NEW YORK (NYTIMES) - Concerns about safety risks of the controversial new Alzheimer's drug Aduhelm have intensified in the wake of the death of a 75-year-old woman who experienced brain swelling after receiving infusions of the drug as a participant in a clinical trial.

The death of the woman, who lived in Canada, occurred in late September and was reported by a doctor to the United States Food and Drug Administration's (FDA) adverse event reporting system this summer.

It is being investigated by both the FDA and Biogen, which makes the drug, also known by its scientific name, aducanumab.

In a statement, Biogen said: "The cause of death is unknown at this time. We know the 75-year-old clinical trial patient was admitted to the hospital with a seizure and diagnosed with brain swelling.

"Following a prolonged hospitalisation, the patient was being prepared for discharge when she deteriorated and was transferred to another facility.

"We have requested missing information, including brain imaging, from the critical last nine days of hospitalisation."

Last week, RBC Capital Markets' biotech analyst Brian Abrahams, who has a medical degree, wrote to clients that he had obtained the woman's case report through a public records request and had concluded that the brain swelling probably caused the woman's death and "that this is likely to have been caused by aducanumab".

Brain swelling and brain bleeding are known to be possible side effects of Aduhelm, a monoclonal antibody that is administered as a monthly infusion.

The FDA approved the drug in June even though a council of senior FDA officials, an advisory committee of outside experts and many Alzheimer's specialists said it was unclear whether Aduhelm could benefit patients and that the drug carried serious risks of harm.

On Aduhelm's label, the FDA warns about these brain side effects, known as amyloid-related imaging abnormalities (Aria).

Most data available about Aduhelm comes from two nearly identical phase three clinical trials that Biogen conducted before applying for FDA approval of the drug.

Safety data from those trials was published on Monday (Nov 22) in the journal Jama Neurology.

The data showed that 425 of 1,029 patients, or 41 per cent, who received the high dose of the drug - the dose that the FDA later approved - experienced either brain swelling or bleeding.