On April 9, the biotech company Genentech announced that it was withdrawing its psoriasis medicine Raptiva from the market because it can cause a rare but often fatal brain infection called progressive multifocal leukoencephalopathy, or PML. Four cases of the disease have been reported in patients taking Raptiva; all had been on the drug for at least three years.
Raptiva is used to control moderate to severe cases of plaque psoriasis, the most common form of the autoimmune disease, which causes red scaly patches on the skin that can itch and hurt. Raptiva, approved in the U.S. in 2003, works by inhibiting the overactive immune cells that give rise to the condition.
What is PML?
PML is a devastating infection of the nervous system caused by a virus. Known as JC virus, it attacks the white matter of the brain and disrupts nervous system activity. Symptoms depend on where in the brain the virus hits and can include deficits in speech, vision, movement and thought processes. The disease progresses rapidly and inexorably toward serious disability and death. "The average survival is generally no more than three months," says Dr. Joseph Berger, a neurologist at the University of Kentucky who has studied and written about the disease.
PML was an extremely rare disease until the 1980s, when it started showing up in AIDS patients. Worldwide, only 230 cases were documented between 1958 -- when the disease was first described -- and 1984. "Then it became explosive," Berger says, with about 1 in 20 AIDS deaths showing signs of PML upon autopsy.
PML does not occur without some underlying risk, and to date the highest risk comes from HIV infection. But it also can arise in people who have certain cancers, as well as transplant patients and those taking a small number of medicines, Berger says. The common thread in all these conditions is immunosuppression, either caused by a virus, cancer or drugs.
What is JC virus?
JC virus has infected an estimated 80% of the world's adult population and often remains latent in the body. For the disease to arise, a series of things -- some with very low probability -- must happen. A mutation must occur that makes the virus capable of attacking brain cells. The transformed virus must be activated and gain entry to the brain. And the brain's immune surveillance must be deficient.
