Biomedical Engineering Reference
In-Depth Information
mice resulted in enhanced pDNA expression in tumor tissue as compared with a
conventional, stable PEG-modified MEND.
3.2 Bioresponsive Lipid Bilayer Interaction
As outlined in previous sections, escape of polyplexes from endosomes to the
cytosol can be a major bottleneck in delivery. Membrane-active polymer domains
or other conjugated molecules can help to overcome this barrier (see Sect.
2.3
), but
they may trigger cytotoxicity when acting extracellularly or at the cell surface.
Therefore membrane-crossing agents either have to be inherently specific for endo-
somal compartments (for example by pH-specificity), or they have to be modified to
be activated in endosomes. For example, the reducing stimulus of intracellular
vesicles has been used to activate formulations containing less active disulfide
precursors of LLO [
163
] or Mel [
170
].
Also, acid-labile masking of membrane-disruptive agents has been pursued.
Copolymers consist of hydrophobic, membrane-disruptive methacrylate polymers
were reversibly masked with PEG chains through pH-sensitive acetal linkers [
172
].
Dialkylmaleic acid derivatives were used for pH-reversible acylation of amino
groups of a membrane-destabilizing polyvinylether polymer [
56
] or the lytic, but
not pH-specific peptide Mel [
57
,
202
-
205
]. Modification of Mel lysines with
dimethyl maleic anhydride (DMMAn) blocks the lytic activity. At endosomal pH,
DMMAn groups are released, unmasking highly lytic Mel for endosome disruption
(Fig.
3
). PLL when covalently modified with DMMAn had a 1,800-fold improved
gene transfer activity over unmodified PLL [
203
] and, in a PEG-modified version,
could also mediate efficient siRNA transfer [
57
,
204
].
3.3 Bioresponsive Polyplex Stability
The polymeric carrier has to stably bind the medicinal nucleic acid outside the cell
to compact and protect it from degradation. Inside the cell, the polyplex has
to disassemble to such an extent that the nucleic acid can be biologically active.
Different chemical characteristics outside the cell, in the endosome, the cytosol,
and the nucleus can be utilized to manage the controlled disassembly process.
Medicinal nucleic acids can be either noncovalently complexed or covalently con-
jugated to the carrier polymer. Release of the nucleic acid at the target location from
the polymer may proceed, for example, by exchange processes against polyions
such as intracellular RNA [
206
], by polymer degradation [
207
], or by cleavage of
the nucleic acid from the polymer attachment sites [
54
].
High nucleic acid/polymer affinity does not necessarily directly correlate with
high efficiency. Apparently, an optimum has to be reached. Also, the big difference
in size of different medical nucleic acids (pDNA has several thousand negative
