The companies are under a time constraint: Trexima is a combination of the generic painkiller naproxen and Glaxo's migraine drug Imitrex, which loses patent protection in 2009. After both of those drugs have generic equivalents available, it may be hard for Pozen to get doctors to prescribe Trexima when prescribing two generics would serve the same purpose.
Essentially Pozen is taking a calculated risk, with the understanding that Trexima is worth a heck of a lot more if it gets an approval in six months or less, than if it runs a six-month clinical trial and gets an approval in well over a year. It's certainly possible that the FDA might be satisfied with the non-trial safety data that Pozen plans to submit in its response -- after all, it was pre-clinical data that triggered the second approvable letter.
Since I haven't sat in on the meetings at the FDA, it's difficult to know if this is what's really going on, or if the FDA has assured Pozen that the drug will be approved after the additional safety data is turned in. The latter is how the companies presented it in their press release, but Encysive Pharmaceuticals
Now, I do I think it's a good idea for the companies to take the risk, but that doesn't mean Pozen is a good investment. I'm no more convinced that the drug will be approved without a clinical trial than I was before the companies announced their intention to submit their response. In addition, taking the gamble now will cost you about 8% more than it would have on Thursday.