Enhanced active liposomal loading of a poorly soluble ionizable drug using supersaturated drug solutions.

Nanoparticulate drug carriers such as liposomal drug delivery systems are of considerable interest in cancer therapy because of their ability to passively accumulate in solid tumors. For liposomes to have practical utility for antitumor therapy in patients, however, optimization of drug loading, retention, and release kinetics are necessary. Active loading is the preferred method for optimizing loading of ionizable drugs in liposomes as measured by drug-to-lipid ratios, but the extremely low aqueous solubilities of many anticancer drug candidates may limit the external driving force, thus slowing liposomal uptake during active loading. This report demonstrates the advantages of maintaining drug supersaturation during active loading. A novel method was developed for creating and maintaining supersaturation of a poorly soluble camptothecin analogue, AR-67 (7-t-butyldimethylsilyl-10-hydroxycamptothecin), using a low concentration of a cyclodextrin (sulfobutylether-β-cyclodextrin) to inhibit crystallization over a 48 h period. Active loading into liposomes containing high concentrations of entrapped sodium or calcium acetate was monitored using drug solutions at varying degrees of supersaturation. Liposomal uptake rates increased linearly with the degree of supersaturation of drug in the external loading solution. A mathematical model was developed to predict the rate and extent of drug loading versus time, taking into account the chemical equilibria inside and outside of the vesicles and the transport kinetics of various permeable species across the lipid bilayer and the dialysis membrane. Intraliposomal sink conditions were maintained by the high internal pH caused by the efflux of acetic acid and exchange with AR-67, which undergoes lactone ring-opening, ionization, and membrane binding in the interior of the vesicles. The highest drug to lipid ratio achieved was 0.17 from a supersaturated solution at a total drug concentration of 0.6 mg/ml. The rate and extent of loading was similar when a different intraliposomal metal cation (sodium) was used instead of calcium. The proposed method may have general application in overcoming the formulation challenges associated with the liposomal delivery of poorly soluble, ionizable anticancer agents.