Pfizer, Johnson & Johnson and Elan ended most plans to develop an Alzheimer's drug after a second trial failure, a blow to the companies' efforts to market the first product to slow progress of the disease.

Bapineuzumab, designed to attack the brain plaques that serve as a hallmark of Alzheimer's, failed to improve symptoms of dementia in the second of four final-stage trials of the drug, Pfizer and J&J said in statements.

Drugmakers have been trying for more than a decade to find therapies to slow Alzheimer's.

While companies have focused on developing drugs to hinder the amyloid deposits, scientists aren't certain whether the clumps cause or are a minor contributor to the disease or merely a consequence.

"Alzheimer's is a tough nut for any drug company to crack," said Erik Gordon, a professor at the University of Michigan's Ross School of Business, in an e-mail.

"We don't know for sure what causes it or even what it really is. There will be more failures before we see a success."

In the Pfizer-J&J trial, the compound called Bapineuzumab didn't help patients who don't have a gene, called ApoE4, that boosts the risk of getting the disease.

New York-based Pfizer announced on July 23 that a final-stage study of the product failed in patients with the ApoE4 gene.

The failure is "a black eye for Pfizer's new 'strong pipeline' happy face," he said.