at a polymer:DNA ratio of 10, while linear PEI completely retarded DNA at a ratio of
6. Polymers containing a large amount of PLLA reduced toxicity on HeLa cells
compared to linear PEI, but also mediated gene transfer less efficiently.
A non-degradable hydrophobic block in a copolymer can be used to introduce
hydrophobicity. A good example is Pluronics and its hydrophobic segment poly
(propylene glycol) (PPG), such as in PEG-PPG-PEG-b-PDMAEMA (Fig. 21e)
[219]. The pK a value of PEG-PPG-PEG-b-PDMAEMA was 7.1, lower than
PEG-b-PDMAEMA and PDMAEMA, due to the effect of Pluronic™ lowering the
dielectric constant of the amino groups. The polymer possessed a CMC of 5 g L
À1
,
which is relatively high. The polymer condensed DNA into polyplexes of 200 nm in
diameter at polymer:DNA ratios of 6 and more and a slightly positive zeta potential.
Compared to PEG-b-PDMAEMA, the condensation was less efficient but the transfection efficiency was much higher and at lower polymer:DNA ratio [220].
LPEI-b-PPG-b-LPEI (Fig. 21f) with various LPEI and PPG block lengths were
studied [221]. Note that these polymers, at least the ones with the highest hydrophilic:hydrophobic ratio, may self-assemble into flower-like micelles. LPEI 50 -bPPG 36 -b-LPEI 50 and LPEI 19 -b-PPG 36 -b-LPEI 19 were able to retard DNA migration
at a polymer:DNA weight ratio of 3:4 and 1:1, respectively, while LPEI 14 -b-PPG 68 -
b-LPEI 14 was not able to retard DNA even at a ratio of 15:1. These data, in
correlation with AFM studies, suggest that LPEI 50 -b-PPG 36 -b-LPEI 50 forms micellar structures where the positive charges are still available for interactions with
DNA, whereas in the case of LPEI 19 -b-PPG 36 -b-LPEI 19 , the positive charges must
be buried in the structure, hindering efficient electrostatic interactions with DNA.
2.2.3 Micelles of Amphiphilic Polymers and Lipopolymers
The polymers presented in this section can form micelles due to their amphiphilic
structure. Moreover, there is the possibility to use these micelles as multicarriers,
with hydrophobic drug loaded in the hydrophobic interior of the micelles and the
genetic material complexed on the positively charged shell, if the micelles can
structurally resist the addition of DNA.
P(MDS-co-CES) (Fig. 22a) is a biodegradable copolymer with a polyester main
chain and containing potentially hydrolytically labile urethano groups to link the
cholesterol moieties [222]. Moreover, this polymer contains both quaternary
ammonium groups (DNA binding) and tertiary amine groups (endosomal buffering). This polymer formed micelles (CMC ¼ 1.9 mg mL
À1 ), which were positively
charged (72 mV) and had a diameter of 96 nm in sodium acetate buffer and these
pre-formed micelles were used for complexation of pDNA. This approach is
different from the approach previously seen, where the polymer was added to
DNA and, consequently, micelle formation was hindered due to the stronger
electrostatic interactions between DNA and the positively charged block of the
copolymer. The obtained polyplexes exhibited decreased mobility in gel electrophoresis and complete retardation at N:P ratio of 2. By studying the changes in the
microenvironment of pyrene entrapped in the micelles, the authors verified the
integrity of the core–shell nanoparticles during the DNA binding process and that
Polyelectrolyte Complexes of DNA and Polycations as Gene Delivery Vectors
173
6. Polymers containing a large amount of PLLA reduced toxicity on HeLa cells
compared to linear PEI, but also mediated gene transfer less efficiently.
A non-degradable hydrophobic block in a copolymer can be used to introduce
hydrophobicity. A good example is Pluronics and its hydrophobic segment poly
(propylene glycol) (PPG), such as in PEG-PPG-PEG-b-PDMAEMA (Fig. 21e)
[219]. The pK a value of PEG-PPG-PEG-b-PDMAEMA was 7.1, lower than
PEG-b-PDMAEMA and PDMAEMA, due to the effect of Pluronic™ lowering the
dielectric constant of the amino groups. The polymer possessed a CMC of 5 g L
À1
,
which is relatively high. The polymer condensed DNA into polyplexes of 200 nm in
diameter at polymer:DNA ratios of 6 and more and a slightly positive zeta potential.
Compared to PEG-b-PDMAEMA, the condensation was less efficient but the transfection efficiency was much higher and at lower polymer:DNA ratio [220].
LPEI-b-PPG-b-LPEI (Fig. 21f) with various LPEI and PPG block lengths were
studied [221]. Note that these polymers, at least the ones with the highest hydrophilic:hydrophobic ratio, may self-assemble into flower-like micelles. LPEI 50 -bPPG 36 -b-LPEI 50 and LPEI 19 -b-PPG 36 -b-LPEI 19 were able to retard DNA migration
at a polymer:DNA weight ratio of 3:4 and 1:1, respectively, while LPEI 14 -b-PPG 68 -
b-LPEI 14 was not able to retard DNA even at a ratio of 15:1. These data, in
correlation with AFM studies, suggest that LPEI 50 -b-PPG 36 -b-LPEI 50 forms micellar structures where the positive charges are still available for interactions with
DNA, whereas in the case of LPEI 19 -b-PPG 36 -b-LPEI 19 , the positive charges must
be buried in the structure, hindering efficient electrostatic interactions with DNA.
2.2.3 Micelles of Amphiphilic Polymers and Lipopolymers
The polymers presented in this section can form micelles due to their amphiphilic
structure. Moreover, there is the possibility to use these micelles as multicarriers,
with hydrophobic drug loaded in the hydrophobic interior of the micelles and the
genetic material complexed on the positively charged shell, if the micelles can
structurally resist the addition of DNA.
P(MDS-co-CES) (Fig. 22a) is a biodegradable copolymer with a polyester main
chain and containing potentially hydrolytically labile urethano groups to link the
cholesterol moieties [222]. Moreover, this polymer contains both quaternary
ammonium groups (DNA binding) and tertiary amine groups (endosomal buffering). This polymer formed micelles (CMC ¼ 1.9 mg mL
À1 ), which were positively
charged (72 mV) and had a diameter of 96 nm in sodium acetate buffer and these
pre-formed micelles were used for complexation of pDNA. This approach is
different from the approach previously seen, where the polymer was added to
DNA and, consequently, micelle formation was hindered due to the stronger
electrostatic interactions between DNA and the positively charged block of the
copolymer. The obtained polyplexes exhibited decreased mobility in gel electrophoresis and complete retardation at N:P ratio of 2. By studying the changes in the
microenvironment of pyrene entrapped in the micelles, the authors verified the
integrity of the core–shell nanoparticles during the DNA binding process and that
Polyelectrolyte Complexes of DNA and Polycations as Gene Delivery Vectors
173