Last checked at 
* Drug fails to help patients walk better in Phase III study
* Prosensa shares slump 70 percent
* Rival Sarepta Therapeutics edges up 2 percent (Adds analyst comments, updates shares)
By Ben Hirschler
LONDON, Sept 20 (Reuters) - An experimental drug forDuchenne muscular dystrophy (DMD) from GlaxoSmithKline and Prosensa failed to meet its goal in a late-stageclinical trial, sending shares in Prosensa tumbling 70 percent.
The slump wiped some $600 million off the market value ofDutch-based Prosensa, which only made its stock market debut onNasdaq in June.
The drug, drisapersen, did not show a statisticallysignificant improvement in the distance that patients could walkin six minutes compared with placebo in the Phase III test runby GSK, the companies said on Friday.
The setback may be good news for Sarepta Therapeutics, which is developing a rival treatment for the raremuscle-wasting disorder, though the news highlights thedifficulties of developing effective treatments.
Sarepta shares rose 2 percent in early trade, afterinitially losing ground in pre-market dealings.
GSK and Prosensa had previously been seen as ahead of thepack in making a drug for the untreatable condition - a beliefunderscored by it having been awarded a "breakthrough therapy"designation by the U.S. Food and Drug Administration.
However, data released on GSK's website last month hadalready indicated a substantial number of patients treated withdrisapersen might not be getting a meaningful drug effect.
NEXT STEPS
The news is a fresh late-stage pipeline blow for GSK -Britain's biggest drugmaker - which reported on Sept. 5 that anexperimental cancer vaccine failed to help melanoma patients.
Up until now, 2013 has been a strong year for GSK research,with new drug approvals in HIV, cancer and respiratory disease.But finding a treatment for DMD, like developing a therapeuticcancer vaccine, was always a difficult prospect.
GSK and Prosensa now plan to study the clinical trialresults in detail to see if drisapersen has a future.
"We appreciate that these results will be disappointing forboys with DMD and their families," said Carlo Russo, head ofrare diseases research at GSK.
"We are committed to evaluating the outcome of this study inthe context of the overall development programme with experts inthe field and we expect such evaluation to help inform our nextsteps for drisapersen. It is our hope that progress will be madein an effort to help boys with DMD."
Industry analysts, however, are not optimistic. James Gordonof JP Morgan said a regulatory filing for the medicine was now"very unlikely".
GSK has an exclusive worldwide licence to develop andcommercialise drisapersen and other experimental DMD drugs fromProsensa. The Dutch firm is eligible for up to 428 millionpounds ($687 million) in milestone payments from GSK andpercentage royalties in the low teens on any future sales.
($1 = 0.6226 British pounds) (Editing by Elaine Hardcastle and David Holmes)
