Sanofi (SNY) Stock Plunges: MS Drug Trial Failure Sends Shares Reeling

Another clinical trial bites the dust. Sanofi's stock just took a nosedive after its multiple sclerosis hopeful hit a brick wall in late-stage testing.

Bad News for the Pipeline

The study flopped. No gentle letdown here—it failed to meet its primary goal, sending a clear signal to investors: back to the drawing board. This wasn't a minor setback; it was a full-stop failure for a key asset in the company's neurological portfolio.

Market Reaction: Instant Sell-Off

Traders didn't wait for the fine print. Shares of SNY got hammered the moment the news hit the wires. It's the classic pharma tango—billions in potential revenue vanish with a single data readout, proving once again that in biotech, the pipeline giveth and the pipeline taketh away. A perfect reminder that Wall Street's love is conditional and entirely data-dependent.

What's Next for Sanofi?

The company now faces a pipeline gap it needs to plug, and fast. Investors will demand a new game plan. While Sanofi has other irons in the fire, this failure forces a recalibration—and likely a few cynical whispers about whether big pharma R&D is just glorified gambling with fancier PowerPoints.

TLDR

• Sanofi’s PERSEUS phase 3 study for tolebrutinib in primary progressive MS failed to meet its primary endpoint
• The company will not pursue regulatory registration for PPMS, which affects 10% of MS patients
• Tolebrutinib received an FDA delay alongside the failed trial results, creating dual setbacks for the drug
• Sanofi shares have declined 15% year-to-date following the news
• The company maintains its 2025 financial guidance and continues pursuing approval for non-relapsing secondary progressive MS

Sanofi announced on December 15, 2025, that its experimental multiple sclerosis drug tolebrutinib failed a key clinical trial. The PERSEUS phase 3 study did not meet its primary goal of delaying disability progression in patients with primary progressive MS.

Sanofi, SNY

The setback represents a double blow for the French pharmaceutical company. Tolebrutinib also faced a delay in its FDA review process around the same time.

Primary progressive MS affects about 10% of all multiple sclerosis patients. These patients experience a slow, steady decline in neurological function without periods of improvement.

Sanofi’s experimental MS drug has been hit with two setbacks: a regulatory delay in the US as well as a failure in a late-stage clinical trial https://t.co/KUsi2lgQit

— Bloomberg (@business) December 15, 2025

The trial tested whether tolebrutinib could delay the time to 6-month composite confirmed disability progression compared to placebo. Results showed it could not.

Financial Impact and Stock Performance

Sanofi shares have dropped 15% year-to-date following the news. The company’s stock has faced pressure as investors digest the clinical disappointment.

Despite the trial failure, Sanofi said the outcome will not affect its main profit measures. The company confirmed there is no change to its financial guidance for 2025.

Sanofi will conduct an impairment test on the intangible asset value attached to tolebrutinib. The company plans to provide a status update with its Q4 and full-year 2025 results in January 2026.

The impairment test will have no impact on business net income or business earnings per share. This accounting adjustment follows standard procedures under international financial reporting standards.

Executive Vice President Houman Ashrafian expressed disappointment with the results. He said the data will still help improve understanding of multiple sclerosis disease biology.

Safety Profile and Continued Development

The safety profile of tolebrutinib in the study was consistent with previous trials. Drug-induced liver injury remains an identified risk that requires strict monitoring.

Sanofi emphasized the importance of liver monitoring requirements and prompt management of elevated liver enzymes. These measures help reduce the risk of liver injury.

Based on the trial results, Sanofi will not pursue regulatory registration for primary progressive MS. The company is abandoning development for this particular patient population.

However, Sanofi remains committed to developing tolebrutinib for non-relapsing secondary progressive MS. This condition affects patients who previously had relapsing MS but now experience steady disability accumulation without relapses.

Tolebrutinib received provisional approval in the United Arab Emirates in July 2025 for non-relapsing secondary progressive MS. The drug is currently under regulatory review in the European Union and other jurisdictions worldwide.

The FDA granted tolebrutinib breakthrough therapy designation in December 2024. This designation is meant to speed development of drugs that show promise for serious conditions.

Full safety and efficacy results from the PERSEUS study will be presented at an upcoming medical meeting. The company thanked study participants, families, and healthcare professionals for their involvement in the research.