US regulators last week cleared Biogen and Elan to resume testing Tysabri in patients for the first time since a link to progressive multifocal leukoencephalopathy, or PML, forced a suspension a year ago. The Food and Drug Administration's action suggests that next month's US government advisory committee meeting may recommend resuming Tysabri sales.

Biogen and Elan introduced Tysabri in November 2004 and about 8,000 patients took the drug. When it was withdrawn in February 2005, after two of three patients who contracted PML died, Biogen and Elan lost a combined $17.8bn in market value in one day.

After Tysabri reduced MS attacks in trials by two-thirds, with few reported side effects, analysts estimated it would generate $3bn in annual sales.

MS is a neurological disorder that robs people of muscle coordination and balance, sometimes leading to damaged vision and paralysis. It affects about 2.5 million people worldwide. Patients want Tysabri back even with the risk of PML, said Howard Rossman, medical director at the Michigan Institute for Neurological Disorders, which cares for about 2,200 MS patients.

About 110 patients at the centre took Tysabri, said Rossman, who was involved in Tysabri trials.

"Their quality of life is so much better with Tysabri that they're willing to take that risk so they can continue to experience what they experienced before."

Biogen is cautioning that patients with impaired immune systems should be advised not to take Tysabri, and that it shouldn't be taken in combination with other MS therapies. Both patients who died of PML were taking Tysabri in combination with Biogen's Avonex, an older MS drug. Biogen now expects the FDA to approve Tysabri with warnings and guidelines.

PML is caused when an infection called the JC virus evades the body's immune defences and penetrates the central nervous system, where it eats away the protective coating of nerve fibres, called myelin, causing irreversible brain damage.

MS is caused when an abnormal immune system response attacks the myelin.

Tysabri, an antibody-based medicine, was designed to prevent that assault by suppressing T-cells, the immune system cells involved in MS.

Researchers theorise that Tysabri may have subdued defences meant to keep the JC virus out of the brain.

Some doctors question returning the drug to market without a test to predict a patient's risk for PML. Those with serious concerns include the drug's inventor, Stanford University neurologist Lawrence Steinman, who says he abandoned his work on the drug in the 1990s because Tysabri's immune-altering mechanism might lead to dangerous infections.

"The drug is dangerous, and there is no way to effectively screen patients before they have the feared clinical complication of PML, " he said.

"The risks far outweigh the benefits."