To assess the relevance of the cuprizone-intoxication model for studying the pro-remyelination effects of siponimod.

Recent clinical observations, using magnetic resonance imaging (MRI) to assess the magnetization transfer ratio (MTR) and T2-weighted signal intensity (T2-WSI), revealed pro-myelination effects for siponimod, the first oral disease-modifying therapy shown to reduce disability progression, cognitive decline, and total brain volume loss versus placebo in SPMS patients. Hence, siponimod appears to be an ideal pharmacological tool to validate/invalidate preclinical mechanistic models for the translational study of remyelination processes.

Typically, cuprizone treatment in mice (5 weeks; 0.2% in diet) produces robust signs of demyelination in the brain corpus callosum (CC) and external capsule (EC), visualized by MRI (MTR and T2-WSI). Upon stopping cuprizone treatment, slow spontaneous remyelination can be observed by quantitative immunohistochemistry (qIHC) for myelin density (Luxol Fast Blue staining [LFB]) and oligodendrocyte numbers (GST-π⁺ cells) in brain sections. This study compared the impacts of siponimod treatment (via drug-loaded diet [10 mg/kg]) with a control diet.

Cuprizone-intoxication resulted in a marked demyelination within the CC/EC, indicated by a 24% increase in mean T2-WSI and 5–7% reduction in mean MTR. Within 2 weeks of cuprizone washout, control mice showed a small but significant reduction (8%) in T2-WSI, not associated with MTR changes. This indicated a weak spontaneous remyelination, confirmed by qIHC.

In siponimod-treated mice, drug levels reached in blood and brain were within the expected range (0.4 and 2.5 µM, respectively) and the level of spontaneous remyelination over 2 weeks, measured by qIHC, was significantly increased (by 26% and 16%, respectively) versus controls, although no significant impact on MRI parameters could be detected.

The mouse cuprizone-intoxication model appears to be a relevant mechanistic model for exploring siponimod-sensitive remyelination processes, with monitoring periods longer than 2 weeks needed for visualization via imaging techniques.