CPX-351 is a liposomal formulation of a fixed combination of the antineoplastic drugs cytarabine and daunorubicin. The two drugs are present inside the liposome in a 5:1 molar ratio. The development of CPX-351 (cytarabine:daunorubicin) Liposome Injection was based on 1) defining a synergistic ratio of the two active moieties, cytarabine and daunorubicin, using cell-based screening assays and 2) designing a liposomal drug carrier to maintain this ratio after intravenous administration. CPX-351 was found to be more active in in vivo models of cancer than combinations of conventional cytarabine and daunorubicin. Both cytarabine and daunorubicin are active chemotherapeutic agents, each approved for clinical use in the United States for the treatment of hematological neoplasms.