The announcement last month that drugmakers Amylin Pharmaceuticals and Eli Lilly were parting ways is only the latest twist in their more than two-year quest for regulatory approval of their once-weekly injectable diabetes medication, Bydureon. The companies first filed for approval with the US Food and Drug Administration (FDA) in May 2009, and in the intervening period they have had to deal with two complete response letters for manufacturing issues and potential heart problems. But with new trial data in the bank and Bydureon's European approval in June, things are looking up, and a decision is expected from the FDA by 28 January.

“I think that the FDA will very likely approve Bydureon,” says senior biotechnology analyst Thomas Wei of the investment firm Jefferies in New York. “And a big part of the reason I think that this will be successful is that the efficacy looks very good.”

Bydureon, a reformulation of the company's already approved twice-daily injectable, Byetta (exenatide), is just one of several diabetes drugs in the pipeline that mimic the gut hormone glucagon-like peptide-1 (GLP-1) to lower overall blood sugar by stimulating insulin and inhibiting the hormone glucagon. And GLP-1 mimics have an edge over other drugs that act further upstream: they not only lower blood sugar but also cause weight loss. In a phase 3, 456-person trial, people taking Bydureon lost 2.6 kilograms (5.7 pounds) on average over a 26-week span—significantly more than those on insulin alone, who had an average gain of 1.4 kilograms (Lancet 375, 2234–2243, 2010). (Similar effects have been reported for other GLP-1 analogs.) This extra benefit of the drug against obesity, which often accompanies type 2 diabetes, could help it beat out its competition.

“To finally have a diabetes drug that not only lowers your glucose but also tackles your weight at the same time—that's a real breakthrough for patients and doctors,” says Daniel Drucker, a diabetes researcher at the Samuel Lunenfeld Research Institute at Mount Sinai Hospital in Toronto, who led phase 3 trials for Bydureon.

San Diego–based Amylin will receive custody of Bydureon in the split, but, even if approved, the drug faces competition down the road. British drug giant GlaxoSmithKline and Indiana–based Eli Lilly each have their own long-acting versions of GLP-1 analogs in phase 3 trials. Meanwhile, Denmark's Novo Nordisk has a pair of once-weekly GLP-1–like drugs in the works: one, currently in phase 2 testing, is a novel molecule called semaglutide, and the other is a reformulation of Victoza (liraglutide), the company's bestselling daily injectable diabetes drug, which is in phase 1 development.

Experts say that the clinical data published so far have not produced a runaway winner among these new drugs. “There aren't huge differences in efficacy or potency in regards to how they work,” says Drucker. The driving factor may come down to how much weight people lose in comparative trials. The different GLP-1 analogs vary in size, so some of the drugs may be less able to cross the blood-brain barrier than others. “A question that all of us have is: how important is it for these drugs to access regions in the brain that control appetite and body weight?” Drucker says.

Another unknown is the long-term effect of each drug on morbidity and mortality— although Amylin and Novo Nordisk both have ongoing long-term trials which should produce results in 2016 if not before. But, until the long-term effects are known, it may not be reasonable to ask patients to pay more for these drugs, despite their convenience, says Nicole Pinelli, a clinical pharmacy specialist at Wayne State University in Detroit. “If there's nothing to say that these drugs reduce cardiovascular morbidity or mortality, it's hard to justify using them above and beyond the generic drugs that are currently on the market,” she says.