Biomedical Engineering Reference
In-Depth Information
introduced by direct, covalent incorporation of PEG into the polymeric carrier
[
68
-
72
], direct PEGylation of the nucleic acid [
55
,
73
,
74
], PEGylation after
polyplex formation [
75
-
77
], or by attachment in a noncovalent manner [
78
,
79
].
PEGylation of polyplexes improves solubility, reduces the interaction with blood
cells and serum proteins, provides a better biocompatibility, and prolongs blood
circulation times [
64
,
80
,
81
]. In addition to PEG, other hydrophilic molecules like
poly(hydroxypropyl methacrylate) (pHPMA) [
82
,
83
], hyaluronan/hyaluronic acid
[
84
,
85
], various polyanions such as
g
-polyglutamic acid [
86
,
87
], or the receptor-
targeting ligand serum transferrin [
88
] have been applied to reduce the positive
surface charge of polyplexes.
Shielded polyplexes with improved blood circulating properties are interesting
tools for systemic cancer therapy (see Sect.
4.2
). Nanoparticles can take advantage
of the “enhanced permeability and retention” (EPR effect) [
89
] for passive tumor
targeting. The EPR effect is based on the leakiness of tumor vasculature, due to neo-
vascularization in growing tumors, combined with an inadequate lymphatic drain-
age. Nanoparticles with an elongated plasma circulation time can extravasate and
passively accumulate at the tumor site.
Polyplex surface shielding solves several crucial problems, but may also create
new problems. Shielding can strongly reduce the efficiency of subsequent cellular
steps of the delivery process [
68
,
69
], and also can negatively alter other polyplex
characteristics. For pDNA/PEI polyplexes with optimum medium size of PEI, PEG
was found to reduce the polyplex stability in vivo [
64
,
65
,
81
]. For a discussion of
these aspects see Sect.
3.1
.
2.2 Cell Targeting and Intracellular Uptake
By targeting cell surface receptors, defined target cells in various tissues can be
addressed. Targeting may be essential for efficiency, mediating cell-binding of
shielded polyplexes, and triggering enhanced polyplex uptake by receptor-
mediated endocytosis or related uptake pathways. Different ligands have been
found to be suitable for targeted nucleic acid delivery [
90
-
94
]. These can be
vitamins, carbohydrates, peptides, proteins and glycoproteins, antibodies in various
modifications, or nucleic acid aptamers. It has to be kept in mind that the mere
presence of targeting ligands does not guarantee targeting; there is no chemotaxis
involved. Polyplexes have to first reach the receptor by other means (e.g., by
passive targeting) to be available for biochemical receptor docking. It has been
observed by several groups that passive accumulation at the target site of targeted
and nontargeted polyplexes can be very similar. Active retention and endocytosis at
the target cells has been seen as a major functional difference and advantage of
receptor-targeted nanocarriers.
The 80 kDa glycoprotein transferrin (Tf) is responsible for intracellular iron
transport via the transferrin receptor (TfR), using a clathrin-dependent endocytosis
process. Tumor tissues frequently overexpress the TfR. While natural Tf recycles to
