Glioblastoma in natalizumab-treated multiple sclerosis patients.

We present two natalizumab-treated multiple sclerosis patients who developed glioblastoma multiforme (GBM) with variable outcomes. One patient had an isocitrate dehydrogenase (IDH)-wildtype GBM with aggressive behavior, who declined treatment and died 13 weeks after symptoms onset. The other patient underwent resection of an IDH-mutant secondary GBM that arose from a previously diagnosed grade II astrocytoma. He is still alive 5 years after the diagnosis of GBM. JC virus was not detected in either case. Whether natalizumab played a role in the development of GBM in those patients deserves further investigation.