Synthesis and characterization of lipid-polymer hybrid nanoparticles with pH-triggered poly(ethylene glycol) shedding

Abstract

Novel lipid-polymer hybrid nanoparticles are designed with a poly(ethylene glycol) (PEG) coating that is shed in response to a low pH trigger. This allows the nanoparticles to be stable during systemic circulation and at neutral pH, but destabilize and fuse with lipid membranes in acidic environments. The hybrid nanoparticles consist of a poly(lactic-co-glycolic acid) core with a lipid and lipid-PEG monolayer shell. To make the hybrid nanoparticles pH sensitive, a lipid-(succinate)-mPEG conjugate is synthesized to provide a hydrolyzable PEG stealth layer that is shed off the particle surface at low pH. The pH-sensitivity of the nanoparticles is tunable using the molar concentration of the lipid-(succinate)-mPEG incorporated in the lipid shell of the particles. Possible uses of these pH-sensitive nanoparticles include aggregating in acidic tumor microenvironments, escaping acidified endosomes, or aggregating in deep lung tissue for improved inhalation administration.

Figure 1

Schematic of a lipid-polymer hybrid nanoparticle with pH-triggered PEG shedding. The hybrid nanoparticle consists of a drug-loaded PLGA polymeric core, a fusogenic DOPE/oleic acid monolayer shell, and a sterically stabilizing PEG corona. The PEG layer comes off in response to environmental acidity, making the particle fusogenic toward lipid membranes.

Figure 2

Synthesis and characterization of lipid-(succinate)-mPEG conjugate. (A) Schematic description of the synthesis of lipid-(succinate)-mPEG conjugate that is sensitive to acidic pH. (B) ¹H NMR spectrum of the synthesized lipid-(succinate)-mPEG conjugate.

Figure 3

Hydrodynamic size (diameter, nm) of the lipid-polymer hybrid nanoparticles synthesized with various amounts of lipid-(succinate)-mPEG replacing a molar percentage of DOPE lipid in the lipid monolayer shell. All samples are at pH = 7.4.

Figure 4

Scanning electron microscopy (SEM) images of lipid-polymer hybrid nanoparticles, in which lipid-(succinate)-mPEG makes up 15 mol% of the lipid monolayer shell. (A) At pH = 7.4, isolated particles show spherical morphology and little aggregation. (B) At pH = 5, particles have aggregated and show irregular morphology.

Figure 5

Time dependent particle aggregation at pH = 7.4 and pH = 5 over 24 hours. Lipid-(succinate)-mPEG was incorporated into the lipid monolayer shell at a molar ratio of 15%. Nonhydrolyzable DSPE-PEG was used in the same ratio as a negative control. At pH = 7.4, both formulations are stable over the 24 hours, with the lipid-(succinate)-mPEG particles increasing in size by only 28 nm (from 107 nm to 135 nm). At pH = 5, the DSPE-PEG particles remain stable while the lipid-(succinate)-mPEG particles begin to aggregate immediately and continue for 6 hours before plateauing at around 2000 nm.

Figure 6

Particle aggregate size (diameter, nm) at varying pH values after 20 hours incubation. Lipid-polymer hybrid nanoparticles show increasing stability with increasing amounts of lipid-(succinate)-mPEG incorporated to the lipid monolayer shell. With 15 mol%, complete aggregation occurs at pH = 6. With 20 mol%, 30 mol% and 40 mol% lipid-(succinate)-mPEG, aggregating begins at pH = 6, 5, and 3, respectively. With 50 mol% lipid-(succinate)-mPEG, aggregation is not observed over the range of pH values tested.