U.S. regulators on Monday approved Biogen Inc's aducanumab as the first treatment to attack a likely cause of Alzheimer's disease despite controversy over whether the clinical evidence proves the drug works, sending its shares soaring, Reuters reports.

Biogen said it has priced the drug, to be sold as Aduhelm, at US$56,000 per year. After jumping more than 50 percent, Biogen shares closed up by 38.3 percent at $395.85 - their highest closing level in over six years.

U.S. traded shares of Japanese partner Eisai Co added 56 percent to US$116.03.

Aducanumab works by removing sticky deposits of a protein called amyloid beta from the brains of patients in earlier stages of Alzheimer's to stave off its ravages, which include memory loss and the inability to care for one's self.

"This is good news for patients with Alzheimer's disease. We've not had a disease modifying therapy approved ever," said Dr. Ronald Petersen, an Alzheimer's disease expert at the Mayo Clinic.

However, he cautioned, "This is not a cure. It's hoped that this will slow the progression of the disease."

"I think this is a big day," Peterson said. "But we can't overpromise."

Alzheimer's is the sixth leading cause of death in the United States.

The Food and Drug Administration on its website said results from clinical trials of Aduhelm were mixed, but the drug was shown to reduce amyloid beta plaques in the brain, which is likely to benefit patients.

"We decided that the accelerated approval pathway fits well here," Dr. Patrizia Cavazzoni, director of the FDA's Center for Drug Evaluation and Research, told Reuters, explaining that this "allows for there being some residual uncertainty on the drug's clinical benefits while making the drug available to patients rather than having to wait."

Under that pathway, Biogen is required to conduct a confirmatory clinical trial, but has several years to do so, she said.

Biogen's drug had been hailed by patient advocates and some neurologists eager to have an effective option for patients with the lethal disease.

Other doctors said clinical trial results were inconsistent and more proof was needed. "I'm surprised and disappointed," said Dr. Caleb Alexander, a drug researcher at Johns Hopkins who was on the FDA's advisory committee that voted against approval of Biogen's drug.

"It's hard to find a scientist, even one with ties to the manufacturer, who believes the totality of the evidence is persuasive."

US approves Biogen Alzheimer's drug Aduhelm, shares pop