Intracellular delivery and antitumor effects of pH-sensitive liposomes based on zwitterionic oligopeptide lipids

pH-sensitive liposomes (HHG2C18-L and PEGHG2C18-L) based on zwitterionic oligopeptide lipids as anticancer drug carriers were developed and evaluated for effective intracellular delivery and enhanced antitumor activity. The amino acid-based lipids, 1,5-dioctadecyl-l-glutamyl 2-histidyl-hexahydrobenzoic acid (HHG2C18) and 1,5-distearyl N-(N-α-(4-mPEG2000) butanedione)-histidyl-l-glutamate (PEGHG2C18), were synthesized, which have the multistage pH-response to tumor microenvironmental pH (pHe, pH 6.0–7.0) and endosomal/lysosomal pH (pHi, pH 4.0–6.0) successively. HHG2C18-L contains HHG2C18, while PEGHG2C18-L includes HHG2C18 and PEGHG2C18. Both of them displayed the capability of charge conversion to the surrounding pH. The zeta potentials of HHG2C18-L and PEGHG2C18-L were negative at pH 7.4, whereas positive at pH 6.5 and more positive at lower pH. Coumarin 6-loaded HHG2C18-L (C6/HHG2C18-L) and PEGHG2C18-L (C6/PEGHG2C18-L) showed higher tumor cellular uptake due to electrostatic absorptive endocytosis at pHe (pH 6.5), produced proton sponge effect for endo-lysosomal escape, and accumulated to the mitochondria based on stronger positive charge by the hydrolysis of a pH-sensitive linker at pHi (pH 5.5 and pH 4.5). Furthermore, temsirolimus (CCI-779)-loaded HHG2C18-L (CCI-779/HHG2C18-L) and PEGHG2C18-L (CCI-779/PEGHG2C18-L) had significantly higher antiproliferative and apoptosis inducing effects toward the human renal carcinoma (A498) cells at pH 6.5 relative to that at pH 7.4. The half maximal inhibitory concentration (IC50) of CCI-779/HHG2C18-L and CCI-779/PEGHG2C18-L were about 3 μg/mL and 5 μg/mL at pH 6.5, 1.67-fold and 1.60-fold improved relative to that at pH 7.4, respectively. The total apoptotic ratio of CCI-779/HHG2C18-L and CCI-779/PEGHG2C18-L increased from 9.90% and 7.78% at pH 7.4 to 19.53% and 12.10% at pH 6.5, respectively. In vivo, CCI-779/PEGHG2C18-L after intravenous administration presented remarkably higher bioavailability and blood persistence compared with unPEGylated CCI-779/HHG2C18-L, and had the strongest antitumor efficacy against xenograft renal cancer (Renca) tumor models. Accordingly, the results provide the feasibility of using pH-sensitive zwitterionic oligopeptide lipids to extend the applications of liposomes to efficient anticancer drug delivery in cancer therapy.