The experimental weight-loss drug Contrave -- a combination of naltrexone (used to treat opium addiction withdrawal) and bupropion (used to treat depression and help people quit smoking) -- has produced promising results in the first six months of a small phase III clinical trial, according to its developer.

San Diego-based Orexigen Therapeutics, Inc. said participants in the trial using Contrave experienced on average more than a 7 percent weight loss over a 24-week period compared to approximately a 1 percent weight loss for patients taking a placebo.

Some 250 patients at 14 clinical sites are taking part in the trial, which was blinded for the first 24 weeks. A second 24 weeks of open-label treatment is currently underway.

Three different versions of Contrave -- containing a high, medium and low dose of naltrexone -- were tested in the trial, which also randomized some patients to naltrexone alone, bupropion alone or a placebo.

As a result, the total number of patients getting any one of the three different doses of Contrave was relatively small.

The bupropion/naltrexone combination is based on research into the brain’s regulation of appetite and energy expenditure, which suggests that combining these two central nervous system drugs may improve the ability to initiate weight loss and block the body’s attempts to compensate for weight loss during treatment.

"Contrave is designed to activate a hypothalmic center in the brain associated with reduced appetite, while blocking beta-endorphin, which may be responsible for limiting weight loss," said Dr. Gary Tollefson, Orexigen Therapeutics CEO.

While patients in the trial who took Contrave with the highest test dose of naltrexone lost an average of 7.52% of weight from baseline, this treatment group also was more likely to have experienced nausea early in the course of their treatment, researchers said.

Patients receiving Contrave with the middle dose of naltrexone lost on average an excess of 7% of weight from baseline, but with improved tolerability -- suggesting that this middle dose may provide the best combination of weight-loss and tolerability.

Patients receiving Contrave with the lowest dose of naltrexone lost considerably less weight, but also experienced the least nausea.

"We believe Contrave represents a unique approach to successful long-term weight loss with a very acceptable safety profile,” Tollefson concluded.