US experts are becoming increasingly skeptical of the safety claims of the diet pills. If Abbott's Meridia received a split verdict last week, another FDA advisory committee has now rejected Arena's lorcaserin altogether. Its side effects outweighed any potential benefits, they have concluded.
The pill is thought to work by stimulating a serotonin receptor in the brain; the receptor has been associated with changes in feelings of satiety and feeding behavior, according to Arena.
Panelists said they were unsure if tumors in lab rats meant the drug would increase brain or breast cancers in people.
Arena's lorcaserin was designed to block appetite signals in the brain in a similar way to the now-withdrawn fenfluramine in the fen-phen diet-drug cocktail. The Arena drug is more selective in the receptors it affects and the company says studies have not found any of the heart-valve problems with lorcaserin that were linked to fenfluramine.
Arena also argued the findings of rat tumors did not apply to people because they resulted from high doses or biological mechanisms specific only to rodents. No increase in cancer cases was seen in people.
Patients who took 10 milligrams of lorcaserin twice daily for a year lost about 5.8 percent of their body weight on average, FDA reviewers said. Placebo patients lost 2.5 percent.
"I do think (lorcaserin) is promising, but there's too much uncertainty at this time," said Jessica Henderson, the panel's consumer representative.
The expert panel called on Thursday for more study of Arena's lorcaserin in people with diabetes, high blood pressure and other health problems, to better reflect patients who would use the drug if it was approved. The FDA usually follows panel recommendations. A final ruling is due Oct. 22.
The probability of lorcaserin ever reaching the market is "really small," BMO Capital Markets analyst Jason Zhang said after the panel vote. "They can run a large trial in a high-risk group and see if that will show similar efficacy but a better risk profile. That's a little remote," Zhang said.
Arena is studying lorcaserin in overweight and obese diabetics and said it expects results later this year.
Drugmakers have failed for decades to produce a pill to help people shed a significant amount of weight without serious side effects.
Prescription weight-loss drugs have failed to gain much traction in the United States, despite the potential for strong sales in a nation where two-thirds of the population are overweight or obese. The current U.S. diet drug market is small at about $381.5 million in 2009, according to data from IMS Health.
In the U.S., where nearly 35% of the people are obese, there would be a potentially lucrative market for a weight-loss drug. But a magic pill isn't about to come on the market. "There is no magic bullet, no ultimately successful weight loss pill," says Vanderbilt University endocrinologist Dr. Kevin Niswender, who specializes in the treatment of obesity. "And ultimately, I don't think that there ever will be."