CHAPEL HILL, N.C. — The Food and Drug Administration has dealt another blow to Pozen Pharmaceuticals and drug development partner GlaxoSmithKline in their bid to win approval for a new migraine treatment drug.

Before the markets opened Wednesday, Pozen said the company and GSK had received a letter from the FDA wanted additional “analysis and supporting information” about Trexima.

The news triggered an immediate 9 percent drop, or $1.62 per share, in Pozen stock from Tuesday’s close of $18.11. Pozen trades on the NASDAQ under the symbol POZN.

By 10:45 a.m., shares were off $2.94, or more than 16 percent. Shares hit a low of $15.06 before rallying in the afternoon. For the day, Pozen lost $2.02, or 11 percent, to close at $16.09.

GSK (NYSE: GSK) was largely unaffected, closing down 9 cents at $52.91.

Pozen said it would submit the requested information before the end of the year. In a conference call, Pozen said GSK still has plans to launch Trexima in the third quarter of 2007.

The FDA first asked for additional information from Pozen and GSK in June. The news sparked a sell0off in Pozen stock, dropping the price to $5.26 — a 52-week low — from $13.90 in one day. More than 25 million shares were traded.

Since then, Pozen stock has rallied strongly. Pozen and GSK submitted the initial additional information the FDA wanted in November. The new letter raised questions about the November submission.

John Plachetka, Pozen’s chairman and chief executive call, said in a conference call before the markets opened that the company and GSK would respond to the FDA’s request before the end of the year.

“There is nothing in the letter that leads us to believe we are in any different situation than we were in November when we sent in our response,” Plachetka told analysts and reporters. “This is more an issue of how the data was presented.” He added that “I wouldn’t read any more into [the letter] than that.”

The FDA sent Pozen a so-called approvable letter in June that raised concerns about the safety of Trexima. Pozen and GSK teamed up to provide additional information in November.

Plachetka said Pozen and GSK executives “felt we had submitted adequate information” and added that the two companies have already agreed of “new data presentations.”

“We feel we now have a much better understanding of how the FDA wants the information submitted,” he explained. “We got the message. We will deliver the information in a different format.”

The FDA wants to be certain that the additional information, which was gathered from trials done by GSK in another study, would allow “apples to apples” comparison with earlier data.

Plachetka reiterated that Pozen and GSK believe Trexima is effective and safe.

GSK and Pozen are planning on Trexima winning approval within six months of receiving the revised data, he added. GSK is looking to launch Trexima in the third quarter of 2007, assuming it is approved, Plachetka said.

In September, GSK and Pozen unveiled data that the partners believed further boosted the case for approval of Trexima. At a migraine conference in London, Pozen, which is based in Chapel Hill, and GSK said new test results “suggest Trexima may address an unmet need among migraine sufferers, who frequently cite inconsistent effectiveness as a reason for their dissatisfaction with their medicine.” The results also indicate Trexima “was generally well tolerated across both studies.”

Trexima is a combination of the GSK drug Imitrex, which is a pain reliever for acute migraines, and a combination of other products formulated with Pozen-developed technology. Trexima also contains naproxen sodium, a non-steroidal anti-inflammatory drug used in over-the-counter drugs such as Aleve.

Pozen received a $20 million milestone payment last fall from GSK following the submission of the new drug application for Trexima to the FDA.

GSK made as $15 million milestone payment to Pozen in May of 2004 based on reaching Phase III clinical trial milestones. At that time, the companies said they hoped to file the new drug application in the second half of 2005.

More than 20 million Americans and more than 300 million people worldwide suffer from migraine headaches.

Tuesday’s drop in price erased considerable gains in Pozen stocks over the past three months. Pozen signed a drug development deal with AstraZeneca in September and received, which included a $40 million upfront payment. The Trexima data announced at the drug show also triggered a rally in the stock price.