Unconfirmed PML Cases Dent Biogen, Elan

Senior writer Adam Feuerstein contributed to the reporting of this article.

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Shares of Biogen Idec (BIIB Quote) and Elan(ELN Quote) sank deeper Friday on investor fears that additional cases of a serious brain infection in patients taking multiple sclerosis drug Tysabri may surface in the near future.

On a conference call early Friday to discuss the two confirmed cases of progressive multifocal leukoencephalopathy (PML) tied to Tysabri treatment, Biogen Idec officials refused to disclose or discuss the number of Tysabri patients overall suspected to be afflicted with the serious and potentially fatal brain infection.

Biogen shares were sinking $18.16, or 26%, to $51.60, while Elan shares were plummeting $8.89, or 44%, to $11.16, in recent trading Friday.

There are currently 12 suspected cases of PML involving Tysabri patients detailed in the U.S. Food and Drug Administration's Adverse Event Reporting System (AERS) database.

Copies of this database information noting the suspected PML cases have circulated for weeks among some institutional investors. When Biogen reported no new PML cases on its July 22 earnings conference call, however, it appeared as if these suspected incidences of PML were false alarms.

The two confirmed PML cases announced Thursday have reignited fears that some of these other possible PML cases might also turn into the harmful disease. The fact that Biogen won't say how many suspected cases of PML exist or how many have turned out to be negative creates uncertainty as to the real incident rate of PML in Tysabri patients.

That unknown is weighing on shares of Biogen Idec and Elan.