Aducanumab

Marketed under the brand name Aduhelm, aducanumab is a human monoclonal antibody engineered to specifically recognize and bind to aggregated forms of the amyloid-beta protein. According to the amyloid hypothesis, the accumulation of these protein aggregates into plaques in the brain is a primary pathological driver of Alzheimer's disease, leading to neurodegeneration and cognitive decline. By binding to these plaques, aducanumab is believed to activate the brain's immune cells, principally microglia, to clear the amyloid deposits.

The drug's development, led by Biogen in partnership with Eisai, was based on this plaque-clearing mechanism. However, its path to approval was marked by significant controversy. Two pivotal Phase 3 clinical trials, EMERGE and ENGAGE, yielded conflicting results. While the EMERGE trial suggested that a high dose of aducanumab could modestly slow cognitive and functional decline in patients with early-stage Alzheimer's, the ENGAGE trial failed to show a similar benefit. This discrepancy fueled intense debate within the scientific and medical communities about the drug's true efficacy.

Aducanumab's development and approval represent a major milestone in the history of Alzheimer's research, which for decades has been dominated by the amyloid hypothesis. For years, treatments for Alzheimer's were purely symptomatic, aiming to manage cognitive symptoms without altering the underlying disease course. Aducanumab was one of the first disease-modifying therapies to receive regulatory approval, shifting the treatment paradigm toward targeting the biological basis of the disease. Its approval paved the way for other amyloid-targeting monoclonal antibodies, such as lecanemab and donanemab, which have since demonstrated clearer clinical benefits alongside plaque reduction.

The significance of aducanumab lies less in its clinical success and more in its impact on regulatory science and patient advocacy. In June 2021, the U.S. Food and Drug Administration (FDA) granted it accelerated approval, a decision based on its proven ability to reduce amyloid plaques (a surrogate endpoint) rather than on conclusive evidence of slowing cognitive decline. This decision was highly contentious, leading to the resignation of several members of the FDA's advisory committee. For patients and families, the approval offered a glimmer of hope but also presented difficult choices, given the drug's high cost, requirement for monthly intravenous infusions, and risk of side effects like amyloid-related imaging abnormalities (ARIA), which can cause brain swelling or bleeding. Ultimately, due to limited insurance coverage and low clinical adoption, Biogen announced in 2024 that it would discontinue the development and sale of Aduhelm, closing a complex chapter in the search for an effective Alzheimer's treatment.