Why does AstraZeneca's (NYSE: AZN) blood thinner Brilinta work better outside the U.S. than here at home? The Food and Drug Administration's advisory panel didn't seem to have a good explanation, but they didn't appear to care much, either. The panel of experts voted 7 to 1 recommending approval of the drug.

I'm a little surprised that the panel blew off the unexplainable data. The clinical trials clearly show that Brilinta works better than Bristol-Myers Squibb (NYSE: BMY) and sanofi-aventis' (NYSE: SNY) Plavix. But for whatever reason, perhaps because of higher levels of aspirin, or maybe a combination of several factors, the drug didn't work very well in the subset of patients treated in the U.S. Doctors can mimic how the drug was used in the clinical trials outside the U.S., but without a definitive reason, that might be tough.

A mixed panel vote would have been the perfect ammunition for the FDA to send AstraZeneca back to the drawing board, asking for a new clinical trial to clear up the issue. With this resounding endorsement, it'll be harder for the FDA to reject Brilinta, although it's certainly in its power to do so.

Oddly enough, if you read the FDA briefing documents, the reviewers at the agency seem fairly conflicted about whether to approve the drug. This isn't like Dendreon's (Nasdaq: DNDN) panel meeting a few years ago, where the FDA was down on the drug, the panel voted yes, and the FDA nixed it anyway. The panel's endorsement may be just what AstraZeneca needed to tilt the agency in favor of an approval when its PDUFA date comes up on Sept. 16.

An approval is just the first hill in a long upward battle toward blockbuster status for Brilinta. Eli Lilly's (NYSE: LLY) Effient also beat Plavix in a head-to-head trial, but it hasn't made much headway into Plavix's market share. Given the questions over whether Brilinta really works in the U.S., I have a hard time seeing sales amounting to much, even if an approval seem more likely than it did earlier in the week.

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