100–300 nm and a slightly positive zeta potential, while PDMAEA and PDEAEMA
were not capable of condensing pDNA to small particles, possibly due to their
relatively low molecular weight (and low solubility of PDEAEMA at pH 7). The
transfection efficiency and the cytotoxicity of the polymers differed widely: the
highest transfection efficiency and cytotoxicity were observed for PDMAEMA.
As PDMAEMA is capable of condensing DNA to small particles and has the lowest
average pK a value (7.5) of all condensing derivatives, PDMAEMA has the highest
buffering capacity and probably behaves as the best candidate for endosomal
escape. Furthermore, molecular modeling showed that, of all studied polymers,
PDMAEMA has the lowest number of interactions with DNA. Therefore, the
authors hypothesized that the superior transfection efficiency of its polyplexes
can be ascribed to the intrinsic property of this polymer to destabilize endosomes
combined with an easy dissociation of the polyplex once present in the cytosol
and/or nucleus.
Hennink and colleagues also studied copolymers of DMAEMA with various
monomers such as the hydrophobic methyl methacrylate (MMA) in P(DMAEMAco-MMA) (Fig. 7f), or hydrophilic N-vinyl pyrrolidone (NVP) in P(DMAEMA-coNVP) (Fig. 7h), and OEGMA in P(DMAEMA-co-OEMA) (Fig. 7g) of M w > 90 kDa
[142]. A copolymer with 20 mol% of MMA showed reduced transfection efficiency
and a substantially increased cytotoxicity compared with PDMAEMA of the same
molecular weight. A copolymer with OEGMA (48 mol%) showed both a reduced
transfection efficiency and a reduced cytotoxicity (presence of OEMA), whereas a
copolymer with NVP (54 mol%) yielded smaller particles at a lower P:DNA ratio
than PDMAEMA or than the other copolymers with equivalent degree of modification (as NVP interacts with DNA via hydrogen bonding) and showed an increased
transfection and decreased cytotoxicity.
Further derivatives of PDMAEMA were synthesized in order to improve its
condensation ability at physiological pH, such as a comb-type polycation consisting
of P(DMAEMA-co-PLL) (Fig. 7i) [144]. This copolymer possessed a
pH-dependent behavior due to the presence of PDMAEMA (pK a 7.5) and PLL
(pK a 10). The presence of the PLL segments prevented precipitation of the copolymer at pH > 7.5, as is the case for PDMAEMA homopolymer, and this comb-type
polymer was capable of DNA condensation at pH 8 (as observed using circular
dichroism).
Poly(hydroxyethyl
methacrylate)
backbones,
onto
which
poly
[2-(dimethylamino)ethyl methacrylate] of various lengths were grafted via click
chemistry [P(HEMA-g-PDMAEMA), M w > 75 kDa; Fig. 7j], were able to condense DNA into small particles (90–190 nm, at a polymer to plasmid mass ratio of 6)
[145] and the sizes as well as the zeta potentials of the P(HEMA-g-PDMAEMA)based polyplexes were independent of the molecular weight of P(HEMA-gPDMAEMA) (for M w of 75 kDa and above). P(HEMA-g-PDMAEMA) showed
more EtBr quenching than the starting PDMAEMA, indicating a weaker binding
of this low molecular weight polymer to the pDNA. However, it showed similar
quenching as high molecular weight PDMAEMA, but less toxicity.
Polyelectrolyte Complexes of DNA and Polycations as Gene Delivery Vectors
143
were not capable of condensing pDNA to small particles, possibly due to their
relatively low molecular weight (and low solubility of PDEAEMA at pH 7). The
transfection efficiency and the cytotoxicity of the polymers differed widely: the
highest transfection efficiency and cytotoxicity were observed for PDMAEMA.
As PDMAEMA is capable of condensing DNA to small particles and has the lowest
average pK a value (7.5) of all condensing derivatives, PDMAEMA has the highest
buffering capacity and probably behaves as the best candidate for endosomal
escape. Furthermore, molecular modeling showed that, of all studied polymers,
PDMAEMA has the lowest number of interactions with DNA. Therefore, the
authors hypothesized that the superior transfection efficiency of its polyplexes
can be ascribed to the intrinsic property of this polymer to destabilize endosomes
combined with an easy dissociation of the polyplex once present in the cytosol
and/or nucleus.
Hennink and colleagues also studied copolymers of DMAEMA with various
monomers such as the hydrophobic methyl methacrylate (MMA) in P(DMAEMAco-MMA) (Fig. 7f), or hydrophilic N-vinyl pyrrolidone (NVP) in P(DMAEMA-coNVP) (Fig. 7h), and OEGMA in P(DMAEMA-co-OEMA) (Fig. 7g) of M w > 90 kDa
[142]. A copolymer with 20 mol% of MMA showed reduced transfection efficiency
and a substantially increased cytotoxicity compared with PDMAEMA of the same
molecular weight. A copolymer with OEGMA (48 mol%) showed both a reduced
transfection efficiency and a reduced cytotoxicity (presence of OEMA), whereas a
copolymer with NVP (54 mol%) yielded smaller particles at a lower P:DNA ratio
than PDMAEMA or than the other copolymers with equivalent degree of modification (as NVP interacts with DNA via hydrogen bonding) and showed an increased
transfection and decreased cytotoxicity.
Further derivatives of PDMAEMA were synthesized in order to improve its
condensation ability at physiological pH, such as a comb-type polycation consisting
of P(DMAEMA-co-PLL) (Fig. 7i) [144]. This copolymer possessed a
pH-dependent behavior due to the presence of PDMAEMA (pK a 7.5) and PLL
(pK a 10). The presence of the PLL segments prevented precipitation of the copolymer at pH > 7.5, as is the case for PDMAEMA homopolymer, and this comb-type
polymer was capable of DNA condensation at pH 8 (as observed using circular
dichroism).
Poly(hydroxyethyl
methacrylate)
backbones,
onto
which
poly
[2-(dimethylamino)ethyl methacrylate] of various lengths were grafted via click
chemistry [P(HEMA-g-PDMAEMA), M w > 75 kDa; Fig. 7j], were able to condense DNA into small particles (90–190 nm, at a polymer to plasmid mass ratio of 6)
[145] and the sizes as well as the zeta potentials of the P(HEMA-g-PDMAEMA)based polyplexes were independent of the molecular weight of P(HEMA-gPDMAEMA) (for M w of 75 kDa and above). P(HEMA-g-PDMAEMA) showed
more EtBr quenching than the starting PDMAEMA, indicating a weaker binding
of this low molecular weight polymer to the pDNA. However, it showed similar
quenching as high molecular weight PDMAEMA, but less toxicity.
Polyelectrolyte Complexes of DNA and Polycations as Gene Delivery Vectors
143