Shares in Ireland’s major biopharmaceutical company, Elan Corp. PLC, suffered their worst fall in more than three years Wednesday after a drug being developed to fight Alzheimer’s Disease reported disappointing results.

Elan shares plummeted over 27 percent to euro14.90 ($23.25) in morning trade on the Irish Stock Exchange, forcing the wider index into negative territory.

The drop followed Tuesday night’s unveiling of full phase-two results of bapineuzumab. Elan and U.S. biopharmaceutical giant Wyeth have been developing the drug in hopes it could become the first to reverse the brain-ravaging effects of Alzheimer’s, which afflicts more than 25 million people worldwide.

Last month the companies said the drug showed signs of working, but the full trial results indicated that the drug was ineffective in treating people who have the ApoE4 gene most commonly found in Alzheimer’s victims. Among those who lacked the gene, the drug did appear to slow down, if not reverse, mental deterioration.

The detailed findings, presented at the International Conference on Alzheimer’s Disease in Chicago, also linked use of the drug to a wide range of potential side effects, including brain swelling, paranoia, anxiety, vomiting and high blood pressure.

Experts appeared divided on the results, but investors dumped Elan and Wyeth shares because the results were much less upbeat than the companies’ advance characterizations. Elan and Wyeth are already pursuing a final-phase trial involving 4,000 Alzheimer’s victims.

Shares in Wyeth fell more than 7 percent Wednesday on Germany’s DAX index in Frankfurt.

Wyeth is much less dependent on the performance of bapineuzumab than its smaller Irish partner, which is banking on delivering an Alzheimer’s breakthrough for its future growth.

Elan currently is pursuing seven trials of potential drugs for Alzheimer’s and two for multiple sclerosis, an incurable disease that attacks the nervous system.

The Dublin-based company nearly went bankrupt in 2002 following the failure of previous Alzheimer’s research and the discovery of loss-hiding accounting practices, and has battled red ink for the past six years.

But the company said last week it is within a year of returning to profit thanks to the success of Tysabri, an MS drug developed in alliance with U.S. drugmaker Biogen Idec Inc. and approved for sale in the U.S. and European markets in mid-2006.

The last time Elan’s shares fell so sharply was in March 2005, after Tysabri was linked to three cases of a rare, deadly brain disease called PML and was withdrawn suddenly from U.S. sale. The drug returned to the market after extensive screening of Tysabri users found no more PML cases.