Abstract
Interest in amyotrophic lateral sclerosis (ALS) biomarkers has grown exponentially over the course of the last 25 years, with great hope that they might serve as tools to facilitate the development of meaningful therapies for this otherwise inexorably progressive and invariably fatal disease. Effective use of biomarkers, however, requires an understanding of what it means for them to be 'fit-for-purpose' as well as an appreciation of the nuances of the clinical context(s) in which they will be applied. Neurofilament light chain (NfL) has emerged as a leading candidate with enormous potential to aid ALS therapy development; it is, however, also profoundly misunderstood. Within the conceptual framework of the BEST (Biomarkers, EndpointS, and other Tools) Resource, developed by the National Institutes of Health and the Food and Drug Administration in the USA, we consider the evidence supporting the use of NfL for a variety of purposes in different clinical contexts. We conclude that: (i) it may serve as a susceptibility/risk biomarker in populations at elevated risk for ALS; (ii) it has value as a prognostic biomarker when measured early in the course of established disease, empowering stratification or dynamic randomization to amplify the signal-to-noise ratio of promising therapeutics; and (iii) there is sufficient evidence to support the use of a reduction in NfL in response to an experimental therapeutic as a pharmacodynamic biomarker that may aid in phase 2 trial go/no-go decisions. Moreover, the basis for expecting that a reduction in NfL is a reasonably likely surrogate end-point (i.e. reasonably likely to predict clinical benefit-which may be more than simply survival) is nuanced, and depends on when in the course of disease the experimental therapeutic is administered.