Transition Metal-Mediated Liposomal Encapsulation of Irinotecan (CPT-11) Stabilizes the Drug in the Therapeutically Active Lactone Conformation

Purpose To determine whether entrapped transition metals could mediate the active encapsulation of the anticancer drug irinotecan into preformed liposomes. Further, to establish that metal complexation could stabilize liposomal irinotecan in the therapeutically active lactone conformation. Materials and Methods Irinotecan was added to preformed 1,2-distearoyl-sn-glycero-phosphocholine/cholesterol (DSPC/chol) liposomes prepared in CuSO₄, ZnSO₄, MnSO₄, or CoSO₄ solutions, and drug encapsulation was determined over time. The roles of the transmembrane pH gradient and internal pH were evaluated. TLC and HPLC were used to monitor drug stability and liposome morphology was assessed by cryo-TEM. Results Irinotecan was rapidly and efficiently loaded into preformed liposomes prepared in unbuffered (∼pH 3.5) 300 mM CuSO₄ or ZnSO₄. For Cu-containing liposomes, results suggested that irinotecan loading occurred when the interior pH and the exterior pH were matched; however, addition of nigericin to collapse any residual transmembrane pH gradient inhibited irinotecan loading. Greater than 90% of the encapsulated drug was in its active lactone form and cryo-TEM analysis indicated dark intravesicular electron-dense spots. Conclusion Irinotecan is stably entrapped in the active lactone conformation within preformed copper-containing liposomes as a result of metal–drug complexation.