In biotech, what goes around comes around.

Yesterday, PTC Therapeutics (NASDAQ:PTCT) jumped 27.5% following positive data for its spinal muscular atrophy drug, risdiplam, which sent shares of competitors Biogen and Ionis Pharmaceuticals down on the news.

Today, it was PTC Therapeutics' turn to take a hit, falling 30.6% today, after competitor Sarepta Therapeutics (NASDAQ:SRPT) presented data for its gene therapy to treat Duchenne muscular dystrophy (DMD).

Connected puzzle pieces with dystrophy and genetics written on them

Image source: Getty Images.

So what

The first three patients treated with Sarepta's gene therapy, which goes by the convoluted codename AAVrh74.MHCK7.micro-Dystrophin, produced expression of micro-dystrophin in 76.2% of fibers with a mean intensity of the fibers of 74.5% compared to normal control.

There wasn't any functional efficacy data presented, but the amount of serum creatine kinase, a measurement of muscle damage that occurs in DMD patients, was reduced by an average of over 87%, suggesting that the expression of micro-dystrophin was compensating for the mutant copy in the patients.

PTC has two drugs approved for patients with DMD -- Emflaza, a steroid that's approved in the U.S., and Translarna, which treats a subset of DMD patients with a certain type of mutation in the EU -- so it's understandable how investors could be worried about a new competitor.

However, unless Sarepta's gene therapy fully compensates for the lack of dystrophin production in DMD patients, it's likely that patients would continue to take Emflaza and/or Translarna after being treated with the gene therapy. Given the different mechanisms of action, it's quite possible the treatments would have additive effects bringing the patients closer to fully functioning.

Now what

Today's move seems like a bit of an overreaction given the limited data available for Sarepta's therapy and the possibility that it could be approved in a few years and still not hamper PTC's sales.