New DMD data give Sarepta an edge in GlaxoSmithKline showdown

Few drug development races attract the kind of heated attention as the showdown between drisapersen and eteplirsen--two drugs angling to become the first approved therapy for lethal, muscle-wasting cases of Duchenne muscular dystrophy (DMD). So when investigators pulled back the covers from a new block of data on GlaxoSmithKline's drisapersen, the analysts immediately went to work to see how it stacked up against Sarepta's eteplirsen.

Sarepta's shares ($SRPT) bounced up on the news--which says a lot about how the latest handicapping went down.

The key section deals with drisapersen's production of dystrophin, the missing protein that causes havoc in  boys who suffer from DMD. The theory behind both drugs is quite simple: If you can replace lost levels of dystrophin in these boys, you can arrest or slow the rapid deterioration of their muscles. GSK's ($GSK) drisapersen--which uses an exon 51 skipping technology similar to that of eteplirsen--spurred dystrophin production in 72% of the patients taking a continuous dose of the drug in Phase II and 59% of boys taking an intermittent dose.  

Viewed in isolation, that data would be considered good, with a solid majority benefiting. But there are a few hitches to consider. Primarily, while eteplirsen has only been tested in a dozen boys in Sarepta's Phase II, they all benefited in terms of dystrophin production. Drisa's Phase II included bigger numbers, but a portion of the boys didn't respond. And there were significant adverse events in the drisa study that did not appear in Sarepta's midstage trial, which has been reported earlier.

In Reuters' follow-up story, that was a "disappointing" result for GSK--but only in comparison. At this point, with drisapersen ready to complete a Phase III study in time for an early 2014 new drug application, it's likely to go head-to-head with Sarepta, which is planning on filing an early application on Phase II data. GlaxoSmithKline, which is partnered with Prosensa (a 2012 Fierce 15 company), also won the FDA's breakthrough drug designation, indicating that the agency is disposed to hurrying the regulatory process along.

Analysts who scored the latest data give Sarepta the edge.

"GSK's newly released dystrophin data indicates that a substantial number of patients treated with the targeted dose of drisapersen may not be getting a meaningful drug effect. We believe this introduces new clinical, regulatory, and commercial risk for this competitive drug, which should benefit Sarepta given the more favorable therapeutic index that eteplirsen appears to have," wrote Baird analyst Brian Skorney in a note, according to a report in Barron's.

Cowen analysts also give eteplirsen the edge but notes that Sarepta's use of an assay to count dystrophin-positive fiber could be more consistent than GlaxoSmithKline's mixed use of methods.Cowen also notes that the data "underscores (the) point that no correlation can be drawn currently between dystrophin and clinical benefit," which may explain why Sarepta's volatile shares slid 6% this morning.

It's important to note that GlaxoSmithKline's positive data still adds up to a likely OK, provided there are no nasty surprises in the Phase III results. GSK investigators have already spelled out an improvement in walking distance, the key measure of efficacy in this group. But an edge on the data would give Sarepta a big advantage in any marketing competition.

- here's the Barron's report (sub. req.)
- read the story from Reuters