VNPs as Tools for Nanomedicine - Viral Nanoparticles: Tools for Materials Science and Biomedicine - page 211

of effective PEGylation of AAV, in which the virus remained fully infective
allowing for efficient gene delivery while benefiting from a stealth effect
(Lee ., 2005). The more the PEG chains and the larger the PEG chains

attached, the better the shielding and stealth effect. On the downside,
shielding is not exclusive to the components of the immune system. As AAV
vectors rely on the native capsid specificity to interact with their target cells,
if decorated too heavily with PEG chains AAV loses its ability to transfect cells
(Lee

et al

et al ., 2005).

8.1.2.2 Stealth efect via geneic alteraion

Studies focusing on AAV showed that a stealth effect could also be achieved
by alterations of the particle surface. Mutant particles with altered antigenic
surface properties have been generated using genetic modification protocols
(Huttner ., 2003). However, compared with chemical modification

protocols these methods can be cumbersome and time-consuming. Mutants
with up to 70% reduced affinity to AAV-specific antibodies demonstrated
antibody evasion

et al

. On the downside, the mutants suffered from
significant losses in infectivity (Huttner

in vitro

., 2003). This example also
underlines the importance in carefully evaluating each formulation. It is
critical to find the right balance between reducing undesired side effects
while still maintaining the desired function of the VNP.

et al

8.1.2.3 Dual-Funcionalized Formulaions That Facilitate Shielding
While Maintaining Eicient Gene Delivery

To overcome the problems of PEGylated formulations being shielded
from their target cells and tissues, targeting ligands that bind to specific
molecular receptors have been utilized (discussed in detail in Sections 8.5
and 8.6). A series of PEG-Ad-RGD vectors have been developed and tested.
The RGD (Arg-Gly-Glu) motif is the targeting ligand that promotes binding
and internalization of the vectors into tumor cells (Sections 8.5 and 8.6). The
RGD motif is highly specific for the integrins overexpressed

on tumor endothelium and cancer cells. Targeting these tumor markers has
proven successful using a range of platforms and is envisaged as a promising
strategy for imaging and drug delivery (Li

α b

and

α b

v 3 v 5

et al ., 2003; Meyer et al ., 2006;

Sipkins ., 2003). PEG-Ad-RGD vectors have been

shown to retain antibody evasion while maintaining efficient gene delivery
rates (reviewed in Eto

et al ., 1998; Winter et al

et al ., 2008). The principles are outlined in Fig. 8.2.