What happened

Shares of Sarepta Therapeutics (NASDAQ:SRPT), a biopharmaceutical company focused on rare genetic diseases, are on the move following a clinical-trial readout from Pfizer (NYSE:PFE). Both are developing potential new gene therapies for the treatment of Duchenne muscular dystrophy (DMD), and recently released clinical trial results suggest Sarepta's candidate has an edge over Pfizer's. Although Pfizer hardly budged, Sarepta shares jumped 17.2% higher as of 3:47 p.m. EST on Friday.

So what 

Last October, Sarepta showed us data from the first four evaluable DMD patients in a study with AAVrh74.MHCK7.micro-dystrophin, a treatment that uses an adenovirus vector to deliver a portion of the dystrophin gene and a promoter called MHCK7 that promotes its expression. Sarepta reported robust expression of micro-dystrophin that appeared to allow the entire dystrophin complex to function normally and protect muscles from deteriorating with every movement.

Man in a suit being propelled by wings, following a blue upward sloping arrow.

Image source: Getty Images.

Pfizer's DMD gene therapy candidate PF-06939926 has a similar mode of action but uses a different mini-dystrophin gene, gene promoter, and viral vector to deliver the goods. There weren't any serious side effects among all four patients treated with Sarepta's therapy, but Pfizer's safety data was troubling, to say the least. 

Among the six boys treated with Pfizer's candidate, one was hospitalized with severe vomiting symptoms and another developed a life-threatening immune reaction that left him in the hospital for 11 days with kidneys that couldn't function on their own. 

Now what

Pfizer noted that the patient hospitalized with a severe immune response wasn't being monitored as closely as he could have been, and the company will pause development of PF-06939926 while developing a risk-mitigation strategy to prevent this from happening again. 

If the dosage that led to severe side effects turns out to simply be too strong for safety, Pfizer's gene therapy candidate could have a hard time competing against Sarepta's with a lower dosage. Average micro-dystrophin production among four patients treated with Sarepta's candidate safely reached 81% of muscle fibers, while just 38% of muscle fibers showed mini-dystrophin production after a lower dose of Pfizer's candidate.

These weren't head-to-head studies, so any comparisons need to be taken with a grain of salt. That said, it looks like Sarepta's DMD gene therapy is firmly in the lead.

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