GlaxoSmithKline and partner Pozen’s bid to get approval for their new migraine treatment Trexima has hit another stumbling block after US regulators once again asked for more information on the drug.

The US Food and Drug Administration has issued a second approvable letter for Trexima (sumatriptan/naproxen), which follows the first missive sent by the agency in December last year. The firms responded to that letter in January by submitting additional safety and preclinical data from trials, but that has still not quite done the trick.

The FDA has requested that Pozen address its concern about the potential implications from one preclinical in vitro chromosomal aberration study (one of four standard genotoxicity assays) in which genotoxicity was seen for the combination of the old painkiller naproxen and sumatriptan, sold by GSK as Imitrex, but not with either component alone. The firms added that none of the other three standard studies demonstrated any genotoxicity, further noting that no additional information regarding the cardiovascular safety of Trexima (which had been the FDA’s earlier concern) was requested. The companies have agreed, however, to conduct a study after approval to evaluate the effects on blood pressure during chronic, intermittent treatment.

GSK and Pozen said they intend to request a meeting with the FDA “as quickly as possible to discuss the necessary steps to address the agency’s concerns” but only getting an approvable letter could delay the launch of the drug until mid-2008 and reduces the time that patients can be switched to Trexima before Imitrex faces generic competition in the USA near the end of 2008. Speaking on a conference call, Pozen’s chief executive John Plachetka said the delay may be no more than two or three months, but it all depends on the type of new information the FDA requires about the genotoxic finding.

Pozen does not think that there is a major problem but the uncertainty has alarmed investors and the company’s shares fell over 43% to close at $9.93. Analysts have been cutting their sales forecasts for Trexima, put originally at around $600 million per year by 2012, and some have suggested that GSK may even pull out of the programme if the delay becomes a large one.