Amylin Pharmaceuticals (NASDAQ:AMLN) released a triple-decker press release yesterday about its diabetes drug, Byetta, but it contained only one piece of information that investors should really care about.

Ironically, it’s the part about how nothing's changed. The company finished its study comparing once-weekly Byetta (used in the clinical trial) with the product that will be sold by Amylin and Eli Lilly (NYSE:LLY) once the drug hits the market, and everything checks out OK. That keeps Amylin on track to submit the marketing application for the once-weekly product by the end of the second quarter.

Manufacturing issues are usually low on the list of things that investors need to worry about, but that's been changing recently, as Genzyme (NASDAQ:GENZ) investors found out earlier this month. Once-weekly Byetta is fairly complicated, because it uses Alkermes' (NASDAQ:ALKS) extended-release technology, so the Food and Drug Administration required the study instead of just allowing Amylin to use laboratory data.

Amylin also said that the regular Byetta doesn't pose a cardiovascular risk for patients. After heart issues were found in diabetics taking GlaxoSmithKline's (NYSE:GSK) Avandia, the FDA changed its guidelines to require this type of study. The results were necessary for the once-weekly marketing application, but heart issues aren't the likely side-effect stumbling block. The twice-daily version has been linked to pancreatitis, and it's unclear how that's going to affect once-weekly Byetta's chances at approval.

The third part of the update had to do with a new trial, named Duration-5, that Amylin is running to test once-weekly Byetta against the twice-daily version to support marketing applications outside the United States. Amylin is running a series of head-to-head trials -- aptly named Duration-1 through -4 -- against Merck's (NYSE:MRK) Januvia, sanofi-aventis' (NYSE:SNY) Lantus, and others to support the marketing of the drug.

Even though it only has to be taken once weekly and has a nice weight-loss side effect, the drug still has a lot of competition from drugs that have been on the market for awhile. Without results showing that the drug is just as good, if not better, than those on the market, sales of once-weekly Byetta could plateau much lower than investors hope they will.

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Fool contributor Brian Orelli, Ph.D., doesn't own shares of any company mentioned in this article. GlaxoSmithKline is an Income Investor pick. The Fool has a disclosure policy.