The Food and Drug Administration's panel of experts voted 9-5 against Arena Pharmaceuticals' drug lorcaserin, saying a series of unresolved safety questions outweighed the drug's modest benefits.
Panelists raised concerns about tumors seen in rats in early stage testing of the drug. Other safety concerns included higher rates of depression and memory loss among patients taking the company's pill compared with dummy pills.
The FDA is not required to follow the panel's vote and will make its own decision by Oct. 22.
Thursday's vote is a major setback for San Diego-based Arena, which currently has no drugs on the U.S. market. Its pill Lorcaserin is one of three drugs racing to be the first new prescription weight loss drug approved by the Food and Drug Administration in more than a decade. In July, the same FDA panel voted against recommending approval for Vivus Inc.'s Qnexa due to safety concerns. Orexigen Therapeutics is scheduled to have its own drug reviewed at a meeting in December.
Both of those drugs showed greater weight loss than lorcaserin, but the drug was favored by analysts because of its perceived safety.
Nearly all panelists agreed the company had met FDA's requirements for achieving weight loss, but the majority said they were "troubled" by tumors seen in animal studies of the drug.
"There wasn't any clear evidence that this was a dangerous drug, but there was enough doubt raised about the risks that I had to take a harder look at the crux of the benefits," said Dr. Edward Gregg, of the Centers for Disease Control and Prevention, who voted against approval.
Additionally, panelists complained that the company did not study the drug in patients with diabetes, heart disease and other conditions which are common among obese and overweight people.
"I voted no because there is too much uncertainty about brain and breast tumor development and too much uncertainty about the actual population this drug would be used in," said Dr. Jessica Henderson, of Western Oregon University.
Arena studied the drug in more than 7,000 obese and overweight patients. After one year, the average weight loss among lorcaserin patients was 5.8 pounds, compared with 2.5 pounds among patients taking a dummy pill. That difference was enough to meet FDA's effectiveness requirements "by a slim margin," according to agency reviewers.
But the chairman of the panel suggested Thursday that the agency should raise the bar on weight loss drugs.
"If we want more effective drugs for weight loss we need to set higher limits," said Dr. Abraham Thomas, of Henry Ford Hospital.
The quest for a blockbuster weight loss drug has eluded nearly every major pharmaceutical firm over the last four decades. A slew of prescription weight loss offering have been associated with cardiovascular problems, most notably Wyeth's diet pill combination fen-phen. The drug was pulled from the market in 1997 because of links to heart valve disease.
Lorcaserin targets the same appetite pathway as fen-phen but in a more selective, and what was thought to be safer, manner.
On Wednesday the same FDA panel was split on whether to withdraw Abbott Laboratories' diet pill Meridia, which has been linked to heart attack and stroke. The drug is one of only three prescription medications approved for weight loss in the U.S. European regulators banned the drug earlier this year.