Development of High-Content Gemcitabine PEGylated Liposomes and Their Cytotoxicity on Drug-Resistant Pancreatic Tumour Cells

Purpose The objective of this study was to develop high-content gemcitabine PEGylated liposomes to reverse gemcitabine resistance in pancreatic tumour cells. The mechanism of drug loading into liposomes was also investigated. Methods To increase the drug entrapment efficiency (EE) and drug loading (DL), a novel passive loading approach named Small Volume Incubation method (SVI) was developed and compared to the reverse phase evaporation (REV) and remote loading methods. The in vitro cytotoxicity was evaluated using MIA PaCa-2 and Panc-1 cell lines. Results The EE for remote loading was 12.3 +/- 0.3%, much lower than expected and a burst release was observed with the resultant liposomes. Using the optimized SVI method, increased EE (37 +/- 1%) and DL (4%, w/w) were obtained. The liposomes (200 +/- 5 nm) showed minimal drug leakage, good stability, and significant improvement in cytotoxicity to the gemcitabine-resistant pancreatic cancer cell lines. Conclusions Remote loading was not suitable for loading gemcitabine into liposomes. pK(a)>4.6 for basic drugs and intra-liposomal precipitation of loaded compounds were suggested as an additional requirement to the current criteria for remote loading using ammonium sulphate gradient (pK(a)<11). High DL is essential for liposomes to reverse gemcitabine resistance in pancreatic cell lines.