If you get rabies and don’t get help in time, you will almost certainly die. While there are a few tantalizing reports of people surviving a case of rabies without medical attention, there are all too many cases of people dying after getting bitten by a rabid animal. The virus sneaks from the bite wound into a nearby nerve and then makes its way through the nervous system. Along the way, it can make people mad and send them into comas before killing them. Officially, over 50,000 people die of rabies each year, but that number is likely a gross underestimate. A lot of rabies cases occur in places where the causes of deaths go unrecorded. In one study in Tanzania, doctors determined that the actual number of deaths from rabies was over 100 times higher than the official count.
The rabies virus is hugely dangerous not only because of its deadliness, but because doctors are so ill-equipped to fight it. The standard way to treat people with rabies is to give them a massive dose of rabies vaccine. The vaccine primes the immune system to fight the rabies virus. On top of the vaccine itself, patients also need to get shots of immunoglobin, a molecule that stimulates the immune system. The treatment leaves a lot to be desired. For one thing, it takes a while for the immune system to rise up against the rabies virus. By the time it has swung into action, the virus may have made too many copies of itself to be wiped out. As a result, getting a rabies vaccine only works if you get to a doctor soon after getting bitten. Making matters worse, the vaccine is expensive and needs to be kept refrigerated. What the world needs is a penicillin for rabies: a pill that’s cheap to make, can sit on a shelf for years, and kill viruses quickly.
In Wired last year, I wrote about one of the teams of scientists who are looking for a rabies antiviral–a group based at a small San Francisco start-up called Prosetta. I paid a visit to Prosetta while researching the story. On the day of my visit, Vishwanath Lingappa, the company’s co-CEO, had a batch of rabies virus protein shells stewing in a flask on his lab bench. I sat in on a meeting where he proudly showed off some of the results of the experiments to his co-workers. Lingappa is an American-born Indian with a deep radio-talk-show-host voice. As he explained the results of his experiment, pointing eagerly to details of his slides, he got more and more excited. Finally, in triumph, he invoked his favorite philosopher, John Dewey, in a bellow: “The givens of experience are not given. They are taken! With great difficulty.”
It would take many more months to get the results ready for publication. Now, finally, Lingappa and his colleagues (including his sister Jaisri, a virologist at the University of Washington, and his daughter Usha) are publishing the study this week in the Proceedings of the Academy of Sciences. They don’t have a cure for rabies yet, but they have found drugs that show a lot of promise. And what makes their study even more interesting is that they took a very unusual path to find those drugs.
Most scientists who look for antivirals hunt for molecules that interfere with a virus’s own molecules. Viruses often make enzymes called proteases, for example, that help prepare new virus proteins. So-called protease inhibitors lock onto the proteases and prevent them from doing their job. These inhibitors have proven effective against HIV and extended the lives of millions of people.
But viruses can’t make new viruses entirely on their own. They need an enormous amount of help from their host cell. The Lingappas have done a number of experiments over the years to figure out the nature of that assistance. They’ve found that many of our own proteins join forces to assemble the pieces of viruses into their final shape.
This figure shows how the Lingappas and their colleagues think the process works. The host cell makes proteins for the virus (A), which begin to stick together (B). But then a swarm of host proteins have to come together around the virus proteins (C ) in order to organize them into their proper shape. This work takes energy, in the form of a molecule called ATP (D). Once the host proteins have finished their work, the new virus is ready to leave the cell (E).
This idea is controversial, because it challenges a long-held view about how viruses form. Many scientists argue that once a host cell makes the parts for a new virus, the parts can assemble themselves without any more help. If you were to illustrate what they do with the figure above, the viruses should jump straight from B to E.
If the Lingappas were right, they reasoned, they should be able to block the formation of rabies protein shells by interfering with the host proteins that build them. The experiment wouldn’t just lend support to their model. It would also lead them to potential drugs against the viruses. They could kill two viral birds with one stone.
To test their idea, the Prosetta team launched a series of experiments on a lot of viruses at once. But rabies is the first one they’re reporting on.
Their first step in search of an anti-rabies molecule was to create a kind of cellular stew. They mixed together proteins and other molecules from cells in a broth known as a cell-free system. They then added rabies viruses to the broth. In a matter of hours, the viruses were using the molecules in the cell-free system to make new viruses.
The scientists then turned to a catalog of candidates for a drug. Chemists have built up a list of tens of thousands of molecules that have promising medical properties, such as being small enough to slip easily into cells. They added some of the molecules to their rabies stew to see if any of them would grab onto the host proteins as they were assembling the rabies viruses.
They found a few. When these molecules were added to the cellular stew, the rabies virus shells grew more slowly. Presumably, the molecules made it harder for the viruses to organize the host proteins to build new viruses.
The Lingappas and their colleagues fine-tuned these molecules. After some adjustments, the molecules would only latch onto to the proteins when they came together to make rabies. That way, the drug wouldn’t cause side effects by interfering with proteins that were doing what they were supposed to be doing for our own health.
One drug, which they dubbed PAV-866, proved to be both safe to host cells and potent against rabies. In a dish of cells infected with rabies, PAV-866 wiped the virus out altogether.
To see if PAV-866 was indeed blocking the host proteins, the scientists ran an experiment. They infected cells with rabies and then waited for different periods of time before adding PAV-866. They found that the drug PAV knocked down the virus if it was added up to six hours after the experiment started. Afterwards, it didn’t do much good.
You wouldn’t expect this result if the drug was interfering directly with the virus itself. If that were the case, then the drug would have been able to attack the viruses throughout the experiment. But if it interfered with the initial assembly of the viruses, then it would need to be present in cells as new viruses were being assembled.
The scientists then probed their cellular stew for proteins that were sticking together and found a couple proteins that PAV-866 consistently latched onto. These may be a couple of the proteins that rabies viruses depend on to be built. It will take more work to figure out exactly how these proteins assemble rabies and how PAV-866 stops them. But the Prosetta researchers already know enough about the drug to move forward with more experiments, in the hopes of getting it someday to the marketplace.
While researching my Wired story, I spoke to one of the Lingappas’ co-authors, Charles Rupprecht, a virologist who heads the rabies program at the Centers for Disease Control. I asked what he thought about this new approach to antivirals. “It’s provocative, it’s intriguing,” he told me. But he immediately listed all of the tasks that lie before him and his colleagues to see if PAV-866 is indeed a cure for rabies.
First off, the cells they infected for the new study were from the kidneys of monkeys. They chose the cells because they’re easy to work with. But now they have to run the same experiment all over again with neurons. If PAV-866 stops rabies in neurons, they will then move to experiments on animals. But to do so, they’ll have to figure out the right dose to give them, and then they’ll have to figure out how to keep infected animals alive long enough to study the effects of the drug.
“How are you going to intubate a mouse? How are you going to keep it alive as it goes into cardiac arrest?” Rupprecht asked me.
Still, he’s ready to keep on working with the Lingappas to see where the research goes. “We have nothing else to fall back on, so of course we’re going to be optimistic,” Rupprecht said. “You’ve got to find a coping mechanism.”
Prosetta is not the only place where scientists are trying to fight viruses by targeting the host. Other researchers are investigating the possibility of stimulating the virus-killing defenses inside all of our cells. Others are developing drugs to rewire the networks of genes in cells to cause them to commit suicide as soon as they are infected. What makes these studies especially exciting is that they might lead to a single antiviral drug that can treat a wide range of viruses–or perhaps even all viruses. Curing rabies would be remarkable enough. But it might be the start of something much bigger.
(For more information on rabies, see Rabid, written by my editor at Wired, Bill Wasik and his wife, Monica Murphy. For more on viruses in general, see my own book, A Planet of Viruses. I also spoke on Fresh Air about research on host-targeted antivirals.)
[Update 4 pm: Corrected timing for vaccine. It has to be given shortly after exposure, not symptoms. Update, 2/12: Clarified that Lingappa was not producing live viruses but rather capsids on my visit, also that his daughter was lead author on the paper.]