Biomedicine

Gene-Silencing Drugs Finally Show Promise

After more than a decade of disappointment, powerful new drugs based on a Nobel-winning idea are getting closer to the market.

About 85 percent of proteins in the body can’t be targeted with conventional drugs.

The disease starts with a feeling of increased clumsiness. Spilling a cup of coffee. Stumbling on the stairs. Having accidents that are easy to dismiss—everyone trips now and then.

But it inevitably gets worse. Known as familial amyloid polyneuropathy, or FAP, it can go misdiagnosed for years as patients lose the ability to walk or perform delicate tasks with their hands. Most patients die within 10 to 15 years of the first symptoms.

There is no cure. The disease is caused by malformed proteins produced in the liver, so one treatment is a liver transplant. But few patients can get one—and it only slows the disease down.

Now, after years of false starts and disappointment, it looks as if an audacious idea for helping these patients finally could work.

In 1998, researchers at the Carnegie Institution and the University of Massachusetts made a surprising discovery about how cells regulate which proteins they produce. They found that certain kinds of RNA—which is what DNA makes to create proteins—can turn off specific genes. The finding, called RNA interference (RNAi), was exciting because it suggested a way to shut down the production of any protein in the body, including those connected with diseases that couldn’t be touched with ordinary drugs. It was so promising that its discoverers won the Nobel Prize just eight years later.

Inspired by the discovery, another group of researchers—including the former thesis supervisor of one of the Nobel laureates—founded Alnylam in Cambridge, Massachusetts, in 2002. Their goal: fight diseases like FAP by using RNAi to eliminate bad proteins (see “The Prize of RNAi” and “Prescription RNA”). Never mind that no one knew how to make a drug that could trigger RNAi. In fact, that challenge would bedevil the researchers for the better part of a decade. Along the way, the company lost the support of major drug companies that had signed on in a first wave of enthusiasm. At one point the idea of RNAi therapy was on the verge of being discredited.

But now Alnylam is testing a drug to treat FAP in advanced human trials. It’s the last hurdle before the company will seek regulatory approval to put the drug on the market. Although it’s too early to tell how well the drug will alleviate symptoms, it’s doing what the researchers hoped it would: it can decrease the production of the protein that causes FAP by more than 80 percent.

This could be just the beginning for RNAi. Alnylam has more than 11 drugs, including ones for hemophilia, hepatitis B, and even high cholesterol, in its development pipeline, and has three in human trials —progress that led the pharmaceutical company Sanofi to make a $700 million investment in the company last winter. Last month, the pharmaceutical giant Roche, an early Alnylam supporter that had given up on RNAi, reversed its opinion of the technology as well, announcing a $450 million deal to acquire the RNAi startup Santaris. All told, there are about 15 RNAi-based drugs in clinical trials from several research groups and companies.

“The world went from believing RNAi would change everything to thinking it wouldn’t work, to now thinking it will,” says Robert Langer, a professor at MIT, and one of Alnylam’s advisors.

Delivering Drugs

Alnylam started with high hopes. Its founders, among them the Nobel laureate and MIT biologist Phillip Sharp, had solved one of the biggest challenges facing the idea of RNAi therapies. When RNAi was discovered, the process was triggered by introducing a type of RNA, called double stranded RNA, into cells. This worked well in worms and fruit flies. But the immune system in mammals reacted violently to the RNA, causing cells to die and making the approach useless except as a research tool. The Alnylam founders figured out that shorter strands, called siRNA, could slip into mammalian cells without triggering an immune reaction, suggesting a way around this problem.

Yet another huge problem remained. RNA interference depends upon delivering RNA to cells, tricking the cells into allowing it through the protective cell membrane, and then getting the cells to incorporate it into molecular machinery that regulates proteins. Scientists could do this in petri dishes but not in animals.

Alnylam looked everywhere for solutions, scouring the scientific literature, collaborating with other companies, considering novel approaches of its own. It focused on two options. One was encasing RNA in bubbles of fat-like nanoparticles of lipids. They are made with the same materials that make up cell membranes—the thought was that the cell would respond well to the familiar substance. The other approach was attaching a molecule to the RNA that cells like to ingest, tricking the cell into eating it.

And both approaches worked, sort of. Researchers were able to block protein production in lab animals. But getting the delivery system right wasn’t easy. The early mechanisms were too toxic at the doses required to be used as drugs.

As a result, delivering RNA through the bloodstream like a conventional drug seemed a far-off prospect. The company tried a shortcut of injecting chemically modified RNA directly into diseased tissue —for example, into the retina to treat eye diseases. That approach even got to clinical trials. But it was shelved because it didn’t perform as well as up-and-coming drugs from other companies.

By 2010, some of the major drug companies that were working with and investing in Alnylam lost patience. Novartis decided not to extend a partnership with Alnylam; Roche gave up on RNAi altogether. Alnylam laid off about a quarter of its workers, and by mid-2011, its stock price had plunged by 80 percent from its peak.

But Alnylam and partner companies, notably the Canadian startup Tekmira, were making steady progress in the lab. Researchers identified one part of the lipid nanoparticles that was keeping them from delivering its cargo of RNA to the right part of a cell. That was “the real eureka moment,” says Rachel Meyers, Alnylam’s vice president of research. Better nanoparticles improved the potency of a drug a hundredfold and its safety by about five times, clearing the way for clinical trials for FAP—a crucial event that kept the company alive.

Even with that progress, Alnylam needed more. The nanoparticle delivery mechanism is costly to make and requires frequent visits to the hospital for hour-long IV infusions—something patients desperate to stay alive will put up with, but likely not millions of people with high cholesterol.

So Alnylam turned to its second delivery approach—attaching molecules to RNA to trick cells into ingesting it. Researchers found just the right inducement—attaching a type of sugar molecule. This approach allows for the drug to be administered with a simple injection that patients could give themselves at home.

In addition to being easier to administer, the new sugar-based drugs are potentially cheaper to make. The combination of low cost and ease-of-use is allowing Alnylam to go after more common diseases—not just the rare ones that patients will go to great lengths to treat. “Because we’ve made incredible improvements in the delivery strategy,” Meyers says, “we can now go after big diseases where we can treat millions of patients potentially.” 

The Next Frontier

In a sixth-floor lab on the MIT campus, postdoctoral researcher James Dahlman takes down boxes from a high shelf. They contain hundreds of vials, each containing a unique type of nanoparticle that Dahlman synthesized painstakingly, one at a time. “It turns out we have a robot in the lab that can do that,” he says. “But I didn’t know about it at the time.”

Dahlman doesn’t work for Alnylam; he had been searching for the next great delivery mechanism, one that could greatly expand the diseases that can be treated by RNAi. Some of the materials look like clear liquids. Some are waxy, some like salt crystals. He points to a gap in the rows of vials, where a vial is conspicuously missing. “That’s the one that worked. That’s the miracle material,” he says.

For all of their benefits, the drug delivery mechanisms Alnylam uses have one flaw—they’re effective only for delivering drugs to liver cells.  For a number of reasons, the liver is a relatively easy target—that’s where all kinds of nanoparticles tend to end up. Alnylam sees the potential for billions of dollars in revenue from liver-related diseases. Yet most diseases involve other tissues in the body.

Dahlman and his colleagues at MIT are some of the leaders in the next generation of RNAi delivery—targeting delivery to places throughout the body. Last month, in two separate articles, they published the results of studies showing that Dahlman’s new nanoparticles are a powerful way to deliver RNAi to blood vessel cells, which are associated with a wide variety of diseases. The studies showed that the method could be used to reduce tumor growth in lung cancer, for example.

Treating cancer is one area where RNAi’s particular advantages are expected to shine. Conventional chemotherapy affects more than just the target cancer cells—it also hurts healthy tissue, which is why it makes people feel miserable. But RNAi can be extremely precise, potentially shutting down only proteins found in cancer cells. And Dahlman’s latest delivery system makes it possible to target up to 10 proteins at once, which could make cancer treatments far more effective. Lab work like this is far from fruition, but if it maintains its momentum, the drugs currently in clinical trials could represent just a small portion of the benefits of the discovery of RNAi. 

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