Intrathecal antisense oligonucleotides (ASOs) have revolutionized the management of genetic motor neuron diseases. Nusinersen is approved for spinal muscular atrophy (SMA) caused by SMN1 mutations, and tofersen for amyotrophic lateral sclerosis (ALS) linked to SOD1 mutations. Since their approval, some studies reported the presence of macrophagic inclusions in cerebrospinal fluid (CSF) of patients treated with ASOs, first in nusinersen-treated patients and more recently in those receiving tofersen. These findings remain poorly characterized, and their clinical significance is unclear.

We first conducted a retrospective study in 21 patients (132 CSF samples): six treated with tofersen (every 4 weeks) and 15 with nusinersen (every 4 months). CSF samples were analyzed for macrophagic inclusions, their time of onset, and persistence over time. To assess clinical and inflammatory correlates of macrophagic inclusions, we then performed an analysis of CSF inflammatory biomarkers and serum ferritin and neurofilament light chain tests in 18 of these patients still under treatment.

In tofersen-treated patients, macrophagic inclusions were consistently observed and persisted over time, except in one case. In nusinersen-treated patients, inclusions were rare and transient. An inflammatory CSF profile was associated with the presence of inclusions, but their cellular nature remained undetermined. Notably, tofersen-treated patients with “tofersenophages” exhibited favorable clinical responses.

Macrophagic inclusions appear more frequent in the CSF of tofersen-treated patients than previously reported. While their origin remains unclear, they seem linked to CSF inflammation without precluding a beneficial therapeutic response.