(Reuters) - Orexigen Therapeutics Inc said patient enrollment in a trial testing the heart risks associated with its experimental obesity drug is expected to complete in the first quarter of 2013, in about half the time originally estimated.

J.P. Morgan Securities analyst Cory Kasimov said the accelerated enrollment could prompt an interim analysis in the second half of 2013 and set in motion the resubmission of application on Orexigen's drug Contrave.

The U.S. Food and Drug Administration requested for an additional trial after it rejected Contrave in January 2011 citing heart risks.

The main goal of the trial, named Light, is to analyze the cardiovascular impact of the drug.

The company is planning to conduct an interim analysis after evaluating 87 incidents of adverse cardiovascular events and to resubmit its drug application if the results are favorable.

With the current enrollment rate, Orexigen expects the 87th event to occur in the second half of 2013.

"We remain confident that there will not be an adverse cardiovascular signal with Contrave," analyst Kasimov said.

Vivus Inc's Qnexa had also faced similar hurdles and was rejected on cardiovascular issues. Vivus now expects a decision by July 17 after the FDA pushed its action date by three months to review the company's risk evaluation plan.

Arena Pharmacetical Inc's obesity drug lorcaserin was approved last month, after initially being rejected on potential cancer risks.

Lorcaserin was the first obesity drug to get approval in 13 years. FDA had set an unusually high bar for approvals because of safety problems.

Orixegen's shares were up 4 percent at $6.22 on the Nasdaq. They rose 10 percent to an 18-month high of $6.58 earlier in the day.

Arena shares were down 5 percent and Vivus shares were down 2 percent on the Nasdaq.

(Reporting By Pallavi Ail; Editing by Joyjeet Das and Don Sebastian)