Obesity drugs with safety issues are as common as celebrities getting fired for saying (or tweeting) something stupid. The Food and Drug Administration wants a cardiovascular safety trial before it'll approve Orexigen's (Nasdaq: OREX) Contrave. Arena Pharmaceuticals' (Nasdaq: ARNA) lorcaserin has potential cancer issues. And VIVUS (Nasdaq: VVUS) is dealing with the potential for its combination drug, Qnexa, to cause cleft lip in babies whose mothers take the drug.

The latest victims are Amylin Pharmaceuticals (Nasdaq: AMLN) and Takeda, which announced yesterday that they were putting a hold on the development of their obesity drug combination pramlintide/metreleptin. Laboratory tests on blood samples from a recently completed clinical trial revealed that two patients had developed antibodies to the drug.

The antibodies can bind to and inactivate the drug, which would make it fairly useless in those patients. If only a few patients are seeing the drug inactivated, it wouldn't be a major problem; the responders that don't make antibodies could overcome the data from those making antibody.

But metreleptin is pretty close to leptin, a naturally occurring hormone that tells the body to stop eating. If the antibodies are inactivating leptin, the side effects, even in a small number of patients, could be enough to derail development.

More investigation is needed before they put this one on the shelf, but investors shouldn't hold their breath. The safety bar is extremely high for obesity drugs.

Amylin's shares ended basically flat yesterday on the news. While the data to date on pramlintide/metreleptin has been fairly strong, I think investors haven't put much value on Amylin's obesity program. Bydureon, which should be back in front of the FDA in the second half of the year, is a much more important to Amylin's near-term success.

If Amylin and partners Eli Lilly (NYSE: LLY) and Alkermes (Nasdaq: ALKS) can get Bydureon approved in 2012, it really won't matter if the obesity program is trashed.

Follow Amylin as it tries to get Bydureon approved and get its obesity drug program back on track. Click here to add it to My Watchlist, which will help you keep track of all our Foolish analysis on Amylin.

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