A Marin County philanthropist who belongs to a pioneer Sonoma County family is donating $3.5 million to help get a prospective cure for Alzheimer's disease through the pharmaceutical industry's "valley of death."

Douglas Rosenberg, a retired real estate developer whose ancestors crossed the Sierra in the mid-1800s, said he hopes to raise an additional $6.5 million or more to back the work of neurobiologist Dale Bredesen at the Buck Institute for Research on Aging in Novato.

"I am picking a horse and going with it," said Rosenberg, a Kentfield resident who calls himself a venture philanthropist.

His foundation's $10 million investment would get a chemical compound to be identified by Bredesen's laboratory into clinical trials, using human subjects, in the next two to three years.

"Valley of death" is the term for the funding gap between scientific discovery and drug development, so large that many potential medical breakthroughs never make it to market, Bredesen said.

Pharmaceutical giant Pfizer Inc. paid $225 million in 2008 for marketing rights to an antihistamine called dimebon hailed as a possible Alzheimer's treatment. Potential sales were estimated at $1 billion or more a year, but last year Pfizer announced that dimebon had failed a clinical trial.

"We can't afford to wait; I can't afford to wait," said Rosenberg, 53, whose father, stepfather and stepmother died of Alzheimer's in the past three years.

Alzheimer's, a memory-robbing disease, now afflicts 5.3 million Americans, and the number is expected to swell to 13 million by 2050 as the baby boomer generation ages.

There were nearly 9,000 Alzheimer's patients in Sonoma County in 2008, and there are an average of 150 deaths a year in the county caused by the disease.

Rosenberg said he "did my due diligence" on Alzheimer's research and decided to back Bredesen, who two years ago abandoned the conventional concept that ties dementia to formation of amyloid plaques in the brain.

Instead, Bredesen believes that Alzheimer's upsets the balance between breaking and sustaining connections between brain cells, which equate with forgetting and remembering.

His lab has tested thousands of compounds on brain cells and lab mice, and found two — which he will not identify — that appear to restore the balance.

But it means nothing, Bredesen said in an interview last month, unless the treatment works in humans.

An Alzheimer's cure would reward investors handsomely, Rosenberg said, describing venture philanthropy as "the ultimate high-risk roll of the dice."

Any financial returns to his foundation would be spent "to do more good for the world," Rosenberg said.

His ancestors, Jewish immigrants from Poland, came over the Sierra in the late 1840s — the first group following the ill-fated Donner Party — settled first in Grass Valley and soon moved to Sonoma County. Wolf Rosenberg, joined later by his nephew, Max Rosenberg, opened the old Rosenberg's department store in Healdsburg in the 1880s, and Max opened his Red Front Store in Santa Rosa in 1896.

That store became Rosenberg's, a fixture for decades on Fourth Street that now houses a book store. Max Rosenberg became a community leader, and served as cantor and acting rabbi of Santa Rosa's first Jewish congregation.

Claude Rosenberg, Douglas' father, made a fortune as a San Francisco money manager and devoted his retirement to urging wealthy people to give more to charity. He died in 2008.