Researchers injected four hepatitis C-infected chimps with the drug once a week for 12 weeks. The animals showed a dose-dependent drop in the number of viruses in their blood that lasted two to three months after the last injection. The treatment also appears to avoid a major problem suffered by almost all other hepatitis C drugs in clinical development--viral resistance. "We have tested a lot of other drugs, and they were good drugs," says Lanford, but resistance appears within days. While these other drugs work initially, the virus mutates to avoid the drugs' attack mechanism and quickly bounces back.

"This paper opens a couple of exciting breakthroughs," says Peter Sarnow, a researcher at Stanford University who was not involved in the research. Notably, "the use of locked nucleic acids to do gene therapy in the liver and the surprising finding that these locked nucleic acids are taken up by the liver in an animal without being [specially packaged for delivery]."

Scientists saw no negative effects during the study period, and analysis of gene expression showed that the livers of treated animals began to look more normal. However, the long-term safety of the drug is not yet clear. MiR-122 controls the expression of hundreds of genes in the liver, among them those involved in regulating cholesterol. Because of this, the Santaris compound has the potentially beneficial side effect of reducing cholesterol levels. But the function of many of the other genes is unknown. Some are linked to cancer, so increasing expression of these genes might lead to overgrowth of liver cells, he says. "Still, I am cautiously optimistic," says Sarnow.

It's also unclear whether the drug will prove as effective in humans. While chimps are the only animal other than humans to be infected with hepatitis C, the virus acts differently in these animals. They do not suffer the long-term liver damage that people do, and the drug may act differently in diseased liver cells. The drug is currently being tested in healthy volunteers, and results of those tests should be reported next year, says Orum. The company does not know when clinical tests of hepatitis-infected patients will begin.

Santaris has developed a number of other locked nucleic-acid drugs for a variety of diseases, including viral infections and cancer. Four of those are being tested in clinical trials in collaboration with Enzon Pharmaceuticals, a drug development company in New Jersey.