MONDAY, June 5 (HealthDay News) — Setting strict guidelines as a condition, the U.S. Food and Drug Administration said Monday that it will allow the return of the multiple sclerosis drug Tysabri.

Tysabri (natalizumab) had received FDA 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).

“This is one of the very rare cases in which a drug withdrawn from the market for safety reasons has been returned back to the market after appropriate steps have been taken,” Dr. Steven K. Galson, director of the FDA’s Center for Drug Evaluation and Research, said at a press conference.

“In this case, we made the assessment that the benefits of this drug outweigh the risks for MS,” Galson added. “We are certain that patients are willing to take this risk because of the potential benefits of the drug.”

According to the new guidelines, Tysabri can only be administered by approved doctors, infusion sites and pharmacies that register and comply with a patient-safety program designed by Biogen-IDEC, the maker of Tysabri, and approved by the FDA.

Multiple sclerosis, which affects some 350,000 Americans, is an unpredictable disease of the central nervous system. Most MS patients experience muscle weakness in their extremities and difficulty with coordination and balance. Symptoms may be severe enough to impair walking or even standing. Its cause is not known and there is no cure, according to the National Institutes of Health.

Studies have found that treatment with Tysabri reduced the rate of relapse in MS patients by as much as two-thirds after two years, and reduced the number of patients whose symptoms worsened.

“Tysabri is a monoclonal antibody used as a single drug for the treatment of patients with relapsing forms of MS,” Galson said. “The drug has been used effectively in patients who have not responded well to, or cannot tolerate, other MS treatments.”

The drug was first approved by the FDA in November 2004, but was taken off the market by Biogen-IDEC in February 2005. The recall occurred after three patients in clinical trials developed PML.

The FDA put the clinical trials on hold in February 2005, but allowed them to resume in February 2006, with no additional cases of PML reported.

With the drug’s return to the market, there will undoubtedly be other cases of PML and deaths, said Dr. Russell Katz, of the FDA’s Center for Drug Evaluation and Research. “This is balanced against the significant benefit the drug confers,” he added.

“Currently, the best estimate we have of the rate of occurrence of PLM is about one in 1,000 patients treated,” Katz said. “We know little about what the risk would be in a larger population or what the risk would be if treatment continues beyond two years,” he said.

In March, the FDA’s advisory committee on drugs for peripheral and central nervous systems recommended reintroducing the drug with a risk-minimization program, including mandatory patient registration and periodic follow-up.

Tysabri will only be available through a risk-management plan, called the TOUCH Prescribing Program. To receive Tysabri, patients must confer with their doctor and understand the risks and benefits of the drug and agree to all of the instructions in the program.

Some seo tips of those restrictions include: only those registered with the program and who agree to comply with it will be able to prescribe the drug; only infusion centers registered and authorized will be able to administer Tysabri intraveneously; only pharmacies registered with the program will be able to dispense Tysabri to authorized infusion centers; only patients enrolled in the program and who agree