Biomedical Engineering Reference
In-Depth Information
association rate constant, whereas the dissociation rate constant was nearly
unchanged. Using a mathematical model, intramolecular and/or intermolecular
blocking by tethered PEG were proposed as the main factors behind the decrease
in the observed association rate constant. The model suggested that more than 90%
of PEGylated ligands are not capable of binding the target, indicating that accessi-
bility to PEGylated molecules is significantly restricted [
43
]. This model can
partially explain the lower immunogenicity and higher enzymatic resistance of
PEGylated molecules.
Although the affinity was decreased fivefold upon PEGylation, PEGylated
antibody fragments showed an 8.5-fold higher accumulation in tumors than unmod-
ified antibody fragments, because of a longer serum half-life [
46
].
3.3 PEGylation on the Surface
PEGylation technology is also relevant for solid surfaces. Immobilization of
proteins, antibody fragments, and whole antibodies has been widely used in
biosensing and bioseparation systems [
47
]. There are several factors that affect
the ability of these systems. These factors include the quantity, density, conforma-
tion, and orientation of the immobilized molecules. A common method of immobi-
lization of a protein is based on the reaction between reactive residues in the
protein, such as lysine, and the reactive surface. Yoshimoto et al. immobilized
antibody fragments (Fab
0
) on a gold surface through S-Au linkage [
48
]. However,
after the initial absorption of Fab
0
onto the gold surface, reactions between the
interactive residues of Fab
0
and the gold surface changed the conformation and
orientation of Fab
0
, resulting in the inactivation of the antigen-binding function of
lnactive
Gold surface
PEG(5 k)
PEG(5 k)
s
Fab Fragment
Active
s
s
Fig. 10 PEGylation on the surface. A highly dense PEG layer composed of mixed-PEG prevents
nonspecific protein interactions and inactivation of Fab
0
