Biomedical Engineering Reference
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physically trapped in small capillaries, particularly in the lung, or taken up by
phagocytosis by macrophages (Chemin et al. 1998 ). Despite these drawbacks, some
success has been obtained in cell culture, including in non dividing cells (Boussif
et al. 1995 ; Dheur et al. 1999 ; Gomes dos Santos et al. 2006 ). Some positive results
have also been obtained in vivo: Grzelinski et al. ( 2006 ) were able to suppress the
expression of the growth factor pleiotrophin and Urban-Klein et al. ( 2005 ) achieved
an anti-tumor effect with a PEI complex of siRNA targeting the HER-2 receptor.
The biocompatibility of PEI-based formulations can be improved by coating
them with PEG (Mao et al. 2006 ) or by glycosylation of the surface (Leclercq et al.
2000 ). Specific targeting ligands have also been attached. One such ligand is the
RGD (Arg-Gly-Asp) peptide, which was used by Schiffelers et al., ( 2004 ) to block
vascular endothelial growth factor receptor-2 (VEGF-R2) expression in vascular
endothelial cells and thereby prevent tumor angiogenesis.
Dendrimers composed of poly (amidoamine) (PAMAM) carry a high density of
positive charge and can therefore be used to bind nucleic acids on their surface
(Zhou et al. 2006 ; Shen et al. 2007 ). Helin et al. ( 1999 ) studied the intracellular
distribution of a fluorescent ODN adsorbed onto dendrimers and found that this
depended on the stage of the cell cycle. Fluorescence was found in the nuclei during
G2/M phase while in other phases it seemed to be concentrated in acidic vesicles.
Inhibition of target gene expression was observed with these systems. Other exam-
ples of inhibition of gene expression by AS-ODN attached to dendrimers are given
by Bielinska et al., ( 1996 ), Yoo et al. ( 1999 ) and Li and Morcos ( 2008 ). The subject
has been reviewed recently by RaviƱa et al. ( 2010 ). Specific targeting of dendrimers
carrying AS-ODN against the Epidermal Growth Factor receptor to glioma cells
was achieved by coupling folic acid to their surface (Kang et al. 2010 ).
SiRNA has also been immobilized on dendrimers (Kang et al. 2005 ; Zhou et al.
2006 ). These systems have been rendered more biocompatible by acetylation
(Waite et al. 2009 ; Patil et al. 2008 ) and they have also been targeted: with the Tat
peptide (Kang et al. 2005 ) and with the peptide hormone LHRH (Minko et al.
2010 ). Other, more water-soluble, polymers have been used to prepare dendrimers
for nucleic acid delivery. Among these are carbosilane (Gras et al. 2009 ) and poly
(L-lysine) (Eom et al. 2007 ).
Nucleic acids can also be complexed with the cationic arginine-rich polypep-
tide protamine. Junghans et al. ( 2001 ) observed uptake of protamine-AS-ODN
particles by endocytosis and delivery to the cytoplasm in Vero cells. More stable
particles were prepared using both protamine and human serum albumin
(Weyermann et al. 2005 ). These showed both uptake and an anti-sense effect in
mouse fibroblasts. Attempts were made to target these particles to the blood-brain
barrier, by coating them with apolipoprotein A-1 to promote transcytosis (Kratzer
et al. 2007 ). More sophisticated delivery systems can be obtained by mixing
protamine, lipids and nucleic acids. Thus, Li et al. ( 2008 ) were able to deliver
siRNA to the cytoplasm of NCI-H460 tumor cells, after modifying the surface
with PEG and anisamide to target a tumor receptor. Similar particles have been
used to downregulate Bcl-2 expression in cells with AS-ODN, using transferrin as
a targeting ligand (Yang et al. 2009 ).
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