Sanofi Shares Slip After Tolebrutinib Misses Key Milestone in MS Trial

Sanofi SA (NYSE: SNY) shares took a hit on Monday following disappointing results from its Phase 3 PERSEUS trial. The company announced that its experimental drug, tolebrutinib, did not achieve the main goal of delaying confirmed disability progression over six months in patients with primary progressive multiple sclerosis (PPMS).The trial, which targeted PPMS-a form that accounts for roughly 10% of MS cases-showed no significant improvement over placebo. This outcome caused the stock to drop by 4.4% during early trading hours in Paris. The setback led Sanofi to scrap plans for seeking regulatory approval of tolebrutinib specifically for PPMS.Despite the results, Sanofi's Executive Vice President of R&D, Houman Ashrafian, expressed hope that these findings could shed more light on the underlying biology of MS. The safety profile of tolebrutinib remained consistent with earlier studies, though liver injury risks will continue to require vigilant monitoring.Not all hopes for tolebrutinib are dashed, however. The drug already gained provisional approval in the United Arab Emirates in July 2025 for treating non-relapsing secondary progressive multiple sclerosis (nrSPMS). It is still under review in the European Union and other regions, and it carries a breakthrough therapy designation from the U.S. Food and Drug Administration (FDA).Sanofi updated that the FDA's decision timeline for tolebrutinib's approval in nrSPMS is now likely to slip beyond the original December 28, 2025 target, with new guidance expected by the end of Q1 2026. The company has also submitted an expanded access protocol, possibly signaling a degree of FDA comfort with the drug's risk-benefit balance.Jefferies analysts highlighted that while the PPMS trial failure is a setback, the larger commercial opportunity lies with nrSPMS. They noted the FDA's extended access request may reflect regulatory caution but not outright disapproval, suggesting it may be premature to write off tolebrutinib entirely.Financially, Sanofi plans to conduct an impairment test on the intangible asset value of tolebrutinib and will disclose findings alongside its Q4 and full-year 2025 earnings in January. The company confirmed that this won't affect its net income or earnings per share forecasts for the year, and the broader guidance remains unchanged.The PERSEUS trial ran globally as a double-blind study, comparing daily oral doses of tolebrutinib against placebo over roughly five years. Tolebrutinib is a brain-penetrant Bruton's tyrosine kinase inhibitor designed to tackle neuroinflammation, a key factor in MS-related disability progression.This mixed bag of clinical results puts Sanofi in an intriguing spot as it balances ongoing regulatory hurdles and market expectations around tolebrutinib's future.