The battle to prevent blood clots is heating up this week as several drugmakers begin releasing study results for their experimental drugs. At stake, of course, is a huge market for stroke prevention that is worth billions of dollars, since the new meds are designed to supplant warfarin and aspirin, which are troublesome or simply less effective.

Toward that end, Bristol-Myers Squibb and Pfizer issued results showing patients taking their apixaban blood thinner were 54 percent less likely to have a stroke or damaging clot than those taking aspirin, and did not show significant signs of bleeding. Their Averroes study, which was presented at the European Society of Cardiology conference in Stockholm, involved about 5,600 people suffering atrial fibrillation and who are unable to use warfarin, which is the standard therapy but can cause bleeding.

Trial results are also expected shortly from a collaboration between Bayer and Johnson & Johnson, which are testing Xarelto (we will provide updates, of course). Yet another entrant is Boehringer Ingelheim’s Pradaxa, which will be reviewed on Sept. 20 by an FDA advisory panel (see study results here).

The real test for apixaban, however, will be results from a study comparing the drug to warfarin, since this is, generally, recommended more widely than aspirin. It is worth noting, though, that warfarin typically reduces the rate of stroke 40 percent more than aspirin, a lower rate than what was shown by the drug from Pfizer and Bristol-Myers Squibb. Results from a study of 18,000 patients comparing apixaban with warfarin in atrial fibrillation are expected next year.

“To understand the importance of Averroes, it is critical to understand the design. Specifically, this was NOT a head-to-head trial comparing apixaban to warfarin (aka Coumadin),” writes Sanford Bernstein analyst Tim Anderson in a research note this morning. “Rather, it was evaluating apixaban versus aspirin in patients at risk of stroke who were – for different reasons – not candidates for warfarin. As such, it is not surprising that Averroes showed a benefit for apixaban, as the comparator group was an easy one, by design.”

But will the results allow Pfizer and Bristol to file for FDA approval months earlier than expected? Anderson suggests this is unlikely. Even though the drug may satisfy an unmet medical need, he posits the FDA may worry about off-label use, given that comparative data - apixaban versus warfarin - is still several months away from being released.

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