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Interruption of natalizumab therapy for multiple sclerosis

Thomas Huxley, the great 19th century British biologist, quipped that “many a beautiful theory was killed by an ugly fact.” In late 2004, the Food and Drug Administration approved natalizumab for relapsing-remitting multiple sclerosis (RRMS), a “magic bullet” that seemed to arrest multiple sclerosis (MS) disease activity. That beautiful idea died a few months later with the discovery of an ugly fact: natalizumab was associated with a risk of progressive multifocal leukoencephalopathy (PML). After removal from the market in early 2005, natalizumab was rereleased in 2006 to a cautious community of neurologists and patients.

Since the rerelease of natalizumab for the treatment of RRMS, neurologists have struggled with some fundamental questions. Which patients are most appropriately treated with natalizumab? How long should treatment continue? Does the disease rebound when the medication is stopped? Is a drug holiday effective at reducing the risk of PML? How can we assess an individual patient's risk of developing PML and mitigate that risk?

In this issue of Neurology ®, O'Connor et al.1 provide information on the kinetics of the return of MS disease activity after natalizumab cessation. When the risk of PML was discovered in 2005, study participants in natalizumab trials and open-label extension phases were followed for safety after the medicine was discontinued. …