Regulating the membrane-fusion effect of liposomes via the protonation of tertiary amines

Membrane fusion, a fundamental biological process enabling precise exchange of biomolecules within and between cells, has inspired the development of various fusogenic model systems for applications in drug delivery and beyond. However, conventional systems typically operate in an "always-on" state, lacking spatiotemporal control. Although cleavable polyethylene glycol coatings can confer temporal regulation through stimuli-responsive activation, such strategies are often unidirectional and synthetically challenging. Here, we report a simple yet reversible strategy to regulate liposomal membrane fusion via protonation of tertiary amines. By replacing the permanently positively charged lipid DOTAP with an ionizable analogue, DODAP, we constructed a pH-switchable fusogenic liposome (PsFul) that remains inactive at neutral pH but activates fusion upon protonation in acidic environments. This transition is fully reversible, allowing adaptive toggling between "on" and "off" states. We demonstrate that PsFul enables direct cytosolic delivery of anticancer drugs via membrane fusion, bypassing lysosomal entrapment and significantly enhancing cytotoxicity under acidic conditions. This work establishes a facile platform for achieving spatially and temporally controlled membrane fusion, offering promising potential for targeted therapeutic delivery.