Cambridge, Massachusetts-based Biogen was informed of the American woman’s death two days ago, company spokeswoman Naomi Aoki said in an interview. The company reported on Oct. 29 that the patient had been diagnosed with the brain illness, progressive multifocal leukoencephalopathy.

Tysabri, which generated $597 million in sales in the first nine months of the year, is Biogen’s second-best-selling medicine after the MS drug Avonex. Biogen and its marketing partner, Irish drugmaker Elan, pulled Tysabri from the market in February 2005 after three patients, two of whom died, contracted PML. The death is the first among four cases of the brain illness reported since the drug was reintroduced in the U.S. in 2006.

The patient had received 14 Tysabri infusions before becoming ill, Biogen reported in a regulatory filing in October. Aoki declined to comment on the type of treatment for PML the patient received.

Since Tysabri’s reintroduction, three European patients have contracted PML. Biogen and Elan reported one case on Dec. 15 and two others on July 31. Elan fell 3 euro cents to 4.42 euros at 9:45 a.m. in Dublin trading.

More than 48,000 patients have taken Tysabri, Biogen told investors on Oct. 21, when the company released its earnings. Chief Executive Officer James Mullen said 100,000 patients will be taking the drug by 2010.

Affects 1 of 1,000

PML is included in Tysabri’s prescribing information as a possible side effect in 1 of every 1,000 patients taking the drug. The condition occurs when the JC virus, named with initials for the first patient diagnosed with it, evades the body’s immune defenses and penetrates the brain, causing irreversible damage.

The U.S. Food and Drug Administration approved Tysabri’s return to the market in June 2006 because research showed the treatment is twice as effective as other MS drugs. At the same time, the agency mandated strict measures to monitor side effects.

MS is a neurological disorder that robs people of muscle control and balance, sometimes leading to damaged vision and paralysis. The disease, which affects about 2.5 million people worldwide, is caused when the body’s immune system attacks the protective coating of nerve fibers.

Tysabri, given intravenously once a month, is designed to suppress the immune assault and is one of five immune-suppressing therapies that are known to cause PML. The others are Roche Holding AG’s CellCept, Biogen and Genentech Inc.’s Rituxan, Genzyme Corp.’s Campath and Genentech’s Raptiva.

http://www.bloomberg.com/apps/news?pid=20601202&sid=a1WzLbyMDGMA