Shares of French pharmaceutical giant Sanofi tumbled sharply on Monday, December 15, 2025, following a dual setback for its experimental multiple sclerosis drug, tolebrutinib. The company announced both a further delay in the U.S. regulatory review process and disappointing results from a key late-stage clinical trial.
The stock dropped as much as 5% in early Paris trading, making it the worst performer on the SBF 120 index, before paring losses to trade about 4% lower. This decline contributes to a year-to-date drop of approximately 15% for Sanofi shares as investors digest the clinical and regulatory hurdles.
The U.S. Food and Drug Administration (FDA) has pushed back its decision on tolebrutinib for non-relapsing secondary progressive multiple sclerosis (nrSPMS) beyond the target action date of December 28, 2025. This marks the second delay, following an initial postponement from September 2025. Sanofi now expects further guidance from the FDA by the end of the first quarter of 2026. The drug had previously received breakthrough therapy designation from the FDA in December 2024.
Compounding the regulatory delay, Sanofi disclosed that the PERSEUS Phase 3 trial for primary progressive multiple sclerosis (PPMS) failed to meet its primary endpoint. The study showed tolebrutinib did not significantly slow disability progression in this form of the disease, which accounts for roughly 10% of MS cases. As a result, Sanofi stated it will not pursue regulatory registration for PPMS and will assess whether to book an impairment charge on the asset's value.
Houman Ashrafian, Sanofi's Head of Research and Development, expressed disappointment but noted the results would improve the understanding of MS disease biology. Despite the PPMS failure, Sanofi remains confident in tolebrutinib's potential for nrSPMS. The company confirmed its 2025 financial guidance remains unchanged, and any potential impairment test would not affect core business net profit.
The drug has received provisional approval in the United Arab Emirates and is under regulatory review in the European Union. The full safety and efficacy results from the failed PERSEUS study are expected to be presented at an upcoming medical meeting.
