Effects of the molecular weight and molar ratio of poly(2-ethyl-2-oxazoline)-based lipid on the pH sensitivity, stability, and antitumor efficacy of liposomes

In this study, we evaluated and screened the effects of the molecular weight (MW) and molar ratio of poly(2-ethyl-2-oxazoline)-cholesteryl methyl carbonate (PEtOz-CHMC) on the pH sensitivity, stability, and antitumor efficacy of liposomes. The pH sensitivity of PEtOz-CHMC with different MWs and molar ratios was screened by drug release and cytotoxicity experiments at different pH levels. Results indicated that the liposomes coated with PEtOz1k-CHMC (7% molar ratio) and PEtOz2k-CHMC (5% molar ratio) exhibited the desirable pH responsiveness. When the MW of PEtOz was relatively low, 7% of the modified ratio obtained the strongest stability, but the turbidity of the liposomes did not obviously change when the molar ratio of PEtOz-CHMC was further increased. A375 cells were used as models to investigate the cellular uptake and intracellular localization of coumarin-6-loaded liposomes (C6-L), PEGylated liposomes (PEG-C6-L), and PEtOzylated liposomes. PEtOz1k-C6-L and PEtOz2k-C6-L presented remarkably stronger fluorescence intensity at low pH than at pH 7.4, whereas C6-L and PEG-C6-L did not achieve any obvious diversity at different pH conditions. Compared with C6-L and PEG-C6-L, PEtOz-C6-L showed efficient intracellular trafficking, including endosomal/lysosomal escape and cytoplasmic release. Pharmacokinetic experiments demonstrated that half-lives of PEG2k-C6-L, PEtOz2k-C6-L, and PEtOz1k-C6-L were 11.89-, 7.00-, and 5.29-fold times higher than those of C6-L, respectively. Among the liposomes, the DOX·HCl-loaded liposomes coated with PEtOz2k-CHMC demonstrated the strongest antitumor efficacy against B16 tumor xenograft models in vivo. These findings provide the feasibility of using PEtOz-CHMC with optimal pH sensitivity and long circulation to extend the application of liposomes to efficient anticancer drug delivery.