Introduction: Limitations of PEGylation in modern biologics
Polyethylene glycol (PEG) has long been the polymer of choice for improving the solubility, stability, and pharmacokinetics of biologics. PEGylation is widely used across peptides, proteins, and antibody-drug conjugates (ADCs) to reduce aggregation and prolong systemic circulation by minimizing nonspecific interactions and rapid clearance.
As the clinical use of PEGylated therapeutics has expanded, however, several limitations have become increasingly evident. Most notably, the presence of anti-PEG antibodies, which was observed in both treatment-naïve and previously exposed patients, can lead to accelerated blood clearance, reduced efficacy upon repeat dosing, and potential hypersensitivity reactions. In addition, PEG is non-biodegradable, raising concerns regarding long-term accumulation in chronic treatment settings.
These challenges have prompted growing interest in alternative hydrophilic polymers that preserve PEG’s favorable “stealth” properties while offering improved immunological profiles. Polysarcosine (pSar) has emerged as a leading candidate in this space.
Figure 1. Conceptual comparison of PEGylation and polysarcosine (pSar) as stealth coatings for biologics.
What is pSar?
PSar is a synthetic polypeptoid composed of repeating N-methylglycine units. It features a peptide-like backbone in which the amide nitrogen is methylated, eliminating hydrogen-bond donors along the polymer chain. These structural characteristics result in a highly hydrated and flexible polymer with minimal nonspecific protein interactions, granting pSar exceptional “stealth” properties in biological environments.
From a material standpoint, pSar offers high water solubility, excellent biocompatibility, and tunable chain length through controlled polymerization. These properties make it well suited for bioconjugation applications where precise control of hydrophilicity and molecular size is required.
Figure 2. Structural comparison between PEG and pSar repeat units.
Why pSar works in bioconjugation
PSar performs effectively in bioconjugation due to its strong hydration and low nonspecific interaction profile. The polymer forms a highly hydrated shell that helps mitigate aggregation and maintain solubility, particularly in conjugates with increased loading or structural complexity.
Importantly, pSar enables improved control over overall hydrophobicity, allowing higher conjugation density without compromising stability. This property is especially valuable in antibody- and peptide-based conjugates, where small changes in linker composition can strongly influence developability.
From a translational perspective, pSar has demonstrated a low tendency towards immune recognition in reported studies, supporting its use in settings that require repeated dosing. In addition, its tunable chain length and readily functionalized termini allow seamless integration into established bioconjugation workflows.
Figure 3. Mechanistic illustration of the pSar shielding in bioconjugation
Case Studies Highlighting pSar as a PEG alternative
Case 1. pSar-enabled High-Drug-Load ADCs
A 2019 Chemical Science study reported the use of monodisperse pSar as a hydrophilicity-masking element to address the developability limitations typically observed in high-drug loaded ADCs. The author designed a β-glucuronidase-responsive, self-immolative linker platform carrying MMAE and coupled it to trastuzumab to generate homogeneous DAR8 ADCs, a loading level that often leads to increased apparent hydrophobicity, aggregation risk, and accelerated clearance.
A key design insight from this work is that the architecture and placement of pSar matter as much as its presence. When pSar was positioned in an orthogonal orientation relative to the payload, the resulting ADC showed a clear reduction in hydrophobicity (as reflected by HIC behavior) compared with a linker lacking the hydrophobicity-masking segment. In contrast, placing pSar in a more linear configuration between the antibody and the payload was reported to be less effective, reinforcing that linker topology can strongly influence the macroscopic properties of the final conjugate.
The study further provided a side-by-side comparison between pSar and PEG at comparable length, suggesting that pSar can offer shielding performance that is at least comparable, or slightly better than PEG. Importantly, pSar-masked DAR8 ADC exhibited restored pharmacokinetic behavior and stronger antitumor efficacy than the non-masked DAR8 control. The pSar chain length (6-24 monomer units) was also explored and an apparent optimal window (around the mid-range) where PK and efficacy were balanced, rather than a “longer is always better” trend was observed. Overall, this research is a strong example supporting pSar as a practical PEG alternative for enabling highly loaded, homogeneous ADCs with improved developability.
Figure 4. pSar-enabled hydrophobicity control in high-DAR ADCs.
Case 2. pSar vs PEG in Therapeutic Protein Conjugation
Hu and coworkers provided a clear head-to-head example showing pSar as a practical alternative to PEG for therapeutic protein conjugation. In this study, human interferon-α2b (IFN) was modified in a site-specific manner (N-terminus) via native chemical ligation to afford a well-defined pSar-IFN conjugate, and a similar prepared PEG-IFN conjugate with comparable hydrodynamic size was used as control. Both pSar and PEG substantially improved IFN’s resistance to protease digestion and prolonged circulation half-life in vivo, indicating that pSar can reproduce PEG’s classic stabilizing and PK benefits. Notably, pSar-IFN retained higher in vitro activity than PEG-IFN, showed enhanced tumor accumulation (with reduced liver exposure) after systemic administration. Most importantly, pSar exhibited a minimal immunogenicity profile, eliciting significantly lower levels of anti-IFN antibodies upon repeated dosing. Consistent with these trends, pSar-IFN achieved stronger tumor growth inhibition than PEG-IFN in an OVCAR3 Xenograft model, supporting pSar as a “stealth” polymer that can match PEG on PK while offering potential advantages in bioactivity retention and immunological profile.
Figure 5. Head-to-Head comparison of PEG-IFN and pSar-IFN conjugates
Case 3. pSar Lipids in PEG-free LNP-mRNA Systems
A 2024 Bioactive Materials study explored pSar lipids as direct substitutes for PEG lipids in clinically relevant lipid nanoparticles (LNP) platforms used for mRNA delivery. The authors engineered ALC-0315 and SM-102 based LNP formulations by replacing the PEG-lipid component with a panel of pSar-lipids designed to provide comparable hydrophilic “stealth” behavior. Across multiple lipid architectures, pSar-containing LNPs maintained similar particle characteristics and, in several formulations, preserved or improved mRNA expression in cell models and in vivo after intramuscular administration. Importantly, the study highlighted that pSar-lipid structure (lipid tail design and pSar chain length) strongly impacts performance, with an apparent trade-off between shielding and cellular delivery as polymer length increases. Safety readouts, including acute cytokine/chemokine responses and liver enzyme levels, were comparable to PEG-LNP controls. Notably, pSar-LNPs showed no detectable binding in anti-PEG antibody assays. Overall, this work supports pSar-lipids as a practical PEG-free design option for LNP-mRNA systems, especially in settings where repeated dosing and pre-existing anti-PEG antibodies as potential concerns.
Figure 6. Comparison of PEG- and pSar-based stealth coronas in lipid nanoparticles
Design Considerations for pSar-Based Conjugates
Collectively, these studies illustrate that pSar can function as a true PEG alternative across diverse conjugation platforms, while its performance remains highly design-dependent. Parameters such as polymer chain length, attachment topology, and placement relative to the payload or carrier strongly influence the balance between shielding, bioactivity and delivery efficiency.
Importantly, pSar does not represent a universal “drop-in replacement” for PEG, but rather a tunable design element that can be leveraged to address specific developability challenges, particularly in systems requiring high loading, preserved activity, or repeated dosing. As such, thoughtful integration of pSar into linker and carrier architectures is critical to fully realize its advantages in translational bioconjugation.
Figure 7. Design considerations for pSar-based conjugates.
Supporting pSar Linker at PrecisePEG
An interest in pSar continues to grow as a PEG alternative, successful implementation relies on precise molecular design and controlled conjugation strategies. At Precise PEG, we support pSar-based bioconjugation through well-defined pSar linkers (https://precisepeg.com/collections/psar-linkers) and customizable linker architectures, enabling partners to translate design concepts into developable conjugates across antibody, protein, and delivery platforms.
Reference
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2. Conilh, L.; Fournet, G.; Fourmaux, E.; Murcia, A.; Matera, E.-L.; Joseph, B.; Dumontet, C.; Viricel, W. Exatecan Antibody Drug Conjugates Based on a Hydrophilic Polysarcosine Drug-Linker Platform. Pharmaceuticals 2021, 14, 247. https://doi.org/10.3390/ph14030247
3. Viricel, W.; Fournet, G.; Beaumel, S.; Perrial, E.; Papot, S.; Dumontet, C.; Joseph, B. Monodisperse polysarcosine-based highly-loaded antibody-drug conjugates. Chem. Sci. 2019, 10, 4048–4053. https://doi.org/10.1039/c9sc00285e
4. Hu, Y.; Hou, Y.; Wang, H.; Lu, H. Polysarcosine as an Alternative to PEG for Therapeutic Protein Conjugation. Bioconjugate Chem. 2018, 29, 2232. https://doi.org/10.1021/acs.bioconjchem.8b00237
5. Kang, D. D.; Hou, X.; Wang, L.; Xue, Y.; Li, H.; Zhong, Y.; Wang, S.; Deng, B.; McComb, D. W.; Dong, Y. Engineering LNPs with polysarcosine lipids for mRNA delivery. Bioact. Mater. 2024, 37, 86. https://doi.org/10.1016/j.bioactmat.2024.03.017