Addressing the polycation dilemma in drug delivery: charge-converting liposomes

The aim of this study was to tackle the polycation dilemma in drug delivery by developing charge-converting liposomes capable of permeating the mucus gel layer and enhancing cellular uptake. Positively charged liposomes containing dioleoylphosphatidylethanolamine (DOPE), cholesterol, and oleyl-oligolysine were formulated via the thin-film method. These liposomes were coated with polyphosphate to create negatively charged phosphorylated liposomes (pp-liposomes). Liposomes were characterized regarding droplet size, zeta potential, stability, cytotoxicity, and hemolytic activity. The cleavage of polyphosphates from the surface of liposomes triggered by intestinal alkaline phosphatase (AP) was monitored via malachite green assay and shift in zeta potential. Mucus permeation was assessed using porcine intestinal mucus in Transwell inserts. Cellular uptake was quantified in Caco-2 cells by flow cytometry and confocal microscopy. Liposomes exhibited an average size of 138.7 ± 2.9 nm and a zeta potential of +35.4 ± 1.5 mV, while the size of polyphosphate-coated liposomes increased to 168.4 ± 1.2 nm with a zeta potential of −24.2 ± 2.5 mV; both remained stable over 24 hours. Liposomes were non-toxic and hemolytic in a concentration of 0.1%. pp-Liposomes were less toxic than uncoated liposomes. Significant phosphate release occurred within first 6 hours of incubation with AP, and the zeta potential converted to +12.9 ± 5.19 mV within 24 hours. Mucus permeation studies showed that pp-liposomes exhibited 12-fold increase in permeability in the absence of AP compared to its presence. Cellular uptake of liposomes and pp-liposomes in Caco-2 cells demonstrated comparable levels of internalization. Accordingly, charge-converting liposomes effectively traversed the mucus barrier and improved cellular uptake, indicating a promising approach to resolving the polycation dilemma.