Sanofi secures European backing for its $3.7 billion MS drug as transatlantic regulators split on liver toxicity

The European Medicines Agency has recommended approval for Sanofi’s tolebrutinib, establishing the first regulatory foothold for a class of brain-penetrant autoimmune drugs that American regulators recently deemed too toxic to clear. The decision creates a sharp transatlantic divergence on how to weight the risk of liver injury against the near-total lack of treatment options for non-relapsing secondary progressive multiple sclerosis.

Tolebrutinib, which Sanofi intends to market as Cenrifki, belongs to a newer generation of Bruton’s tyrosine kinase (BTK) inhibitors. Unlike earlier oncological BTK blockers, these molecules are engineered to cross the blood-brain barrier. The mechanism was theorized to halt the smoldering neuroinflammation characteristic of advanced MS. But the entire class has struggled in late-stage testing; a rival compound from Merck KGaA failed outright, and Sanofi’s own drug stumbled in two trials for relapsing disease before showing a statistically meaningful delay in disability progression for the secondary progressive cohort.

The EMA’s positive opinion salvages a portion of the $3.7 billion Sanofi spent acquiring Principia Biopharma in 2020. The European regulator explicitly acknowledged the drug's hepatotoxicityChemical-driven liver damage. In drug development, severe hepatotoxicity is a common cause for clinical trial failures and regulatory rejections. profile—the exact mechanism that prompted the US Food and Drug Administration to issue a complete response letterA formal communication from the FDA indicating that it will not approve a new drug application in its present form, typically detailing specific safety or efficacy deficiencies. in January. Where the FDA cited the potential for severe or fatal drug-induced liver injury as a hard stop, the EMA concluded that the delay in physical disability outweighed the hepatic risks for a patient population that currently has virtually no disease-modifying therapies.

The immediate winner is Sanofi, which now has a path to recoup its Principia outlay, and European MS patients who gain access to the first therapy targeting this specific disease stage. The losers are the harmonized regulatory frameworks that pharmaceutical companies rely on to model global drug rollouts. The FDA’s January rejection, which Sanofi characterized at the time as a sudden shift in agency feedback, now stands in stark contrast to the EMA's risk-tolerance calculus.

What the European decision forecloses is the assumption that the BTK inhibitorA class of drugs that block Bruton's tyrosine kinase, an enzyme crucial for B-cell development. Originally developed for blood cancers, they are increasingly being tested for autoimmune and neurological diseases. pipeline for neurology is dead. What it opens is a fragmented market where Roche and other drugmakers advancing similar molecules must navigate fundamentally different safety thresholds across the Atlantic. For neuroimmunology, the regulatory baseline is no longer global efficacy, but regional appetite for risk.