Sanofi's Multiple Sclerosis Drug Faces Regulatory Hurdle, Stock Barely Budges

Sanofi (SNY) received disappointing news from U.S. regulators regarding tolebrutinib, its experimental oral treatment for non-relapsing secondary progressive multiple sclerosis. The FDA issued a complete response letter, signaling the agency will not approve the drug at this stage, pushing the regulatory timeline into 2026.

The stock closed at $48.32 on December 23, 2025, inching up just 0.60% despite the setback. The muted market reaction suggests investors may have already priced in potential headwinds for the tolebrutinib program.

What Tolebrutinib Does

Tolebrutinib is an oral medication designed to cross the blood-brain barrier—a key advantage for treating neurological conditions. It works by targeting Bruton’s tyrosine kinase, an enzyme involved in neuroinflammation. In multiple sclerosis relapse scenarios, controlling this inflammation is critical to slowing disability progression, particularly in the non-relapsing secondary progressive form where disease activity can occur without obvious relapses.

The company had originally expected FDA guidance by December 28, 2025, but that timeline has now extended into early 2026. Sanofi submitted an expanded access protocol in response to FDA feedback, keeping dialogue open with regulators.

Financial and Regulatory Path Forward

Sanofi announced it would conduct an impairment test on tolebrutinib’s intangible asset value under IFRS standards, with results coming in January 2026 alongside fourth-quarter earnings. However, the company confirmed this assessment will not impact business net income, business earnings per share, or its 2025 financial guidance.

On a positive note, tolebrutinib already secured provisional approval in the United Arab Emirates in July 2025 for treating non-relapsing secondary progressive multiple sclerosis and slowing disability accumulation independent of relapse activity. The drug is also under review across Europe and other international markets.

Houman Ashrafian, Executive Vice President of Research & Development at Sanofi, expressed disappointment with the FDA’s decision, emphasizing the company’s commitment to working with regulators to find a resolution and ultimately serve the multiple sclerosis community.