Biomedical Engineering Reference
In-Depth Information
efficiency was observed, which is likely attributed to steric shielding effects. The
cytotoxicity of both gal-PEI and polyplexes significantly decreased with increasing
galactosylation. Other ligands that have been used to target PEI/DNA polyplexes
include mannose(Diebold et al. 1999 ), RGD-containing peptide (Erbacher et al.
1999 ), transferring (Kircheis et al. 1997 ), epidermal growth factor (Blessing et al.
2001 ), anti-CD3 antibody (O'Neill et al. 2001 ), and anti-platelet endothelial cell
adhesion molecule antibody (Li et al. 2000 ). Interestingly, the conjugation of nega-
tively charged transferring protein to PEI is able to reduce the surface positive
charge of PEI/DNA polyplexes, thereby produce shielded formulations without
PEGylation (Kircheis et al. 2001a ).
Furthermore, PEGylation and ligand conjugation can be combined to give poly-
mers with even more excellent performance for gene transfections. Ligands like
galactose (Sagara and Kim 2002 ), transferrin (Kursa et al. 2003 ), epidermal growth
factor(Lee et al. 2002 ), and folate (Benns et al. 2002 ) have been used for targeting
PEGylated PEI/DNA polyplexes. These ligands can be conjugated to the PEG
chains of PEGylated PEI either before or after polyplex formation. Alternatively,
the ligands can be covalently coupled to PEI, while the formed ligand-PEI com-
plexes are then modified with PEG chains. The related studies have demonstrated
that combination of PEGylation and cell targeting can favorably modify the behav-
iors (such as decreased toxicity, prolonged circulation time, and tissue-selective
accumulation) of PEI/DNA polyplexes after system administration.
3.2.4
PEGylation of PEI
Despite its efficacy, in vivo applications of PEI for gene therapy have been largely
baffled by its toxicity (Kichler 2004 ). The LD50 (median lethal dose) of a linear
PEI (lPEI, 22 kDa) in Balb/C mice is 4 mg/kg. The systemic toxicity of PEI is
considered to, at least in part, relate to the unspecific interactions with erythro-
cytes and other blood components such as albumin, fibrinogen and complement
C3, mainly owing to the excess positive charges of the formed polyplexes (Ogris
et al. 1999 ).
Conjugates of PEI with PEG (~5,000 Da) were shown reduced unspecific binding
capacity of PEI itself. Polyplexes based on these 'shielding' conjugates displayed
improved solubility and stability as well as strongly reduced in vitro and in vivo
toxicity (Table 5 ) (Ogris et al. 2003 ). Shielding the positive surface charges pro-
vides the polyplexes with longer half-lives in circulation, due to the reduced
opsonization and reticuloendothelial system (RES) uptake (Kircheis et al. 1999 ).
As a result, these 'stealth' polyplexes allow the injection of highly concentrated
formulations. As a successful case, the polymer of lPEI (22 kDa) conjugated with
PEG (~5,000 Da) has shown significant levels of transgene expression in a mice
model post-nasal instillation (Kichler et al. 2002 ). PEGylation, however, may sac-
rifice the DNA-binding capability of modified PEI, sterically hinder interactions of
the polyplexes with the target cells, and therefore may impair the transfection effi-
ciency of resulting formulations.
Search WWH ::




Custom Search