Revenue down 6% year over year. Prescriptions of the company's top-selling drug, Byetta, down 5.9% quarter over quarter. Amylin Pharmaceuticals' (Nasdaq: AMLN) fourth quarter wasn't pretty.

Yet shares are up 6% today. Investors are a forward-looking bunch.

Their optimism owes to the timeline for Bydureon, the company's once-weekly version of Byetta, designed using Alkermes's (Nasdaq: ALKS) extended-release technology. The drug received a complete response letter from the Food and Drug Administration in October, because the agency wants a thorough study to check the heart rhythms of patients taking Bydureon. At the time, the company said its goal was to resubmit before the end of the year.

After getting the FDA to sign off on its trial design, Amylin plans to start the trial early next month, and it's now guiding for resubmitting the drug in the second half of the year. While that’s technically the same timeline as in a worst-case scenario, at least there's a possibility of an earlier submission.

Yet on the conference call yesterday, CEO Dan Bradbury cautioned investors not to be overly optimistic. If everything goes perfectly as planned, maybe the company gets it completed in the third quarter. However, science is rarely perfect. Investors should plan accordingly.

Once Bydureon gets approved, it should be able to do more than just cannibalize Byetta sales. Byetta has been steadily losing market share to Novo Nordisk's (NYSE: NVO) Victoza, but the whole class of drugs is growing. Victoza is injected once daily, while Byetta has to be injected twice daily. But if we extrapolate that out to once-weekly Bydureon, Amylin and marketing partner Eli Lilly (NYSE: LLY) should be able to take market share back once it's approved sometime in 2012.

Amylin is still considerably cheaper than it was before Bydureon was turned down by the FDA. It should be able to get back to the $20-per-share range, but that's going to take some time.

Interested in keeping track of Amylin as it approaches its third attempt at an FDA approval for Bydureon? Click here to add it to My Watchlist, which will help you keep track of all our Foolish analysis on Amylin.

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Fool contributor Brian Orelli, Ph.D., doesn't own shares of any company mentioned in this article. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Fool has a disclosure policy.