Alexion Pharmaceuticals ( ALXN) shares were taking a pounding after the company's heart drug failed to meet the main target of a study in which it was aiming to prevent heart attacks in a moderate- to high-risk group of patients.

Lately, the shares were plunging $8.19, or 27.7%, to $21.37. Volume was extremely heavy, with 20 times more shares trading than the daily average for the last three months. By midsession, more than 10 million shares had changed hands, compared with about 458,000 on a normal day.

The trial of Alexion's pexelizumab was designed to determine whether the drug could reduce the rate of heart attacks and death 30 days after coronary artery bypass surgery, with or without another valve surgery, compared with a placebo. The drug did reduce both outcomes, but fell short of the threshold for statistical significance.

The trial, which was also conducted to study the drug's safety, involved 4,250 patients at 250 sites worldwide.

"We are clearly disappointed that pexelizumab did not meet its primary endpoint in the study ," said Leonard Bell, chief executive of Alexion. "We look forward to completing an analysis of the data and obtaining a more in-depth understanding of these results." Alexion expects to present the trial results at an upcoming scientific meeting.

The Cheshire, Conn., company says it's looking at the implications of the study on another phase III trial in which the drug is used in patients who've had an angioplasty procedure. During an angioplasty, a small balloon is inflated in a blocked artery to improve blood flow.

Alexion works on a number of therapeutic products aimed at treating patients with a wide array of severe diseases. The company says it remains on track to complete a late-stage trial on its leading drug candidate, eculizumab, for a rare blood disorder called paroxysmal nocturnal hemoglobinuria. Results from that trial are expected in the first quarter of 2006.