Risk of natalizumab-associated progressive multifocal leukoencephalopathy in patients with multiple sclerosis: a retrospective analysis of data from four clinical studies.

Background: Previous estimates of risk of progressive multifocal leukoencephalopathy (PML) in patients with multiple sclerosis receiving natalizumab were stratified by three risk factors: anti-John Cunningham virus (JCV) antibodies in serum, previous immunosuppressant use, and treatment duration, which were estimated using population-based assumptions. We aimed to calculate PML risk estimates from patient-level risk-factor data and to stratify risk by concentrations of anti-JCV antibody in serum (anti-JCV antibody index).

Methods: Data on natalizumab-treated patients were pooled from four large, observational, open-label studies: STRATIFY-2, STRATA, TOP, and TYGRIS.