updated 3/6/2006 12:52:16 PM ET 2006-03-06T17:52:16

The risk of a rare brain disease and other infections remains the primary concern of federal regulators as they seek independent advice on whether to allow a promising multiple sclerosis drug back on the market, according to documents released Monday.

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If Tysabri (pronounced ty-SAH-bree) returns to the market, its use should be restricted and monitored to curtail the risk patients run of developing progressive multifocal leukoencephalopathy, or PML, and other opportunistic infections, according to Food and Drug Administration officials.

The pharmaceutical companies Biogen Idec Inc. and Elan Corp. PLC voluntarily withdrew Tysabri in February 2005 after three patients developed PML, which is rare and often fatal. Two of the patients died.

Now, the two companies want permission to start selling the drug again, saying its benefits outweigh its risks. They have proposed a risk-management plan to guard against further cases of the brain disease.

FDA officials said such a program should be mandatory. But they remain skeptical about which patients could safely use the drug: “Primarily because of the risk of PML, which is not well-quantified, it is unclear for which patients the risk-benefit profile would be acceptable,” FDA staff wrote in briefing documents released ahead of a meeting of the agency’s Peripheral and Central Nervous System Drugs advisory committee.

The outside panel is to spend Tuesday and Wednesday discussing whether to recommend the FDA allow Tysabri back on the market. The FDA typically follows the advice of its advisory committees.

One concern of FDA officials is the “absence of tools with documented effectiveness to manage” the risks associated with use of the drug.

The tools, which include regular neurological examinations, MRI scans and the testing of patient spinal fluid and blood serum, might not be useful in monitoring for the presence of the JC virus that is believed to cause PML, FDA staff wrote.

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