This is a prospective, single-center, open-label, 3+3 dose escalation Phase 1 safety study. Adults with newly diagnosed GBM or gliosarcoma will receive macitentan in addition to the standard of care treatment for GBM. The study consists of a screening period, a treatment period, and a 30-day safety follow up period. The treatment period includes 6 weeks of concurrent therapy (macitentan+RT+TMZ), 4 weeks of monotherapy (macitentan) and 12 cycles of maintenance therapy (macitentan+TMZ). The study will end when the last treated subject has completed study treatment and the 30-day safety follow-up period.