Design of functional dendritic polymers for application as drug and ...

Sideratou et al: Dendritic polymers for application as drug and gene delivery systems
determined in detail, but it is presumably based on
acid-base interactions between the anionic phosphate
moieties on the DNA backbone and the primary and
tertiary amines of the dendrimers, which are positively
charged under physiological conditions. It has also been
found (Tang et al, 1996) that partially degraded (or
fragmented) dendrimers, are more appropriate for gene
delivery than the intact dendrimers and a fragmentation (or
activation step) consisting of hydrolytic cleavage of the
amide bonds is needed to enhance the transfection. These
dendrimers are characterized as activated and are shown in
Figure 24. It has been concluded from several
investigations that the spherical shape of dendrimers is not
advantageous in gene delivery. This agrees with earlier
work, where ‘fragmented’ PAMAM dendrimers show
NH2 H2N O N
H
N
N
H
O
N
O
O
H
N O
H
N N
N N
N
H
O
H
N
O
O
N
N N
H
H
O N
O
H
N
N O
O
N
H
N
O
N
H
H
H
NH H 2 2N H
N
H N
O
N H H
O N
NH2 H2N H
N O O
O
N
N
O
H
H
H
N
2N
H NH2 NH
N
2
N
O
N
N H
H
O H
N
2N
O
H N
N
O
H H
N
2N
O
N O
H
N
H
H
O N
N
N NH2 H2N N
O
N O
O
NH2 H
N O
H
H
O
N
H
N N
H
N N
O
O
H N
N H
2N
O
H
N
H N
NH
2N O
O
2
N
H
H N
O N
O
H
N
H
N
N
O
H H
2N
N O
N
H O
2N
O O
N
H
N
N N
H H
H
O N
N
N
O
O
O
N
H O
H
O N
2N H2N N N
H N
N H
H
H
H
N O
O N
N
O
N
NH O O
2 H H
N
2N H N N H NH2 H
O NH2 N
N H
H
O
N
O NH2 N N NH2 H
H
O N
O
N
NH2 N NH2 H
O
N H
H2N NH2 H2N Figure 24. Structural features of intact vs activated PAMAM Dendrimers.
superior transfection efficacy in comparison with the
spherical ‘intact’ dendrimers (Boas and Heegaard, 2004).
In comparison to the intact dendrimers, the partially
degraded dendrimers have a more flexible structure (fewer
amide bonds) and form a more compact complex with
DNA, which is preferable for gene delivery by the
endocytotic pathway (Dennig and Duncan, 2002). In
addition, it is generally found that the maximum
transfection efficiency (Figure 25) is obtained with an
excess of primary amines to DNA phosphates, yielding a
positive net charge of the complexes The more flexible
higher generation DAB dendrimers (containing no amides)
are found to be too cytotoxic for use as non-viral gene
vectors, however, the lower generation dendrimers are
well-suited for gene delivery (Zinselmeyer et al, 2002).
H 2N
H 2N
H 2N
O
N
H
HO
H 2N
HO
N
O
N O
H N
O
O
N
H
N
H
N
O
H
HO
N O
O
HO
H
O
N
O
O
N
H
O
N
HO
N
O
N
N
H
HO
N O
O
N
H
N
H
N
O
O
HO
O
O
N
H H
N N
H N
O
H
N
N
O
O
N
N
H
O
N H
O
N
N
H
N
O
H N
N
O
H N
NH 2
H 2N
N H
O
N H
O H
N
O
O
OH
N
N
OH
O
O
O
N
OH
O
N
O
N
H
N
H
N
O
Figure 25. Transfection efficacy of poly(propylene imine) dendrimers of various generations (G1-G5) relative to N-[1-(2,3dioleoyloxy)propyl]-N,N,N-trimethylammonium
methylsulphate (DOTAP) in the A431 cell line studied in 96-well plates. DAB G1 and
DAB G2 were dosed at a dendrimer: DNA ratio of 5:1, and a DNA dose of 20 µg per well was used. DAB G3 was also dosed at a
dendrimer: DNA ratio of 5:1, but a DNA dose of 5 µg per well DNA was used; DAB G4 and DAB G5 were dosed at a dendrimer: DNA
ratio of 3:1 and a DNA dose of 20 µg per well. DOTAP was dosed at a DOTAP:DNA ratio of 5:1, and a DNA dose of 20 µg per well.
Data represented as the mean ±SD of at least 3 replicates. Reproduced from Zinselmeyer et al., 2002 with kind permission from
Springer.
88
N
H
N
O
OH
H
N
O
O
OH
H
O N
NH 2
N
O
N
N
H
H
O N
O N
H
NH 2
O
NH 2
NH 2