VNPs as Tools for Nanomedicine - Viral Nanoparticles: Tools for Materials Science and Biomedicine - page 210

PEG chains have been covalently attached to Potato virus X (PVX),

Tobacco mosaic virus (TMV), and MS2 (Bruckman et al ., 2008; Kovacs et al .,

2007; Steinmetz ., 2009c) using NHS chemistries, click reactions, or

oxime ligation (recall Chapter 4). PEGylation has been extensively studied
using various CPMV formulations (Destito

et al

et al ., 2007; Lewis et al ., 2006;

Raja ., 2003; Steinmetz & Manchester, 2009). PEGylation of CPMV before

administration effectively shielded the particles from inducing a primary
immune response (Raja

et al

., 2003). PEGylating CPMV is also an effective
strategy to avoid CPMV-cell interactions

et al

in vitro and in vivo (Destito et al .,

2007; Lewis ., 2006; Steinmetz & Manchester, 2009). It was shown that

attaching as few as 30 PEG chains of a molecular weight of 2000 Da (this
is a PEG chain consisting of

et al

14 PEG monomers; see Fig. 8.1) to CPMV
surface Lys side chains allowed effective shielding

n =

in vitro using cells and ex

vivo using tumor tissues (Steinmetz & Manchester, 2009). Only 10% of the

available surface Lys groups were modified with PEG [CPMV displays 300
addressable Lys side chains (Chatterji ., 2004)]. Modeling suggested

that less than 1% of the available surface area of the VNP was covered and
effectively shielded (Steinmetz & Manchester, 2009). Similar data were
obtained studying PEGylated PVX formulations (Steinmetz

et al

., 2009c).
This implies that limited PEGylation of VNPs is sufficient for shielding, and
thus a large surface area and many attachment sites remain available for
further modification with targeting, imaging, and therapeutic ligands.

et al

8.1.2.1 Stealth efect of PEGylated Ads and AAV for gene therapy

Ad vectors and AAV have been developed for gene therapy for many years.
Broad clinical utility has been challenging because of (i) rapid clearance
(the plasma half-life of Ad is less than 2 min), (ii) liver toxicity, and (iii)
prevalence of Ad-neutralizing antibodies in an individual's serum (reviewed
in Eto ., 2008).

A variety of PEGylated Ad vectors have been analyzed

et al

, and PEG
attachment was beneficial for addressing each of the above-mentioned
points: (i) PEGylated Ads had a fourfold higher clearance ratio compared
with native Ads; (ii) enhanced tumor uptake and lower uptake in the liver
implied reduced side effects and increased therapeutic success; and (iii)
evasion of antibody neutralization by PEG-Ad formulations has been
reported (reviewed in Eto

in vivo

., 2008).
Careful design and evaluation of a particular formulation is critical, as
shown in a study using AAV. It was found that there is only a small window

et al