By Ben Hirschler

LONDON (Reuters) - Shares in Irish drugmaker Elan (ELN.I: Quote, Profile, Research) lost a third of their value on Wednesday as detailed clinical trial results for its new Alzheimer's drug bapineuzumab raised safety issues and disappointed bullish investors.

Stock in Wyeth, the group's bigger U.S. partner, fell 14 percent in early U.S. trade.

Bapineuzumab is one of the most keenly watched drugs under development in the pharmaceuticals industry, reflecting the huge need for effective dementia treatments.

But researchers told the International Conference on Alzheimer's Disease in Chicago that, while it helped some patients with a certain genetic profile, the drug raised the risk of a potentially serious side effect in the brain.

There were also huge variations in its effectiveness among patients and no clear signs of a better response according to the dose given.

"We believe that today's results leave unanswered questions about the efficacy profile of bapineuzumab," JP Morgan analyst Chris Schott said in a research note.

Elan shares were down 32 percent at 13.96 euros by 9:32 a.m. EDT. Wyeth lost 14 percent to $38.85, as Citi analysts cut the stock to "sell" from "hold" and other brokerages slashed price targets for the stock.

If proven to work, Elan and Wyeth's antibody medicine, also known as AAB-001, could be the first to modify the course of Alzheimer's disease, rather than just offering symptom relief.