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First Tysabri, Now Rituxan

The same rare, but fatal viral infection that forced Biogen Idec and Elan to recall, and recently relaunch, the multiple sclerosis drug, Tysabri, may also be a side effect of Rituxan, also from Biogen Idec and Genentech .

In an SEC filing -- not a press release -- and on the FDA website, the companies and the agency disclosed yesterday evening that two people with lupus died after taking Rituxan. The drug is approved for non-Hodgkin's lymphoma and rheumatoid arthritis, but Genentech says doctors have used it on an estimated 10,000 lupus patients "off-label". The companies say there's no concrete evidence Rituxan caused the central nervous system infection, but that they can't rule out the possibility.

The stocks took a hit in after-hours yesterday and early trading today, but they've trimmed their losses on a lot of analyst commentary that this does not represent a huge threat to the $1.8 billion Rituxan franchise.

Genentech gets 60%, Biogen Idec 40%, but the damage to BIIB shares is greater because analysts say the company bears less of the Rituxan expenses, so more of the money falls to the bottom line than at Genentech. Genentech says it's forging ahead with late-stage clinical trials of Rituxan on lupus, but some analysts think this disclosure could make some patients afraid to enroll.

Several other companies are also working on lupus drugs in mid to late stage development, including (and some of these have very small market value) Teva Pharmaceuticals , GlaxoSmithKline, Human Genome Sciences , Immunomedics, AlexionPharmaceuticals, and Aspreva Pharmaceuticals .

Lupus affects about a quarter-million people in the U.S., most of them women and minority women are considered to be most at risk.

Questions? Comments? PharmasMarket@cnbc.com

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