Gene Delivery Using Chemical Methods - Challenges in Delivery of Therapeutic Genomics and Proteomics - page 136

(550 Da) do not exhibit any shielding effect. The surface charge of PEI- g -PEG-DNA
complexes reduced significantly when PEG molecular weight was more than 5 kDa.
It has been demonstrated that the presence of a PEG segment on the surface of
the polyelectrolyte complexes enhances stability of the complexes by preventing the
interparticular aggregation [55] . Transmission electron microscope (TEM) studies of
cellular internalization of the complexes revealed that PEGylated PEI-DNA com-
plexes preserved a small and distinct particular structure when the complexes entered
the cells, whereas unmodified PEI-DNA complexes were present as large aggregates
upon cellular entry.
The cytotoxicity of PEI was reduced by PEG grafting. An in vivo study was per-
formed to evaluate the effect of PEI- g -PEG on gene expression by repeated intra-
thecal injections of PEI- g -PEG-DNA complexes and unconjugated complexes [56] .
Following repeated intrathecal injections, PEI- g -PEG-DNA complexes did not show
any reduction in gene expression, whereas unconjugated complexes exhibited a 70%
reduction in gene expression. Hence, cytotoxicity was the reason for cell death and
lesser gene expression.
4.2.1.2.1.2 FOL-Conjugated PEIs FOL-conjugated PEI was synthesized by graft-
ing FOL-PEG-FOL (FPF) to PEI to confer FOL-polyethylene glycol-FOL-grafted-
polyethylenimine (FPF- g -PEI) [57] . FPF- g -PEI was able to condense the DNA to
form oblique spheroids with a mean diameter of 150 nm. The FPF- g -PEI exhibited
lower cytotoxicity with higher transfection efficiency than unmodified PEI when
studied in CT-26 colon tumor cells [57] . Nevertheless, enhanced transfection was not
observed in normal smooth muscle cells, indicating specificity of FPF- g -PEI toward
FOL receptors overexpressing tumor cells. Polyethylenimine-polyethylene glycol-
FOL (PEI-PEG-FOL) conjugate was used as a vector for a plasmid encoding small
interfering RNA (siRNA) for targeting green fluorescence protein (GFP) [58] . The
complex formed PEI-PEG-FOL-siRNA demonstrated superior suppression of GFP
expression compared to unmodified PEI complexes in FOL receptor overexpressing
cells.
In another study, luciferase pDNA and PEI were complexed to form slightly
positively charged nanoparticles, onto which FOL-PEG-PLL conjugate was surface
coated. The FOL-PEG-PLL/PEI-DNA complexes exhibited higher gene expression
level, even in the presence of serum, than the PEI-DNA or lipofectamine-DNA com-
plexes [59] .
4.2.1.2.1.3 Antibody-Conjugated PEIs Conjugation of the antibody to PEI can
provide target-specific transfection without loss of the original properties of PEI. A
monoclonal antibody for human epidermal growth factor receptor-2 (HER-2) was
conjugated to linear PEI for targeted delivery to cancer cells [60] . The HER-2 anti-
body-PEI complex demonstrated enhanced transfection efficiency in HER-2, overex-
pressing human breast adenocarcinoma cells (Sk-Br-3) compared to unmodified PEI.
The OV-TL16 antibody against the OA3 surface antigen expressed in ovarian
carcinoma cells was conjugated to PEI at antigen-binding fragment FabV [61] . The
incorporation of PEG may increase the stability, solubility, and site specificity of
the polymer-DNA complexes. PEGylated antibody conjugated PEI-DNA complex