Biomedical Engineering Reference
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will interact spontaneously with DNA (Figure 14.9). Initially, the negatively charged plasmid DNA
probably acts as a bridge between adjacent vesicles. Further DNA/vesicle interactions quickly
generate a complex three-dimensional lattice-like system composed of fl attened vesicles (some of
which probably rupture) interspersed with plasmid DNA. The lipid component of such 'lipoplexes'
should, therefore, provide a measure of physical protection to the therapeutic gene.
Gene therapy results to date using this approach have been mixed. The process of lipoplex formation
is not easily controlled; hence, different batches made under seemingly identical conditions may not be
structurally identical. Furthermore,
in vitro
test results using such lipoplexes can correlate very poorly
with subsequent
in vivo
performance. Clearly, more research is required to underpin the rational use
of lipoplexes for gene therapy purposes. The same is true for other polymer-based synthetic gene
delivery systems, the most signifi cant of which is the polylysine-based system. Polylysine molecules,
due to their positive charge (Figure 14.8), can also form electrostatic complexes with DNA. However,
the stability of such 'polyplexes' in biological fl uids can be problematic. Furthermore, polyplexes tend
to be rapidly removed from circulation, prompting a low plasma half-life. These diffi culties can be
alleviated in part by the attachment of PEG molecules. PEG attachment is also used to increase the
serum half-life of various therapeutic proteins, such as some interferons (Chapter 8).
No matter what their composition, such synthetic gene delivery systems also meet various biologi-
cal barriers to effi cient cellular gene delivery. Viral vector-based systems are far less prone to such
problems, as the viral carrier has evolved in nature to overcome such obstacles. Obstacles relate to:
•
blood-related issues;
•
biodistribution profi le;
•
cellular targeting;
•
cellular entry and nuclear delivery.
Whereas lipoplexes/polyplexes generally protect the plasmid from serum nucleases, the overall
positive charge characteristic of these structures leads to their non-specifi c interactions with cells
Figure 14.9
Initial interaction of plasmid DNA with cationic (positively charged) vesicles. Refer to text for
further details
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