Pharmaceutical company Biogen is halting two global phase-three trials testing the once-promising Alzheimer’s drug aducanumab, delivering a late-stage blow to researchers searching for therapies for the incurable degenerative disease.

“This disappointing news confirms the complexity of treating Alzheimer’s disease and the need to further advance knowledge in neuroscience,” Biogen CEO Michel Vounatsos said in a statement Thursday. “We are incredibly grateful to all the Alzheimer’s disease patients, their families and the investigators who participated in the trials and contributed greatly to this research.”

Biogen and its Japanese partner Eisai decided to end the aducanumab trials after an independent data-monitoring committee determined that the drug was unlikely to provide benefit to Alzheimer’s patients compared to a placebo, the company said in its statement. The committee did not find safety issues associated with the drug.

Aducanumab was meant to slow the rate of cognitive decline and functional impairment in people with mild Alzheimer’s disease by clearing amyloid from the brain. Amyloid protein is thought to contribute to Alzheimer’s-related decline by forming sticky plaques in the brain, potentially compromising nerve cells and leading to dementia and memory loss. The trial’s failure, however, calls that amyloid hypothesis — and potential treatment path — into question.

It’s also yet another failure in the search for a drug that can fight Alzheimer’s disease, which has been riddled with bad news in recent years. In 2018, pharmaceutical companies including Eli Lilly, AstraZeneca, Pfizer and Merck halted research or development into possible Alzheimer’s and dementia therapies due to disappointing results.

Write to Jamie Ducharme at jamie.ducharme@time.com.