of bioreducible polymers include PEI, polyion complex (PIC) micelles, polyamidoamine (PAA) and polypeptides.
PEI and PEI Conjugates
PEI is polycationic polymer that can polyplex DNA at low PEI:DNA ratios and is
easily taken up by cells through endocytosis. Within the endosome, the amine
groups of PEI can buffer the protons to undergo mechanical swelling (proton
sponge effect), increase intra-endosomal osmotic pressure and, thereby, promote
endosomal disruption that can lead to efficient endosomal escape of the polyplex
[40]. Thus, PEI can result in increased success of gene delivery. Unfortunately, PEI
was shown to have high cytotoxicity that increased with increase in its molecular
weight.
Studies have shown that conjugation of PEI with disulfite linkages in their
polymeric structure can impart bioreducible properties, increase intracellular release
of the DNA and reduce cytotoxicity. Crosslinking of low molecular weight PEI with
a homo-bifunctional and amine-reactive crosslinker such as dithiobis(succinimidylpropionate) (DSP) and dimethyl 3,30-dithiobispropionimidate.2HCl (DTBP)
[41, 42], cystamine bisacrylamide (CBA) [43] or methylthiirane (thiolation) [44]
significantly reduced cytotoxicity and improved gene delivery efficiency. In
a comparative study, Breunig et al. [45] reported that disulfide crosslinked low
molecular weight linear PEIs (polycationic bioreducible PEIs) had higher transfection efficiency and lower cytotoxicity than commercial transfection reagents such as
PolyFect, SuperFect, Lipofectamine, FuGENE6 or JetPEI. These beneficial effects
were observed with both linear [46] and branched PEIs [47].
Acetylation [48] and PEGylation [49, 50] were also shown to influence the gene
delivery efficiency of PEI. Hosseinkhani et al. [48] reacted PEI with acetic anhydride to acetylate 80% of the primary and 20% of the secondary amines. This
acetylated PEI was shown to have enhanced gene delivery efficiency over unmodified PEI for MSCs. Chen et al. [51] showed that a PEG–PEI copolymer had better
gene delivery efficiency than cationic liposomes and did not affect the bionomics,
proliferation and differentiation potential of MSCs.
Others have used PEI to coat the biopolymers to form NPs. The PEI coated on
biopolymers caused polyplexing of DNA [52, 53] while the biopolymers
increased the cellular uptake [54] and reduced the cytotoxicity [55] by modifying
the surface charge and dispersing the stability and buffering capacity of the
resulting NPs [55]. The PEI coated on biomaterials such as hyaluronan (HA)
also helped in controlled, sustained and prolong release of the DNA [55]. Park
et al. [56] polyplexed four genes (SOX5, SOX6 and SOX9 genes fused to GFP,
YFP or RFP marker) with PEI coated onto PLGA NPs and obtained ~80%
transfection efficiency in human MSCs. By polyplexing with PEI, the cell-uptake
ability of the DNA-loaded NPs was enhanced for both in vitro and in vivo culture
systems, including human MSCs [56]. Jeon et al. [54] achieved co-delivery of
DNA and siRNA into human MSCs by complexing them with PEI coated on
Nanoparticles for Gene Delivery into Stem Cells and Embryos
59
PEI and PEI Conjugates
PEI is polycationic polymer that can polyplex DNA at low PEI:DNA ratios and is
easily taken up by cells through endocytosis. Within the endosome, the amine
groups of PEI can buffer the protons to undergo mechanical swelling (proton
sponge effect), increase intra-endosomal osmotic pressure and, thereby, promote
endosomal disruption that can lead to efficient endosomal escape of the polyplex
[40]. Thus, PEI can result in increased success of gene delivery. Unfortunately, PEI
was shown to have high cytotoxicity that increased with increase in its molecular
weight.
Studies have shown that conjugation of PEI with disulfite linkages in their
polymeric structure can impart bioreducible properties, increase intracellular release
of the DNA and reduce cytotoxicity. Crosslinking of low molecular weight PEI with
a homo-bifunctional and amine-reactive crosslinker such as dithiobis(succinimidylpropionate) (DSP) and dimethyl 3,30-dithiobispropionimidate.2HCl (DTBP)
[41, 42], cystamine bisacrylamide (CBA) [43] or methylthiirane (thiolation) [44]
significantly reduced cytotoxicity and improved gene delivery efficiency. In
a comparative study, Breunig et al. [45] reported that disulfide crosslinked low
molecular weight linear PEIs (polycationic bioreducible PEIs) had higher transfection efficiency and lower cytotoxicity than commercial transfection reagents such as
PolyFect, SuperFect, Lipofectamine, FuGENE6 or JetPEI. These beneficial effects
were observed with both linear [46] and branched PEIs [47].
Acetylation [48] and PEGylation [49, 50] were also shown to influence the gene
delivery efficiency of PEI. Hosseinkhani et al. [48] reacted PEI with acetic anhydride to acetylate 80% of the primary and 20% of the secondary amines. This
acetylated PEI was shown to have enhanced gene delivery efficiency over unmodified PEI for MSCs. Chen et al. [51] showed that a PEG–PEI copolymer had better
gene delivery efficiency than cationic liposomes and did not affect the bionomics,
proliferation and differentiation potential of MSCs.
Others have used PEI to coat the biopolymers to form NPs. The PEI coated on
biopolymers caused polyplexing of DNA [52, 53] while the biopolymers
increased the cellular uptake [54] and reduced the cytotoxicity [55] by modifying
the surface charge and dispersing the stability and buffering capacity of the
resulting NPs [55]. The PEI coated on biomaterials such as hyaluronan (HA)
also helped in controlled, sustained and prolong release of the DNA [55]. Park
et al. [56] polyplexed four genes (SOX5, SOX6 and SOX9 genes fused to GFP,
YFP or RFP marker) with PEI coated onto PLGA NPs and obtained ~80%
transfection efficiency in human MSCs. By polyplexing with PEI, the cell-uptake
ability of the DNA-loaded NPs was enhanced for both in vitro and in vivo culture
systems, including human MSCs [56]. Jeon et al. [54] achieved co-delivery of
DNA and siRNA into human MSCs by complexing them with PEI coated on
Nanoparticles for Gene Delivery into Stem Cells and Embryos
59