Polyethylene Glycol (PEG) and Its Derivatives: End-Group Design and Applications in PROTACs, ADCs, and Medical Device

Introduction to Polyethylene Glycol and PEGylation

Polyethylene glycol (PEG) is a hydrophilic polymer composed of repeating –CH₂–CH₂–O– units. Owing to its excellent water solubility, low toxicity, low immunogenicity, and good biocompatibility, pharmaceutical-grade PEG has been widely used in injectable formulations, sustained/controlled-release preparations, and various drug delivery systems.

In pharmaceutical applications, medical/pharmaceutical-grade PEG with controlled molecular weight, high purity, and low polydispersity is particularly important:

1. A narrow molecular weight distribution helps achieve stable and reproducible pharmacokinetic behavior;

2. Low levels of impurities help reduce potential toxicity and the risk of immune responses.

PEGylation refers to a class of modification technologies in which PEG chains are covalently attached to drug molecules (proteins, peptides, small molecules, oligonucleotides, nanoparticles, etc.). Its typical advantages include:

1. Improving aqueous solubility and formulation stability;

2. Increasing the apparent molecular weight and forming a “hydrophilic shield,” thereby slowing glomerular filtration and enzymatic degradation and prolonging systemic circulation time;

3. Reducing exposure of the drug to the immune system, and thus lowering the risk of immune recognition and clearance;

4. Altering tissue distribution, increasing drug concentration at target sites, and enabling long-acting therapy.

It is important to emphasize that, although PEG is generally regarded as having low toxicity and low immunogenicity, multiple studies in recent years have reported the presence of anti-PEG antibodies and associated hypersensitivity or accelerated blood clearance (ABC). Therefore, when designing PEGylated drugs or PEG-based nanomedicines, immunological risks should be evaluated comprehensively rather than assuming PEG to be completely “non-immunogenic.”

PEG Functionalization and Derivative Design

1. Importance of End-Group Functionalization

Linear PEG chains naturally carry hydroxyl (–OH) groups at both termini. Under physiological conditions, these groups are relatively low in chemical reactivity, making it difficult to achieve efficient and site-specific conjugation to proteins, peptides, or small-molecule drugs using unmodified PEG directly. Consequently, in practical applications, the PEG chain ends are usually functionalized to introduce more reactive and more selective end groups, thereby generating a variety of PEG derivatives.

PEG derivatives bearing different end groups can participate in distinct types of conjugation reactions and materials construction. They are the key structural elements that enable PEG to “do its job” in the fields of drug modification and polymer materials.

2. Common Types of End-Functional PEG Derivatives

Function type / Abbreviation	Representative end group / Structural example	Primary reaction partner	Reaction / performance features	Typical applications
NHS-type products (NHS-PEG)	PEG capped with N-hydroxysuccinimide ester (NHS ester), e.g., mPEG–NHS	Primary amines (–NH₂), such as lysine side chains and protein N-terminal amino groups	Highly reactive toward amines, readily forms stable amide bonds. Sensitive to water and prone to hydrolysis; requires anhydrous / mildly basic conditions and freshly prepared solutions.	Amino modification on the surface of proteins, peptides, and nanoparticles; construction of PEGylated conjugates and crosslinked networks.
MAL-type products (Maleimide-PEG)	PEG capped with maleimide (MAL), e.g., mPEG–Mal	Thiols (–SH), such as cysteine residues and free thiol-containing small molecules	Highly selective toward thiols at pH 6.5–7.5, suitable for site-specific thiol conjugation and precise protein/antibody modification.	PROTAC linkers, ADC linkers, selective conjugation to thiol-containing small molecules or peptides.
ALD-type products (Aldehyde-PEG)	PEG capped with aldehydes (–CHO), e.g., CHO–PEG–CHO (Aldehyde–PEG–Aldehyde)	Primary amines (–NH₂), hydroxylamines (–NHOH) via Schiff base formation / reductive amination	Forms imines (Schiff bases) with amines, which can then be reduced with NaBH₃CN and similar reagents to give stable C–N bonds. Operates under mild conditions (pH 5.5–9.5); commonly used for relatively site-selective N-terminal protein modification and hydrogel crosslinking.	PEGylation at protein/peptide N-termini; injectable hydrogel crosslinkers; surface immobilization.
NH₂-type products (Amine-PEG)	PEG with terminal primary amines (–NH₂), e.g., H₂N–PEG–NH₂	Carboxyl groups (–COOH), NHS esters, aldehydes, etc.	Can be further coupled with COOH, NHS, ALD, etc. Multi-arm NH₂–PEG can be crosslinked with NHS–PEG to form three-dimensional hydrogel networks.	Construction of crosslinked gels; introducing amino groups on surfaces for subsequent coupling; bridging molecules with different functional groups.
SH-type products (Thiol-PEG)	PEG with terminal thiols (–SH), e.g., HS–PEG–SH	Thiols (–SH) on proteins/peptides; acrylates and vinyl sulfones containing double bonds (Michael addition)	Can participate in disulfide exchange or oxidation to form –S–S– bonds, which are cleavable under reducing conditions, enabling degradable linkages. Can add to acrylates / VS to form hydrogels.	Degradable crosslinkers; introduction of thiols onto protein/nanoparticle surfaces; modification of colloidal gold, quantum dots, and related materials.
Azide / alkyne-type products (Azide-PEG & Alkyne/DBCO-PEG)	N₃–PEG–N₃, N₃–PEG–X; DBCO–PEG–X (strain-promoted alkyne)	Alkynes (for classical CuAAC click reaction); azides (with terminal alkynes or strain-promoted alkynes)	Azide–PEG is used in Cu(I)-catalyzed azide–alkyne “click” chemistry (CuAAC). DBCO–PEG reacts with azides via SPAAC under copper-free, mild, and more biocompatible conditions.	Modular assembly of PROTAC/ADC linkers; bioprobes and labeling; “click” functionalization of surfaces.
Amino acid-type PEG derivatives	PEG chains linked to amino acids (e.g., Lys, Glu), bearing both NH₂ and COOH functionalities	Multiple reactive groups (COOH, NHS, ALD, SH, etc.)	Provide both amine and carboxyl “anchors,” making them suitable as multifunctional “intermediate nodes.” Can be further used to introduce fluorophores, ligands, drugs, etc.	Construction of multi-branched / multifunctional carriers; fine-tuning ligand density; complex PROTAC/ADC linker architectures.
Acid-type products (Carboxyl-PEG, etc.)	PEG with terminal carboxyl groups (–COOH) and variants, e.g., HOOC–PEG–COOH (Acid–PEG–Acid)	Hydroxyl groups (–OH) and amino groups (–NH₂) in the presence of coupling reagents	Can be coupled to OH/NH₂ via ester or amide formation, serving as an “acid-terminated” scaffold for further grafting of drugs, small molecules, or other polymers.	Synthesis of PEG–small-molecule prodrugs; introduction of carboxyl groups onto surfaces; crosslinking with NH₂–PEG / NHS–PEG.
Acrylate-PEG / VS-type products (Acrylate-PEG / VS-PEG / AA-PEG-AA)	Acrylate-terminated PEG: PEG–acrylate, AA–PEG–AA; vinyl sulfone-terminated PEG: VS–PEG–VS	Acrylates: thiols, amines via Michael addition or free radical polymerization; vinyl sulfone: highly reactive toward thiols and other nucleophiles	Contain C=C double bonds that can undergo free radical polymerization or Michael addition. Multi-arm PEG–ACLT / PEG–VS can rapidly form crosslinked networks, yielding hydrogels or coatings.	Injectable, in situ-forming hydrogels; tissue engineering scaffolds; surface modification; thiol-reactive crosslinkers.

3. Design at the Polymer Architecture Level: Linear, Multi-Arm, and Block Copolymers

In addition to end-group chemistry, the architecture of the PEG backbone itself is another important dimension in design:

1. Linear PEG

The most classical form, widely used for PEGylation, surface modification, and solubilization.

2. Multi-arm / star-shaped PEG (e.g., 4-arm, 8-arm PEG)

1. Provides multiple reactive sites and can be used to construct highly crosslinked hydrogels.

2. Can serve as a “multi-site drug-loading platform,” increasing overall drug loading capacity.

3. Block copolymers composed of hydrophilic PEG segments and hydrophobic polyester segments (e.g., PEG–PLA, PEG–PLGA, PEG–PCL)

1. Formed by alternating or sequential polymerization of two or more monomers with different chemical structures.

2. Combine the hydrophilicity of PEG with the degradability and hydrophobicity of polyesters such as polylactic acid and polycaprolactone, enabling the preparation of various carrier materials including nanomicelles, microspheres, and biodegradable scaffolds.

Applications of PEG Derivatives in Drug Molecule Modification

1. Use of PEG Derivatives in PROTAC Linkers

PROTACs (Proteolysis Targeting Chimeras) are a class of novel therapeutic molecules that exploit the cell’s endogenous ubiquitin–proteasome system to selectively degrade target proteins. They generally consist of three parts:

1. A ligand for the target protein;

2. A ligand for an E3 ubiquitin ligase;

3. A linker connecting the two ligands.

The rigidity, polarity, hydrophobicity, and length of the linker all significantly influence the efficiency of ternary complex formation and the resulting degradation activity. Statistical analyses show that in a considerable proportion of reported PROTAC molecules, PEG segments are incorporated as components of the linker.

Roles of PEG linkers in PROTACs:

1. Improving solubility and tuning cell permeability

1. Introduction of PEG usually enhances the overall aqueous solubility of the molecule;

2. For highly hydrophobic ligands, PEG segments can reduce hydrophobicity and improve formulation properties and absorption;

3. However, excessively long PEG chains increase molecular polarity, which may compromise membrane permeability and oral absorption, so a balance must be struck.

2. Enabling systematic tuning of linker length

1. By selecting PEG segments of different lengths (e.g., PEG2, PEG4, PEG6, …), the impact of linker length on degradation efficiency can be systematically evaluated;

2. This helps identify the structural “window” most favorable for forming the ternary complex between the target protein and the E3 ligase.

3. Accelerating structural optimization with bifunctional PEGs

1. Various heterobifunctional PEGs (i.e., bearing different functional groups at the two termini, such as distinct N- and C-terminal functionalities) enable modular assembly, where:

(1) one end is conjugated to the target protein ligand;

(2) the other end is conjugated to the E3 ligase ligand;

2. This greatly accelerates the synthesis and screening of PROTAC molecules.

Design considerations:

1. Parameters such as molecular weight, topology, flexibility/rigidity, polarity, and LogP need to be considered comprehensively;

2. While improving solubility and synthetic accessibility, one should avoid excessive polarity that leads to insufficient intracellular exposure.

2. Use of PEG Derivatives in ADC Linkers

Antibody–drug conjugates (ADCs) are composed of three elements:

1. A monoclonal antibody (mAb);

2. A cytotoxic small-molecule payload;

3. A linker connecting the two.

The linker is one of the critical determinants of ADC success. It not only affects the stability of the conjugate and the mode of payload release, but also directly influences pharmacokinetics, efficacy, and safety.

In ADC development, incorporating PEG segments as linkers or spacers has become very common:

1. Tuning physicochemical properties

(a) PEG segments can balance the extreme hydrophobicity of certain payloads, reducing ADC self-aggregation and nonspecific binding;

(b) They improve overall aqueous solubility and formulation stability;

(c) They help achieve higher drug-to-antibody ratios (DAR) without markedly worsening PK or safety.

2. Improving pharmacodynamic characteristics

(a) The length of the PEG linker, the presence of degradable motifs, and the connection mode all influence the efficiency and site of payload release;

(b) For example, release under intracellular reducing conditions or at specific enzymatic cleavage sites can ensure that the toxin acts predominantly within tumor cells, thereby enhancing efficacy and reducing systemic toxicity.

3. Optimizing pharmacokinetic (PK) properties

(a) By tuning overall molecular size and hydrophilicity via PEG, ADC clearance, half-life, and tissue distribution can be modulated;

(b) Rational design can increase exposure in tumor tissue and reduce accumulation in non-target tissues.

In addition, multi-branched PEG derivatives can be used to construct multi-drug-loaded ADCs:

1. On one hand, they increase the drug loading capacity;

2. On the other hand, they allow different types of payloads to be borne on the same antibody, enabling the development of next-generation multi-payload ADCs.

Key design trade-offs:

1. PEG chains that are too short may be insufficient to resolve hydrophobicity and aggregation issues;

2. PEG chains that are too long may render the molecule overly hydrophilic and introduce excessive steric hindrance, impairing antibody internalization or tumor penetration;

3. Therefore, the optimal linker design must be determined through systematic structure–property–in vivo behavior studies tailored to the specific antibody and payload.

3. Applications of PEG Derivatives in Small-Molecule Drug Design

PEGylation has also been widely investigated for small-molecule drugs, especially in the prodrug design of anticancer agents such as taxanes and camptothecin derivatives.

Small-molecule drugs often face the following challenges:

1. Relatively low molecular weight, leading to rapid renal filtration and clearance;

2. Strong hydrophobicity and poor water solubility, requiring excipients or organic solvents for dissolution;

3. Narrow therapeutic windows and substantial toxicity.

PEGylation can help address these issues in several ways:

1. Increasing solubility

(a) Covalently attaching PEG to hydrophobic small molecules via cleavable linkers can markedly increase their aqueous solubility;

(b) This facilitates formulation as injectable or infusion products and improves convenience of administration.

2. Reducing toxicity and local irritation

(a) PEGylated drugs are often in a prodrug form, with the active small molecule released slowly under specific conditions (e.g., ester hydrolysis, amidase action, reducing environments);

(b) By controlling the release rate, peak plasma concentration (C_max) can be reduced, thereby attenuating acute toxicity and local irritation.

3. Extending half-life and prolonging exposure

(a) PEG increases molecular weight and slows renal clearance, acting as a “long-acting reservoir”;

(b) The drug is released gradually over a certain period, allowing the lesion site to maintain effective drug concentrations throughout the dosing interval and thus reducing dosing frequency.

4. Achieving sustained release and targeted delivery

(a) By designing linkers that are sensitive to pH, enzymes, or redox conditions, environment-responsive release can be realized;

(b) For nanoparticle–PEG–drug systems, tumor targeting can be further enhanced by exploiting the enhanced permeability and retention (EPR) effect or by ligand modification.

Core design element:

The site of PEG attachment and the type of cleavable linkage (e.g., ester, acetal, peptide bond, disulfide bond) determine the release rate and release site. These are therefore the key parameters in the design of PEGylated small-molecule prodrugs.

Applications of PEG Derivatives in Medical Devices and Tissue Engineering

1. Characteristics of PEG Hydrogels

PEG is one of the best-known synthetic polymers with favorable biocompatibility. PEG-based hydrogels have the following features:

1. High water content and softness, with mechanical properties close to those of soft tissues;

2. Tunable degradation rate and mechanical strength by adjusting molecular weight and crosslinking density;

3. Markedly reduced nonspecific adsorption of proteins and cells, exhibiting characteristic “low protein adsorption” and “low cell adhesion” behavior.

Through appropriate crosslinking strategies and formulation design, medical-grade PEG and its derivatives can be processed into:

1. Surgical sealants and hemostatic materials;

2. Anti-adhesion barrier materials;

3. Tissue fillers and spacers for radiotherapy;

4. Components of implantable medical devices, partially replacing plant-derived, animal-derived, or human-derived materials.

2. Applications in Surgical Sealing and Hemostasis

In ophthalmic, neurosurgical, spinal, cardiothoracic, and abdominal procedures, injectable hydrogels based primarily on PEG derivatives have already been used in a variety of surgical settings, and several related products have received regulatory approval.

A typical mode of application is as follows:

1. Two components (e.g., multi-arm PEG–NHS and an amine-containing crosslinking component) are delivered simultaneously onto the wound surface via a spray or mixing device;

2. Rapid crosslinking occurs in vivo under mild physiological conditions, forming a dense hydrogel layer;

3. At the wound site, the hydrogel:

(a) provides mechanical sealing and hemostasis;

(b) prevents leakage of body fluids (e.g., cerebrospinal fluid, bile, intestinal fluid, etc.);

(c) reduces the risk of postoperative adhesions;

4. As tissue healing progresses, the hydrogel gradually degrades, and its degradation products together with residual PEG are eliminated via body fluids.

Because PEG and its degradation products generally exhibit good tissue compatibility, these products tend to be well tolerated in clinical use. However, it should still be emphasized that:

1. Different formulations may vary in terms of safety profile and degradation behavior;

2. Local inflammation, immune reactions, and other adverse events may still occur, and use should strictly follow the instructions for use and relevant clinical guidelines.

3. Use as Tissue Spacers in Radiotherapy

In tumor radiotherapy, PEG hydrogels have been introduced as tissue spacers to reduce radiation exposure to surrounding healthy tissues while maintaining the prescribed dose to the tumor.

A typical example is in prostate cancer radiotherapy, where an injectable PEG hydrogel is placed between the prostate and rectum:

1. The hydrogel forms a stable local space, physically displacing the rectum away from the high-dose region;

2. It maintains its shape throughout the course of radiotherapy, significantly reducing the radiation dose to the rectum;

3. After completion of radiotherapy, the hydrogel gradually degrades and is cleared.

Similar tissue-spacing strategies are being explored in other anatomical regions as well, providing new tools to improve dose conformity and safety in radiotherapy.

Summary and Outlook

Over the past decades, polyethylene glycol and its derivatives have evolved from “simple hydrophilic polymers” into an extremely important class of tool molecules in drug development and medical device design:

1. At the drug-molecule level, PEGylation has been successfully applied to multiple protein therapeutics, small-molecule prodrugs, and nanoscale delivery systems, achieving improved solubility, long-acting delivery, and reduced toxicity;

2. At the structural design level, end-group functionalization (NHS, MAL, ALD, SH, azide/DBCO, etc.) and control of polymer architecture (linear, multi-arm, block copolymer) provide rich design space for PROTACs, ADCs, long-acting small molecules, and injectable hydrogels;

3. In the medical device field, PEG-based hydrogels are now widely used for surgical sealing, hemostasis, anti-adhesion barriers, and tissue spacing in radiotherapy, with indications continuing to expand.

At the same time, as clinical use increases, immunological issues such as anti-PEG antibodies and accelerated blood clearance (ABC) are drawing growing attention. Future research is expected to achieve further advances in the following areas:

1. More precisely defined monodisperse PEGs and sophisticated structural designs;

2. Material systems in which PEG works synergistically with, or is partially replaced by, other hydrophilic polymers (such as polysaccharides and poly(amino acids));

3. More systematic immunosafety assessments and in-depth studies on the relationship between structure and immune response.

Aladdin: https://www.aladdinsci.com/