A bloodthinner called Xarelto worked as well as a standard injection to prevent blood clots from forming in critically ill hospital patients, but also caused a surprisingly high rate of bleeding in a trial of patients with acute illnesses. The findings amount to a setback for Bayer and Johnson & Johnson, which are hoping the med will grab a big chunk of the multi-billion-dollar bloodthinner.

Although atrial fibrillation patients is the largest indication for which the drugmakers hope to receive FDA approval for Xarelto, they have been developing the pill for other potential blockbuster uses - including the stroke prevention among patients with atrial fibrillation. In fact, usage among acutely ill patients might have represented a market as large as $2.8 billion.

The study of 8,101 patients compared Xarelto bleeding risk and effectiveness with Lovenox, which is the standard injectable given patients hospitalized for acute illness, including heart failure, infectious disease and breathing difficulty. After 10 days of treatment, both were considered equally effective in preventing dangerous blood clots in the extremities and in the lungs - 2.7 percent of patients getting either Xarelto or Lovenox for 10 days developed a blood clot or died as a complication of one.

However, there was significantly higher major bleeding with Xarelto at day 10 (0.6 percent versus 0.3 percent) and at day 35 (1.1 percent versus 0.4 percent). The net clinical benefit at day 35 was inferior for Xarelto compared to Lovenox at day 35 (9.4 percent versus. 7.8 percent). Although disappointing for the drugmakers, one analyst was not surprised (read the Bayer statement).

“Although the patient population in Magellan represents a large opportunity, we have had low expectations for Xarelto in this indication,” writes Wells Fargo Securities analyst Larry Biegelsen in an investor note. “Xarelto US approval could happen around mid-year. We expect approval and launch of Xarelto in the US for venous thromboembolism prevention in orthopedic surgery in July. The more important stroke prevention in atrial fibrillation approval will likely occur in the fall.” The upshot? He forecasts US sales of $400 million in 2012, and $1.5 billion by 2015.

UPDATE: And Credit Suisse analyst Catherine Arnold writes this: “While the results are not too surprising from a medical perspective, they are somewhat unexpected given the relatively bullish stance we felt Bayer’s management had taken on the data ahead of today’s presentation. It is possible that there may be additional information present in the subgroup analyses that will support the company’s optimism but, based on today’s data, we believe it will be difficult for the product to obtain regulatory approval in this indication.”