Shares in pharmaceutical firm Pozen plunged 46 percent Thursday after the company disclosed that the Food and Drug Administration wants more information about its proposed migraine drug Trexima.

Pozen and GlaxoSmithKline, which has paid Pozen millions to develop Trexima, said they had requested a meeting with the FDA to discuss the situation.

Pozen (NASDAQ: POZN) shares dropped $7.59, or 466 percent, to $9.36 in morning trading. GSK (NSYE: GSK) shares were not affected, with the price increasing 47 cents to $52.08.

The “approvable letter” as it is called from the FDA is the second one issued about Trexima. Pozen and GSK responded to the first letter with additional data in January.

The latest letter expressed concerns about genotoxicity, or damage to DNA, that was “seen for the combination of naproxen sodium and sumatriptan,” Pozen said in a statement. Those are the ingredients Pozen has combined in one pill. The genotoxicity was observed in one of four studies, Pozen said.

“It appears the FDA is concerned about this one test,” Pozen Chairman and Chief Executive Officer John Plachetka said in a conference call. He noted that Pozen believes the results are “unique to this test” and could be related to test conditions.

Pozen told analysts that it was Pozen’s belief the genotoxicity issue had been addressed and that the three other “genetox” tests were negative.

The FDA had earlier expressed concerns about cardiovascular side effects of the drug. Pozen said the FDA did not express that concern in the latest letter.

Pozen and GSK executives did discuss the FDA letter Wednesday night, and Plachetka said GSK was “rigorous” in its support of Trexima. “I believe Glaxo was committed,” he added. Plachetka described GSK as “fully engaged.”

Plachetka also affirmed Pozen’s commitment to Trexima.

“We have a good product here,” he said. Pozen plans to “move forward in this difficult regulatory environment.”

Pozen used its proprietary technology to combine an existing GSK migraine drug Imitrex with naproxen, a non-sterodial anti-inflammatory drug.

“The companies intend to request a meeting with the FDA as quickly as possible to discuss the necessary steps to address the Agency’s concerns,” Pozen said.

According to the American Medical Association, migraines affect an estimated 28 million people.

The FDA’s decision comes nearly two months after Pozen disclosed two trials demonstrated the efficacy of its proposed migraine drug Trexima.

Pozen stock rallied throughout the day Wednesday on expectations that the FDA would approve Trexima. It closed up 80 cents at $17.45.

The FDA letter triggered a huge sell off just as occurred in June of 2006 when Trexima was rejected. Pozen shares, which collapsed to $5.26 from $13.90. More than 25 million shares traded hands that day.

Data from the most recent trials released in June showed Trexima was “nearly twice as effective as placebo” and also “eliminated all traditional migraine symptoms” at two hours and four hours after patients took one table. Symptoms include nausea, vomiting and light sensitivity.

"I believe this key endpoint will have more meaning to patients, as it measures when the whole migraine is gone rather than just the head pain," said Paul Winner, lead author and director of the Palm Beach Headache Center, in a statement released by Pozen. "In these studies we found Trexima consistently eliminated migraine symptoms in more patients without the need for rescue medication."

Pozen also announced positive test results in April.