GlaxoSmithKline's Duchenne MD drug mirrors placebo effect in PhIII

Over the weekend GlaxoSmithKline ($GSK) detailed the decisive failure of its ambitious Phase III study for drisapersen, a new drug designed to treat Duchenne muscular dystrophy. John Kraus, GSK's medical leader for drisapersen, unveiled more data from the failed study on the exon-skipping technology, which fell short not only on its primary endpoint for improved walking distances in a 6-minute test but also flunked out on three separate secondary endpoints, with the outcomes looking almost identical in each case with a placebo.

After 48 weeks of therapy, says Kraus, the drisapersen group suffered a decline in walking distance, just as the placebo arm did. The decline was 10.3 meters in the drug arm's favor, falling short of a significant improvement and well off the more than 30-meter improvement seen in Phase II. Using the North Star Ambulatory Assessment test, the placebo arm scored 6.7 compared to a close 7.2 score for drisapersen. A velocity test gauging the time to climb four stairs was also virtually identical--a 0.12 decline in the placebo arm and a 0.14 decline in the drug group. There was a near mirror-image decline in the rate of patients' 10-meter walk/run velocity gauge.

Kraus says the trial was well run and the quality outcomes they measured were good, providing a clear read out on the drug for the group they studied. Now investigators are going back over the data to see if a subset of patients benefited. And they are also running analyses on dystrophin expression to see how the therapy--in-licensed from the Dutch biotech Prosensa--influenced a key biomarker for the disease.

"We need to understand if there may be a subpopulation that could have derived a benefit from drisapersen," Kraus tells FierceBiotech. "We are currently running analyses to help us understand."

Subpopulation analysis is common in drug development, but it's far from ideal. A positive sign in a particular group might open the door to a new study geared for a narrow segment of the patient population, which has no meaningful therapy to rely on. But it's no guarantee of even limited success.

Investors have been paying particularly close attention to the drisapersen data to see how it might reflect on eteplirsen, a DMD drug in development at Sarepta ($SRPT). Sarepta shares have soared on the positive outcome seen in a very small study of the drug, and the biotech believes it has a shot at an accelerated approval based on that study. Right now, eteplirsen is the leading therapy in the DMD field, and there's nothing in GlaxoSmithKline's latest data that changes any of that.  

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