Costa GD, et al. - In 2 European cohorts of 312 patients with multiple sclerosis (MS), researchers analyzed serum neurofilament light chains (NfL) to determine whether NfL is a biomarker of progressive multifocal leukoencephalopathy (PML) during treatment with natalizumab. The cohort included 25 patients with PML, 136 patients treated with natalizumab and 151 untreated patients with MS. Investigators found that patients who subsequently developed PML had NfL similar to other MS patients treated with natalizumab. At the onset of PML, NfL were 10-fold higher than in the pre-PML condition and in MS patients treated with natalizumab or untreated, and NfL continued to increase until the onset of immune reconstitution inflammatory syndrome. Findings suggested that NfL could be an early and accessible marker of PML in patients treated with natalizumab.