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16 Beyond PEGylation “PEGylation andits Alternatives”
477
(c) Additionally, polypeptides can be customized with specic sequences and
lengths, allowing for precise control over their properties and interactions with other molecules, making them valuable for tailoring drug delivery systems to specic therapeutic needs.
(d) Polypeptides offer diverse functionalities, including amino acids with side
chains, which can be used for drug conjugation and targeting ligands.
(e) They are naturally compatible with biological systems, allowing them to inter-
act more effectively with cells, tissues, and biological molecules than synthetic polymers.
(f) Some polypeptides, like β-sheet-forming peptides, can self-assemble into sta-
ble secondary structures, enhancing drug delivery and controlled release. Polypeptides are particularly useful in applications like tissue engineering, wound healing, and regenerative medicine, as they mimic extracellular matrix components and are biocompatible with biological structures (Hosseinkhani etal. 2013).
Excluding hydrophobic and highly polar amino acids from a polypeptide sequence increases drug delivery or the system’s solubility. Taking advantage of this benet, the circulation half-life of the drug can be enhanced by using homopoly­mers of genetically encoded amino acids (Bolze etal. 2016; Falvo etal. 2016). PAS, a polypeptide composed of proline, alanine, and serine, was biodegradable and hydrophilic, with a large hydrodynamic volume and surface charge. The highly soluble formulation not only showed antihemolytic activity but was also distributed and cleared similarly to PEG.PASylation is dened as “the genetic infusion of a conformationally unfolded homopolymer of PAS with therapeutic proteins or drugs” (Harari etal. 2014; Kuhn et al. 2016). In mouse models, the stability of PASylated products was demonstrated in serum, with no organ distribution and minimal immunogenicity and toxicity. Though this class of products is yet to be investigated in the clinic, they have been widely employed to extend the systemic circulation of leptins, antibody fragments, chemotherapeutics, interferons, and growth hormones and for various therapeutic as well as diagnostic purposes (Mendler etal. 2015; Morath etal. 2015; Schlapschy etal. 2013). Another study reported an intriguing genetic fusion of XTEN, a polypeptide sequence, with a pro­tein that resulted in a 60-fold increase in plasma half-life. XTEN is a hydrophilic, stable homopolymer with an indenite structure. It is non-immunogenic and con­sists of certain amino acid sequences (alanine, glutamic acid, glycine, proline, ser­ine, threonine). XTEN fusions have been further extended to glucagon, factor VII, green uorescent protein, and human growth hormone to improve the efcacy of the respective systems in diverse invivo models (Podust etal. 2016; Schellenberger etal. 2009; Strohl 2015).
Polypeptides offer advantages like enzymatic degradation and structural stabil­ity, but may also face challenges in large-scale production. The choice between PEG and polypeptides depends on the drug delivery system’s requirements and desired properties, and researchers carefully consider these factors when selecting the most suitable polymer (Fig.16.14).
478
Fig. 16.14 Chemical structure of cyclized polypeptide as an alternative to PEG
S. Mohanta et al.
Fig. 16.15 Structure of carbohydrate-based polymers

16.5.9 Carbohydrate-Based Systems

Carbohydrates, nucleic acids, and proteins are important natural macromolecules for life. Synthetic carbohydrates can be good candidates for applications in polymer and pharmaceutical research. They can have a linear, comb-type, branched, or dendrimer- like structure (Tolstyka etal. 2016) (Fig.16.15).
Carbohydrate-based systems exhibit a PEG-like effect due to their hydrophilicity and steric hindrance, which form a protective layer on drug delivery systems or biomaterials. This layer minimizes protein adsorption, extending the circulation time and enhancing bioavailability. Carbohydrate-based systems can also mimic glycoproteins and glycolipids found on cell surfaces, effectively “camouaging” themselves from the immune system. This molecular mimicry can reduce immune responses and prevent immune-mediated clearance of the drug delivery system. Carbohydrates are biocompatible, making them suitable for drug delivery reducing the likelihood of adverse reactions or immunogenicity. The chemistry in their con­jugation is crucial in understanding these mechanisms (Moros etal. 2012).
16 Beyond PEGylation “PEGylation andits Alternatives”
479
Carbohydrate-based systems can be conjugated to drug carriers, nanoparticles, or other substrates through various chemical reactions. The specic chemistry used depends on the functional groups on the carbohydrate and the substrate’s surface. Carbohydrates can be functionalized with specic reactive groups, such as amino (-NH2), carboxyl (-COOH), or thiol (-SH) groups, during synthesis or chemical modications. Chemical conjugation can occur through amidation, thiol-disulde exchange, or carbodiimide-mediated coupling. If both carbohydrate and substrate have compatible functional groups, they can be covalently linked to form a stable bond. Some carbohydrate-based systems can be conjugated using enzymatic or gly­cosylation reactions, mimicking natural glycosylation processes in living organ­isms. The carbohydrate-based system and chemistry choice depend on the application and desired drug delivery properties. Carbohydrate-based systems offer an attractive alternative to PEG, providing similar benets in drug delivery and biomaterials while aligning with the biocompatibility of natural carbohydrates (Su etal. 2021).
Aside from their biological activities, they are suitable candidates for PEG sub­stitutes due to intrinsic qualities that render them chemically well-dened, compact, biocompatible, highly hydrated, synthetically exible, and less susceptible to aggre­gation and protein adsorption. Both polysaccharides (dextran, starch, hydroxyethyl starch (HES), and hyaluronic acid) and oligosaccharides (from one to twelve sugar units) have been widely investigated. Dextran-coated nanocarriers, like PEG and HES, have demonstrated low protein adsorption and prolonged circulation dura­tions (Besheer etal. 2007). Carbohydrates, combined with their active biological role of interacting with cell surfaces or specic proteins, have emerged as desirable candidates for biotherapeutic applications (Sizovs etal. 2013; Luyckx and Baudouin
2011). Trehalose is an exciting example of a naturally occurring disaccharide. It is
a nonreducing sugar with glucose units joined by α,α’-1,1-glucosidic bond, another compelling example of a carbohydrate. It has shown fascinating biochemical prop­erties, rendering them appealing for research in academia and industry (Gu etal. 2014).

16.5.10 Hydrophilic Polymers

Hydrophilic polymers have numerous applications, including drug delivery (Knop et al. 2010; Hildebrand et al. 2017), self-assembly (Willersinn et al. 2017), and catalysis (Ge et al. 2007). As a result, research into hydrophilic polymers is an essential aspect of polymer science. Signicant areas of investigation have been structure-property relationships and their interactions with biological entities (Rudolph etal. 2013; Takahashi etal. 2018; Cuomo etal. 2012). Synthetic polymer chemistry offers new possibilities for advanced characteristics and prospects using innovative hydrophilic polymers and structures (Liu etal. 2018; Mccormick and Lowe 2004). Hydrophilic polymers have essential features not just when dissolved but also when crosslinked, as in the case of hydrogels. This class of materials has various intriguing features as well as customizable size and morphology, which are
480
S. Mohanta et al.
of great importance for pharmaceutical applications (Thiele etal. 2014). Hydrophilic polymers include poly(acrylamide) and poly(ethylene glycol) (PEG). It has to be noted that some polymers, such as poly(2-hydroxyethyl methacrylate), are hydro­philic and water-swellable albeit with poor aqueous solubility. Even for hydrophilic polymers, water solubility varies according to polymer concentration, type, and molecular weight. As a result, they may not be distinguished based just on solubility (Schmidt 2019) (Fig.16.16).
Hydrogels are hydrophilic polymer materials known for their biocompatibility, swelling, and soft texture. They nd applications in areas such as tissue engineering and implants. Reports have described polymer self-assembly combined with hydro­gel formation has been described, with the double hydrophilic block copolymer poly(N-vinylpyrrolidone)-b-poly (oligo ethylene glycol methacrylate) coupled with α-cyclodextrin (α-CD). This results in thermo-responsive hydrogels that exhibit dis­tinct mechanical properties upon cooling to room temperature (Jiang etal. 2016). Cyclic cyclodextrin (CD) can not only be applied for gelation but can also be uti­lized in stimulus-responsive hydrogels for drug entrapment. Bian etal. synthesized an amphiphilic polymer by combining hydrophilic acrylamide, hydrophobic N-dodecyl acrylamide, and sodium acrylate, which delayed the formation of pheno­lic network crosslinking points (Xu etal. 2019). Cuomo and coworkers described a
Fig. 16.16 Chemical structure of epoxy poly(AAG-AA), a hydrophilic polymer
16 Beyond PEGylation “PEGylation andits Alternatives”
481
hydrogel based on an emulsion of alginate and lemon grass essential oil, which was studied for mechanical properties.
Thiele and coworkers reported a polysaccharide hydrogel targeting microgels via microuidics (Hauck etal. 2018). Combining chitosan or hyaluronic acid with tert­butyl isocyanide and a PEG crosslinker allowed for the fabrication of mono- disperse microgels with sizes in the 70–100μm range. Functional microgels were formed by adding functional carboxylic acids in the Ugi crosslinking reaction, potentially valuable for drug delivery. Lensen and coworkers described the microstructure of PEG-based hydrogels, revealing the strong effect of the polymerization process on the size of crystalline domains. With this understanding, crystallization behavior in PEG-based hydrogels can be ne-tuned to improve material properties (Zhang etal. 2018).
Reinforcement via nanoparticles has also been explored to investigate the forma­tion and mechanical properties of Laponite/cellulose-based hydrogels (Xie etal.
2018). When Laponite was combined with silanized hydroxypropyl methylcellu-
lose, a signicant reinforcement effect was observed, increasing the storage modu­lus by an order of magnitude. Fluorescent microsphere tracking analysis was used to investigate the hydrogel architecture, revealing two domains: loose aggregates in the periphery and dense Laponite structures in the core.
Hydrophilic polymers have been applied in the biomedical eld with advances in drugs and vaccines in the form of polymeric nanomaterials. These nanoparticles exhibit targeted delivery and sustained release (Secker etal. 2015). Poly(peptoids) have gained attention, with Barz et al. developing siRNA polyplexes with poly(sarcosine) as a shielding agent. They reported the formation of lipo-oligomer structure and siRNA polyplexes, with the oligomer consisting of cholanic acids coupled through a bioreducible disulde linker. The ability to shield was tested using invivo imaging and biophysical assays, revealing prolonged systemic circula­tion time on par with the gold standard PEG (Klein etal. 2018) (Table16.1).

16.5.11 Non-PEGylated Nanoparticles

Non-PEGylated microparticles and nanoparticles (Fig. 16.17), mainly inorganic nanoparticles such as iron, gold, silver, hafnium, ceramic silica, and carbon nano­tubes, have sparked considerable interest in tissue engineering, drug delivery, and diagnostic formulations (Anselmo and Mitragotri 2015, 2016). Metal nanoparticles with about 1–100nm diameters are appealing owing to their distinct physicochemi­cal characteristics, such as enhanced surface-area-to-volume ratio, small size, and conductivity. They are also amenable to surface modication and can be designed to respond to external stimuli with pharmacokinetics suitable for systemic delivery (Huang etal. 2011). Because of their increased permeability and retention, nanopar­ticles preferentially concentrate at malignant tumor sites.
Non-PEGylated nanoparticles can be modied to achieve a PEG-like effect through chemical reactions and surface functionalization. These methods include hydrophilic coating, ligand attachment, surface modication, and stealth coatings.
482
Table 16.1 Comparison between different alternative polymers for PEG
Name of Polymer Polyzwitterions Chemical exibility,
Polyglycerols (PGs) Biocompatibility, easy
Polyvinyl pyrolidones (PVP)
Polyoxazolines (Pox) Low PDI values,
Polyacrylamides Easy functionalization
Poly[oligo(ethylene glycol) methyl ether methacrylate] (POEGMA)
Polyamino acids Biodegradable,
Carbohydrate-based Biodegradable, low
Advantages Disadvantages
Nondegradable, high zwitterionic nano­vectors, biocompatibility, non-fouling, prolonged blood circulation, and multifunctionality
synthesis, immune invasion, and prolonged circulation due to hyperbranched polyglycerol (HPG)s
Biocompatible and stabilizer
exible end-group chemistry, biodegradability, and biocompatibility
due to hydroxyl groups, Biocompatible and fouling
Biocompatible, Easy chemical modication, Thermoresponsive, Biodegradable
Decreased ABC phenomenon, Biocompatible
immunogenicity, nontoxic, easy chemical modication, biocompatible
synthesis and
modication cost
Nonbiodegradability,
accumulation in tissues
Enhanced protein
adsorption, slower
biodegradation
Higher synthesis cost
and no FDA approval
Nonbiodegradability,
degradation by thermal
and photolytic effect,
resultant monomer gives
neurotoxic effect
Microbial
contamination, excessive
hydration, and reduced
viscosity on storage
Variable chemistry,
broad or mixed
molecular weights
S. Mohanta et al.
Reference Qian etal.
(2022), Zheng etal. (2017)
Hoang Thi etal. (2020)
Hadjesfandiari and Parambath (2018), Knop etal. (2010)
Hoang Thi etal. (2020)
Hadjesfandiari and Parambath (2018), Knop etal. (2010)
Lutz (2008)
Boddu etal. (2021)
Barclay etal. (2019), Hoang Thi etal. (2020)
Hydrophilic polymers like dextran, chitosan, or albumin can be conjugated to the nanoparticle’s surface through covalent bonding (Salatin and Yari Khosroushahi
2017). Ligands, peptides, or biomimetic materials can enhance biocompatibility
and enable targeted drug delivery. Surface modications, such as adsorption or encapsulation of hydrophilic materials, create a hydrophilic shell around the nanoparticles. Stealth coatings can be formed using materials like zwitterions or synthetic polymers.
16 Beyond PEGylation “PEGylation andits Alternatives”
Fig. 16.17 Structure of a non-PEGylated nanoparticle
483
Non-PEGylated nanoparticles should be synthesized and characterized to meet high standards of quality, purity, and consistency. Biocompatibility testing should be conducted to evaluate safety and interactions with biological systems. Sterility should be maintained during preparation and handling, especially in medical and biological applications. Chemical compatibility should be veried to avoid degrada­tion, reduced efcacy, or undesired by-products. Dosage optimization should be determined for each specic application, as excessive amounts may not improve effectiveness or lead to undesirable side effects.
Non-PEGylated gold nanoparticles are used in cancer therapeutic applications like drug delivery and photothermal therapy due to their high biocompatibility, min­imal cytotoxicity, and low immunogenicity. They are compatible with biological systems and have limited inammation and foreign body responses. Toxicity stud­ies show no acute toxicity, but thorough characterization and purication are cru­cial. Sterile handling is essential to prevent contamination. The optimal dosage and concentration should be determined for each cancer therapeutic application. The safety prole varies depending on the nanoparticles used and their application, and comprehensive safety assessments and studies are essential. Researchers should ref­erence peer-reviewed sources for safety assessments and research (Gamucci etal. 2014).
Metal nanoparticle-based therapeutic systems are being investigated in various stages of preclinical and clinical development for the therapy as well as diagnosis of diseases such as cancer, liver cirrhosis, and anemia. While more research is war­ranted to understand the long-term effects of nanoparticles, current treatment options can be improved by enhancing bioavailability and efcacy via an approach that combines inorganic metal nanoparticles conjugated to small molecule entities (Na etal. 2009).

16.6 Future Prospects

Although the FDA has approved natural polymers such as polypeptides, polysac­charides, and nucleotides for various indications, scientists have tried to develop synthetic polymers that can perform better. Among other synthetic polymers, linear PEG has been demonstrated to be superior for multiple applications.
484
S. Mohanta et al.
Even though PEGylation provides numerous benets, new research has shown critical shortcomings such as immune activation, poor degradability, and potential aggregation. The enormous advances in macromolecular engineering and polymer chemistry have aided in exploring substitutes for PEG that can address the above­mentioned limitations. Polymeric systems such as poly(glycerol)s, poly(vinylpyrrolidone), poly(amino acid)s, poly(N-(2-hydro-xypropyl) methacryl­amide), and poly(2-oxazoline)s have shown potential as far as hydrophilicity, aque­ous solubility, stealth properties, and biocompatibility are concerned. However, none of them have been approved by the FDA yet, and further research is required to properly evaluate and compare them with PEG through a comprehensive analysis of polymer structure as well as cellular interactions.

16.7 Conclusion

PEGylation is, without a doubt, a very versatile and effective bioconjugation method. PEGylation of nanoparticles allows for effective transport of hydrophobic medicines, prolongs systemic circulation, enhances mucus penetration, and improves the stability of biotherapeutics. As a result, PEGylation has emerged as an invaluable technique for nanomedicine modication to enable the development of safe and efcacious drug delivery systems. Nonetheless, the anti-PEG immune response is a vexing problem that compromises the safety of drugs. So far, potential PEG alternatives are yet to be successfully translated to the clinic. Studies investi­gating the immunogenic potential of PEG have demonstrated that the clearance of subsequent doses of the drug can be primarily attributed to the anti-PEG antibodies produced after the rst dose. If the patient has preexisting antibodies due to prior exposure to PEG in domestic or cosmetic goods, the rst dose can elicit an immune response. It is, therefore, critical to understand the parameters that inuence the development of anti-PEG antibodies to design innovative drug vectors or change the injection schedule and/or administration route and achieve therapeutic efcacy. While PEG alternatives will be aggressively investigated in the near future, we believe that PEG will continue to be extensively used for bioconjugation to enable effective drug delivery.
Acknowledgments The author RKT acknowledges the Department of Pharmaceuticals, Ministry
of Chemicals and Fertilizers, India, for supporting the drug discovery and formulation research at NIPER Ahmedabad. R.K.T also acknowledges the Department of Science and Technology, Government of India, for a Core Research Grant funding (File No. CRG/2021/005402) and also acknowledges the Indian Council of Medical Research (ICMR), New Delhi, for the grant File Id: 2021-14161 and grant File Id: IIRP-2023-4849/F1 for supporting research in RKT lab.
16 Beyond PEGylation “PEGylation andits Alternatives”
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