Updated from 11:51 a.m. EDT

Pozen

(POZN)

, a drug-development firm with no commercial products, had nearly half of its market cap erased after U.S. regulators said they couldn't yet approve the company's proposed treatment for migraines.

The Food and Drug Administration issued what's called an approvable letter for Trexima, meaning certain conditions must be satisfied before the drug can be granted full marketing clearance.

Specifically, the FDA determined that Trexima is effective as an acute treatment for migraine headaches, but requested additional safety information. As a result, Trexima might require new studies.

Shares of Pozen plunged $6.87, or 49%, to $7.24 following the announcement. Trading volume was extremely heavy, with 6.7 million shares changing hands two hours into the day. That's more than 15 times the average for a full session.

Pozen, Chapel Hill, N.C., is developing Trexima with the U.K.'s

GlaxoSmithKline

(GSK) - Get Report

. The drug's developers plan to ask for a meeting with the FDA as quickly as possible to discuss the approvable letter and determine the next steps they should take.

Trexima combines the active ingredient in another migraine drug, Glaxo's Imitrex, and the painkiller naproxen sodium. Naproxen sodium is found in several brand-name prescription and over-the-counter medications, including Anaprox and Aleve.