What happened

In response to reporting disappointing top-line results from an important phase 3 trial, shares of PTC Therapeutics (NASDAQ:PTCT), a commercial-stage biotech focused on rare diseases, fell by 10% as of 11 a.m. EST Thursday.

So what

PTC announced the results of its ACT CF trial, which was testing its product Translarna (ataluren) as a hopeful treatment for nonsense mutation cystic fibrosis. Unfortunately, data from the trial showed that Translarna was unable to meet its primary or secondary endpoints. In response, management announced its intentions to discontinue all clinical development evaluating Translarna in cystic fibrosis and withdraw its application in Europe.

A woman giving a thumbs down.

Image source: Getty Images.

PTC's CEO Dr. Stuart Peltz said:

"We are disappointed with the outcome of this trial as there are no treatments that target the underlying cause of nonsense mutation cystic fibrosis, one of the most difficult forms to treat."

However, he did reaffirm the company's commitment to studying Translarna in other indications.

Given the bummer news, it is easy to understand why shares are tumbling on Thursday.

Now what

There's no doubt that this news is a big disappointment for shareholders. The company estimated that the worldwide addressable market for Translarna to treat CF was more than 9,000 patients. That's a few thousand patients larger than its addressable market in treating nonsense mutation Duchenne muscular dystrophy (DMD). Nabbing an extra approval would have gone a long way toward expanding the drug's peak sales potential.

Nonetheless, PTC investors still have a few reasons to remain bullish. Management believes that Translarna's sales will grow strongly in 2017, which is why they are guiding for sales to land between $105 million to $125 million. That's a nice bump from the $81 million in sales that was recorded in 2016. The company also plans to resubmit Translarna to the FDA. Given the agency's controversial decision to approve Sarepta Therapeutics DMD drug Exondys 51 last year, they might have a better chance of success. Finally, the company is moving forward in its phase 2 trials studying Translarna as a treatment for Mucopolysaccharidosis I, Aniridia, and Dravet Syndrome. Success in any of those indications would be a big win for investors.

Advancing those clinical studies and expanding access to Translarna won't come cheap, so the company will still be burning through cash for the foreseeable future. It is also hard to handicap the company's chances of success with the FDA given its history with the agency.

All in all, PTC has a few things going for it, but it remains a high-risk stock. That's why my plan remains to root for this company's success from the sidelines.

This article represents the opinion of the writer, who may disagree with the “official” recommendation position of a Motley Fool premium advisory service. We’re motley! Questioning an investing thesis -- even one of our own -- helps us all think critically about investing and make decisions that help us become smarter, happier, and richer.