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6 PEGylation asaTool toAlter Immunological Properties ofNanocarriers
177
ultimately reduces the immune recognition of such molecules, thereby signicantly
reducing their immunogenicity (Shi etal. 2022).
Enhanced Circulation Time
PEGylation can increase the overall circulation time of the proteins or drugs upon
administration. PEGylation increases the overall molecular weight of the proteins or
drugs, which reduces their renal clearance (Du etal. 2020). Reduced renal clearance
coupled with declined immune recognition owing to PEGylation combined enhances
the circulation time of the PEGylated molecules.
6.2.3.2 Disadvantages ofPEGylation
Despite all the lucrative advantages that the technique of PEGylation offers, it still
presents some disadvantages. Careful consideration and thorough examination of
these disadvantages are recommended while adapting the strategic PEGylation
approach for proteins, peptides, and drugs of interest. A few leading disadvantages
of PEGylation are mentioned in following section.
Heterogeneity
PEGylation is a complex process necessitating the careful monitoring of various
process parameters such as reaction temperature, reaction time, molecular weight of
PEG, and pH.PEGylation reactions are time-consuming and involve forming heterogeneous nal products over a monosubstituted product. This further makes the
downstream processing cumbersome and increases the cost of production. Nonspecic coupling reactions usually lead to the formation of heterogeneous products.
However, this can be minimized by site-specic PEGylation of proteins and peptides, which specically carries out PEGylation at the particular amino acid residue
of the protein or peptide under consideration.
Risk ofImmune Response
Although PEG is generally considered safe for human use, some individuals may
develop an immune response to PEGylated molecules, leading to allergic reactions
or other adverse effects. Multiple reports have suggested the formation of immediate and delayed anti-PEG antibodies in individuals after exposure to PEG.De Groot
etal. have reported instances of anaphylactic shock with a marketed PEGylated
contrast agent (De Groot etal. 2004). The cases of urticaria are also reported by
Perez et al. using polysorbate 80, a PEG-containing polymer (Pérez-Pérez
etal. 2011).
Alteration ofBiological Activity
PEGylation can alter the biological activity of a molecule, either positively or negatively, depending on the specic application. The steric hindrance created by PEG
may interfere with the interaction of the PEGylated molecules and their target,
thereby reducing the desired pharmacological response. The drugs or proteins
requiring cellular internalization to demonstrate their pharmacological activity may
suffer signicantly due to PEGylation. This effect can be prominently observed in
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A. Tiwari et al.
the case of PEGylated enzymes, wherein the interaction between the therapeutic
enzyme and the substrate is severely affected due to PEGylation.
Overall, PEGylation is a valuable tool for modifying the properties of molecules
for various applications. However, the advantages and disadvantages of PEGylation
should be carefully considered, along with other factors, such as the specic application, target molecule, and regulatory requirements.
6.3 Immunological Properties ofNanocarriers
Nanocarriers are engineered structures that are used to deliver drugs, genes, or other
therapeutic agents to specic sites in the body. While nanocarriers have shown great
promise as drug-delivery vehicles, they can also have immunological properties that
must be carefully considered. To understand the immune response to the externally
administered nanocarriers, it is essential rst to understand the immune system. The
immune system is a complex network of cells, tissues, and organs that protect the
body against pathogens and foreign substances. It is responsible for identifying and
eliminating harmful invaders while maintaining tolerance to self-antigens (Chaplin
2010). The immune system can be divided into two main parts—the innate immune
system and the adaptive immune system. The innate immune system is the rst line
of defense against invading pathogens. It includes physical barriers, such as the skin
and mucous membranes, as well as cellular components, such as phagocytic cells
(e.g., macrophages and neutrophils) and natural killer (NK) cells. These cells can
recognize and eliminate foreign invaders through non-specic mechanisms, such as
phagocytosis, cytokine production, and complement activation (Anaya etal. 2013).
The adaptive immune system is a more specialized and targeted response that
develops over time in response to specic pathogens. It includes B, T, and antigenpresenting cells (APCs). B cells produce antibodies that specically recognize and
neutralize foreign antigens, while T cells recognize and destroy infected or abnormal cells. APCs, such as dendritic cells and macrophages, present antigens to T
cells to initiate an immune response (Den Haan etal. 2014). The humoral immunity
and cell-mediated immunity are two branches of adaptive immunity. Humoral
immunity involves the production of antibodies by B cells, which can neutralize
pathogens in the bloodstream or other extracellular spaces.
In contrast, cell-mediated immunity consists of activating T cells, which can
destroy infected or abnormal cells. The immune system can also remember previous
exposure to pathogens, allowing for a faster and more efcient response upon subsequent encounters. This is the basis for vaccination, which involves the introduction of a non-pathogenic form of a pathogen to elicit an immune response and
establish memory (Cooper and Eleftherianos 2017).
Depending on the therapeutic application, nanocarriers can play an essential role
in modulating the immune response, either by enhancing or by suppressing immune
responses. The immune response to nanocarriers can depend on several factors,
including their size, shape, surface chemistry, and cargo.
6 PEGylation asaTool toAlter Immunological Properties ofNanocarriers
179
6.3.1 Enhancing Immune Responses
Nanocarriers can be designed to enhance immune responses by delivering antigens,
adjuvants, or immunomodulatory agents to antigen-presenting cells (APCs), such as
dendritic cells and macrophages. This can activate T and B cells, generating an
adaptive immune response against infectious diseases or cancer. For example, nanocarriers can be used as a platform for cancer vaccines to deliver tumor-associated
antigens and adjuvants to APCs, which can enhance the immune response to cancer.
Castro et al. developed chitosan/poly(γ-glutamic acid) nanoparticles (Ch/γ-PGA
NPs) to modulate the inammatory prole of macrophages and dendritic cells to
impair their ability to promote cancer evasion (Castro etal. 2017). Ch/γ-PGA NPs
demonstrated the development of immunostimulatory phenotype of dendritic cells,
thereby enhancing the expression of co-stimulatory molecules such as CD86, CD40
and secretion of pro-inammatory cytokines such as IL-6, IL-12p40, and TNF-α.
Ch/γ-PGA NPs also promoted the macrophages to a pro-inammatory prole,
decreasing the expression of CD163. These developments observed using Ch/γ-
PGA NPs inhibited the antigen-presenting cells’ ability to invade colorectal cancer.
6.3.2 Suppressing Immune Responses
Nanocarriers can also be designed to suppress immune responses, such as treating
autoimmune diseases or transplant rejection. This can be achieved by delivering
immunosuppressive agents, such as siRNA or small molecule drugs, to immune
cells, such as T cells or dendritic cells, which can reduce their activation and proliferation. Tang etal. developed PEG-b-poly(,-lactide-co-glycolide) (PEG-PLGA)
nanoparticles and encapsulated cyclosporine A (CsA) into them (Tang etal. 2012).
T-cell assay was performed to assess the suppression of T-cell proliferation and
production of inammatory cytokines. Both free CsA and CsA encapsulated PEGPLGA NPs demonstrated dose-dependent production of inammatory cytokines
and suppression of T-cell proliferation. The release kinetics of CsA from CsA
encapsulated PEG-PLGA NPs (55.6% release on day 1) suggested their ability to
maintain the therapeutic concentration of CsA invivo. This PEGylated NP-based
delivery system for the delivery of immunosuppressive drugs presents the potential
application of PEGylated techniques for the targeted delivery of immunosuppressive drugs.
6.3.3 Immune Evasion
Nanocarriers can also be designed to evade the immune system by using stealth
coatings, such as PEG, or by mimicking the properties of natural cells, such as
erythrocytes or platelets. This can increase the circulation time of nanocarriers and
reduce their recognition and clearance by the immune system.
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A. Tiwari et al.
6.4 Immunological Barriers toNanocarrier-Mediated
Drug Delivery
PEGylation is the procedure of adhering PEG chains to a nanocarriers’ surface,
such as liposomes or nanoparticles. By inhibiting immune system detection and
clearance, PEG chains can lengthen the duration that nanocarriers are in circulation
in the bloodstream. Nanocarriers are foreign substances, and when they are introduced into the body, they can elicit an immune response. The immune system can
recognize nanocarriers as foreign and try to eliminate them. This immune response
can limit the effectiveness of nanocarriers as drug-delivery systems (Zolnik etal.
2010). By disguising the surface of the nanocarriers and preventing the immune
system from recognizing them, adding PEG chains to nanocarriers can aid in lowering the immunological response. The “stealth effect” is the name given to this phenomenon. The stealth effect can prolong the circulatory circulation duration of
nanocarriers, allowing for their accumulation in the target tissue or organ (Zalba
etal. 2022). However, prolonged circulation of PEGylated nanocarriers can also
lead to the formation of anti-PEG antibodies in some patients. These antibodies can
recognize and eliminate PEGylated nanocarriers, limiting their effectiveness as
drug delivery systems. The formation of anti-PEG antibodies may also lead to
hypersensitivity reactions in some patients (Zhang etal. 2016a).
Nanocarrier-mediated drug delivery has shown great potential for the treatment
of various diseases. However, there are several immunological barriers that can
limit their efcacy and safety. The immune system can recognize nanocarriers as
foreign invaders, which can trigger an immune response. This can lead to the activation of immune cells, such as macrophages and dendritic cells, and the production
of inammatory cytokines (Liu etal. 2017). When nanocarriers are recognized by
the immune system, they can be coated with opsonins, such as antibodies and complement proteins. This opsonization can promote their uptake by phagocytic cells,
such as macrophages, which can limit their circulation time and therapeutic efcacy
(Gamucci etal. 2014).
Furthermore, nanocarriers can be cleared from the body by the reticuloendothelial system (RES), which includes the liver and spleen. The rate of clearance can
depend on the size, shape, surface charge, and surface chemistry of the nanocarrier
(Duan and Li 2013). Some nanocarriers can be immunogenic, meaning they can
stimulate an immune response even without an antigen. This can be due to adjuvants, contaminants, or impurities in the nanocarrier (Zarreen Simnani etal. 2023).
Although the nanocarriers offer a variety of advantages for drug delivery purposes, their utility is often limited by the immunological responses that are elicited
upon their administration. Some strategies, such as surface modication and
PEGylation, can be undertaken to overcome the immunological barriers associated
with nanocarriers.
6 PEGylation asaTool toAlter Immunological Properties ofNanocarriers
181
6.4.1 Strategies toOvercome Immunological Barriers
There are several strategies that can be employed to overcome immunological barriers in nanocarrier-mediated drug delivery, including the use of biocompatible
materials and surface modications. Nanocarrier surface modications possess a
distinct advantage as they allow non-biocompatible materials to be delivered without eliciting an immune response. Surface modications include using macromolecules, membranes from endogenous cells, receptors, and steric polymers. The aim
of utilizing the surface modication strategies is to synthesize the stealth nanocarriers, which are undetectable to the immune system, allowing them to reach the target
site and exert their desired therapeutic response. The commonly utilized surface
modication strategies are summarized in Table6.1.
6.4.1.1 PEGylation
PEGylation, as described earlier, refers to the method of coating the surface of
nanocarriers with PEG.Various proteins, such as complement proteins, plasma proteins, and antibodies, are known to aggregate on the surface of foreign bodies and
lead to protein corona formation, which marks the initial step in the process of
immune recognition. PEG has shown a potential to hinder the formation of protein
corona on the surface of nanocarriers by forming a steric barrier (Schöttler etal.
2016). Additionally, PEG-coated nanocarriers can be designed to smartly shed the
PEG coating in response to a stimulus upon reaching the target site. Qiao etal. successfully developed a nanosystem by combining matrix metalloprotease-2 sensitive
peptides with HLAH, a pH-sensitive peptide, and PEG (Qiao etal. 2017). The PEG
ensured the enhanced circulation time for the compound, whereas, upon reaching
the acidic tumor microenvironment, the PEG coating was released, leading to the
delivery of the cytotoxic compound to the target site.
Although PEGylation possesses various merits over non-PEGylated systems, it
still presents certain limitations. PEG is susceptible to oxidative degradation, which
Table 6.1 Strategies to overcome immunological barriers
Surface
modication
strategy
PEGylation PEG Form steric barrier and
Coating with cell
membranes
Coating with
carbohydrates
Coating with
proteins
Coating material
Cell membranes derived from
RBCs, leukocytes
Heparin, hyaluronic acid,
polysialic acid, glucosamine,
and dextran
Albumin, CD47 ligand,
zwitterion of lysine, and
glutamic acid
Mechanism of immune
evasion
prevent protein corona
formation
Mimic circulatory cells
and increases circulation
time
Mimic cell surface and
prevent protein corona
formation
Form steric barrier and
prevent protein corona
formation
References
Schöttler etal.
(2016)
Parodi etal.
(2013)
Bellido etal.
(2015)
Nowinski
etal. (2014)
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A. Tiwari et al.
will lead to the formation of reactive oxygen species, and repeated administration of
the PEGylated system might also lead to the formation of anti-PEG antibodies
(Sung et al. 2010; Verhoef et al. 2014). However, compared to other polymers
potentially used to meet the same purpose, PEG is preferred choice.
6.4.1.2 Cell Membranes
Using cell membranes as a coating material for the nanocarriers is another area of
research that primarily focuses on creating biomimetic systems to evade the immune
response. The initial investigations in this area involved the development of nanocarriers coated with the membrane of red blood cells. This method exhibited promising applications as the nanocarriers prepared by membrane coating retained their
original structure with enhanced circulation time and reduced clearance (Hu etal.
2011). The coating with cell membranes allows multiple administrations of the
developed formulation without developing a signicant immune response (Hu etal.
2011; Piao etal. 2014).
Apart from RBCs, the membranes of various leukocytes have been utilized to
demonstrate immune evasion by the nanocarriers. Leukocyte-derived membranes
showed immune evasion of the nanocarriers and provided unique functions to the
nanocarriers, such as receptor-ligand afnity and diapedesis (Parodi etal. 2013).
The nanocarriers coated with platelet membranes showed stealth properties and the
ability to adhere to the damaged blood vessels (Hu etal. 2015). Using cell membranes to coat the nanocarriers’ surface opened new avenues of opportunities in
engineered biomimetic nanosystems. Future investigations in this domain are
expected to develop synthetic biomimetic membranes with specic properties and
targeting abilities.
6.4.1.3 Carbohydrates
Macromolecules have been thoroughly investigated to develop coated nanocarriers
that can remain signicantly undetectable by the immune system. Heparin and hyaluronic acid are the natural glycosaminoglycan polysaccharides found on the surface
of various cells. This imparts the ability to trick the immune system and prevents the
development of an immunogenic response against the coated nanocarriers (Bellido
etal. 2015; Peer etal. 2003; Toole 2004). Monosaccharides such as polysialic acid
have demonstrated similar functions by shielding the nanocarriers and allowing targeted delivery of nanocarriers (Wilson et al. 2014; Fernandes and Gregoriadis
2001). A precursor to glycosaminoglycans, glucosamine, is another carbohydrate
that reduces complement activation and thereby minimizes immunogenic response
to the coated nanocarriers (Thasneem etal. 2013). Besides the above-mentioned
polysaccharides and monosaccharides, dextran has also demonstrated the ability to
evade complement activation (Thomas etal. 2011).
6.4.1.4 Proteins
Like carbohydrates, the presence of various proteins, as an integral part of anatomical and physiological aspects of human beings, makes them a suitable choice for
coating nanocarriers. Albumin, an abundantly present plasma protein, has been
6 PEGylation asaTool toAlter Immunological Properties ofNanocarriers
widely utilized as a coating material owing to its ability to shield the nanocarriers
and form a steric barrier, which prevents the plasma proteins from forming a protein
corona around the nanocarriers (Gulati etal. 2017, 2018). CD47 ligand is another
commonly used coating material found on the surface of RBCs and is known to
induce M1 phagocytic activity (Rodriguez etal. 2013). Zwitterions can also be utilized to coat the surface of nanocarriers effectively. Zwitterionic coating consisting
of lysine and glutamic acid repeated chains has demonstrated signicant promise as
a coating material by preventing protein corona formation on the surface of nanocarriers (Nowinski etal. 2014).
183
6.5 Effects ofPEGylation onInVivo Behavior
ofNanocarriers
PEGylation plays a crucial role in tailoring the invivo behavior of nanocarriers,
profoundly impacting their efcacy as drug delivery systems. PEGylation imparts
stealth properties to the nanocarriers, thereby preventing their immune recognition
and enabling them to remain in systemic circulation for longer. Various effects of
PEGylation on the invivo behavior of nanocarriers are discussed below.
6.5.1 Pharmacokinetics andBiodistribution ofNanocarriers
PEGylation can signicantly affect the pharmacokinetics (PK) and biodistribution
of nanocarriers. By attaching PEG chains to the surface of nanocarriers, the physicochemical properties of the nanocarriers can be altered, leading to changes in their
PK and biodistribution. One of the main effects of PEGylation is the extension of
the circulation time of nanocarriers in the bloodstream. PEG chains on the surface
of nanocarriers can create a steric barrier that reduces their recognition and uptake
by the reticuloendothelial system (RES), resulting in increased systemic circulation
time. The longer circulation time can allow nanocarriers to accumulate in the target
tissue or organ and improve their therapeutic efcacy (Suk etal. 2016).
Another effect of PEGylation is the reduction of non-specic interactions
between nanocarriers and biological components, such as serum proteins, cell membranes, and extracellular matrix components. This can decrease the rate of clearance
and uptake by the RES, which can lead to improved biodistribution and reduced
accumulation in non-target organs (Donahue etal. 2019). The size of PEG chains
and their density on the surface of nanocarriers can also impact PK and biodistribution. Higher PEG density on the surface of nanocarriers can lead to a more signicant reduction of RES uptake, but it may also lead to faster kidney clearance.
Conversely, lower PEG density may increase non-specic interactions, leading to
rapid clearance by the RES (Perry etal. 2012).
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A. Tiwari et al.
6.5.2 Modulation oftheImmune Response
PEGylation can also modulate the immune response by reducing the recognition of
nanocarriers by the immune system and altering the interaction between nanocarriers and immune cells. As mentioned earlier, nanocarriers can elicit an immune
response, which can reduce their efcacy as drug delivery systems. By adding PEG
chains to the surface of nanocarriers, the immune response can be reduced due to
the “stealth effect.” The PEG chains can create a hydrophilic layer on the surface of
nanocarriers, which can reduce their recognition and uptake by immune cells, such
as macrophages and dendritic cells (Liu etal. 2017).
Moreover, PEGylation can also affect the cytokine prole of the immune
response. Some studies have shown that PEGylation can lead to a shift toward an
anti-inammatory cytokine prole, such as interleukin 10 (IL-10) and transforming
growth factor-beta (TGF-β), which can reduce inammation and promote tissue
repair. This shift in cytokine prole can be benecial for treating inammatory diseases, such as rheumatoid arthritis (Naing etal. 2016). However, prolonged circulation of PEGylated nanocarriers can also lead to the formation of anti-PEG antibodies,
which can elicit an immune response and limit the effectiveness of PEGylated nanocarriers. In some cases, these anti-PEG antibodies can also lead to hypersensitivity
reactions.
Therefore, the immunomodulatory effects of PEGylation need to be carefully
considered in developing nanomedicines. The PEGylation strategy needs to be optimized to achieve the desired immune response while minimizing the risk of adverse
effects, such as forming anti-PEG antibodies (Xia etal. 2021).
6.5.3 Immunogenicity ofPEGylated Nanocarriers
Although PEGylation can reduce the immunogenicity of nanocarriers, it can also
lead to the formation of anti-PEG antibodies, which can affect the safety and efcacy of PEGylated nanocarriers. The formation of anti-PEG antibodies is a concern
because it can lead to accelerated clearance of PEGylated nanocarriers from the
bloodstream and reduce their circulation time. This can decrease the therapeutic
efcacy of PEGylated nanocarriers (Li etal. 2018). Several factors, including the
size and shape of PEG chains, the density of PEG chains on the surface of nanocarriers, the route of administration, the frequency of administration, and the immune
status of the patient, may inuence the formation of anti-PEG antibodies. For example, smaller PEG chains may be more immunogenic than larger ones, and higher
densities of PEG chains on the surface of nanocarriers may increase the likelihood
of antibody formation (Kozma et al. 2020). Furthermore, it has been reported that
some patients with pre-existing PEG-specic antibodies may be at an increased risk
of developing hypersensitivity reactions to PEGylated nanocarriers. Hypersensitivity
reactions can range from mild to severe, including anaphylaxis, and can limit the
clinical application of PEGylated nanocarriers (Sellaturay et al. 2021).
6 PEGylation asaTool toAlter Immunological Properties ofNanocarriers
Fig. 6.2 Applications of
PEGylated nanocarriers
185
While PEGylation can reduce the immunogenicity of nanocarriers, it can also
lead to the formation of anti-PEG antibodies, which can impact the safety and efcacy of PEGylated nanocarriers. Careful consideration of the immunogenicity of
PEGylated nanocarriers and optimization of the PEGylation strategy is necessary to
minimize the risk of adverse immune responses.
6.6 Applications ofPEGylated Nanocarriers
inDrug Delivery
Considering the advantages PEGylation offers, the PEGylated moieties are expected
to possess a diverse spectrum of applications in drug delivery. PEGylated nanocarriers have been widely investigated as drug delivery systems for various therapeutic
applications. The applications of PEGylated nanocarriers are elaborated in Fig.6.2.
6.6.1 Cancer Therapy
PEGylated liposomes have been approved for treating various cancers, including
ovarian and multiple myeloma. PEGylation can increase the circulation time of
liposomes and enhance their accumulation in tumors through the enhanced permeability and retention (EPR) effect. Furthermore, PEGylation can reduce the toxicity
and immunogenicity of liposomes (Fulton and Najahi-Missaoui 2023).
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A. Tiwari et al.
6.6.2 Gene Therapy
PEGylated nanoparticles have been used for gene therapy to deliver nucleic acids,
such as DNA and RNA.PEGylation can protect the nucleic acids from degradation
and increase their stability in circulation. Moreover, PEGylation can enhance the
cellular uptake of nanoparticles and reduce their interaction with serum proteins and
immune cells (Hatakeyama etal. 2011).
6.6.3 Immunotherapy
PEGylated nanoparticles have been used for immunotherapy to deliver immunomodulatory agents, such as cytokines and Toll-like receptor agonists. PEGylation
can increase the half-life of nanoparticles and improve their bioavailability.
Moreover, PEGylation can reduce the toxicity and immunogenicity of immunomodulatory agents (Debele etal. 2020).
6.6.4 Central Nervous System (CNS) Drug Delivery
PEGylated nanoparticles have been investigated for the delivery of drugs to the
CNS, which is challenging due to the blood-brain barrier (BBB). PEGylation can
enhance the penetration of nanoparticles through the BBB and improve the distribution of drugs in the brain. Furthermore, PEGylation can reduce the clearance of
nanoparticles by the RES in the liver and spleen (Crawford etal. 2016).
6.6.5 Pulmonary Drug Delivery
PEGylated nanoparticles have been investigated for pulmonary drug delivery, a
non-invasive route of administration for treating respiratory diseases. PEGylation
can increase the retention time of nanoparticles in the lungs and enhance their penetration through the mucus layer. Moreover, PEGylation can reduce the clearance of
nanoparticles by the immune system in the lungs (Almeida and Souto 2007).
6.6.6 Ocular Drug Delivery
PEGylated nanocarriers have shown potential in ocular drug delivery, allowing for
sustained release of drugs to the eye. These nanocarriers can improve the bioavailability of drugs, prolong therapeutic effects, and enhance patient compliance in
treating ocular diseases (Tsai etal. 2018).