A biotech company erased $3 billion in value overnight after one of its drugs failed a key trial

RNAVossman on WikipediaComputer generated image of what a single strand of RNA looks like.

Biotech company Alnylam Pharmaceuticals is down almost 50% Thursday after stopping a late-stage clinical trial.

The drug, revusiran, was being developed to treat a condition called hereditary ATTR amyloidosis with cardiomyopathy. It’s a rare disease in which amyloid proteins build up in the body, in particular the heart.

Alnylam discontinued the trial after 18 patients died who were part of the trial.

“Patient safety comes first. We have stopped all dosing and are actively monitoring patients across revusiran studies to ensure their safety,” Alnylam CEO John Maraganore said in a release issued Wednesday evening, adding that the company would be looking into what caused these deaths.

Revusiran is a type of treatment called an RNA interference (RNAi). These types of drugs aim to go in and silence the gene that’s making a certain dysfunctional gene. This isn’t the first time RNAi has faltered: it originally won the Nobel Prize back in 2006, and it’s taken longer than expected to get it to work in humans.

However, Alnylam said in the release, this shouldn’t affect patisiran, another RNAi drug the company has in late-stage clinical trials for a hereditary ATTR amyloidosis with polyneuropathy, a condition that’s fairly similar, but affects the nervous system more.

Even so, the news hit the company hard. Its market cap dropped from around $6 billion to around $3.27 billion.

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