Biogen Idec and Genentech have reported that two patients suffering from lupus have died from a rare brain infection after taking Rituxan, the firms' successful cancer and rheumatoid arthritis drug.

The two companies have issued a letter to doctors and other prescribers concerning the patients who died from progressive multifocal leukoencephalopathy, or PML after taking Rituxan for systemic lupus erythematosus (SLE), an indication for which the treatment is not approved.

Biogen and Genentech, which noted that the patients had longstanding SLE with multiple courses of immunosuppressant therapy prior to receiving Rituxan, said they were working with the US Food and Drug Administration to update prescribing information. They also added that while studies of Rituxan were ongoing for lupus, the two patients who died were not enrolled in any clinical trial. The FDA said that it is "working to gather more information about Rituxan and PML and to strengthen the warnings" on the product label.

Although rare, PML has been in the news recently and it was the reason why Biogen and Elan's Tysabri (natalizumab) was voluntarily pulled from sale in 2005 after three cases of the brain infection, two of which were fatal, were found in patients taking the multiple sclerosis drug. Tysabri is now back on the market but its sale is heavily restricted.

Rituxan is a major earner for Biogen and Genentech, with total sales for the third quarter of $551 million and the news sent the firm’s shares lower in after-hours trading. Biogen was down 5% and its partner’s stock slipped 2%.

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