European regulators yesterday said they were examining 23 cases of patients who developed potentially fatal brain infections after taking multiple sclerosis drug Tysabri manufactured by Biogen Idec Inc, a global leader in innovative therapies, and Elan Corp, an Irish drug maker that markets Tysabri in Europe.

The European Medicines Agency's Committee for Medicinal Products for Human Use, known as CHMP, reported there have been a higher number of progressive multifocal leukoencephalopathy, or PML, cases linked to Tysabri (natalizumab).

The agency, which regulates drugs in Europe, said it will review the risks and benefits of Tysabri and discuss whether added measures are needed to assure safety.

The move sent the stock of Biogen Idec, based in Cambridge, down $3.42, or 7.2 per cent, to $43.81 on the Nasdaq exchange. Shares of Elan plunged $1.13, or 17.5 per cent, to $5.31 on the New York Stock Exchange.

Biogen Idec spokeswoman Jennifer Neiman said the incidents of the brain infection, progressive multifocal leukoencephalopathy, remain extremely rare - well within the range of one in 1,000 cases noted on the drug's label.

''Tysabri is a very compelling option for the treatment of multiple sclerosis,'' Neiman said, ''and overall, the benefit-risk profile for the product continues to be very favorable.''

She said Tysabri is the only MS therapy that has been shown to stop - and in some cases reverse - the progression of the disease.

Al Sandrock, senior vice president and head of neurology research and development at Biogen Idec, said the company was discussing with global regulators a change to the wording on Tysabri's label that would reflect an increased risk the longer patients took the drug.
After all, Tysabri's current label states a risk for PML that occurs in an estimated 1 in 1,000 patients.

But heightened monitoring of the drug could slow the rate of new patient accruals in Europe, where it has been growing faster than in the US.
That could lead to less aggressive treatment, particularly in Germany, the second-largest Tysabri market.

During its earnings call last week, Biogen reported that the number of new patients on Tysabri slowed during the third quarter, averaging 230 additions per week vs. 260 per week in the second quarter, with the biggest slowdown in Europe.

As on 30 September, a total of 46,200 patients were on Tysabri treatment, with about 23,000 of those on the drug for 18 months or longer.

Biogen originally hoped to have 100,000 patients on therapy by the end of 2010 but acknowledged, even before the latest PML count, that the goal likely was unrealistic.

The drug, which was first approved by the FDA in 2004, was pulled off the market in 2005 after it was linked to PML, an often-fatal brain infection caused by activation of the JC virus in patients who are immune-suppressed.

The FDA issued a rare second approval in 2006 - along with a strict risk-management plan.

Tysabri sales for the third quarter totaled $207 million, up 21 per cent over the same quarter last year.