Shares in US drugmaker Alexion fell through the floor yesterday after the company announced disappointing results from a Phase III trial of its cardiovascular drug pexelizumab.

The antibody-based treatment, designed to reduce the rate of heart attacks and death in patients undergoing coronary artery bypass graft surgery, did show some activity but was unable to demonstrate a statistically-significant improvement over placebo in the study, called PRIMO-CABG2.

Alexion and its development partner for the drug, Procter & Gamble Pharmaceuticals, aid the results were a disappointment and that they would now assess what impact the results might have on a second Phase III trial – APEX-AMI - that is testing pexelizumab in patients undergoing angioplasty.

Alexion’s shares fell by more than a quarter to close at $21.53 yesterday, reflecting the importance of pexelizumab to the company. The US company had estimated before the publication of the CABG trial data that pexelizumab could bring in peak sales of around $500 million dollars.

Alexion’s other main pipeline project is eculizumab, an antibody for the treatment of a rare blood disorder known as paroxysmal nocturnal haemoglobinuria, which is also in a Phase III trial, called TRIUMPH. This is due to generate results during the first quarter 2006.

Analysts said eculizumab had a better chance of successfully reaching the market than pexelizumab, although its sales potential is lower at around $150 million at peak.