A drug with the startling power to mute defective genes raises hopes for a new class of therapies — but hurdles abound

The experimental drug has startling powers: It can turn down a mutant gene in a patient’s body, stopping the production of proteins that cause a terribly painful rare disease.

A crucial, late-stage clinical trial showed that the drug works — and that it’s safe. And now the biotech company behind it, Alnylam, is poised to bring this first-of-its-kind therapy to market.

The news has thrilled both patients and scientists, who have been working for decades on the technology to mute misbehaving genes, known as RNA interference, or RNAi. They’ve understood for two decades how the biology works. But it’s been a long, slow slog to figure out how to deliver RNAi therapies to the right cells safely and effectively. Alnylam alone has spent 15 years, and more than $1 billion, on the effort.

So does the company’s recent success herald an explosion of new drugs that can shut down troublesome genes?

Maybe.

The RNAi delivery systems remain highly complex — and the most effective technologies are still protected by patents that make, one of its most advanced drugs. Rather than alleviating it, the drug exacerbated pain in a rare nerve disease called transthyretin amyloidosis. And several patients died in the clinical trial, though it’s exactly why. Alnylam’s stock plummeted by half on that news.