A federal advisory committee Thursday voted against recommending approval of a new weight loss drug developed by Arena Pharmaceuticals.

In their 9-to-5 vote, the advisers said there were some uncertainties about the safety of the drug, called lorcaserin, particularly findings of tumors in rats.

While some said the drug was not clearly unsafe, even the possibility that it might cause harm seemed enough to outweigh what some panel members characterized as the drug’s minimal effectiveness in causing weight loss.

“I voted no because there’s too much uncertainty,’’ said Jessica W. Henderson, a professor of community health education at Western Oregon University who was the consumer representative on the committee.

The negative vote is the second setback this year in attempts to win approval for what would be the first new prescription weight loss drug in more than a decade. In July, citing safety concerns, the same committee voted 10 to 6 against approval for the drug Qnexa from Vivus, even though that drug produced a much greater weight loss than lorcaserin.

The F.D.A. is expected to decide on whether to approve lorcaserin and Qnexa in October.

Some experts say the need is great for new medicines to treat obesity, with roughly a third of American adults classified as obese and another third overweight. Obesity contributes to many other health problems like diabetes, heart disease, cancer and arthritis, and there is a hope that drugs to help people lose weight would help mitigate some of those other problems as well.



Yet the F.D.A. is very safety conscious in this area since the drugs will be taken by millions of people and because of past safety problems with weight-loss drugs.

Lorcaserin mimics the effect of the brain chemical serotonin and has an effect on suppressing appetite. Its mechanism of action is similar to that of fenfluramine, which was part of the fen-phen diet pill combination that was taken off the market for causing damage to heart valves.

Arena, which is based in San Diego, said its drug was carefully designed to work in the brain and not the heart. In its clinical trials, those who got the drug did not have a significantly higher rate of valve problems than those who got a placebo. The F.D.A., however, said that statistically the company could not rule out a 50 percent increase in the risk of valve problems.

The F.D.A. also expressed concern that the drug appeared to cause tumors in rats, particularly in the mammary glands of females. Arena argued that those tumors were not caused by the drug and that the findings would not apply to people. In the clinical trials, there was no increase in cancer rates in those who got the drug.

Safety concerns aside, lorcaserin met only one of the two F.D.A. standards for weight loss drugs, and it did so only by what the agency termed a “slim margin.’’

Those taking the drug lost 5.8 percent of their weight after a year, compared to 2.5 percent for those getting a placebo. The difference, 3.3 percent, is below the 5 percent criterion set by the F.D.A.

Forty-seven percent of those taking lorcaserin lost at least 5 percent of their weight, compared to 22 percent of those getting a placebo. That just exceeded the F.D.A. standard that twice as many people on a drug versus a placebo lose 5 percent of their weight.

Jack Lief, the chief executive of Arena, defended the drug in a statement issued after the meeting.

“Arena and Eisai believe that lorcaserin has a positive benefit-risk profile,’’ he said in a statement. “Arena will work closely with the F.D.A. as the agency completes its review.’’

Eisai, a Japanese drug company, acquired the rights to market the drug in the United States if it is approved.

On Wednesday the advisory panel split 8 to 8 on whether the diet drug Meridia should be removed from the market. A study showed that it increased the risk of cardiovascular problems. The drug has already been removed from the market in Europe.

Abbott Laboratories, which makes Meridia, said the patients in the clinical trial already had cardiovascular risks and should not be given the drug but that the product was useful for other people.