Poly(ethylenimine) and its role in gene delivery

Purpose: Gene therapy still lacks safe and efficient delivery vectors. PEI has emerged as a promising non-viral polycation because it condenses DNA and mediates endosomal escape, but its transfection mechanism, targeting potential, and toxicity require clearer understanding. This review summarizes PEI chemistry, PEI/DNA complex characterization, transfection considerations, targeting approaches, mechanism of action, and toxicity.

Hypothesis: This is a review article and does not test a single formal hypothesis. Its central premise is that PEI’s high density of protonatable amines enables DNA condensation, endosomal buffering, and nuclear delivery, making it a promising non-viral gene delivery vehicle, but that efficiency, targeting, and toxicity must be better understood and controlled for clinical utility.

Aims: Briefly review PEI chemistry and the characterization of PEI/DNA complexes. - Discuss transfection considerations, including polymer molecular weight, charge ratio, serum effects, and cell-type range. - Examine cell-targeting approaches using ligands and cell-specific promoters. - Review the proposed mechanism of PEI-mediated transfection, including endocytosis, endosomal escape, and nuclear entry. - Survey toxicity issues related to PEI and PEI/DNA complexes.

Delivery system: Polymer: Poly(ethylenimine) (PEI), linear and branched forms; branched PEI is more commonly used for transfection. - Complex type: PEI/DNA polyplexes formed by electrostatic interaction between cationic PEI amines and anionic DNA phosphates. - Payload: Plasmid DNA; potential for larger genetic cargo than adenoviral or adeno-associated viral vectors. - Targeting ligands: Transferrin, RGD peptide, anti-CD3, galactose, folate, anti-CD4, gp120, LDL, and integrin-binding peptides. - Targeting strategy: Ligand-mediated endocytosis and cell-specific promoters for targeted gene expression. - Key physicochemical features: Size, shape, surface charge, buffering capacity, N/P ratio.

Approach: Review of in vitro and in vivo literature. In vitro studies include many cell types; PEI-mediated transfection documented in 25 different cell types, including 18 human cell lines and pig/rat primary cells. In vivo studies include newborn and adult mice and Sprague-Dawley rats, with delivery to brain, kidney, and lung. Clinical gene therapy trials referenced for cystic fibrosis and cancer. No new primary experiments are reported.

Key methods: Gel fractionation chromatography for PEI molecular weight and polydispersity. - Atomic force microscopy and electron microscopy for polyplex size and morphology. - Dynamic light scattering for apparent diameters in solution. - Zeta potential for surface charge. - Transfection assays for gene expression. - Confocal microscopy for intracellular trafficking. - Toxicity and membrane permeabilization studies.

Key results: PEI forms: Branched PEI condenses DNA more extensively than linear PEI and has yielded greater transfection success. - PEI/DNA size: 20–40 nm by AFM; toroidal structures of 55 ± 12 nm by electron microscopy; apparent diameters of 90–130 nm by dynamic light scattering. - Surface charge: Branched PEI zeta potential ~37 mV; PEI/DNA complexes ~31.5 mV at 7.5:1 N/P; centrifugation lowered to ~29.2 mV. - Charge ratio: Maximal in vitro transfection with branched PEI at 9–13.5 PEI nitrogens per DNA phosphate; ratios 5:1–13.5:1 used successfully. - Molecular weight: In vivo, PEI 25 kDa gave higher transfection than higher molecular weight PEIs; in vitro, transfection efficiency increased with molecular weight for low-MW PEIs. Polydispersity differences may explain conflicting results. - Cell targeting: Ligand coupling improved uptake and transfection when cells recognized the ligand. - Mechanism: PEI/DNA complexes are endocytosed, buffer endolysosomal pH, escape into cytoplasm, and enter nuclei as intact ordered structures. - Toxicity: Free PEI harms cells, but DNA-bound PEI is less toxic; low PEI concentrations do not appear to disrupt plasma membranes, though endosomal concentrations may cause permeabilization.

Interpretation: PEI is a successful non-viral transfection agent that spontaneously condenses DNA into toroidal complexes, is readily endocytosed, buffers endolysosomal pH, and delivers DNA to the nucleus. Its transfection mechanism and physicochemical properties provide a basis for designing future non-viral vectors. However, increasing transfection efficiency while reducing toxicity is necessary before PEI can be used for efficacious gene therapies.

Limitations: This is a review, not a primary study; no new experimental data or meta-analysis. - The mechanism of PEI-mediated nuclear entry remains speculative. - Toxicity, especially at endosomal concentrations, is not fully resolved. - Many studies use different PEI sources, molecular weights, and polydispersities, complicating comparisons. - Clinical translation is not established; no FDA-approved PEI gene therapy is presented. - Long-term effects on host-cell transcription and in vivo safety are not fully known. - No large-animal validation or detailed clinical trial data are presented.