Enhancing cancer immunotherapy through polymer-based antibody conjugation technologies

Single or dual immune checkpoint therapy

Monoclonal immunotherapeutic antibodies have achieved remarkable success in cancer treatment with the wide clinical use of immune checkpoint inhibitors (anti-PD-1/PD-L1 and anti-CTLA-4) and tumor-targeting antibodies (anti-CD20 and anti-HER2). Nevertheless, these therapies face challenges, including limited response rates, suboptimal tumor targeting, and systemic toxicity. Polymer-based antibody conjugation technology offers a versatile strategy to overcome these barriers through multivalent binding and modular assembly, enabling better mono-, bi-, or multi-specific architectures that strengthen immune cell–tumor interactions.

Xue et al. developed a polymer-assembled PD1/PDL1 bispecific antibody achieving 90.1% tumor suppression³. BsAbαPD1/αPDL1-treated mice maintained stable body weight while the free antibody groups showed weight loss, indicating reduced toxicity. BsAbs enhanced tumor accumulation through multivalent binding, activated CD8⁺ T cells more effectively (90% killing rate), and preserved organ function.

To reduce off-target issues, Zhao et al. designed a switchable immunomodulator (Sw-IM) incorporating environment-responsive chemistry⁴. The polymer uses reversible masking through disulfide bonds or acid-labile maleic anhydride linkers, maintaining therapeutic inactivity during circulation. Only upon encountering tumor microenvironment conditions (high glutathione or acidic pH) do linkers degrade, selectively unleashing immune activation at tumor sites. This spatial control preserved anti-tumor efficacy while significantly reducing hepatosplenic toxicity.

Immune checkpoint inhibitors reinvigorate T cells by releasing inhibitory signals, which restores cytotoxic function. Once effective, these therapies induce durable responses and prolong survival. However, clinical benefit remains limited with response rates typically <30%. Dual checkpoint blockades (anti-PD-1 with anti-CTLA-4 or anti-LAG-3) demonstrate superior activity but cause high-grade immune-related adverse events from systemic T cell activation. Polymer-based conjugation provides a solution by enabling multivalent binding, stoichiometric control, and sustained activation, while simplifying bispecific assembly. Liu et al. developed polymeric multivalent Fc-binding peptide (PLG-Fc-III-4C) creating BsAbs by mixing polymer with anti-PD1 and anti-CTLA-4 antibodies in aqueous solution⁵. This innovation achieved the optimal 3 αPD1:1 αCTLA-4 ratio, matching the Opdivo plus Yervoy combination. BsAb_PD1/CTLA-4 achieved 96.8% tumor suppression vs. 77.3% for free combination in MC38 models. The system demonstrated 6.3-fold enhanced tumor accumulation through PD1-mediated targeting, while reducing colitis-related adverse events compared to conventional therapy, addressing a major limitation of dual checkpoint blockade.

CD3 antibody-based T cell engager

CD3 is commonly used as a key target in the design of bispecific T cell engagers (BiTEs), which activate T cell-mediated cytotoxicity by simultaneously binding CD3 on T cells and tumor-associated antigens on cancer cells. Conventional BiTEs are typically low-valency constructs with limited binding avidity to targets and efficient and durable T cell activation. To address these limitations, polymer-mediated trispecific T cell engagers (TiTEs) have been developed. Xue et al. improved this approach by constructing PDL1/CD3ε/4-1BB TsAbs using polymer scaffolds⁶. This system integrates 11.4 antibodies on average, delivering triple immune signals (CD3ε for T cell activation, 4-1BB for co-stimulation, and PDL1 for tumor targeting). The multivalent presentation dramatically stabilizes immune synapses, increasing tumor killing from 45% to 80% while minimizing off-target effects. TiTE achieves increased binding avidity, prolonged immunologic synapse formation, and amplified cytotoxic T cell responses, thereby offering a promising strategy to enhance the efficacy of tumor immunotherapy.

Therefore, polymer-antibody conjugates offer key advantages over conventional mono-, bi-, and tri-specific antibodies, such as enhanced affinity through multivalent binding, facile assembly with precise ratio control, spatiotemporal activation in tumor microenvironment, and superior immune synapse stabilization.

Small molecule immunomodulators

Toll-like receptor (TLR) agonists have emerged as potent immunostimulatory adjuvants, bridging innate and adaptive immunity¹⁵. By activating pattern recognition pathways in dendritic cells (DCs) and other APCs, TLR agonists enhance antigen processing and presentation, thereby promoting de novo CTL priming and expansion^16,17. However, the broader clinical translation of TLR agonists is hampered by rapid systemic clearance, insufficient tumor accumulation, and dose-limiting inflammatory toxicities. Antibody-polymeric drug complexes (pADCs) could be constructed for reducing adverse toxicity, while increasing therapeutic efficiency.

Chu et al. developed αPDL1-PLG-IMDQ by conjugating imidazoquinoline TLR7/8 agonists to anti-PD-L1 antibodies via polyglutamic acid carriers¹³. This design achieves multiple objectives simultaneously, as follows: tumor-specific targeting through PD-L1 binding; controlled agonist release within the acidic tumor microenvironment; local dendritic cell activation without systemic inflammation; and synergistic checkpoint blockade. The spatial restriction of immune activation to tumor sites enabled 97% and 89.4% tumor inhibition in CT26 and 4T1 models, respectively, effectively converting cold tumors to hot tumors without systemic toxicity.

Similarly, Cui et al. developed aPDL1-PLG/R848 nanoparticles addressing the poor solubility and high toxicity of free R848¹⁴. The polymer formulation not only solubilizes the hydrophobic agonist but also provides tumor-specific delivery, achieving 97.3% tumor inhibition with 50% complete responses when combined with ultrasound-triggered release. These systems demonstrate how polymer platforms transform systemically toxic immune agonists into precisely targeted immunotherapeutics.

Cytotoxic drug-induced immunogenic cell death (ICD)

Some cytotoxic drugs, such as anthracyclines, oxaliplatin, and camptothecin derivatives, can induce ICD, releasing damage-associated molecular patterns (DAMPs) and activating anti-tumor immune responses. However, the non-specific distribution of traditional chemotherapy drugs leads to severe normal tissue toxicity, while drug concentrations at tumor sites are often insufficient to effectively induce ICD. Conventional ADCs are constrained by drug-to-antibody ratios below 10, limiting their ability to deliver sufficient cytotoxic agents for effective ICD induction.

Zhang et al. shattered this limitation with aPDL1-NPLG-SN38, achieving DAR = 72 through polyglutamic acid modification¹⁰. This increase in drug loading ensures sufficient SN38 concentration at tumor sites to trigger robust ICD and damage-associated molecular pattern (DAMP) release. In addition, the Fc-III-4C conjugation strategy preserves antibody function, maintaining PD-L1 checkpoint blockade, while delivering cytotoxic payloads. This dual mechanism-simultaneous ICD induction and immune checkpoint inhibition-resulted in 2.8-fold higher tumor accumulation and significantly enhanced therapeutic efficacy.

The concept extends beyond simple cytotoxicity to metabolic immune regulation. Xie et al. developed αCD73-PLG-MMAE targeting the adenosine pathway, a critical metabolic checkpoint in tumor immunity¹¹. CD73 catalyzes adenosine production, creating a profoundly immunosuppressive microenvironment. The polymer conjugate delivers MMAE for tumor killing, while simultaneously blocking CD73-mediated immunosuppression, effectively converting “cold” tumors to “hot” immune-responsive states. When combined with αPD-1, this metabolic reprogramming achieved 92% tumor suppression vs. only 21% for αPD-1 monotherapy, demonstrating the power of targeting metabolic checkpoints alongside conventional immune checkpoints.

Polymer platforms enable the use of less toxic payloads while maintaining efficacy for hematologic malignancies. CD38-targeting pADCs (ISA-P-EPI and DARA-P-EPI) achieved DAR = 30 using HPMA carriers, allowing epirubicin with lower-toxicity to replace traditional highly toxic payloads, while achieving complete tumor eradication¹². This toxicity reduction expands the therapeutic window and improves patient tolerance.

Polymer-based ADCs have the potential to overcome conventional ADC limitations primarily through dramatically enhanced drug loading, which directly translates to improved therapeutic efficacy by ensuring sufficient drug concentration for robust immunogenic cell death and enhanced anti-tumor immune activation. In addition, polymer platforms preserve antibody functionality via non-covalent conjugation strategies, like Fc-III-4C, and minimize organic solvent exposure through aqueous bioorthogonal chemistry, maintaining both antibody integrity and immune effector functions throughout the conjugation process.

Carbodiimide chemistry (EDC/NHS)

EDC/NHS conjugation represents the widely adopted antibody modification strategy, using 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide (EDC) for carboxyl activation. The unstable O-acylisourea intermediate formed by EDC alone exhibits poor reactivity with amines. However, N-hydroxysuccinimide (NHS) enables formation of semi-stable NHS ester intermediates that efficiently react with lysine ε-amino groups, generating stable amide bonds. For example, trastuzumab (anti-HER2) and UCHT1 (anti-CD3) were modified on lysines with BCN via NHS-ester chemistry, then coupled to azido-modified poly-ADP-ribose through copper-free SPAAC⁸. The engineered multivalent complexes bridged HER2⁺ tumor cells and CD3⁺ T cells, increasing local effective concentration and binding avidity.

In addition, Bu et al. demonstrated this principle through PAMAM dendrimer-based PD-L1 antagonist (G7-aPD-L1), clustering multiple anti-PD-L1 antibodies on a single nanoplatform⁷. This multivalent architecture improved the dissociation constant (KD) by an order of magnitude from 9.6 × 10⁻¹⁰ M to 8.5 × 10⁻¹¹ M, while koff decreased 32-fold. These kinetic improvements translated to 1.9-fold increased IL-2 secretion and 2.5-fold enhanced tumor targeting in vivo. However, NHS/EDC coupling is inherently non-site-specific, yielding heterogeneous DARs and potentially impairing binding/Fc function.

Maleic anhydride-amine ring-opening amidation

Maleic anhydrides react directly with antibody surface lysines through nucleophilic ring opening to form stable amide bonds with pendant carboxyl groups. This approach offers direct conjugation without pre-activation steps and achieves high drug-antibody ratios through multisite modification. The pH-responsive SwpH(S) a4-1BB used methylmaleic anhydride chemistry-mPEG-MMMA and was directly conjugated to antibodies at pH 9.5, yielding conjugates stable at physiologic pH but selectively cleaving in tumor-acidic environments⁴. Limitations include non-site-specific modification leading to product heterogeneity, anhydride susceptibility to hydrolysis, and conversion of positively charged lysines to negatively charged carboxylates potentially affecting antibody stability.

Click chemistry

Click chemistry refers to a class of highly efficient, reliable, and user-friendly reactions characterized by modularity, high yields, excellent selectivity, mild reaction conditions, and rapid kinetics. Common coupling strategies in click chemistry include copper-catalyzed azide-alkyne cycloaddition, strain-promoted azide-alkyne cycloaddition, thiol-alkene reactions, photoinduced cycloadditions between tetrazoles and alkenes, and thiol-maleimide reaction, also exhibit click-like characteristics. Among these strategies, thiol-maleimide conjugation and strain-promoted azide-alkyne cycloaddition have been widely adopted in the development of ADCs. Thiol-maleimide conjugation encompasses two approaches (natural disulfide reduction and genetically engineered cysteine conjugation). Natural disulfide reduction uses reducing agents to cleave hinge disulfides, generating thiols for maleimide conjugation, while genetic engineering introduces specific cysteine codons at predetermined positions. Advantages include directional conjugation, mild reaction conditions, and stable thioether linkages. Huang et al. developed PLA-based bispecific nano-engagers via thiol-maleimide chemistry, revealing that controlling αCD3 valence within 25–50 enables stable T-cell pre-binding without premature activation⁹. The Huang et al. study revealed that controlling αCD3 valence within an intermediate range (25–50) enables stable T-cell pre-binding without premature activation, while αPD-L1 mediates tumor-specific localization.

Strain-promoted azide-alkyne cycloaddition (SPAAC) repre-sents advanced bioorthogonal conjugation. Antibodies are pre-modified with NHS-azide or DBCO-NHS reagents, followed by cycloaddition with functionalized polymers forming stable triazole linkages. The reaction proceeds efficiently under physiologic conditions without copper catalysts, achieving high DAR. Limitations include required antibody pre-modification and high synthesis costs for DBCO linkers.

Fc-binding peptide (Fc-III-4C)-mediated conjugation

Fc-III-4C is a 15-amino acid bicyclic peptide that binds specifically to the Fc region of antibodies with high affinity (Kd < 5 nM). This peptide enables the conjugation of antibodies to polymer carriers through non-covalent interactions. This approach offers several advantages. Specifically, this approach completely preserves antibody structure and function because no direct modification is required, enables ultra-high drug loading, allows rapid assembly in simple buffer conditions (2–4 h in PBS), and provides precise control over antibody ratios for creating multi-specific combinations. The multivalent presentation enhances binding strength and therapeutic potency. While this platform shows great promise for clinical translation due to simplicity and versatility, potential limitations include peptide synthesis costs, possible immunogenicity, and competition from endogenous antibodies in the circulation. Xue et al. established a universal Fc-III-4C-poly [_L-glutamic acid] (PGLU) platform that allows one-step, purification-free self-assembly of multi-specific antibodies in aqueous solution⁶. Using this modular system, Xue et al. constructed PD-1/OX40 BsAbs and PD-L1/CD3ε/4-1BB TsAbs, achieving high valence.