Table 5.

Summary of strategies for reducing siRNA carrier nanotoxicity.

Strategy	Example	Ref.
Optimization of carrier materials
Use linear, low-molecular-weight polymer	Linear PEI induced less proinflammatory cytokines than the branched counterparts	[26]
Use polymer with higher biodegradability	siRNA nanocarriers made of biodegradable poly(amino esters) demonstrated significantly higher transfection efficiency and lower toxicity to lung cells when compared with 25-kDa PEI	[54]
Use well-established natural polymer	Human lung cancer cells transfected with a chitosan nanosystem demonstrated higher viability than when using a commercial lipid transfection agent	[61]
Use bioreducible polymer	Nanocarriers prepared by cross-linking low-molecular-weight linear poly(ethylenimine sulfide) quickly degraded in the reductive intracellular environment and produced degradants that were essentially nontoxic (cell survival 98.69%)	[104]
Engineer cationic lipids based on structure–toxicity analysis	Lipids containing biodegradable linker had lower inherent cytotoxicity	[79]
Optimization of physical properties
Screen for right size	Gold NPs with the smallest sizes (3 and 5 nm) and the largest sizes (50 and 100 nm) are nontoxic, but the intermediate sizes (8–37 nm) had lethal effects on mice	[89]
Masking the cationic charges	PAMAM dendrimer was engineered to keep the cationic charges inside the carrier instead of on the surface, resulting in significantly reduced dendrimer toxicity while improving the in vivo tumor-targeting properties when conjugated with LHRH	[90]
Increase surface hydrophilicity with PEGlyation	After pulmonary administration, it was shown that hydrophilic modifications of PEI nanocarriers with PEG grafting induced fewer proinflammatory effects without depleting macrophages and disrupting the epithelial/endothelial barrier in the lungs	[106]
Alternative delivery routes	Spherical nucleic acid nanoparticle conjugates applied topically did not cause clinical or histological signs of toxicity or trigger cytokine activation in mouse blood or tissue samples	[107]
Low-dose approach based on combinatorial synthesis and screening	Highly potent lipidoid-based siRNA nanoparticles allowed low-dose treatment that was well tolerated in mice	[108]
Hybrid nanocarrier made of two or more classes of materials	Solid lipids in lipid–polymer hybrid nanocarriers reduced the toxicity of PEI components in human breast epithelial cells as indicated by less membrane damage, improved mitochondrial function, reduced reactive oxidative species production and lower caspase-3 activity	[30]
Active targeting
EGFR targeting	Asymmetric liposome particles showed no cell toxicity	[114]
Folate receptor targeting	Folate-PEG-grafted siRNA nanocarriers demonstrated favorable cytotoxicity, transfection efficiency and cancer cell specificity compared with PEG-grafted nanocarriers	[115]
LHRH peptide-based targeting	NPs containing superparamagnetic iron oxide and poly(propyleneimine) dendrimer functionalized with LHRH peptide and PEG demonstrated enhanced serum stability, specific cellular uptake and low toxicity in A549 cells	[90]
Cell-penetrating peptide-based targeting	Chitosan-based siRNA nanocarriers functionalized with PEG and the cell-penetrating peptide TAT exhibited maximum cell transfection ability and very low cytotoxicity in Neuro2a neuroblastoma cells	[116]

NP: Nanoparticle; PAMAM: Polyamidoamine; PEI: Polyethylenimine.