Serum biomarkers remain stable after transitioning from intravenous to subcutaneous natalizumab in multiple sclerosis

Background: Subcutaneous natalizumab offers greater convenience than intravenous administration, but pharmacokinetic differences have raised concerns about potential subclinical disease activity. Serum neurofilament light chain (sNfL) and glial fibrillary acidic protein (GFAP) are sensitive biomarkers of neuroaxonal damage and astroglial activation. Methods: In this prospective, single-center cohort study, consecutive patients with relapsing–remitting multiple sclerosis who transitioned from intravenous to subcutaneous natalizumab were followed for 12 months. Serum sNfL and GFAP were measured at baseline (prior to switch), and at 6 and 12 months using SIMOA technology. Additional clinical outcomes included annualized relapse rate (ARR), EDSS, and MRI activity. Results: 23 patients were included (mean age 43.7 years; 91% female). Median disease duration was 14 years (IQR 8.1–22), median time on natalizumab was 5 years (IQR 3.3–7.9), baseline 2-year ARR was 0.09 ± 0.2, and median EDSS was 2.0 (IQR 2.0–3.5). Median sNfL Z-scores were 0.2 (IQR − 0.3–0.6) at baseline, 0.1 (IQR − 0.4–1.0) at 6 months, and 0.5 (IQR − 0.4–1.1) at 12 months, with no significant change over time (p = 0.401). GFAP levels were similarly stable (87.7, 86.8, and 90.2 pg/mL; p = 0.957). ARR remained low and unchanged (0.09 pre- and post-switch; p = 1.0), with no radiological activity observed. EDSS remained stable over follow-up. Conclusions: In this small real-world cohort, switching from intravenous to subcutaneous natalizumab was associated with stable sNfL and GFAP levels over 12 months, alongside stable conventional clinical and MRI outcomes. These findings provide supplementary biomarker evidence broadly consistent with existing clinical trial and real-world data, but should be interpreted cautiously given the small sample size.