[184], or a decrease in the polarity of the polyion environment due to the presence
of the PEG chains [185]. Moreover, the presence of PEG prevented the aggregation
of polyplexes (as found in PDMAEMA polymer) and colloidally stable stoichiometric polyplexes were obtained. The capacity of a polymer to condense DNA is
dependent on the DMAEMA content, but where the DMAEMA units are placed
in the copolymers and the length of the PEG chains seem to play a role. Increasing
DMAEMA content led to better condensing properties, i.e., smaller complexes.
Despite, the propensity of these PEG-based copolymers to form small sterically
stabilized complexes, their transfection ability in A549 cells at a monomer:nucleotide ratio inferior or equal to 5 was inferior to that of PDMAEMA homopolymer,
maybe due to the presence of PEG, which provides a steric barrier around the
polyplexes that inhibits contact with cells. In a subsequent publication [186],
Stolnik and colleagues studied the cellular association and uptake and transfection
efficiency of these copolymers at a fixed monomer:nucleotide molar ratio on three
cell lines (A459, hepG2 and COS-7 cells) because transfection efficiency can be
cell type-dependent. The transfection efficiencies were similar in the three cell lines
for a given polymer but the rate of uptake of the complexes depended on the cell
line used. Comb polymer P(DMAEMA-stat-PEGMA) consistently showed lower
transfection efficiency than the linear PDMAEMA-b-PEG and bottle-brush
PDMAEMA-b-POEGMA; the latter two giving similar transfection levels with
all three cell lines. Indeed, P(DMAEMA-stat-PEGMA) showed less interaction
with the cells (in flow cytometry studies) as well as less cell uptake (as shown by
confocal microscopy) than the two other polymers. The authors explained this
as being due to the different structures of the particles formed in the presence of
DNA. Indeed, linear PDMAEMA-b-PEG and bottle-brush PDMAEMA-bPOEGMA have both diblock architectures, with the main difference being the
spatial distribution of the EG units (linear PEG chain versus branched with
OEG chains) but both polymers can form a micelle-like polyion complex with
DNA. P(DMAEMA-stat-PEGMA) on the other hand, has several long pendant
PEG chains (45 units) randomly distributed along the DMAEMA-based backbone;
this statistical structure leads to rather soluble complexes (as shown by light
scattering).
Given that a fraction of EG units much more than 20% in polymers seemed to
hinder complexation in previous studies, Yang and colleagues conducted a systematic study [181] using a fixed fraction of EG units (~20%). This allowed comparison
of various structures of PDMAEMA-PEG copolymers of the same molecular
weight (M n ~ 11 kDa): block with linear PEG (PDMAEMA-b-PEG, Fig. 12a),
brush block copolymer (PDMAEMA-b-POEGMA, Fig. 12b), and statistical copolymer P(DMAEMA-stat-OEGMA); for this last structure, the effect of molecular
weight was also tested. All these polymers were found to complex well with DNA
and completely retarded DNA migration at an N:P ratio of 2, showing that in this
case (low fraction of EG), the structure of the PEG block did not affect the gene
binding capacity of the polymers (nevertheless surprising for stat and brush
copolymers). Similar results were obtained for polyanion exchange, but the difference can be seen in the capacity of DNA compaction, where the hydrodynamic
156
A. Bertin
of the PEG chains [185]. Moreover, the presence of PEG prevented the aggregation
of polyplexes (as found in PDMAEMA polymer) and colloidally stable stoichiometric polyplexes were obtained. The capacity of a polymer to condense DNA is
dependent on the DMAEMA content, but where the DMAEMA units are placed
in the copolymers and the length of the PEG chains seem to play a role. Increasing
DMAEMA content led to better condensing properties, i.e., smaller complexes.
Despite, the propensity of these PEG-based copolymers to form small sterically
stabilized complexes, their transfection ability in A549 cells at a monomer:nucleotide ratio inferior or equal to 5 was inferior to that of PDMAEMA homopolymer,
maybe due to the presence of PEG, which provides a steric barrier around the
polyplexes that inhibits contact with cells. In a subsequent publication [186],
Stolnik and colleagues studied the cellular association and uptake and transfection
efficiency of these copolymers at a fixed monomer:nucleotide molar ratio on three
cell lines (A459, hepG2 and COS-7 cells) because transfection efficiency can be
cell type-dependent. The transfection efficiencies were similar in the three cell lines
for a given polymer but the rate of uptake of the complexes depended on the cell
line used. Comb polymer P(DMAEMA-stat-PEGMA) consistently showed lower
transfection efficiency than the linear PDMAEMA-b-PEG and bottle-brush
PDMAEMA-b-POEGMA; the latter two giving similar transfection levels with
all three cell lines. Indeed, P(DMAEMA-stat-PEGMA) showed less interaction
with the cells (in flow cytometry studies) as well as less cell uptake (as shown by
confocal microscopy) than the two other polymers. The authors explained this
as being due to the different structures of the particles formed in the presence of
DNA. Indeed, linear PDMAEMA-b-PEG and bottle-brush PDMAEMA-bPOEGMA have both diblock architectures, with the main difference being the
spatial distribution of the EG units (linear PEG chain versus branched with
OEG chains) but both polymers can form a micelle-like polyion complex with
DNA. P(DMAEMA-stat-PEGMA) on the other hand, has several long pendant
PEG chains (45 units) randomly distributed along the DMAEMA-based backbone;
this statistical structure leads to rather soluble complexes (as shown by light
scattering).
Given that a fraction of EG units much more than 20% in polymers seemed to
hinder complexation in previous studies, Yang and colleagues conducted a systematic study [181] using a fixed fraction of EG units (~20%). This allowed comparison
of various structures of PDMAEMA-PEG copolymers of the same molecular
weight (M n ~ 11 kDa): block with linear PEG (PDMAEMA-b-PEG, Fig. 12a),
brush block copolymer (PDMAEMA-b-POEGMA, Fig. 12b), and statistical copolymer P(DMAEMA-stat-OEGMA); for this last structure, the effect of molecular
weight was also tested. All these polymers were found to complex well with DNA
and completely retarded DNA migration at an N:P ratio of 2, showing that in this
case (low fraction of EG), the structure of the PEG block did not affect the gene
binding capacity of the polymers (nevertheless surprising for stat and brush
copolymers). Similar results were obtained for polyanion exchange, but the difference can be seen in the capacity of DNA compaction, where the hydrodynamic
156
A. Bertin