For the second time, the FDA has issued an approvable letter for the Trexima migraine med, which combines Glaxo’s Imitrex with naproxen sodium, an older anti-inflammatory drug. The agency wants more info on results from a study that found the treatment can be toxic in lab tests, according to Glaxo.

The news sent Pozen stock plummeting 44 percent. And of course, this isn’t good news for Glaxo, either. The drugmaker needs new meds to fill its pipeline. Imitrex and a related drug generated $1.4 billion in sales last year, but faces generic competition in 2009. Trexima has been heralded as a savior.

In January, the companies responded to the FDA’s first approvable letter by submitting additional safety data from clinical trials, data from Glaxo’s database, and additional in vitro preclinical data. In the second approvable letter, no additional info about cardiovascular safety was requested. But the companies agreed to conduct a prospective study after approval to evaluate the effects on blood pressure during chronic, intermittent treatment, according to a Glaxo statement. “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.”