Arena Pharmaceuticals Inc. has said the Food and Drug Administration rejected the company's application for lorcaserin, one of three drugs seeking to become the first new FDA-approved prescription weight loss drug in more than a decade.

The federal agency's rejection came after an FDA panel of experts on Sept. 16 recommended against approving lorcaserin in a 9-5 vote. Panelists raised concerns about tumors seen in rats in early stage testing, one of the factors that Arena Pharmaceuticals said the FDA had cited in a letter responding to the company's application.

After last month's FDA panel vote, shares of Arena plunged nearly 47 percent, to $1.99. San Diego-based Arena currently has no drugs on the U.S. market.

A group of Arena investors later launched a campaign arguing that the FDA panel's review had relied on faulty scientific data. On Friday, as the full FDA's review was expected to conclude, shares of Arena rose nearly 12 percent to close at $1.63.

Arena said on Saturday that the FDA determined "that it cannot approve the application in its present form."

In addition to citing safety concerns about tumors, the FDA's letter said the agency found that lorcaserin's weight loss efficacy "in overweight and obese individuals without type 2 diabetes is marginal," Arena said.

Arena said the letter stated that the FDA may require additional clinical studies if the company can't provide further evidence to address the concern about tumors.

The company plans to hold a conference call on Monday to discuss the FDA's rejection. Arena President and CEO Jack Lief said in a news release that his company plans to request a meeting with the FDA "to obtain further clarity on the approval path and timeline."

Lorcaserin's backers have argued it offers a safer way to shed pounds than older medications linked to dangerous side effects. Notable among them was Wyeth's diet pill combination fen-phen, which was pulled from the market in 1997 because of links to heart valve disease.

Lorcaserin is part of a new wave of weight loss drugs from several small drugmakers seeking FDA approval.

However, in July, the same panel that recommended against lorcaserin also voted against approval for Vivus Inc.'s Qnexa after studies showed signs of heart palpitation and suicidal thoughts. 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.