What: After the company received a "Dear John" letter from the Food and Drug Administration, shares in PTC Therapeutics (PTCT -3.58%) crashed 52% at 12:00 p.m. ET today.

So what: PTC Therapeutics markets the Duchenne muscular dystrophy drug Translarna in the EU, and patients have been hoping that DMD drugs like Translarna would similarly become available here in the United States.

However, PTC Therapeutics reported earlier today that the FDA has sent the company a "refuse-to-file" letter stating that Translarna's application for approval wasn't complete enough to permit a substantive review of the drug by the regulator.

The news throws cold water on any existing timeline for a potential Translarna launch in the U.S. and continues a string of bad news from the FDA for DMD drugs. In January, it opted against approving BioMarin's (BMRN 0.83%) drug Kyndrisa and sent notes to a key advisory committee that were harsh enough to have many anticipating an eventual FDA no-go for Sarepta Therapeutics' (SRPT -1.27%) DMD drug eteplirsen.

Now what: PTC Therapeutics says it's evaluating its options and that conceivably keeps the window open a crack for another filing. However, chances for any eventual approval could be hindered by arguably shaky efficacy in late-stage trials of Translarna that were reported in October.

Overall, the DMD market has an unquestionably massive need for new treatment options, but it appears the current generation of up-and-coming therapies just doesn't deliver the efficacy and safety necessary to pass muster with regulators. Given Translarna uncertain future, investors are right to head to the sidelines to await clarity.