Sanofi reported that Perseus, a Phase III study of its BTK inhibitor tolebrutinib in primary progressive multiple sclerosis, failed to meet its primary endpoint. The company also warned the U.S. regulatory review for a related submission in non‑relapsing secondary progressive MS will likely extend beyond the targeted PDUFA date. Sanofi has submitted an expanded‑access protocol and maintains belief in the drug’s risk‑benefit profile while preparing further analyses. The twin setbacks follow a year of high expectations after Sanofi’s acquisition of the asset and underscore the difficulty of producing definitive outcomes across multiple MS subtypes. Sanofi said it will continue to engage regulators and explore additional data that could support approval in select populations. BTK inhibitors aim to modulate B‑cell signaling and innate immune pathways; differential efficacy across MS phenotypes remains a central scientific and regulatory question. The news will influence Sanofi’s MS portfolio planning and investor sentiment around BTK programs.