PEGylated Drugs: Definition, Structure, Classification and Benefits

Polyethylene glycol (PEG) is a versatile polymer commonly utilized for the covalent alteration of biopolymers like proteins and peptides. PEGylation represents a pharmaceutical technique where PEG is covalently linked to medications to enhance their pharmacokinetic, pharmacodynamic, and immunological attributes, thereby boosting their therapeutic efficacy. This process alters the physical and chemical characteristics of drugs, influencing factors such as conformation, electrostatic binding, and hydrophobicity. These modifications extend drug retention in the body, elevate plasma half-life, prolong absorption duration, influence drug binding to cell receptors, and enhance tumor targeting. PEG-modified drugs can decrease dosing frequency, enhance effectiveness, improve tolerability, and lower the severity and occurrence of adverse effects. Additionally, PEG can enhance protein solubility and stability, aiding in drug production and storage. Consequently, PEG is commonly employed in drug delivery and modification technologies, either directly linked to drugs or attached to their surface and enclosed within nanomaterials.

What are PEGylated Drugs?

Since the early 1990s, PEGylation has emerged as a leading technology for extending the half-life of drugs, with a proven safety record in human use spanning over 30 years. PEGylated medications have received approval for various administration routes in numerous countries. This modification technique is versatile, applicable to a range of substances including proteins, peptides, oligonucleotides, antibody fragments, small organic molecules, and nanoparticles. Understanding the pharmacokinetics and bioanalysis of PEG and PEGylated drugs is crucial for ensuring their safety and advancing technological progress. However, the intricate nature of PEGylated drugs, intrinsic interferences, and limited analytical methods present significant challenges in studying their pharmacokinetics and bioanalysis.

Structure of PEGylated Drugs

PEG polymers are polymerized from ethylene oxide and can form linear or branched structures. The molecular formula of linear PEG is H-(O-CH₂-CH₂)_n-OH, and only 2 ends can be used to modify new conjugated drugs, so the drug loading capacity is low. The branched chain structure has functional groups on one or more ends, such as branched PEG, forked PEG and multi-arm PEG with branched terminal structures, thus realizing a variety of conjugation possibilities and greatly increasing the drug loading capacity.

Fig. 1. PEG-modified biopharmaceuticals (Expert Opin Drug Deliv. 2009, 6(1): 1-16).

PEGylated drugs generally include PEG, coupled drugs and/or linkers and other components. PEGylation optimizes the solubility, immunogenicity and biological functions of drugs through various conjugated chemical components and/or linkers. The diversity of PEG couplings comes from the use of stable or hydrolyzable bonds. PEGylation technology has appeared in the 1970s. It initially used succinimidyl succinate (ss) as a linker, and later gradually evolved into a variety of linkers. In recent years, with the continuous deepening of research and development, new PEGylated drugs such as PEG bifunctional heterodoxy modified drugs (heterobifunctional PEG, X-PEG-Y) have emerged.

Types of PEG Modified Drugs

The main applications of PEGylation modification in medicine include PEGylated protein drugs, PEGylated peptide chain compounds, PEGylated small molecule drugs, PEGylated liposomes, etc.

PEGylated Protein Drugs

The modification pathways of PEGylated protein drugs mainly include amino modification (including acylation modification of the N-terminal amino group, acylation modification of the lysine side chain amino group, and alkylation modification of the N-terminal amino group), carboxyl modification, thiol modification, etc. At present, research on PEGylated protein drugs mainly focuses on adenosine deaminase, asparaginase, interferon, granulocyte colony-stimulating factor, interleukin, etc. PEGylated macromolecule drugs are currently mainly used to treat cancer, chronic kidney disease, hepatitis, multiple sclerosis, hemophilia and gastrointestinal diseases.

PEGylated Peptide-based Compounds

Peptides generally have short plasma half-life and low oral bioavailability. This is due to the presence of a large number of peptidases and their excretion mechanisms in the body, which inactivate and eliminate peptides. This instability allows the body to quickly regulate hormone levels to maintain homeostasis, but is detrimental to many therapeutic developments. In addition, the low bioavailability of oral peptides is due to the fact that digestive enzymes in the oral cavity can decompose the amide bond of the protein, and can also effectively cut off the same bond of the peptide hormone. At the same time, the high polarity and large molecular weight of the peptide also severely limit intestinal permeability.

Chemical modification of peptides with PEG can improve various physical and chemical properties and pharmacokinetic properties of the peptide with minimal increase in manufacturing costs. The impact of PEG modification on peptide pharmacokinetics has potentially beneficial biodistribution changes, including avoiding reticuloendothelial system (RES) clearance, reducing immunogenicity, and reducing enzymatic and renal filtration losses. These effects can significantly increase the half-life of the peptide in the body and indirectly improve the bioavailability, but will not adversely affect the binding and activity of the peptide and the ligand. PEGylated peptide chain compounds, such as calcitonin and epidermal growth factor, have a longer half-life and higher biological activity than the parent drug. Especially in the site-directed modification of PEG, peptide compounds are easier to obtain than proteins. The most common application in PEGylation studies of peptide compounds is mPEG.

PEGylated Small Molecule Drugs

Currently, many small molecules, especially anti-tumor drugs, can be modified using PEGylation technology. PEG-loaded small molecules can transfer many of their excellent properties to the conjugate, giving the polymer good biocompatibility. Not only can its solubility and biodistribution be improved, but its metabolism and toxicity can also be reduced by changing the drug's exposure to enzymes and vital organs. Many anti-tumor drugs are modified with high molecular weight PEG to achieve targeted delivery to tumor tissues. Small molecule anti-tumor drugs such as irinotecan, camptothecin, doxorubicin, and paclitaxel are prepared into prodrugs through PEG modification. Their solubility, half-life in vivo circulation, adverse reactions, etc. are greatly improved, and they also have significant enhancement. The penetration and retention effects, and the targeting effect on tumor tissue have also been improved.

PEGylated Liposomes

Lipids are amphiphilic molecules with two parts: hydrophilic and hydrophobic. When lipids come into contact with water, unfavorable interactions of hydrophobic segments of the molecule with the solvent lead to the self-assembly of lipids, usually in the form of liposomes. Liposomes are spherical self-sealing structures formed by one or more concentric lipid bilayer membranes, with an aqueous phase wrapped between the center and the bilayer membrane, and are composed of natural lipids or synthetic lipids. In the 1960s, Alec D Bangham of the Babraham Institute of the University of Cambridge first discovered liposomes and proposed the idea of using liposomes as drug delivery vehicles. Due to their size, hydrophobic and hydrophilic properties (in addition to biocompatibility), liposomes are promising drug delivery systems with many advantages. Liposomes can improve the therapeutic indicators of new or already marketed drugs by changing drug absorption, reducing metabolism, extending biological half-life, or reducing toxicity. Drug distribution is mainly controlled by the properties of the carrier, not just by the physical and chemical properties of the raw material.

Liposomes also have many disadvantages, such as high production costs, easy leakage and fusion when encapsulating drugs/molecules, and phospholipids sometimes undergo oxidation and hydrolysis reactions. The main drawback of liposomes is that they are quickly captured by RES, resulting in short half-life, low solubility, and short stability period. PEGylated liposomes (PEGylated long-circulating liposomes) can solve these problems. After PEGylation, the PEG chain builds a hydrophilic protective film on the liposome surface, which increases the hydrophilicity of the liposome surface and reduces the affinity with mononuclear phagocytes, thereby evading RES recognition and reducing the liposome surface. Capture and prevent liposomes from interacting with other molecules, such as various serum components, so they are also called stealth liposomes. A well-known example of the application of this technology is Doxil®, which was developed by the American company Sequus. It is the first liposome drug approved by the US FDA and the first nanomedicine.

Other Applications of PEGylated

PEGylated affinity ligands and cofactors are used for the purification and analysis of biomacromolecules and cells in aqueous two-phase distribution systems. PEGylated sugars can be used as materials and carriers for new drugs. PEGylated oligonucleotides can improve solubility, resistance to nucleases, and cell membrane permeability. PEGylated biomaterials can reduce thrombus formation and reduce protein and cell adhesion.

Featured Services from BOC Sciences

In addition to the PEGylated drugs mentioned above, BOC Sciences also supports PEGylation services for the following biomolecules:

Advantages of PEGylated Drugs

The effects of PEG on proteins are mainly reflected in two aspects: reducing renal clearance and enhancing protection against protein degradation, both of which reduce the total clearance of the drug. Therefore, the main advantage of PEGylated protein drugs is to extend the half-life.

Fig. 2. Main advantages of PEGylation: improved pharmacokinetic, pharmacological and toxicological properties (Expert Opin Drug Metab Toxicol. 2014, 10(12): 1691-702).

Enhancing pharmacodynamic properties and mitigating established toxicities

PEGylation reduces epitope exposure, inhibits the generation of neutralizing antibodies, and diminishes antigenicity and immunogenicity while preserving maximum biological activity. Subcutaneous injection of PEGylated proteins can yield a smoother pharmacokinetic profile for drugs linked to plasma peak toxicity. PEGylation also aids in reducing immune-related adverse reactions triggered by specific protein drugs.

Enhancing drug stability

In aqueous solutions, PEG creates a robust hydration film through hydrogen bonding with water molecules. This film, in conjunction with the flexible PEG chain, thwarts protein adsorption to underlying surfaces, preventing protein aggregation and precipitation. Altering the linkage between PEG and lipid derivatives (acyl, ether, disulfide bonds, etc.) can enhance liposome stability. The flexible PEG chain induces a steric hindrance effect, shielding the modified product from protease attacks and bolstering stability. PEGylation can enhance the thermal and mechanical stability of molecules.

Improving drug distribution and pharmacokinetic properties within the body

Post-PEG modification, the drug's molecular weight increases, significantly reducing glomerular filtration during systemic administration, thereby lowering renal clearance and urinary excretion. Simultaneously, it evades the RES clearance mechanism, extending the drug's plasma half-life and enhancing its release within the body. PEGylated drugs enhance systemic circulation stability, prolonging residence time, which aids in improved drug distribution, particularly in tumor and inflammatory site accumulation of macromolecular drugs. PEGylation enhances bioavailability by minimizing losses at the subcutaneous injection site. This modification alters drug circulation time, safeguarding against biological inactivation by proteolysis or metabolism, potentially reducing dosages and enhancing patient compliance by reducing injection frequency.

Increasing solubility

PEG exhibits solubility in water and various organic solvents like toluene, methylene chloride, ethanol, and acetone. One application involves phase separation of target molecules or cells using PEG-modified antibodies. PEGylated antibody fragments can be concentrated to over 200 mg/mL, expanding formulation and delivery options, such as subcutaneous administration of high-dose proteins, contrasting with intravenous administration of many therapeutic antibodies.

References

1. Bailon, P. et al. PEG-modified biopharmaceuticals. Expert Opin Drug Deliv. 2009, 6(1): 1-16.
2. Zhang, X. et al. Effects of pharmaceutical PEGylation on drug metabolism and its clinical concerns. Expert Opin Drug Metab Toxicol. 2014, 10(12): 1691-702.