BOSTON - An experimental obesity drug made by Vivus Inc showed dramatic results in two late- stage clinical trials, potentially offering patients a much- needed weight-loss aid and sending the company's shares soaring more than 70 percent.

Mountain View, California-based Vivus said a trial of 1,267 morbidly obese patients treated with its drug, Qnexa, lost an average of 37 pounds, or 14.7 percent of their body weight in the course of a year, compared with a body weight loss of 2.5 percent for patients taking a placebo.

The results are among the strongest ever seen in an obesity drug trial and came without the serious side effects seen in other obesity products such as Acomplia, an obesity drug developed by Sanofi-Aventis SA which never reached the U.S. market because it caused mental problems.

The results will form the basis of a marketing application with U.S. regulators later this year.

I think this is a game-changer, said Leland Wilson, the company's chief executive, in an interview. This will change the way people look at obesity, the way payers pay for it, and we think it will have a major impact on health.

Wilson said the company plans to begin immediate partnership discussions with big pharmaceuticals companies and given the huge potential patient population -- more than a third of Americans are obese -- he expects to negotiate a state of the art deal.

We've had high interest from pharmaceutical companies, but we were all waiting for these results, he said. Now serious negotiations can begin.

The drug will be labeled to treat patients with a body mass index of 27 or more -- which will likely include many people who are not obese. The body mass index is a number calculated from a person's weight and height.

I myself am 5 feet 9 inches, I weigh 195 pounds, and am the best conditioned athlete that I know, Wilson said. Yet my BMI is 30.

Analysts expect the drug to be a blockbuster, meaning it will generate annual sales of more than $1 billion.

The weight loss is much more impressive than that seen with other drugs now on the market or in development, said Adam Cutler, an analyst at Canaccord Adams. I think it's safe to call this the best.

The marketed diet drugs Xenical and Meridia cut body weight by 4 percent to 5 percent, while products being developed by Arena Pharmaceuticals Inc and Orexigen Therapeutics Inc produce high-single digit weight loss, he said.

A second year-long trial of 2,487 obese patients with concomitant conditions such as diabetes, showed those who took Qnexa had an average weight loss of 30 pounds, or 13.2 percent of body weight, compared with a percentage weight loss of 2.4 percent for the placebo group.

Patients who took a lower dose of the drug lost 24 pounds, or 10.5 percent of their body weight.

In addition to the weight loss, patients saw improvements in other risk factors for heart disease, including decreases in blood pressure, bad cholesterol and triglycerides, a type of body fat.

Qnexa combines relatively low doses of the generic stimulant phentermine and epilepsy drug topiramate, which is sold under the brand name Topamax. It is a capsule designed to release the components in a way timed to minimize side effects.

Phentermine was part of the infamous fen-phen diet drug cocktail. Two other drugs used in the cocktail were recalled after they were linked with cardiovascular damage.

Side effects in the Qnexa trials included dry mouth, tingling, constipation, altered taste and insomnia.

Importantly, episodes of moderate to severe depression were less than 2 percent, in line with those for patients taking a placebo. The company said there were no suicide attempts or suicidal thoughts or behaviors -- a side affect that hurt Acomplia, known generically as rimonabant.

Positing a hypothetical example of what patients might expect from Qnexa, Vivus said a typical 51-year-old female weighing 250 pounds would lose 37 pounds, reduce her blood pressure and risk for diabetes, increase good cholesterol 21 percent and reduce bad cholesterol 18 percent.

In clinical terms, that means the average risk of having a major heart event within the next 10 years drops from 27 percent to 7 percent, Vivus said.

A recent study carried out in part by the U.S. Centers for Disease Control and Prevention found that obesity-related diseases account for nearly 10 percent of all medical spending in the United States, or an estimated $147 billion a year.

Vivus's shares rose as high as $12.49 on Nasdaq before slipping back to trade at $11.74 in midday trading.

But not all Vivus investors were winners. Short interest representing bearish bets amounted to 4.7 million Vivus shares as of Aug. 14, or about 6.7 percent of the company's roughly 70 million shares outstanding. (Reporting by Toni Clarke with additional reporting by Esha Dey and Deena Beasley, Ransdell Pierson and Esha Dey, editing by Dave Zimmerman and Andre Grenon)