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

Biogen said it has priced the drug, to be sold as Aduhelm, at $56,000 per year. Its shares jumped nearly 52% to $434.52 when trading resumed following the approval. U.S. traded shares of partner Eisai Co were already up $33.50, or 45%, to $107.75

Aducanumab aims to remove sticky deposits of a protein called amyloid beta from the brains of patients in earlier stages of Alzheimer’s in order 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.”

“It think this is a big day,” Dr. 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 said on its website that clinical trials for the treatment showed a reduction in the plaques that is expected to lead to a slower decline in patients.

“Although the Aduhelm data are complicated with respect to its clinical benefits, FDA has determined that there is substantial evidence that Aduhelm reduces amyloid beta plaques in the brain and that the reduction in these plaques is reasonably likely to predict important benefits to patients,” the agency said in a statement.

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.

‘A WIN-WIN FOR ALL’

Current Alzheimer’s treatments including AbbVie’s Namenda and Pfizer’s Aricept are intended to ease Alzheimer’s symptoms, while the Biogen drug targets the underlying cause and aims to slow disease progression.

The FDA said Biogen will need to conduct a post-approval trial to verify Aduhelm’s clinical benefit, and that it could be pulled from the market if the drug does not work as intended.

“I think they made the right decision. It makes the drug available to patients while requiring the company to do more research to prove its benefit. I think it’s a win-win for all,” said Dr. Howard Fillit, chief science officer of the Alzheimer’s Drug Discovery Foundation.

Not all were celebrating the approval.

Consumer watchdog Public Citizen in a news release decried the decision, accusing the FDA of collaborating with Biogen to rush a “lucrative unproven treatment” to market, “despite the nearly unanimous conclusion of an independent panel of experts convened by the agency in November that there was not sufficient evidence that the drug was effective.”

Aducanumab was studied in patients with early disease who test positive for a component of amyloid brain plaques.

Some trial patients experienced potentially dangerous brain swelling. The FDA advised that patients who experience the side effect should be monitored but not necessarily taken off the drug.

The FDA decision did not define the specific Alzheimer’s patients for who the drug is appropriate – such as very early stage disease – raising the possibility that the treatable population could be larger than anticipated.

Biogen has estimated that around 1.5 million Americans would be eligible for treatment with aducanumab, which is given by monthly infusion, raising concerns about costs to the healthcare system.

The number of Americans living with Alzheimer’s is expected to rise to around 13 million by 2050 from more than 6 million currently, according to the Alzheimer’s Association.

“We think the drug has the potential to generate (at least)$10 billion in peak sales, and believe a successful launch of aducanumab has the potential to completely change the profile of the company,” said Guggenheim analyst Yatin Suneja.

The medicine is seen as critical to Biogen’s revenue growth prospects as competition has hurt sales of its mainstay multiple sclerosis drug, Tecfidera, as well as its other growth driver, spinal muscular atrophy treatment Spinraza.

Biogen and partner Eisai Co had scrapped development of the drug in March 2019, then reversed course in October, saying a more detailed data analysis showed some patients with very early forms of the disease benefited from taking higher doses over an extended period of time.

Wall Street analysts had forecast that an FDA approval of Biogen’s drug could reinvigorate a field that has been abandoned by many large pharmaceutical companies.

The amyloid theory has been at the center of Alzheimer’s research for many years with a long list of failures to show for it, and seemed on life support until Biogen revived its aducanumab program.

Shares of companies developing Alzheimer’s treatments also jumped, with AC Immune SA and Anavex Life Sciences up between 14% and 17%, while shares of Eli Lilly and Co and Axsome Therapeutics rose more than 5%.