Drisapersen's role in treating DMD is being investigated in two randomized, placebo-controlled clinical trials. A six-month, phase II study of 54 patients is complete. A one-year, phase III study of 180 patients is expected to have results ready for release in the fourth quarter. The Glaxo spokesperson said results from the phase II study are being withheld until the third quarter in order to preserve the data integrity of the larger, phase III study.

Prosensa, a Dutch drug firm, is co-developing drisapersen in partnership with Glaxo.

Both Sarepta's eteplirsen and Glaxo's drisapersen are designed to "skip over" a section of damaged gene in DMD patients and restore the gene's ability to produce partially functioning dystrophin, a protein involved with muscle repair. The absence of functional dystrophin is the hallmark of DMD and causes the progressive loss of muscle function in patients.

The respective drugs from Sarepta and Glaxo differ in their chemical makeup, explained Nate Sadeghi, an institutional healthcare investor and former contributor to TheStreet:

Glaxo and Prosensa's compound belongs to an older class of molecule -- it's a 2'-O-methyl phosphorothioate antisense oligonucleotide, or 2'OMe AO, for you nerds out there -- which has the advantage of having been studied extensively in preclinical and clinical studies. The disadvantages are a relatively narrow therapeutic index and apparent uptake of filtered oligonucleotide by the proximal tubules in the kidney. This latter observation does not appear linked to kidney toxicity, although it's something to watch.

Sarepta uses newer, so-called morpholino chemistry for the design of eteplirsen. This chemistry affords eteplirsen a wide therapeutic window, good stability in vivo, and no obvious toxicity signal. The downside is the unknown, since morpholino chemistry has limited preclinical and clinical data.

-- Reported by Adam Feuerstein in Boston.

Adam Feuerstein writes regularly for TheStreet. In keeping with company editorial policy, he doesn't own or short individual stocks, although he owns stock in TheStreet. He also doesn't invest in hedge funds or other private investment partnerships. Feuerstein appreciates your feedback; click here to send him an email.

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