Poly(1-vinylimidazole)s alkylated with various chain lengths (R) and
quaternization degree β (PVIm-R-β, Fig. 20g) were studied by the group of
Asayama [212]. Due to the fact that imidazole groups are negligibly charged at
physiological pH, quaternary nitrogen atoms as strong electrolytes were introduced
in the structure. In order to still benefit from the pH buffering capacity of imidazole
groups at endosomal pH, the quaternization was only partial. Taking PVIm-Bu as
an example, complete DNA retardation occurred at a ratio of [butylated imidazole]/
[phosphate] of around 1. Given that the pK a of the unmodified imidazole groups is
around 6, the efficiency of DNA complexation at pH 6 (retardation of DNA
migration occurred at a ratio of [butylated imidazole]/[phosphate] of 0.5) was
greater than at pH 7.4, benefiting from the protonation of these imidazole groups.
Moreover, the polyplexes based on PVIm-Bu caused negligible hemolysis at pH 7.4
but had a membrane disruptive activity at endosomal pH, and their stability (against
polyanion exchange) was between that of PVIm-NH 2 and PVIm-Oct. Concerning
the cytotoxicity, PVIm-R with short alkyl chains (methyl, ethyl, butyl) were
relatively non-cytotoxic, while PVIm-Oct caused significant cytotoxicity. Gene
transfection of the luciferase gene to Hep2 cells was dependent on the length and
density of the alkyl chains: for PVIm-Bu; the higher the density of butylated
imidazole groups, the better was the transfection efficiency at low charge ratios
(+/À < 12). For a middle density of alkylated imidazole groups (~20%), PVIm-Me
and PVIm-Et mediated a higher gene expression than PVIm-Bu even at lower
+/À charge ratio. This could be explained, as in the last case, by too much screening
of the charges with the butyl chains.
2.2.2 Amphiphilic Polymers and Lipopolymers
Lipopolymers are polymers containing lipid moieties such as a fatty acid or a
steroid such as cholesterol. At least some of the polymers presented in this section
could eventually form micelles due to their amphiphilic structure, but either the
concentration of their solution is under the CMC or the micelles are diluted and/or
destabilized during their addition to the DNA solution.
Hydrophobicity can be introduced onto the side chains with hydrophobic moieties
such as cholesterol and 1,2-dioleoyl-sn-glycero-3-phosphoethanolamine (DOPE) as
will be presented in the next two examples. P(QuatDMAEMA-co-Chol) (Fig. 21a) is
a copolymer containing quaternary ammonium units and cholesterol but no tertiary
amines [213]. As previously seen, the range where the polyplexes are stable is
diminished if the content of hydrophobic groups is too high. By slow addition of
polycation to DNA, at N:P ratio close to 1, flocculation was observed for polyplexes
based on the polymers with the highest cholesterol content (6.3 and 8.7 mol%) and
only negatively charged polyplexes could be prepared. By fast addition of the
polycation with highest cholesterol content to DNA, the M w of the aggregates showed
the inverse tendency and decreased with increasing N:P ratios, even close to unity as
already observed in the studies by Kabanov and Kabanov [214] and Oupicky et al.
[215], while staying relatively constant above N:P ¼ 1 for the polyplexes based on
170
A. Bertin
quaternization degree β (PVIm-R-β, Fig. 20g) were studied by the group of
Asayama [212]. Due to the fact that imidazole groups are negligibly charged at
physiological pH, quaternary nitrogen atoms as strong electrolytes were introduced
in the structure. In order to still benefit from the pH buffering capacity of imidazole
groups at endosomal pH, the quaternization was only partial. Taking PVIm-Bu as
an example, complete DNA retardation occurred at a ratio of [butylated imidazole]/
[phosphate] of around 1. Given that the pK a of the unmodified imidazole groups is
around 6, the efficiency of DNA complexation at pH 6 (retardation of DNA
migration occurred at a ratio of [butylated imidazole]/[phosphate] of 0.5) was
greater than at pH 7.4, benefiting from the protonation of these imidazole groups.
Moreover, the polyplexes based on PVIm-Bu caused negligible hemolysis at pH 7.4
but had a membrane disruptive activity at endosomal pH, and their stability (against
polyanion exchange) was between that of PVIm-NH 2 and PVIm-Oct. Concerning
the cytotoxicity, PVIm-R with short alkyl chains (methyl, ethyl, butyl) were
relatively non-cytotoxic, while PVIm-Oct caused significant cytotoxicity. Gene
transfection of the luciferase gene to Hep2 cells was dependent on the length and
density of the alkyl chains: for PVIm-Bu; the higher the density of butylated
imidazole groups, the better was the transfection efficiency at low charge ratios
(+/À < 12). For a middle density of alkylated imidazole groups (~20%), PVIm-Me
and PVIm-Et mediated a higher gene expression than PVIm-Bu even at lower
+/À charge ratio. This could be explained, as in the last case, by too much screening
of the charges with the butyl chains.
2.2.2 Amphiphilic Polymers and Lipopolymers
Lipopolymers are polymers containing lipid moieties such as a fatty acid or a
steroid such as cholesterol. At least some of the polymers presented in this section
could eventually form micelles due to their amphiphilic structure, but either the
concentration of their solution is under the CMC or the micelles are diluted and/or
destabilized during their addition to the DNA solution.
Hydrophobicity can be introduced onto the side chains with hydrophobic moieties
such as cholesterol and 1,2-dioleoyl-sn-glycero-3-phosphoethanolamine (DOPE) as
will be presented in the next two examples. P(QuatDMAEMA-co-Chol) (Fig. 21a) is
a copolymer containing quaternary ammonium units and cholesterol but no tertiary
amines [213]. As previously seen, the range where the polyplexes are stable is
diminished if the content of hydrophobic groups is too high. By slow addition of
polycation to DNA, at N:P ratio close to 1, flocculation was observed for polyplexes
based on the polymers with the highest cholesterol content (6.3 and 8.7 mol%) and
only negatively charged polyplexes could be prepared. By fast addition of the
polycation with highest cholesterol content to DNA, the M w of the aggregates showed
the inverse tendency and decreased with increasing N:P ratios, even close to unity as
already observed in the studies by Kabanov and Kabanov [214] and Oupicky et al.
[215], while staying relatively constant above N:P ¼ 1 for the polyplexes based on
170
A. Bertin