For the PNAS study, the Alnylam researchers designed short, double-stranded RNA molecules to silence the gene for PCSK9 in rodents, monkeys, and humans. They packaged the molecules into lipid-based nanoparticles developed by biomedical engineer Robert Langer and his group at MIT. The nanoparticles protect the molecules in the bloodstream and escort them to liver cells.

Injecting the drug into mice and rats lowered total cholesterol by up to 60 percent, and in monkeys, a single dose cut LDL cholesterol by 50 to 60 percent. The reduction lasted about three weeks. Although PCSK9's importance was clear from genetic studies in rodents and humans, "what was not known was, if you were to acutely knock down the level of PCSK9, how long would it take for cholesterol to go down," Fitzgerald says. "The answer was, if you knock it down today, then your cholesterol is down tomorrow."

It's not yet clear how well an RNA-interference-based drug that requires injections could compete with existing medicines for lowering cholesterol. No such drugs have yet been approved by the Food and Drug Administration, although several are in clinical trials. While there have been some safety concerns with RNA-based therapeutics, scientists at Alnylam say that they saw no unacceptable side effects in animals given the cholesterol-lowering treatment, and people who naturally lack PCSK9 seem healthy.