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More Reasons to Invest in Sarepta With Caution

Stocks in this article: SRPT GSK

Investor pushback on this issue seems to rely largely on the interpretation of management comments from semi-public breakout sessions or other small group settings. Unfortunately, I wasn't present at those meetings.

In general, it's usually not a good sign when the smaller biotech company appears to be taking a more aggressive path to market than the large-cap pharmaceutical company. Having said that, the question of what might be acceptable to the FDA, especially in the face of what will likely be significant pressure from patient advocates, remains murky.

Some investors suggested that FDA concern about the observation of proteinuria -- protein in the urine -- among patients treated with GSK-2402968 forced Glaxo to conduct long-term randomized studies. That's certainly reasonable, and requires that we return to the literature for more information.

As a reminder, both Sarepta's eteplirsen and Glaxo's GSK-2402968 induce the body to "skip" a section of the exon (a sequence of nucleic acids) that codes for dystrophin, a protein critical to muscle repair that is dysfunctional in patients with DMD. Think of a row of puzzle pieces with one missing. Eteplirsen and GSK-2402968 facilitate creation of a semi-functional protein by hiding a mismatched puzzle piece (exon 51), which in turn enables matching puzzle pieces (sections of protein) on either side of the mutation to connect.

Although more-or-less functionally equivalent, the two drug candidates use different chemical maneuvering to outwit the body's immune system. 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. Those looking to dig deeper on the chemistry of these drugs should start by reading the excellent review article published in the July 2011 American Journal of Pathology (Hoffman E, et al. Am J Pathol 2011, 179:12-22.)

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