Cellular uptake of CPX-351 by scavenger receptor class B type 1-mediated nonendocytic pathway

The proper uptake of drugs in Liposome formulations into target cells markedly impacts therapeutic efficacy. The protein corona (PC), formed by the adsorption of serum proteins onto the Liposome surface, binds to specific surface receptors of target cells, influencing the uptake pathway. We investigated the uptake pathway into leukemia cells based on PC analysis of CPX-351, a Liposome containing cytarabine and daunorubicin in a fixed 5:1 synergistic molar ratio. The PC of CPX-351 mixed with Fetal Bovine Serum was analyzed by nanoflow liquid chromatography-tandem mass spectrometry. CPX-351 uptake in HL-60, K562, and THP-1 leukemia cell lines was measured by flow cytometry using daunorubicin fluorescence. The major components of CPX-351 PC include apolipoproteins A-I and A-II, which bind to scavenger receptor class B type 1 (SR-BI), a nonendocytic pathway that takes up only Liposome contents. SR-BI was expressed in each cell, and its expression correlated with CPX-351 uptake. The uptake was significantly decreased by the inhibition of clathrin-mediated endocytosis and macropinocytosis. Additionally, blocks lipid transport-1 (BLT-1), a selective inhibitor of SR-BI, decreased the uptake; however, high-dose BLT-1 addition significantly increased the uptake, which was more strongly inhibited by macropinocytosis suppression compared with clathrin-mediated endocytosis. BLT-1 enhances the binding of SR-BI to liposomes in a dose-dependent manner. These findings indicate that the enhancement of binding between SR-BI and CPX-351 activates different pathways, such as macropinocytosis, distinct from CPX-351 alone. SR-BI may be a biomarker for CPX-351 therapy, and the combination of CPX-351 with high-dose BLT-1 may augment therapeutic efficacy.