Exanta has proven to be as effective as warfarin in preventing strokes and/or clots, and there was no statistically significant difference in the number of major and minor bleeding events between the drugs. But the same trial that showed these strengths also showed that 6.3% of Exanta patients had an elevated liver enzyme, called ALT, compared to 0.8% of warfarin patients. An elevated level is defined as more than three times the normal ALT level. This test was reported in The Lancet in November, and the company said "nearly all enzyme changes occurred within the first six months of treatment and decreased with or without drug discontinuation."

These mixed results have investment banks worried. "We remain concerned about the approvability of Exanta," said Alexandra Hauber, of Bear Stearns' European equity research unit, in an Aug. 16 report. "We remain concerned about Exanta's liver profile and would not be surprised if the FDA requested further safety data prior to approval in chronic indications."

The FDA doesn't always follow the recommendations of its advisory panels, but it usually does. The agency is expected to make a ruling on Exanta in late October. If the FDA required additional safety tests for chronic conditions such as atrial fibrillation, Hauber fretted that the drug could be delayed until 2006.

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