Phase III data reported yesterday at the AHA Meeting in Chicago showed that new blood thinner Xarelto (rivaroxaban) stacks up well against warfarin in preventing strokes in patients with atrial fibrillation. Less certain is whether Johnson & Johnson/Bayer can claim their drug is superior to the standard anticoagulation therapy, something which would afford a marketing edge over Boehringer Ingelheim’s recently approved blood thinner, Pradaxa.

Researchers from the Duke Clinical Research Institute, sharing results from the Rocket AF study, said Xarelto was essentially just as good as warfarin, with similar rates of bleeding and adverse events. While patients were taking Xarelto, the drug was actually superior to the 60-year-old anticoagulant.

J&J, in its press release on the Rocket AF results, stressed the superiority point. Analysts, however, noted that this alone will not earn Xarelto the right to make such a claim in its label.

“While the company press release suggests that rivaroxaban achieved superiority over warfarin, this is only in the ‘per protocol’ analysis that includes patients who completed the whole study,” Credit Suisse analyst Catherine Arnold wrote in a note to investors. “In the more important ‘intent to treat’ analysis that includes all patients randomized into the trial (and that regulators generally focus on), the drug showed non-inferiority to warfarin.”

Pradaxa, on the other hand, has shown superiority over warfarin, Arnold noted, even though RE-LY, the trial on which Pradaxa’s approval was based, was a non-inferiority trial like Rocket AF. Nevertheless, should Xarelto get approved in the US, it will most likely be a viable choice for anticoagulation therapy. Xarelto is a once-a-day pill, compared to Pradaxa’s twice-daily dosing. Neither of the new drugs requires coagulation monitoring. This factor, along with limited potential for food interactions, makes both more convenient than warfarin.

Boehringer Ingelheim was first to the US market with a warfarin replacement. The drug received FDA approval October 20 to reduce stroke risk in patients with AFib. J&J/Bayer filed their NDA for rivaroxaban for the prevention of deep vein thrombosis and pulmonary embolism in patients undergoing hip or knee replacement surgery in 2008. In 2009, the FDA issued a complete response letter requiring no new trials but additional data from completed and ongoing studies.

Emerging warfarin alternatives also include Bristol-Myers Squibb/Pfizer’s apixaban, in phase III development, and Merck/Portola’s betrixaban, in phase II.