Active loading liposomal irinotecan hydrochloride: Preparation, in vitro and in vivo evaluation

The aim of the present study was to investigate the effect of various transmembrane ammonium salt gradients and different lipid composition on the loading efficiency of liposomal formulations of irinotecan hydrochloride (CPT-11), their behavior in vivo and cytotoxicity. Among ammonium salts studied, ammonium sulfate was successfully used to load CPT-11 into liposomes with the highest encapsulation efficiency. Subsequently, liposomal CPT-11 with different lipid composition was prepared by ammonium sulfate gradient method. CPT-11 can be loaded to a level over 90% into liposomes composed of soybean phospholipids/cholesterol (SPC-L) or hydrogenated natural soybean phospholipids/cholesterol (HSPC-L). In vitro release profiles were also investigated, indicating that HSPC-L had a lower release than that in SPC-L. In vivo, encapsulation of CPT-11 in both liposomal formulations showed higher area under the curve (AUC), a lower rate of clearance (CL) and smaller volume of distribution for CPT-11 than those of irinotecan hydrochloride solution (CPT-11-S). However, CL and AUC of 7-ethyl-10-hydroxycamptothecin (SN-38) were moderately improved in HSPC-L group. Based on the results of comparative pharmacokinetics of liposomal CPT-11 with different lipid composition, the in vitro cytotoxicity of HSPC-L was evaluated with human tumor cell. The result indicated that liposomal CPT-11 showed a great enhancement in vitro cytotoxicity. The results suggested that entrapment of CPT-11 in liposomes especially in those with high phase-transition temperature lipid by ammonium sulfate gradient would be a promising formulation with a better in vivo behavior.