In earlier, Dams et al. [10] demonstrated that rats transfused with serum from rats
pretreated with PEGylated liposomes rapidly cleared the dose when they were
treated with PEGylated liposomes for the first time. Moreover, this accelerated
clearance of the PEGylated liposomes was not observed upon pre-heating the serum
at 56 °C for 30 min prior to transfusion. Later on, in a series of our studies [49, 50,
53], we have demonstrated that, following incubation with PEGylated liposomes,
extensive IgM binding with subsequent complement consumption was remarkably
detected only in sera from rats exhibiting an enhanced clearance of the second dose.
In the same context, Yang et al. [51] also emphasized the consumption of a massive
amount of complement in the serum obtained from rats pretreated with PEGylated
liposomes upon incubation with PEGylated liposome. These results suggest that
complement activation plays a crucial role in the induction of the accelerated blood
clearance of subsequent dose of PEGylated liposome. Nevertheless, the influence of
other serum factor(s) rather than complement in the elicitation of the ABC phenomenon should not be excluded.
20.6 Properties of Anti-PEG Antibody Epitope
Many reports have highlighted the contribution of anti-PEG antibodies to the
accelerated blood clearance of PEGylated therapeutics. However, the antigenic
determinant of the PEG polymer still remains a mystery. Based on its highly
repetitive structure, PEG is acceptably classified as TI-2 antigen. TI-2 antigens are
composed of identical repeating epitopes that act by cross-linking the cell-surface
immunoglobulins of specific B cells leading to a significant and prolonged activation of B cells without co-stimulation by T cells [54]. In an earlier study, Richter
and Akerblom [26] observed the inhibition of anti-PEG antibody precipitation with
PEG 300 , suggesting that the antigenic determinant of PEG may be a sequence of 6–
7 –CH 2 -CH 2 -O- units. Cheng et al. [55] found that the monoclonal anti-PEG
antibody (IgM) generated by immunization with PEGylated b-glucuronide recognizes the repeating sequence –(O–CH 2 –CH 2 ) n – subunit (16 units) of PEG. Saifer
et al. [56] later reported that triethylene glycol (MW 150–160) was bound by
anti-PEG antibodies in direct and competitive ELISAs. These data suggest that the
minimum epitope recognition by anti-PEG antibodies could range from 3 to 4
repeated oxyethylene units.
Nevertheless, several studies showed that PEG polymer per se does not elicit an
anti-PEG response. On the other hand, PEG conjugated to proteins and/or
nanocarriers would induce an anti-PEG immune response. Accordingly, the antigenic determinant for anti-PEG antibodies is supposed to ensue at the linkage
between PEG and other materials. Shiraishi et al. [57] observed cross-reactivity
between anti-PEG IgMs elicited by hydrophobic PEGylated micelles and liposomes, but hydrophilic PEGylated micelles did not induce either an anti-PEG IgM
response or the ABC phenomenon. Accordingly, they suggested that the anti-PEG
antibody epitope lies at the interphase between a hydrophobic core and conjugated
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A. S. Abu Lila and T. Ishida
pretreated with PEGylated liposomes rapidly cleared the dose when they were
treated with PEGylated liposomes for the first time. Moreover, this accelerated
clearance of the PEGylated liposomes was not observed upon pre-heating the serum
at 56 °C for 30 min prior to transfusion. Later on, in a series of our studies [49, 50,
53], we have demonstrated that, following incubation with PEGylated liposomes,
extensive IgM binding with subsequent complement consumption was remarkably
detected only in sera from rats exhibiting an enhanced clearance of the second dose.
In the same context, Yang et al. [51] also emphasized the consumption of a massive
amount of complement in the serum obtained from rats pretreated with PEGylated
liposomes upon incubation with PEGylated liposome. These results suggest that
complement activation plays a crucial role in the induction of the accelerated blood
clearance of subsequent dose of PEGylated liposome. Nevertheless, the influence of
other serum factor(s) rather than complement in the elicitation of the ABC phenomenon should not be excluded.
20.6 Properties of Anti-PEG Antibody Epitope
Many reports have highlighted the contribution of anti-PEG antibodies to the
accelerated blood clearance of PEGylated therapeutics. However, the antigenic
determinant of the PEG polymer still remains a mystery. Based on its highly
repetitive structure, PEG is acceptably classified as TI-2 antigen. TI-2 antigens are
composed of identical repeating epitopes that act by cross-linking the cell-surface
immunoglobulins of specific B cells leading to a significant and prolonged activation of B cells without co-stimulation by T cells [54]. In an earlier study, Richter
and Akerblom [26] observed the inhibition of anti-PEG antibody precipitation with
PEG 300 , suggesting that the antigenic determinant of PEG may be a sequence of 6–
7 –CH 2 -CH 2 -O- units. Cheng et al. [55] found that the monoclonal anti-PEG
antibody (IgM) generated by immunization with PEGylated b-glucuronide recognizes the repeating sequence –(O–CH 2 –CH 2 ) n – subunit (16 units) of PEG. Saifer
et al. [56] later reported that triethylene glycol (MW 150–160) was bound by
anti-PEG antibodies in direct and competitive ELISAs. These data suggest that the
minimum epitope recognition by anti-PEG antibodies could range from 3 to 4
repeated oxyethylene units.
Nevertheless, several studies showed that PEG polymer per se does not elicit an
anti-PEG response. On the other hand, PEG conjugated to proteins and/or
nanocarriers would induce an anti-PEG immune response. Accordingly, the antigenic determinant for anti-PEG antibodies is supposed to ensue at the linkage
between PEG and other materials. Shiraishi et al. [57] observed cross-reactivity
between anti-PEG IgMs elicited by hydrophobic PEGylated micelles and liposomes, but hydrophilic PEGylated micelles did not induce either an anti-PEG IgM
response or the ABC phenomenon. Accordingly, they suggested that the anti-PEG
antibody epitope lies at the interphase between a hydrophobic core and conjugated
376
A. S. Abu Lila and T. Ishida