A new published last week in Nature is raising doubts about one of the hottest biotech fields known as RNAi, or RNA interference, which offers a way to turn specific genes on and off. And, of course, finding a way to turn off a gene that triggers disease is the sort of thing that attracts biotechs and drugmakers, such as Merck, which paid $1 billion to buy Sirna Therapeutics two years ago.

But the study suggests that at least some drugs now being tested in clinical trials actually work not by silencing genes but by activating the immune system. And as The New York Times notes, this could mean the drugs are not really precise tools and could have unexpected side effects.

“It seems to be working by a completely different mechanism that’s unrelated to the underlying premise,”’ Jayakrishna Ambati, a professor of ophthalmology at the University of Kentucky and senior author of the study, tells the Times.

The researchers examined two drugs being developed by Opko Health and Allergan to treat macular degeneration. The companies say that their drugs are safe and that they have their own evidence the meds worked through the intended mechanism. But as the Times notes, those assurances didn’t convince investors.

Opko shares fell nearly 10 percent since the study was published online on March 26, before regaining much of that ground during Tuesday’s broad market rally. Opko shares closed at $2.19, up 13 cents on the day, although still off 3 percent since before the Nature article appeared. Shares of Alnylam, a start-up specializing in RNA interference, dropped nearly 10 percent, although have climbed back up a bit since then.

For added insight, take a peek at In the Pipeline

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