From conventional to pH-sensitive formulations: increasing complexity, increasing characterization

pH-responsive liposomal systems based on 1,2-dioleoyl-sn-glycero-3-phosphoethanolamine (DOPE) stabilized with oleic acid (OA) have been proposed as effective drug delivery platforms due to their ability to release payloads under acidic conditions. In these systems, the deprotonated form of OA (R-COO - ) becomes protonated (RCOOH) at low pH, potentially triggering membrane destabilization. In this study, a multi-technique approach was employed to investigate the mechanisms underlying the pH sensitivity of DOPE–OA liposomes and the effect of poly(ethylene glycol) (PEG) incorporation on their behavior. Specifically, we examined (i) how OA stabilizes DOPE bilayers in a lamellar phase at physiological pH (7.4), and (ii) whether the protonation of OA under acidic conditions induces bilayer destabilization. Additionally, the liposomes were functionalized with a targeting peptide, a cyclic RGD (cRGD) peptide, to evaluate how surface modification influences key properties such as pH sensitivity, colloidal stability, and overall liposome behavior and to assess their potential for targeted drug delivery. Our findings demonstrate that OA stabilizes the DOPE bilayer by enhancing the hydrophilicity of the membrane surface. Upon acidification, the transition from RCOO − to R-COOH reduces surface hydrophilicity and decreases the effective area per lipid, leading to bilayer destabilization and promoting drug release. Furthermore, PEG incorporation was found to reduce the pH sensitivity of the liposomes, in agreement with previous reports. These results provide mechanistic insights into the design of pH-sensitive liposomal systems for targeted therapeutic applications.