Mechanistic Insight into How PEGylation Reduces the Efficacy of pH-Sensitive Liposomes from Molecular Dynamics Simulations

Abstract

Liposome-based drug delivery systems composed of DOPE stabilized with cholesteryl hemisuccinate (CHMS) have been proposed as a drug delivery mechanism with pH-triggered release as the anionic form (CHSa) is protonated (CHS) at reduced pH; PEGylation is known to decrease this pH sensitivity. In this manuscript, we set out to use molecular dynamics (MD) simulations with a model with all-atom resolution to provide insight into why incorporation of poly(ethyleneglycol) (PEG) into DOPE-CHMS liposomes reduces their pH sensitivity; we also address two additional questions: (1) How CHSa stabilizes DOPE bilayers into a lamellar conformation at a physiological pH of 7.4? and (2) how the change from CHSa to CHS at acidic pH triggers the destabilization of DOPE bilayers? We found that (A) CHSa stabilizes the DOPE lipid membrane by increasing the hydrophilicity of the bilayer surface, (B) when CHSa changes to CHS by pH reduction, DOPE bilayers are destabilized due to a reduction in bilayer hydrophilicity and a reduction in the area per lipid, and (C) PEG stabilizes DOPE bilayers into the lamellar phase, thus reducing the pH sensitivity of the liposomes by increasing the area per lipid through penetration into the bilayer, which is our main focus.

Keywords: PEGylated pH-sensitive liposomes; bilayer hydrophilicity; cholesteryl hemisuccinate; molecular dynamics simulations; phase transition.

Figure 1

Chemical structure of the components of the bilayer showing atom numbering for the acyl chains of DOPE and the partial charges derived for CHOL, CHS, and CHSa.

Figure 2

Average area per molecule.

Figure 3

Partial mass density profiles of water relative to the position of the phosphate head group peak (A). Number of bilayer–water contacts (shown in bar graphs with values on the left-hand side Y-axis) and number of bilayer–water H-bonds (in line graphs with values on the right-hand side Y-axis) (B). Percentage of area of the steroid head groups covered by other components of the system (C).

Figure 4

Deuterium order parameter along the Sn-1 tail (A) and mass density profile results for the lipid component (B).

Figure 5

Percentage of Na⁺ ions in contact with the bilayers in non-PEGylated systems (A) and percentage of Na⁺ ions in contact with the bilayers and PEG chains of the PEGylated systems (B).

Figure 6

Mass density profile of PEG relative to the position of the phosphate head group peak (A) and visualization of the final configurations of the system in the last frame of the simulation of PEG–DOPE–CHOL (B), PEG–DOPE–CHS(C), and PEG–DOPE–CHSa (D). Phosphate atoms are colored as green and PEG chains as red. All other components of the systems were removed for clarity. The inset in Figure 6A represents the PEG mass density profile of PEG_5%–DSPC–CHOL_33% studied in our previous publication (ref (44)).