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If numbers tell a story, then Bayer (OTCPK:BAYN) and Johnson & Johnson (NYSE:JNJ) may have some explaining to do. The complete results of a widely anticipated study known as Rocket were just published and show their Xarelto blood thinner is not only just as safe as warfarin - the standard treatment - but 21 percent better at preventing stroke in patients with atrial fibrillation.

But there was more, and it was sobering. An editorial in the same issue of The New England Journal of Medicine maintains the use of multiple statistical analyses to assess the medication “muddied the waters” when it comes to fully understanding efficacy and effectiveness compared with warfarin.

The point raised by Gregory Zoppo of the University of Washington and Misha Eliasziw of the University of Calgary may prompt closer scrutiny of Xarelto than had previously been expected. The medication was approved last month for preventing blood clots in patients after hip or knee surgery, but approval for the all-important market for stroke prevention is expected later this year.

This could become a sticking point for the drugmakers. The market for blood thinners used for stroke prevention is variously estimated to be worth anywhere from $15 billion to $20 billion. Meanwhile, Boehringer Ingelheim has already launched Pradaxa with an aggressive promotional campaign and Pfizer (NYSE:PFE) and Bristol-Myers Squibb (NYSE:BMY) recently released study results that has some analysts speculating their apixaban bold thinner will be the one to beat (see this).

The problem, according to the editorial, is that the Rocket study did not show any superiority for Xarelto on an “intent-to-treat” basis. Instead, the study authors used a different group of patients - the as-treated safety population - to identify a benefit. Hence, interpreting the data was more difficult. And the editorial noted warfarin was not administered as effectively as it might have been (here is the abstract for the Rocket study).

Time spent in therapeutic range for warfarin patients in the study was 55 percent, which is lower than other studies of atrial fibrillation patients. In short, Xarelto was found to be as effective as warfarin, but should not be considered superior. “I think we can say safely that (Xarelto) is not inferior to warfarin. I don’t think we can say it is superior,” Zoppo tells Reuters.

UPDATE: In an investor note this morning, ISI Group analyst Mark Schoenebaum writes that: “Overall, we think the journal publication CLEARLY (his emphasis) leaves the door open for apixaban. According to Bristol and Pfizer’s press release … apixaban achieved ’superiority’ on efficacy and major/non-major clinically relevant bleeding to warfarin. Thus, if the data are clean – that is, the superiority findings were generated via an (intent-to-treat) analysis, we think apixaban will emerge as the obvious winner in the novel oral blood thinner space.”

We have asked Bayer and J&J for comment and will update you accordingly.

UPDATE: A J&J spokesman writes us this: “The NEJM publication of the ROCKET AF study re-emphasizes that (Xarelto) delivered a significantly lower rate of stroke and non-CNS systemic embolism versus dose-adjusted warfarin in the on-treatment population, and comparable results to dose-adjusted warfarin in the intent-to-treat population.”

Disclosure: None

Source: Bayer and J&J 'Muddied The Waters' With Xarelto