Experimental drugs that clear clumps of proteins from the brains of Alzheimer's patients haven't panned out yet.
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An experimental drug dramatically reduced the toxic plaques found in the brains of patients with Alzheimer's disease, a team reports in the journal Nature.

Results from a small number of patients who received a high dose of the drug, called aducanumab, hint that it may also be able to slow the loss of memory and thinking.

"If that hint of a clinical benefit is confirmed, it would be a game changer in the fight against Alzheimer's disease," says Dr. Eric Reiman, executive director of the Banner Alzheimer's Institute in Phoenix. Reiman wrote a commentary that accompanies the study in Nature.

But it will take much larger studies to show for sure whether aducanumab really does slow down Alzheimer's disease, Reiman says.

Officials of Biogen, which is developing the drug, were also cautious about interpreting the results of the study, which included 165 patients in the early stages of Alzheimer's disease.

"We think we have something important here," says Dr. Alfred Sandrock, chief medical officer for Biogen. "We hope we're right because if it's true it would benefit millions of patients. But we don't know we're right yet."

The results are encouraging for pharmaceutical companies that have spent billions of dollars on efforts to come up with the first drug to treat the underlying cause of Alzheimer's. Those efforts have failed to produce a single approved drug so far.

But last summer, Biogen began presenting its results with aducanumab at scientific meetings, including the Alzheimer's Association International Conference in July. These early reports suggested the drug had a remarkable ability to remove plaques in the brain.

"It was surprising, encouraging and thought-provoking to see such a striking reduction of existing plaques," Reiman says.

The publication in Nature on Wednesday provides details about those early reports.

Biogen has already begun two much larger studies of aducanumab. They will include a total of 2,700 patients, and results are still several years off.

But there are reasons to think aducanumab may succeed where other drugs have failed.

One is that the drug appears to ignore benign forms of amyloid protein while attacking the toxic forms thought to damage brain cells. Another is that aducanumab seems to enhance the ability of existing immune cells in the brain to devour toxins, including amyloid.

But there's a downside. The process of removing plaque sometimes causes fluid to build up in the brain. In rare cases, it can also cause bleeding. These side effects are known as amyloid-related imaging abnormalities, or ARIA.

"We were actually anticipating that we would see it," Sandrock says. And the researchers did. Twenty patients dropped out of the trial because of adverse effects.

If aducanumab works in larger studies, it could help settle a long-running debate about whether amyloid is really the root cause of Alzheimer's. This idea is known as the amyloid hypothesis.

And large studies showing that removing amyloid can preserve memory and thinking "would go a long way toward validating the amyloid hypothesis," Sandrock says.

Results like those could also lead to the first approved drug to treat the underlying cause of Alzheimer's rather than just the symptoms.