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More Reasons to Invest in Sarepta With Caution

Stocks in this article: SRPT GSK

The Italian physicians observed a 12-month 6MWT decline of only 42.3 meters among the 71 boys that were more than seven years old. That's far less than the decline observed in the McDonald study. Even if we exclude patients receiving continuous steroid treatment -- a high-risk therapeutic option that has modest, temporary efficacy in DMD -- the Italian cohort performed far better than the Americans, with 6MWT scores declining by 66.4 meters.

There are disadvantages to both citations. Investors should rightly worry about the quality of data from a single European country, although the large cohort size and relentless progression of DMD may lessen this concern. On the other hand, compared to the independently funded, multi-center Italian group, the American study -- paid for by PTC Therapeutics and including only patients from a single institution -- has an increased risk of bias.

My second review of the literature raises an important point. Despite the steady loss of function over time, DMD patients experience more clinical variability than I would have expected. In the Italian study, the standard deviation -- a measure of data dispersion around the mean -- exceeded the mean 6MWT decline (-42.3 meters ± 73.9 meters for the subgroup of patients more than seven years old). McDonald and colleagues also note the high degree of variability as patients approach loss of ambulation. Finally, the Phase I-IIa trial of Glaxo and Prosensa's GSK-2402968 (formerly PRO-051) show a standard deviation nearly as wide as the mean 6MWT result itself.

These data all suggest that progression in DMD can vary widely, which makes it especially difficult to tell whether Sarepta's results show a drug effect or just unexplained progression in an unblinded study cohort. I'm inclined to believe it's a drug effect, but I don't think it's as clear-cut as do the bulls. As a result, I'm standing by my recommendation that investors keep Sarepta long positions relatively modest.

I'm not sure how to think about the rebuttals to my concern that Glaxo's Phase III trial suggests global regulatory agencies might require a long-term, placebo-controlled study. On the one hand, I agree with my colleague Adam Feuerstein that the new Prescription Drug User Fee Act (PDUFA) regulations give the FDA increased leeway to rapidly approve promising drugs for devastating diseases. Sarepta's eteplirsen may clear this hurdle, although so might Glaxo's GSK-2402968. After all, although Glaxo doesn't have data correlating dystrophin levels with clinical outcomes, the patients enrolled were 9.2 years old on average -- beyond the point at which recovery would be expected -- and yet showed an absolute improvement in 6MWT results.

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