Amylin Pharmaceuticals Inc. (AMLN) said Monday that its diabetes drug Byetta doesn't share a connection to thyroid cancer risk seen in potential competitor, Novo Nordisk A/S's (NVO) Liraglutide - a concern that contributed to Amylin losing $1.7 billion in market value last week.

Despite belonging to the same class of drugs, the San Diego-based Amylin stressed that the two drugs are distinctly different molecules and that data already show Byetta doesn't carry similar risk.

Separately, the company said it remains on track to file a longer-acting version, in development with Byetta partner Eli Lilly & Co (LLY) and Alkermes Inc. (ALKS), and that the Food and Drug Administration should agree to key Byetta label changes soon.

Amylin's attempt to clear the air comes as its shares dropped 19% last week and are down 72% in the last 12 months. They recently traded Monday down 3.6% to $9.33.

Last week, an FDA panel came to a split vote on whether the occurrence of a type of thyroid cancer in rodent studies permitted marketing of once-daily Liraglutide. A panel member, and subsequently some Wall Street analysts, raised concerns that the cancer risk could be present in the currently marketed twice-daily Byetta, or the once-weekly version in development.

"This is not a problem with Byetta," said Orville Kolterman, head of research and development at Amylin. He noted that the FDA itself made that statement following the panel meeting.

He added that the company looked for similar effects in both versions of Byetta, but only saw abnormal amount of those thyroid cancers in female rats at very high doses.

In Liraglutide, the higher risk was seen in both rats and mice, and across multiple dosages.

Separately, Kolterman said, Amylin remains on track to file for marketing approval of the longer-acting version, called Byetta LAR, by the end of June.

The launch and timing of that new drug, which Lazard Capital Markets expects to have $1.8 billion in sales as soon as 2012, is seen as vital to the unprofitable Amylin as sales growth of Byetta rapidly decelerate.

Though it wasn't a topic at the Liraglutide advisory meeting, both versions of Byetta continue to be overshadowed by concerns that they increase the risk of pancreatitis, an inflammation of the pancreas.

The FDA is expected to approve Byetta for use as a monotherapy, but that decision has been delayed for months by ongoing discussions for increased label warnings about pancreatic risks.

"We have had extensive discussions with the FDA regarding the pancreatitis issue," Kolterman said. "Based on those discussions, we believe that resolution of that issue is imminent and will be forthcoming shortly."

Kolterman pointed to data that show that pancreatitis occurs more often in the type-2 diabetes patients and findings from an insurance company database to show that neither Byetta nor Merck & Co.'s (MRK) Januvia increased pancreatitis risk.

Despite the company's efforts, data released Monday from IMS Health Inc. (IMS) and cited by multiple Wall Street analysts show that total Byetta prescriptions for the week ended March 27 fell 14% from a year ago.

The bigger worry for investors is that FDA concern about pancreatic side effects related to the longer-acting version, which remains in the body longer, could delay the drug's launch.

"We have not had an indication from the agency of a concern about LAR," Kolterman said. "But I just need to caution that they could have concerns and they haven't been expressed yet."

Any questions from regulators will likely come as they review the application, and Kolterman said he believes the company can address any issues with the data it continues to collect on the drugs.

-By Thomas Gryta, Dow Jones Newswires; 201-938-2053; thomas.gryta@dowjones.com