This is the second approvable letter that Pozen and marketing partner GlaxoSmithKline
The FDA is no longer worried about the potential cardiovascular side effects from the combination of the drugs. Now it's worried about a preclinical test for cancer that Pozen performed. The test involved putting a high level of the drug on cells grown in a laboratory. At a drug concentration that is 100 times higher than would be used in humans, the combination of drugs killed the cells.
The results may be an artifact, or an unusual result, of the test; in fact, the FDA acknowledged that in its approvable letter. If Pozen and GlaxoSmithKline can explain the reason for the weird result, they could receive an approval fairly quickly.
More likely, Pozen will need to run a small clinical trial to show that the effect isn't seen in humans. The trial might be as small as a single dose given to subjects, followed by analyzing blood cells for chromosomal aberrations. That would take a few months at most, and then the companies could submit the data.
Given that the drugs will be given at a much lower dose, I expect there wouldn't be a safety problem with the new trial. Drugs can have weird effects at high concentrations. Vitamin C, for instance, is fairly toxic at high doses, but certainly has benefits at normal concentrations.
On average, safety issues result in less of a delay than when the FDA has issues with a drug's effectiveness, which doesn't seem to be a problem here. Sure, there's times when the FDA requires lifetime carcinogenicity studies, but in this case, this issue appears to be fairly minimal, and Pozen should be able to wrap it up relatively quickly.
As a fellow Fool pointed out, Pozen's biggest problems may begin after the drug is approved. A cheap generic for naproxen is already available, and one for sumatriptan will be available soon. Pozen would need to convince patients to take its overpriced combination pill when doctors could just prescribe the two drugs separately.