Leukemia Cell–Selective Uptake of Cytarabine and Daunorubicin in the Bone Marrow Compartment Mediated by CPX-351 Liposome Injection

Abstract

Abstract Background Anticancer drug combinations such as cytarabine plus daunorubicin can act synergistically, additively or antagonistically depending on the ratio of the agents being combined. We have shown that delivering synergistic cytarabine-to-daunorubicin drug ratios in vivo using nanoscale liposomes (CPX-351 liposome injection) provides dramatic efficacy improvements compared to the free drug cocktail in a wide range of preclinical leukemia models without exacerbating myelosuppressive effects. This enhanced efficacy is associated with increased and prolonged delivery of the synergistic drug ratio to bone marrow. Promising signs of efficacy have been observed with CPX-351 in previously treated patients with AML with poor prognosis. We aim to investigate the pharmacodynamic basis for the potent therapeutic activity of CPX-351. Patients and Methods A bone marrow–engrafting CCRF-CEM human leukemia xenograft model was developed. CPX-351 was injected intravenously (I.V.), and the bone marrows of femurs from tumor bearing mice were subsequently harvested. Cells were analyzed by flow cytometry, and leukemia cells were separated from normal bone marrow cells using anti-CD45 coated magnetic nanoparticles. Cells were analyzed for intracellular cytarabine-to-daunorubicin and liposome content using 3 H-Cyt, HPLC, and 14 C-lipid, respectively. Results Twenty-eight days after I.V. tumor inoculation, femurs contained approximately equal numbers of CCRF-CEM and normal bone marrow cell populations that could be readily separated and quantitatively harvested by antibody-coated magnetic nanoparticle-mediated isolation. Eighteen hours after CPX-351 I.V. injection, bone marrows of the tumor bearing mice contained 82.6 ng cytarabine, 63.0 ng daunorubicin, and 1.2 μg liposomal lipid per total femur aspirate. Within the separated cell populations, leukemia cells contained 5.9 ng cytarabine, 8.2 ng daunorubicin, and 0.16 μg liposomal lipid per 10 6 cells. These intracellular levels were 9.5-, 2.2-, and 1.9-fold higher than those observed in the normal bone marrow cell population. Confocal microscopy on cells incubated with CPX-351 in vitro demonstrated that CPX-351 liposomes are taken up intact by human leukemia cells and subsequently release the drugs intracellularly. Conclusion CPX-351 was designed to enhance the antitumor efficacy of cytarabine-to-daunorubicin combination therapy by encapsulating both agents within a drug carrier that maintains the synergistic 5:1 molar ratio for extended times after injection. The potent anti-leukemic activity obtained in the absence of significant nonhematologic toxicity with this formulation may be due, in part, to the selective accumulation of CPX-351 into leukemia cells.