So how will Lilly ring the register after its Zyprexa anti-psychotic loses patent protection in 2011? The drugmaker is counting on a blood thinner called Prasugrel, which is designed to prevent recurring heart attacks and strokes in people with cardiovascular disease, The Wall Street Journal reports (subscription required).

The results of a 14,000-patient study are due in November. “If prasugrel really is demonstrably better, it can get a major share of that business,” says Stephen Laveson, an analyst with Becker Capital Management, which owns Lilly shares. Wall Street analysts predict annual prasugrel sales could approach $4 billion. By comparison, Plavix generated $6 billion in sales last year for Bristol-Myers Squibb and Sanofi-Aventis.

Earlier studies have suggested Prasugrel is more potent than Plavix in blocking blood-platelet activity, but potency can cause also cause bleeding. The latest trial must also show that the risk of serious bleeding with Prasugrel isn’t significantly worse than that for Plavix, if Prasugrel is to gain regulatory approval and market acceptance, the Journal writes.

“One of the questions is, ‘What’s the trade-off?’ ” Chris Granger, director of the cardiac-care unit at Duke University Medical Center, tells the paper. “How potent can we get before we go too far and have caused intolerable bleeding?”

Of course, Bristol-Myers is watching closely. “We are taking Prasugrel seriously, but we believe it will be focusing on a much smaller set of patients,” Lamberto Andreotti, the drugmaker’s chief operating officer of world-wide pharmaceuticals, told analysts on a conference call last month.