The more infusions of natalizumab a patient receives, the greater the risk of developing progressive multifocal leukoencephalopathy (PML), according to a US Food and Drug Administration (FDA) advisory.
Natalizumab is approved by the FDA for the treatment of multiple sclerosis and Crohn disease. PML is a rare and deadly neurological infection caused by the JC virus, a nearly ubiquitous and usually benign virus that typically affects immunosuppressed individuals. The disorder is also associated with use of natalizumab, and the FDA has received 31 reports of such cases. Because of the risks, natalizumab is currently only available for use through a carefully monitored prescribing program.
Bridget M. Kuehn. Natalizumab Risks. JAMA. 2010;303(12):1139. doi:10.1001/jama.2010.301