Jan. 8 (Bloomberg) -- Orexigen Therapeutics Inc. fell 34 percent in Nasdaq extended trading after releasing study results on its Contrave obesity pill that failed to show enough difference between patients taking the drug and those taking a placebo.

Orexigen fell to $4 at 5:35 p.m. New York time on the Nasdaq Stock Market after earlier closing down 3 cents at $6.05. The company’s shares dropped 55 percent in the past 12 months.

In order to gain approval, the U.S. Food and Drug Administration has said that any obesity drug must demonstrate at least 5 percent more weight loss in patients taking the drug than treated with a placebo in two trials, said Adam Cutler, an analyst with Cannacord Adams in New York, in a telephone interview today. The difference in the average weight loss between the drug and placebo patients was 4.2 percent.

“The data looked good but not as good as a lot of people were expecting,” Cutler said. “They missed” hitting the FDA threshold, he said.

Contrave combines a sustained-release version of the antidepressant bupropion with naltrexone, a drug used to manage alcohol and opiate addiction. The combination pill helped obese patients who took at least one dose in the study to lose 9.3 percent of their body weight, about 20 pounds, while those taking a placebo lost 5.1 percent of their weight, said Graham Cooper, chief financial officer for San Diego-based Orexigen, in a telephone interview today.

Almost 800 patients participated in the study. All the patients in the trial went on diets, walked for at least 30 minutes a day and took part in group counseling sessions. Three- fourths of the patients in the trial took Contrave and the rest received placebos.

Impact of Diet, Exercise

The fact that the placebo patients took part in the diet- and-exercise program made it hard for the drug to outperform the placebo, Cutler said.

The results released today are from the first of four late- stage medical trials under way involving 4,500 patients in all. That means the company will have three more chances to get two studies that meet the FDA goals, Cutler said. The other planned trials are straight comparisons of drug-against-placebo tests and don’t include diet and exercise components.

The average person in the trial was a woman in her 40s who weighed about 220 pounds.

“These were people who have really struggled with their weight,” Cooper said.

Orexigen hopes to complete the other late-stage medical trials and seek marketing approval from U.S. regulators by the end of the year, Cooper said.

To contact the reporter on this story: Rob Waters in San Francisco at rwaters5@bloomberg.net.

Last Updated: January 8, 2009 18:50 EST