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Dudley Shaw places his hand over his wife’s hand, in this AP photo. Eveyln was diagnosed with Alzheimer’s disease in 1992. The two were married in 1936.

Treatment continues to be tested for Alzheimer’s disease. The most recent drug to be tested is being developed by Biogen Idec.

An experimental drug for Alzheimer’s disease sharply slowed the decline in mental function in a small clinical trial, researchers reported, reviving hopes for an approach to therapy that until now has experienced repeated failures.

The drug, being developed by Biogen Idec, could achieve sales of billions of dollars a year if the results from the small trial are replicated in larger trials that Biogen said it hoped to begin this year. Experts say that there are no really good drugs now to treat Alzheimer’s.

Biogen’s stock has risen about 50 percent since early December, when the company first announced that the drug had slowed cognitive decline in the trial, without saying by how much. Analysts and investors had been eagerly awaiting the detailed results, some of them flying to France to hear Biogen researchers present the findings at the International Conference on Alzheimer’s and Parkinson’s Diseases and Related Neurological Disorders in Nice.

The drug, called aducanumab, met and in some cases greatly exceeded Wall Street expectations in terms of how much the highest dose slowed cognitive decline. “Out of the ballpark efficacy, acceptable safety,” Ravi Mehrotra, an analyst at Credit Suisse, wrote.

Shares of Biogen rose $42.33, or 10 percent, to $475.98.

Alzheimer’s specialists cautioned that it is difficult to read much from a small early-stage, or Phase 1 trial, designed to look at safety, not the effect on cognition. Also, other Alzheimer’s drugs that had looked promising in early studies ended up not working in larger trials.

“It’s certainly encouraging,” said Dr. Samuel Gandy, of the Center for Cognitive Health at Mount Sinai Hospital in New York, who was not involved in the study. He said the effect of the highest dose was “pretty impressive.”

Aducanumab, until now has been called BIIB037, is designed to get rid of amyloid plaque in the brain, which is widely believed to be a cause of the dementia in Alzheimer’s disease. However, other drugs designed to prevent or eliminate plaque have failed in large clinical trials, raising questions about what role the plaque really plays.

In the reported results — for 166 patients, randomly assigned to get one of several doses of the drug or a placebo — the drug slowed cognitive decline and substantially reduced plaque in the brain. Additionally higher doses were better than lower doses, a sign that the effects seen were from the drug.