Just one week after Biogen (BIIB) reported its ninth case of PML (progressive multifocal leukoencephalopathy) in a patient taking Tysabri, BIIB has announced a 10th case. This is the 13th case of PML in all, the third this month and the 10th since its re-introduction in 2006. In all, three patients have died of PML since Tysabri’s introduction to the market in 2005.

Shares of BIIB fell $2.75 (5.5%) to close at $47.32. BIIB said the patient lives overseas and had been taking Tysabri for more than 30 months.

Deutsche bank also restated its rating on BIIB, to “hold” from “buy” over the weekend and now places the risk of PML infection with Tysabri at 1:2,000 vs. past estimates of 1:4,200.

To potentially avoid PML, some MS docs have started putting patients on Tysabri holidays, but that has decreased sales of the drug. According to the WSJ, neurologist John Richert with the National Multiple Sclerosis Society did not seem swayed by the infection risk, however:

“There are a number of neurologists who have started taking people off Tysabri for periods of time after they’ve been on it for a couple of years. (But) when the odds that the person with MS is going to have significant disability, the 1 in 1,000 risk of PML, which was estimated early on, seems like a reasonable risk for many people.”

Stay tuned…

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