Strategies to Maximize Liposomal Drug Loading for a Poorly Water-soluble Anticancer Drug

Purpose To develop a liposomal system with high drug loading (DL) for intravenous (i.v.) delivery of a poorly water-soluble basic drug, asulacrine (ASL). Methods A thin-film hydration and extrusion method was used to fabricate the PEGylated liposomal membranes followed by a freeze and thaw process. A novel active drug loading method was developed using ammonium sulphate gradient as an influx driving force of ASL solubilized with sulfobutyl ether-P-cyclodextrin (SBE-beta-CD). DL was maximized by optimizing liposomal preparation and loading conditions. Pharmacokinetics was evaluated following i.v. infusion in rabbits. Results Freeze-thaw resulted in unilamellar liposome formation (180 nm) free of micelles. Higher DL was obtained when dialysis was used to remove the untrapped ammonium sulphate compared to ultracentrifuge. The pH and SBE-beta-CD level in the loading solution played key roles in enhancing DL. High DL ASL-liposomes (8.9%w/w, drug-to-lipid mole ratio 26%) were obtained with some drug "bundles" in the liposomal cores and were stable in a 5% glucose solution for >80 days with minimal leakage (<2%). Surprisingly, following administration of ASL-liposomes prepared with or without SBE-beta-CD, the half-lives were similar to the drug solution despite an increased area under the curve, indicating drug leakage from the carriers. Conclusions High liposomal DL was achieved with multiple strategies for a poorly-water soluble weak base. However, the liposomal permeability needed to be tailored to improve drug retention.