Feb. 16 (Bloomberg) -- Biogen Idec Inc.’s multiple sclerosis drug Tysabri has been linked to four new cases of a life-threatening brain illness, a spokeswoman said.

The latest cases raise to 35 the number of patients found with progressive multifocal leukoencephalopathy, or PML, since Biogen and its partner, Elan Corp., were allowed to resume selling the drug, said Naomi Aoki, a spokeswoman for Cambridge, Massachusetts-based Biogen. Eight of the patients had died as of Feb. 9, Aoki said.

Biogen and Dublin-based Elan pulled Tysabri from the market in 2005 after three patients, two of whom died, developed the brain infection. The U.S. Food and Drug Administration allowed sales to resume in July 2006 after deciding benefits for slowing MS relapses outweighed the risk. There were no reported infections for about two years after the drug’s reintroduction.

“The overall picture is consistent with the rate described in the label,” Aoki said in an interview. “PML remains rare.”

The drug’s labeling indicates the infection rate is 1 in 1,000, and the new cases fall within that rate, Aoki said. The likelihood of infection increases with the time patients are using the drug, she said. Of the 35 cases, 11 were in the U.S., 21 in European Union countries and 3 in the rest of the world, she said.

Tysabri is Biogen’s fastest-growing product and had total sales of $1.1 billion in 2009, an increase of 30 percent from the year before, the company said on Feb. 9.

To contact the reporter on this story: Elizabeth Lopatto in New York at elopatto@bloomberg.net.

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