Tag: General Medical Oncology & Hematology

Written by: Jaffer A. Ajani, MD, FASCO
This educational opportunity is sponsored by: Jazz Pharmaceuticals

Concept and Technology

Bispecific antibodies (BsAbs) transcend conventional limitations of therapeutic protein engineering by simultaneously engaging two distinct biological targets. Rooted in molecular cooperation, BsAbs combine two functional antigen-binding fragments (often Fab arms) into a single molecule.¹ A considerable novelty over traditional monoclonal antibodies (mAbs), which target a single epitope, BsAbs lead to forced cellular proximity or receptor clustering.^1–3  Technological challenges of manufacturing BsAbs for optimal pharmacokinetics (PK), stability, and purity remain. Yet, their dual-targeting allows BsAbs to mediate synergistic effects and intervene in complex, multi-factorial disease pathways—for example, in oncology, where we find multiple redundant receptors, ligands, and evasion mechanisms.¹ The following review will review BsAb structures, mechanisms of action, safety profiles, and future directions.

Structural Variants

1. Non-IgG-like (Fc-Silent) variants are characterized by a lack of the Fragment crystallizable (Fc) domain, resulting in small molecules that are rapidly cleared by the kidneys, necessitating frequent dosing. Their advantage is high potency and efficient tissue penetration.⁴ These include:
   • Bispecific T-Cell Engagers (BiTEs): Typically constructed as tandem single-chain variable fragments (scFv) that link a tumor-associated antigen (TAA) binder and a CD3 binder via a peptide linker.⁴ Blinatumomab is a well-known example that achieves potent cellular redirection.
   • Dual Affinity Re-targeting Molecules (DARTs): Similar to BiTEs, DARTs incorporate an additional disulfide bridge to improve structural stability.
   • Killer Cell Engagers (BiKEs/TriKEs): These target the innate immune system by engaging CD16 on NK cells. Trispecific Killer Engagers (TriKEs) feature a third component, such as an IL-15 crosslinker, to sustain NK cell proliferation and cytotoxicity.⁴
2. IgG-like (Fc-Containing) formats retain the Y-shaped IgG structure, including the Fc domain, conferring prolonged serum half-life via FcRn recycling.⁴ However, assembling two different heavy chains and two different light chains into a functional heterodimer without forming undesirable mispaired byproducts demands intensive engineering—e.g., CrossMab and/or Knobs-into-Holes (KiH) technologies.^4–6

Mechanisms of Action (MOA)

The therapeutic power of BsAbs lies in their ability to execute mechanisms categorized as acting in-trans or in-cis, based on their molecular or cellular target configuration.

1. In-Trans Mechanisms: The core in-trans function is creating a physical linkage between two distinct molecular or cellular entities. These include:
   • Cellular Bridging (T-Cell Engagers; TCEs): This is the hallmark of oncology BsAbs. By simultaneously binding a TAA and CD3 on T cells, the BsAb forces a physical link, forming a cytolytic synapse.⁶ This mechanism bypasses the need for natural T-cell receptor (TCR) clustering and Major Histocompatibility Complex (MHC) presentation, allowing the T cell to attack regardless of the tumor’s MHC status.^4,6
   • Co-factor Mimicry: Outside of cytotoxicity, BsAbs can direct components to form a functional complex. Emicizumab, approved for Hemophilia A, is an example.^2–6
2. In-Cis Mechanisms: Involve targeting components that reside on the same cell or act within the same signaling pathway. These include:
   • Dual Signaling Inhibition (Dual Blockade): Simultaneously blocks two different receptors or ligands to suppress synergistic pathways crucial for disease progression.
     • Examples: Targeting HER2/HER3 (Zenocutuzumab) or EGFR/MET (Amivantamab) to halt parallel proliferation cascades in cancer.^1–6
   • Biparatopic Engagement: By binding two distinct, non-overlapping epitopes on the same antigen^4,5, biparatopics intensify control over one oncogenic “addiction” pathway via geometry-driven clustering, internalization, and boosted Fc effector functions. Biparatopics enhance binding avidity and promote superior functional modulation of the target, such as forced receptor clustering and internalization, the latter being highly beneficial for Antibody-Drug Conjugates (ADCs). Biparatopic binding drives dense clustering of the same receptor, leading to “caps” on the cell surface, resulting in potent receptor internalization and degradation. This yields deeper and more durable signal blockade than a single monoclonal antibody or cocktail.⁷ The high local receptor and antibody density also enables multimodal effector functions, and helps overcome resistance within a single pathway by engaging distinct functional domains to block both ligand-dependent and ligand-independent signaling and interfere with heterodimerization (e.g., HER2/HER3). They also retain efficacy when tumors escape mono-epitope antibodies through epitope masking or mutation.
     • Example: Zanidatamab, which targets two distinct HER2 epitopes, and is unique in its ability to induce receptor clustering and “capping.”^8-9

Clinical Landscape

The BsAb landscape has rapidly expanded since the first approval of Blinatumomab in 2014, reflecting a growing therapeutic impact across multiple disease areas. As of late 2025, fifteen bispecific molecules have secured FDA approval, spanning both oncology and non-oncology indications. This surge underscores the versatility of BsAbs and their ability to address complex biological pathways through innovative mechanisms of action.

Limitations, Safety, and Risk Mitigation

1. Manufacturing Challenges: The inherent complexity of BsAbs introduces challenges related to stability, manufacturability, and impurity control.¹ The fusion of exogenous antigen-binding domains can decrease biophysical stability, and the complex assembly process frequently results in the formation of product-related impurities and mispaired species, which are difficult to remove during purification. These factors are not merely manufacturing hurdles; they directly influence the biological activity and, critically, the immunogenic potential of the final drug product.
2. MOA-Specific Toxicity: The safety profile of BsAbs is highly dependent on their MOA
   • T-Cell Engager Toxicity: The highly potent, acute T-cell activation triggered by TCEs results in two major, distinct safety concerns. The first is Cytokine Release Syndrome (CRS), a serious acute toxicity caused by the mass release of systemic cytokines. CRS has been reported in up to 70% of patients receiving BsAbs, often necessitating hospitalization and precise management protocols. Severe cases (Grade ≥ 3) occur in 5–10% of patients.⁶ The second concern is Neurotoxicity (ICANS), which, while less frequent than CRS, affects 10–15% of patients and can range from mild confusion to cerebral edema.⁶ In addition, there is an Immune Regulation Paradox. Paradoxically, T-BsAb therapy can trigger the expansion and activation of inhibitory Regulatory T (Treg) cells in the tumor microenvironment, leading to the production of anti-inflammatory cytokines like IL-10. This critically inhibits the desired effector T-cell response, suggesting that combination strategies—such as transient Treg ablation—may be necessary to maximize efficacy.⁶
   • Pathway Blocker and Biparatopic Toxicity: These agents generally do not induce acute, systemic cytokine surges. Instead, their adverse event profiles reflect the targeted receptors. For example, the dual signaling blocker Amivantamab (targets EGFR/MET) exhibits EGFR-inhibition-related dermatologic toxicities, like paronychia, skin fissures, and pruritus, as well as infusion reactions. Dual checkpoint inhibitors can have classic IO toxicities. Biparatopic antibodies, like Zanidatamab, demonstrate a manageable profile but frequently cause gastrointestinal toxicities (such as diarrhea and nausea/vomiting) and infusion-related reactions (IRRs).⁹ Importantly, clinical data for Zanidatamab confirmed no reports of CRS.⁹
3. Mitigating Immunogenicity Risk: The complex structures, engineered sequences, and immunostimulatory MOAs of oncology BsAbs contribute to an increased risk of immunogenicity compared to mAbs. Mitigation must begin at the engineering stage, utilizing in silico prediction and in vitro assays to guide the selection of low-risk antibody constructs through deimmunization and tolerization methods.

Future Directions

The BsAb pipeline remains robust, reflecting a continuous drive toward addressing current clinical limitations and expanding into novel biological territories. The future of BsAbs is characterized by a strategic shift toward overcoming the immunosuppressive tumor microenvironment (TME). Emerging candidates are now focused on targets that modulate the innate immune system and TME suppression, such as LILRB1/2 bispecific IgG1 antibodies for advanced solid tumors.^10-11 Furthermore, BsAbs are expanding beyond simple blockade, with molecules like SAR446422 (CD28xOX40 bispecific) in trial for inflammatory indications, demonstrating the potential for BsAbs to achieve synergistic co-stimulatory agonism.^10-11 The continuous innovation in structural design, focused now on minimizing impurity-driven immunogenicity and maximizing the therapeutic window, ensures that BsAbs are poised to become the standard for highly tailored, multifunctional therapeutic intervention across diverse and complex diseases. The future of BsAbs is very promising.^10-11

References:

1. Shan KS, Musleh Ud Din S, Dalal S, Gonzalez T, Dalal M, Ferraro P, Hussein A, Vulfovich M. Bispecific Antibodies in Solid Tumors: Advances and Challenges. International Journal of Molecular Sciences. 2025; 26(12):5838. https://doi.org/10.3390/ijms26125838.
2. The Bispecific 2024 Landscape Review. Beacon Intelligence. 2024. https://beacon-intelligence.com/landscape-reviews/bispecific/. Accessed November 23, 2025.
3. Ai Z, Wang B, Song Y, Cheng P, Liu X, Sun P. Prodrug-based bispecific antibodies for cancer therapy: advances and future directions. Front Immunol. 2025;16:1523693. Published 2025 Jan 22. doi:10.3389/fimmu.2025.1523693.
4. Amash A, Volkers G, Farber P, et al. Developability considerations for bispecific and multispecific antibodies. MAbs. 2024;16(1):2394229. doi:10.1080/19420862.2024.2394229.
5. Shui L, Wu D, Yang K, Sun C, Li Q, Yin R. Bispecific antibodies: unleashing a new era in oncology treatment. Mol Cancer. 2025;24(1):212. Published 2025 Aug 4. doi:10.1186/s12943-025-02390-y.
6. Dewaele L, Fernandes RA. Bispecific T-cell engagers for the recruitment of T cells in solid tumors: a literature review. Immunother Adv. 2025;5(1):ltae005. Published 2025 Jan 27. doi:10.1093/immadv/ltae005.
7. Kast F, Schwill M, Stüber JC, et al. Engineering an anti-HER2 biparatopic antibody with a multimodal mechanism of action. Nat Commun. 2021;12(1):3790. Published 2021 Jun 18. doi:10.1038/s41467-021-23948-6.
8. Elimova E, Ajani J, Burris H, et al. Zanidatamab plus chemotherapy as first-line treatment for patients with HER2-positive advanced gastro-oesophageal adenocarcinoma: primary results of a multicentre, single-arm, phase 2 study. Lancet Oncol. 2025;26(7):847-859. doi:10.1016/S1470-2045(25)00287-6.
9. Ziihera Safety Information. Ziihera HCP (Jazz Pharmaceuticals). https://www.ziiherahcp.com/safety. Accessed November 23, 2025.
10. Wen J, Cui W, Yin X, et al. Application and future prospects of bispecific antibodies in the treatment of non-small cell lung cancer. Cancer Biol Med. 2025;22(4):348-375. doi:10.20892/j.issn.2095-3941.2024.0470.
11. Engineering the Next Generation of Bispecific Antibodies. PEGS Europe 2024 Archive. https://www.pegsummiteurope.com/24/engineering-bispecifics. Accessed November 23, 2025