MONDAY, Jan. 14 (HealthDay News) -- Tysabri, a controversial drug used to treat multiple sclerosis, may also be used for patients with a moderate to severe form of another autoimmune illness, Crohn's disease, the U.S. Food and Drug Administration announced Monday.

But the drug also comes with a rare but serious risk of a potentially deadly brain infection, as well as other side effects, so it must be used carefully, the FDA said in a press teleconference.

"With the addition of Tysabri to the treatment options of sufferers of Crohn's disease, we make an important step in the armamentarium for treatment, but one that carries serious risks," said Dr. Joyce Korvick, deputy director of the division of gastroenterology products at the FDA's Office of New Drugs.

Crohn's is a serious, often painful, inflammatory bowel disorder that affects about 600,000 people in the United States. It can involve intestinal bleeding, diarrhea, weight loss, arthritis, skin problems, fever and anemia.

Tysabri (natalizumab), a monoclonal antibody used to treat MS, has been under a cloud of controversy for some time. The drug works by attaching itself to white blood cells called lymphocytes and preventing them from entering the brain, where they do damage that causes the disabling symptoms of MS.

But the drug has a checkered past. It first received U.S. Food and Drug Administration approval in November 2004, only to be pulled from the market three months later after several patients in clinical trials developed a rare but deadly viral infection of the brain called progressive multifocal leukoencephalopathy (PML).

In June 2006, the FDA allowed the drug to return to the market but with strict conditions. According to the new guidelines, Tysabri can only be administered by approved doctors, at infusion sites and pharmacies that register and comply with a patient-safety program called CD Touch, designed by Biogen Idec, the maker of Tysabri, and approved by the FDA.

Last August, an FDA advisory panel voted 12-3 in favor of the use of the drug for Crohn's disease during a joint meeting of the Gastrointestinal Drugs Advisory Committee and the Drug Safety and Risk Management Advisory Committee.

At the time, the panel found that Tysabri could help ease the symptoms of the disease in patients who don't respond to standard therapies such as steroids or immunosuppressants.

The FDA is only approving Tysabri for Crohn's patients with moderate to severe symptoms, and this use will be subject to strict controls.

Korvick said that, besides the risk of PML, Tysabri also raises a patient's odds for hypersensitivity reactions, liver injury and severe herpes infections, especially in those on immunosuppressant therapy, so people taking immuno-suppressing drugs should not take Tysabri, Korvick said.

Crohn's patients who begin Tysabri therapy should also taper off their use of steroids, she added.

Side effects from the drug include headache, fatigue, infusion reactions, rash, and joint and limb pain.

"Because of these risks, patients, prescribers, pharmacies and infusion centers must all be enrolled in the [drug makers'] 'CD Touch' program and agree to comply with the company's strict monitoring guidelines," Korvick said. Doctors should also "evaluate Crohn's patients after three months of treatment to determine if they have improved on Tysabri. If not, then patients should be discontinued," she added.

According to the Wall Street Journal, Biogen Idec earlier this month noted that more than 21,000 patients were using Tysabri without any additional reports of PML.

More information

There's more on Crohn's disease at the U.S. National Institute of Diabetes and Digestive and Kidney Diseases.