Europe’s medicines agency yesterday said it is currently in the process of assessing the two additional reports of the rare brain disease progressive multifocal leukoencephalopathy in multiple sclerosis patients receiving Elan Pharma’s Tysabri (natalizumab).

The two cases reported at the end of July occurred in patients who had been receiving Tysabri for more than 12 months, but renewed old fears for investors and the firm’s share price – along with that of its US partner Biogen Idec – sank on the news. The Ireland-based firm crashed almost 75% - reflecting the sensitivity and importance of the medicine to its future prospects, though it was also impacted by concerns over its Alzheimer’s disease drug bapineuzumab being developed with Wyeth.

The agency has also asked Elan to provide any additional information it may have, at which point its scientific advisory panel will decide whether the medicine’s product labeling needs updating. Tysabri has been under intense scrutiny since it was reintroduced in the USA and approved in Europe two years ago – and these are the first new cases to surface since that time. It was originally withdrawn from the market in 2005 after three patients developed PML, but following its reintroduction has gone on to be a big earner – pulling in some $200 million in the second quarter of this year alone. Almost 32,000 patients were receiving Tysabri as of the end of June.

Most analysts do seem to remain convinced of the value of Tysabri – certainly the firms do not intend to withdraw the drug again, stressing the stringent safety monitoring programme in place.

Icahn takes stake in Biogen on Tysabri woes
And rather than backing off from the stock, maverick investor Carl Icahn has seen the dip in Biogen’s value as an opportunity to snap up some cut-price shares – increasing his stake in the company to 6% from 4.3%. All eyes are on his movements: earlier on this year he criticised the board for not looking hard enough to find a buyer as well as its decision to take down the ‘for sale’ sign in December last year month after no serious offers were made. He could be gearing up to push for another sale.