Despite reservations expressed by FDA medical reviewers over the effectiveness of the Xarelto bloodthinner, an FDA advisory panel this afternoon voted 9-to-2 to recommend that the agency approve the Bayer and Johnson & Johnson drug for the marketplace.

The endorsement came after FDA staffers wrote in briefing documents that there is a “lack of substantial evidence” that Xarelto will “have its desired effect” when used to prevent stroke in people with atrial fibrillation. The FDA reviewers had reviewed a trial comparing the drug with warfarin and concluded warfarin was not given in the proper dosing. They also worried that when patients stopped taking Xarelto and resumed warfarin, there was a higher risk of stroke.

And so the FDA medical reviewers suggested the FDA issue a complete response letter, which generally means approval is denied until additional data is provided. The FDA is expected to make a decision by November 5 and could issue such a letter, although the agency is not bound by recommendations from its panels (back story).

In any event, the vote is a boost for the drugmakers. Although Xarelto is presumed to have an advantage because the pill can be taken once a day, its clinical trial results were eclipsed by data for Eliquis, a forthcoming bloodthinner from Pfizer and Bristol-Myers Squibb, both of which are heavy hitters in marketing cardiovascular meds. Meanwhile, Boehringer-Ingelheim has already begun an aggressive marketing campaign for its own Pradaxa.

Nonetheless, Bayer and Johnson & Johnson may find that the approved labeling for Xarelto places them at a disadvantage, given concerns expressed by the FDA staffers. UPDATE: In an investor note, Wells Fargo Securities analyst Larry Biegelsen suggests there’s a 50/50 chance the FDA will require addditional clinical data on transitioning Xarelto patients to warfarin before approving Xarelto for stroke prevention in atrial fibrillation, which would delay approval to late 2012 from late 2011.

“This would allow Bristol and Pfizer to launch Eliquis ahead of Xarelto as we expect Eliquis to be approved in the US for SPAF in mid-2012. Although we expect Xarelto to receive a claim that it is an effective alternative to warfarin, we think the Xarelto label will probably be sub-optimal,” he writes. “At this point, we think Xarelto’s image among clinicians is tarnished due to the highly negative and well publicized FDA briefing documents which were issued this past Tuesday.”

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