Phase I single-arm trial in Japan testing tirabrutinib (ONO-4059) plus R-MPV for untreated primary CNS lymphoma. Drugs and mechanisms (type): tirabrutinib—selective, irreversible small-molecule BTK inhibitor blocking B-cell receptor signaling, rituximab—anti-CD20 monoclonal antibody depleting B cells via CDC/ADCC, methotrexate—antifolate antimetabolite inhibiting DHFR and thymidylate synthesis, procarbazine—alkylating/DNA-methylating agent causing DNA damage, vincristine—vinca alkaloid microtubule inhibitor arresting mitosis. Targets: malignant CD20+ B cells in CNS, pathways affected include BCR/BTK signaling, CD20-mediated immune clearance, folate/DNA synthesis, DNA integrity, and microtubule/mitotic machinery.