U.S. regulators on Monday approved a new drug for Alzheimer's disease developed by Biogen Inc. and Japan's Eisai Co., calling it the first therapy to work on the underlying cause of the illness.

The drug, named Aduhelm, is the first new treatment greenlit for Alzheimer's since 2003, the U.S. Food and Drug Administration said in a news release. The companies also filed an application for the drug in Japan in December.

File photo pictured on Feb. 10, 2015 shows Eizai Co.'s headquarters in Tokyo's Bunkyo Ward. (Kyodo) 

"We believe this first-in-class medicine will transform the treatment of people living with Alzheimer's disease and spark continuous innovation in the years to come," Biogen CEO Michel Vounatsos said in a joint press release.

Noting that the Japanese pharmaceutical company has been working on the creation of new treatments for Alzheimer's disease since the 1980s, Eisai CEO Haruo Naito said in the release, "This approval has the potential to bring hope to the future of global health, society and, most importantly, the patients and their families, and represents a great step toward the advancement of holistic ecosystem solutions for this devastating disease."

Naito added that the company is "very pleased to be able to open a new chapter in the history of Alzheimer's disease treatment."

According to the FDA, the drug's efficacy was evaluated in three separate studies and patients receiving the treatment had "significant reduction" of amyloid beta proteins in the brain, which are believed to play a key role in causing Alzheimer's.

Aduhelm is an amyloid beta-directed antibody. It will be given as an infusion once every four weeks with a yearly cost of $56,000 for the treatment, said Biogen, which is headquartered in Cambridge, Massachusetts.

The Associated Press reported that the regulatory approval came despite warnings from independent advisers that the much-debated treatment hasn't been shown to help slow the brain-destroying disease.

Under the terms of approval, the FDA is requiring Biogen to conduct a new clinical trial to verify the drug's benefit. If the trial fails to verify effectiveness, the FDA said it may initiate proceedings to withdraw its approval of the drug.

Alzheimer's is an irreversible, progressive brain disorder that slowly destroys memory and can seriously affect the ability to carry out daily activities.

More than 30 million people worldwide live with Alzheimer's disease, and the number is expected to rise in the years ahead to a level that could outpace the healthcare resources needed to manage it and cost billions of dollars, Biogen said, citing data from the World Health Organization.

The annual cost of care for Alzheimer's disease and other dementias in the United States is over $600 billion and lifetime care for someone with the illness is estimated to cost approximately $500,000 per patient, according to the company.