For the first time, a drug has been shown to slow the rate of cognitive and functional decline in those in the early stages of Alzheimer's disease. Biogen (BIIB  ) and Eisai Co Ltd (ESALY  ) teamed up to develop the drug, lecanemab, which slowed the progression of Alzheimer's by 27% in a large-scale study.

"It's not a huge effect, but it's a positive effect," said Ronald Petersen, director of the Mayo Clinic Alzheimer's Disease Research Center.

Eisai, the leader of the 50-50 partnership, hopes to receive U.S. Food and Drug Administration (FDA) approval in an accelerated process that could result in a decision by early next year. The pharmaceutical company wants to start selling the drug in the United States, Europe, and Japan by the end of 2023. Patient advocates are also pushing for a speedy approval process.

If lecanemab does receive accelerated approval, Eisai will need to continue to run trials on the drug in order to gain full approval.

The 1,800-patient trial compared the effectiveness of lecanemab against a placebo. The drug is meant to delay the progression of the disease by removing sticky protein deposits from the brains of Alzheimer's patients, a solution that has existed as an unproven theory for some time.

Lecanamab isn't the first drug to target these protein deposits, known as amyloid beta. Aduhelm also targeted amyloid, but lecanemab aims for amyloid that has yet to form a deposit. Last year, Aduhelm became the first Alzheimer's drug to receive FDA approval in two decades.

While lecanemab has resulted in slowed progression, Aduhelm was approved by the FDA on the sole basis that it targeted the theorized problem causing the disease. That decision received pushback from the scientific community, and the FDA's own expert panel advised against approving Aduhelm.

However, patients and patient advocates were still ecstatic when Aduhelm became available. After so long with no drugs showing any promise, even just the potential for improvement was enough to convince many of those suffering from this disease to take the risk, something that pharmaceutical companies hoped to cash in on.

After facing widespread criticism for the $56,000 price tag on a year's worth of Aduhelm, Biogen eventually reduced the price to $28,000 per year. According to The New York Times, Aduhelm is a commercial failure, despite the fact that any successful Alzheimer's drug is expected to rake in billions of dollars for its developer.

Aduhelm's failure is at least partly to blame on a lack of health care coverage. Medicare will only cover the cost of Aduhelm and other similar Alzheimer's drugs if the patient has been enrolled in a valid clinical trial. Whether or not Medicare coverage of lecanemab will be available hasn't been decided.

One issue that is still present in the lecanemab study is brain swelling. Removing amyloid build-up is often accompanied by brain swelling, and the study participants treated with lecanemab experienced 12.5% swelling compared to 1.7% for those on the placebo. Still, only 2.8% of participants experienced any symptoms associated with brain swelling.

On the other hand, microhemorrhages in the brain affected 17% of lecanemab participants compared to 8.7% for the placebo group. This rate is still far below the rate seen during the Aduhelm studies, according to Petersen.

Studies on the effectiveness of Alzheimer's drugs are also underway at Roche (RHHBY  ) and Eli Lilly (LLY  ). After repeated failures in the development of Alzheimer's treatments, lecanemab's success is expected to reinvigorate investment in these types of studies.

While the surge in research is promising, the number of people affected by Alzheimer's disease has been growing at a steady rate. By 2050, the number of Americans living with Alzheimer's is expected to double to 13 million people, according to the Alzheimer's Association.

Globally, the rate of increase is even higher: Alzheimer's Disease International reports that there are currently around 55 million people around the world with Alzheimer's disease. That figure could reach as high as 139 million by 2050.