The present disclosure relates generally to compositions and methods of treating neoplastic diseases or cancers, such as glioblastoma and non-Hodgkin's lymphomas, or other cancers in which the subject suffers from an advanced solid tumor, comprising a combination of, or administering a combination of, a bromodomain and extra-terminal protein (BET) inhibitor and at least one chemotherapeutic agent, which does not inhibit BET directly. The BET inhibitor/chemotherapeutic agent combination, or combination therapy, can yield synergistic effects, thereby increasing the effectiveness of the cancer treatment as compared with the administration of either the BET inhibitor or the chemotherapeutic agent alone.