updated 3/3/2010 2:50:18 PM ET 2010-03-03T19:50:18

A promising experimental Alzheimer's disease drug Pfizer Inc. and a partner are developing worked no better than a placebo in a late-stage study, an unexpected disappointment after the potential blockbuster stopped symptoms from worsening for a year in a prior test.

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Pfizer and partner Medivation Inc. said Wednesday that the drug, Dimebon, failed to meet its primary and secondary goals — improving thinking ability and overall daily function over six months in patients with mild to moderate Alzheimer's disease.

Still, the results don't necessarily spell the end for Dimebon, one of New York-based Pfizer's key prospects. Medivation and Pfizer, the world's biggest drugmaker by revenue, are continuing three ongoing studies that could prove Dimebon helps patients in combination with other Alzheimer's drugs or when used for a longer period.

Setback — not a 'death knell'
Given strong results of the prior, midstage study, some doctors thought — or at least hoped — Dimebon might be able to stop or reverse the mind-robbing disease, which would be a first.

"It's a setback, because it was the drug nearest to approval," said Dr. Ronald Petersen, chairman of the Alzheimer's Association's medical and scientific advisory council. He said it could have been on the market in two years, and anything else will take much longer.

The news sent shares of Medivation and Pfizer down.

Still, Petersen said the result is "not a death knell," as the other Dimebon studies continue.

Dr. Lynn Seely, chief medical officer of San Francisco-based Medivation, said, "The battle did not go well, but the war isn't over."

The four existing drugs, led by Pfizer's heavily advertised Aricept, can only temporarily reduce symptoms: memory problems, confusion, aggression and a general decline in ability to function. The newest one, Namenda, was approved in 2003.

"It is a short-lived, finite benefit, a year, two years, somewhere in that ballpark," that these drugs reduce symptoms, Petersen said.

Already, about 5.3 million Americans have Alzheimer's disease, and it strikes nearly a half million new patients a year, mainly as people hit their 70s or 80s. In 2050, up to 16 million Americans may have the disease.

"The baby boomers are just getting there now, so that's why its so crucial that we find something now," said Petersen, who heads Alzheimer's research at the Mayo Clinic.

Dimebon was expected to at least lessen symptoms more than the existing drugs, though its full potential was unclear because its mechanism of action is not entirely understood.

It works differently than the four Alzheimer's drugs already on the market, which act on particular chemicals in the brain to improve nerve cell function. So Dimebon could have been used along with any one of those to ease symptoms even more.

Either way, Dimebon was seen as a probable blockbuster, given the desirability of any drug that can delay worsening of symptoms — and the need for expensive nursing home care.

Yet in the final-stage study, which included 598 patients with an average age of 74, symptoms didn't improve in either patients getting Dimebon or those getting dummy pills.

"We believed that this drug would improve the symptoms of Alzheimer's disease, as well as stabilize the symptoms over time," said Seely. "We are very surprised by these results and (are) analyzing the data, trying to understand what went wrong."

Petersen said there are 90 to 100 other drugs in various stages of testing at drug companies and universities around the world, but this failure comes at a bad time, given the disease's increasing prevalence and the recession-related decline in government and foundation funding for Alzheimer's research.

Analyst Steve Brozak of WBB Securities said failures of drugs in final patient testing aren't that rare, but it is "unusual in something that they have such high hopes for."

"Nobody was going out there and saying, 'This would be the be-all and end-all,'" he added.

Seely noted experimental drugs sometimes fail in one study but produce good results in others and ultimately are approved, as happened with Namenda.

The two companies are testing Dimebon, known chemically as latrepirdine, in three additional studies involving patients with moderate to severe Alzheimer's, and in one involving patients with Huntington's disease. That's a fatal genetic disorder that causes muscle twitches and eventual deterioration in mental abilities.

The companies entered a partnership in September 2008, and if Dimebon is approved, Medivation could get hundreds of millions in milestone payments, as well as royalties on sales of the drug.

Copyright 2010 The Associated Press. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.

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