However, the extraordinary measures she had to employ—injecting a dose of siRNA equal to one-fifth of the mouse’s blood volume at high pressure—would never work in humans. But the following year, a group from Alnylam managed to inject siRNA into a mouse at a normal pressure and volume to silence a gene called apoB, which causes high LDL cholesterol. The researchers altered the siRNA slightly, chemically stabilizing the strands and attaching them to a molecule of cholesterol to make it easier for them to pass into cells. This approach also kept the siRNA from being quickly degraded by enzymes—normally a problem—and the treatment had the desired effect, entering the liver and slowing production of LDL. Alnylam followed up with a 2006 study in primates that also reduced cholesterol.

Yet this success hasn’t really solved the biggest problem of using RNAi to treat human disease. Delivery is the elephant in the room, and progress has been slow. In addition to Alnylam’s cholesterol work, Lieberman is developing an approach using antibodies, while Rossi at the Beckman Research Institute is trying to attach siRNA to a molecule called an aptamer that can bind to various parts of a cell. But none of these has been tested in people yet.

Until the delivery issue is sorted out, researchers say, they’re left to pluck the low-hanging fruit, targeting tissues to which siRNA can be applied directly rather than depending on systemic delivery through the bloodstream. For example, three years ago, Acuity (now called OPKO Health) began the first clinical trial of an siRNA for the wet form of age-related macular degeneration (AMD). Researchers injected siRNA for the gene that codes for the protein VEGF directly into the back of the eye. VEGF causes leaky blood vessels to grow in the eye, damaging the macula—the part of the retina with the most vision cells—and harming the ability to see fine detail. In these small trials, the siRNA proved effective in reducing expression of VEGF. Now, in a larger Phase III trial (the first for an RNAi therapeutic), the siRNA drug is being compared with another AMD drug already on the market.

The lung, reached via inhaled drugs, is also an easy target. Alnylam is conducting a clinical trial that attempts to silence a gene important for the replication of the respiratory syncytial virus (RSV). The company’s main drug, ALN-RSV01, was found to be safe and well tolerated in Phase I trials, and it’s now in Phase II trials to test how well it knocks down the virus in the upper respiratory tract.

For RNAi to live up to its hype, however, researchers will have to find a way to go beyond direct delivery. There are other issues too, not least the worry that siRNAs could trigger a damaging immune response in humans. And what would happen if someone took RNAi drugs for a lifetime, a requirement for many diseases such as HIV or hepatitis? Still, hopes are high. “All of us developing RNAi-based drugs think that this will add a whole new class to our arsenal,” says Fruehauf of Cequent Pharmaceuticals.

Rossi hopes his HIV treatment will be one of those. He has just recruited the first of five patients for a small trial. The patient has lymphoma and AIDS, and as part of the treatment for lymphoma, he’ll receive a blood stem-cell transplant. But before the transplanted cells go into the patient, Rossi will add an anti-HIV siRNA, which “we hope will make the other drugs the patient is on more potent. That could let us lower the dosage of those drugs, or enable patients to go on drug holidays.”

Rossi will follow the patients in his trial indefinitely to monitor whether siRNA continues to combat the virus. But he also thinks RNAi will benefit AIDS patients in another major way. Because HIV mutates rapidly, drugs that were once effective eventually lose potency. RNAi could offer an important answer to this persistent problem. “You could just make a new RNA that would counter the resistance mutation,” he says. “It would be so easy to change the drug. We might even be able to develop an injectable once-a-month treatment using siRNA that would take the place of conventional drugs.”

Dossier

1. Presentation about the 2006 Nobel Prize in Physiology or Medicine [nobelprize.org/nobel_prizes/medicine/laureates/2006/index.html]. An impressive multimedia depiction of Craig Mello and Andrew Fire’s pivotal work with RNAi, including what RNAi is, why it’s important and what its discovery means to the future of medicine.

2. “Duplexes of 21-Nucleotide RNAs Mediate RNA Interference in Cultured Mammalian Cells,” by Sayda M. Elbashir, Jens Harborth, Winfried Lendeckel, Abdullah Yalcin, Klaus Weber and Thomas Tuschl, Nature, May 24, 2001. Tuschl shows that RNAi works in mammalian cells, writing the recipe for successful gene silencing and sparking the field of RNAi therapeutics.

3. “Strategies for Silencing Human Disease Using RNA Interference,” by Daniel H. Kim and John J. Rossi, Nature Reviews Genetics, March 2007. A fascinating summary of the cutting-edge ways in which researchers are currently using RNAi to treat disease, as well as the challenges they face.

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