Peptide therapeutics are expanding quickly worldwide, with over 100 approved drugs and 200 more in clinical trials for diseases such as diabetes, cancer, Alzheimer's disease, and rare disorders. Their high potency, strong target specificity, and favorable safety profiles make them an important drug class.

Despite their therapeutic promise, peptides administered without a carrier system face major limitations in vivo, particularly poor stability and limited duration of action. Nearly 50% of peptide molecules degrade within an hour in the digestive tract, and roughly 25% of development projects fail due to formulation challenges.

To solve this, long-acting peptide technologies have been widely used. Advanced strategies such as PEGylation and lipidization have successfully shifted dosing schedules from daily injections to weekly or monthly dosing, greatly improving patient compliance. Currently, over 55% of new peptide formulations utilize these stabilization techniques to ensure long-lasting therapeutic impact.
 

| PEGylation of peptides

Polyethylene glycol (PEG) is a hydrophilic and biocompatible polymer widely used in pharmaceutical and biomedical applications. It is valued for its high water solubility, low toxicity, low immunogenicity, and excellent safety profile. Structurally, PEG consists of repeating ethylene oxide units [-(O-CH2-CH2)n-], and its terminal hydroxyl groups can be chemically modified with various functional ligands, making PEG highly versatile for drug conjugation and delivery systems.

PEGylation is the process of covalently attaching PEG chains to therapeutic drug molecules. Since the first PEGylated drug (Adagen) was approved by the FDA in 1990, more than 40 PEGylated therapeutics have reached the market.
 

| Key Benefits of PEGylation for Peptides

Enhanced Solubility and Stability: PEGylation significantly improves the water solubility of hydrophobic peptides. By surrounding the peptide with a hydration shell, PEG linkers mask greasy regions and create steric hindrance. This prevents peptide molecules from sticking together (aggregation), ensuring the drug remains stable and easy to formulate for clinical use.

Extended Circulation and Half-Life: Small peptides are typically cleared by the kidneys within minutes. PEGylation increases the molecule's effective size (hydrodynamic radius), making it too large for rapid renal filtration. Additionally, the PEG chain acts as a physical shield that protects the peptide from being broken down by digestive enzymes, extending its circulation time from hours to days.

Reduced Immunogenicity: PEG shields antigenic epitopes on the peptide surface, reducing recognition by the immune system. This reduces the risk of patients developing neutralizing antibodies or allergic reactions, which is essential for the safety and efficacy of long-term treatments.

Improved Patient Compliance: By making drugs longer-lasting and more stable, PEGylation allows for a move from daily injections to weekly or even monthly dosing. This significantly reduces the treatment burden on patients, leading to better compliance and more consistent therapeutic results. Currently, over 55% of new peptide formulations utilize these stabilization techniques to improve the patient experience.

Several PEG-modified peptide drugs have already been approved for clinical use.

Yuviwel (navepegritide): TransCon CNP (navepegritide), is a pegylated long-acting CNP prodrug consisting of CNP-38 (the 38 C-terminal amino acids of human CNP-53) conjugated to a multi-arm, branched 40 kDa  PEG moiety via a cleavable linker, enabling sustained release of CNP-38. In February 2026, the FDA has approved Yuviwel (navepegritide) for injection to improve growth in pediatric patients aged two years and older with achondroplasia with open epiphyses (growth plates).

Figure 1. Structure of Yuviwel (navepegritide)

Yorvipath (Palopegteriparatide): YORVIPATH (palopegteriparatide), developed by Ascendis Pharma, was granted FDA approval on August 9, 2024, for the treatment of adult hypoparathyroidism (HP). It is a prodrug of teriparatide (PTH(1-34)) consisting of PTH(1-34) transiently conjugated to a branched 40 kDa (2×20 kDa) methoxypolyethylene glycol (mPEG) carrier via a proprietary TransCon Linker.

Figure 2. Structure of Yorvipath (Palopegteriparatide)

Syfovre (Pegcetacoplan): On February 17, 2023, the FDA announced approval Syfovre (pegcetacoplan, Apellis Pharmaceuticals) for the treatment of geographic atrophy (GA) secondary to age-related macular degeneration (AMD). It is a complement inhibitor comprising two identical pentadecapeptides covalently bound via its C terminus to the ends of a linear 40 kD PEG linker.

 
Figure 3. The structure of Syfovre (Pegcetacoplan)

SKYTROFA® (Lonapegsomatropin): In 2021, the FDA approved Skytrofa, a prodrug of somatropin (human growth hormone, or hGH) administered once weekly, for the treatment of pediatric GHD. Skytrofa consists of somatropin transiently conjugated to a methoxypolyethylene glycol carrier. (mPEG) via a proprietary TransCon Linker. 

Empaveli (Pegcetacoplan): In May 2021, the FDA approved Empaveli (pegcetacoplan), a complement C3 inhibitor that is the same ingredient in Syfovre, for adults with paroxysmal nocturnal hemoglobinuria (PNH). 

Omontys (Peginesatide): Peginesatide is a synthetic, pegylated dimeric peptide comprised of two identical 21-amino acid chains covalently bonded to a single lysine-branched bis-mPEG chain (40,000 Da). It was approved in 2012 by FDA for the treatment of symptomatic anaemia associated with chronic kidney disease (CKD).

Fulaimei (PEG loxenatide): PEG loxenatide is a once-weekly GLP-1RA formulation approved by China in May 2019. It is obtained by modifying the chemical structure of exendin-4 on the 2nd (glycine alanine), 14th (methionine norleucine), 28th (asparagine glutamine), and 39th (serine cysteine) N-terminal positions, and linking them to a Y-shaped PEG molecule (mPEG2-MAL). The half-life of PEG loxenatide is 104~121 h.

| Main PEGylation Strategies for Peptides

PEGylation can be divided into two main types: site-specific PEGylation and non-site-specific PEGylation. In early development, non-site-specific PEGylation was often used because it is easier to prepare. However, site-specific PEGylation has become more popular in recent years because it gives better control over the final product, helps keep the peptide activity, and improves product consistency.

Here are the primary strategies used to achieve this precision:

1. N-Terminal Modification

N-terminal PEGylation is one of the most widely used site-specific strategies. By controlling the reaction conditions, especially pH, PEG can preferentially attach to the N-terminus of the peptide rather than to lysine side chains. This approach is relatively straightforward and can help preserve the peptide’s biological activity while improving half-life and stability.

2. Cysteine PEGylation

Cysteine-based PEGylation uses the thiol group of cysteine residues as the attachment site. Because cysteine is relatively rare in natural peptide sequences, pairing it with maleimide-functionalized reagents allows for exceptionally efficient reactions and highly defined, homogeneous conjugates. In some cases, cysteine residues may also be introduced through peptide design or protein engineering to create a defined PEGylation site.

3. Lysine PEGylation

Lysine is common in most peptides and proteins, making it a highly accessible site. While lysine modification was traditionally random, modern synthetic techniques—such as using protection/deprotection (such as Fmoc) steps during solid-phase synthesis—now allow us to target a specific lysine residue with surgical precision, preventing multiple attachments.

4. Carboxyl group PEGylation

When the amino or thiol groups are essential for the peptide's activity, carboxyl groups (found on Aspartic acid, Glutamic acid, or the C-terminus) provide an alternative. While less common than amine-targeting, carboxyl-specific PEGylation is a vital tool for specific designs where other reactive sites must remain untouched.

5. Click chemistry and enzyme-based PEGylation

These represent the frontier of peptide engineering:

Click Chemistry: By incorporating unnatural amino acids (UAAs) that only react with a complementary PEG linker (azide, alkyne, DBCO etc.), developers can achieve Lego-like docking precision. These reactions are fast, high-yielding, and occur with minimal side reactions.

Enzyme-Mediated PEGylation: Utilizing catalysts such as transglutaminase (TGase) provides biological-grade specificity under exceptionally mild reaction conditions, ensuring the structural integrity of the peptide remains completely untouched.

| Key Design Factors in PEGylated Peptides

In peptide and protein PEGylation, the physicochemical properties of PEG modifiers themselves can significantly influence the structural characteristics, homogeneity, biological activity, and pharmacokinetic behavior of the final conjugates. In addition to PEGylation sites and conjugation strategies, factors such as PEG molecular weight, chain architecture, polydispersity, and terminal group design all play important roles in determining the performance of PEGylated therapeutics.

PEG Molecular Weight (MW)

PEG molecular weight is one of the most important parameters affecting the half-life and biological activity of PEGylated drugs. In general, higher molecular weight PEG can increase the hydrodynamic size of the conjugate, thereby reducing renal clearance and prolonging systemic circulation time. However, increasing PEG size may also introduce stronger steric hindrance, which can interfere with receptor binding and reduce biological activity in some systems.

In addition, PEGs with different molecular weights may exert distinct effects on drug conformational stability and thermal stability, depending on the structure of the peptide or protein and the conjugation approach used.

PEG Chain Structure and Architecture

The geometric configuration of the PEG polymer—primarily categorized into linear and branched/multi-arm architectures—directly dictates its surface-shielding efficiency and spatial orientation.

Compared with linear PEG, branched PEG generally provides stronger steric shielding and can more effectively reduce proteolytic degradation and immune recognition, thereby improving in vivo stability.

In some studies, multi-site conjugation with several shorter PEG chains has demonstrated broader surface coverage and improved pharmacokinetic performance compared with single-site conjugation using one larger PEG chain of similar total molecular weight.

PEG Polydispersity (PDI)

As a polymeric material, conventional (polydisperse) PEG typically exhibits a certain degree of molecular weight distribution. Broad polydispersity can increase the complexity of purification, characterization, and batch-to-batch consistency control for PEGylated products.

In contrast, PEGs with narrower molecular weight distributions or monodisperse PEGs are more favorable for producing structurally defined and highly homogeneous conjugates, facilitating downstream analysis and quality control.

Related Articles: Monodisperse (Discrete) PEGylation vs. Polydisperse PEGylation: Future Trends

PEG Terminal Groups and Linker Design

The terminal groups and linker structures of PEG can affect interactions between PEG chains and peptide or protein surfaces, thereby influencing the spatial conformation of the final conjugate. Differences in hydrophobicity, flexibility, and rigidity of PEG terminal groups or linkers may further alter the shielding efficiency toward antigenic epitopes and proteolytic cleavage sites.

In some studies, more hydrophobic or rigid linker structures promoted the formation of more compact PEG conformations, contributing to improved stability, reduced immunogenicity, and prolonged circulation time.

| Conclusion and Outlook

As a key strategy for peptide drug optimization, PEGylation has demonstrated significant value in improving the pharmacokinetic and therapeutic profiles of peptide therapeutics.

However, PEGylation is not without limitations. In some cases, PEG conjugation may reduce the pharmacological activity of a peptide or protein by masking key binding sites or introducing steric hindrance. It may also bring safety concerns, including injection-site reactions such as lipodystrophy and cytoplasmic vacuolization.

In addition, while PEGylation was originally designed to lower immunogenicity, especially for non-human proteins, it is now clear that PEG itself can sometimes trigger immune responses. Anti-PEG antibodies may accelerate plasma clearance of PEGylated drugs, and in some cases, PEG may also activate the complement system.

Because the window between optimal therapeutic efficacy and these biological limitations is narrow, selecting the appropriate polymer architecture is absolutely critical. Successful peptide PEGylation begins with a rational, structurally guided modification strategy where the choice of PEG reagent directly dictates both conjugation efficiency and downstream therapeutic performance.

Biopharma PEG provides a comprehensive portfolio of high-purity, structurally defined PEG reagents tailored to modern peptide engineering requirements, including linear PEG, branched PEG, and multi-arm PEG. Also, we can also provide PEG linkers with a variety of functional groups, such as NHS-activated PEG for lysine or N-terminal conjugation, maleimide PEG for cysteine-selective PEGylation, aldehyde PEG and hydrazide PEG for site-specific N-terminal modification, and click-chemistry-compatible PEG reagents such as azide- or alkyne-functionalized PEGs.
 

References:
[1] Jang, W., & Bong, K. W. Strategies for Loading and Releasing Peptide Therapeutics in Biodegradable Carriers. Advanced Functional Materials, e31987. https://doi.org/10.1002/adfm.202531987 
[2] Gao, Y., Joshi, M., Zhao, Z., & Mitragotri, S. (2023). PEGylated therapeutics in the clinic. Bioengineering & Translational Medicine, 9(1), e10600. https://doi.org/10.1002/btm2.10600
[3] Li, C., Li, T., Tian, X., An, W., Wang, Z., Han, B., Tao, H., Wang, J., & Wang, X. (2024). Research progress on the PEGylation of therapeutic proteins and peptides (TPPs). Frontiers in Pharmacology, 15, 1353626. https://doi.org/10.3389/fphar.2024.1353626 
[4] Ma, M., Di, J., Wang, C., Xie, Y., Cui, F., Zhai, Y., Zhu, S., & Gao, J. (2025). Site-specific PEGylation of proteins: Insights into structural and functional changes. Acta Pharmaceutica Sinica. B, 15(12), 6253. https://doi.org/10.1016/j.apsb.2025.10.014