Sanofi SA (SNY) delivered some disappointing news Monday about tolebrutinib, its experimental multiple sclerosis treatment that's been running into obstacles on multiple fronts.
The pharmaceutical giant announced results from its PERSEUS Phase 3 study, which tested tolebrutinib against placebo in patients with primary progressive multiple sclerosis. This is the particularly nasty version of MS where things just get steadily worse from day one, no breaks, no remissions, just relentless neurological decline that typically hammers mobility.
To understand what's at stake here: Multiple sclerosis is an autoimmune condition where your body attacks myelin, the protective coating around nerve cells in your brain and spinal cord. Primary progressive MS (PPMS) represents about 10% of all MS cases, but it's among the most severe forms because there's no reprieve from the worsening symptoms.
The Trial Results
In the PERSEUS study, participants were randomly assigned in a 2:1 ratio to receive either a daily oral dose of tolebrutinib or placebo for approximately 60 months. The trial was looking to see if the drug could delay what's called "6-month composite confirmed disability progression," essentially whether it could slow down how quickly patients lose function.
It didn't work. The trial missed its primary endpoint, and Sanofi is now scrapping plans to seek regulatory approval for tolebrutinib in primary progressive MS. The preliminary safety analysis was at least consistent with previous studies, so no new surprises there.
But here's an important caveat: drug-induced liver injury (DILI) is a known risk with tolebrutinib. That means patients need strict liver monitoring and quick action if enzyme levels start climbing. It's manageable, but it requires vigilance.
Where the Drug Still Has a Shot
Tolebrutinib isn't completely out of the game. The drug received provisional approval in the United Arab Emirates back in July 2025 for non-relapsing secondary progressive multiple sclerosis (nrSPMS), which is a different flavor of MS. Think of nrSPMS as the gradual decline version without the dramatic flare-ups, where damage and disability accumulate steadily over time.
The drug is currently under regulatory review in the European Union and other markets worldwide. The FDA even granted it breakthrough therapy designation in December 2024, which typically signals the agency sees meaningful promise.
FDA Review Hits a Speed Bump
Speaking of the FDA, Sanofi announced that the U.S. regulatory review for tolebrutinib in non-relapsing secondary progressive MS is taking longer than expected. The target action date was December 28, 2025, but that's not happening anymore. The company expects to hear more from the FDA by the end of the first quarter of 2026.
In response to an FDA request, Sanofi has submitted an expanded access protocol for tolebrutinib in nrSPMS, which would allow certain patients to access the drug before full approval.
Financial Implications
The French pharmaceutical company will conduct an impairment test on the intangible asset value attached to tolebrutinib. Translation: they need to figure out if the drug's value on their books needs to be written down given these setbacks. Results of that assessment will come in January 2026 when they report fourth quarter and full-year 2025 results.
The good news for investors is that whatever write-down might occur won't affect business net income or business EPS, and Sanofi isn't changing its 2025 financial guidance.
SNY Price Action: Sanofi shares slipped 1.77% to $47.82 during premarket trading on Monday.