Risk of Biologic Therapy-Associated Progressive Multifocal Leukoencephalopathy: Use of the JC Virus Antibody Assay in the Treatment of Moderate-to-Severe Crohn's Disease.

For treatment of moderate-to-severe active Crohn's disease, clinicians generally rely on immunosuppressants (including azathioprine and 6-mercaptopurine), corticosteroids, and antibodies against tumor necrosis factor α. However, a significant proportion of patients do not respond to these therapies, lose response over time, or are intolerant to these therapies. In such cases, one of the only remaining pharmacologic treatment options is natalizumab, an α4 integrin-targeted antibody. Unfortunately, 3 cases of progressive multifocal leukoencephalopathy (PML) were reported in natalizumab-treated patients in 2005, shortly after natalizumab's approval by the US Food and Drug Administration (FDA). Natalizumab was subsequently withdrawn from the market but was then reintroduced in 2006 under close supervision by the FDA. Careful review of postmarketing data revealed 3 major risk factors for the development of natalizumab-associated PML, the most significant of which is prior exposure to the JC virus (JCV). To help identify patients who may be at higher risk for developing natalizumab-associated PML, a JCV antibody assay was developed that can detect anti-JCV antibodies in patients' blood. Clinicians can now consider a patient's anti-JCV antibody status together with the other major risk factors for natalizumab-associated PML-duration of natalizumab therapy and prior immunosuppressant use-to more accurately gauge the risks and benefits of natalizumab therapy in a particular patient.