Amylin said Tuesday that quarterly sales fell 5.3% to $165 million for the period ended December 31 as Byetta sales declined. Sales for the year fell 2%, to $651 million for the year ended December 31, compared with $669 million the prior year.

The drug maker provided its latest figures during its earnings call on Tuesday. The results came a week after the FDA approved its weekly diabetes 2 drug, Bydureon (exenatide extended release), which the company said will be on store shelves February 17.

The company is banking on the xenatide franchise, which includes the weekly shot Bydureon as well as the twice-daily Byetta (exenatide) to put the company on solid ground. It has added 325 employees to its sales force to promote the category.

CEO Daniel Bradbury would not provide long-term guidance for the category but said Bydureon is walking into a prepped market. “We’ve never actually been involved in a launch where awareness of a new product is as high as it was for Bydureon,” he said.

“Physicians actually call out the Bydureon brand by name, so you know it’s not just talking to a generic name or a theoretical product,” he added.

Amylin saw sales dip for twice-daily Byetta during the quarter, to $133 million, compared with $136 million for the same period last year. The drug’s annual sales were down 8%, hitting $518 million, compared with $560 million the year before.

Sales for diabetes injection Symlin (Pramlintide) rose 8%, to $28 million for the quarter, compared to $26 million for the same period last year. Symlin’s annual sales were up 13%, to $104 million, compared with $92 million the year before.

CFO Mark Foletta attributed these changes to the impact of pricing and volume shifts, with Byetta losing volume and Symlin’s remaining flat.

Company executives and analysts tussled over the use of GAAP vs. Non-GAAP numbers after the call, with many analysts attempting back-of-the-envelope calculations to get an idea of what the company was forecasting.

“It seems to imply Amylin expects 2012 [year over year] revenue growth of about 11% versus the Street at 6%. So the guidance looks better than we had originally calculated,” ISI Group’s Mark Schoenebaum wrote in an email, referring to a slide that was part of the company’s earnings presentation.

The company said the Bydureon pen should be on the market between the fourth quarter of 2012 and the first of 2013. The pen would let patients skip a preparation step. They also said they expect the send monthly version of Bydureon into stage 3 trials next year.

Former partner Eli Lilly remains part of Amylin’s financial picture. Lilly will receive 15% of global net sales of xenetide, up to $1.2 billion, as part of their separation agreement. Lilly will also receive a one-time payment of $50 million dollars if the monthly form of Bydureon is approved, and will promote the exenatide drugs outside of the US over the next two years.