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RNA interference appears to be more potent than previous versions of antisense because it hijacks some of the cell's internal regulatory machinery that determines whether genes get turned on or off. This special potency was discovered in 1998 by scientists at the Carnegie Institution of Washington working on worms, but it works in plants, fruit flies and humans, as well.

The machines that RNA interference take over appear to be part of an ancient evolutionary defense mechanism against viruses, whose perverted genes sometimes come in a double-stranded RNA form.

"It's part of how creatures learned to resist viruses before there ever was an immune system with antibodies," said Mario Stevenson, a virologist at University of Massachusetts Medical School.

There are hints that RNA interference machinery still has an anti-viral function because influenza virus, for example, tries to knock it out when it invades cells, he said.

Big business —— in the lab

RNA interference quickly gained popularity among university biologists who wanted a way to shut off specific genes. In 2002, it was proclaimed "Breakthrough of the Year" by Science magazine.

Carlsbad-based Invitrogen, which sells tools to biology researchers, has jumped into the field in a big way, becoming one of the leading suppliers of RNA interference products.

"In the past year, it's really gained acceptance as a standard method in the lab," said Marilyn Datta, vice president of RNAi for Invitrogen. The earliest adopters of the technique were university scientists, but an increasing number of industrial researchers are using it, she said.

Invitrogen can make synthetic RNA for scientists to use on cells growing in dishes, as well as more elaborate tools for introducing RNA interference agents into animals.

Consulting firm Frost & Sullivan predicted this year that RNA interference sales revenues will grow by 30 percent a year, growing to $328 million in 2010 from around $48 million in 2003.

The largest part of that money would come from pharmaceutical firms using RNA interference for "target validation," the firm projected. Target validation means figuring out whether a potential drug a company's scientists are studying will turn genes on and off in the way they want.

Some venture capital firms such as Boston's Oxford Bioscience have invested in start-up companies that use RNA interference, such as Alnylam and Sirna Therapeutics in Colorado. But Forward Ventures, San Diego's premier biotech venture capital firm, has stayed clear so far.

"It's too soon," said Ivor Royston, a partner at Forward Ventures. "I don't think the current RNA interference companies are going to generate viable products soon enough to reward our investors."

Barriers to clinical use

Antisense and RNA interference both face similar obstacles before they can be effective ways to fight disease.

"It's all about delivery," said Stevenson with the University of Massachusetts.

It's no coincidence that the first RNA interference drug tested in a clinical trial gets injected into the eye, the same site of action of the first antisense drug, he said.

"It's the easiest place to go; the delivery problems aren't there because you're delivering the RNA to a specific place," he said.

The authors of Alnylam's recent Nature paper used a clever trick by chemically attaching the RNA to a molecule of cholesterol, which gets taken up by the liver.

"It's frustrating," he said. "Most of any RNA agent gets taken up by the liver, anyway."

Stevenson is working on using RNA interference to fight HIV; in his case, the task is to deliver a drug specifically to the lymph node.

In addition to the specificity problem, unprotected RNA lasts only minutes in the blood because enzymes break it down. In high quantities, double-stranded RNA also can activate a general anti-viral defense system called the interferon response.

Isis has spent years developing chemical modifications that protect RNA from the body's enzymes, Frank Bennett said. The modifications even enhance the inhibitory effects, he said.

Other companies, such as Rosetta Inpharmatics in Seattle, have also detected glitches where RNA interference can shut off genes that are similar to, but not the same as, the target gene.

"It will take some time and effort to overcome the hurdles," Bennett said. "But we're making a bet that it will be an exciting area in the future."
