Biomedical Engineering Reference
In-Depth Information
manner. Some of these pH-regulated polymers, although mainly designed for
incorporation into liposomal drug delivery systems [
174
,
175
], might be also useful
in the context of polyplexes.
Intracellular pathways after escape from the endolysosomal system into the
cytosol are less clear. Obvious bottlenecks include, in the case of gene transfer
(pDNA delivery), cytosolic transport to the perinuclear area, nuclear uptake, and
nuclear presentation of the pDNA to the transcriptional machinery in bioactive form.
In the case of siRNA (or mRNA and some other nucleic acids such as oligo-
nucleotides), cytosolic accessibility for the required function is essential. Besides
cytosolic transport [
176
,
177
] and the nuclear import of large nucleic acid molecules
[
178
-
180
], incorporation of functional nuclear import peptide domains has been
evaluated [
181
-
186
]. Another bottleneck, nucleic acid unpackaging [
187
], i.e., partial
or complete dissociation from the polymeric carrier, which is required for biological
accessibility of the delivered nucleic acid, will be discussed in Sect.
3.3
.
3 Polymer Design for Bioresponsive Activity
Natural viruses provide us with perfect demonstrations of how effective nucleic
acid transfer into mammalian cells can proceed. The “secret” of their efficiency is
their dynamic, bioresponsive behavior during delivery, which distinguishes them
from classic synthetic nanoparticles. Thus, it has been tempting for us and many
research colleagues [
69
,
92
,
164
,
188
-
194
] to design nucleic acid nanoparticles
with virus-like characteristics (“synthetic viruses”).
Polymers can be designed to be bioresponsive and to change their properties in
various biological compartments, for example their conformation and charge. They
may contain chemical bonds that are cleaved under temporal or spatial control, or
they may associate/dissociate in a controlled fashion. Bioresponsiveness of the
carrier is required in several steps of the transport (Fig.
1
).
A highly stable and shielded polyplex should circulate in the blood stream
without undesired interactions until it reaches the target cell. At that location,
specific interactions with the cell surface should trigger intracellular uptake.
While lipid membrane interaction is undesired at the cell surface, it should happen
subsequently within the endosomal vesicle and mediate polyplex delivery into the
cytosol. During or after intracellular transport to the site of action, the polyplex
stability should be weakened to an extent that the nucleic acid is accessible to exert
its function.
The following sections discuss how polymers and polyplexes can be chemically
designed to be bioresponsive in three key delivery functions: (1) polyplex surface
shielding, (2) interaction with lipid bilayers, and (3) polyplex stability.
