The Importance of Poly(ethylene glycol) Alternatives for Overcoming PEG Immunogenicity in Drug Delivery and Bioconjugation

Abstract

Poly(ethylene glycol) (PEG) is widely used as a gold standard in bioconjugation and nanomedicine to prolong blood circulation time and improve drug efficacy. The conjugation of PEG to proteins, peptides, oligonucleotides (DNA, small interfering RNA (siRNA), microRNA (miRNA)) and nanoparticles is a well-established technique known as PEGylation, with PEGylated products have been using in clinics for the last few decades. However, it is increasingly recognized that treating patients with PEGylated drugs can lead to the formation of antibodies that specifically recognize and bind to PEG (i.e., anti-PEG antibodies). Anti-PEG antibodies are also found in patients who have never been treated with PEGylated drugs but have consumed products containing PEG. Consequently, treating patients who have acquired anti-PEG antibodies with PEGylated drugs results in accelerated blood clearance, low drug efficacy, hypersensitivity, and, in some cases, life-threatening side effects. In this succinct review, we collate recent literature to draw the attention of polymer chemists to the issue of PEG immunogenicity in drug delivery and bioconjugation, thereby highlighting the importance of developing alternative polymers to replace PEG. Several promising yet imperfect alternatives to PEG are also discussed. To achieve asatisfactory alternative, further joint efforts of polymer chemists and scientists in related fields are urgently needed to design, synthesize and evaluate new alternatives to PEG.

Keywords: PEG immunogenicity; PEGylation; anti-PEG antibody; bioconjugation; cancer; drug delivery; nanomedicine; vaccine.

Figure 1

Different materials at the nanoscale: the various types of synthetic nanoparticles (NPs) are generally sized at 1–100 nm, which is comparable with native nanostructures including proteins, viruses and DNA. Poly(ethylene glycol) (PEG) is conjugated to NP surface to form PEG-NPs, providing a number of advantageous properties within a drug delivery system. A limitation of PEGylated NPs at the biological interface is the development of anti-PEG antibodies: anti-PEG immunoglobulin M (IgM) is induced after the first dose of PEG-NPs, and it quickly recognizes PEG-NPs in the second dose. Kupffer cells in the liver are then directed to activate the complement system and remove PEG-NPs.

Figure 2

Repeated administration of PEGylated drugs (PEG-drugs) or PEGylated nanoparticles (PEG-NPs) triggers anti-PEG IgM production in animal models (a) and in humans (b), and it causes the accelerated blood clearance (ABC) phenomenon. In the case of patients with pre-existing anti-PEG IgM, the ABC phenomenon occurs upon the first administration of PEG-drugs or PEG-NPs. (c) Illustration of the increase of pre-existing anti-PEG IgM over the last couple of decades.

Figure 3

Anti-PEG IgM production as a function of time interval was tested in sera of rats intravenously injected with PEGylated liposomal gambogenic acid (PEG-GEA-L) at 1.500 mg/kg (a) [57]. Mice were treated with PEGylated liposomes of 1 μmol/kg, and the production of anti-PEG IgM in mouse sera was determined as a function of time (b) [58]. The effect of PEG-GEA-L doses on anti-PEG IgM production was recorded on the fifth day after the first injection. Rats were treated with low (1.125 mg/kg), medium (1.500 mg/kg), and high (1.875 mg/kg) dosages. ELISA was utilized to determine anti-PEG IgM (c). Anti-PEG IgM in organ tissues of rats that were intravenously injected with various PEG-GEA-L doses (d). Used with permission from [57]. (* p < 0.05, ** p < 0.01).

Figure 4

Production of anti-PEG IgM in the serum of mice injected PEGylated liposomes (PL) with various functional groups were determined by ELISA on the fifth day after administration. The lowest immunogenicity was exhibited by PL–OH (a). C3 binding in the serum of mice injected P–OCH₃ and PL–OH on the fifth day, with non-treated mice as the control sample. Sera were diluted with a gelatin veronal buffer with Mg⁺⁺ and Ca⁺⁺ (GVB⁺⁺) buffer (b) and an Mg²⁺/ethylene glycol-bis(2-aminoethylether)-N,N,N′,N′-tetraacetic acid (EGTA) buffer (c). Mice treated with PL–OH demonstrated the highest compliment activation, driving the ABC phenomenon (d). Used with permission from the European Journal of Pharmaceutics and Biopharmaceutics [65]. Production of anti-PEG IgM rat sera against PEGylated emulsions (DE) with various functional groups seven days post-administration (e). Used with permission from the European Journal of Pharmaceutics and Biopharmaceutics [66]. (* p < 0.05, ** p < 0.01, *** p < 0.001).

Figure 5

Summary of the factors that impact the accelerated blood clearance of PEGylated nanoparticles, as caused by the anti-PEG antibody generation after the first dose of the treatment agent, or through pre-existing anti-PEG antibodies that are generated by prior exposure to PEG from various sources.

Figure 6

Chemical structures of synthetic and natural polymers as alternative candidates to PEG.

Figure 7

Chemical structures of some zwitterionic polymers.

Figure 8

Chemical structure of the zwitterionic carboxybetaine functionalized polypeptide (PepCB) [79] (a). The preparation of poly(2-methacryloyloxyethyl phosphorylcholine)/poly(β-amino ester) (PMPC/PAE) mixed shell micelles (MSMs) with pH-sensitive behavior: MSMs were in zwitterionic state at a physiological blood pH (7.4) and in a cationic form within the tumor microenvironment (pH 6.5) (b).