The injectable drug is a potential rival to Eli Lilly and Amylin Pharmaceuticals' established treatment Byetta and Novo Nordisk's Victoza, which the Danish drugmaker expects to launch in Europe this summer.

"In our view, Syncria is beginning to look like a me-too drug, assuming exenatide LAR (long-acting Byetta), Victoza and (Roche and Ipsen's) taspoglutide make it to market first," Citigroup analyst Peter Verdult said in a note.

Clinical results from a Phase II trial on Syncria, which Glaxo is developing with Human Genome Sciences, were presented at the annual meeting of the American Diabetes Association.

They showed Syncria was more effective than the existing twice-daily version of Byetta in lowering blood sugar. However, a spokesman for Lilly and Amylin said the Byetta data in the study were inconsistent with results from previous published trials.

Piper Jaffray analysts said they were "underwhelmed" by the Syncria data, since there was only a low absolute reduction in HbA1C -- a common measure of blood sugar -- and modest weight loss.