Tirabrutinib (ONO-4059) hydrochloride is an orally active Bruton’s Tyrosine Kinase (BTK) inhibitor (can cross the blood-brain barrier (BBB)), with an IC50 of 6.8 nM. Tirabrutinib hydrochloride irreversibly and covalently binds to BTK and inhibits aberrant B cell receptor signaling. Tirabrutinib hydrochloride can be used in studies of autoimmune diseases and hematological malignancies.