2020-03-01 22:18:29
Marwa Mohamed, Amr S. Abu Lila, Taro Shimizu, Eman Alaaeldin, Amal Hussein, Hatem A. Sarhan, Janos Szebeni & Tatsuhiro Ishida
1.Introduction
PEGylation, the covalent linking of polyethylene glycol (PEG) chains, is currently considered as an effective approach to increase stability and prolong liposomes in vivo circulation time. Nanocarriers, such as liposomes and biodegradable polymeric nanoparticles are promising tools for targeted drug delivery in treating cancer and many other diseases and a number of them have received clinical approval for the delivery of a variety of therapeutics [1–5]. They combine the advantages of being biocompatible and relatively nontoxic, and having an inherent ability to protect the encapsulated payload from enzymatic degradation or other unfavorable conditions [6,7]. In addition, they have been shown to increase the therapeutic index of encapsulated drugs and decrease their toxicity by altering the pharmacokinetic profile of the drugs [8,9]. Collectively, these advantages have led to the approval of nanoscale drug carriers in a number of clinical settings involving a variety of small molecule therapeutics and recently siRNA. Nevertheless, the widespread utilization of nanocarrier-based therapeutics in medicine is occa- sionally constrained by concerns regarding potential toxicities of the carrier.
Nanocarriers with certain physical and chemical characteristics, in some circumstances in animal experiments, have been reported to prime the host- immune system, resulting in severe adverse effects and/or lack of therapeutic efficacy of the encapsulated drug [10–12]. For instance, surface electrostatic charges and/or particle size of developed nanocarriers are fundamental to define such immune responses [13]. In these experiments, the nanoparticles are essentially recognized as foreign particles by immune cells, which induce multilevel immune responses. In addition, the nanoparticles have been reported to interact with circulating serum proteins, such as com- plement proteins belonging to the primary humoral immune system, resulting in their rapid systemic clearance by the cells of mononuclear phagocyte sys- tem (MPS) [14,15]. Mononuclear phagocyte system (MPS), composed of dendritic cells (DCs), monocytes and macrophages, is a part of the innate immune system that plays a critical role in phagocytosis of pathogen and foreign substances in all phases of the immune response. Taken together, these interactions with the immune system have the potential to influ- ence the in vivo fate of administrated nanocarriers and might impair their therapeutic performance.
PEGylation, the covalent linking of polyethylene glycol (PEG) chains, is currently considered as an effective approach to increase stability and prolong liposomes in vivo circulation time. PEGylation has been reported to hinder the adsorption of protein opsonins in the circulation onto liposome surfaces, which results in increased clearance of liposomes by the mononuclear phagocytic cells in the liver and spleen [16,17]. Consequently, PEGylation improves the residence time of liposome, as well as encapsu- lated therapeutic agents, in the circulation. In the drug delivery field, PEGylated and non-PEGylated liposomes have received a number of clinical approvals. The most successful example of a PEGylated liposomal formulation is Doxil® (PEGylated liposomes encapsulating the anticancer drug doxorubicin (DXR)), approved for Kaposi’s sar- coma, ovarian cancer, breast cancer and multiple myeloma [18–20]. Nevertheless, PEGylated liposomes have caused some lipid-related side effects. Doxil®, in some patients, caused immediate hypersensitivity reactions upon first injection. A number of studies confirmed the role of complement activation in ana- phylactic reactions observed in up to 25%–45% of patients treated with Doxil® upon the first injection [12,21,22]. This reaction is well controlled by slowing the rate of Doxil® infusion and giving the patient pre- medications [23,24]. The hypersensitivity reaction is rarely seen upon the second and subsequent injec- tions of Doxil® [24].
2.Immunogenicity of PEG
Polyethylene glycol (PEG) is a bio-inert, thermoelastic linear hydrophilic polymer with the molecular formula (C2nH4n+2On+1). It is a non-toxic and non-ionic ether diol with a GRAS (generally recognized as safe) desig- nation and a growing use as a food additive and an additive in pharmaceutical formulations. Over the past decades, PEG has been considered to be non- immunogenic. However, there is growing evidence that PEG might be more immunogenic than previously recognized. This is supported by the growing existence of anti-PEG antibodies in healthy humans who are increasingly exposed to PEG additives [25,26]. For example, Armstrong et al. [27] reported the existence of anti-PEG antibodies in 25% in healthy blood donors. Later on, Yang and Lai [28] documented even higher concentrations of anti-PEG antibodies in about 42% in patients with no history of treatment with PEGylated products. The higher prevalence of pre-existing anti- PEG antibodies in healthy individuals, not receiving PEG-containing drugs, is being attributed to the fre- quent use of PEG-containing or PEG-coupled products that are common ingredients in personal care, beauty, and household cleaning products (e.g. soap, sunblock, cosmetics), as well as processed foods. Accordingly, Yang and Lai proposed a tentative mechanism for the formation of PEG antibodies in which various condi- tions like ulcerations, abrasions and skin tears may result in local inflammatory reactions and induction of immune responses in the presence of PEG. The frequent use of products containing, or coupled to, PEG, such as, soaps, shampoos, toothpastes, lotions, or detergents, results in penetration of PEG to sites of inflammation where contact with immune cells stimu- lated the production of anti-PEG antibodies [28]. The presence of pre-existing anti-PEG antibodies might trigger further immunogenic responses to PEG when the human subjects receive PEGylated therapeutics [25,26]. These pre-existing and induced anti-PEG anti- bodies together may compromise the response to PEGylated medicines in the clinical settings [29–31].
3.Function of grafted PEG on nanocarriers
As described above, PEG has been used for surface modification of nanocarriers such as liposomes, nanoparticles and therapeutic proteins, to increase their circulation half-lives [32–36]. It is reported that PEG grafted on surfaces increases their hydrophilicity and functions as a steric barrier, hindering the interactions between the nanocarriers and serum protein opsonins that are involved in the recognition of the carriers by the cells of MPS [17]. PEGylated nanocarriers such as PEGylated lipo- somes, PEGylated micelles and PEGylated proteins that are not prematurely taken up by the cells of MPS have a greater chance of reaching, and deliver- ing increased levels of therapeutics to target dis- eased organs, compared to non-PEGylated one. This could be attributed to the stealth property imparted by PEGylation on nanocarrier surfaces and consequently decreased uptake of PEGylated nanocarriers by the cells of MPS, which explains in part why they have received a number of clinical approvals [1,19,37,38].
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