The company's chief executive said, "We don't know if this will be sufficient for a phase 3 registration trial," said CEO Ginger Graham during an investors conference.
San Diego-based Amylin announced in August positive preliminary results from a 15-week study of the drug, a once-a-week injection called Exenatide LAR.
Byetta, which is given twice daily by injection, was approved by the U.S. Food and Drug Administration in April 2005 as a treatment for Type 2 diabetes in patients who no longer respond adequately to pills commonly used to control blood sugar.
Graham said the drug, which is co-marketed with Eli Lilly & Co., is now the eighth most commonly prescribed drug for Type 2 diabetes.
But a large portion of prescriptions have been for refills, rather than new patients, according to prescription tracking services.
She also said the prescription figures have been skewed by several factors, including many new patient starts that are free samples.
The company is adding to its sales force next year and expects to raise the number of physicians it targets.
Graham said Amylin is also talking with the FDA about plans to launch in the first half of this year a study of Byetta as a single treatment in newly-diagnosed diabetes patients.
"It might work better if it is used earlier," said Lynn DePippo, managing partner at Sherbrook Capital Management LLC. In addition to Byetta, Amylin also sells diabetes drug Symlin for patients who are already on insulin.
Sales of the two drugs totaled $21.9 million in the third quarter of last year.
Novo Nordisk is developing a drug in the same class as Byetta that is given by daily injection. The Danish company is expected to start a large pivotal stage trial of its drug this year, Graham said.