Washington - Federal regulators said Thursday one case of a deadly brain infection has been reported in a patient taking Genentech and Biogen Idec's blockbuster arthritis and cancer drug Rituxan.

The Food and Drug Administration said the woman died of the rare viral infection more than a year and a half after discontinuing the drug. Genentech and Biogen Idec comarket the drug in the U.S.

Cases of PML have previously been reported in patients taking Rituxan for unapproved uses, including blood cancer. But FDA said the latest case is the first reported in a patient taking the drug for arthritis. The drug is also approved for non-Hodgkin's lymphoma.

A company spokeswoman for Genentech noted the drug's label already mentions risks of the infection.

"The patient had a number of confounding factors that make it difficult to assess the potential role, if any, that Rituxan exposure may have played," said Tara Cooper. Genentech first disclosed the death during its July earnings call, Cooper added.

According to the posting on FDA's Web site, the patient was undergoing chemotherapy and radiation treatment for cancer in the months before she developed the infection.

South San Francisco-based Genentech sent a letter to doctors about the case earlier this month.

Rituxan is Genentech's second-best selling drug with $2.28 billion in sales last year, just behind the cancer therapy Avastin.

Reports of the brain disease, called progressive multifocal leukoencephalopathy, or PML, have hurt shares of Biogen Idec in recent months. In July the company reported two new deaths from the disease in European patients taking its multiple sclerosis drug Tysabri. The drug was pulled from the market in 2005 after being linked to the rare brain disease but was reintroduced under restricted sales conditions in mid-2006.

Shares of Genentech Inc. rose $1.08 Thursday to $96.77 in afternoon trading. Shares of Biogen Idec Inc. fell 18 cents to $47.12.