Three patients taking Genentech's psoriasis drug Raptiva are thought to have died of a rare brain infection, known as a possible side effect of the drug, federal health officials reported.

On Thursday, the Food and Drug Administration said there have been three confirmed and one possible case of progressive multifocal leukoencephalopathy or PML in people taking Raptiva. Three of the patients died. All four had been treated with the drug for more than three years.

Raptiva also known as efalizumab is a once-weekly injection prescribed for adults with moderate to severe plaque psoriasis. The drug works by suppressing the immune system to reduce psoriasis flare-ups, but, by suppressing the body's natural defense system, it can also increase the risk of serious infections and malignancies in patients. The drug was approved in 2003.

In October last year, the FDA added a black box warning on the drug's label saying it increases the risk of life-threatening infections, such as bacterial sepsis, viral meningitis, invasive fungal disease and even PML, a disorder mostly seen in patients with immune deficiencies causing swelling of the brain, usually leading to death. The company said by the time that it was only aware of one case of PML which they reported earlier in October. It involved a 70-year-old patient who received Raptiva for more than four years.

The decision of adding a black box warning to the drug's label followed reports to the FDA's Office of Surveillance and Epidemiology of hospitalizations and deaths resulting from infections linked to the used of the drug.

The FDA announcement on Thursday came the same day that European Union regulators recommended a ban on marketing Raptiva saying "its benefits no longer outweigh its risks, because of safety concerns."

Genentech spokeswoman Tara Cooper released a statement saying "we take the risk of PML very seriously and are working diligently with the FDA to put the right plans in place that will help protect patient safety."

Raptiva is sold outside the US by Germany's Merck KGaA. Phyllis Carter, the company's spokeswoman said they "will work closely with the European health officials to undertake all necessary measures to comply."

The FDA said health care professionals should carefully monitor patients taking Raptiva and those who have stopped the treatment for any signs or symptoms of neurologic disease. PML symptoms may include weakness, loss of coordination, changes in vision, difficulty speaking and personality changes.