We've Had an Approved Alzheimer's Drug for Two Years. We Just Found Out How It Actually Works.

A drug that's been slowing Alzheimer's disease in real patients for two years just had its actual mechanism revealed for the first time. The finding, published in Nature Neuroscience by scientists at VIB and KU Leuven in Belgium, rewrites what we thought we knew about how these treatments work — and it opens a clear path to fixing the drug's most dangerous side effect.

What Lecanemab Does (and Why No One Knew How)

Lecanemab — sold as Leqembi — is an antibody therapy that clears amyloid beta plaques from the brain. These toxic protein clusters damage neurons and eventually cause dementia. The drug received full FDA approval in 2023 and showed a 27% slowing in cognitive decline in clinical trials. Millions of eligible Alzheimer's patients now have access to it in the US.

The uncomfortable truth? Until last week, researchers didn't fully understand why it worked.

The standard assumption was that the antibody latched onto amyloid plaques directly and dragged them away. Some scientists even suggested the therapy's effectiveness had nothing to do with the immune system at all. This mattered, because guessing wrong about a mechanism means guessing wrong about everything you try to improve next.

The Fc Fragment Is the Key

The Belgian research team built a specialized mouse model that contained actual human microglial cells — the brain's resident immune cells — and tested lecanemab directly against human immune responses. What they found was stark.

Antibodies have two main parts: one that binds to a target (amyloid plaques, in this case) and one called the Fc fragment, which signals the immune system. When the team removed the Fc fragment entirely, lecanemab did nothing. The plaques stayed. Microglia didn't respond.

"The Fc fragment works as an anchor that microglia latch onto when they are near plaques," said Dr. Giulia Albertini, co-first author of the study. "These cells are reprogrammed to clear plaques more efficiently."

In other words, the antibody isn't clearing plaques itself. It's recruiting the brain's own immune cells to do it. The drug works as a signaling mechanism — a flag that tells microglia exactly where to act and exactly how.

Why This Changes the Treatment Equation

The stakes of this discovery are enormous, and they come back to one number: about 1 in 5 patients on lecanemab develops amyloid-related imaging abnormalities (ARIA) — brain bleeding or swelling that can range from asymptomatic scan findings to seizures and severe cognitive setbacks. In rare cases, ARIA has been associated with death.

This side effect has been the drug's central problem. It's why take-up has been slower than expected despite FDA approval. It's why some neurologists hesitate to prescribe it for patients taking blood thinners. It's why insurers initially balked.

Now that researchers know the Fc fragment is the active mechanism — and that it works by triggering a specific immune program in microglia — the next question becomes: can you tune that immune response to maximize plaque clearance while minimizing the inflammation that causes ARIA?

The short answer is: probably yes. Antibody engineering can modify the Fc fragment. Knowing exactly what it does makes that modification tractable in a way it simply wasn't before. Researchers are already working on bispecific antibodies that target both amyloid and tau (another protein linked to Alzheimer's progression) simultaneously — and this mechanism discovery gives those projects a far more precise target.

The Access Problem No One Is Solving

Here's where the story gets more complicated.

Lecanemab's annual list price is $26,500. The drug also requires regular MRI scans, infusion center visits, and monitoring labs that can add tens of thousands of dollars more to the total cost. Medicare and Medicaid patients make up roughly 92% of the eligible US market. For the rest of the world, the drug barely exists.

More than 55 million people live with Alzheimer's globally. The burden is rising fastest not in wealthy countries with insurance systems, but in Africa, Asia, and Latin America — where demographic aging is accelerating without corresponding healthcare infrastructure. In India, one study suggests 90% of dementia cases remain undiagnosed. In sub-Saharan Africa, the figure is likely higher.

Lecanemab requires PET imaging or cerebrospinal fluid tests to confirm amyloid pathology before a patient qualifies for the drug. Most of the world doesn't have PET scanners in routine clinical use. The treatment assumes a healthcare baseline that simply doesn't exist in most of the planet.

The Belgian mechanism discovery doesn't fix this. But it points toward a future where next-generation antibody therapies — engineered with the Fc fragment insight — might be simpler, more targeted, and potentially cheaper to manufacture and monitor. A therapy that works better and produces less ARIA might not require the same level of intensive MRI surveillance. That matters most where MRI access is limited.

What Comes Next

The scientific community already knew microglia were involved in amyloid clearance in some way. What this study proves is that microglia are the mechanism — not a byproduct. The plaque-clearing is a learned cellular program, triggered specifically by the Fc fragment, and the therapy's efficacy is entirely dependent on that immune activation.

Prof. Bart De Strooper's team at KU Leuven has effectively provided a biological blueprint. Future Alzheimer's antibody therapies can now be built around Fc fragment optimization rather than treating the immune response as a side effect to manage.

For the 55 million people living with Alzheimer's today — and the projected 139 million expected by 2050 as the world ages — this isn't a cure. It's a proof of mechanism. But in drug development, understanding why a treatment works is often the most important step toward making it work better, and eventually, making it work for everyone.

The brain has its own cleanup crew. We just learned how to give it orders.

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Related: What the global health equity gap means for access to Alzheimer's treatments — and how the information divide shapes who even knows these drugs exist.