US drugmaker Pozen and partner GlaxoSmithKline said on Friday that the US Food and Drug Administration had issued an ‘approvable’ letter for their migraine treatment Trexima.

Trexima combines GSK's Imitrex (sumatriptan), which uses Pozen's RT Technology allowing drugs to quickly dissolve in the stomach, with the nonsteroidal anti-inflammatory, naproxen sodium.

The agency has asked for more safety information on Trexima, and this may require additional clinical studies to be carried out, according to the companies. Both Pozen and GSK said they hoped to meet with the FDA soon to determine what additional information will need to be generated to support full approval.

Pozen shares plunged nearly 60% on the news to $5.84.