Biomedical Engineering Reference
In-Depth Information
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Vector
DNA
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Endosome
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Cell
Nucleus
Figure 7.8
Gene delivery system. 1: Transfection of target cells with plasmid DNA is dependent on the condensation of the
plasmid DNA by a suitable vector system; 2: the specific binding of the vector-DNA complex to the target cell;
3: the uptake of the complex into intracellular endosomes; 4: successful release from endosomes into the cyto-
plasm; 5: intracellular transport and nuclear localization of the DNA; 6: followed by decomplexation.
3. Coencapsulation of other bioactive agents for multiple plasmids.
4. Improvement of DNA bioavailability due to the protection from serum nuclease
degradation by the polymer matrix.
5. Lyophilization of nanospheres for storage, without losing too much activity.
Chitosan was first used as a carrier of DNA for gene delivery applications in 1995 [40] by
mixing a solution of the respective chitosan with plasmid DNA. Chitosan has an average
amino group density of 0.837 per monosaccharide d-glucosamine unit. This amino group
density depends on the degree of deacetylation. The cationically charged chitosan will form
polyelectrolyte complexes with the negatively charged plasmid DNA. Chitosan-DNA com-
plexes could be protected from DNase to improve the bioavailability of the plasmid DNA
delivered into the body for gene therapy. The complex sizes yielded (150-500 nm) depends
on both the molecular weight of the chitosan (108-540 kDa) used and the chitosan/DNA
ratio [41,42].
Objectively speaking, few of the researchers have made efforts to correlate the trans-
fection efficacy with the size of chitosan-DNA complexes, although there are several
reports supporting the use of chitosan for gene transfection [43,44]. Typically, the parti-
cles smaller than 100 nm can be enclosed within endocytic vesicles, allowing entry into
target cells via transferrin cytosis [45]. For chitosan-DNA complexes systems, Erbacher
et al. [43] proposed that zeta potential was close to 0 mV (N:P 52) with a size range of
1-5 mm. In studying cationic cholesterol derivative-mediated gene transfection,
Nikanishi found that moderate sizes of particles (0.4-1.4 mm) yielded the highest trans-
fection [46], while small vesicles less than 400 nm in size showed lower transfection effi-
ciency. The results obtained by different authors seem to be contradictory. Thus, the
mechanism of cationic polymer-mediated transfection is still underappreciated.
The DNA delivered by nonviral carriers is vulnerable to degradation by DNase. A criti-
cal parameter of DNA delivery systems therefore is the ability of the carrier material to
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