"The main reason why our [psoriasis] patient developed PML after three years of treatment with Fumaderm is prolonged lymphocytopenia [low white blood cell counts]," Schulz said. "In both BG-12 trials published in the New England Journal of Medicine in 2012, 4 percent to 5 percent of patients developed this kind of severe lymphocytopenia and are in my view at risk to also develop PML."
Another expert noted another caveat.
About 3 percent of patients in the Tecfidera trials were taken off the medication when their white blood cell counts dropped too low, said Dr. Robert Fox, a staff neurologist at the Mellen Center for Multiple Sclerosis at the Cleveland Clinic in Ohio.
"The intensive monitoring of the clinical trial may have been what helped to prevent PML," said Fox, who was on the steering committee for one of the studies that tested the medication and who has been a paid consultant for Biogen.
Fox added that the PML case reports are "very important additions to our understanding of this class of therapy."
"Although we haven't seen any PML cases in patients treated with Tecfidera, I think there's good reason to apply the lessons learned here to Tecfidera," he said.
For more on the risks and benefits of multiple sclerosis treatments, visit the National Multiple Sclerosis Society.
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