Biomedical Engineering Reference
In-Depth Information
hydrazone linkage showed 100-fold higher gene expression in vitro , and 10 times
higher gene expression in a mouse model, when compared to the stably shielded
control polyplexes. For the PEI polyplexes, their peripheral decoration using
amine-reactive pyridylhydrazone-based PEGs also significantly increased transac-
tion efficiency in both in vitro and in vivo models, as compared to stably shielded
polyplexes (Fella et al. 2008 ).
Due to its acid-labile character, imine linker, also called as Schiff base, has been
used to cross-link low molecular weight PEI (1,800 Da) using glutadialdehyde
(Kim et al. 2005b ). In vitro transfection in 293 T cells and A7R5 cells showed that
the transfection efficiency of the acid-labile polycations was comparable to that of
PEI25. In addition, the acid-labile PEI was much less toxic than PEI25, due to the
degradation of acid-labile linkage.
Alternatively, pH-dependent charge reversal moieties can be used to modulate
the gene delivery performance of synthetic vectors. Maleic amide derivatives, like
cis-aconitic and citraconic amide have negative charges at neutral pH, while they
may degrade promptly at mildly acidic pH 5.5 to expose positively charged amines.
Materials bearing these functional moieties are called masked endosomolytic
agents (Wolff and Rozema 2008 ). Namely, when they enter the acidic environment
of endosomes or lysosomes, their pH-labile bonds are broken, releasing the agent's
endosomolytic capability. Polymer vectors with cis-aconitic or citraconic amide
can significantly improve the transfection activity of conventional polyplexes both
in vitro and in vivo as evidenced by Wolff's and Kataoka's groups (Rozema et al.
2007 ; Lee et al. 2008 ).
3.9.2
Redox-Responsive Disulfide
It is widely accepted that the cytoplasm is a highly reducing environment, where
the glutathione concentration is about 50-1,000 times higher than that in the extra-
cellular milieu. Disulfide can be effectively reduced in the intracellular environ-
ment, and therefore it can function as a responsive linkage to create redox-triggerable
delivery systems. There have been considerable studies concentrating on intracel-
lular delivery of nucleic acids based on the reductive cleavage of disulfide bonds.
As aforementioned, cross-linking of LMW-PEI has been implemented using various
compounds bearing disulfide. Once inside the cell, disulfide linkages will be
reduced, resulting in the cleavage of disulfide-containing high molecular weight
PEI into low molecular weight fragments which are less toxic. In addition, PEG and
polycations can be coupled with disulfide linkage to formulate polyplexes with
enhanced colloidal stability and prolonged circulation, while PEG chains will be
detached after endocytosis to facilitate intracellular trafficking.
Kataoka's group have synthesized a block catiomer (PEG-SS-P[Asp(DET)]), in
which PEG was linked with PAsp(DET) (Takae et al. 2008 ). In vitro transfection study
showed that PEG-SS-P[Asp(DET)] based polyplex micelles displayed 1-3 orders of
magnitude higher transfection efficiency than those based on a structurally similar
polymer without disulfide linkages. A more rapid onset of gene expression was also
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