A fatal case of progressive multifocal leukoencephalopathy, or PML, in a patient treated with arthritis drug Rituxan has prompted pharmaceutical company Genentech to revise the labeling of the drug.

Rituxan, also known as rituximab, was approved to treat non-Hodgkin’s lymphoma in 1997 and rheumatoid arthritis in 2006 and had sales last year of $2.3 billion in the US alone.

PML, a virus infection that usually occurs in patients with impaired cell-mediated immunity, was previously registered in patients taking Rituxan for unapproved uses. This is the first case when PML occurs in a patient taking the drug for arthritis. The woman developed the infection more than a year and a half after discontinuing treatment with Rituxan, the US Food and Drug Administration said. However, it is not sure if the disease was provoked by Rituxan since the woman had also received chemotherapy agents about nine months earlier.

“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,” Tara Cooper, a representative for Genentech said. She also wanted to underline the fact that the risk of developing PML is already mentioned on Rituxan’s label. Whether or not the woman’s disease was an effect of Rituxan, the US Food and Drug Administration recommended doctors administering Rituxan to their patients look for neurological problems.