The drug company Genentech announced that it was voluntarily withdrawing its biologic drug for psoriasis, efalizumab (Raptiva), from the market because of the drug’s link to a rare but often fatal brain disorder.

In a letter to patients posted on the Web site of the Food and Drug Administration (FDA), the company said that since September it has received three reports of progressive multifocal leukoencephalopathy (PML) in patients taking Raptiva. Two of those cases were fatal. A fourth patient died while on the drug of an unknown cause after developing neurologic symptoms. All of the patients who developed PML had been taking the drug for more than three years.

PML is caused by a common virus carried by a majority of people that normally lies dormant in the body. When the immune system is suppressed by medication or disease, however, the virus can reactivate, causing a brain infection.

The first symptoms of PML are diverse because their development depends on which parts of the brain are affected. Typically, however, people with PML become clumsy and then muscles progressively weaken over days or months. These can be accompanied by changes in vision, speech or personality.

Raptiva, a once-weekly injection, treats psoriasis by suppressing the immune system.

“Although we believe that there are many psoriasis patients who benefit from Rapitva, there is no way to know ahead of time who will get PML,” said Hal Barron, the Chief Medical Officer for Genetech, in a statement. “There is no treatment or cure for PML. People who do live with PML are severely disabled.”

The company estimates that 2,000 people in the U.S. are taking Raptiva.

Genetech and the FDA are advising people who take Raptiva not to stop abruptly, but to talk to their doctors about transitioning to other therapies. 

By June 8, 2009, Raptiva was no longer be available in the U.S., the FDA said.