Lipid Modified with Pyridinium Betaine Manipulates Liposomal Membrane Fusion Behavior for Spatially Confined Cytoplasmic Delivery

Abstract Spatially confining the membrane‐fusion effects of fusogenic delivery systems in tumors can enhance therapeutic efficacy and reduce systemic toxicity. However, current existing fusion‐regulating strategies often suffer from insensitivity and unidirectional control, leading to suboptimal antitumor effects and potential safety risks. Here, we designed a pyridinium betaine‐capped lipid, 2‐(4‐((2,3‐di(stearoyloxy)propyl)carbamoyl)pyridin‐1‐ium‐1‐yl)acetate (DSPCPA)), as a pH‐responsive regulator to precisely manipulate the membrane‐fusion behavior of fusogenic liposomes. Incorporating DSPCPA, we optimized a formulation of s elf‐a d aptive, en vironment‐responsive fu sogenic l iposome (SENDFUL) capable of rapidly and reversibly switching membrane‐fusion activity responding acidic tumor microenvironments. At physiological pH, SENDFUL remains negatively‐charged due to the zwitterionic nature of DSPCPA, undergoing in a non‐fusogenic state. In acidic tumor regions (pH 6.5), protonation of DSPCPA rapidly converts surface charge to positive, triggering tumor‐specific fusion via strengthened electrostatic interactions with cellular membranes. This reversible transition highly confines lysosome‐bypassing cytosolic delivery to tumors, minimizing systemic toxicity. We utilized SENDFUL to construct a nanoagonist for the stimulator of interferon genes (STING) pathway, which elicited potent antitumor immunity with negligible toxicity in vitro and in vivo. Thus, SENDFUL represents an advanced strategy for spatially precise regulation of liposomal membrane fusion, providing a safe and efficient platform for tumor‐specific intracellular drug delivery.