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Hepatitis C Drug Targets RNA

A new drug suppresses the virus in chimps without generating resistance.
December 4, 2009

An experimental drug developed by Danish startup Santaris effectively controls the hepatitis C virus in chimpanzees without creating drug-resistant forms of the virus–a major advantage over other compounds in clinical development. The compound, a synthetic nucleic acid that binds to a microRNA molecule required for viral reproduction, is now in early-stage clinical trials. It is the first microRNA-targeting drug to be tested in humans.

Viral: The hepatitis C virus.

Approximately 170 million people across the globe are infected with the hepatitis C virus, a chronic infection that can lead to cirrhosis, liver cancer, and the need for a liver transplant. While drugs exist to treat the virus, they carry serious side effects and work in fewer than half of all infected patients. “The treatment is very harsh and needs to be taken for 48 weeks,” says Robert Lanford, the lead author on the new study, which was published online today in Science. “Most people can’t tolerate it that long, especially if they have liver disease.”

Existing drugs suppress the virus by boosting the patient’s immune system. The Santaris drug targets the hepatitis C virus more directly by binding to a short piece of RNA called a microRNA, which the virus needs to replicate. The research is part of a larger effort over the last decade to develop methods of selectively targeting and silencing RNA molecules to treat a number of diseases.

DNA and RNA are made of a series of chemical letters. In an approach called “antisense therapy,” molecules designed to complement a sequence of these chemical letters in a target piece of RNA or DNA bind to the target, thereby blocking its function.

One of the major challenges in developing RNA- and DNA-based drugs is creating molecules that are stable enough to remain in the bloodstream until they reach the target tissue. One option is to encase the molecules in special molecular packaging, but that approach adds another layer of complexity to drug development. Santaris has developed a novel chemistry that creates stable DNA molecules that can be injected into the blood and remain there long enough to be taken up by the liver, where the virus resides.

To create the molecule, Santaris scientists altered the structure of a subset of bases within a short strand of DNA, using a technology called “locked nucleic-acid chemistry.” The alterations make the molecule highly stable and give it a strong affinity to its RNA complement–in this case, a microRNA called miR-122 that is made by the human genome and which the virus needs to replicate.

“Whereas other chemistries invented in the last 20 years as a means to improve the [binding] properties of oligonucleotides [short strands of RNA or DNA] provide one degree of improved binding, locked nucleic acids provide fivefold to tenfold improvement,” says Henrik Orum, Santaris’s vice president and chief scientific officer. “It’s really a quantum leap in affinity.”

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.

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