A third member of a key Food and Drug Administration advisory panel has resigned over the agency's controversial decision to approve Biogen's new Alzheimer's drug, Aduhelm, CNBC has learned.

"At the last minute, the agency switched its review to the Accelerated Approval pathway based on the debatable premise that the drug's effect on brain amyloid was likely to help patients with Alzheimer's disease," he wrote in resigning from the FDA's Peripheral and Central Nervous System Advisory Committee.

The FDA approved the drug under a program called accelerated approval, which is usually used for cancer medications, expecting the drug would slow the cognitive decline in Alzheimer's patients.

Federal regulators have faced intense pressure from friends and family members of Alzheimer's patients asking to fast-track the drug, scientifically known as aducanumab, but the road to regulatory approval has been a controversial one since it showed promise in 2016.

When Biogen sought approval for the drug in late 2019, its scientists said a new analysis of a larger dataset showed aducanumab "reduced clinical decline in patients with early Alzheimer's disease."?