Biomedical Engineering Reference
In-Depth Information
at bone healing with PTH1-34 alone was not included in this study, the healing time
following PTH1-34 expression alone is not known.
Delivery of 0.1 mg or 1 mg of VEGF expressing plasmid in a collagen sponge
was investigated to a rabbit critical-sized radial defect (Geiger et al. 2005 ). VEGF
delivery resulted in bone formation at 6 and 12 weeks compared to controls, where
no defect bridging was observed. In addition to bone formation, delivery of VEGF
plasmid led to increased vasculature at 6 and 12 weeks compared to controls.
Although an increase in vasculature was seen from 0.1 mg to 1 mg of VEGF plas-
mid, this did not appear to correspond to increased regeneration, as no difference
was seen with respect to the volume of new bone.
These studies demonstrate the feasibility of plasmid delivery for bone regenera-
tion; however, the large amount of plasmid DNA required for a measurable biological
response led groups to investigate gene carriers to facilitate delivery, which may
increase the efficiency of gene delivery. In addition, these results highlight the impor-
tance of gene selection for bone regeneration studies of gene treatment success. This
is evident when comparing delivery of BMP-4 (Fang et al. 1996 ) and VEGF (Geiger
et al. 2005 ) genes in a collagen sponge for healing of rat femur and rabbit radial
defects. Despite potential differences between the healing and regenerative potential
of each of the defect sites, only 0.1 mg of VEGF plasmid was required to bridge the
defect at 6 weeks, whereas 0.5 mg of BMP-4 plasmid was required to bridge the gap
at 9 weeks. Given the importance of sufficient blood supply for fracture healing and
regeneration, a growth factor able to induce angiogenesis may be more potent.
Recent studies have also shown that ultrasound can also be used to introduce
genes into different cell types in the absence of a gene carrier (Li et al. 2009 ). This
process, also known as sonoporation, creates pores in the cell membrane and facili-
tates uptake of genes both in vitro and in vivo . Using a BMP-2 expression system,
de novo induction of bone was demonstrated at intramuscular sites (rodent) using
microbubble-enhanced transcutaneous sonoporation (Osawa et al. 2009 ). No carrier
was used in that study since muscle tissue is unique in picking up DNA molecules.
BMP-9 expression was also shown to lead to bone induction with a similar approach
(Sheyn et al. 2008 ). The nature of the microbubbles used to enhance the sonopora-
tion process was shown to be critical in optimizing gene expression at muscular
sites (Kodama et al. 2010 ), although a therapeutic effect was not explored in the
latter study. It remains to be seen if sonoporation could be effectively used for gene
expression in deep tissues, so that an effective bone repair could be achieved.
4.2
Plasmid DNA Delivery with Synthetic Carriers
The success of branched 25 kDa PEI for in vitro gene delivery led to its investiga-
tion for in vivo applications. A poly(lactic-co-glycolic acid) (PLGA) scaffold was
used to deliver 0.2 mg of BMP-4 plasmid condensed with PEI to a rat skull critical
defect (Huang et al. 2005 ). At 8 weeks, defects with scaffolds containing the
PEI-condensed plasmid showed bone formation around the edges of the defect,
compared to naked (uncondensed) plasmid and the empty scaffolds which showed
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