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The three treatments being tested are not even drugs in the traditional, chemical sense. They are antibodies _ proteins made by the immune system that promote clearance of amyloid, the stuff that forms the plaque.

It’s a strategy with a checkered history, and scientists aren’t even sure that amyloid causes Alzheimer’s or that removing it will do any good in people who already have symptoms. But there are some hopeful signs they may be on the right track.

“Everybody in the field is probably holding their breath that there is something positive to come out of these trials,” said Dr. Ronald Petersen, director of the Mayo Clinic’s Alzheimer’s Disease Research Center.

“It may not be a home run” in terms of improving memory and cognition, but if brain imaging or spinal fluid tests show the drugs are hitting their target, “they will be regarded as successes,” he said.

William Thies, scientific director of the Alzheimer’s Association, agreed.

Even if there is just a small effect, “that would be a huge finding because that would let you know you had a drug that worked,” he said. It then could be tried as a preventive medicine or given earlier in the course of the disease when it may have more impact.

The three drugs and their developers are:

• Bapineuzumab, by Pfizer Inc. and Johnson &Johnson’s Janssen Alzheimer Immunotherapy unit. • Solanezumab, by Eli Lilly & Co. • Gammagard, by Baxter International Inc.

All are given as periodic intravenous infusions; some companies are trying to reformulate them so they could be given as shots. If a major study shows that one of the drugs works, there will be a huge effort to make it more convenient and practical, Thies predicted.