Polymers for Drug Conjugation: From PEGylation to Next-Generation Polymer Therapeutics

In polymer–drug conjugates, the polymer is not only a passive carrier. Its chemistry, molecular weight, architecture, degradability, and functional groups directly influence key parameters circulation time, biodistribution, drug loading, release kinetics, manufacturability, and long-term safety.

As polymer therapeutics keep evolving, the choice of backbone has become one of the most critical decisions in drug conjugation. PEG remains the most established polymer in this field, but growing interest in repeat dosing, biodegradability, reduced immunogenicity, and improved molecular control is accelerating the development of next‑generation alternatives.

This article explores the main polymer backbones used in drug conjugation, including PEG, polysarcosine (PSar), poly(glutamic acid) (PGA), PLGA, HPMA copolymers, and poly(2‑oxazoline)s (POx). It also introduces PSarylation as an emerging alternative to PEGylation for biologics, nanoparticles, and advanced polymer–drug conjugate systems.

The role of polymer selection in the drug conjugation strategy

The performance of a polymer–drug conjugate depends not only on the API, but also on the polymer backbone itself. Polymer composition, molecular weight, architecture, hydrophilicity, degradability, and functionalization capacity directly influence how the conjugate behaves in real‑world conditions.

In modern polymer therapeutics, the polymer backbone influences several critical properties simultaneously:

• Circulation time and biodistribution: Hydrophilic polymers with strong stealth behavior can reduce opsonization and prolong systemic circulation, increasing exposure to target tissues.

• Drug loading and conjugation flexibility: The number and accessibility of reactive groups determine how efficiently APIs, targeting ligands, or imaging agents can be incorporated.

• Release kinetics and intracellular activation: Polymer hydration, conformation, and degradability influence linker exposure and therefore impact how and where the payload is released.

• Biodegradability and long‑term safety: Non‑degradable systems may accumulate after repeated dosing, while biodegradable backbones can improve clearance and reduce long‑term toxicity concerns.

• Manufacturability and scalability: Polymer dispersity, reproducibility, purification complexity, and GMP compatibility are increasingly important for clinical translation.

Historically, PEG has been the gold standard in the field by providing the perfect balance of stealth behavior, manufacturability, and clinical familiarity. However, the evolution of polymer therapeutics is driving demand for more sophisticated systems that combine prolonged circulation with biodegradability, controlled architecture, reduced immunogenicity, and improved functional versatility.

As a result, modern drug conjugation platforms need a broader toolbox of polymer backbones, including polypeptides, polypeptoids, biodegradable polyesters, and precision copolymers engineered for specific therapeutic applications.

The Main Polymers Used in Drug Conjugation

Different polymer backbones have been explored for polymer–drug conjugates (PDCs), each offering distinct advantages and associated challenges. While PEG remains the most established polymer in clinically validated systems, newer materials such as polysarcosine (PSar), poly(glutamic acid) (PGA), PLGA, HPMA copolymers, and poly(2‑oxazoline)s (POx) are expanding the design possibilities for next‑generation therapeutics. Rather than a single universal solution, modern drug conjugation increasingly relies on selecting the polymer backbone according to the biological, pharmacokinetic, and manufacturing requirements of each therapeutic modality.

PEG: The Historical Gold Standard for Drug Conjugation

Polyethylene glycol (PEG) has been the dominant polymer in drug conjugation and polymer therapeutics for more than three decades. Its strong hydrophilicity, flexible structure, and stealth behavior made PEG the reference material for improving circulation time, reducing protein adsorption, and increasing the solubility of therapeutic payloads.

PEGylation has been widely applied across multiple therapeutic classes, including:

• biologics and proteins,
• peptide therapeutics,
• antibody fragments,
• small-molecule cytotoxics,
• and more recently, nucleic acid delivery systems.

One of PEG’s main advantages is its manufacturing maturity. PEG is available in multiple architectures and molecular weights, enabling scalable GMP production and relatively straightforward conjugation chemistry. Its extensive clinical history has also contributed to broad regulatory familiarity.

However, despite its success, several limitations have become increasingly important in modern therapeutic development. Growing evidence surrounding anti‑PEG antibodies, accelerated blood clearance (ABC), potential hypersensitivity reactions, and the non‑biodegradable nature of PEG has accelerated interest in alternative stealth polymers with improved long‑term safety and repeat‑dosing potential.

These challenges are driving the development of next‑generation polymer systems designed to preserve PEG’s favorable pharmacokinetics while improving biodegradability, immunological profile, and molecular precision.

PSar and PSarylation: A New Generation of Stealth Polymer Therapeutics

Polysarcosine (PSar) is emerging as one of the most promising next‑generation polymers for drug conjugation, biologics, and nanoparticle systems. As a polypeptoid based on N‑methylated glycine units, PSar combines PEG‑like stealth behavior with improved chemical stability, low immunogenicity, and potential biodegradability.

Like PEG, PSar forms a highly hydrated shell that reduces protein adsorption and macrophage uptake, helping prolong systemic circulation and improve biodistribution. However, unlike PEG, PSar has shown minimal immune recognition and reduced risk of accelerated blood clearance (ABC) in multiple preclinical studies, making it particularly attractive for repeat‑dosing applications.

These properties are driving growing interest in PSarylation as a next‑generation alternative to PEGylation for advanced therapeutics.

PSarylation refers to the conjugation or surface modification of therapeutic systems using polysarcosine‑based materials in order to improve:

• circulation time
• colloidal stability
• solubility
• stealth behavior
• overall biological compatibility

This strategy is increasingly explored across:

• polymer–drug conjugates
• biologics
• antibody‑based systems
• nanoparticles
• lipid nanoparticles (LNPs)
• multifunctional theranostic platforms

Another important advantage of PSar is its high degree of architectural control. Controlled polymerization techniques enable the synthesis of well‑defined molecular weights and functional end groups, supporting precise conjugation strategies and reproducible manufacturing workflows.

Compared with PEG, PSar also demonstrates:

• oxidative stability
• excellent aqueous solubility
• ultralow protein adsorption
• enhanced compatibility with modern multifunctional conjugate designs

As the field moves toward more programmable and repeat‑dose‑compatible therapeutics, PSar is increasingly positioned not simply as a PEG substitute, but as a broader platform for next‑generation stealth polymer engineering.

At Curapath, this evolution aligns closely with ongoing efforts in advanced polymer development, including high‑molecular‑weight polysarcosine materials designed for drug conjugation, stealth coatings, and next‑generation delivery systems.

PGA: Biodegradable High-Loading Polymer Systems

Poly(glutamic acid) (PGA) is one of the most established biodegradable polymers used in polymer–drug conjugates, particularly in oncology applications where high drug loading and controlled release are critical.

As a polypeptide composed of glutamic acid units, PGA offers a dense distribution of carboxylic acid groups that enable efficient conjugation of therapeutic payloads, targeting ligands, and imaging agents. This multifunctional chemistry makes PGA highly attractive for advanced conjugate architectures and theranostic systems.

One of PGA’s key advantages is its biodegradability. Unlike non‑degradable stealth polymers, PGA can be enzymatically degraded into glutamic acid, a naturally occurring metabolite, supporting improved biological clearance and long‑term tolerability.

PGA has been widely explored for the conjugation of:

• hydrophobic chemotherapeutics
• camptothecin derivatives
• paclitaxel
• doxorubicin analogues
• peptides
• imaging probes

Its ability to maintain aqueous solubility even with highly hydrophobic payloads makes it especially valuable for cytotoxic drug delivery.

Another important feature of PGA systems is their compatibility with stimuli‑responsive linkers. PGA conjugates are frequently engineered using:

• enzyme‑cleavable peptide linkers
• hydrolytically degradable ester bonds
• pH‑sensitive chemistries designed to enhance intracellular release within tumor microenvironments

Beyond classical drug conjugation, PGA is increasingly explored in multifunctional nanomedicine platforms that combine therapy, targeting, and imaging within a single polymer system.

Compared with PEG‑like stealth polymers, PGA provides lower stealth behavior but significantly improved biodegradability and drug‑loading flexibility, making it particularly attractive for applications where controlled degradation and high payload density are priorities.

As interest grows in safer and more programmable polymer therapeutics, biodegradable polypeptide backbones such as PGA are becoming increasingly important components of next‑generation conjugation strategies.

PLGA: Clinically Validated Controlled Release Platforms

Poly(lactic‑co‑glycolic acid) (PLGA) is one of the most clinically validated biodegradable polymers in drug delivery and controlled release applications. With multiple FDA‑approved products and decades of translational use, PLGA has become a benchmark material for long‑acting formulations, implants, nanoparticles, and advanced therapeutic systems.

Unlike stealth‑oriented polymers such as PEG or PSar, PLGA is primarily valued for its tunable degradation behavior and its ability to provide sustained release over extended periods.

PLGA is composed of lactic acid and glycolic acid monomers linked through hydrolytically degradable ester bonds. By adjusting parameters such as:

• the lactic acid:glycolic acid ratio
• molecular weight
• end‑group chemistry
• polymer architecture

developers can precisely modulate degradation kinetics, release profiles, and overall formulation behavior.

This flexibility has made PLGA highly attractive across:

• oncology
• long‑acting injectables
• local drug delivery
• peptide therapeutics
• nanoparticle systems
• hybrid conjugate platforms

PLGA systems are frequently used with:

• small‑molecule chemotherapeutics
• peptides and proteins
• imaging agents
• targeting ligands
• multifunctional nanocarriers

Although PLGA has historically been associated with encapsulation‑based delivery systems, interest in PLGA‑containing conjugate architectures continues to grow. Modern approaches increasingly combine covalent conjugation strategies with erosion‑controlled release, enabling more sophisticated pharmacokinetic control and stimuli‑responsive behavior.

PLGA also supports a broad range of linker and surface‑engineering strategies, including:

• hydrolytically cleavable ester bonds
• pH‑sensitive linkers
• redox‑responsive chemistries
• PEG or ligand functionalization for targeting applications

One of the major advantages of PLGA is its strong regulatory familiarity and manufacturing maturity, which simplifies translational development compared with many emerging polymer systems.

However, PLGA also presents several formulation challenges. Its degradation process can generate localized acidic microenvironments that may destabilize sensitive payloads, while burst release effects and complex erosion kinetics require careful formulation optimization.

Despite these limitations, PLGA remains one of the most important biodegradable polymers in modern drug delivery and continues to play a central role in next‑generation controlled release and hybrid therapeutic platforms.

HPMA and POx: Precision and Multifunctional Polymer Platforms

Beyond PEG, PSar, PGA, and PLGA, several advanced polymer systems are expanding the design space of modern polymer therapeutics.

HPMA copolymers remain one of the benchmark platforms for multifunctional polymer–drug conjugates due to their highly modular architecture and exceptional linker engineering flexibility. Their copolymer structure enables precise incorporation of:

• therapeutic payloads
• targeting ligands
• imaging agents
• stimuli‑responsive release chemistries

This versatility has made HPMA particularly relevant in oncology and theranostic applications where controlled activation and multifunctionality are required.

Poly(2‑oxazoline)s (POx) have also emerged as highly promising next‑generation polymer systems. POx materials combine PEG‑like stealth behavior with precise architectural tunability, supporting the development of:

• high‑drug‑loading micelles
• advanced nanocarriers
• programmable polymer therapeutics

Compared with more traditional polymers, POx platforms offer strong flexibility in molecular design, hydrophilicity control, and functionalization strategies.

Together, HPMA and POx reflect a broader trend in drug conjugation toward increasingly programmable and precision‑engineered polymer systems tailored for specific biological and therapeutic requirements.

Comparing Polymer Backbones for Modern Drug Conjugation

Different polymer backbones offer distinct advantages depending on the therapeutic modality, release strategy, dosing regimen, and manufacturing requirements. While no single polymer is universally optimal, understanding the trade‑offs between stealth behavior, degradability, linker compatibility, and scalability is essential for designing effective polymer therapeutics.

Among these systems, PEG and PSar currently dominate the discussion around stealth polymer engineering, while biodegradable platforms such as PGA and PLGA continue to play central roles in controlled release and oncology‑focused conjugates.

The Future of Polymer Therapeutics

Polymer–drug conjugates are rapidly evolving from relatively simple carrier systems into highly engineered therapeutic platforms where the polymer backbone, linker chemistry, architecture, and degradation profile are designed together to achieve precise biological performance.

As the field advances, several major trends are reshaping the next generation of polymer therapeutics:

• increasing demand for repeat‑dose‑compatible systems
• biodegradable stealth polymers
• precision‑controlled architectures
• multifunctional conjugates
• scalable GMP‑compatible manufacturing strategies

In this context, polymer selection is becoming a central design parameter rather than a secondary formulation decision. Different therapeutic modalities increasingly require different polymer behaviors, whether the goal is prolonged circulation, high drug loading, controlled intracellular release, improved tolerability, or advanced targeting capabilities.

While PEG continues to play a major role in clinically established systems, next‑generation materials such as polysarcosine (PSar), biodegradable polypeptides, and precision copolymers are expanding the possibilities for safer, more programmable, and biologically optimized therapeutics.

The growing interest in PSarylation further reflects this evolution, particularly in applications involving biologics, nanoparticles, lipid nanoparticles (LNPs), and advanced polymer–drug conjugates where stealth behavior, repeat dosing, and molecular precision are becoming increasingly important.

At Curapath, these advances align closely with ongoing efforts in advanced polymer development, drug conjugation technologies, and next‑generation delivery systems designed to support the evolving needs of modern therapeutics.