Assess associations between serum neurofilament light chain (sNfL) levels and step count in a large international cohort of people with progressive multiple sclerosis (MS) who participated in a randomized controlled trial.

A post-hoc analysis of SPI2 (high-dose biotin [MD1003] vs. placebo) was conducted. The two treatment arms were analyzed together because no therapeutic benefit of MD1003 was observed. Participants with paired sNfL (z-scores) and step count data were included. Daily steps were averaged within ±15-day windows around each timepoint. The associations between sNfL z-scores and step count were assessed using bootstrapped (1000x) mixed-effects negative binomial regression models (accounting for multiple measurements/person), adjusting for age and sex.

630 participants contributed 1,985 observations over 15 months. 54% were female, mean 52.7 [SD 7.7] years old, median Expanded Disability Status Scale (EDSS) score: 6 [IQR 4.5-6], and mean sNfL of 15.2 pg/mL (SD: 9.6). The median daily steps were 3,047 (IQR: 1,621–4,982). 10% exceeded 7,200 steps/day, consistent with prior cohorts with comparable disability. Fewer daily steps were significantly associated with higher sNfL: each one-unit sNfL increase corresponded to 3.4% fewer steps (IRR=0.97, 95% CI 0.94–0.99 p=0.014). This association was not impacted by EDSS category (≥5.0 or <5.0) or treatment group (MD1003 or placebo) on the steps outcome (p>0.05).

Reduced ambulatory activity was associated with elevated sNfL levels, linking real-world mobility to molecular markers of neuroaxonal damage. Both measures appeared more sensitive than EDSS in capturing subtle changes in disease status. Results support the integration of fluid and digital biomarkers to enhance sensitivity for detecting progression in clinical trials and routine practice.