UPDATE 4-Orexigen revises obesity drug data; shares fall

* Fewer patients met weight loss goals than reported

* Says primary and secondary trial goals still met

* Shares fall 11 percent. (Updates with closing share price)

By Toni Clarke

BOSTON, April 13 (Reuters) - Orexigen Therapeutics Inc (OREX.O) said on Tuesday that fewer patients achieved key weight loss goals when taking its experimental obesity drug than originally reported, sending the company's stock down 11 percent.

The company said only 50.5 percent of patients who took the drug, Contrave, lost 5 percent or more of their body weight after 56 weeks, compared with a previously reported figure of 56.3 percent.

Only 28.3 percent of patients lost 10 percent or more of body weight, compared with a previous figure of 32.9 percent.

Michael Narachi, the company's chief executive, said in an interview that the downward revision was the result of an omission in the program used to calculate the numbers.

"It is odd that we didn't catch it," he said. "But it doesn't materially change our story."

Contrave combines the antidepressant Wellbutrin, known generically as bupropion, and naltrexone, which is used to treat alcoholism.

The company recently filed for approval of the drug with the U.S. Food and Drug Administration. It is waiting to hear whether the agency deems the application complete and ready for review. The revision to the data is unlikely to affect the drug's timeline for approval, he said.

"The FDA told us to handle this as a clinical amendment," he said -- a technique used to make small changes to a drug application. "It's not a resubmission, or something major."

Moreover, he said, the results still meet the benchmark set by the agency for clinically significant weight loss.

If approved, Contrave would likely compete with Qnexa, an experimental drug from Vivus Inc (VVUS.O) and lorcaserin, an experimental drug from Arena Pharmaceuticals Inc (ARNA.O). [ID:nN09101428]

Orexigen, based in La Jolla, California, is seeking a partner to help launch the drug. Narachi said the company is running a competitive bidding process and would rather sign a deal before the drug is approved, even if it means receiving a smaller upfront payment, in order to hit the ground running once it is approved.

He declined to say who the company is negotiating with.

William Tanner, an analyst at Lazard Capital Markets, speculated that the mistake in the figures might have been identified by a potential partner.

"It seems a little late in the game to notice the error," he said. "We wonder whether it might not have been identified by a "fresh" set of eyes," he said in a research note.

Weight loss drugs have been notoriously difficult to develop and the road is littered with failures. Wyeth, which was acquired by Pfizer Inc (PFE.N) last year, was forced to set aside more than $21 billion to settle litigation related to fen-phen, a cocktail that included the stimulant phentermine and either fenfluramine or dexfenfluramine.

Fenfluramine and dexfenfluramine were recalled in 1997 after being linked with heart valve damage.

Arena's drug, lorcaserin, works in a similar way to fenfluramine, but appears to be safer. One trial of lorcaserin showed that patients who took the drug twice a day for a year lost, on average, 5.9 percent of their body weight, compared with 2.8 percent for those taking a placebo.

Vivus' drug, Qnexa, combines relatively low doses of phentermine with the epilepsy drug topiramate, sold under the brand name Topamax. One late-stage trial showed that patients who took Qnexa lost an average of 14.7 percent of their body weight, compared to a loss of 2.5 percent for patients taking a placebo.

Thomson Pharma has forecast 2013 annual sales of $418 million for lorcaserin, $357 million for Qnexa and $325 million for Contrave.

Orexigen said that for patients who completed 56 weeks of treatment with Contrave, and did not drop out, 64.9 percent lost 5 percent or more of body weight, as opposed to the company's previously reported number of 75.8 percent.

It said that, of patients who completed treatment, 39.4 percent lost 10 percent or more of body weight, compared with a previously reported figure of 48.2 percent.

Orexigen's shares closed down 11 percent at $5.10 on the Nasdaq. Earlier in the day they fell as low as $4.94. Vivus' shares rose 1.8 percent to $8.87 while shares of Arena fell 2.5 percent to $3.07. (Reporting by Toni Clarke; Editing by Derek Caney, Gunna Dickson and Bernard Orr)