Sanofi’s Bruton’s tyrosine kinase (BTK) inhibitor tolebrutinib failed to meet the primary endpoint in the Perseus Phase III trial for primary progressive multiple sclerosis and faces a delayed U.S. regulatory decision in non‑relapsing secondary progressive MS (nrSPMS). The company said the Perseus miss and the ongoing review will push regulatory timelines beyond the targeted PDUFA date, and it has submitted an expanded‑access protocol to the FDA. BTK inhibitors have been watched as potential oral disease‑modifying options in MS; these setbacks raise questions about clinical differentiation and regulatory strategy for the asset.