Sanofi’s tolebrutinib falls short in Phase III for primary progressive MS

Sanofi disclosed topline results from the PERSEUS Phase III study evaluating tolebrutinib in primary progressive multiple sclerosis (PPMS), revealing that the investigational Bruton’s tyrosine kinase (BTK) inhibitor did not meet its primary endpoint of delaying the time to 6-month composite confirmed disability progression (cCDP) compared with placebo. Based on these findings, the company will not pursue regulatory approval for tolebrutinib in PPMS, which accounts for roughly 10% of the MS population.

Sanofi emphasized its ongoing commitment to MS research and innovation, despite the disappointing PPMS outcome. The company reiterated confidence in tolebrutinib’s potential in non-relapsing secondary progressive multiple sclerosis (nrSPMS), where the agent is currently under review by regulators and received a breakthrough therapy designation from the US FDA in December 2024. Tolebrutinib was also provisionally approved in July 2025 in the United Arab Emirates for nrSPMS and remains under review in the EU and other jurisdictions.

The PERSEUS results mark a setback for expanding tolebrutinib’s label across MS subtypes, particularly in a form of the disease with limited effective treatments. However, Sanofi’s broader MS strategy retains focus on unmet needs in progressive disease and disability accumulation independent of relapse activity.

Sanofi will conduct an impairment test on the intangible asset value associated with tolebrutinib, with results to be disclosed in its Q4 and full-year 2025 financial reports. The company noted that this assessment will not impact business net income or EPS guidance for 2025.

What PERSEUS setback means for MS landscape

Sanofi’s failure to meet the primary endpoint in the PERSEUS Phase III trial of tolebrutinib in primary progressive multiple sclerosis (PPMS) underscores the persistent difficulty of demonstrating disease-modifying efficacy in progressive MS.

PPMS represents around 10% of the MS population and is characterized by steady disability accumulation with limited overt inflammatory activity. This biology has historically limited the effectiveness of therapies developed for relapsing disease.

To date, ocrelizumab remains the only approved disease-modifying therapy for PPMS, based on modest but statistically significant slowing of disability progression. Even so, treatment effects are most evident in patients with residual inflammatory activity, leaving a large proportion of PPMS patients without meaningful benefit.

Against this backdrop, PERSEUS joins a long list of late-stage failures in PPMS, including agents targeting broad immunosuppression, neuroprotection, anti-fibrotic and anti-oxidative pathways.

Unlike earlier PPMS programs, tolebrutinib was designed as a brain-penetrant BTK inhibitor, aiming to suppress chronic, compartmentalized CNS inflammation driven by microglia and B cells. This mechanism was widely viewed as a rational next step beyond peripheral immune modulation.

However, the PERSEUS outcome suggests that:

• Targeting innate immune signaling alone may be insufficient to meaningfully slow disability in established PPMS
• The window for therapeutic intervention may be earlier than most PPMS diagnoses allow
• Composite disability endpoints remain difficult to shift over realistic trial durations

Notably, Roche’s fenebrutinib, another BTK inhibitor, has shown encouraging results in relapsing MS, but has yet to deliver definitive late-stage success in PPMS, reinforcing concerns that mechanistic promise does not readily translate in progressive disease.

Divergence between PPMS and nrSPMS

Sanofi’s strategy now clearly differentiates PPMS from non-relapsing secondary progressive MS (nrSPMS), a distinction regulators have increasingly accepted. In nrSPMS, disability progression often follows a relapsing phase; residual inflammatory biology may be more amenable to intervention; and trial populations can show clearer separation from placebo.

This biological and clinical distinction underpins Sanofi’s continued confidence in tolebrutinib for nrSPMS, even as the PPMS program is discontinued.