WASHINGTON (AP) – A multiple sclerosis treatment is about to return to the market, a year after being derailed by a rare but life-threatening side effect.

Government advisers voted unanimously Wednesday that the Food and Drug Administration should let Tysabri resume sales – but only with strong restrictions designed to ensure that patients understand that as many as one in 1,000 users could get a potentially deadly brain disease.

Among the restrictions: A mandatory patient registry that would limit where monthly infusions of the drug are given and require regular exams for signs of trouble.

The move delighted patients who had clamored for a second chance at Tysabri because, while not a cure, it does work differently than standard treatments for nerve-damaging multiple sclerosis.

“We have our hope back,” said Christy Cooksey-Dressell, who told FDA’s advisers how her mother, Janet Dressell, improved so much after a single Tysabri dose that she abandoned her walker – but the Klamath Falls, Ore., woman is deteriorating badly again.

“We’re willing to take that 1-in-1,000 risk,” Cooksey-Dressell said. “Right now, it’s quality of life vs. quantity of life for my mom.”

The FDA usually follows its advisers’ recommendations, and agency drug chief Dr. Robert Temple signaled Wednesday that it would do so with Tysabri.

“Devastating diseases often are treated by drugs that have problems of their own,” Temple said.

But, he cautioned, deciding to use Tysabri will be “a tough decision” because the brain side effect – called progressive multifocal leukoencephalopathy, or PML – is often fatal and, worse, there’s no evidence that diagnosing it early improves survival.

“I think our major objective will be to explain as clearly as possible to people the nature of the risk and its severity,” Temple said. “It may not kill you all the time, but it doesn’t leave you in good shape. … The patient has to have the presumption that if he or she gets this (the side effect), that it’s going to be very bad.”

Only one other prescription drug has ever returned to the market after being pulled because of dangerous side effects – the irritable bowel treatment Lotronex.

Tysabri’s manufacturers voluntarily pulled the drug from the market last year, only four months after FDA initially approved it, because three users were diagnosed with PML. Two of them died.

Now, calling the risk remote and the drug’s promise great, manufacturers Biogen Idec, based in Cambridge, Mass., and Elan Corp. want to resume sales, and pledged Wednesday to work closely with FDA on a risk-management program.

Multiple sclerosis afflicts about 350,000 Americans. The cause is unknown and there is no cure. The most common form causes periodic flare-ups of symptoms, which include trouble seeing and walking.

Recently published studies found that Tysabri alone or in combination with standard interferon treatment cut the rate of relapse by as much as two-thirds, and reduced the number of people whose MS got worse. It appears to work by blocking destructive immune cells leaving the bloodstream and entering the brain, where they can inflame and damage nerves.

PML, the apparent side effect, is caused by a common virus that usually lies dormant. When it does cause disease, the most common victims are people with very weak immune systems, such as AIDS patients.

The question for would-be Tysabri users is how big a risk PML really poses. The one-in-1,000 risk that Temple cited could be higher, depending on the overall health of Tysabri users and how long they take the drug.

Also, Dr. Karl Kieburtz of the University of Rochester, who chaired the FDA advisory committee, cautioned that there are complicating factors: Clinical trials of Tysabri have included fairly young people whose MS wasn’t too severe. And PML’s symptoms are hard to distinguish from those of an MS relapse, because the two diseases both attack the coating of nerve cells.

All those unknowns helped push FDA’s advisers to call for fairly stringent restrictions, Kieburtz said: In addition to a mandatory patient registry, the drug should be administered only in specially designated sites, such as infusion centers trained in Tysabri’s use – and only to patients with relapsing MS who take no other MS medicines.

AP-ES-03-08-06 1904EST