It's never a good sign when a lead drug's sales growth starts to deteriorate, but for one-hit wonders such as Amylin Pharmaceuticals (Nasdaq: AMLN), it's enough to get them singing the blues.

Sales of diabetes drug Byetta, which Amylin comarkets with Eli Lilly (NYSE: LLY), were up 8.2% year over year, but down 10% from the last quarter. Some of the sequential drop can be attributed to wholesalers increasing inventories at the end of last year and then using that to supply some of their customers in the first quarter. Correcting for the estimated impact as given by management in the conference call, this gets Amylin to 4% sales growth of Byetta, at best.

Amylin's other drug, Symlin for diabetics, saw sales shoot up 30% year over year. Customers are clearly digging the new convenient pen injector. Unfortunately sales of Symlin only contribute about 10% of revenue, so the increase doesn't move the needle much.

The weak revenue growth plus increased spending as the company prepares its once-weekly version of Byetta led to a net loss of almost $69 million. Fortunately, Amylin still has more than $1 billion in cash reserves on the balance sheet, so it won't run out anytime soon.

Even so, Amylin really needs to kick it into high gear during the last three quarters of this year. It's in a race to get as many diabetic patients on Byetta before Novo Nordisk (NYSE: NVO) gets its own glucagon-like peptide-1 drug, liraglutide, approved. Novo's marketing application should be submitted this quarter, so Amylin doesn't have much time.

What will really help Amylin recruit patients is its once-weekly version of Byetta, but ... that drug is still more than a year away.

Amylin is taking steps to help this more patient-friendly drug compete once it's approved for sale. For instance, the company is running a pair of studies designed to show that the drug works better than Merck's (NYSE: MRK) Januvia or Takeda's Actos in one, and better than Sanofi-Aventis' (NYSE: SNY) Lantus in the other. But management doesn't expect results until the first half of 2009.

Right now, Amylin is just treading water. It needs to perk up -- quickly -- or else it's likely headed to the bottom of the ocean to hang out with other shipwrecked drug developers.

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Fool contributor Brian Orelli, Ph.D., doesn't own shares of any company mentioned in this article. Eli Lilly is a selection of the Income Investor newsletter. The Fool has a disclosure policy.