Unconfirmed PML Cases Dent Biogen, Elan

Biogen spokeswoman Naomi Aoki declined to provide any more specifics about the suspected PML cases, stating that a "vast majority of suspected cases are not PML but are part of the [effects of] multiple sclerosis. But we don't want to get into the whole business of discussing suspected cases."

The two cases of Tysabri-related PML, disclosed in a regulatory filing Thursday, are the first confirmed reports since approval of the multiple sclerosis drug in the EU and the re-entry of the drug into the U.S. market two years ago.

The patients were on Tysabri for 17 months and 14 months, respectively. One is clinically stable and ambulatory at home and the other is hospitalized.

PML is caused by a virus that damages the white matter of the brain. In healthy people, the virus lays dormant and isn't dangerous, but it can be reactivated in people with weakened immune systems.

Tysabri was previously removed from the market after being linked to three cases of PML. The drug was subsequently relaunched with a label that warns doctors and patients about the risk of PML.

"We've said in the past, and the FDA has also said, as is clearly stated in our label that we've anticipated seeing additional cases of PML," Biogen spokeswoman Aoki said Thursday night. The label currently indicates that the expected rate is 1 occurrence in 1,000 patients, which was calculated at the time of the drug's reintroduction to the market.

According a survey conducted in October 2007, physicians indicated that they may reduce Tysabri usage by 26% in the instance of a PML case, according to Citi analyst Yaron Werber, who has a hold rating on the stock. "We thus expect slowdown in Tysabri weekly growth rate and possibly scaling back in usage. However, we believe the damage will really depend if this is the first two cases of many to come and what the true incidence will be."