An advisory panel to the U.S. Food and Drug Administration voted 17-6 that regulators should make companies conduct heart impact studies in order to sell their drugs in the United States, even if clinical trials do not initially show evidence of increased heart risk.

"Anti-obesity drugs have a bad track record of cardiovascular risk," said Dr. Marvin Konstam, a professor at Tufts University School of Medicine and panel member.

The FDA usually follows panel recommendations, although it is not required to, and will make a final decision later.

The news could affect Vivus Inc and Arena Pharmaceuticals Inc, which are vying to get the first new obesity drug to the market in more than a decade. The FDA previously ordered Orexigen Therapeutics Inc to conduct a heart safety study before it would consider approving its weight loss pill, Contrave.

Shares of Vivus, whose drug Qnexa is up for an approval decision from the FDA in mid-April, fluctuated wildly ahead of the panel vote and were also a subject of high-volume options trading.

Vivus shares closed up 4.5 percent at $21.28 on the Nasdaq following the decision but gave back about half of those gains in extended trading.

Shares of Arena, whose lorcaserin drug is up for review from a separate FDA panel in May, closed 4.1 percent higher at $3.04 on the Nasdaq and slipped to $2.96 in after-hours trading.

Orexigen shares dipped 1.1 percent to $4.54. Orexigen's heart safety studies of its drug are not likely to conclude before sometime in 2013 or 2014.

VARIABLE SIGNALS

Panelists were divided on whether companies could do a meta-analysis of existing data prior to approval, or whether they would have to do a full clinical trial focused on heart safety before the obesity drugs could reach the market.

A new clinical trial would be far more costly and require more time, and some advisers cautioned against crimping investment in obesity pharmaceuticals research.

Most panelists agreed the need for a pre-approval trial would depend on how much weight loss a drug can bring, and what other risks it has shown. They also advised enrolling patients at higher risk for heart disease in trials, such as the elderly.

The advice largely mirrored the FDA's guidance for makers of pills for Type 2 diabetes, which requires companies to prove their drugs do not increase the risk of heart attacks or other heart problems.

Obesity has become an epidemic in the United States, leading to a huge increase in diabetes and a host of related health problems, including heart disease. But many potential weight-loss drugs have either failed or been abandoned due to safety issues.

In public health studies, weight loss has generally been tied to positive effects on the heart, including lower blood pressure, FDA staff said in a review earlier this week.

But a few trials of the long-term heart safety of obesity drugs have shown they actually increase heart risks, or have no effect on heart health, the FDA staff said.

The infamous diet drug combination known as "fen-phen" was pulled from the market in 1997 after reports of sometimes fatal heart-valve problems. And Abbott Laboratories withdrew its weight-loss drug Meridia from the U.S. market in 2010 after a study showed it increased the risk of heart attacks and strokes in some patients.

However, Michael King, analyst for Rodman & Renshaw, bet the FDA is feeling more pressure to approve obesity drugs, and would avoid delaying Vivus's Qnexa with more pre-approval trials.

Despite the panel vote, it is up to the FDA to decide whether a drug shows signs of heart safety problems.

"It's just impossible to run a trial that's large enough that will conclude in a reasonable amount of time," he said.

"If you don't care if the drug (Qnexa) gets approved five years from now that's fine. (But) I think it's clear ... that the FDA does want this drug on the market."

(Reporting by Anna Yukhananov, additional reporting by Bill Berkrot; Editing by Gary Hill and Steve Orlofsky)

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