Heterogeneous lipid membranes tuned by pH were evaluated at 37^o C in the form of PEGylated vesicles composed of lipid pairs with dipalmitoyl (n = 16) and distearoyl (n = 18) chain lengths. One lipid type was chosen to have the titratable moiety phosphatidic acid on its headgroup, and the other lipid type to have a phosphatidylcholine headgroup. The effect of pH on formation of lipid heterogeneities and on membrane permeability was studied on vesicles composed of lipid pairs with matching and non-matching chain lengths.

Formation of lipid heterogeneities increases with decreasing pH in membranes composed of lipid pairs with either matching or non-matching chain lengths. Increased permeability with decreasing pH was exhibited only by membranes composed of lipid pairs with non-matching chain lengths. Permeability rates correlate strongly with the predicted extent of interfacial boundaries of heterogeneities suggesting defective packing among non-matching acyl chains of lipids. In heterogeneous mixtures with one lipid type in the fluid state (n = 12), dependence of membrane permeability on pH is weaker. In the presence of serum proteins, PEGylated gel-phase vesicles containing lipid pairs with non-matching chain lengths exhibit faster release rates with decreasing pH compared to measured release rates in phosphate buffer, suggesting a second mechanism of formation of separated phases. PEGylated vesicles composed of lipid pairs with non-matching chain lengths labeled with internalizing anti-HER2/neu antibodies that target overexpressed antigens on the surface of SKOV3-NMP2 ovarian cancer cells exhibit specific cancer cell targeting, followed by extensive internalization (more than 84 % of bound vesicles) and fast release of contents intracellularly. These PEGylated vesicles composed of rigid membranes for long blood circulation times that exhibit pH - dependent release of contents intracellularly could become potent drug delivery carriers for the targeted therapy of solid tumors.