Sanofi reported that tolebrutinib, its BTK inhibitor acquired in a prior buyout, failed to meet the primary endpoint in the phase III Perseus trial for primary progressive multiple sclerosis, the company said in investor releases and regulatory filings. The setback follows a likely U.S. regulatory review delay for another tolebrutinib indication and prompted Sanofi to submit an expanded‑access protocol. Sanofi said it remains committed to evaluating additional data and emphasized ongoing dialogue with regulators. The news has immediate pipeline and commercial implications for the company’s neurology strategy and underscores the high bar for BTK inhibitors in neurodegenerative indications.