Because of ethical rules, there's no way to fully test the drug in humans unless there actually is a smallpox epidemic. But researchers are preparing to try it out on monkeys after successfully protecting mice from a virus similar to smallpox.
"This is really pretty exciting," said Dr. Karl Hostetler, a professor of medicine at the University of California at San Diego and a member of a research team working to convert an existing intravenous drug, cidofovir, into an oral form.
He said the drug could protect the many people -- including AIDS patients, cancer patients, and those with skin infections -- who can't be vaccinated against smallpox.
A member of Hostetler's team was scheduled to report its findings Tuesday at the American Society for Microbiology's Biodefense Research meeting in Baltimore.
Cidofovir, which prevents viruses from reproducing, didn't exist in the mid-1970s, the last time anyone saw a case of smallpox. At that time, doctors had no treatment for the disease and could only try to keep patients comfortable until they recovered or died.
Antiretroviral drugs like cidofovir -- used to treat a devastating eye infection in AIDS patients -- began appearing in the 1980s and 1990s. While researchers aren't sure if cidofovir is effective against smallpox in humans, doctors are using it to treat people who get bad reactions from the smallpox vaccine, said Dr. Henry Shinefield, co-director of the Kaiser Permanente Vaccine Study Center.
The problem is that the drug is available only in an intravenous form.
"The patient has to be hydrated prior to the drug administration; it has to be given slowly over a protracted period of time, and a patient has to be treated with another drug to protect the kidneys," Hostetler said. "There's a lot of fiddling around."
Hostetler and his colleagues are trying to create an oral form of the drug that wouldn't have as many side effects.
"It's always better to have an oral form," Shinefield said. "You've got better control of the drug under those circumstances, and generally the tolerance is better."
Two oral forms of the drug successfully prevented mice from dying of cowpox, a sister virus to smallpox, Hostetler said. The drugs prevented infection if they were given three to five days before exposure to the virus. They also stopped the disease in its tracks if given within three days after infection.
According to the researchers, the findings suggest that patients could take several staggered doses a month to protect themselves from becoming infected in an outbreak.
The smallpox vaccine infects people with a virus that is similar to smallpox itself. Most recover easily from the infection. But people with compromised immune systems -- due to AIDS, cancer treatment, organ transplants or even previous skin infections -- may face serious illness or death because they can't fight off the infection caused by the vaccine.
Doctors are also wary of using the vaccine on pregnant women and people who live with anyone who is in a risk group. (The infection caused by the vaccine can be contagious in close living quarters.)
Chimerix Inc., a company that is developing the oral forms of cidofovir, is moving to tests in primates and hopes to try the drug on humans within 18 to 22 months, said president and chief operating officer George Painter.
Researchers can't infect humans with smallpox to see if the drug works, so they'll only be able to test it on healthy subjects to check for side effects, Painter said.
But what if a smallpox outbreak occurs before the testing is finished? Doctors can use drugs for unapproved, "off-label" purposes. But cidofovir is not commonly used, so the supply is limited.
"There's hardly any of it around," Hostetler said. "If there was really a crisis, there's not enough of the drug."
Another study presented at the conference Tuesday says that interferons may also work at treating smallpox. Researchers found that mice infected with a virus close to smallpox were successfully treated 95 percent of the time when given interferon in their noses.