Eli Lilly's (NYSE:LLY) decision to end its partnership with Amylin Pharmaceuticals over diabetes drugs Byetta and Bydureon is turning out to be a good move. Its similar GLP-1 drug, dulaglutide, beat Byetta in a head to head.

And metformin and Merck's (NYSE:MRK) Januvia, too.

In the three phase 3 trials, dulaglutide reduced hemoglobin A1c, a measure of long-term blood sugar levels, more than the comparator drug. Other than the top-line it's-superior result, Lilly didn't give any details about the magnitude that dulaglutide beat its future competitors.

Dulaglutide is in two additional phase 3 trials that are still ongoing. In both trials, Lilly is comparing dulaglutide to Sanofi's (NASDAQ:SNY) long-acting insulin Lantus. In one trial, patients are already well into their disease progression because they're also taking metformin and glimepiride. In the other trial, they're even further along because they require Lilly's fast-acting insulin Humalog.

Assuming the last two trials come up positive -- and they only have to show that dulaglutide works as well as Lantus to be considered a success -- Eli Lilly should have enough data to get dulaglutide approved when it submits an application to the Food and Drug Administration, hopefully next year. The only remaining issue is that all new diabetes drugs -- thanks to Avandia -- have to show that they don't pose a cardiovascular risk.

Notably absent, though, is a head-to-head trial with Bydureon, which Bristol-Myers Squibb (NYSE:BMY) and AstraZeneca (NASDAQ:AZN) now own after purchasing Amylin. Both drugs are dosed once weekly, so they'll be the most direct competitors to each other.

Without a direct comparison, doctors will be left with extrapolating how each drug did in their individual clinical trials. One of Byetta's and Bydureon's selling points is that they promote weight loss, which is important since being overweight is a risk factor for developing type 2 diabetes. In addition to looking at HbA1c levels when the full data is presented, investors should pay close attention to how much weight patients taking dulaglutide lost.

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