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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.

Dr. Aaron Kesselheim, a professor of medicine at Harvard Medical School, said the agency's decision on Biogen "was probably the worst drug approval decision in recent U.S. history," according to his resignation letter obtained by CNBC.

"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.

He wrote it was "clear" to him that the agency is not "presently capable of adequately integrating the Committee's scientific recommendations into its approval decisions."

"This will undermine the care of these patients, public trust in the FDA, the pursuit of useful therapeutic innovation, and the affordability of the health care system," he said.

Shares of Biogen surged 38% on Monday after the FDA approved the biotech company's drug, the first medication cleared by U.S. regulators to slow cognitive decline in people living with Alzheimer's and the first new medicine for the disease in nearly two decades.

Biogen's drug targets a "sticky" compound in the brain known as beta-amyloid, which scientists expect plays a role in the devastating disease.

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. The agency granted approval on the condition that Biogen conducts another clinical trial.

The agency's decision was a departure from the advice of its independent panel of outside experts, who unexpectedly declined to endorse the drug last fall, citing unconvincing data. At the time, the panel also criticized agency staff for what it called an overly positive review of the data.

At least two other FDA panel members have resigned as a result of the agency's decision on the drug. Mayo Clinic neurologist Dr. David Knopman and Washington University neurologist Dr. Joel Perlmutter have also submitted resignation letters.

"I was very disappointed at how the advisory committee input was treated by the FDA," Knopman told Reuters. "I don't wish to be put in a position like this again."

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.

In March 2019, Biogen pulled development of the drug after an analysis from an independent group revealed it was unlikely to work. The company then shocked investors several months later by announcing it would seek regulatory approval for the drug after all.

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."

Alzheimer's experts and Wall Street analysts were immediately skeptical, with some wondering whether the clinical trial data was enough to prove the drug works and whether approval could make it harder for other companies to enroll patients in their own drug trials.

Some doctors have said they won't prescribe aducanumab because of the mixed data package supporting the company's application.