U.S. regulators on Thursday formally approved an Alzheimer's drug developed by Japanese pharmaceutical company Eisai Co. and U.S. firm Biogen Inc. that has become the world's first medicine shown to slow the onset of the degenerative brain disease.

The Food and Drug Administration granted its approval for the new drug, known chemically as lecanemab and branded Leqembi, which can reduce the amount of a sticky brain protein called amyloid beta, seen by experts as a hallmark of the disease.

Treatment with the drug, which Eisai expects to cost $26,500 annually before insurance coverage, is intended to be started for patients with mild cognitive impairment or who have entered the mild dementia stage.

Photo shows image of Alzheimer's drug, lecanemab, that is to go on sale in the United States. (Photo courtesy of Eisai Co.)(Kyodo)

"Today's action is the first verification that a drug targeting the underlying disease process of Alzheimer's disease has shown clinical benefit in this devastating disease," Teresa Buracchio, acting director of the Office of Neuroscience in the FDA's Center for Drug Evaluation and Research, said in a statement.

"This confirmatory study verified that it is a safe and effective treatment for patients with Alzheimer's disease," Buracchio said.

The antibody, administered intravenously every two weeks, is not a cure for the disease. An 18-month clinical trial involving about 1,800 patients showed that Alzheimer's symptoms advanced 27 percent more slowly in those administered the medication than those who received a placebo, according to Eisai.

The Japanese drugmaker also said 13 to 17 percent of those who received the drug experienced side effects, such as brain edema and bleeding. Still, it said most of the side effects were mild to moderate and temporary.

Speaking at a press conference Friday in Tokyo, Eisai CEO Haruo Naito vowed to make the drug widely available, saying, "We will do our utmost to provide the product to as many patients as possible and as quickly as possible."

Haruo Naito, CEO of Japanese pharmaceutical company Eisai Co., speaks during a press conference in Tokyo on July 7, 2023, after the U.S. Food and Drug Administration granted full approval for Eisai and Biogen Inc.'s anti-Alzheimer's drug lecanemab. (Kyodo) ==Kyodo

Japanese health minister Katsunobu Kato told a separate press conference Japan will examine results from the U.S. review and complete its own review of the drug "appropriately and swiftly."

Eisai estimates that the drug's peak annual sales will reach about 1 trillion yen ($6.9 billion) globally in the business year from April 2030 as it expects approval also in other major markets, including Europe and China.

Among other drugmakers, U.S. pharmaceutical group Eli Lilly and Co.'s donanemab, currently under development, is seen as a major rival for lecanemab.

The FDA granted lecanemab fast-track conditional approval in early January based on observed reductions of the toxic protein that builds up in the human brain. For full approval, the drugmakers were required to present additional data for the agency to review the medicine's safety and clinical benefits.

With the FDA now giving the drug formal approval, patients under the federal government's Medicare insurance program could have greater access to the treatment.

The World Health Organization states that over 55 million people worldwide are affected by dementia-related illnesses, currently the seventh leading cause of death.

In the United States, an estimated 6.7 million people aged 65 and older are living with the disease in 2023. Seventy-three percent are 75 or older, according to the Alzheimer's Association, a U.S.-based nonprofit.

Eisai and Biogen have applied with health authorities for approval of the drug in a number of countries. They hope to gain approval in Japan by the end of the year.

A major group supporting dementia patients in Japan welcomed the news and said they hope the drug will be available at an affordable price.

"It has cleared the way for treating Alzheimer's and given us hope," the Kyoto-based group with more than 10,000 members said in a statement.