PH-Controlled Liposomes for Enhanced Cell Penetration in Tumor Environment

An innovative pH-switchable colloidal system that can be exploited for site-selective anticancer drug delivery has been generated by liposome decoration with a new novel synthetic non-peptidic oligo-arginine cell-penetration enhancer (CPE) and a quenching PEGylated counterpart that detaches from the vesicle surface under the acidic conditions of tumors. The CPE module (Arg₄-DAG) is formed by four arginine units conjugated to a first-generation (G1) 2,2-bis(hydroxymethyl)propionic acid (bis-MPA)/2,2-bis(aminomethyl)propionic acid (bis-AMPA) polyester dendron terminating with 1,2-distearoyl-3-azidopropane for liposome bilayer insertion. The zeta potential of the Arg₄-DAG-decorated liposomes increased up to +32 mV as the Arg₄-DAG/lipids molar ratio increased. The Arg₄-DAG liposome shielding at pH 7.4 was provided by methoxy-PEG_{5 kDa}-polymethacryloyl sulfadimethoxine (mPEG_{5 kDa}-SDM₈) with 7.1 apparent pK_a. Zeta potential, surface plasmon resonance and synchrotron small-angle X-ray scattering analyses showed that at pH 7.4 mPEG_{5 kDa}-SDM₈ associates with polycationic Arg₄-DAG-decorated liposomes yielding liposomes with neutral zeta potential. At pH 6.5, which mimics the tumor environment, mPEG_{5 kDa}-SDM₈ detaches from the liposome surface yielding Arg₄-DAG exposure. Flow cytometry and confocal microscopy showed a 30-fold higher HeLa cancer cell association of the Arg₄-DAG-decorated liposomes compared to non-decorated liposomes. At pH 7.4, the mPEG_{5 kDa}-SDM₈-coated liposomes undergo low cell association while remarkable cell association occurred at pH 6.5, which allowed for the controlled intracellular delivery of model macromolecules and small molecules loaded in the liposome under tumor conditions.