In fall 1976 the first recorded Ebola outbreak ravaged a small village in the Democratic Republic of the Congo (formerly Zaire). The virus, named for the river valley where it was found, causes a deadly hemorrhagic fever. It spread quickly via contact with blood and contaminated needles killing nearly 90 percent of the 318 villagers it infected. Since then about 2,300 human cases have been reported, according to the U.S. Centers for Disease Control and Prevention, 85 percent of which were fatal.
After identifying a new strain called "Ebola-Reston" during a 1989 outbreak scare in Reston, Va., (and earning a central role in Richard Preston's book, The Hot Zone ) Thomas Geisbert had tried everything to quash the virus, which continues to threaten civilians and medical aid providers in Africa as well as scientists who work in the highest level biocontainment facilities around the world. Vaccines have protected monkeys, and therefore might protect humans from the ensuing fever when given prophylactically (before exposure), but such treatments offer little hope for those already exposed to the virus.
After 15 years of trial and error, Geisbert and colleagues at the National Emerging Infectious Diseases Laboratory Institute at Boston University and the U.S. Army Medical Research Institute of Infectious Diseases at Fort Detrick in Frederick, Md., changed their strategy. Instead of boosting host immunity, they would target the virus itself by silencing its genes. If they could mute the genes responsible for its replication perhaps they could prevent Ebola's rapid and rampant spread though its host, at least buying time for other treatments, such as vaccines, to kick in.
The silent treatment
Gene silencing is a naturally occurring process, also known as RNA interference (RNAi). Some stretches of our DNA are transcribed into RNA sequences, which are then translated into the proteins that keep our cells working. But other, shorter stretches are transcribed into small interfering RNA (siRNA). These tiny molecules aren't translated into proteins. Rather, they exist as short strands of RNA that bind to portions of other RNA strands with complementary sequences, preventing them from being translated into protein. The blocking process allows gene expression to be finely controlled, like the volume on a radio.
Shortly after the process of RNAi broke into the scientific literature at the turn of the millennium, scientists began looking for ways to induce the process artificially to combat diseases. In 2004 Geisbert and colleagues applied this strategy to Ebola, selecting the genes they would target. To stop the virus in its tracks they developed synthetic siRNAs that would bind to and silence a gene essential for replication—polymerase L . In preliminary cell culture studies the researchers successfully silenced the gene and inhibited replication. The next step was to test the approach in Ebola-infected animals (guinea pigs), but this introduced a new problem.
Enzymes in blood and bodily tissues break down "naked" siRNAs. In order for the treatment to make it to the virus intact, it had to be packaged into a molecular vehicle that would be taken up by infected cells without breaking down en route or provoking an immune response. The team's first siRNA packaging attempt (which used synthetic vehicles called "polyethylenimine polyplexes") didn't work, and only 20 percent of the infected guinea pigs survived.
Then Geisbert got an unexpected call. "We would often get calls from 'snake oil salesmen'—people who wanted us to try their products in our models," Geisbert says. Ian MacLachlan, chief scientific officer for the British Columbia–based biotech company Tekmira, told Geisbert about a new delivery system for siRNAs that used SNALPs—stable nucleic acid lipid particles. "I don't know why I gave him a chance," Geisbert recalls.
After MacLachlan and colleagues at Tekmira packaged the siRNAs into SNALPS, Geisbert repeated the guinea pig experiment and waited. Thirteen days after the infection guinea pigs in the control group had died. But those treated with the SNALP-packaged siRNAs were still alive. "It turns out that a lot of the cells that are targeted by Ebola [immune cells like macrophages and dendritic cells] take up SNALP," Geisbert says. "We got 100 percent protection."
Geisbert knew from experience that success in guinea pigs didn't necessarily translate to nonhuman primates (the gold standard in Ebola animal models), which was crucial for developing a human-ready version. Last year, Geisbert and his team decided to test their approach in monkeys infected with the most deadly strain of Ebola—Ebola-Zaire. To increase the chance of success, they included two other siRNAs targeting other viral genes thought to placate the host's immune response, called VP24 and VP35 . They injected the siRNA-loaded SNALPS 30 minutes after injecting Ebola, and then every day for a week. All of the treated monkeys survived and were virus-free after seven days.
Taking it to the finish line
Although this small "proof of concept" study, published May 29 in The Lancet, is only the first of many steps toward a post-exposure Ebola treatment for humans, Geisbert couldn't be more excited. "This is the holy grail for me," he says. "You always want to do something you didn't think could be done. We could never get across that last hurdle of complete protection after exposure." He and his team are already pushing the limits of their treatment. They hope it will be as effective if administered 24 hours after infection—a more realistic time window in an outbreak or lab injury scenario.
But getting the treatment approved for use in humans will be no easy task, Geisbert says. "You're talking millions and millions of dollars to take a product like this across the finish line, and it's a very small market for something like Ebola." Their study was funded by the Defense Threat Reduction Agency, which is part of the U.S. Department of Defense and is dedicated to countering weapons of mass destruction. Geisbert hopes the agency will commit more money for follow-up studies, along with the National Institutes of Health. "This is a great product and I just want to see it do some good," he says.