Synergistic Cytarabine:Daunorubicin Ratios Delivered by CPX-351 to Human Leukemia Xenografts Is Associated with Liposome-Mediated Bone Marrow Drug Accumulation, Intracellular Delivery of Encapsulated Agents to Leukemia Cells, and Increased Efficacy.

CPX-351, a liposomal formulation of cytarabine:daunorubicin co-encapsulated at a synergistic 5:1 molar drug ratio, has been previously shown to be highly active in various preclinical leukemia models. In the present investigations, we utilized a bone marrow engrafted human lymphocytic leukemia xenograft model in order to elucidate the pharmacodynamic basis for its superior efficacy compared to un-encapsulated cytarabine:daunorubicin combinations. In normal bone marrow, the un-encapsulated cytarabine:daunorubicin saline-based cocktail and CPX-351 provided comparable cell suppression after intravenous treatment at maximum tolerated dose. In contrast, when these formulations were administered to tumor bearing mice, CPX-351 completely ablated leukemia cells from the bone marrow for multiple weeks whereas the saline-based drug cocktail induced only transient leukemia suppression. Examination of drug levels in the bone marrow compartment revealed that cytarabine:daunorubicin concentrations were significantly elevated for CPX-351. More striking was the observation that whereas bone marrow drug concentrations over time appeared comparable after each dose of the saline-based drug cocktail, the first dose of CPX-351 promoted elevated bone marrow drug accumulation for subsequent doses. Under these conditions, the cytarabine:daunorubicin ratio in bone marrow was maintained near the administered 5:1 molar ratio for CPX- 351 while the drug ratio changed more than 100-fold within 4 hours after administration of the saline-based drug cocktail. Confocal fluorescence microscopy of leukemia cells exposed to CPX-351 in vitro revealed that CPX-351 liposomes were taken up into cytoplasmic vacuoles and subsequently released their drug contents intracellularly. Taken together, these results indicate that the improved efficacy observed following CPX-351 administration in vivo is related to liposome-mediated prolonged and direct exposure of leukemia cells to synergistic drug ratios.