Gene Delivery Using Chemical Methods - Challenges in Delivery of Therapeutic Genomics and Proteomics - page 167

compared to standard lipidic and polymeric vectors. Lipopolyplexes were usually
prepared by a two-step process in which the first step was condensation of DNA with
a cationic polymer to form polyplexes, followed by complexation of anionic lipo-
somes to form lipid-protamine-DNA (LPD)-type lipopolyplexes. Both steps were
driven by spontaneous charge interaction. The two key parameters for LPD-type
lipopolyplexes were the N-P ratio during polyplex formation and the lipid-DNA
ratio during lipopolyplex (or LPD) formation. Lipopolyplexes displayed superior
properties compared to lipoplexes or polyplexes.
Usually, the size of self-assembled lipoplexes is quite large because DNA cannot
be efficiently condensed by lipid alone, particularly when monovalent cationic lipid
is used. This large size of lipoplex confers practical difficulty for systemic delivery
of DNA. Lipopolyplexes comprised of polycationic polymer such as PLL or prot-
amine solve the problem related to size of the lipoplex. In addition, it also improves
transfection efficiency and reduces cytotoxicity of the complex. The polycations
commonly used are PLL [249,250] , PEI [250] , spermidine [251] , or spermine [252] .
The lipopolyplexes prepared using anionic lipid and PLL demonstrated reduced
cytotoxicity as compared to cationic liposome [250] . Lipopolyplexes prepared using
neutral lipid and spermidine displayed higher transfection efficiency and protection
against nuclease degradation as compared to cationic liposomes during in vitro stud-
ies [251] . Later on, similar results were obtained using lipopolyplexes comprised of
spermine as polymer [252] . It was observed that IV administration of polycation-
condensed DNA resulted in efficient gene transfer to most highly perused organs,
with highest transfection efficiency in the lung.
LPD complexes, lipopolyplex, were investigated for their toxicity and transfection
efficiency [253,254] . The prepared lipopolyplexes demonstrated superior transfection
efficiency as compared to cationic liposomes. Further, modified LPD lipopolyplex
was prepared by attaching asialofetuin (AF), a target ligand specific for the receptor
found on hepatocytes. The AF-conjugated LPD lipopolyplex demonstrated approxi-
mately 3- and 10-fold higher transfection efficiency over AF-conjugated lipoplex and
unmodified lipoplex, respectively. LPD lipopolyplexes have also proved promising
for cancer treatment [255,256] .
RGD-conjugated LPD lipopolyplexes were prepared for targeting tumor cells [257] .
Incorporation of RGD conjugated to PEGL-ylated lipid (DSPE-PEG-RGD), rather
than PEGylated lipid, into LPD lipopolyplex improves particle binding and targeting of
LPD formulations to tumor cells. In addition, transfection of LPD-PEG-RGD was spe-
cific for integrin-expressing cells. However, no comparison was done with lipoplexes.
Incorporation of FOL-targeting ligands into lipopolyplexes improves gene transfer effi-
ciency [249] . Hence, it is evident from the above studies that lipopolyplexes are suit-
able as successful gene delivery systems for a wide variety of treatments.
4.5 Peptides
The use of synthetic and semisynthetic peptides and polypeptides for nonviral gene
delivery has gained researchers' interest over the last few years. A while ago they