The long wait for a disease-modifying treatment option for millions of Americans suffering from Alzheimer's disease is finally over. The FDA on Monday approved aducanumab from Biogen (BIIB 1.82%) and its longtime collaboration partner Eisai (ESALY 0.10%). Biogen will market the drug under the brand name Aduhelm.

Aduhelm acts in the brain to remove amyloid protein fragments before they can form sticky plaques associated with Alzheimer's disease. Unfortunately, aducanumab failed to outperform a placebo in one of two identical clinical trials designed to prove its ability to help preserve patients' cognitive ability. Biogen also failed to convince a panel of independent experts that Aduhelm provided a disease-modifying benefit.

Elderly patient with a caregiver.

Image source: Getty Images.

However, the FDA granted Aduhelm an accelerated approval based on its ability to improve amyloid protein fragment concentrations. In theory, this approval will be rescinded if Biogen doesn't provide clinical evidence of a cognitive benefit at some point in the future. In practice, though, drugs that are granted accelerated approvals tend to remain on pharmacy shelves even when such evidence fails to materialize.

Aduhelm isn't intended for patients who have already progressed to a severe stage of Alzheimer's disease, but the FDA didn't specify disease severity on Aduhelm's prescribing label. This means there are around 6.2 million Americans now eligible to begin treatment with the drug.

Don't be surprised if sales of Aduhelm top $10 billion annually long before Biogen provides any further evidence of efficacy. Patients who receive monthly infusions of the drug will need regular MRIs to monitor for signs of dangerous brain swelling that occurred in some clinical trial participants, but the label contains no black box warnings to hinder its uptake.