Controlling Endosomal Escape Using Nanoparticle Composition: Current Progress and Future Perspectives

Purpose: Endosomal escape remains a major roadblock for delivering biological therapeutics such as DNA, siRNA, and proteins. Polymeric nanoparticles can protect cargo and target cells, but they often remain trapped in endosomal/lysosomal compartments, where cargo is degraded. A better understanding of how nanoparticle composition controls endosomal escape is needed to design more effective delivery systems. ---

Hypothesis: The review’s central premise is: if nanoparticle composition and architecture—including hydrophobicity, pKa, disassembly pH, crosslinking, and polymer architecture—are systematically tuned, then endosomal escape can be controlled and improved. The proton sponge effect alone is insufficient to explain escape; multiple mechanisms likely act in concert. ---

Aims: - Review recent advances in understanding endosomal escape of polymer nanoparticles. - Discuss how nanoparticle structure and composition can control endosomal escape. - Focus on pH-responsive mechanisms, including proton sponge, membrane interaction/disruption, and particle swelling. - Highlight future perspectives and the need for better assays and fundamental understanding. ---

Delivery system: Platform: Polymer nanoparticles for therapeutic delivery. Key polymer systems discussed: - pH-responsive polymers: PDEAEMA, PDPAEMA, DEAEMA, DPAEMA, DMAEMA, BMA, PAA, PLGA blends, polyaspartamides, PEG-b-PDEAEMA, pHlexi nanoparticles, ultra pH-sensitive (UPS) copolymers, cross-linked/hyperbranched/linear architectures, core–shell gel particles, polymer blend particles, nanogels. - Payloads: DNA, siRNA, mRNA, proteins/OVA antigen, model cargo calcein, luciferase, proapoptotic peptide. Endosomal escape mechanisms: - Proton sponge effect - Membrane disruption/destabilization - Particle swelling - Osmotic pressure - pH-triggered disassembly ---

Approach: Narrative review of published literature. No primary experimental groups. Model systems discussed include: - In vitro: RAW 264.7 murine macrophages, JAWSII murine dendritic cells, A549 cells, red blood cell hemolysis assays, and other cell lines. - Disease/therapeutic context: Gene delivery, vaccination, cancer therapy, cytosolic protein delivery. - No in vivo primary studies are reported by the review itself; it synthesizes mechanistic and preclinical findings. ---

Key methods: Techniques highlighted across cited studies: - Hemolysis assay for membrane interaction. - Dynamic light scattering (DLS) for nanoparticle disassembly pH. - Calcein assay for endosomal escape. - Flow cytometry for transfection and antigen presentation. - MHC class I/II antigen presentation assays. - Luciferase knockdown for siRNA function. - Split-GFP complementation assay for cytosolic delivery. - Redox-activatable sensor for quantifying cytosolic protein delivery. - Fluorescence microscopy and cell viability assays. ---

Key results: - Hydrophobicity: 30% BMA was optimal for hemolysis at endosomal pH 6.2; higher BMA prevented efficient nanoparticle disassembly. 30% BMA also improved DNA transfection in RAW 264.7 and JAWSII cells. - Polymer architecture: Cross-linked and hyperbranched nanoparticles showed significantly higher hemolysis than linear counterparts; hyperbranched variants enhanced MHC class I antigen presentation. - Protonation rates: Odd-numbered aminoethylene repeats in N-substituted polyaspartamides improved mRNA transcription and endosomal escape; even-numbered repeats behaved differently, and the trend reversed for pDNA. - Particle size: 40 nm nanoparticles showed different cell trafficking and luciferase knockdown compared with 100 nm particles; only the smaller particles achieved knockdown. - pHlexi nanoparticles: Endosomal escape depended on core polymer molecular weight; 7 kDa PDEAEMA showed limited escape, while >25 kDa showed significant escape. Tuning pKa showed highest escape at pH 7.2 or pH 4.9, and lowest at pH 6.4 (1:1 PDEAEMA:DPAEMA). - UPS copolymers: pKa tuned from pH 7.4 to 4.0 in 0.3 pH increments; a cyclic seven-membered ring copolymer gave the greatest cytosolic delivery, similar to JetPEI. - Polymer blends: MHC class II presentation peaked at 0.2 terpolymer weight ratio; MHC class I presentation was highest at 0.5 wt ratio. - Proton sponge controversy: PEI did not change lysosomal pH over 24 h; buffering polymers do not always enhance endosomal escape. ---

Interpretation: The authors conclude that endosomal escape is a significant bottleneck for biological therapeutics and that nanoparticle composition, architecture, hydrophobicity, pKa, and disassembly behavior can be tuned to control escape. However, the relationship between nanoparticle composition and endosomal escape is not fully understood, and the field must move beyond synthesizing new nanoparticles toward understanding fundamental nanoparticle/endosomal interactions. New, more sensitive assays are needed to quantify endosomal escape. ---

Limitations: - Poor understanding of the mechanisms governing endosomal escape. - Difficulty in conclusively isolating one escape mechanism from another; multiple mechanisms likely act together. - Quantification of low levels of endosomal escape remains challenging. - Current assays (e.g., calcein) are subjective and hard to quantify; functional assays may not distinguish escape failure from other biological inhibition. - Cell-type differences in nanoparticle/endosomal interactions are often dismissed or poorly understood. - Limited studies on swelling-controlled endosomal escape. - No primary in vivo validation or large-animal data in this review. - As a review, it does not provide systematic meta-analysis or original experimental data.