Stealth Coating of Nanoparticles in Drug-Delivery Systems

Purpose: Nanoparticles (NPs) interact extensively with plasma proteins after intravenous administration and are rapidly cleared by the mononuclear phagocyte system (MPS) or complement system, causing premature drug release at off-target sites. A stealth coating layer can improve blood circulation half-life by escaping immune recognition and clearance.

Hypothesis: No formal experimental hypothesis. Central thesis: grafting a stealth coating—synthetic polymers such as PEG, POx, or polyzwitterions, or biomimetic strategies such as cell-membrane camouflaging and CD47 functionalisation—onto NP surfaces can suppress opsonisation, evade MPS/complement clearance, prolong blood circulation, and improve targeted drug delivery. Coating molecular weight, surface chain density, and surface conformation are critical determinants of stealth efficacy.

Aims: Review the basic concept underlying stealth behaviour of polymer-coated NPs. - Discuss fundamental surface coating characteristics: molecular weight, surface chain density, and polymer chain conformation. - Summarise commonly used stealth polymers: PEG, poly(2-oxazoline) (POx), and poly(zwitterions). - Highlight biomimetic stealth strategies: cell-membrane camouflaging and CD47 functionalisation. - Discuss opsonisation, phagocytosis, and complement activation as barriers to NP circulation.

Delivery system: Stealth polymers: PEG (gold standard), POx [poly(2-methyl-2-oxazoline) (PMeOx), poly(2-ethyl-2-oxazoline) (PEtOx)], polyzwitterions [poly(sulfobetaine) (PSB), poly(carboxybetaine) (PCB)]. - Biomimetic stealth coatings: red blood cell membranes, platelet membranes, immune cell membranes, cancer cell membranes, Escherichia coli membranes; CD47-mimicking “self” peptides. - NP cores: liposomes, micelles, dendrimers, hydrogels, virus-like nanoparticles, PLGA, PLA, PLGA-lecithin-PEG-biotin, gold NPs, mesoporous silica nanocapsules, graphene oxide nanosheets, hydrogel NPs, polystyrene NPs. - Payloads: doxorubicin, paclitaxel, vincristine, recombinant human tumour necrosis factor-α (rHuTNF-α), superoxide dismutase 1 (SOD1), model active pharmaceutical ingredient (API). - Routes: intravenous administration primarily; subcutaneous and intratracheal examples also cited.

Approach: Review of preclinical literature; no primary experiments. Discusses in vitro macrophage uptake, plasma protein adsorption, complement activation, and in vivo biodistribution/circulation in mouse and rat models. Examples include tumour-bearing mice and healthy animals. No clinical trials are reported; the review is focused on mechanistic and material design principles.

Key methods: Opsonisation and protein corona: plasma protein adsorption, complement activation pathways (classical, lectin, alternative), C3/C4/C5 opsonins. - Phagocytosis: macrophage uptake, MPS/RES clearance. - NP characterisation: size, surface charge, hydrophobicity, polymer molecular weight, surface chain density, conformation. - Circulation and biodistribution: blood half-life, liver/spleen/lung accumulation, tumour accumulation. - Polymer analysis: ELISA for antibody recognition, TEM/freeze-fracture TEM, Flory radius, mushroom vs brush conformation. - In vitro/in vivo correlation: macrophage association, phagocytic blood cell association, antitumor efficacy.

Key results: Au-PEG10k: >95% still circulating 1 h after intravenous injection; Au-PEG750 did not show prolonged circulation. - Uncoated Au NPs and Au-PEG750: ~90% captured in liver and spleen after 1 h; Au-PEG10k: <5% at 1 h and 54% at 24 h. - RBC membrane-coated PLGA NPs: elimination half-life 39.6 h vs 15.8 h for PEG-coated NPs in mice. - PEG MW: at least 2 kDa generally required for efficient shielding; protein adsorption decreased from 2 to 5 kDa, with no further reduction above 5 kDa in one study. - 100 nm polystyrene NPs: 559 Da PEG at density >1.2 PEG/nm² effectively shielded particles. - 5 kDa PEG on 100 nm polystyrene NPs: dense brush (R_F/D ≥ 2.8) required for maximal macrophage uptake suppression; ≥10 kDa PEG required R_F/D > 8. - Highly dense PEGylated particles (R_F/D ≥ 6.6): circulation half-life 14 h, with eventual liver accumulation. - PEGylated hydrogel NPs: mushroom (R_F/D ~0.9) and brush (R_F/D ~1.5) both reduced macrophage uptake; half-lives 15.5 h and 19.5 h vs 0.89 h uncoated. - POx: SOD1-POx conjugates enhanced neuronal uptake, crossed intact blood-brain barrier, and increased plasma circulation half-life vs native and PEGylated SOD1; PMeOx-coated tobacco mosaic virus showed better antibody shielding and reduced macrophage uptake vs PEGylated. - CD47-functionalised nanobeads: prolonged in vivo circulation, enhanced drug retention in tumours, and improved antitumor efficacy.

Interpretation: Stealth coating is essential for long-circulating NPs. PEG remains the benchmark, but anti-PEG antibodies and accelerated blood clearance (ABC) motivate alternatives. POx and polyzwitterions show promising stealth properties, while cell-membrane camouflaging and CD47 functionalisation provide biomimetic immune evasion. The authors conclude that molecular weight, surface density, and conformation must be optimised together; no single parameter is sufficient. Further studies are needed to evaluate safety and clinical translation.

Limitations: Review article; no primary data. - PEG immunogenicity: anti-PEG antibodies reported in 20–25% of normal individuals; ABC phenomenon on repeated dosing. - POx: no FDA approval; detailed immune interactions and biodistribution in blood remain incompletely understood. - Polyzwitterions: super-hydrophilic, insoluble in most organic solvents; research still at proof-of-concept stage. - Cell-membrane and CD47 strategies are nascent; large-scale manufacturing, characterisation, and clinical validation are limited. - Coating density threshold depends on particle size and surface curvature; different quantification methods cause variability. - Correlation between PEG molecular weight and surface density is difficult to control; increasing PEG content does not always improve circulation. - Long-term safety and efficacy of stealth NPs for clinical applications require further study.