Pfizer Inc. and Johnson & Johnson said they are ending development of an intravenous formulation of a drug to treat Alzheimer's disease after the treatment failed in two late-stage clinical trials.

The companies said bapineuzumab intravenous did not work better than placebo in two late-stage trials in patients who had mild to moderate Alzheimer's disease.

The drug is designed to prevent the buildup of plaque in the brain.

J&J said Monday it is not discontinuing development of the compound and noted it has ongoing studies including a mid-stage neuroimaging study with bapineuzumab delivered subcutaneously. Johnson & Johnson made a big bet on bapineuzumab in 2009, agreeing to invest up to $1.5 billion US initially.

The two companies said July 23 that the drug had failed in a different trial.

Current treatments for Alzheimer's can only temporarily ease symptoms of the disease, which include increasing memory loss, confusion, wandering and aggression.

In the latest trial, bapineuzumab was tested on about 1,300 patients who lacked a gene that is associated with a greater risk of Alzheimer's.

Last month the companies said the drug also didn't work on patients who do have that gene.

The announcement that the drug failed was predictable, said Dr. Barry Greenberg, director of neuroscience drug discovery and development at Toronto's University Health Network.

Enrolling people with mild-to-moderate Alzheimer's disease symptoms for "anti-amyloid therapies" is generally considered to be too late, Greenberg said.

Researchers believe that as the amyloid accumulates, brain cells begin to die, leading to disorientation and progressive memory loss.

"When patients come into the clinics already with an identifiable dementia, the amyloid dependent phase of the disease is by and large over," Greenberg said.

"So what you're doing is clearing out a vestige of what caused the initiating consequences of the disease, but the disease itself has already moved on to a subsequent phase and will require other therapies in order to halt the progression."

Pfizer and Johnson & Johnson were running two other late-stage trials as part of a very large testing program for the drug.

Worldwide, about 35 million people already have dementia, of which Alzheimer's is the most common type.

Finding a drug that could at least slow the disease has become a sort of Holy Grail in the pharmaceutical industry. A successful medicine would be guaranteed to generate billions in annual sales, given the world's aging population.

With files from CBC's Marijka Hurko