GlaxoSmithKline plc (ADR) (GSK), Sarepta Therapeutics Inc (SRPT): The Good and Bad of a Competitor’s Failure

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GlaxoSmithKline plc (ADR) (NYSE:GSK)On the surface, GlaxoSmithKline plc (ADR) (NYSE:GSK)‘s release of phase 2 data on drisapersen looks like good news for Sarepta Therapeutics Inc (NASDAQ:SRPT)‘s competing Duchenne Muscular Dystrophy drug, eteplirsen.

Investors seem to think so; shares of Sarepta Therapeutics Inc (NASDAQ:SRPT) were up almost 3.3% Friday, while Prosensa, which developed drisapersen and licensed it to GlaxoSmithKline plc (ADR) (NYSE:GSK), was down 8%.

But be careful though. Sometimes, one man’s trash is just another man’s trash.

First, the blah data
Keep in mind this is phase 2 data. GlaxoSmithKline plc (ADR) (NYSE:GSK) is already running a phase 3 trial that should read out later this year. The phase 2 data will have little influence on the approvability of drisapersen, but it could give hints as to whether the phase 3 trial will be successful and how it might compete with Sarepta Therapeutics Inc (NASDAQ:SRPT)’s eteplirsen.

The study looked at both continuous dosing of drisapersen, defined as once weekly, and intermittent dosing, which consisted of alternating between once and twice a week for six weeks and then taking the seventh to 10th weeks off before restarting the cycle.

The continuous dosing clearly worked better after 25 weeks on the drug, with 72% of patients showing an increase in dystrophin, the protein missing in Duchenne muscular dystrophy patients that leads to muscle wasting. The intermittent dosing produced increases in dystrophin levels in 59% of patients. Only one out of 18 of the patients treated with placebo registered positive, so drisapersen is clearly doing something.

The problem for Galxo and Prosensa is Sarepta Therapeutics Inc (NASDAQ:SRPT)’s data indicate that all of patients taking eteplirsen had an increase in dystrophin levels. One hundred percent trumps 72%, although the two companies measured dystrophin levels in different ways, and it isn’t clear which one is more stringent.

Be careful
In addition to the apples-to-oranges comparison across clinical trials, there’s the issue of using dystrophin as an endpoint for approval. Sarepta Therapeutics Inc (NASDAQ:SRPT) has two options for approval using its phase 2 data. It could gain accelerated approval based on the surrogate endpoint of increasing dystrophin or the Food and Drug Administration could award full approval based on the boys taking eteplirsen walking further, a clinically meaningful endpoint.

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