The maker of a weight loss drug once seen as a potential blockbuster said it is scrapping its bid for U.S. approval because of "unprecedented" demands by regulators on safety trials.

Orexigen Therapeutics Inc. said Friday that it will focus on developing its drug candidate Contrave and another drug candidate, Empatic, in non-U.S. markets until there is a clear pathway to approval in the U.S. The Food and Drug Administration refused to approve Contrave in February because of concerns about its effects on the heart.

As recently last year, Contrave was seen as a highly promising obesity drug in a nation where more and more people are overweight or obese. Friday's announcement indicated that Orexigen believes Contrave's prospects are dim, at least in the U.S., which is the world's largest pharmaceutical market.

Shares of Orexigen tumbled $1.03, or 32.3 percent, to $2.15 in midday trading.

Orexigen said the FDA wants it to conduct a study on Contrave's heart side effects that is unprecedented and would generate much more information than is necessary or feasible.

In February, the FDA said Orexigen would need to conduct the heart study to get Contrave approved. Orexigen proposed a more limited study, saying it wanted to run a trial to demonstrate that cardiovascular problems linked to Contrave did not present an unreasonable risk. The company also expressed interest in trying to get the drug approved for patients with a lower risk of heart problems.

However, the FDA's Division of Metabolic and Endocrinologic Products refused to approve Contrave even for low-risk patients without data from a cardiovascular trial.

WBB Securities analyst Steve Brozak said Orexigen will either need a partner to help develop Contrave, or it will need to adopt a different approach to approval, which means seeking approval in other markets.

"I would say they are basically acknowledging they're currently not configured to continue on this dance with the FDA," he said in an interview.

Orexigen said the FDA plans to hold an advisory committee on cardiovascular risk assessment for obesity drugs in early 2012. According to Orexigen, the FDA said it might change any agreements about cardiovascular studies based on that advisory committee meeting.

Orexigen said it plans to appeal the FDA's responses through a formal dispute resolution process. The company will also will speed up exploration of non-U.S. markets. Orexigen said it plans to evaluate new opportunities, but did not specify what those opportunities are.

The company said Wednesday that it would have an announcement about Contrave Friday morning. Its shares jumped 18 percent that day as investors anticipated good news.

Contrave was considered the most promising of a group of three obesity drug candidates that were in development in the last few years. However the FDA said it was not willing to approve any of the products because of concerns about their side effects.

The agency was concerned about the effectiveness of Arena Pharmaceuticals Inc.'s drug lorcaserin, as well as a link to tumors when lorcaserin was studied on rats. The FDA hasn't approved Vivus Inc.'s drug Qnexa because one of its ingredients is linked to birth defects.