In December 2010, Harvard postdoctoral scientists Lior Zangi and Kathy Liu looked on as the hearts of their lab mice began to glow. They had injected mRNA — the molecule that bears the genetic orders to guide our cells in making proteins —  to compel the heart into producing the light-emitting enzyme found in fireflies. When it worked, they knew it meant they could guide the heart to create any protein, even one that might be able to heal it after a heart attack. 

They were still missing a crucial ingredient to carry the idea forward. Zangi scoured the lab’s freezer, thumbing through little vials labeled with genes that they could transcribe into mRNA, until one caught his eye. The label read “VEGF-A,” for vascular endothelial growth factor A. He knew that this gene produces proteins that encourage the growth of new blood vessels and thus might help regenerate the heart. “That was the only thing that we had,” says Zangi. “I said okay, like let’s give it a shot.” 

Once the mouse hearts started expressing VEGF-A protein, Zangi and Liu took their results to their mentor Kenneth Chien, now a professor of cell and molecular biology at the Karolinska Institute in Sweden. Chien, a cardiologist by training, previously proved that VEGF-A was essential for a healthy heart. They decided to pursue the project further and see if VEGF-A mRNA could become the first mRNA drug to treat heart failure. Their VEGF-A mRNA would  become the first mRNA therapeutic to enter humans in a 2019 clinical trial. 

Success hasn’t always come this fast for mRNA. Back in the 1990s, biologists battled a backlog of problems, from too little protein being made to unexpectedly strong immune responses that attacked mRNA molecules and thwarted their ability to prevent or cure sickness. “When I said to somebody, I’m working with mRNA, they always felt sorry for me,” says Katalin Karikó, an adjunct professor at the University of Pennsylvania who has studied mRNA for over thirty years. But Karikó believed in the power of mRNA as a protein therapy that could treat an expansive array of illnesses — and ultimately change the world of medicine — and so she stuck with it. 

In 2005, Karikó and biologist Drew Weissman discovered a way to modify mRNA so that it doesn’t activate such a vigorous immune response. In doing so, they laid the foundation for safe and effective mRNA vaccines, including the COVID-19 mRNA vaccines. These powerful vaccines train cells to mount an immune response against infections without using a live virus, which makes them both safer and faster to mass produce. (Karikó was until 2022 a senior vice president at BioNTech, the mRNA-based biotech company that created the first FDA-approved mRNA COVID-19 vaccine with Pfizer.) 

Now, with mRNA vaccines off to a roaring success, many scientists are designing mRNA as a treatment for diabetes, heart failure, cancers, and other conditions. The excitement is ever-growing, but unique challenges keep mRNA therapeutics trailing far behind their vaccine counterparts. Researchers still struggle to turn mRNA into a stable, deliverable drug. Currently, only between one and five percent of the mRNA makes it into cells to create proteins. Without the immune system kicking in to amplify the protein signal  (like it does for mRNA vaccines), scientists must use extremely high doses of mRNA for therapy — up to 1,000 times higher than for a vaccine dose. 

Kariko, who has witnessed many challenges facing mRNA work, believes that the hurdles facing mRNA therapeutics will be cleared in her lifetime. “I am very optimistic that I will still be alive … and I will get one,” she says. mRNA therapeutics are still far from ready for primetime. But creative solutions are starting to open doors that researchers have been knocking on for decades. 

A Disappearing Act

Before Zangi began making mRNA in Chien’s lab, there had not been a safe and effective way to treat the heart with VEGF-A. “The problem was not if the gene is good or not,” says Zangi. Instead, it was “how the protein is delivered.” 

When delivered as a protein, VEGF-A is usually short-lived in the heart. Inject it, and it’ll vanish within hours; inject a harmless virus to deliver VEGF-A and it’ll stick around too long. (Researchers have previously shown that long-lived VEGF-A caused heart tumors in mice.) Chien’s lab needed to find a sweet spot. 

mRNA offered the perfect Goldilocks scenario: this way, the heart cells produced VEGF-A protein for up to ten days. Mouse experiments succeeded right off the bat. Blood vessels regenerated, and the heart was stronger for it. They published their results in 2013.

“That was the first evidence, quite frankly, that this mRNA technology could work in the cardiovascular system,” says Chien.

It was especially exciting because, before they got the results, Chien’s colleague at Harvard, Derrick Rossi, approached him at a wedding about starting an mRNA-based company together. In 2010, Rossi and Chien co-founded Moderna with two other colleagues. Chien says that the early VEGF-A data served as a foundation for Moderna. (The company went on to create the first mRNA flu vaccine, and one of the first vaccines against Covid-19.) 

But while the transient nature of mRNA — only lasting a few days in the heart —made it a good delivery vehicle to avoid the long-term toxicity of VEGF-A, it would prove to be a huge hurdle for the rest of mRNA therapeutics. 

“It was the question all of us mRNA biologists asked: how are you going to make this work?” says Jeff Coller of Johns Hopkins University, who has studied mRNA stability for nearly twenty years. Using mRNA made more sense for vaccines, he adds, which only require mRNA to stick around briefly before the immune system uses the protein to start making its own cells. But many therapeutics need to remain in the body’s cells for more than just a few days to adequately treat a disease. 

Like VEGF-A, mRNA doesn’t stick around very long. Cells will only use it to make proteins for a few hours. After that, enzymes chew up each mRNA molecule from both ends. Today, even with the latest strategies from Coller’s lab and others that slow down the biological machinery that wipes out mRNA so quickly, injected mRNA still only sticks around for a few days.

“We’d like mRNA therapeutics to work such that once you inject it, it gives you a sustained response for weeks, or months, but it’s not there yet,” says Coller.

Some researchers hope to get around this barrier — by redesigning the mRNA itself. 

Lost in Translation

One of the most obvious ways to redesign mRNA is to change its shape. Biotech startups like Orbital Therapeutics and others make circular mRNA that’s meant to dodge destructive enzymes that would otherwise latch onto both ends. But so far, circular mRNA doesn’t last long enough inside cells yet.

Another alternative that scientists are exploring is a “self-amplifying” mRNA. These are derived from an alphavirus, which are a group of viruses usually transmitted by mosquitoes. The self-amplifying mRNA builds off the virus’ excellent capability to make copies of themselves inside the cell to stick around longer and keep building more proteins. Anna Blakney, a professor of biomedical engineering at the University of British Columbia, Canada, says that her lab has seen protein expression for up to 30-60 days with self-amplifying mRNAs, but their research is still in the early stages. (Blakney cofounded VaxEquity, a biotech company focused in part on creating self-amplifying mRNA for therapeutics and vaccines.)

Despite these advances in mRNA design, researchers face another critical challenge: How does one get these little molecules into cells?

The mRNA vaccines developed for Covid-19 were packaged in a protective casing of fat molecules, known as a lipid nanoparticle. But lipid nanoparticles present their own issues. They cause immune responses and accumulate in the liver or spleen, rather than go to the organ they’re supposed to treat.

“It’s kind of like real estate; location, location, location. How do you get it there?” says Chien. 

“It’s really only within the last couple of years that people have started really looking at how we can engineer the other components of the lipid nanoparticle to get delivery beyond the liver and spleen,” says Jilian Melamed, a postdoctoral researcher in Weissman’s lab at the University of Pennsylvania. As a postdoctoral researcher at Carnegie Mellon, Melamed routed mRNA lipid nanoparticles to a mouse’s pancreas for the first time.

Chien’s team skirted around the delivery bottleneck issue — with help from the heart. It turns out that the heart can take up mRNA directly without a protective casing. Chien was shocked and impressed that it worked so well. But they soon ran into other challenges.

In Search of Better

After the VEGF-A mouse results were published in 2013, Chien and his colleagues at Moderna went looking for partners, but the work wasn’t welcomed with open arms. “When we started, people just didn’t want to believe this,” says Chien. 

They presented the work to several large pharmaceutical companies, hoping to find a collaborator who would help them carry the work forward in large animals, like pigs. (A pig’s heart is about the size of a 14 year old’s human’s heart.) Every company turned them down until the CEO of AstraZeneca, Pascal Soriot, saw the potential and became an early believer.

Encouraging data from studies in pigs fueled more optimism. AstraZeneca helped push the mRNA therapeutic into a phase I clinical safety trial in humans in 2019. Though VEGF-A proved safe, the phase II trial hasn’t been a slam dunk. Results in a small group of 11 patients show improvements in the heart’s ability to pump out more blood, but scientists hoped for better.

The first mRNA therapeutic to enter humans may have fallen victim to moving too quickly, Zangi says. 

The gene for VEGF-A tested in clinical trials was the same one — the only one — he’d pulled from the freezer. “To go forward without really trying a couple of other genes,” he says, may have been a “fundamental problem” from the start. 

Zangi wonders if they may have benefited from slowing down and finding other genes that help regrow heart muscle, in addition to the blood vessels that VEGF-A regenerates. Zangi, now a professor at the Icahn School of Medicine at Mount Sinai, has his eye on a promising candidate that may do just that: a gene called Pkm2. “I think that this is a key target, and it will be interesting to see how we do in the clinic. Hopefully it will get there,” Zangi says. Zangi’s lab plans to publish the results of a Pkm2 drug in pigs later this year. 

The long list of scientific barriers has not discouraged researchers from trying to invent the first big mRNA therapy. But in anticipating challenges like the ones Zangi and others have faced, they’re adapting—by reimagining what mRNA can do. As a growing list of mRNA therapies line up to prove themselves in clinical trials, many are employing mRNA in unexpected ways.

The excitement is ever-growing, but unique challenges keep mRNA therapeutics trailing far behind their vaccine counterparts.

mRNA’s Next Moves

The nascent revolution of mRNA medicine is pivoting toward cancer. Cancer treatments are one of the most exciting directions for mRNA to go, according to experts who spoke with Grow

Researchers refer to these as “vaccines,” but they train immune cells to attack someone’s individual cancer cells rather than an infectious disease. The mRNA cancer vaccines teach the immune system to look out for the specific type of cancer already spreading in the person’s body.  

“That’s where mRNA therapeutics are beautiful,” says Coller, “because if you had to make personalized medicine from a protein standpoint or other gene therapy, it’d be very costly and very difficult to do.” Instead, with mRNA, Coller notes that scientists can take a sample of a patient’s tumor, sequence it, and design a novel mRNA for that patient all within a month or so.

In May, a team led by scientists at Memorial Sloan Kettering Cancer Center and BioNTech announced success with an mRNA vaccine against pancreatic cancer — one of the deadliest cancers, killing nearly 90 percent of those diagnosed. In a small phase I clinical trial, 16 patients with pancreatic cancer were given an mRNA cancer vaccine tailored to their unique tumor. When the therapeutic was delivered intravenously, half of the patients mounted an immune response against their own tumor, and those patients did not relapse during the study’s 18 months.

More approvals could follow within the decade. BioNTech has several phase I clinical trials to treat tumors intravenously or through direct injection. In addition to mRNA cancer vaccine trials, Moderna also has two clinical trials supplying mRNA to treat types of diseases that cause issues in breaking down proteins and fats, leading to toxic buildup. Intellia Therapeutics, a biotech company based in Cambridge, Massachusetts, has a phase I trial to treat transthyretin amyloidosis, a fatal disease in which globs of toxic proteins get lodged in the heart and nerves, using mRNA to deliver gene therapy. And mouse studies are underway to test mRNA drugs for a range of ailments including blood clotting disorders, autoimmune conditions, and cystic fibrosis. 

Still, Coller cautions that it’s important to recognize that it won’t be able to cure every disease out there. “It’s a new type of technology that pushes open a whole new realm of possibilities. But it’s not a holy grail,” he says.

The many contenders suggest that mRNA therapies are more than possible — they’re imminent. It took a global pandemic to rapidly accelerate progress for mRNA vaccines, and mRNA therapeutics still have several hurdles to clear. Zangi and Chien remain as excited as they were when they saw the results of their first mRNA therapeutic healing the hearts of mice. 

For mRNA therapeutics, if the heart regeneration work from Chien’s lab is any indication, it might take a little bit of luck to get started, but ultimately will require looking outside the (ice) box to find new solutions.

“mRNA has already been disruptive to the vaccine field, that’s clear,” says Chien. “Will mRNA be disruptive to the therapeutic field? That’s the big question. And I’m optimistic that we will see this play out in this decade.”