Active Loading and Release of Cationic Drugs into/from Cationic Liposomes by Utilizing a Concentration Gradient of NaCl

Liposomes are widely used as drug delivery systems (DDS), enabling the encapsulation of therapeutic agents to improve drug stability, reduce toxicity, and facilitate targeted delivery. Conventional active loading methods for weak base drugs rely on pH or salt gradients. However, these methods require column separation to replace the external aqueous phase, increasing the complexity. In this study, we propose a novel active loading method that enables the spontaneous accumulation of cationic drugs into liposomes by utilizing a sodium chloride (NaCl) concentration gradient. Unlike traditional methods, our approach does not require column separation, as high concentrations of NaCl and a target drug are added only in the external aqueous phase of the liposomes, while the internal aqueous phase contains only sucrose. The cationic lipid dioleoyltrimethylammoniumpropane (DOTAP) is incorporated into liposome membrane to enhance chloride ion permeability, which facilitates the accumulation of cationic drugs, including epirubicin (Epi), daunorubicin (Dau), and idarubicin (Ida). Fluorescence microscopy analysis revealed that the presence of NaCl in the external phase significantly increased the drug accumulation within cationic liposomes. Additionally, we observed that drug retention was sustained under high NaCl conditions, but triggered release occurred under low NaCl conditions. This suggests that our NaCl-based loading method provides an effective strategy for both drug accumulation in physiological conditions (e.g., plasma) and controlled release in low Cl^- conditions (e.g., interstitial tumor tissues).