Cooke. “If you could instead inject mRNA encoding the receptor targeted to T cells, it would provide a much simpler way of treating the patient.”
In a recent study, researchers successfully generated transient CAR T cells entirely inside the body by injecting modified mRNA in lipid nanoparticles targeted to T cells. The ability to produce CAR T cells in the body using mRNA may have several therapeutic applications for treating a variety of diseases.
A new era in medicine
The clinical success of COVID-19 mRNA vaccines has thrust the technology into the limelight.
“Before the pandemic, it was seen as quite a high-risk technology because nobody had ever done a Phase III trial,” says Blakney. “But we’ve now seen they have excellent efficiency and safety profiles – that’s been a total game-changer for the field.”
The door is now wide open for the development of a new raft of next-generation RNA-based therapeutics with the potential to replace existing therapies or provide cures for currently intractable diseases.
“It’s a new arrow in our quiver,” enthuses Cooke. “The field is going to continue to grow and deliver many revolutionary new therapies for patients.”