Physicochemical approaches to enhance the liposomal loading and retention of hydrophobic weak acids

Clinical development of highly potent lipophilic neutral camptothecins has been impeded by the poor solubility, stability and non-specific toxicity of these compounds. Liposomal encapsulation offers a promising formulation route for tumor site-specific delivery of these novel drug candidates. However, formulation efforts for liposomal loading and retention of amphiphilic weak acids such as the neutral camptothecins have been unsuccessful. In this study, several novel physicochemical approaches were explored to enhance the liposomal loading and retention of hydrophobic weak acids, using a neutral camptothecin, 7-tert-butyldimethylsilyl-10-hydroxycamptothecin (DB-67), as a model solute. Mathematical models were developed to rationalize the in vitro experimental results and to predict performance under various conditions. A dynamic dialysis method was initially developed and employed to study the membrane permeability of hydrophobic solutes and mathematical models were developed to estimate membrane permeability coefficient from the kinetics of liposomal drug release. The first formulation approach explored the potential of liposomes comprised of rigid gel phase lipid bilayer to retain the therapeutically active (lactone) form of DB-67. The second formulation approach explored the potential of intravesicular pH modification to improve the liposomal retention of hydrophobic weak acids by retaining the drug in a membrane impermeable ionized form. The third formulation approach explored the role of a complexing agent (hydroxypropyl-beta-cyclodextrin) on the membrane partitioning and permeability of DB-67 and the role of intravesicular pH modification in the presence of a complexing agent to enhance liposomal retention. The fourth formulation approach explored the ability of intravesicular divalent metal ions (calcium) to enhance the retention of hydrophobic weak acids. All of these formulation approaches were evaluated in vitro either with or without a trans-bilayer pH gradient (extravesicular pH = 7.4), as appropriate. The half-life for DB-67 release from the vesicles prepared by the various approaches ranged from approximately 3 to 28 hrs in vitro in the presence of a trans-bilayer pH gradient. These formulation strategies were also evaluated in vivo in a mouse model and were found to enhance the retention of DB-67 in liposomes to varying degrees.;Keywords. Camptothecins, Membrane Permeability, Liposomes, Hydrophobic Compounds, Controlled Release...