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 <i>via</i> 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 <i>via</i> 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.