Roche Holding has begun an appeal before a US health panel to retain the approval of Avastin for the treatment of breast cancer.
The US Food and Drug Administration (FDA) proposed removing the treatment in December, but Genentech, a unit of Roche, is expected to argue the drug should be kept on the market pending new studies that show a more significant clinical benefit.
Taking the drug off the market could cut Roche sales by almost $1bn annually and jeopardise treatment for an estimated 17,000 women currently using the medicine.
The appeals panel will make a recommendation during the two-day hearing, which began yesterday, and the FDA will make the final decision.
At least 40 patients held signs and chanted outside the FDA building on day one of the hearing, arguing that Avastin saved their lives.
Roche won US clearance for Avastin to be used as a treatment for breast cancer in 2008, based on a study showing the drug stalled cancer growth by 5.5 months.
But later studies found only a one- to three-month delay in breast cancer growth, and no studies showed Avastin extended the lives of patients.