WASHINGTON — Government health officials on Monday approved the first new drug for Alzheimer’s disease in nearly 20 years, disregarding warnings from independent advisers that the much-debated treatment hasn’t been shown to help slow the brain-destroying disease.

The Food and Drug Administration approved the drug from Biogen based on study results showing it seemed “reasonably likely” to benefit Alzheimer’s patients. It’s the only therapy that U.S. regulators have said can likely treat the underlying disease, rather than manage symptoms like anxiety and insomnia.

The decision, which could impact millions of Americans and their families, is certain to spark disagreements among physicians, medical researchers and patient groups. It also has far-reaching implications for the standards used to evaluate experimental therapies, including those that show only incremental benefits.

The new drug, which Biogen developed with Japan’s Eisai Co., did not reverse mental decline, only slowing it in one study. The medication, aducanumab, will be marketed as Aduhelm and is to be given as an infusion every four weeks.

Dr. Caleb Alexander, an FDA adviser who recommended against the drug’s approval, said he was “surprised and disappointed” by the decision.

“The FDA gets the respect that it does because it has regulatory standards that are based on firm evidence. In this case, I think they gave the product a pass,” said Alexander, a medical researcher at Johns Hopkins University.

The FDA’s top drug regulator acknowledged that “residual uncertainties” surround the drug, but said Aduhelm’s ability to reduce harmful clumps of plaque in the brain is expected to help slow dementia.

“The data supports patients and caregivers having the choice to use this drug,” Dr. Patrizia Cavazzoni told reporters. She said the FDA carefully weighed the input of people living with the “devastating, debilitating and deadly disease.”

Under terms of the so-called accelerated approval, the FDA is requiring Biogen to conduct a follow-up study to confirm benefits for patients. If the study fails to show effectiveness, the FDA could pull the drug from the market, though the agency rarely does so.

Biogen said the drug would cost approximately $56,000 for a typical year’s worth of treatment, and said the price would not be raised for four years. Most patients won’t pay anywhere near that thanks to insurance coverage and other discounts. The company said it aims to complete the FDA-mandated follow-up trial by 2030.

Biogen shares jumped 38% in trading Monday on the news, with analysts forecasting billions in future sales. The Cambridge, Massachusetts-based company plans to begin shipping millions of doses within two weeks.

The non-profit Institute for Clinical and Economic Review said that “any price is too high” if the drug’s benefit isn’t confirmed in follow-up studies.

Some 6 million people in the U.S. and many more worldwide have Alzheimer’s, which gradually attacks areas of the brain needed for memory, reasoning, communication and basic daily tasks. In the final stages of the disease, those afflicted lose the ability to swallow. The global burden of the disease, the most common cause of dementia, is only expected to grow as millions more baby boomers progress further into their 60s and 70s.

Aducanumab (pronounced “add-yoo-CAN-yoo-mab”) helps clear a protein called beta-amyloid from the brain. Other experimental drugs have done that but they made no difference in patients’ ability to think, care for themselves or live independently.

The pharmaceutical industry’s drug pipeline has been littered for years with failed Alzheimer’s treatments. The FDA’s greenlight Monday is likely to revive investments in therapies previously shelved by drugmakers.

The new medicine is manufactured from living cells and will be given via infusion at a doctor’s office or hospital.

Researchers don’t fully understand what causes Alzheimer’s but there’s broad agreement the brain plaque targeted by aducanumab is just one contributor. Evidence suggests family history, education and chronic conditions like diabetes and heart disease may all play a role.

“This is a sign of hope but not the final answer,” said Dr. Richard Hodes, director of the National Institute on Aging, which wasn’t involved in the Biogen studies but funds research into how Alzheimer’s forms.

Patients taking aducanumab saw their thinking skills decline 22% more slowly than patients taking a placebo.

But that meant a difference of just 0.39 on an 18-point score of cognitive and functional ability. And it’s unclear how such metrics translate into practical benefits, like greater independence or ability to recall important details.

The FDA’s review of the drug has become a flashpoint in longstanding debates over standards used to evaluate therapies for hard-to-treat conditions. On one side, groups representing Alzheimer’s patients and their families say any new therapy — even one of small benefit — warrants approval. But many experts warn that greenlighting the drug could set a dangerous precedent, opening the door to treatments of questionable benefit.

The approval came despite a scathing assessment in November by the FDA’s outside panel of neurological experts. The group voted “no” to a series of questions on whether reanalyzed data from a single study submitted by Biogen showed the drug was effective.

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